Magnetic Fabric and Palaeomagnetic Studies in the Texas ... · MAGNETIC FABRIC AND PALAEOMAGNETIC STUDIES IN THE TEXAS AND COFFS HARBOUR BLOCKS, NEW ENGLAND OROGEN Palaeomagnetism

J

MAGNETIC FABRIC AND

PALAEOMAGNETIC STUDIES

IN THE TEXAS AND COFFS

HARBOUR BLOCKS,

NEW ENGLAND OROGEN

• -jo rnm:,JCATIONS COMP (LEJ.~IUNG SECnON) :ACTVS

BYC AUBORG, CT. KLOOTWIJK& R.J. KORSCH

RECORD 1994/58

AUSTRALIAN GEOLOGICAL SURVEY " ' ORGANISATION

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• 3

• • •

1994/58

MAGNETIC FABRIC AND PALAEOMAGNETIC STUDIES IN THE

TEXAS AND COFFS HARBOUR BLOCKS,

NEW ENGLAND OROGEN

Palaeomagnetism Project 224.03

Sedimentary Basins of Eastern Australia Mapping Accord Project 112.05

C. AUBOURG1,2,3, C.T. KLOOlWIJK1 and R.J. KORSCH1

Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT 2601, Australia. Laboratoire de Geophysique Interne et Tectonophysique, Universite J. Fourier, BP 53X, 38041 Grenoble, France. Present address: Laboratoire de Structures des Materiaux Geologiques, Universite de CergyPontoise, 8 Avenue du Parc, Le Campus, Batiment I, BP 8428, 95806 Cergy, France.

II II ~II" ~II ~ *R9405801*

DEPARTMENT OF PRIMARY INDUSTRIES AND ENERGY

Minister for Resources: The Hon. David Beddall, MP Secretary: Greg Taylor

AUSTRALIAN GEOLOGICAL SURVEY ORGANISATION

Executive Director: Harvey Jacka, AM

© Commonwealth of Australia, 1994

ISSN: 1039-0073 ISBN: 0 642 22233 9

This work is copyright. Apart from any fair dealing for the purpose of study, research, criticism, or review, as permitted under the Copyright Act, no part may be reproduced by any process without written permission. Copyright is the responsibility of the Director, Australian Geological Survey Organisation. Inquiries should be directed to the Principal Information Officer, Australian Geological Survey Organisation, GPO Box 378, Canberra City, ACT, 2601.

It is recommended that this publication be referred to as:

AUBOURG, c., KLOOTWIJK, C.T., and KORSCH, RJ., 1994. Magnetic fabric and palaeomagnetic studies in the Texas and Coffs Harbour blocks, New England Orogen. Australian Geological Survey Organisation. Record 1994/58.

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Contents

CONTENTS

ABSTRACT •....................................................•........... 1

INTRODUCTION ............................•......................•..•.•..•. 3

ANISOTROPY OF MAGNETIC SUSCEPTIBILITY: A BRIEF REVIEW ..•............•...•..• 3 AMS parameters •...........................•..........•.....•.•.•..•.• 3

Directional data ........•.............•..................•....•.. 3 Anisotropy parameters .............................•.....•..•.••.. 3

AMS measurement techniques .............................•.....•.•....•. 4 Spinner method ......................................•.......... 4

(1) Specimen shape .........................•....•....•.... 5 (2) Intercalibration of spinner magnetometer and impedance bridge ..... 5

Impedance bridge method .................................•....... 5 Sensitivity and precision .........................................•....... 5 Specimen shape and measurement time .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 5 MagnetiC carriers ...................................................... 6 Basic rules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

(1) Orientation of AMS axes versus the petrofabric . . . . . . . . . . . . . . . • . . . . . . . . 7 a) Scattering of AMS axes ...................•.....•.......... 7 b) Interchange of AMS axes .................................. 7

(2) Length of AMS axes versus deformation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 a) The intrinsic anisotropy ....................•.............. 9 b) The density function .... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 c) The type of deformation ................................... 9

Some complementary techniques ......................................... 10 (1) Magnetic mineralogy ...................................•...... 10 (2) Remanence anisotropies ....................................... 10 (3) Anisotropy of magnetic susceptibility in high field ..................... 10

Concl usion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 10

APPLICATION OF AMS TO THE NEW ENGLAND OROGEN . . . . . . . . . . . . . . . . . . . . . . . . . . .. 12 Tectonic framework ................................................... 12 Deformational history .................................................. 12

Eastern Coffs Harbour block . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . • . .. 12 Central Coffs Harbour block ....................................... 12 Texas block ................................................... 14

Oroclinal bending . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 14 Sampling ........................................................... 15

AMS sampling ................................................. 15 Palaeomagnetic sampling ......... . . . . . . . . . . . . . . . . . . . . . . . . . • . . . . .. 17

AMS results, general comparison KLY-2 and DIGICO ........................... 19 General observations ....•....................................... 19 Specific examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 21

Contents

a) Sampling parallel to the foliation plane . . . . . . . . . . . . . . . . . . . . . . . . 21

b) Sampling perpendicular to bedding . . . . . . . . . . . . . . . . . . . . . . . . . . 21

AMS results, KL Y -2 impedance bridge ..................................... 23

I nterpretation of KL Y -2 results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

KLY-2 anisotropy parameters ...................................... 25

a) Susceptibility .......................................... 25

b) Magnitude of anisotropy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 25

c) Shape of the magnetic fabric. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 25

d) Presentation of site results ................................ 25

AMS results, DIGICO anisotropy spinner .................................... 25

DIGICO anisotropy parameters .........•........................... 27

a) Susceptibility .......................................... 27

b) Shape of the magnetic fabric .............................. 27

c) Foliation parameter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 27

d) Lineation parameter ..................................... 27

e) Presentation of site results ................................ 27

Comparison of magnetic fabric (AMS) results and structural observations ......... . .. 27

General procedure .............................................. 27

Eastern Coffs Harbour block . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 28

Central Coffs Harbour block ....................................... 33

Texas block ................................................... 34

Terrica beds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 40

Gilgurry Mudstone .............................................. 42

Origin of magnetic fabric . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 42

Origin of lineation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 42

Shape of the magnetic fabric . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 44

a) The Texas block ........................................ 44

b) The central Coffs Harbour block . . . . . . . . . . . . . . . . . . . . . . . . . . .. 44

c) The eastern Coffs Harbour block ........................... , 44

MAGNETIC MINERALOGY .................................................... 45

Intrinsic anisotropy of magnetic carriers ..................................... 46

Magnetic mineralogy constraints eastern Coffs Harbour block ..................... 47

SOME PALAEOMAGNETIC RESULTS ..... , ...................................... 47

Background ... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

Alum Rock palaeomagnetic results ........................................ 49 Low blocking temperature (LT) components ........................... 52

High blocking temperature (HT) components . . . . . . . . . . . . . . . . . . . . . . . . . .. 52

DISCUSSION .............................................................. 55

AMS interpretation premises ..................... . . . . . . . . . . . . . . . . . . . . . . .. 55

Origin of magnetic fabric . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 59

Regional lineation observations .... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 59

D1-lntersection lineations, eastern Coffs Harbour block ................... 59

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Contents

Stretching lineations, Texas and eastern Coffs Harbour blocks .............. 60 Intensity and shape of magnetic anisotropy in the Texas block .................... 60 Magnetic fabric versus lineament patterns ................................... 60

CONCLUSIONS ......................................................... , .. 62

ACKNOWLEDGEMENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 63

REFERENCES .......................................................•..... 64

APPENDIX: DIGICO AMS results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 71

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Abstract

ABSTRACT

A magnetic fabric study was carried out on clay-rich rocks from the Texas beds and the Coffs Harbour• Association of the southern New England Orogen in order to test the hypothesis of an oroclinal origin

•(Flood and Ferguson, 1982) for the Z-shaped outline of the Texas and Coffs Harbour blocks.Measurements were carried out on a DIGICO spinner anisotropy unit (Canberra) and on a KLY-2

• impedance bridge (Grenoble, France). Results from the two instruments showed acceptable agreementfor most anisotropy parameters other than lineation. This discrepancy is probably due to shape-

• sensitivity of the DIGICO unit. The magnetic fabric results showed good agreement with previousstructural observations in the Texas and Coffs Harbour blocks. The limited set of results for the Texas

• block showed an increase in anisotropy from the limbs toward the hinge of the megafold, with a more

• prolate magnetic fabric pattern on the interior and a more oblate pattern on the exterior side of themegafold. The magnetic foliation pattern is related to the D1-cleavage throughout the Texas beds and

• the Coffs Harbour block and rotated passively with bedding and cleavage during D3 oroclinal bending.Change of the foliation pattern with strike is particularly well demonstrated from the southeastern Coffs

• Harbour block. Magnetic lineation patterns in the eastern Coffs Harbour block are interpreted as being

• mainly accretion- and subduction-related D1 intersection lineations. In the central Coffs Harbour blockand the Texas block, in contrast, the magnetic lineation patterns are interpreted as being mainly D1

• stretching lineations. A north-to-south increase in anisotropy of the eastern Coffs Harbour block agreeswith previous field observations for a north-to-south increase in intensity of deformation.

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A magnetic remanence study of Early Permian (Sakmarian) rhyolitic igninribrites from the Alum Rockinlier in the Texas block showed a clearly-defined characteristic, presumed primary, magnetization

• component. Its direction is offset at least eighty degrees clockwise from the direction expected from theapparent polar wander path for cratonic Australia (Klootwijk, Giddings & Percival, 1993), and accounts

• for about half of the oroclinal bending. Oroclinal bending was, thus, already in progress during extrusion

• of the Alum Rock ignimbrites in the Early Permian.

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Introduction

INTRODUCTION

This study provides magnetic fabric and palaeomagnetic data for the Texas and Coffs Harbour blocksto test and constrain models that view bending of the southern New England Orogen as oroclinal inorigin. We studied a large collection of oriented cores (276 cores, 21 sites) in shales and other clay-richrocks throughout the Texas and Coffs Harbour blocks in order to determine anisotropy of magneticsusceptibility (AMS). This technique gives fast and precise information on the preferred orientation ofmagnetic grains in rock and, hence, provides information on deformational history. An adjunct

palaeomagnetic study was made on felsic volcanic rocks of Early Permian age to provide additionalconstraints on the magnitude and timing of oroclinal bending of the Texas block.

ANISOTROPY OF MAGNETIC SUSCEPTIBILITY : A BRIEF REVIEW

Analysis of anisotropy of magnetic susceptibility patterns is a powerful tool in structural geology. Thetechnique is well-established in Europe and North America, but not so in Australia, despite pioneeringstudies by Stacey et al. (1960) on rocks from the Canberra-Cooma region. Major improvements in bothinstrumentation and measurement and analysis techniques have been achieved during the last decade,and reliable and precise results can now be obtained. Comprehensive reviews of the technique arepresented in Hrouda (1982), Borradaile (1988), Jackson and Tauxe (1991), Rochette et al. (1992), andTarling and Hrouda (1993).

AMS parametersReduction of AMS measurements provides an ellipsoid of magnetic susceptibility K ii , defined by thelength and orientation of the principal axes Kmax >Kint >Kmin (Fig. 1). The AMS measurements thusprovide two types of information (Hrouda, 1982):

••• Directional data

Kmax = maximum susceptibility;O Kmin = minimum susceptibility;

Kim = intermediate susceptibility;

•the axis of easy magnetizationthe axis of hard magnetizationdirectionally, relative to Kmax and Kmin used to definewhether the fabric is characterized by a lineation(prolate) or a foliation (oblate)

•Anisotropy parameters

Km ' (Kmax + Kint + Kmin)/3;^(SI units)Km = mean susceptibilityP = degree of anisotropy P = Kmax/KminL = lineation parameter L = Kmax/KintF = foliation parameter F - Kint/KminT = shape parameter T = (2logF/logP) -1

(-1 <T<1 = prolate - ellipsoid - oblate)

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Follows p. 1

Anisotropy of magnetic susceptibility: a brief review

North

Kmax>Kint>Kmin, Km= (Kmax+Kint+Kmin)/3 P= KmaxlKmin : degree of anisotropy L = KmaxlKint : stretching F = Kint/Kmin : compaction

North

Stereoplot (bedding horizontal)

Figure 1 Schematic overview of the relationship between the ellipsoid of susceptibility and sedimentary fabric. The ~n axis is

perpendicular to the bedding, whereas the Kmax axis lies within the bedding plane.

AMS data are generally displayed according to the convention of Ellwood et al. (1988). The Kmax and

Kmin axes are plotted as filled squares and filled circles respectively, on the lower hemisphere of an

equal area stereographic projection (Schmidt net). The statistically appropriate way to process the data

is through determination of a tensorial mean (Jelinek. 1978). This method provides a mean tensor with

three principal axes ~*, Kint* and Kmin*. These axes may be biased, however, towards those

specimens with higher susceptibilities (which can be corrected using a mean susceptibility normalisation

of the tensors) and higher anisotropy parameters (Aubourg et aI., 1991).

AMS measurement techniques Two measurement techniques are common (see Collinson, 1983): the spinner magnetometer technique

and the impedance bridge technique.

Spinner method (DIG/CO. MOLSPIN) Elements of the anisotropy tensor are determined by rotation of individual specimens successively within

three orthogonal planes. Bulk susceptibility is measured independently with an impedance bridge. The

combination of these two datasets determines the susceptibility ellipsoid for individual specimens. The

mean tensor for a site or location and the corresponding eigenvalues and eigendirections can be

calculated using the susceptibility ellipsoids for the member specimens of the site or location. There are,

however, two problems with this technique:

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••• Anisotropy of magnetic susceptibility: a brief review

• (1) Specimen shape• The length/diameter ratio of the sample has to be chosen such that the sample best approximates a

sphere. For cylindrical specimens, the optimal length/diameter ratio is 0.845 (Porath et al., 1966). The• optimal length for the standard 2.54 cm diameter cylindrical sample is therefore about 2.25 cm. Longer

samples will result in a distorted ellipsoid with too-high a susceptibility value for the length-axis of the• core.•

(2) Intercalibration of spinner magnetometer and impedance bridge• Significantly different shapes for the susceptibility ellipsoid may result from the use of different bulk

susceptibility values. Although not expected on theoretical grounds, this may be the result of a numerical• imperfection in the tensor-diagonalisation method. DIGICO spinner measurements provide the deviatoric

• tensor. Incorrect cross-calibration of the DIGICO bulk susceptibility impedance bridge and the DIGICOanisotropy spinner unit, resulting in inappropriate values for the bulk susceptibility IC, should not

• change the orientation and shape of the ellipsoid. Only the magnitude of the eigenvalues should change,i.e. the susceptibilities along the three principal axes.

•

•Impedance bridge method (KLY-2 and SAPPHIRE) Impedance bridges are considered superior to the above mentioned spinner devices, in both precision

• and sensitivity (e.g. Rochette, 1988). The main advantage of the impedance bridge is avoidance of cross-calibration errors because all susceptibility measurements are carried out on the same instrument. The

• measurement sequence requires 9 (for the SAPPHIRE) to 15 measurements (for the KLY-2), which

• permits calculation of the susceptibility ellipsoid from an overdetermined set of tensor elements. Theimpedance technique is far less shape-sensitive than the spinner method, but requires a longer

• measurement cycle, about 5 minutes for the KLY-2.•Sensitivity and precisionThe high sensitivity of commonly used devices (down to 5*10 -8 SI for the KLY-2) allows measurementof a wide range of rock types, from diamagnetic quartzite or limestone (Km in the order of -10 -5 SI) toferromagnetic volcanic rocks with a high magnetite content (Km in the order of 10 1 SI).

Susceptibility anisotropies of 0.1% can be detected with the best commercial device, the KLY-2. Mostdevices can determine with reasonable precision the susceptibility ellipsoid for rocks that havesusceptibility values as low as 10 to 10 -6 SI and with less than 1% anisotropy. Such high sensitivityallows detection of only marginally preferred orientations of magnetic grains, well below the detectionlevel of microscopic methods and immeasurable on mesoscopic and macroscopic scales.

Specimen shape and measurement timeAMS measurements can be carried out on the same specimens that are used for palaeomagneticstudies, subject to the shape guidelines. The AMS measurement procedure is non-destructive and thespecimens can be used subsequently for any other purpose (e.g. palaeomagnetic or rockmagneticstudies, reflected/refracted light studies of polished thin sections, MOssbauer spectometry).

The measurement cycle takes about two minutes for spinner magnetometers (DIGICO, MOLSPIN),

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provided that the bulk susceptibility IC has been determined already, and about five minutes forimpedance bridges (KLY-2, SAPPHIRE). A statistically acceptable determination of the magnetic fabricrequires measurement of a minimum of eight independently oriented specimens per site. In general,eight to sixteen specimens are measured per site.

The complete processing of a suite of specimens from a single site can thus be summarized as follows:(i) sampling and orientation in the field, —10 min/sample; (ii) preparation of specimens from the cores,—10 min/core; (iii) specimen measurements, —5 min/specimen. For an average of 10 specimens persite, it will take about four hours to obtain all the magnetic data required for determination of the meansusceptibility ellipsoid.

Magnetic carriersThe important contribution that non-remanent magnetic carriers may make to AMS measurements hasbeen recognised only during the past 10 years (Hrouda, 1982; Hounslow, 1985; Rochette, 1987a; Henry,1988). The paramagnetic, diamagnetic and antiferromagnetic minerals are commonly referred to as"matrix" because these phases usually represent a large volume fraction of the rock. The AMS tensor,K both a magnetic grain F ii and "matrix" MI contribution:

Kii =Fii +M ii

The diamagnetic contribution of the matrix (contributed by all grains, because diamagnetism is auniversal property) is expected to be isotropic with a negative susceptibility close to -10 -5 SI (Rochetteet al., 1983; Ihm16 et al., 1989). The phyllosilicates generally have paramagnetic properties. Theseminerals have a mean susceptibility Km of about 10 -3 to 104 SI and have an intrinsic magnetocrystalline

anisotropy. The degrees of anisotropy (P= Kmax/Kmin) of biotite, chlorite and muscovite are respectively1.35 (Zapletal, 1990), 1.2 (Borradaile and Sarvas, 1990) and 1.41 (Borradaile, 1988).

