Structural Analyses of Hamadan Metamorphic Tectonites: A Palaeotectonic Discussion. Berberian, M., and Alavi-Tehrani, N., 1977

Geol. Surv. Iran, 40, 263-279, 1977 263

STRUCTURAL ANALYSES OF HAMADAN METAMORPHIC

TECTONITES: A Paleotectonic Discussion.

by

M. BERBERIAN and N. ALAVI-TEHRANI

GEOLOGICAL AND MINING SURVEY OF IRAN

ABSTRACT

A preliminary note on the structural analysis of the Hamadan metamorphicsis presented here for the first time. These rocks are found to the south of Hamadanbetween Hamadan and Malayer, in the northern part of the Main Zagros ReverseFault (Fig. 1).

The oldest exposed rocks in this area are metamorphics of Paleozoic? to Creta-ceous age, consisting of phyllite, schist, crystallized limestone, paragneiss, hornfelsetc. Younger than these, but still Mesozoic, is the Orbitolina-bearing limestone ofCretaceous age. The Orbitolina limestone is overlain by the fossiliferous Eocene andthe Miocene Qom Formation in the northern part of the area.

Analyses of tectonites in Hamadan metamorphics enabled us to define a sequenceof events and their effects. Interpretation of the lineations as B-lineations and deduc-tions concerning their chronology were arrived at by examining the lineations in thefield, and relating them to the fold axes. The local compression axes were then calcu-lated from B-strikes.

From the tectonic analysis of the area, two major orogenic phases were recognized:

1. First phase (S ~, L2) :The Laramidian orogenic phase, responsible for the foldingand flattening of bedding and the formation of the first schistosity (S 1) and the first linea-tion (L1),with predominantly NW-SE striking B-axes (B-axes of folds and B-lineations),resulting from locally regional metamorphism in greenschist facies.

264 BERBERIAN and ALAVI

2. Second phase ($2, L2): The pyrreneean (?) orogenic phase, responsible forthe formation of the second B-lineation (L;), and the second schistosity ($2). B-axes related to the orogenic structure are predominantly N 140°E (parallel to theZagros Reverse Fault), with folding and change in orientation of L 2. The quartz veinswith metamorphic minerals and the andalusite-sillimanite crystals (thermic flow phase)were then folded in this phase.

In the central part of the area, thermic flow took place between the two phases(Upper Cretaceous to Paleocene), forming metamorphic crystals such as andalusiteand sillimanite far from the later granitic intrusions (like Kamari). This thermic domewas followed by gabbroic and granitic intrusions (Paleocene), which are responsiblefor the formation of the hornfels.

The tectonic movements of the second phase, which commenced as early asPyreneean, continued during the Tertiary and Quaternary, and appears to have beenmost extensive during late Tertiary (Zagros Orogeny) although they continued untilthe present time.

1. INTRODUCTION

The purpose of this research was to study several structural sections of HamadanMetamorphics around the Alvand granite, to determine and then separate the tectonicphases and the geological events, and to give the chronology of the events in this area.This problem was approached on the base of micro-tectonic studies. The results ofthe microtectonic investigations were then related to the megatectonics.

The area investigated for Hamadan metamorphics forms part of the HamadanQuadrangle Map, and lies half-way between the cities of Hamadan and Malayer.It is limited by 34o.30, and 34°. 50 northern latitudes, and 48°. 15’ and 480.45, easternlongitudeds (Fig. 1).

The oldest known exposed rocks are metamorphic rocks of presumbly Paleozoic(?) and Mesozoic age. The exact age of these rocks, especially those of the lower part,is, however, unknown because no fossil has yet been found in the lower units. In allprobability, however, its upper boundary age is Lower Cretaceous to Jurassic.

These metamorphics are composed of phyllite; different kinds of schists such asandalusite, sillimanite, staurolite,garnet schists ;crystallized and marmorized limestone,paragneiss etc. Younger than these series are the Orbitolina and marly-platy limestonesof Lower Cretaceous age which are overlain by Eocene fossiliferous rocks, or by thereddish conglomerate and whitish fauna-bearing limestone of the Qom Formation.