The most common magnetic minerals are magnetite, hematite and pyrrhotite with Km values of about1, 10-3 to 1 0-2, and 0.3 SI respectively. Hematite and pyrrhotite have a magnetocrystalline anisotropy

giving P values greater than 100 (Uyeda et al., 1963). Magnetite has a shape anisotropy giving P valuesup to 5 (Uyeda et al., 1963; Hrouda, 1982). However, for magnetite grains that are of detrital origin, Pvalues are expected of about 1.5-2.5 (Borradaile, 1987; Aubourg et al., 1991). A notable feature ofmagnetite grains is the possible interchange of the maximum and the minimum susceptibility axes thatdepends on domain size. Whereas coarse multidomain (MD) grains follow the general rule that the K max

axis is parallel to the long axis of the grain, fine single domain (SD) grains can have a Kmax axisperpendicular to the long axis of the grain (Daly, 1970; Potter and Stephenson, 1988). In both cases (SD

and MD), however, the remanence direction remains parallel to the long axis.

Rochette (1987a), Henry (1988) and Hrouda and Jelinek (1990) recognised the importance of the matrixcontribution in rocks with Km values less than 350*10 -6 SI and P values below 1.35. Aubourg et al.(1995) demonstrated the importance of the AMS matrix contribution in sediments, even in cases wherethe susceptibility of the magnetic grains exceeds the matrix susceptibility. Their study underlined thatthe AMS contribution of a magnetic phase depends not only on its magnetic properties (susceptibility,anisotropy), but of course also on the preferred orientation of grains in the rock.

6

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Supporting magnetic mineralogy studies therefore should be carried out routinely in order to determine

• the magnetic carriers (see Rochette, 1987a; Jackson et al., 1989). This is similar to the requirement ofpalaeomagnetic studies that the magnetic carriers be characterized for a more robust interpretation.

•

• Basic rules

• Three basic AMS rules have been established (see Rochette et al., 1992). Unfortunately, these generalrules do not always apply, and the assumption that the magnetic fabric relates directly to the structural

• fabric may be useful as a rule of the thumb, but is not universally applicable.

• /. The orientation of AMS axes is coaxial with the structural fabric.

• In sedimentary rocks for example (Fig. 1), there is a hard axis of magnetization (Kmin) that isperpendicular to the bedding with an easy axis of magnetization (Kmax) that is aligned within the bedding

• plane and is often parallel to a sedimentary or structural lineation.

• 2. The lengths of AMS axes are related to the X>Y>Z axes of finite deformation.

• In a weakly deformed sedimentary rock such as shale (Fig. 1), the foliation parameter F is greater thanthe lineation parameter L, reflecting a planar preferred orientation of the magnetic carriers. The F>L

• relationship can be related to the Flinn diagram, used in structural geology, where Y/Z > X/Y (Flinn,1962).

3. Both orientation and length of the AMS axes are independent of the natural remanent magnetization(NRM) vector.Some dependencies for magnetite-rich volcanic rocks, however, have been discussed recently (Park etal., 1988; Rochette et al., 1992).

• (1) Orientation of AMS axes versus the petrofabricTwo types of perturbation can occur: scattering and interchange of AMS axes.

•a) Scattering of AMS axes

• Figure 2 displays some classic examples of scattering of AMS axes that are not due to fabric scattering

•(e.g. fluctuation of bedding or cleavage). Figure 2A shows dispersion of the K min axes in a planeperpendicular to well-grouped Kmax axes. This results from the strongly preferred linear orientation of

III^magnetic carriers. Analysis of the anisotropy parameters as obtained from statistical treatment of theanisotropy tensors should show the relation L*> F* (* relates to mean tensor data for a site or location)

• between the tensorial anisotropy parameters, where L* is the linearity parameter and F is the foliation

•parameter as defined previously. Figure 28 displays a wide scattering of Kmax axes. This could be dueeither to a purely planar fabric, in which case the L* value should be close to 1 (Kmax*,-.Kint*), or to the

• combined effect of several preferred linear orientations of magnetic carriers. The latter case has beenrecognised by Aubourg et al. (1991).

0b) Interchange of AMS axes

• There is growing interest in the interchange of AMS axes. This phenomenon has been observed in

• synthetic samples (Potter and Stephenson, 1988), in experimentally deformed clays (Borradaile andPuuamala, 1989), and in calcite-rich rocks (Ihmle et al., 1989; Aubourg et al., 1995). Rochette (1988)

I• 7

•

•

••••


A) SITEATCH

Kmax • Kmin •

B) SITENIO [Au bourg et al., 1991]

Figure 2 Typical example of scattering of AMS axes. A) zonal dispersion of Kunn axes around the Kxnax axes related to an

intersection lineation between bedding and cleavage. B) large dispersion of Kxnax axes. This could reflect a pure planar

anisotropy or the combined effect of multiple fabric patterns.

reviewed the cause of this interchange, whereas Rochette et al. (1992) and Aubourg et al. (1995)

proposed a model that relates the various interchanges of AMS axes to the relative content of single

domain (SO) grains. In this model, inversion of Kmax * I~nt*, ~nt* IKmin * and Kmax * IKmin * axes is referred to as anomalous lineation, intermediate fabric, and inverse fabric respectively. The anisotropy

of the remanent magnetization fraction can be used to identify the cause of these inversions.

(2) Length of AMS axes versus deformation

The anisotropy parameters relate both to the preferred orientation of the magnetic carriers (sedimentary

and deformation effects) and to their intrinsic magnetic properties {susceptibility and anisotropy}.

Rochette (1987a) and Borradaile and Sarvas (1990) have shown that a relationship between the degree

of anisotropy P and the mean susceptibility Km can be interpreted as the effect of magnetic mineralogy

on the magnitude of the anisotropy parameters. Henry (1983, 1988) proposed a model that applies this

relationship in order to discriminate between the contributions of various magnetic phases, for example,

to separate the matrix: contribution from the magnetic mineral contribution.

Another effect of the magnetic mineralogy on the anisotropy parameters was recognised by Bernier et

al. {1987} in a case where the isotropic diamagnetic contribution exponentially enhanced the P

parameter for susceptibility values close to zero. Recently Rochette et al. (1992) showed that rocks with

a mixture of SO (single domain) and MD (multidomain) grains can show considerable variations of the

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anisotropy parameter resulting from only minor changes in the SO jMO ratio.

Early in the development of AMS techniques, several authors tried to correlate the magnitude of the AMS

axes to the X-Y-Z axes of deformation. Wood et al. (1976) found a good correlation and established the

empirical law:

= (A/>'t, where

= principal axes of the susceptibility ellipsoid

= principal quadratic extension J X1 = IXI (Ramsay and Huber, 1983)

Some incorrect correlations have been reported, however. For instance, Rathore (1979) and K1igfield et

al. (1981) attempted to correlate between the overall groupings of the three principal susceptibility axes

and the deformation axes, without due consideration to the spread of the axes within each group. This

empirical approach may lead to acceptable results in some specific cases (e.g. Hrouda, 1982), but often

a more refined method is needed that separates the anisotropy contribution of the matrix from that of

the magnetic minerals. Although such observations show that the anisotropy parameters will tend

towards their finite intrinsic values with increasing deformation, Vergne et al. (1988) demonstrated that

such a correlation has no theoretical justification.

Recently, much attention has been given to theoretical attempts to quantify deformation in terms of AMS

results (e.g. Rochette and Lamarche, 1986; Henry and Hrouda, 1989; Hrouda and Schulman, 1990). This

approach has met with moderate success. Such theoretical considerations are based on the following

considerations:

a) The intrinsic anisotropy of the magnetic carriers

The intrinsic anisotropy, i.e. the anisotropy upon perfect alignment of the magnetic grains, can be well

estimated in clay-rich rocks where the intrinsic anisotropy of the matrix is well known. It can only be

determined approximately, however, in magnetite-rich rocks where the anisotropy parameters (P, L, F,

T) may be affected considerably by the shape and size of the samples, the intensity of NRM, and

variations in the ratio of SO to MO grains.

b) The density function in relation to rotation of magnetic carriers in the matrix

Two density functions are commonly used: the Fischer density function (e.g. Henry and Hrouda, 1989),

and the March (1932) function, as improved recently by Fernandez (1984). The March function assumes

the matrix to be ductile, and the markers rigid with symmetry and without interaction between them.

However, for clay-rich rocks, it is well known that rotation of the phyllosilicate fraction reaches a

maximum level at moderate deformation (''tiling'' of grains, e.g. Housen and Van der Pluijm, 1990). In this

case, the Fisher or March functions represent no more than very rough approximations.

c) The type of deformation

Owens (1974) and Fernandez (1984) proposed different equations relating to different deformational mechanisms such as pure shear and simple shear. Rocks generally have undergone a combination of

such deformations, and it may be difficult to correlate a particular magnetic fabric with its corresponding

increment of deformation.

9


Despite these reservations, the theoretical approach seems appropriate. It requires, however, majorimprovements.

Some complementary techniquesFor succesful interpretation of AMS results covering a range of rock types and different stages ofdeformation, additional information needs to be gathered using different techniques:

(1) Magnetic mineralogy^0It is essential to determine the magnetic carriers. The Curie temperature, coercivity, remanence ratio(Jackson and Tauxe, 1991), Lowrie-Fuller test (Johnson et al., 1975), and high-field susceptibility^•(Rochette et al., 1983) are now routinely determined in conjunction with AMS studies.

•

f2) Remanence anisotropiesThe anisotropy contribution of the magnetic grains must be separated from that of the matrix(Stephenson et al., 1986; Jackson, 1991). There is a growing interest in the determination of remanenceanisotropies using anhysteretic remanence techniques (McCabe et al., 1985; Jackson et al., 1988;Aubourg et al., 1995). This artificial magnetization is easily imparted and erased in a palaeomagneticlaboratory without the chemical and/or coercivity changes that often respectively accompanythermoremanent and isothermal magnetization studies (e.g. Tame et al., 1990).

(3j Anisotropy of magnetic susceptibility in high fieldIn order to measure the matrix contribution M.1.1.' and thereby separate the magnetic grain contribution

using the relationship K ii =M ii + Fir The advantages of this technique are considerable as reviewed inRochette and Fillion (1988) and Hrouda and Jelinek (1990). Major improvements in instrumentation needto be achieved, however, before this method can become realy effective.

ConclusionAMS is a powerful technique for quick and accurate determination of preferred orientations of magneticgrains. It has considerable potential to contribute to solving a wide range of structural geologicalproblems. The direct relationship "magnetic fabric equals structural fabric" has been shown to be validin numerous studies (e.g. Hrouda, 1982). In cases where the relationship is not applicable, for instancedue to scattering or interchange of AMS axes, further magnetic investigations are necessary tocharacterize the origin and disposition of the magnetic carriers. •

•Figure 3 Present-day distribution of the Texas, Coffs Harbour and Manning Oroclines in the southern New England Orogen,

including interpretation beneath younger cover. Heavy solid line separates the Tamworth Belt (Terrane) from theaccretionary wedge sequence, and heavy dashed line separates the Sandon Association from the Coffs Harbour Association.In places, particularly in the south, the stipple pattern for the Woolomin and Sandon Associations also covers areas ofpresently exposed Permian sequences. The boxes show the study areas of the Texas block (a), central Coffs Harbour block(b) and eastern Coffs Harbour block (c), see Figures 4 and 12. After Korsch and Harrington (1987).

•10^ S

•

••S••

•

••I•

I

Application of AMS to the New England Orogen

Tamworth Terrane^

Coffs Harbour Association

Woolomin and Sandon Associations

Idealised strike of bedding16/N131

11

•


APPLICATION OF AMS TO THE NEW ENGLAND OROGEN

Tectonic frameworkThe southern New England Orogen has been interpreted by many workers as a late Palaeozoic volcanicarc-forearc basin-accretionary wedge complex related to a convergent plate margin; for a recent reviewsee Korsch et al. (1990). The Texas and Coffs Harbour blocks (Fig. 3) form part of the accretionarywedge and hence have suffered severe deformation. The two blocks are separated by the Demon Fault,a strike-slip fault along which the only demonstrable displacement is Triassic in age (Korsch et al., 1978).

Deformational historyEastern Coffs Harbour blockIn the eastern Coffs Harbour block, Korsch (1973, 1981) recognised three distinct deformational events.The first two events produced mesoscopic structures whereas the third is obvious only on a regionalscale, producing a complex, regional-scale syncline. The first deformation, D i , produced upright foldsin bedding and an associated axial-plane cleavage. Intensity of deformation in structures of this phaseincreases towards the south of the block (Korsch, 1973), and is accompanied by an increase in themineralogical grade of regional metamorphism (Korsch, 1978). The second deformation, D2, althoughwidespread, was not as intense as the D1-deformation and formed gentle flexures, kinks and chevronfolds.

The mesoscopic structures produced by D i and D2 have quite different orientations north and south ofRed Rock (Figs 3, 4). To the south, both bedding and cleavage strike approximately east-west, with thestrike becoming more northwest-southeast in the southernmost part of the block; younging directionsare dominantly to the north. To the north of Red Rock, the strikes of bedding and cleavage areapproximately north-south; younging directions are dominantly to the west. This led Korsch (1981) topropose that the D i and 02 structures had been folded around a regional-scale, complex syncline (D 3)that thus developed at some stage after the D i and D2 events. Structural data from the Solitary Islandslocated offshore of the Coffs Harbour block, such as bedding and cleavage orientations and youngingdirections, confirm the presence of the syncline and help to constrain its geometry (Korsch, 1993). •In a tectonic environment such as an accretionary wedge, the style of deformation should remainconsistent, but the timing of the deformation would be diachronous, with the locus of deformation^•migrating oceanwards towards the palaeo-trench and subduction zone. Nevertheless, Korsch (1978)demonstrated that a low-grade regional metamorphism accompanied the first mesoscopic deformationalevent (D 1 ), and that this was followed some time later by a regional-scale static thermal metamorphic^•event, which was post-D3 in age. These metamorphic events have been dated by Graham and Korsch(1985) using the Rb-Sr whole-rock technique as 318+8 Ma and 238±5 Ma respectively.^ •

•Central Coffs Harbour blockIn the central Coffs Harbour block, Fergusson (1982a, 1982b) also documented the presence of two^•mesoscopic deformations which he correlated with the D i and D2 deformations of the eastern CoffsHarbour block. In this area, however, the structures are dominated by northwest-southeast strikingbedding and cleavage. Fergusson also described transected D 1 mesoscopic folds and macroscopicanticlines, synclines and faults that are associated with the D i deformation. The Gundahl Complex was •

12^ •

•

153°

Great Artesian Basin(Mesozoic)

– 29°

\\k` ANTE \

–

TDAN ■ \ \ \-,\\Ct \

ATC ---- ,.: , \ \ \-...,.....;-- -..., \ 'N. 111(T

To-n3-7&.\\■ ,7. '- ------- p`\ PN\ —

------___, .,A,.-No\^P ^‘

I' \ ■^0\^\^\

^

TB < \l /^\ \^1 \

\ \ \\\

^

\ 1^\^,\ CI^\\^k \

\ i I TA

Warwick

Mt Barney

Emu Creek block

Baryulgil Serpentinite

Clarence - MoretonBasin

(Mesozoic)


20/W7

S-DwDsDwD-Cs

Woolomin beds

Silveiwood Group

Willowie Creek beds

Sandon beds

CmCgCtCc

Leyburn beds

Gundahl Complex

Texas beds

Coffs Harbour succession

?C?P

D-C

Carboniferous (?) undifferentiated

Permian (?) undifferentiated

Permian sequences

Devonian and Carboniferous sequenceof the Tamworth Belt

Late Carboniferous Bundarra Suite A Permian sediments

III Alum Rock

Texas beds and Coffs Harbour Association

Permian-Triassic intrusives and volcanics

Fault

— — Trend lines

Figure 4 Geology of the Texas-Coifs Harbour region (after Fergusson, 1982a) and location of the sampling sites (Table 1). SeeFigure 3 for legend to boxes.

13


also described from this area by Fergusson (1982a, 1984). It is a classic tectonic mélange, typical of thatfound in subduction-related accretionary wedges elsewhere in the world.

Texas blockLucas (1960) noted that the rocks in the Texas block (Figs 3, 4) constitute a thrust pile that was foldedinto a regional-scale antfformal structure, with the younging directions facing predominantly outwardsfrom the core. Olgers et al. (1974) considered that the Texas beds were folded into a large synclinalstructure, based on bedding orientations. Nevertheless, Butler (1974) confirmed the original, anticlinalinterpretation of Lucas (1960). The anticlinal structure and consistent outward-facing younging directionswas later supported by the regional mapping of Fergusson and Flood (1984).

The presence of cleavage was reported by Olgers et al. (1974) and Butler (1974), who mapped differentorientations in three structural domains. Butler also noted that the cleavage had been deformed by alater event that produced kinks and gentle mesoscopic folds. This is presumably equivalent to the D2

structures recorded from the Coffs Harbour block.

Oroclinal bendingKorsch (1975, 1978) suggested that the southern part of the Coffs Harbour block had been deformedby oroclinal bending. Later, Flood and Fergusson (1982) and Fergusson and Flood (1984) suggestedthat mélange and tectonostratigraphic units in the Texas block could be correlated with similar unitsmapped in the central Coffs Harbour block by Fergusson (1982a). They suggested that the rocks of theTexas and Coffs Harbour blocks were once continuous, and that the regional-scale anticline describedby Lucas (1960) and the syncline described by Korsch (1975) were produced during the samedeformational event. They proposed that the rocks had been folded into a Z-shaped megafold, whichthey termed the Texas-Coffs Harbour Megafold. The geometry of this megafold is a classic example oforoclinal bending (Carey, 1958).

Mesoscopic structures associated with development of the oroclines, such as minor folds or axial-planecleavage, have not been recognised in the field. No clear evidence has been recognised for thedevelopment of these structures. It is likely, therefore, that this deformation was not pervasive at outcropscale. Korsch (1975) suggested that the Coffs Harbour orocline was folded around an axis that plungedsteeply to the northwest. Flood and Fergusson (1982) suggested that the Texas orocline was foldedabout a northwest-striking axial-plane with a near vertical fold axis.

The model of oroclinal bending (Fig. 3) in the New England Orogen was refined by Korsch andHarrington (1987) and Murray et al. (1987). Although the bending occurred principally in rocks of theaccretionary wedge, Korsch and Harrington (1987) proposed that part of the forearc basin (Tamworthand Yarrol Belts) was involved as well. Interpretation of aeromagnetic and gravity anomalies (Wellmanand Korsch, 1988; Wellman, 1990) and lineament analysis (Vinayan et al., 1993) support the outline ofthe Z-shaped megafold of the accretionary wedge and the forearc basin succession, so that there isgeneral consensus on the geometry of the oroclines.

The time of bending remains vigorously debated, however, although some agreement is now developingfor an Early Permian age previously argued for by R.J. Korsch (Korsch and Harrington, 1987; Korsch,

14

•

•

•

•

•

••• Application of AMS to the New England Orogen•

in Murray et al., 1987). Murray et al. (1987) and Lennox and Roberts (1988), in contrast, proposed Late

• Carboniferous (310-300 Ma) oroclinal bending associated with 400-500 km dextral strike-slipdisplacement (which is not obvious on aeromagnetic profiles, Korsch et al., 1990). Korsch and coworkers

• (e.g. Korsch and Harrington, 1987) argue that oroclinal bending occurred during the Early to mid-Permian (280-265 Ma). They relate this to the intense deformation of the Nambucca block (Leitch, 1978;

• Coney et al., 1990), and infer a north-to-south 450-500 km relative movement above a subhorizontal

• decollement (Korsch et al., 1990). Fergusson and Leitch (1993) now favour a slightly earlier initiation ofthe megafold at about 290 Ma, with continuing development until about 280 Ma. Alternatively, a Late

• Permian age for oroclinal bending has been suggested by Collins (1990, 1991,1994) and Collins et al.(1993).

•

• Preliminary palaeomagnetic data from the Texas block, Coffs Harbour block and Tamworth Belt(Klootwijk, 1985) indicate a pervasive "Kiaman" overprint which postdates oroclinal bending. This

• overprint has been approximately dated as Late Carboniferous to Early Permian from comparison withthe Australian APWP (e.g. Klootwijk et al., 1993; Klootwijk and Giddings, 1993). However, the shape of

• the Permian part of the Australian APWP is not well-defined at present, so that a younger limit of Early

• Permian age cannot be excluded on the basis of current knowledge. Extensive studies are currentlybeing undertaken on Carboniferous and Permian ignimbrite successions from the Tamworth Belt in order

• to refine the reference APWP in this crucial late Palaeozoic interval. Such a refinement of the AustralianAPWP may lead to a better constrained younger age limit on oroclinal bending.

•

•Thus, in the Texas and Coffs Harbour blocks, deformation can be related to two very distinctive tectonicevents: (i) Subduction and development of the accretionary wedge produced pervasive deformation (D 1

• and D2, Korsch, 1981) at the mesoscopic scale, and (ii) Oroclinal bending formed a Z-shaped megaf oldat the macroscopic scale. Anisotropy of magnetic susceptibility studies and some remanence studies

• were undertaken to identify and to further constrain the timing of these deformation phases.

•• Sampling

AMS sampling• Twenty-one sites (276 cores) were sampled in shale-rich rocks from the Texas and Coils Harbour blocks

(Table 1). The sites were selected to provide a broad coverage of strike variations around the oroclines• (Fig. 4, Tables 1-3). A site, for palaeomagnetic purposes, is an outcrop of fresh rocks with substantial

• stratigraphic and topographic coverage.

• Seven sites were sampled in the Texas beds (Fig. 4, Table 1). This unit was derived from a volcanic arcand forms part of an accretionary wedge containing melange facies and highly-deformed turbidites with

• pervasive cleavage (Flood and Fergusson, 1982). Korsch and Harrington (1987) revised the interpretation

• by Korsch (1977) of the distribution of stratigraphic associations within the southern New EnglandOrogen, and correlated part of the Texas beds with the Coffs Harbour Association (Fig. 3). A Late

• Devonian to Early Carboniferous depositional age was suggested by Korsch (1977) and confirmedrecently through radiolarian dating by Aitchison (1988) and Aitchison and Flood (1990).

•

•

Four sites were sampled in the central Coils Harbour block and seven sites at headlands in the easternCoffs Harbour block (Fig. 4, Table 1), in areas studied and described by Fergusson (1984) and Korsch

•

• 15


TABLE 1: DescriptioDs of AMS sampling sites, Texas and Coffs HaIbour blocks, New &gland Orogen

Sites CocmIiDatcs1 N2 Lithology Type of Outcrop

Texas beds

ATTA 151.43/28.98 13 silicified shale creek

ATTB 151.15128.81 12 silicified shale road cut

ATrC 151.33128.39 14 sandstone/silstone road cut

ATTD 151.48128.32 12 silicified shale road cut

A1TE 151.53/28.26 12 mudstone road cut

A'lTF 151.98128.21 15 sandstonelsilstone field crop

ATTN 151.60/28.67 13 silicified shale gully

central Coffs Harbour Association

ATCK 152.55129.58 14 silicified shale, melange creek

ATCL 152.58/29.75 15 massive sandstone creek

ATCI 152.43129.86 12 shale old road cut

ATCJ 152.35129.84 12 silstone field crop

castcm Coffs Harbour Association

ATCE 153.14/30.21 13 mudstone Moonee Beach

ATCD 153.19/30.18 13 soft mudstone Emerald Beach

ATCC 153.20/30.08 11 cracked mudstone Mullaway Headland

ATCB 153.20/30.06 13 siltstone and mudstone Arawarra Headland

ATCF 153.29/29.82 13 mudstone Diggers Camp

ATCH 153.30/29.78 16 high carbonate mudstone Minnie Waters

ATCG 153.34129.61 15 soft mudl siltstone Broomes Head

Teuica beds

A1TO 151.48128.51 14 mudstonelsandstone creek

ATTP 151.48128.5 1 12 mudstonelsandstone creek

Gib!unv mudstone

ATCA 152.29128.92 12 massive mudstone gully

Alum Rock

ASAR. 151.67128.50 59 myolitic ignimbrite creek

1 Longitude in decimal degrees E; Latitude in decimal degrees S.

2 N=number of samples per site or locality (ASAR).

(1971. 1981). respectively. A younger limit on the age of deposition of these rocks is given by a Rb-Sr

whole rock isochron of 318 ± 8 Ma (Graham and Korsch. 1985) for a regional metamorphic event in

the Coffs Harbour area.

One site (ATCA) was sampled east of the Demon Fault in the Late Permian (Fauna IV) Gilgurry Mudstone

(Thomson. 1976) and two sites (AnO. Anp) were sampled in the Permian Terrica beds (Olgers et al..

1974) in the Texas block (Fig. 4). with the aim of further constraining the time of oroclinal bending.

16


TABLE 2: KLY-2 AMS rcsu1ts, Texas and Coffil Hmbour blocks, New England Orogen

Sites Km L· Kmax· Kmn·

Texas beds

ATTA 309 1.014 1.002 56/63 239/27

ATTB 197 1.013 1.037 348/49 247/9

ATfC 179 1.006 1.068 90/35 184/5

ATTD 232 1.021 1.235 264/85 8/1

AlTE 469 1.034 1.173 331132 224/26

ATfF 256 1.011 1.076 185/34 80122

ATfN 260 1.016 1.016 259/3 350/15

ceutral Coffil Hatbour Association

ATCK 191 1.021 1.049 174/48 23/38

ATCL 275 1.018 1.020 308125 214/9

ATCI 328 1.017 1.060 118/86 59/2

ATCJ 339 1.042 1.030 323/81 6912

eastern Coffil Hatbour Association

ATCE 120 1.006 1.038 292/16 2418

ATCD 119 1.011 1.050 270/4 179/16

ATCC 309 1.030 1.025 85122 182/17

ATCB 286 1.025 1.060 94/8 316

ATCF 244 1.006 1.008 82/51 173/1

ATCH 230 1.013 1.042 143/59 286/26

ATCG 179 1.012 1.013 216/9 123119

Terrica beds

ATfO 190 1.011 1.006 102/2 195154

ATfP (original data could not be retrieved)

GilI!UIrV mudstone

ATCA 1008 1.052 1.050 17113 78/46

Mean tensorial results :

Km mean susceptibility 10-6 SI; L" = Kmax */Kint * lineation parameter of the mean tensor; P" = Kint *1Kmin" foliation parameter of

the mean tensor; Kmax", Kmin * orientation of principal axes of the mean tensor as dip direction/dip; * indicates mean tensorial result.

Palaeomagnetic sampling

A collection of samples from rhyolitic ignimbrites in the Permian inlier at Alum Rock (site ASAR, sampled

by K100twijk in 1986; Fig. 4, Table 1) was also studied to further constrain the time of deformation. Pilot

studies of ignimbrites in the Rouchel area of the Tamworth Belt (Klootwijk, 1985) have shown that

deuterically oxidised ignimbrites are more likely than associated sedimentary rocks to retain a primary

magnetization despite the occurrence of often pervasive "Kiaman" overprinting in the Tamworth Belt

succession. Sediments associated with the rhyolitic ignimbrites at Alum Rock were dated as Early

Permian (Fauna II) by Dickens (in Olgers et aI., 1974). The faunas have been re-examined by Briggs

17


TABLE 3: DIGICO AMS results. Texas and Coffs Baroour blocks, New England Orogen

Sites KIn sKm L· sL P sF Kmax· K.m.n· So SI GF

Texas beds

ATIA 304 107 1.090 24 1.010 18 34177 251110 280/30 225125 undef

ATIB 182 27 1.050 13 1.070 21 355163 250/8 252127 undef 263/1

ATIC 153 74 1.050 12 1.140 44 286/61 18217 17015 18015 186/20

ATID 207 46 1.060 22 1.490 85 114/67 1614 undef 188/0"· 1313

ATIE 374 347 1.110 70 1.250 68 343/50 226/21 252150 218/47 244147

ATIF 234 62 1.060 19 1.130 51 186152 75115 undef 50125 undef

ATIN 238 37 1.040 16 1.040 6 239171 352/8 32515 17015 undef

central Coffs Harbour Association

ATCK 191 63 1.066 41 1.022 45 190/65 34123 230/36 undef 81/52

ATCL 264 55 1.050 17 1.052 21 309171 20515 242116 undef 225144

ATCI 394 24 1.089 18 1.094 24 111188 24111 undef 238/10 62/0

ATC] 346 60 1.063 12 1.238 33 348/68 8413 55/30 undef 81130

eastern Coffs Harbour Association

ATCE 84 41 1.060 12 1.080 28 95/87 21111 220/2 undef 180/47

ATCD 126 50 1.030 10 1.130 20 281171 177/5 182/14 undef 184/30

ATCC 314 18 1.010 7 1.080 19 78/50 179/9 180/20 17015 17019

ATCB 289 44 1.020 17 1.150 36 101147 317 173126 180/20 344/25

ATCF 261 48 1.070 8 1.050 20 120/65 289/24 270/10 undef 250158

ATCH 251 48 1.070 15 1.070 30 156174 299113 268/36 35015? 319/8

ATCG 145 98 1.050 6 1.040 10 257/53 127/26 276/24 undef 117/44

Terrica beds

ATIO 183 14 1.050 8 1.010 5 266/87 711 237172

ATIP 216 19 1.050 8 1.010 5 263/83 1513 10/80

Gi12urrv mudsrone

ATCA 1033 306 1.040 18 1.050 40 349110 86/35 subver.

Alum Rock

A5AR-l 56 1.085 1.009 227/12 320114 68171

ASAR-2 36 1.016 1.025 218/6 126118 68171

A5AR-3 102 1.053 1.023 87118 189/33 68171

A5AR-4 66 1.065 1.043 47/8 308/46 68171

ASAR-5 81 1.087 1.057 39122 297/28 81136

A5AR-6 78 1.060 1.053 46119 311114 81/36

Mean tensorial results :

Km mean susceptibility 10-6 51; sKm standard deviation; L" = Kmax ·/Kint • lineation parameter of the mean tensor; sl standard deviation

(10- 3); F· = Kint ·'Kmin • foliation parameter of the mean tensor; sF standard deviation (10- 3); Kmax .. , Kmin" orientation of the principal

axes of the mean tensor as dip direction/dip; 50 pole of the bedding plane (field measurement); 51 pole of the cleavage plane (field

measurement); GF indication of cleavage plane detennined in the laboratory from goniometer measurements on individual specimens;

• indicates mean tensorial result; •• Lineation N95-30W.

18

•• Application of AMS to the New England Orogen

• (1993). One locality low in the Alum Rock Conglomerate contains faunas of the Trigonotreta n. sp. zone

• of Briggs, which is Early Sakmarian. Near the top of the stratigraphic succession at Alum Rock, thefaunas are of the Late Sakmarian Strophalosia subcircularis zone. The ignimbrites have recently been

• dated through the U-Pb (SHRIMP) technique at —293 Ma (J. Claoue-Long, pers. comm., 1993; Robertset al., 1994).

•

•AMS results, general comparison KLY-2 and DIGICO

• Numerous studies have been conducted on possible problems comparing the results determined by thedifferent AMS devices available on the market (e.g. Veitch et al., 1985). These studies documented the

• DIGICO's shape-sensitive problem along the length-axis (Z) of the core-specimens. This effect was tested

•in specimens from the southern New England Orogen, through progressive reduction of the length ofthe specimen, but no effect was observed on the anisotropy parameters nor on the orientation of the

• AMS axes. For this reason AMS measurements of all specimens were carried out in late 1991 at theBlack Mountain Palaeomagnetic Laboratory using a DIGICO spinner anisotropy unit and DIGICO bulk

• impedance bridge. The results are documented below. A subset of the specimens was subsequently

•remeasurecl in 1992 on the KLY-2 impedance bridge at the Laboratoire de Geophysique Interne etTectonophysique (LGIT, Grenoble, France), for comparison of results. Discrepancies between the two

• sets of results show that the DIGICO results indeed may have been affected by the shape-effect. Adescription of discrepancies between both sets of results and an interpretation that is mainly based on

• the KLY-2 results is presented below.

General observations

• Bias of DIGICO anisotropy results due to a non-optimal specimen shape (length/diameter ratio) can bereduced by drilling cores perpendicular to the magnetic foliation plane (Heller and Schultz-Krutisch,

• 1988), which is often the bedding or cleavage plane. Unfortunately, field conditions forced us to drillmost cores sub-vertically, often parallel to bedding or cleavage. We checked, therefore, the reliability of

• the DIGICO results, through comparison of results for a representative subset of specimens with

• measurements that were carried out on a KLY-2 impedance bridge at LGIT.

• We generally measured 3 specimens per site on the KLY-2 impedance bridge. These were selected asfollows: one specimen with the minimum susceptibility, one with the maximum susceptibility, and a third

• specimen with susceptibility parameters close to the tensorial mean. The KLY-2 results are shown in

• Table 2, for comparison with the DIGICO results as presented in Table 3. Although the KLY-2 site-meanresults are based on fewer specimens than the DIGICO site-mean results, a comparison is warranted

• because of the higher sensitivity and presumably also higher reliability of results of the KLY-2 bridge.

• Comparison of scalar results between KLY-2 and DIGICO (Figs 5, 6) shows overall agreement for the

• mean susceptibilities and for the susceptibilities of the principal AMS axes. Despite the good overall inter-calibration between the two units, the comparison of the susceptibility data (Fig. 5A) shows some scatter

• (r2 = 0.868). This probably results from the lower sensitivity of the DIGICO spinner compared with theKLY-2 bridge, and from the DIGICO's rather awkward reliance on combining results from two separate

• units, i.e. the bulk bridge and the spinner anisotropy unit. The anisotropy parameter (P*= Kmax*/Kmin*,

•Fig. 5B) and the foliation parameter (F*--,-Kint*/Kmin*, Fig. 6B) also show reasonable correlations, butnot so for the lineation parameter (V= Kmax*/Kint*) (Fig. 6A).

••^19•

•

•

.-rJ:J


800

~ 600 -r."",;;:.;"",.,";.;,.,,,' cO ~ --E ~ 400

200

• • • • • • • • • • • • • • •

o • u ~ ~ m n u w cr ~ a c cr ~ IT ~ ~ ~ 00

TA TB TC TD TE TF TN CK CL CI CJ CE CD CC CB CF CH CG

Sites

20

• • • • • • • • • • • • • • • • • •

••• Application of AMS to the New England Orogen

• Comparison of vectorial results shows general agreement for the declinations (no polarity) of the AMS

• axes (Figs 7A, 8A), particularly for the Krnin* axes (Fig. 7A), but not so for the inclinations (Figs 7B, 8B).The KLY-2 results show steeper inclinations for the Km • n* axes (Fig. 7B), and more moderate inclinations

• for the Krnax* axes than the DIGICO results. This probably reflects shape sensitivity of the DIGICO withover-estimation of susceptibility along the length-axis of the core.