Samples from schistosed and metamorphosed crystalline Cydaris spine-bearinglimestone fo Kuh-e-Khan Gormoz (below the shistosed Aptian limestones) in theHamadan metamorphics were collected and then studied by M. Mehrnush, who deter-mined a Middle Jurassic (to probably Upper Jurassic) age. The samples contained

Fig. 1. Position of the Hamadan metamorphics and Quadrangle on the seismotectonic map ofIran (after Berberian 1976).


Globovalvulina sp., Trocholina el, Plastiniensis, Agathammina sp., and Acieulina sp.In the central part of the area some intrusions ranging from gabbro to granite

crop out. This intrusive mass of Alvand batholite has intruded through the regionalmetamorphosed rocks and has formed distinct contact-metamorphic haloes.

The stratigraphy of the area is only briefly reviewed here, since it is not the mainissue of the present study (for further information refer to the explanatory text ofHamadan Quadrangle, G.S.I., in preparation).

2. Structural analysis

For structural analysis, about seventeen structural sections of metamorphics aroundthe Hamadan granite were established and precisely surveyed. Each section was thendivided into several structural units. Care was taken to choose sections close together,in order to ensure a comprehensive view of the metamorphic tectonites and stresspatterns on the one hand, and to find out the changes in orientation due to tectonicaccidents on the other hand. These sections are indicated on the enclosed map (see.Map No. 8).

To approach the problem, about 2860 tectonic elements, such as B-lineations ofdifferent tectonic phases, B-axes of the folds, C-axes orientations of metamorphiccrystals, schistosities of different tectonic phases, and the old bedding, were measured.In order to determine the relationships between the metamorphic tectonites and therelated principal stress orientations, the tectonite configurations were analyzed geo-metrically by plotting the orientations on the upper hemisphere of the Schmidt equal-area stereonet. Maximum concentrations were selected from each diagram of the unitsto represent dominant tectonite sets at their respective sections.

In order to determine the principal stress orientations present during orogenies,the B-tectonites were selected.

2.1. Sequence of defromations:

The interpretation of the tectonic deformation depends on the determinationof the proper chronological sequence of the tectonic activity. When several structuralelements occur in an area, their chronological sequence may be determined whereone element transects or deforms the others. Using this method, detailed studies ofmicrotectonites of the Hamadan area made it possible to recognize two successivephases of deformations.

The stereographic projection of different orientations (S1,L1,L2, etc.) is givenin the enclosed map (see Map No. 8, in pocket).

2.2. First Deformational phase (S

The Laramidiam diastrophic phase, which affected the Jurassic-Cretaceous rocksof Hamadan region, produced the following deformations in the Mesozoic rocks:

1. Folding and flattening of the bedding (B)

Fig. Recumbent folds in the old sedimentary bedding (metamorphosed Jurassic ShemshakFormation : schist with silty-sandy laminae) and formation of the first schistosity (S1)in the axial-plane of the folds. Sample from the Hamadan metamorphics, Kamary village(photograph by M. Berberian, 3/4 natural size).


2. Formation of the first schistosity ($1).3. Formation of the first lineation (L~).

During this phase, a regional syntectonic metamorphism, generally of greenschist facies, took place.

2.2.1. Bedding:

First phase movements deformed (folded and flattened) the bedding. This obvious in the Kamary village section, east of’tl~e Alvand batholite. From themicrotectonic point of view, this section should, from the entire investigated area,be chosen as the type section of Hamadan metamorphic tectonites.

Fig. 3. As Fig. 2 (l~natural size).

HAMADAN METAMORPHIC TECTONITES 269

In the axial plane of the deformed bedding, penetrative axial plane schistosity($1) striking in different orientations has been formed (Figs. 2,3).

The bedding is rarely visible in the field due to the strong development of thefirst schistosity, but in a few sections, like Kamary-Zamanabad, the remnants ofsandy competent bands are to be found in the incompetent shaly (now phyllite andschist) matrix (Fig. 2). In the entire investigated area, only in 5 units we were luckyenough to find the traces of old bedding, consisting of flattened-isoclinal and tightflexural folds (Section No. V, Unit 1,2 - S.VI, 2.3 and S.VIII, U.2, in enclosed map).