•

Specific examplesa) Sampling parallel to the foliation plane

• For cores drilled parallel to bedding or cleavage, the combination of the DIGICO's shape effect and theorientation of the core parallel to the magnetic foliation (Km * — Kint*) plane can result in both an

• apparent steep magnetic lineation parallel to the length-axis of the specimen and a high lineation

•parameter. Site ATCE (mudstones from the eastern Coffs Harbour block) clearly shows this effect (Fig.9, Tables 2, 3), with an appreciable sub-vertical magnetic lineation for the DIGICO results, and an almost

• horizontal lineation for the KLY-2 results. Clearly the length-axis shape effect has resulted in anoverestimation of the DIGICO Kmax* value, resulting in an apparent magnetic lineation that has no

• structural significance. It should be noted that there is a reasonable grouping of the DIGICO Km * axes,

•and that presence or absence of grouping cannot be used as a criterion for absence or presence ofinstrumental bias.•b) Sampling perpendicular to beddingDrilling of samples perpendicular to bedding may reduce the DIGICO's shape bias, but it cannot betaken for granted that the effect can be ignored. As an example we show results from site ATTO, takenon a gently folded mudstone from the Terrica beds. This Permian inlier in the northeastern Texas block(Olgers et al., 1974; Briggs, 1993) is deformed less than the surrounding Texas beds. The KLY-2 resultsshow good agreement with the local fold structure. The Kmin* axes are perpendicular to bedding andfold axis, and the 1<max* axes are close to the westnorthwest-eastsoutheast aligned fold axis (Fig. 10B).The DIGICO results, however, show quite a different pattern with the K max* axes perpendicular to thefold axis and the Kmin* axes close to the bedding plane (Fig. 10A). Thus assuming a depositional fabric,this can be described as an interchange of the Kmax* and Kmin* axes. It is possible that this interchangeis an artifact induced by a combination of the DIGICO's shape effect and the very low anisotropy of this•^site (KLY-2 = 1.009). However, such shape effect has not been established previously as a likely cause•^for an inverse magnetic fabric (see review in Rochette et al., 1992).

• Pending further study on the magnetic fabric of the Terrica beds, it is not clear what interpretation valuecan be attached to the DIGICO results. If the present DIGICO results prove to be valid observations, two

• of us (CTK, RJK) would argue that an alternative interpretation for the DIGICO data may haveconsiderable significance for control on the time of oroclinal bending. In such an interpretation thecoincidence of the DIGICO Kmin* axis and the pole to the axial-plane suggests development of apreferred orientation and may be an incipient cleavage, expressed in the field as no more than aweathered-out joint pattern parallel to the axial-plane. This joint pattern/incipient cleavage" is parallel

Figure 5 Comparison between results obtained by KLY-2 and DIGICO for (A) the mean susceptibility (Km*=[Kmax*-i-Kint*+Kmin1/3) and (8) the anisotropy parameter ( 1:'*= K max*/Kinin*). a= Texas block, b = central CoffsHarbour block, c= eastern Coffs Harbour block, see Figures 3, 4 and 12.

•

21




Sites

22


• to the regional strike, and was probably induced prior to or during the oroclinal bending process. If so,

• the age of the Terrica beds, as yet no better defined than Permian (studied samples provedpalynologically barren; Clinton Foster, pers. comm., 1993), may provide an important maximum age

• constraint on the time of oroclinal bending.

• Alternatively, it is possible that the interchange of AMS axes indicated by the DIGICO results represents

•a mineralogical effect (Rochette, 1988; Ihmle et al., 1989; Borradaile and Puumala, 1989). For instance,a mixture of single domain and multidomain magnetic grains can result in an abnormal fabric with an

• interchange of the AMS axes and a large variation in anisotropy parameters (Aubourg, 1990; Rochetteet al., 1992; Aubourg et al., 1995). The DIGICO and KLY-2 instruments may measure different fractions

• of the magnetic grains, resulting from the difference in frequency of the driving magnetic fields: 10 Khz

•and 1 Khz for DIGICO and KLY-2 respectively. The lower frequency signal tends to enhance thecontribution of the coarser grains over the finer grains. Hence, the presence of a substantial fine fraction

• in the Terrica beds would show up as a larger amount of SD grains in the DIGICO results than in theKLY-2 results.

•

•In order to define the magnetic carriers of site ATTO rocks, we carried out hysteresis loop studies(Micromag-2900, Centre des Faible Radioactivit6s, Gff-sur-Yvette, France) and the modified Lowrie-Fuller

• test (Dunlop et al., 1973). The hysteresis parameters indicated the presence of low coercivityferromagnetic grains (H 0 = 14 mT) within the pseudo-single domain (PSD) range (M r/Ms = 0.17, H 1/H0

0^= 2.24), with the ferromagnetic fraction contributing no more than 35% to the low-field susceptibility. The

•paramagnetic susceptibility contribution is about 65%. Study of anhysteretic magnetization andisothermal magnetization under increasing AF fields showed a positive Lowrie-Fuller test (Fig. 11)

• indicating a predominance of small magnetic grains within the pseudo-single domain range, rather thanlarge multidomain grains.

A strong matrix contribution to the susceptibility could be expected to show up as a planar anisotropy• (F*>L*) due to the planar anisotropy of phyllosilicates (Zapletal, 1990). It is thus surprising that both the

• DIGICO and the KLY-2 measurements for these moderately deformed sedimentary rocks with a dominantmatrix susceptibility show a predominant lineation (L*> F*). It is possible that both instruments do not

• properly estimate the contribution of magnetic grains to the low-field susceptibility with the rather lowalternating fields that are applied in both the DIGICO (10 Khz) and the KLY-2 (1 Khz) unit.

•

AMS results, KLY-2 impedance bridgeinterpretation of KLY-2 resultsThe sensitivity of the KLY-2 impedance bridge (-5*10 -8 SI; detection of about 0.1% anisotropy) is anorder of magnitude greater than that of the DIGICO anisotropy unit (10-1e SI, detection of about 1%anisotropy). This high sensitivity enables absolute measurement of susceptilibity in specific directionsto be made with the KLY-2 unit. In contrast, the DIGICO anisotropy unit measures the difference in

Figure 6 Comparison between KLY-2 and DIGICO results for: (A) Lineation (L*= K max*/Kint*); (B) Foliation (F*= Kint*/Kmin*).Note the change in scale of the vertical axes between the two graphs. a= Texas block, b= central Coffs Harbourblock, o= eastern Coifs Harbour block, see Figures 3, 4 and 12.

•

23

360°

300'

120°

60°

90°

60°

CCI

P.*

30°


TA TB TC TD TE TF TN TO TP CA CK CL CI CI CE CD CC CB CH CG

TA TB TC TD TE TF TN TO TP CA CK CL CI CJ CE CD CC CB CH CG

Sites

24

••

• Application of AMS to the New England Orogen• susceptibility between specific directions, and combines these observations with a single measurement

• of bulk susceptibility Ka, along the length-axis of the core, in order to arrive at "absolute" measurements.This procedure compounds the observational errors for the two sets of data with the calibration errors

• for the two individual DIGICO units. This instrumental and procedural difference favours the KLY-2 resultsas being more reliable than the DIGICO results. Although the KLY-2 results are based on far fewer

• specimens per site (3) than the DIGICO results (12-16) we regard the KLY-2 results, in general, to be

• representative at site level, given that the DIGICO results do not show any significant inhomogeneity atsite level. We discuss here the KLY-2 results as detailed in Table 2. For comparison with the DIGICO

• results we refer to Table 3 and to the discussion below.

• KLY-2 anisotropy parameters

•a) Susceptibility

Mean susceptibility (Km*) values range from 1*10 -4 SI (mostly for the mudstones from the Coffs Harbour

• region) up to 5*104 SI (mostly for silicified shales), and are in good agreement with the DIGICO bulksusceptibility results (Fig. 5A). Such values are typical for clay-rich sediments that are enriched in

• ferromagnetic grains (Rochette, 1987a).

0b) Magnitude of anisotropy

• The magnitude of anisotropy ranges from a low value P*=1.03 (mostly mudstones from the CoffsHarbour block) up to a high value P*= 1.28 for site ATTD in the Texas block. Comparison of degree of

• anisotropy P* (Fig. 5B) and mean susceptibility Km* (Fig. 5A) shows poor correlation. This suggests that

•the anisotropy parameter mainly reflects the preferred orientation of magnetic grains rather than themagnetic mineralogy.•c) Shape of the magnetic fabric

• The shape of the magnetic fabric is mostly oblate (F*>L*) and indicates a preferred planar orientation

•of the grains (Table 2). Site ATTA is a notable exception with a prolate fabric (L*> F*).

• d) Presentation of site resultsAMS results for individual sites obtained with the KLY-2 impedance bridge are displayed in Figures 12

• and 13. The Krnax and Kmin axes of individual specimens and the K max* and Kmin* axes for the site-mean results are shown as equal area projections in geographic (present-day) coordinates in Figures

• 12 A,C and 13. Mean results for individual sites are summarized in Table 2. A comparison between KLY-

• 2 and DIGICO magnetic fabric results and petrofabric results is presented in one of the followingsections.••ÂMS results, DIGICO anisotropy spinnerSÊighteen sites in the Texas beds and Coffs Harbour Association and three in the Permian Gilgurry

•^Mudstone and Terrica beds were measured with the DIGICO anisotropy unit.

•^Figure 7 Comparison between KLY-2 and DIGICO results for: (A) Declination of K min* axes; (B) Inclination of Kmie axes. a=•^Texas block, b= central Coffs Harbour block, c = eastern Coffs Harbour block.

•S^25

••


00TA TB TC TD TE TF TN TO TP CA CK CL CI CJ CE CD CC CB CH CG

90°

0°TA TB TC TD TE TF TN TO TP CA CI( CL CI CJ CE CD CC CB CH CG

Sites

26


• DIGICO anisotropy parameters

• a) SusceptibililyMean susceptibilities and standard deviations are shown in Figure 5A. Susceptibility values range

• generally from 1*10-4 SI (mostly for the mudstones of the Coifs Harbour block) up to 5*10-4 SI, with a

high value for the Gilguny Mudstone (10 -3 SI). Only site ATTE shows a large variation in susceptibility• values.

•b) Shape of the magnetic fabric

• Figures 6A and 6B show that the majority of the sites have an oblate magnetic fabric with F*» L.Notable exceptions are site ATTA with a prolate fabric (L*>F*), taken as confirmation of the linear

• preferred orientation of the magnetic carriers discussed above (orientation of Kmax* axes), and sites

•ATTO and ATTP (Terrica beds) with a relationship L*>F* that is not related to the shape of the magneticfabric but possibly to an interchange of the AMS axes. Indeed, for a planar preferred orientation of SD

• grains the relation L*> F* may hold (Potter and Stephenson, 1988).

• c) Foliation parameter

•Results are displayed in Figure 6B.

• d) Lineation parameterThe standard deviations of the lineation parameter L* (Fig. 6A) are far larger than those for the foliation

• parameter F*. Site ATTE, for example, has large standard deviations for both the mean susceptibility

•Km* and lineation L* parameters. Hence, the lineation parameter is thought to be more sensitive tovariations in the magnetic mineralogy than the foliation parameter. The eastern Coils Harbour block

• shows generally low values for the lineation L* parameter, probably due to a low amount offerromagnetic carriers. The magnitude of the lineation parameter (L*-=, 1.06) seems nevertheless

• significant, and may reflect a tectonic origin.

0 e) Presentation of site results

• AMS results for individual sites obtained with the DIGICO spinner are displayed in the Appendix. TheKmax, Kint and Kmin axes of individual specimens are shown as equal area projections in geographic

• (present-day) coordinates. Mean results for individual sites are summarized in Table 3. A repeat of theearlier caution regarding the DIGICO results is appropriate here. Bias in the DIGICO results due to

• shape-sensitivity along the length-axis of the core is a well-known instrument problem. Although we did

• not recognize such a bias in a shape-variation test on several samples, it may be wise to treat theDIGICO data with caution. A comparison between KLY-2 and DIGICO magnetic fabric results and

• structural results is presented below.

•

• Comparison of magnetic fabric (AMS) results and structural observationsGeneral procedure

• For individual sites, comparisons between structural data (particularly poles to cleavage and fold axes)

•

•Figure 8 Comparison between KLY-2 and DIGICO results for: (A) Declination of Kmax* axes; (B) Inclination of Km * axes. a=

Texas block, b= central Coffs Harbour block, c= eastern Coffs Harbour block.

•

• 27

••


Figure 9 Comparison of AMS results for site ATCE (eastern Coffs Harbour block) obtained with the DIGICO spinner anisotropy

unit (A) and the KLY-2 impedance bridge (6). The hatched arrow (6) indicates the directional shift of the ~ax axes

away from the DIGICO results towards the KLY-2 results. The samples were drilled about parallel to bedding and cleavage.

and magnetic fabric data (as determined with the KLY-2 impedance bridge and the DIGICO anisotropy

unit) are shown in Figure 14 and summarized in Table 4. Further detail on the KLY-2 and DIGICO data

is shown in Figures 12 and 13 and the Appendix and detailed in Tables 2 and 3 respectively. Field

observations from the eastern Coffs Harbour block are by Korsch (1975), from the central Coffs Harbour

block by Fergusson (1982b) and from the Texas block by Butler (1974).

Eastem Goffs Harbour block

SiteATCB

At this site, SO dips 260 to the south, with a subvertical axial-plane cleavage striking east-west. Fold axes

28

• • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • •

ürai frame of samPlin


Figure 10 Comparison of AMS results for site ATTO (Terrica beds, Texas block) obtained with the DIGICO spinner anisotropyunit (A) and the KLY-2 impedance bridge (B). The hatched arrows (B) indicate the directional shift of the Kmin andKmax axes away from the DIGICO results towards the KLY-2 results. The samples were drilled about perpendicularto bedding.

to the D1 mesoscopic folds are subhorizontal with an east-west trend.The DIGICO Kmin* axis coincides

• with the pole to cleavage, rather than the pole to bedding, and hence Kmax* and K•nt* lie in the cleavageplane and F*>L*. The Kmax* and Kint* DIGICO axes define a great-circle girdle, indicating that the AMS

• ellipsoid is oblate in shape. The Kmax axes determined by the KLY-2, in contrast, form a tight cluster,

•approximately within the girdle determined by the DIGICO data, and close to the orientation of the

mesoscopic fold axes. These results indicate that at this site, the observed magnetic fabric was

111^produced during the D1 deformation of Korsch (1973), with K max* and Kit* lying in the cleavage plane

and Kmax* being parallel to the fold axes.•

• 29

••


Figure 11 Results of the modified Lowrie-Fuller test for samples of site ATTO (Terrica beds, Texas block). ARM= AnhystereticRemanent Magnetization, IRM= Isothermal Remanent Magnetization.

Site ATCCThe results for this site are very similar to site ATCB. The DIGICO and KLY-2 results for Kmin* arecoincident, which are also coincident with the pole to the cleavage. Although the DIGICO Kmax

orientations are spread in the cleavage plane, the KLY-2 K max results cluster as a subhorizontal axistrending to the east. This coincides with mesoscopic fold axes measured at Mullaway Headland.

Site ATCDKmin* axes for both the DIGICO and KLY-2 are coincident with the poles to cleavage. There arenoticeable differences, however, between the orientations of the Kmax* axes determined by the twoinstruments. This probably results from the gross similarity of Kmax* and Kint* values and possibleinterchange of the axes depending on instrumental characteristics. The DIGICO produced an axisplunging steeply to sub-vertically westwards, whereas the KLY-2 produced a subhorizontal Kmax* axiswith an east-west trend. Mesoscopic fold axes measured at Emerald Beach plunge either to the east orwest with subhorizontal to moderate plunges. Hence Kmax* is probably parallel to the fold axes.

Site ATCEAgain, Kmin* axes determined by the two instruments are coincident and in reasonable agreement with

30

• • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • •


the pole to cleavage. There is a significant discrepancy, however, between the orientations of the Kmax *

axes. For the KLY-2, the ~ax * axis plunges gently to the northwest, in contrast to the mesoscopic fold

axes which are all very steeply plunging. Hence Kmax * lies in the cleavage plane but is perpendicular

to the fold axes. Note that for the DIGICO, Kmax * is parallel to the mesoscopic fold axes. It should be

noted that LKLy-2* is very low. So with Kmax *:::::Kint*, interchange of axes may occur, in particular for the

results from the lower resolution DIGICO spinner.

Korsch (1973) observed increasing intensity in the D1 deformation towards the south within this part of

the Coffs Harbour block, that is from site ATCB to site ATCE (Fig. 14). There is a distinct and progressive

tightening of the folds coupled with an increase in metamorphic grade (Korsch, 1978), and also there

is a gradual increase in the plunge of the fold axes from subhorizontal in the north to steeply plunging

in the south. This corresponds with an increase in ~ax * inclinations determined with the DIGICQ, but

not with the KLY-2 bridge (Fig. 14). Some caution is thus warranted in interpreting Kmax* as an

intersection lineation.

It is gratifying to note that the variation in magnetic mineralogy observed for sites ATCB to ATCE

conforms with the southwards increase in metamorphic grade (Korsch, 1978). Pyrrhotite has been

identified as the main magnetic mineral in the more northern sites ATCB and ATCC, and magnetite in

the more southern sites ATCD and ATCE, as detailed in Figure 15. The change from pyrrhotite to

magnetite indicates an increase in metamorphic grade, schematically expressed by Rochette and

Lamarche (1986) as:

Pyrite ~ Pyrrhotite ~ Magnetite

300°C 500°C

increasing metamOlphic grade ~

The highest metamorphic grade reached in the Goffs Harbour block is lower greenschist facies. The

estimated maximum temperature for this grade is about 3500G in a low to intermediate pressure

environment, which is lower than the temperature specified by Rochette and Lamarche for the pyrrhotite

magnetite transformation.

SiteATCF

The Kmin * axis for the DIGIGO result coincides with the pole to cleavage, and is consistent with results

from other coastal sites in the Goffs Harbour block. The KLY-2 result, however, shows a subhorizontal

~in * axis trending to the south (Fig. 14), that cannot be related easily to any structures in the rocks

at this site. The low value of the foliation F* and lineation L* parameters determined with the KLY-2

bridge may have led to interchange of the Krnin * and ~nt * axes. The Krnax * axes, however, are in agreement for the two instruments, and are close to the fold axes of the mesoscopic 0 1 folds.

SiteATCH There is a spread in declination of the Krnin axes obtained by the OIGICO, which may be due to the combined effects of a constant cleavage and a considerable variation in bedding attitude (Table 3 lists

mean bedding attitudes only). Such a zonal dispersion of ~in axes around the ~ax * axis is common

for an intersection lineation between cleavage and bedding. Despite this spread the groupings of Kmax

31

B

.. u..

c

TN

NEW ENGLAND FOLD BELT

13fCb i CJ :

1.2 ••••••••••.••••.•••.•••.•••••••• +. ...... . .. I a i u.. , ' • 1.1 ........ "'cr:,'

CKT •

100 120 140 160 180 1.0 I T

Strike of SO

T'

Ca)

Kmax* • in situ

Expected increasing deformation

CJ

32

CE CD CC CB CF QI CG

Red Rock->Coffs-Harbour

• • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • ,-

•


and Kmin axes for individual specimens show good agreement between the two instruments. The Kmin * axis for KLY-2 is again coincident with the pole to the cleavage plane.

The structural data presented by Korsch {1975} was collected from two adjacent headlands, Rocky Point

and Tree Point, but the structural orientations were consistent between the two sites. The fold axes of the mesoscopic folds show a spread within the cleavage plane. With such a spread, the Kmax * axis will

be parallel to at least some of the fold axes.

SiteATCG

There is good agreement between the ~in axes and the poles to the axial-plane cleavage. There is some discrepancy, however, between the orientations of the Kmax axes determined by the two

instruments. Interchange of the ~ax and ~nt axes may have occurred because of the low values for the foliation F* and lineation L * parameters as determined with the KL Y-2 bridge. Alternatively, the mesoscopic fold axes show a spread in the cleavage plane and hence the Kmax axes determined by both the DIGICO and KLY-2 could be parallel to the fold axes. The KLY-2 ~ax* axis is in good agreement with the intersection lineation (Figs 12, 13).

Central Coffs Harbour block

SiteATCJ

This site falls within Structural Domain 3 of Fergusson (1982b, fig. 4) with subhorizontal, northnorthwestsouthsoutheast trending fold axes. The DIGICO and KL Y -2 results are in reasonable agreement for both the ~ax * and the ~in * axes. The ~in * axis is coincident with the pole to cleavage, and also with the pole to bedding. The ~ax * axis is steeply plunging to subvertical, and lies within the cleavage plane but perpendicular to the fold axis. The foliation F and lineation L parameters for this site are very high (Fig. 6) and the ~ax * axis could thus represent a stretching lineation that formed during D1. Note, however, that this lineation has not been recorded in the field.

SiteATCI

This site falls within Structural Domain 7 of Fergusson (1982b, fig. 4). The AMS axes determined by the

two instruments are in good agreement. The ~in * axes are coincident with the pole to the cleavage plane. As for site ATCJ, the ~ax * axes are subvertical whereas the fold axes are subhorizontal. The foliation L* parameter for this site is very high (Fig. 6), so ~ax * may represent a stretching lineation. The KL Y -2 and the DIGICO ~ax * axes are, however, in good agreement with the intersection lineation (Figs 12, 13, Appendix).

Figure 12 Regional overview of susceptibility anisotropy data across (a) the Texas block, (b) the central Coffs Harbour block and (c) the eastern Coffs Harbour block: A) Overview of I<xnax" and I<xnin" axes for all individual Sites, shown in equalarea prOjection, KLY-2 data: full square= I<xnax" axis, full circle = I<xnin" axis. Bedding (SO), cleavage (S1) and gonio fabric are indicated where determined (Table 3). Gonio fabric= estimate of cleavage plane from goniometer measurements on specimens. See legend to figures 13 and 14; B) Overview of intensity of magnetic foliation (F* = ~nt" Il<xnin ") and degree of anisotropy (P*) and intrinsic anisotropy (Pi") for selected sites within the boxed regions a and band c, DIGICO data; C) Overview of magnetic lineation directions (I<xnax ") for selected sites in equal area projection. ~ax" directions are shown in situ. Tentative small circles are drawn through some of the results. KLY-2

data. See text for discussion.

33

•

•

Application of AMS to the New England Orogen^ •

Site ATCL^ •This site falls within Structural Domain 5 of Fergusson (1982b, fig. 4). The Kmin* axes for the two

•instruments are in good agreement and coincide with the pole to cleavage. There is some discrepancybetween the orientations of the Kmax* axis. The DIGICO results are effectively perpendicular to the fold^•axes, whereas the KLY-2 results are coincident with one of the clusters of fold axes measured in the fieldby Fergusson (1982b). The values for the lineation L* and foliation F* parameters determined with theKLY-2 bridge are rather low (Fig. 6), so interchange of the Kmax* and Kint* axis may have occurred.

•

Site ATCK^ •There are no published structural data that can be used for a direct comparison with the AMS axes fromthis site. The Kmax* and Kmin* axes are in reasonable agreement between the two instruments. The^•

Kmin* axis lies in the bedding plane, and hence Kmin* is perpendicular to the pole to bedding. Based•on the results for the other sites in the Coffs Harbour block we assume that the K min* axis is coincident

with the pole to the axial-plane cleavage. Hence this site is situated in the hinge zone of a minor fold^•because bedding is perpendicular to the cleavage.

•

Texas block^•Published structural data from the Texas Orocline is scarce, although Butler (1974) divided the region •into three structural domains, with a significant change in the orientation of the cleavage planes betweeneach domain. Using Butler's subdivisions, sites ATTA, ATTB, ATTE, ATTF and ATTN are in his Domain^•1, site ATTD is in Domain 2, and site ATTC is in Domain 3. For each of the sites in the Texas beds, thereis good agreement between the orientations of the K min* axes determined by the two instruments.^•

•Site A7TAThis site falls in Domain 1 of Butler (1974). The Kmin axes of individual specimens show considerable^•scatter as demonstrated by the more numerous DIGICO data in the Appendix. This scattering is due toa strongly preferred linear orientation of the grains (the lineation parameter L* is here larger than the^•

foliation parameter F*), and is particularly evident from the DIGICO results (Table 3) but not so obviousfrom the KLY-2 results (Table 2). There is general agreement between the orientations of the Kmax* and

Kmin* axes determined by the two instruments. The Kmin* axis coincides with the pole to cleavage. The^•Kmax* axis plunges moderately to the northeast, whereas Butler (1974) measured the orientations ofmesoscopic fold axes plunging 50-75 ° to the south. The Kmax* axis and the fold axis are thus^•

approximately perpendicular. The DIGICO and the KLY-2 results show a very low value for the foliation•

F* parameter, so the Kmin* and the K•ra* axes could be interchanged. However, the values of the

lineation L* parameter for the DIGICO and the KLY-2 bridge are average to above average, and^•interchange of the Kmax* and K t* axes is thus unlikely. The Kmax* axis could possibly represent a

stretching lineation which would be related to the D1 deformation phase.^ •••Figure 13 Detailed overview of susceptibility anisotropy data obtained with the KLY-2 anisotropy bridge. Data from individual

sites (Table 2) are shown in equal-area projection, as individual specimen results (Km, Kmm) and as site-mean

results (Km *, Kmm*). Bedding (SO) and cleavage (Si) planes are shown for comparison, gonio fabric= estimate

of cleavage plane from goniometer measurements on specimens (see Table 3). See Figure 12A for overview of data

against the regional structure.

34

• • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • •

• Kmin @ Kmin*

• Kmax !Ill Kmax* BEDDING CLEAVAGE

------. GONIO FABRIC

••

Application of AMS to the New England Orogen^ •

Site ATTB^ •

This site falls in the western part of Domain 1. Orientations of the K max* and Kmin* axes are in good 41agreement between the two instruments. The Kmin* axis coincides with the pole to cleavage, but theKmax* axis is approximately perpendicular to the mesoscopic fold axes. The KLY-2 and DIGICO valuesfor F* and L* are about average (Fig. 6), and interchange of axes is unlikely. The Kmax* axis, therefore,could represent a D1 stretching lineation.^ •

Site ATTCThis site falls in Domain 3. The Km' axis shows good agreement between the two instruments, but theKmax* axis determined by the DIGICO coincides with the K im* axis determined by the KLY-2, and viceversa. The lineation parameter L* determined by the KLY-2 is rather low, thus interchange of the Kmax*^•and Kint* axes is possible. It is interesting to note that the orientation of the K min* axis is more akin tothe poles for the cleavage in Domain 2 rather than Domain 3.

Site ATMThis site falls in Domain 2. There is good agreement for all AMS axes between the two instruments. The^•Kmin* axis coincides with the pole to cleavage, but it is not possible to relate the orientation of the K max*

•axis to the mesoscopic folds because of the absence of published structural data. The foliationparameter F* for this site is far higher than for other sites (Fig. 6).

•

Site A7TEThis site falls close to the boundary between Domains 1 and 2 and hence could occur in either domain.There is good agreement for the AMS axes between the two instruments. Comparison with Butler's(1974) structural data shows the Kmin* axis to coincide with the pole to cleavage for Domain 1, and theKmax* axes to be perpendicular to the mesoscopic fold axes. This site shows the highest lineationparameter L* for all sites measured and rather high values for the foliation parameter F*. Interchangeof axes is thus unlikely, and the orientation of the 1<max* axes may be explained as a D1 stretchinglineation.

Site AT1FThis site falls in Domain 1. The AMS axes show good agreement between the two instruments. Theintensity of deformation at this site is certainly less than at sites ATTD and ATTE (Figs 4-6), yet the valuesfor the foliation F* and lineation L* parameters are about average for the sites measured andinterchange of axes is unlikely. The Kmin* axes coincide with the poles to the regional cleavage (Fig. 14).Unlike most of the other sites measured, the Kmax* axis coincides with the mesoscopic fold axes (Butler,1974) and could represent the D1 intersection lineation.

Figure 14 Opposite and following three pages. Comparison between mesoscopic structural observations (left) and magneticfabric data obtained with the DIGICO spinner anisotropy unit (centre) and the KLY-2 impedance bridge (right). NI

figures represent equal area projections on the lower hemisphere. Bedding (SO) and cleavage (S1)are indicated withthe KLY-2 data where determined, gonio fabric= estimate of cleavage plane from goniometer measurements onspecimens (see Figure 13 and Table 3). See Appendix for further detail on the DIGICO magnetic fabric data.^4110A) eastern Coifs Harbour block (opposite): Structural data according to Korsch (1975), the full squares represent foldaxes BS1so (number indicated), the solid lines represent contours of poles to cleavage (number of poles and contourlevels indicated, 1% counting area); DIGICO data: Kmax (full squares), Kim (full triangles), Kmin (full circles); KLY-2data: Kmax (full squares), Kinin (full circles), Kmax* (large open square), Kmin* (large open circle).^ 11/

36^ •

••••••••••

•

0 1 Structure

ATCG

ATCH

ATCF

-87n8,(1,10,30%)

o

ATCB +

42n 8,(2,4,8,16,> 16%)

• ATCC T .,. ....

• 10n8,

• • ..

ATCO • + •

• •• • 25nS, • • 8FA,

• • ••

ATCE '+. ••

\it • 15nS, • • 5FA,

Oigico AMS axes

.. .. ~} .. .. • •

rh =', • • ~ . .. , . ..

.. .. • -"\ .. 'I!. • • .. • ,. • •

• 4: •

.. • .... .... \. ....

.. • •

'< -+- •• -• . .". .. ..

•

• .-..

.. :jl ... • ••• -. 4j. •• • 'II •

•

. • I.! • __ ...... I ...

•

•

• • "j. .:~

..

-... •

!A. 'H

·:W •

.. • •

37

•

•

•

KLY-2 AMSaxes

~ .

• o

•

20·7/212 (1 of 4)

••


Di StructureDigico

AMS axesKLY-2

AMS axes

•••

• 93n S^11FAi

• 93n Si^• 2FA

• 98nS^6FA1

^ Bedding^• Krnax^0 Km *Cleavage^• Kim

— — Gonio fabric • Kmin^0 Kmin *

••ATCK

ATCL

ATCI

ATCJ

Figure 14B) central Coffs Harbour block: Structural data according to Fergusson (1982b); fold axes (full squares), poles tocleavage (full circles). See A) for representation of DIGICO and KLY-2 data. No structural data for site ATCK. See maincaption for general information.

Figure 14C) Texas block (opposite): Structural data according to Butler (1974), presented as data for Domain 1(sites ATTF,ATTE, ATTB, ATM, ATTA), for Domain 2 (site ATTD, ATTN) and Domain 3 (site ATTC). Site ATTN falls in Domain 1but close to the boundary with Domain 2. The Kmin* axes for both site ATTN and AUG (Domain 3) are more akinto the pole for the cleavage in Domain 2 rather than Domain 1 or Domain 3 respectively. For this reason structuraldata for Domain 2 are shown for these two sites. The solid lines represent contours of poles to cleavage (numberindicated and contour levels per 1% counting area indicated). See A) for representation of DIGICO and KLY-2 data.See main caption for general information. •

38^ ••

•••••••••••••••••••••

•


0 1 Structure

• • • • • • ,. • ATTO -:-, . . ~ . .....

• t.t. \ • 4711: 8 0

• • • • • • ,. • ATTP -, .

• f. · ... ,. . .". • 4711: 8 0

ATCA

• 2411: 8 0

Oigico AMSaxes

... r# ........

• I .. •• ...... • '\ ...

.. •

,,,, .;..

• • •

+

• •

-- Bedding -- Cleavage - - Gonia fabric

• Kmax ... Kin!

• Kmin

KLY-2 AMSaxes

-L I

• I

.~

20-7/212 (4 of 4)

o Kmax*

o Kmin*

Figure14D) Terrica beds and Gilguny Mudstone: Structural data according to Olgers et al. (1974), Lucas (1957), P.G. Flood

(pers. comm., 1993), K100twijk (unpublished) for the Terrica beds, and according to Thomson(1976) for the Gilgurry

Mudstone. The full circles indicate poles to bedding. See A) for representation of DIGICO and KLY-2 data. No KLY-2

data for site ATTP. See main caption for general information.

Site ATTN This site falls in Domain 1, but close to the boundary with Domain 2. The Kmin * axes for the two

instruments are in good agreement. However, the KLY-2 Kmax * axis is subhorizontal towards the west,

wherea8 the DIGICO's ~ax * axis is steeply plunging to subvertical. The ~in * axes coincide with the

poles to cleavage in Domain 2. Because of the lack of published structural data, no clear relationship

of the ~ax * axes to the structure can be determined.

Terrica beds Sites ATTO. ATTP DIGICO AMS results are shown for both sites (Figs 10, 140, Appendix, Table 3), and KLY-2 data for site

ATTO only (Figs 10, 140, Table 2). These show good agreement between the two sites. Structural data

for bedding orientations in the Permian Terrica beds of the Terrica inlier (see Fig. 14) was presented

40

• • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • •


TABLE 4: Comparison of magoctic Dbric aDd structural Dbric data, Texas aDd Coflilllubour blocks, New &gland Orogen

SITE Kmax·

easb:m Coflil Barbour AsmciaIion

ATCB !/FAl for KLY-2 .LSI

.LSI

.LSI

ATCC

ATCD

ATCE

ATCF

ATCH

ATCG

!/FAl for KLY-2

-//FAI for KLY-2

//FAI for DlGICO, Hor KLY-2 .LSI

-//FAI for DlGICO and KLY-2 .LSI for DlGICO only

8 some FAI .LSI

8 some FAI for KLY-2 .LSI

ceutral Coflil Barbour Association

ATCJ

ATCI

ATCL

ATCK

Texas beds

ATrA

AlTB

ATTC

ATID

ATrE

ATI'F

ATTN

Terriea beds

A'ITO

ATfP

Gilgurry IIIIldstoDc

ATCA

.LFAI .LSI

-.LFAI .LSI

-8 some FAI for KLY-2 .LSI

.LFAI .LSI

.LFAl .LSI

no data .L S I for Domain 2

no data .LSI

.LFAI .LSI for Domain I

UFAI .LSI

no data .L S I for Domain 2

.LFAI DIGICO,IIFAI KLY-2 usa DlGICO,.LSO KLY-2

.LFAI DlGICO,//FAI KLY-2 USO DlGICO, .LSO KLY-2

USO oblique to SO

Comments

Kmax· DlGICO .L KLY-2

Kmax· DlGICO .L KLY-2

Kmin· DlGICO .L KLY-2

Kmax· DIGICO .L KLY-2

no structural data

Kmax· DlGICO .LKLY-2

interchange of Kmax· and Kmin· axes

between DIGICO and KLY-2 measurements

fabric not depositional

by Lucas (1957), Olgers et al. (1974, fig. 35), P.G. Flood (unpublished data, pers. comm., 1993),

Klootwijk (unpublished data, 1993), with some additional data presented in Table 3. The beds at the two

sampled sites come from two gently dipping limbs of a small symmetric fold, with an axis plunging gently towards the westnorthwest and with a subvertical axial-plane striking westnorthwest -eastsoutheast.

The DIGICO ~in * axis is in good agreement with the pole to this axial-plane. The ~nt * axis is parallel

to the fold axis; hence the t(,y,ax * is perpendicular to the fold axis. This pattern, however, is not

recognizable in the KLY-2 data which show comparatively low L* and F* values and Kmax axes approximately parallel to the fold axis.

41

••••••e•0••••••••••••


Gilguny Mudstone

Site ATCAStructural data (24 bedding orientations) for the Late Permian Gilgurry Mudstone are taken from the1:100 000 geological map of Thomson (1976). There is reasonable agreement between the AMS axesfor the DIGICO and KLY-2 instruments. The bedding has not been strongly folded, and still has relativelyshallow dips. The Kmax* axis is subhorizontal and hence probably lies in the bedding plane. The K min*axis is oblique to bedding. Thus the fabric is not depositional in origin.