In section No. V, unit 1, the average strike of the old bedding is about N 140°E,and dip 28° NE. In the same unit the average trend of the first schistosity ($1) is boutN 120°E, and dips 353 NE. This average trend is based on 14 observations of schistosityin this unit. The comparison of the bedding trend with the schistosity shows that thedip of the schistosity is more than the dip of the folded limb of the bedding; thus inthis unit the limb of the folded bedding "’is not overturned".

In unit 2 of the Zamanabad secton, about 37 different schistosity planes have beenplotted on the stereonet. The schistosity planes are more or less horizontal and themaximum dip is about 15-20 degrees (for example N60°E-15°SE). In this unit, thegeneral trend of bedding is about N 163°E and the average dip is about 40°NE. Withrespect to this net, the dip of the folded limb of the bedding is bigger than the dip ofschistosity, which is more or less horizontal. In this unit, the limb of the folded bed-ding is consequently "overturned" (see enclosed map).

In unit 2 and 3 of section No. V1, the folded limb of the bedding overturned insome places (in the field), and in some places is normal. In unit 3 (section No. VIII)the folded limb is normal.

2.2.2. First Schistosity (S 3):

As noted before, the penetrative axial plane schistosity in the axial plane consitsof deformed (folded and flattened) bedding in the first deformation and metamorphicphase. The sections, in which the remnants of folded and flattened bedding have beenobserved, show that the compression (tlattening) direction is perpendicular to theplane of the scistosity (S 1). The schistosity in the investigated area comprises differentstrikes and dips (see enclosed map). The average strike and dip of S1 in the differentunits and sections are as follows:

- Section No. I : N154E-70NE- Section No. II : Unit 1, N5E-50SE; unit 2, N150E-70NE. This section

shows a rotation in the trend of S 3, which is due to a faultbetween the two units of this section.

- Section No. III : Unit 1, N146E-34SW; unit 2,N135E-34 SW. The schis-tosity in this section is uniform and no great amount ofrotation of S~ trend exists.

- Section No. IV : Unit 1, N 172E-64 SW; unit 2, N 156E-65 NE.- Section No V : Unit 1, N 120 E-35 NE; unit 2, more or less horizontal;

unit 3, N 140E-35NE.- Section.No. VI : Unitl, N80E-25NE;unit2, variable;unit3,N164-50NE;


- Section No. VII:

- Section No VIII :

- Section No. IX :- Section No. X :- Section No. XI :- Section No. XII:- Section No. XIII:

- Section No. XIV:- Section No. XV :

Section No. XVI :

Section No.XVII:

unit 4, N 168-46NE. In this section too, a rotation existsin the trend of S a.Unit 1, N 5 E-35 SE; unit 2, N 64E-40SE; unit 3, N44E-60SE. A small rotation is observed in this section.Unit 1, N 134 E-50SW; unit 2, N 140 E-70SE; unit 3,N160 E-40NE; unit 4, N 32E-45SE. A fault is thoughtto be between the second and the third, and also betweenthe third and the fourth units.N16 E- 40SE.N120 E- 60 NE.N120 E - 50 NE.N138 E - 50 NE.Unit 1, N 66 E - 30 SE: unit 2, N 64 E-40SE; unit 3, N135 E - 40 NE. rotated.N10E, 80 SE.Unit 1, N 162 E - 35 NE: unit 2, N 62 E-30 SE; unit 3,N 140E-30 SE. There is a clockwise rotation.Unit 1, N 70 E- 75 NW ; unit 2, N 22E-30 SE; unit 3,N 90 E - 30 S.Unit 1, N 135 E - 25 NE; unit 2, N 95 E - 25 NE.

2.2.3. First Lineation (L 1):

During the first phase of deformation, a marked B-lineation was formed on mostparts of the schistosity surface. In the axes of the minor folds, the schistosity wasformed and cut the old bedding (Figs. 2, 3, 4, 5, 6, 7).

It is clear that microtectonic investigation s can provide evidence of the orientationof principal strain axes, but detailed interpretation of the exact primary trend of L1is generally difficult, since there is a big change in direction from N-S to E-W (seethe enclosed map). This seems to be consistent with the local variations in the strikeof the schistosity. The variations in the trend of L1 may be due either to the faultingprocess or to the effects of the second deformation phase, which deformed the first one.