Origin of magnetic fabric

In clastic sedimentary rocks that are undeformed to only weakly deformed, the magnetic fabric will bedue principally to compaction. Kmax* and Kint* will lie in the bedding plane, and Kmin* will be parallelto the pole to bedding. Because the rocks of the Texas and Coffs Harbour blocks have been intenselydeformed, it is likely that the original magnetic fabric has been overprinted by new magnetic fabric(s)developed during the deformation (phases). The resultant fabric should be related predominantly to thestructures that formed, and hence be mainly tectonic in origin. The comparison of the AMS andstructural data clearly shows that, in almost every case, Kmin* is parallel to the pole of the axial-planecleavage. The magnetic fabric is thus tectonic in origin (the main requirement for AMS to be applicableto structural problems), and was developed mainly during the D1 phase of deformation that alsoproduced the cleavage.

Origin of lineation

Lineations can have various origins, such as intersection lineations, stretching lineations, or combinationsthereof. For the majority of the sites, Si is very steep (dip > 600) and SO is moderately steep (dip > 400)with a different strike. Moderate to steep intersection lineations are therefore to be expected, but this isnot a unique characteristic. Stretching lineations also will be steeply dipping if they result from transportin a near-vertical direction. The DIGICO results show for most sites a steep to very steep magneticlineation Kmax* (plunge > 40°) (Table 3, Fig. 8), whereas the KLY-2 results show moderate to steep dipsfor only some of the sites (Table 2, Fig. 8). However, both the DIGICO and the KLY-2 results for theKmax* axes suffer from some limitations. As discussed earlier, the DIGICO results may suffer from ashape-effect bias, whereas the KLY-2 results are based on a limited number of observations.Interpretation of the origin of the lineations is facilitated by a comparison of the observed Kmax* axeswith the SO and Si planes. This is shown for the KLY-2 results in Figures 12A and 13.

Despite some reservations about the reliability of the SO and Si measurements, several sites (ATCI,ATCB, ATCC, ATCE, ATCG, ATTB, ATTC, and ATM) show good agreement between the magneticlineation and the intersection between cleavage and bedding. In the Texas block, the bedding controlwas fairly good, with slaty cleavage being generally subparallel to bedding. In site ATTD we measuredintersection lineations with 250 plunges, which are clearly different from many of the Kmax axes. This

Figure 15 Compilation of magnetic mineralogy constraints for selected sites, obtained according to Lowrie's (1990) procedure^•and from Micromag measurents, see text. Km*= mean susceptibility (Km*= [ Kmax*+Kint*+ Kmin1/3); Pi= degreeof intrinsic anisotropy (Pi*= Kmax*/Kmin*).^ •S

42^ SS•

• • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • •

~ "{

; l5 = : = §, E

loo~~~~------------~---,

E o g6ethi~ o riagnepte o Km= 8.4.1 o ~ Paral: 72 oPi:1.l

50 .......• r-··TI--'·-----0

0 100 200 300 400 Soo 600 7oo(OC) Sc+3

4c+3

3c+3

1c+3

0 I

0 100 200 300 400 SOO 700 (0C) 300

i 1 i A 'CF 10 magneti~ ! 0 (pyj:rhoti~e)

200

100 200 300 400 SOO 600 7ooeC)

30

10

0 0 100 200 300 400 SOO 600 7oo(°C)

Temperature

43


20~----~--~--~~--~~

. A~CD

0 0 100 200

1000

800

400 4UU •••• ..;. ................

~ ~ : :

300

agneiite ! • ( yrrhcjtite) i

m= li26.1Qi.6 SI parai: 67 !

400 Soo 600 7oo("C)

200

~-0

0 100 200 300 400 SOO 600 7oo(°C) 300

i iA o r$.gne~te

200

100

100 200 300 400 500 600 7oo(OC)

Temperature

o --2.7T o Hard component • -- O,5T o Medium component [J -- 50 mT o Soft component


site is located close to the apex of the orocline (Fig. 4) and has a far more pronounced magnetic fabric

than sites located on the limbs (Fig. 6). It is likely that ~ax * represent a stretching lineation, resulting

from near-vertical transport in the apex-to-be of the orocline. Thus, sometimes Kmax * is parallel to the

fold axes (intersection lineation) and sometimes Kmax * is parallel to a stretching lineation (which is

usually at some angle other than parallel to the fold axes), particularly in rocks that received more strain.

We have demonstrated above that ~in * is perpendicular to the 01-cleavage in virtually every site in the

Texas and Coffs Harbour blocks. ~ax * therefore lies in the cleavage plane. Hence the logical

interpretation is that both ~in * and ~ax * formed during the 01 phase.

Shape of the magnetic fabric

A regional comparison of foliation and lineation parameters (Fig. 6) suggests:

a) The Texas block shows a variation in the intensity of the foliation parameter F* in accordance with

the location of the sites on the megafold, with higher values of F* at sites closer to the inferred axial

zone (ATTO and ATTE, Fig. 6A). Such an evolution is less clear for the lineation parameter L * (Fig. 68),

although a higher value is shown for site ATTE on the apex of the orocline and more moderate values

for sites ATTA and ATTN in the more central part of the orocline;

b) The central Coffs Harbour block shows a weak southward increase in the value of the foliation

parameter from site ATCK to ATCJ (Fig. 128), reflecting a southwestward increase in the degree of

preferred planar orientation of the grains. The lineation parameters are rather badly defined for two of

the four sites, but do not appear to mimic the weak regional variation of the foliation parameter;

c) The eastern Coffs Harbour block shows no clear variation in the value of the foliation parameter (Fig.

126). The lineation parameter, however, shows a steady southwards increase from site ATCG towards

site ATCC as does the metamorphic grade, with high values for L * close to the inferred axial zone of

the megafold.

Figure 12 shows a regional overview of AMS axes as obtained with the KLY-2 bridge, and bedding

planes as observed for the sites in the Texas and Coffs Harbour blocks. The sites show a magnetic

foliation, the ~ax * - ~nt plane, that generally follows the bedding strike (compare Figure 7A and Table

2 with Figure 12), but with a much steeper dip (compare Figure 78 with Figure 12). Locally the strike

of bedding and the magnetic foliation plane can be quite different (Fig. 12A, Coffs Harbour block). The

cleavage is more or less parallel to the bedding, but with steeper inclination. Comparison of the ~jn *

axes and the poles to the 50 and 51 planes indicates a relation between magnetic foliation and cleavage. The magnetic foliation is thus obviously of tectonic origin.

The origin of the magnetic lineation, i.e. the grouping of ~ax * axes, seems different for the various

regions that have been studied. In nearly all of the sites in the eastern Coffs Harbour block, Kmax * has

been interpreted as a lineation, reflecting intersection between bedding and cleavage. However, in the

sites of the central Coffs Harbour block, the ~ax * axes, at least where well-determined, seem to reflect

a stretching lineation perpendicular to the fold axes. A stretching lineation origin has been interpreted

for a majority of the sites in the Texas block, in particular for sites AnA and AnD that are situated in

the more central part of the Texas megafold. Thus, the high inclinations that are often observed forthese

44

• • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • •

Magnetic mineralogy

Rgure 16 Method followed for determination of intrinsic susceptibility anisotropy parameters.

magnetic lineations (Fig. 88) either represent the intersection between steeply dipping So and S1 planes. or may represent vertical stretching. It should be noted that some of the sites with steep lineations. i.e.

AnD and ATCJ (Fig. 12). show high values for the anisotropy parameter P of 1.28 and 1.14 (Table 2)

respectively. With such pronounced anisotropies. the anisotropy ellipsoids should be well-determined.

The orientations of these ~ax * axes. therefore. cannot be discarded as mathematical abnormalities without physical reality resulting from diagonalisation of a poorly-defined tensor. nor as artifacts

introduced by the DIGICO's shape bias.

MAGNETIC MINERALOGY

The magnitude of the anisotropy parameters and thus the intensity of the magnetic fabric are dependent

on the magnetic mineralogy. in particular the intrinsic susceptibility of the magnetic carriers. and on the

measure of alignment of the magnetic grains. Pilot studies were carried out to determine constraints on

the intrinsic anisotropy (Pi). i.e. anisotropy upon perfect alignment of magnetic grains. and on the

magnetic mineralogy of selected samples.

45

Magnetic mineralogy

.- 2 Q.c

T A TB TC TD CE CD CC CB CF CH CG

Sites

1.35

_Figure 17 Degree of intrinsic anisotropy for selected sites from the Texas block (a) and the eastern Coffs Harbour block (c),

determined with the ANU-susceptibility bridge, following the method described in the text and summarized in Figure

15. Pi= 1.35 is the maximum level for phyllosilicates. (Note that Pi represents the bulk intrinsic susceptibility

anisotropy of the magnetic carriers, assuming that the magnetic grains are well-aligned in the experiment).

Intrinsic anisotropy of magnetic carriers Aubourg (1990) proposed a simple method to estimate bulk intrinsic anisotropy behaviour. Figure 16

shows schematically how this method was implemented in the Black Mountain Palaeomagnetic

Laboratory. A maximum preferred orientation of the magnetic carriers is created through use of a rock

powder. Acetone is preferred as a solvent because of its non-magnetic properties and fast evaporation.

A rather weak direct and horizontally aligned field of 0.3 Tesla proved sufficient to orient high coercivity

grains such as hematite and fine-grained magnetite_ Susceptibility measurements along three specimen

axes were carried out with the ANU-susceptibility bridge. One measurement (M1) was taken parallel to

the direction of the applied field, a second measurement (M3) along the length axis of the sample

(influence of gravity during setting of grains), and a third measurement (M2) in the foliation plane and

perpendicular to Ml. The results of this preliminary experiment are shown in Figure 17. Pi values range from 1.35 for the clay-rich sites such as ATCH, ATCD and ATCE up to 2.6 for site ATCB. The value of

-1.35 is typical for the intrinsic anisotropy of phyllosilicates (e.g. Borradaile and Sarvas, 1990) and it is likely that the preferred orientation of the phyllosilicates is close to maximum. The higher values (Pi = 2.6)

are found for pyrrhotite-rich rocks (see following section), in agreement with observations by Rochette

et al. (1992) who observed bulk intrinsic anisotropy values up to 100 for pyrrhotites.

46

• • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • •

••• Some palaeomagnetic results• Magnetic mineralogy constraints eastern Coffs Harbour block• In order to characterize the different minerals that make up the magnetic mineralogy of the sites from

the eastern Coffs Harbour block, stepwise demagnetization of IRM was carried out following Lowrie's• (1990) procedure. Results are shown in Figure 15. The susceptibility contribution of the (phyllosilicate)

matrix was determined, using Micromag hysteresis studies, at about 70% in the 7 sites studied. Following• Aubourg et al. (1995), it is thus likely that the phyllosilicates have a marked effect on the susceptibility

• anisotropy. The ferromagnetic fraction is composite. Titanium-enriched magnetite is dominant in sitesATCE, ATCD, ATCF and ATCH. Pyrrhotite with a blocking temperature up to 340° C is clearly detectable

• in sites ATCC, ATCB and ATCG. It is probably of secondary origin, formed during metamorphism (e.g.Rochette, 19874

•

• Recent studies (Borradaile, 1987; Aubourg et al., 1995) show that detrital magnetite in sediments hasa low intrinsic anisotropy with a Pi parameter of about 1.5. Pyrrhotite, however, is well known as a

• magnetic carrier with a huge intrinsic anisotropy (e.g. Rochette et al., 1992) and a Pi parameter of about100. The correlation we observe between magnetic mineralogy of the seven sites and their intrinsic

• anisotropy agrees well with the difference in intrinsic anisotropies of pyrrhotite and magnetite. The high

• content of phyllosilicates explains the generally depressed intrinsic anisotropy parameters. Wherephyllosilicates and magnetite are the main magnetic carriers, Pi is in the order of 1.3, but where

• pyrrhotite is present this parameter is enhanced to 2.7.

•• SOME PALAEOMAGNETIC RESULTS

• BackgroundInterpretation of palaeomagnetic data from the southern New England Orogen is impeded by the

• occurrence of numerous overprints, in particular a Late Carboniferous-Early Permian "Kiaman" overprint

•of reverse polarity. In the New England region, this overprint is characterized by a steeply downwardand southwesterly directed component of magnetization (e.g. Klootwijk, 1985, 1988; Schmidt et al., 1990;

• Klootwijk et al., 1993; Klootwijk and Giddings, 1993). A pilot palaeomagnetic study on oroclinal bendingin the southern New England Orogen (Klootwijk, 1985, 1988) focused on competent sandy beds from

• the Texas and Coffs Harbour blocks, that is, from the Texas beds and the Coffs Harbour Association

•respectively, in an attempt to constrain expected oroclinal rotations in those areas. Preliminary analysisof the data (Klootwijk, 1985) showed that no primary magnetization components could be isolated.

• Instead, very pervasive overprints were isolated. The directions are similar for both regions, and are alsosimilar to the pervasive overprint observed in the Carboniferous succession from the Rouchel block of

• the Tamworth Belt. This secondary component thus can be interpreted with some confidence topostdate oroclinal bending. Comparison with the Australian late Palaeozoic APWP (Fig. 18) as outlined

• by Klootwijk et al. (1993) and Klootwijk and Giddings (1993), and with the general Gondwana APWP (see

• Klootwijk and Giddings, 1993, fig.3), suggests that acquisition of these overprints in the regions studiedoccurred sometime within the Late Carboniferous to Early Permian interval. Although these overprint

• magnetizations cannot be used to document the reality and evolution of oroclinal bending, importantlythey may provide an Early Permian younger age constraint on the time of the bending. This favours themodels of Murray et al. (1987), Korsch and Harrington (1987) and Korsch et al. (1990) for oroclinal

• bending during Late Carboniferous-Early Permian in preference to the model of Collins (1990,1991,1994;Collins et al., 1993) who preferred a Late Permian age of oroclinal bending. Furthermore, the results

•

• 47

•

•

Some palaeomagnetic results

• Secondary

[J Uncertain

<> Lachlan Orogen t:;,. Southern New England Orogen 'V Northern New England Orogen 0 Adelaide Fold Belt 0 Cratonic basins

Rgure 18 Late Palaeozoic APWP for Australia (after K1ootwijk, 1988; Klootwijk et aI., 1993; Klootwijk and Giddings, 1993). Pole

positions for the mean "Kiaman" overprints in the Texas block, the Goffs Harbour block and the Rouchel block are

indicated by TEX, CHB and ROU respectively. The pole position for the Alum Rock ignimbrite is indicated by SARG

Qn situ, geographic coordinates) and SARS (bedding corrected, stratigraphic coordinates). The magnitude of the

interpreted clockwise rotation is indicated by the cross-hatched area and the arc between pole SARS and the

intersection of the locus of the pole around the sampling site with the Late Carboniferous - Early Permian part of the

APWP. Oblique Al""toff projection. Gondwana reconstruction after Lawver and Scotese (1987), with Australia in its

present-day position. Only poles with ~5 or ~5 values less than 20° have been plotted. See Tables 9 and 11 in

Klootwijk et al. (1993) for data details.

suggested that no major deformational phase has affected the Texas and Coffs Harbour blocks since

formation of the oroclines, although the region underwent a major thermal event during Late Permian

to Early Triassic times (Graham and Korsch, 1985). This event has not been idenitified in the

palaeomagnetic results as yet.

A preliminary study of the Carboniferous succession from the Rouchel block (Klootwijk, 1985) showed

pervasive "Kiaman" type overprinting of late Palaeozoic sedimentary successions of the Tamworth Belt, which severely reduces their usefulness for documenting oroclinal bending of the New England Orogen.

48


TABLE 5: SampIiDg dcIaiIs Alum Jlock ignimbrites

Location (Grid coordinares) Acronym Agel Site Samples

(0) (0)

(fexas, 9140, 1:100,(00)

151.67E, 28.5OS (514449) ASAROI-12 Sakmarian 12 157.9/29.5

151.67E, 28.50S (514449) ASAR13-22 Sakmarian 2 10 157.9/29.5

151.67E, 28.50S (514449) ASAR23-32 Sakmarian 3 10 157.9/29.5

151.67E, 28.5OS (514449) ASAR33-41 Sakmarian 4 9 157.9/29.5

151.67E, 28.50S (514449) ASAR42-50 Sakmarian 5 9 171.2153.5

151.67E, 28.5OS (514449) ASARSI-59 Sakmarian 6 9 171.2/53.5

1) U-Pb (SHRIMP) age -293 Ma (Jon C1aoue-Long, pers. comm., 1993; Roberts et ai., 1994).

2) Bedding detailed as strike and dip in degrees (left hand rule).

However, Carboniferous and Permian ignimbrite successions that occur widely throughout the Tamworth

Belt and some Permian volcanics that occur within inliers in the Texas block, are more likely to preserve

a primary magnetization because deuteric alteration effects may have resulted in a hard primary

magnetization that is not easily erased by later overprints. Subsequent studies. therefore, have focused

on ignimbrite successions of the Tamworth Belt (Klootwijk, 1994). Here we detail results from a study

of ignimbrites from the Early Permian Alum Rock inlier in the Texas block.

Alum Rock palaeomagnetic resuHs

The moderately deformed Early Permian (Sakmarian: Briggs, 1993) rhyolitic ignimbrites of the Alum Rock

inlier are moderately dipping towards west-southwest. Bedding control was determined from conformably

intercalated sediments and is well-constrained. Six sites were sampled (59 samples) in at least two

separate flow units with different dips (Table 5). Some AMS measurements were made with the DIGICO

spinner unit in order to compare the magnetic foliation with the bedding measurements (see Appendix

and Tables 1 and 3). The fabric is dominated by general grouping of ~ax axes and planar streaking

of ~in and ~nt axes. The ~ax axes verge towards the pole to the bedding plane. Such an "inverted" fabric could be due to shape bias of the DIGICO as the specimens inadvertently departed from optimum

shape with a length of about 2.3 cm.

All remanence measurements were carried out on cryogenic magnetometers at the Black Mountain

Palaeomagnetic Laboratory (ScT and 2G-Enterprises 760R). A large-volume oven (McElhinny et a!..

1971), a small-capacity Schonstedt furnace (TSD-1) and a Schonstedt tumbling-specimen AF

demagnetizer (GSD-5) were used for thermal and alternating field demagnetization respectively. Thermal

demagnetization generally was carried out on two specimens per sample in 15 to 22 steps up to

between 580°C and 650°C. Alternating field (AF) demagnetization, of at least one specimen per site, was

carried out in 22 to 26 steps up to 100 mT. Linear and planar principal component analysis techniques

(Kirschvink, 1980) were used to separately determine magnetization directions of overprint and

characteristic components.

49


NRM 0-

N

I 80'

NRM

P' 0

E~p)

N (up)

1; A~At1 U".l

S

N 80' NRM

! '. J S

'LT' 'HT'ro

°50

E(up) W(dn)

N 80' NRM E

LTs

35

100mT 20-7/210

1 unit=1,292mAm,1 600'e

Figure 19 Representative examples of Zijderveld (1967) plots in geographic coordinates (in situ). The low temperature

components (LTs' LTh) have directions close to the present field, whereas the high temperature components (HTs'

HT h) generally have a steeply-downward northeast direction. Subscript s (h) refers to softer (harder) component. Full

(open) symbols refer to projections in the horizontal (vertical) plane.

The thermal demagnetization results show the general presence of two groups of magnetic components:

one group with low blocking temperature ranges (LT) and obviously of recent origin, and a second

group with high blocking temperatures representing the charactistic components (HT) (Figs. 19, 20A)The

shape of the magnetization decay curves shows a very narrow range of blocking temperatures near

50

10

NRM

1 unit = 0.616 mAmÊ (up)

600°C

20

100 mT

NFihe

1 unit = 0.833 mAm -1

W(dn)

35


Figure 19 continued

the Curie point, i.e. indicative of fine-grained magnetite. The recent field and the characteristiccomponents could both be subdivided into softer (LTs, HTs) and harder (LTh, HTh) subcomponents (Fig.21). The susceptibility (Fig. 20C) increased strongly after heating at 500 °C, but a concomitant increase

in viscous magnetization generally did not impede the measurement of remanence. The few alternatingfield demagnetization studies carried out (Figs 19, 20B) also show the presence of two groupings ofcomponents, one with lower coercivity and the other with higher coercivity (Fig. 21).

51


20-7/208

Figure 20 Representative examples of thermomagnetic curves, showing changes in NRM-intensity upon thermal (A) andalternating field demagnetization (B), and changes in susceptibility (C) versus temperature. The shape of the intensity-decay curves indicates the occurrence of low and high temperature components, see Figure 19, with the hightemperature part of the curves suggesting the presence of more-or-less titanium-enriched magnetite (Tb < 580 0C).The susceptibility-change curves show a very strong increase around 500 0C, which is accompanied by thedevelopment of viscous magnetization. Key to specimens A) 1= ASAR02.1, 2= ASAR10.1, 3= ASAR18.1, 4=ASAR31.1, 5= ASAR40.1, 6= ASAF150.1, 7= ASAR58.2; B) 1= ASAR18.3, 2= ASAR31.3; 3= ASAR40.2, 4= ASAR50.2,5= ASAR58.1; C) 1= ASAR02.1, 2= ASAR10.1, 3= ASAR18.1, 4 =ASAR31.1.

Low blocking temperature (L7) componentsAll samples showed LT components of normal polarity that were broken down below 150 0 to 25000 forthe softer LTs component and below 3500 to 40000 for the harder LTh component (Figs 19-21). Thesetwo LT components have mean directions (Table 6) that are close to the recent field direction (LTs :Dec= 1.00, Inc= -57.0, a95 = 2.70, N=36; LTh : Dec= 7.30, Inc= -63.70, «95 = 1.40, N= 99) and areclearly of recent origin.

High blocking temperature (HT) componentsThese well-determined components were broken down between 3500 and 5000C for the softer HT,component and between 500° and 600°C for the harder HTh component (Figs 19-21). Thedemagnetization trajectory for the HTh component generally passes through the origin of orthogonalprojections and is clearly uni -vectorial. The softer HTs component may be of hybrid origin representingthe composite breakdown of a very dominant characteristic vector and a minor vector of recent origin.Both the softer and the harder component have mean directions (Table 6) that are steeply -down,northeast -directed in geographic coordinates (in situ, HTs: Dec= 34•90, Inc= 58.80, a95 = 2.0, N= 66;HTh : Dec= 46.30, Inc= 59.7°, «95 = 1.90, N=66), and steeply-down, northwest-directed in stratigraphiccoordinates (with respect to palaeo-horizontal, assuming a single deformation event; HT s: Dec= 30810,

e•••

52

•

•Some palaeomagnetic results

• Inc= 63.9°, a95 = 3.20, N=66; HTh: Dec= 304.1 0, Inc= 70.1 0, «95 = 2.4°, N= 66). Two of the six sites

• (Table 6) showed directions that were considerably (site 1) or slightly (site 2) off the main grouping and,therefore, have not been included in the above results. The directions for both the in situ and the

• "bedding corrected" results have not been documented previously for this area and their pole positionsfall outside the main grouping of "Kiaman" overprints (Figs 18, 22). Their reverse polarity and steep

• inclination, however, suggests a "Kiaman" origin. Several hypotheses can be proposed:

•(i)^The HTh component is of post-tilting origin. The in situ direction of this component has a far

• shallower inclination than the Permian direction expected for the region, although its inclination iscomparable to the expected inclination for the Early Mesozoic. An Early Mesozoic origin, however, would

• require invoking a substantial, post-Early Mesozoic counterclockwise rotation. Such a sense of rotation

• and timing is highly unlikely. Available lithological, structural (Murray et al., 1987; Korsch et al., 1990),dating, and basement magnetic data (Wellman and Korsch, 1988) strongly suggest a pre-Mesozoic

• clockwise rather than counterclockwise rotation. An in situ origin for this component is, therefore,considered unlikely.

•(ii)^The bedding-corrected pole position for the harder HTh component is in agreement with oneof two alternative trajectories for the Permian part of APWP for Australia (Klootwijk and Giddings, 1988,

• 1993; Klootwijk et al., 1993 [fig. 12]), namely the trajectory that follows a clockwise and partial looparound the Australian continent from the west to the north. This option would imply the absence of any

• significant rotation since its acquisition. This Permian alternative trajectory, however, is hardly, if at all,

•constrained by Australian results and is mainly based on some Indian palaeomagnetic results fromregions that have undergone considerable tectonism. Recently acquired late Palaeozoic results from

• northeastern Queensland (Klootwijk et al., 1993; Klootwijk and Giddings, 1993) and the Tamworth Belt(Klootwijk, 1994) do not support this alternative trajectory, and the hypothesis of no significant rotation

• can be discarded as a non-viable option.

(iii)^The HTh component does not represent an accurate record of the Earth's magnetic field at the

• time of extrusion of the Alum Rock ignimbrites. This could be due to significant non-dipole components(Schneider and Kent, 1990) of the Earth's magnetic field or to rheomorphic activity that must have been

• of a surprisingly consistent nature throughout the volcanic succession. Whilst possible, such an ad-hoc

•interpretation is not a likely alternative.

• (iv)^Comparison of the bedding-corrected pole position with the generally accepted Permiantrajectory of the APWP for Australia, that is, the trajectory that bypasses southern Australia from west

• to east (Schmidt et al., 1990; Klootwijk and Giddings, 1993, Fig. 18), indicates a significant clockwiserotation of at least 80°. This is about half of the rotation required for complete unfolding of the TexasOrocline and suggests that extrusion of the rhyolitic ignimbrites during the Early Permian (Sakmarian)

• was possibly synchronous with oroclinal bending of the Texas and Coffs Harbour blocks. Thishypothesis conflicts with an earlier interpretation, based on preliminary data (Klootwijk, 1985), that the

• "Kiaman" overprint, which is so pervasively present in the Texas beds and Coffs Harbour Association butnot in the ignimbrites at Alum Rock, postdates oroclinal bending. It was argued on geological groundsthat acquisition of this "Kiaman" overprint most probably occurred during the Late Carboniferous - Early

• Permian (Klootwijk, 1985, 1988; Klootwijk and Giddings, 1988, 1993). Interpretation by Klootwijk et al.(1993) using U-Pb SHRIMP age determinations from the Late Carboniferous Bulgonunna Volcanics

•

• 53

a••^ Discussion

• (Black, 1994) constrains acquisition of the "Kiernan" overprint to the 290-305 Ma interval in the Late

• Carboniferous. This dating constrain applies strictly speaking to the Bulgonunna Volcanic Field only. Itwill have to be further established whether the Australia-wide acquisition of "Kiernan" overprints

• represents a synchronous rather than a diachronous event (Schmidt and Leckie, 1993; Lackie andSchmidt, 1994). Assumption of a synchronous event would date the overprinting mostly prior to

• extrusion of the ignimbrites at Alum Rock in the Sakmarian (293 Ma; J. Claoue-Long, pers. comm., 1993;

• Roberts et al., 1994). Such an early acquisition could account for absence of a clearly identifiable"Kiernan" overprint in the ignimbrites at Alum Rock. However, the two lines of evidence: (i) origin of the

• "Kiaman" overprint as post-oroclinal but pre-extrusion of the ignimbrites at Alum Rock, and (ii) extrusionof the ignimbrites at Alum Rock synchronous with oroclinal bending, but after acquisition of the "Kiernan"

• overprint are clearly incompatible. This dilemma may possibly be resolved with a forthcoming more

• detailed directional analysis of the "Kiaman" overprint throughout the Texas and Coffs Harbour blocksand the Terrica beds in particular. The hypothesis of a clockwise rotation could become consistent if

• acquisition of the "Kiernan" overprint proved to have occurred in, or to have extented into, the Permian(Schmidt and Leckie, 1993; Leckie and Schmidt, 1994), and if oroclinal bending occurred during the

• earliest to Early Permian (Korsch and Harrington, 1987; Korsch et al., 1990) rather than the Late

•Carboniferous (Murray et al., 1987). This hypothesis has fundamental implications for the late Palaeozoicevolution of the New England Orogen, but its internal inconsistency needs to be resolved before the

• hypothesis can be proposed as a viable option.

•• DISCUSSION

• Two major late Palaeozoic tectonic events have been recognised in the New England Orogen,subduction-related accretion and oroclinal bending. Outcrop-scale structures, however, have been

• recognized for only one of the two events, the deformation that occurred during subduction and

•accretion (Korsch, 1981). The geometry of the Z-shaped megafold is clearly indicated by regional scalepatterns, such as outcrop geology, continuous change in regional strike patterns of bedding and

• cleavage, and gravity and aeromagnetic data. Outcrop-scale deformation related to oroclinal bending,however, has not been documented. There is no obvious field evidence for structures that could be

• related to this event, such as axial-plane cleavages or stretching lineations. In this respect, what can beconcluded from the AMS data?

•

•AMS interpretation premises

• For the purpose of this discussion it is appropriate to highlight two features of the magnetic fabric.Firstly, the magnetic fabric determined with the AMS instrumentation is the vector sum of all individual

• fabrics, acquired during different deformation phases. It is thus to be expected that a particularly severe

• deformation will be reflected clearly and be readily identifiable, but multiple deformations of comparable

•Figure 21 Histograms of stability range distributions upon AF and thermal demagnetization for magnetization components with

• low (LT,,LTh,LT) and high (HT,,HTh,HT) blocking temperature intervals. The histograms were constructed by raisingthe height of individual histogram bars by one unit for every sample whose component stability range, as determined

• from principal component analysis, covered that bar-interval. The 2mT and 10 °C intervals were chosen for optimal

resolution only and bear no relation to the intervals between demagnetization steps.••

55

••

TABLE 6: Mean directions Alum Rock

IN SITU BEDDING CORRECTED SOUTH POLE POSITIONS IN SITU BEDDING CORRECTED

SITE/COMBINED DECL INCL K «95 n(N)2 DECL INCL K «95 LAT LONG LAT LONG dp dm PALLAT (0) (0) (0) (0) (0) (0) (OS (OE) (OS) (OE) (0) (0) (OS)

Recent field cOllJl)OJlent, soft ASAROI-12 11.5 -63.8 180.5 2.3 22(12) ------------------------------ 70.8 126.6 -------------- 2.9 3.7 45.5' ASAR13-22 [18.9 -47.5 123.6 22.7 2(10)]2 ASAR23-32 0 -62.6 470.4 2.8 7(10) ------------------------------ 74.5 151. 6 -------------- 3.4 4.4 44.0' ASAR33-41 6.5 -55.7 69.9 5.8 10(9) ------------------------------ 80.5 118.0 -------------- 5.9 8.3 36.2' ASAR42-50 355.1 -55.6 145.4 4.0 10(9) ------------------------------ 81. 3 178.9 -------------- 4.1 5.7 36.1' ASAR51-59 355.3 -61. 8 88.6 6.5 7(9) ------------------------------ 75.0 165.1 -------------- 7.8 10.1 43.0' ASAR01-59 1.0 -57.8 78.8 2.7 36(59) ------------------------------ 80.0 147.2 -------------- 2.9 4.0 38.5'

Recent field cOllJl)OJlent, hard ASAR13-22 3.9 -59.3 74.9 4.6 14 (10) ------------------------------ 78.0 137.2 -------------- 5.2 6.9 40.1' ASAR23-32 10.1 -68.4 169.6 2.8 17 (10) ------------------------------ 65.7 136.4 -------------- 4.0 4.7 51.6' ASAR33-41 1.9 -62.3 78.4 4.1 17(9) ------------------------------ 74.8 146.4 -------------- 5.0 6.4 43.6' ASAR42-50 9.5 -61.5 174.8 2.9 15(9) ------------------------------ 73.9 125.7 -------------- 3.5 4.5 42.6' ASARSl-59 6.2 -65.9 83.1 4.4 14(9) ------------------------------ 69.7 139.7 -------------- 5.9 7.2 48.2' ASAROl-59 7.3 -63.7 99.1 1.4 99(59) ------------------------------ 72.2 134.7 -------------- l.8 2.2 45.3'

Characteristic component, soft ASAR23-32 27.2 61.2 338.4 2.6 10(10) 318.0 70.5 336.7 2.6 14.9N 172.2 0.7 128.9 3.9 4.5 54.7 ASAR33-41 21.4 58.8 39.7 9.7 7(9) 321.5 55.9 39.8 9.7 19.1N 169.0 15.6N 120.3 10.0 13.9 36.5 ASAR42-50 42.5 56.0 204.6 4.2 7(9) 301. 9 57.6 205.3 4.2 13.7N 185.6 4.0N 109.7 4.5 6.2 38.2 ASARSl-59 45.2 56.9 93.6 5.0 10(9) 299.2 58.6 93.6 5.0 l1.6N 186.7 1.7N 109.2 5.5 7.4 39.3 ASAR23-59 34.9 58.8 71.4 2.9 34(59) 308.7 63.9 59.5 3.2 14.6N 178.8 2.5N 118.5 4.0 5.1 45.6 Sites 3-6 34.6 58.7 206.7 5.3 5 309.4 61.6 112.7 7.2 14.8N 178.7 4.9N 117.0 8.6 11.1 42.8

Characteristic component, hard ASARO 1 12 103.0 36.0 6.6 14.6 18(12) 126.2 56.8 6.6 14.6 20.4 49.4 44.6 215.9 14.6 15.4 21.2 ASAR13-22 18.5 65.4 23.2 6.8 21(10) 304.0 67.6 23.1 6.8 12.2N 164.3 3.2 119.8 9.5 11.4 50.5 ASAR23-32 45.0 62.4 713.7 l.3 18(10) 318.2 78.9 719.2 1.3 6.9N 182.6 11.8 137.3 2.3 2.5 68.6 ASAR33-41 36.5 63.4 42.0 5.6 17(9) 313.2 75.1 42.0 5.6 9.4N 176.9 8.0 131. 4 9.4 10.2 62.0 ASAR42-50 51.3 54.9 337.1 2.1 15(9) 299.7 62.5 337.8 2.1 9.9N 191. 9 1.0 112.9 2.6 3.3 43.9 ASAR51-59 50.8 56.8 164.2 2.9 16(9) 296.4 61.3 164.0 2.9 8.7N 190.2 1.9 110.2 3.4 4.5 42.4 ASARO 1-59 54.1 60.4 12.0 4.2 105(59) 303.1 77 .1 10.3 4.5 4.1N 189.2 14.2 130.1 7.8 8.4 65.4 ASAR13-59 40.7 61.5 41.7 2.4 87(47) 304.0 69.6 41.0 2.4 9.6N 180.8 5.1 121. 9 3.5 4.1 53.4 ASAR23-593 46.3 59.7 86.7 1.9 66 (37) 304.1 70.1 53.4 2.4 8.7N 185.4 5.6 122.5 3.6 4.1 54.1 Sites 1-6 56.1 59.7 17.2 16.6 6 302.6 77.5 12.7 19.6 3.6N 190.8 14.2 131.1 18.8 25.0 66.2 Sites 2-6 41. 8 61.1 107.5 7.4 5 303.7 69.3 96.2 7.8 9.5N 181.8 4.9 121.4 7.2 8.5 52.9 Sites 3-63 46.5 59.5 227.7 6.1 4 303.6 69.6 72.3 10.9 8.8N 185.7 5.3 121.7 16.0 18.6 53.4

-----------------') Virtual Geomagnetic Pole 2) Inaccurate result 3) Preferred Mean Direction/Palaeo Pole

• • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • •

N B

_ .. ., W I I I I I I I I + E W

s

N

s

Discussion

E

20-7/207

• Alum Rocks BIC

,. Reference APWP

Figure 22 Equal-area projection of site-mean directions (Table 6) in geographic (present-day, A) and stratigraphic (bedding

corrected, B) coordinates. Result EPE in Figure B indicates the expected Early Permian directions recorded by rocks

at the sampled site according to the late Palaeozoic APWP shown in Figure 18 and assuming absence of deformation.

The heavy shaded double arrow indicates the magnitude of clockwise rotation that may be concluded from

comparison of the expected and observed declination data.

intensity may be less well easy to discriminate in such a composite AMS fabric. Secondly, the high

resolution power of the AMS technique allows identification of weak magnetic fabric patterns that may

result from (a) deformation phase(s) too weak to be identifiable by other means.

A question that deserves some attention is whether the data may indicate a magnetic fabric that could

only have been acquired during oroclinal bending (03), even though evidence for outcrop-scale, related,

deformation has not been established. We also need to ask whether we could separate such a 03-fabric

from a magnetic fabric related to the subduction and accretion process that is identifiable at outcrop

scale as a 01-cleavage. The premises for identification of the various options are straightforward. On the

one hand, a magnetic fabric pattern generated during the 01 phase and hence before oroclinal bending,

would be expected to behave in a passive manner during oroclinal bending and would be rotated along

with the rest of the rocks. With outcrop-scale evidence for a 01-cleavage, a magnetic lineation could be

related either to an intersection lineation or to a stretching lineation not visible macroscopically. No between-site consistency in orientation of 01 magnetic lineations that have undergone 03 oroclinal

bending is to be expected. On the other hand, a magnetic fabric pattern that may have originated during

the 03 oroclinal bending process would be expected to have undergone no, or vel}' limited, passive

rotation related to the 03 phase, so between-site consistency of 03 magnetic lineations would be expected.

57

Discussion

120°80°

60°

0.0

Si

40

•

20°

0eô'sc

10 20

DISTANCE30 40 Km

20°40 Km20

•

58

100°

CPtU)

80°

C.)60° 4±

0

40°

Discussion

Origin of magnetic fabricWe have shown that the Kmin* axes are perpendicular to cleavage at virtually every site in the Texas and

Coffs Harbour blocks. The Kmax* axes therefore lie in the cleavage-plane. The main cleavage is of D1origin, hence the logical conclusion is that the magnetic fabric described by the Kmin* and Kmax* axes

formed during Dl. It is obvious from the distribution of the cleavages (Kmin* axes) around the oroclines(Fig. 12) that the D1 magnetic foliation has been passively rotated during the D3 oroclinal bendingphase. Hence, the D1 magnetic lineation (Kmax* axes) must also have been passively rotated by thesame event. If we assume, for the moment, that the formation and orientation of the magnetic lineationsis in some way related also to development of oroclinal bending, then one would expect that thecomposite D1 — D3 Km * axes should show some consistent pattern. If a D3 magnetic lineation formedas a stretching lineation during orodinal bending, it should have one constant orientation parallel to thedirection of tectonic transport. If such a D3 magnetic lineation originated from some flexural slipmechanism rather than orodinal bending, it should be similar in orientation to a series of slickensides.If we examine the regional pattern of dip directions and dips of Kmax* axes across the Texas and CoffsHarbour blocks we find they do not show a consistent pattern (Figs 12-14, Tables 2-4). Since there isno evidence for regional deformation of post-oroclinal bending origin, it is thus difficult to relate theKmax* axes, partially or wholly, to D3 oroclinal bending. Therefore, our preferred interpretation is thatboth the Kmin* and Kmax* axes formed during the D1 phase. Depending on the level of strain that therocks underwent, the Kmax* axes may represent either intersection lineations or stretching lineations.During the D3 oroclinal bending both the Kmin* and Kmax* axes were rotated passively with bedding andcleavage. This is illustrated by the tendency of Kmax* axes from individual areas to lie on small or greatcircles (Fig. 12C).

Regional lineation observationsDI-intersection lineations. eastern Coifs Harbour blockResults from the eastern Coffs Harbour block show good agreement between structural observationsat outcrop-scale and magnetic fabric determinations. The magnetic fabric can be related to the axial-plane cleavage of D1 origin. The Kmin* axes are perpendicular to the cleavage, and the Kmax* axes aregenerally in good agreement with the intersection of bedding and cleavage according to the structuraldata of Korsch (1975). The magnetic fabric can thus be identified as essentially induced during the D1deformation phase that is attributed to subduction and accretion, and has no bearing on the subsequentoroclinal bending (D3) deformation process.

The regional magnetic fabric pattern for the eastern Coffs Harbour block shows some interesting detail.There is a distinct north-south increase in the magnitude of the foliation parameter, which is inagreement with field observations of Korsch (1973, 1975) of an increase in the intensity of deformation.In the eastern Coffs Harbour block, Korsch (1973) observed that from Red Rock southwards an increaseof the plunge of the fold axes is accompanied by a decrease in the interlimb angle (Fig. 23A). This north-to-south relationship is not visible in the anisotropy parameter (Fig. 23B), but normalization of thisparameter with the intrinsic parameter (Fig. 23B) does demonstrate an increase from north to south,

Figure 23 A) Structural data for the eastern Coffs Harbour block to the south of Red Rock, from Korsch (1973); B) Degree ofanisotropy and normalized degree of anisotropy in the same framework.

••

59

••••••••

Discussion

which is in agreement with the southward increase in intensity of deformation. This observation, althoughbased on only a small number of sites (4) and clearly of preliminary nature, interestingly suggests astrong magnetic mineralogical control on the intensity of the anisotropy parameter. This could be dueto the characteristic turbid ite facies of the area. Its mixture of mudstone and sandstone may lead to largevariations in ferromagnetic and paramagnetic fractions. It is more likely, however, that this effect resultsfrom formation of secondary pyrrhotite upon metamorphism, which increases in grade from north tosouth.

Stretching lineations. Texas and eastern Colts Harbour blocksThe directional and scalar AMS data show evidence for stretching lineations that are most probablyrelated to the subduction and accretion process (D1) and not to the oroclinal bending process (D3) asdiscussed in the previous section. The Texas block data show large differences between the observedsteep plunges of the magnetic lineations and the more moderate plunges that are to be expected forintersection lineations, clearly suggesting the presence of stretching lineations. DIG ICO data for sitesATCB and ATCC close to Red Rock in the eastern Coffs Harbour block suggest the presence of two setsof magnetic lineations (Appendix). One set shows a shallow plunge and may be related to theintersection lineations previously measured by Korsch (1973) and attributed to the D1 phase. The secondset shows steeper plunges, not observed in the field, which may reflect stretching lineations in rocks thathave undergone higher levels of strain than those showing intersection lineations.

Intensity and shape of magnetic anisotropy in the Texas blockScalar anisotropy data for the Texas block show changes in intensity of foliation and shape of theanisotropy ellipsoid around the megafold that seemingly conform with the deformation pattern expectedupon oroclinal bending. Deformation theory (e.g. Ramsay, 1967), predicts that the strain in a fold, oftenbut not always, increases from the limbs toward the axial zone, with the expected shape of the petro-fabric being oblate at the periphery of the fold and prolate in the centre of the axial zone (Fig. 24). Sucha pattern could be interpreted from the limited set of magnetic fabric data obtained for the Texas blockmegafold. (i) Plots of magnitude of the foliation parameter versus the strike of the bedding (Fig. 12B),show a close relationship with the fold geometry, with maximum intensity near to the hinge of the fold.(ii) The variation in shape of the AMS ellipsoid (Fig. 12B,C) accords with the pattern expected for amegafold. Site ATTA, supposedly in the centre of the axial zone (P.G. Flood, pers. comm., 1990) showsa very prolate magnetic fabric L*> F*, whereas the other sites exhibit an oblate fabric F*>L*.

These two observations are suggestive for (further) development of a magnetic fabric during oroclinalbending, but clearly conflict with the earlier discussed observation that the pattern of Kniax* axes aroundthe orocline does not show the regional consistency that is to be expected for a magnetic fabric of 03origin. These conflicting observations warrant further study. Pending the outcome it seems wise tointerpret the above observations (i,ii) as apparent patterns based on a small and non-representative set

of observations.

Magnetic fabric versus lineament patternsA study of lineament patterns in the Texas block based on interpretation of images processed fromLandsat TM data (P.K Vinayan, pers. comm., 1993; Vinayan et al., 1993) identified a noth-northeast to

••••

••••••••

60

61

Figure 24 Schematic representation of evolution of the deformation ellipsoid in a fold with vertical axis, with the highest degree

of deformation localized in the hinge zone of the megafold. The corresponding location of some sites in the Texas

block are indicated, compare with Figure 12. It should be realized that exterme flattening also can occur in the limbs

of a fold (e.g. Ramsay, 1967). In the case of the Texas Orocline it is most likely that the fabric which had already been

formed during the 01 deformation phase became enhanced during the D3 oroclinal bending phase. Note that for site

AnD the Kmin* axis is perpendicular to 81, and 81 is vertical and east-west oriented. Hence the orientation is related

to passive rotation of the 01 magnetic fabric during oroclinal bending. Also it cannot be excluded that site AnA is

located on an elongated limb of the Texas megafold rather than in its axial-plane.

south-southwest lineament axis for part of the Texas block. This axis differs notably from the northwest

southeast oriented axial-plane for the Texas Orocline suggested by Flood and Fergusson (1982) from

field observations. It may be interesting to note that a simple tilt correction of Kmax* axes, using rotation

parameters for individual sites that rotate bedding to horizontal, shows "corrected" Kmax* axes that tend

to line up along such a north-northeast-south-southwest axis. Application of such a tilt correction,however, is purely heuristic. Interpretation of the ·corrected" Kmax* axes as an indication of regionaltransport during oroclinal bending could be justified only if tilting was restricted to the 01 phase, with

oroclinal bending eventuating purely through rotation around vertical axes without any further tilting of

the beds. Such a scenario is highly unlikely. The heuristic tilt correction therefore is probably

unwarranted and the observation may be purely coincidental.

Discussion

highly deformatedand prolate magneticfabric site ATTa

highly deformatedand oblate magneticfabric· site ATTD

~ finite deformation ellipsoidAMS ellipsoid

••••••••••••••••••••••••••••••••••

Conclusions

CONCLUSIONS

This exploratory study of the anisotropy of magnetic susceptibility (AMS) in highly-cleformed rocks of the southern New England Orogen has shown a clear relationship between magnetic fabric and

structural fabric, and has shown considerable potential for further application to the region. The main conclusions reached are:

(1) The magnetic foliation is related to the D1 cleavage throughout the Texas beds and Coffs

Harbour Association;

(2)

(3)

The magnetic lineation is interpreted as a subduction and accretion related D1 intersection

lineation between the cleavage and the bedding in the eastern Coffs Harbour block, and as a D1 stretching lineation in the Texas block and the central Coffs Harbour block;

The magnetic anisotropy data for the eastern Coffs Harbour block mimic the southwards

increase in intensity of deformation and metamorphism observed by Korsch (1973, 1978);

(4) The scalar anisotropy data for the Texas block show a relationship with the shape of the

megafold. This may be purely coincidental as the vectorial pattern of I<,nin * and I<,nax * axes is

of D1 and not of 03 origin although it may have been modified during the later phase;

(5)

(6)

Palaeomagnetic data from the Alum Rock inlier support oroclinal bending ofthe Texas and Coffs Harbour blocks although the timing of the oroclinal bending remains contentious. The "Kiaman" overprint widely observed throughout the Texas and Coffs Harbour blocks shows no evidence

of significant rotations subsequent to its acquisition. The age of this overprint, so far, has been assumed to be Late Carboniferous - Early Permian on the basis of limited geological,

palaeomagnetic, and radiometric evidence, and has been thought further constrained to the 290-305 Ma interval on the basis of recent U-Pb (SHRIMP) zircon dates from northeastern

Queensland. Such an age for oroclinal bending would be supported by the agreement of the

Alum Rock pole position (Sakmarian) with an earlier suggestion of an alternative west-to-north trajectory for the Early Permian APWP. Comparison, however, of the Alum Rock pole position

with the generally accepted west-to-east trajectory suggests a considerable clockwise rotation

of post-Sakmarian age over at least 80 degrees. This is about half of the rotation expected for complete unwinding of the orocline, and suggests that extrusion of the Alum Rock ignimbrites

may have occurred synchronous with the oroclinal bending process. Such an interpretation

could become acceptable if either "Kiaman" overprinting was diachronous across Australia

(Schmidt and Lackie, 1993; Lackie and Schmidt, 1994) and proves to have extended to post

Sakmarian times rather than be confined to the Late Carboniferous - Early Permian, or if further

study of the "Kiaman" overprint shows a more complex directional pattern than hitherto

interpreted;

The Gilgurry Mudstone shows a magnetic fabric pattern that is different from the Texas beds

and Coffs Harbour Association. Their low anisotropy and horizontal magnetic lineation suggest that formation of these Late Permian (Fauna IV) rocks postdated oroclinal bending. Attribution

of this magnetic fabric to deformation related to the Demon Fault is speculative at this stage.

62

• • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • •

•••Âcknowledgements•ÂCKNOWLEDGEMENTS•Peter Flood (University of New England) was a rich source of information on the geology of the Texas

• block, which he generously shared with us. Peter Percival, David Edwards and Richard Clark (AGSO

•Palaeomagnetic Laboratory) provided technical assistance during the measurements. We thank ClintonFoster (AGSO) for palynological analysis of samples from the Terrica beds, Rex Bates and Lindell

• Emerton (AGSO) for cheerful and expert drafting of various figures, and David Denham, Charlie Bartonand in particular John Giddings (AGSO) for constructive criticism of the manuscript.

•

•

•

•

•

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•

•

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•••••••••••• 63

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•

References

REFERENCES

Aitchison, J.C., 1988. Late Paleozoic radiolarian ages from the Gwydir terrane, New England Orogen, eastern Australia. Geology,

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70

Appendix

APPENDIX DIGICO AMS results(See also Tables 1,3 and Figure 4)

AA • A

1.17 Equal area

BULK v AnistroPu

Lineation v Foliation

• • ••• ••^•

e •

Anistropy v T factor

1.19 Equal area

BULK v Anistropy .

29_28Lineatjon v Foliation

V4


•

Site ATTA - Texas beds

Site ATTB - Texas beds

4-

+• •• • •

41410^4,

Equal area^ S

^.^. +.. • •

1^.*^• " I^•

^1.30

ooN

..

•

'

',.

t•

oo• •* •

• 0 • •

S.,,,,....

1.77 Equal area

BULK v AnistroPU

1^1^139_02

t_ineation v Foliation

1111111.87


N

N

BULK v Anistropy Anistropy Axis Dataax

rintin

N

• °

+

IÎ^l35.52


C.

'..

C .

IÎ1.23


ALA• •

+ÎI 17 •^1'^+ALA A4. A

• E el a

AL AL

±

CON

14

Site ATTC - Texas beds

Site ATTD - Texas beds

133.20

1.36 •• •

•

1_71 Equal area

BULK u Anistroow

Lineation u Foliation

IÎ

BULK u Anistropy Anistropg Axis Data

pit

ax

Equal area

43.234ineation u Foliation

•

1.73Anistropy v T factor

. -0 .•

1.92

C17

Site ATTE - Texas beds

Site ATTF - Texas beds

• •

BULK v AnistroPy

Lineation v.Foliat.ion

•° • •

1_07Anistropy v T factor

1_12 Equal area

fl

Site ATTN - Texas beds

1.23 Equal area

51.00

•

BULK u Anistroww

•

Lineation u Foliation

1 .15Anistropy v T factor

•

14

BULK u Anistropy.•

. s.^°

Anistropy Axis Data

11Wax

114ptin

Lineption u Foliation

<4 •

07


A

1.39 Equal area

Site ATCI - central Coffs Harbour Association

Site ATCJ - central Coffs Harbour Association

•

BMX v Anistropy

•

1 38.76

Lineation v Fgliation

•

• •4 .

•.

1.30

Anistropy Axis Dataax

Entin

Equal area

•

AAA


Intin

• Ill• ME• A

• ••+^-1-. ±^+^+• •Â•

AA

A

A• •*^,s,A. • • •

Equal area

A

BULK v AnistroPU

•

•

Lineation v FoliAtion

•• • •


1.16

Site ATCK - central Coffs Harbour Association

Site ATCL - central Coffs Harbour Association

BULK v Anistropy.

•

Lineation to Foliation•


• •

• 0^• . •0^ • •0

Equal area

•

+ • •

•.^.°

1_12 Equal area

BULK v Anistropy .• 0

< . •


in

N

Ltneation v Foliationa

•


A

• 111A+ A +

•

O.

AAîn

A +AA

Site ATCB - eastern Coffs Harbour Association

Site ATCC - eastern Coffs Harbour Association

•

•••■•■ Equal area1.22

BULK v AnistroPU

•

1 25.64

Lineation v Foliation•

•

1.17__Anistropy v T factor

0•

•4

BULK p.AnistroPY

•4.

21_29Lineatipn.v .Foliation

••• 4.

•

•


1.21 Equal area

Site ATCD - eastern Coffs Harbour Association

Site ATCE - eastern Coffs Harbour Association

BULK v Anistropu

0

Lineation v Foliation•


0

Csi

1_20 Equal area

A

1614.± A AAA

A

A

A

Site ATCF - eastern Coffs Harbour Association

BULK v AnistroPW Anistropy Axis Data

giptax

in

U1

Lineation v Foltption


0

• •

•^+ sr • +^+^+^+

•% •• •IN^M^ *

ÎI. ^ 0^•+^• is •• *• • *

A

co•^•

•

Equal area

^* ^• • •• •^:.

1.15

Site ATCG - eastern Coffs Harbour Association

Equal area1.22

gr+ ••

BULK v Anistropy .• .

Anistropy Axis Data

giptin

ax

• O .

A


••

A

••4111 •••• ••

••

•

•


Site ATCH - eastern Coffs Harbour Association

■■■ • ••■•1.08 Equal area

BULK v Anistropy .• •^••

• .4?COC

26_63Lineation v F9liation

• .•

(00


fl

• •• •

Anistropy Axis Data

:Entin

axBULK v Anistropy

CO0

•

Equal area1.08

•



Site ATTO - Terrica beds

Site ATTP - Terrica beds

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Sites ASAR1 to ASAR4 - Alum Rock

Sites ASAR5 and ASAR6 - Alum Rock

Documents

Magnetic Fabric and Palaeomagnetic Studies in the Texas ... · MAGNETIC FABRIC AND PALAEOMAGNETIC STUDIES IN THE TEXAS AND COFFS HARBOUR BLOCKS, NEW ENGLAND OROGEN Palaeomagnetism