In some undisturbed sections the N 125°E and N155°E trends seem to be persis-tent. This feature can be given only approximately, because neither is the initial andprimary trend of La known, nor is the phenomenon which deformed the first lineationknown exactly at present.

In a section near Kamary, the first lineation (older=L1) was measured as N155°Eand the second (younger=L2) as NI40°E (Fig. 9). If we consider the N 65°E directionof local stress responsible for the formation of L a (N 155°E), and the N50°E directionof local stress responsible for the formation of L2 (N140°E), we may postulate counter-clockwise rotation of the stress field from the period of LI (Laramidian)to the period of L2 (Pyrenecan) (Fig. 9). But if we assume a N120°E trend for and a N140°E trend for L 2, the rotation of the stress field will be clockwise. Researchfor finding the exact trend of L a is in progress; with better data, reconstruction the realstress field will be possible.

Fig. 4. Deformational phase (L1) developed in Hamadan metamorphics (3/4 natural size, photographby Berberian 1971)


2.3. Thermic dome and intrusions

After the first phase of deformation and before the second phase, during UpperCretaceous-Paleocene times, a thermic flow arose in the central part of the area.The field evidence which led us to this conclusion is the presence of metamorphicminerals such as andalusite, sillimanite, garnet etc., far from the intrusive bodies.The andalusite, schists of Zamanabad (15 km east of the Alvand intrusion), for example,constitute one significant piece of evidence. Since the andalusites of the thermic domephase were folded by the B-lineation and folds of the second phase (L2), the thermiceffect should have occurred before the second deformation phase.

The plutonic activity of Hamadan area, which is a Laramidian post-tectonicmagmatism, had three phases:

- Intrusion of basic masses- Intrusion of Alvand granitic batholite- Aplitic and pagmatitic veins.

Fig. Two different B-lineations (L1 : older lineation = NIOS°E; L2: younger lineation N145E) in the metamorphics of the Hamadan region (photograph by M. Berberian, 3/4natural size).


2.4. Second deformational phase (S2,Lz,K2; a dynamometamorphic phase)

The subsequent phase of deformations (second phase) corrugated the first schis-

tosity plane and produced the penetrative schistosity which developed axial-planar

Fig.

Quartz Vein( ÷Kyanite

6.~ Schematic drawing of a section through a part of the Hamadan metamorphics northof Kamary, showing the generation of two schistosities (S1,$2) and B-lineations (L1,L2).(By M. Berberian, October 1971).

Fig. 7. Two different B-lineations (LI, L2) in Hamadan m2tamorphics(photograph by M. Berberian.½ natural size).

Fig. 8. Kink-band developed during the second deformational phase (FI) in Hamadan metamorphics(photograph by M. Berberian 1971, natural size).


to the corrugated St: at the same time the second lineation, kinkbands and foldingscame into being (Figs. 5, 6, 7’ 8). This phase produced the following deformation andstructures:

1. Folding of S 1 and possibly a change in the orientation of L1.2. Formation of the second schistosity (Sz).3. Formation of the second Lineation (Lz).4. Formation of the kink-bands (K2).

2.4.1. Folding of $1 and formation of $2:

As already mentioned, the movements of the second deformational phase (Py-reneean) corrugated the plane of the first schistosity and produced small-scale foldsand B-lineation (S 1 in Fig. 6; and Figs. 5,7).The second schistosity is badly developedthroughout the area. Only in the phyllites is it clear and well-developed, and less soin the competent metamorphics.

The movements of this phase might have changed and rotated the trend of thefirst schistosity. Confirmation of this hypothesis, however, must await a further visitto the area for detailed microtectonic studies.

2.4.2. Second lineation (L~)

During the second phase of deformation, possibly during the Pyreneean phase,the second marked B-lineation(L 2)was formed on the first schistosity plane(Figs. 5,6,7,8).This lineation transects and folds the B-lineation of the first phase (L0, folds themetamorphic crystals and quartz veins of the first phase, and also bends the andalusitecrystals of the thermal phase (Zagheh and Kandelan units; and Fig. 5); thus L~ younger than L1 and the thermic dome.

The most prominent and uniformaly developed structural features were formedduring this stage. Since the trend of the movements of the 2rid deformational phaseis not rotated, a detailed study has been made of the Lz strain geometry.

Throughout the Mesozoic rocks, the B-lineation (Lz) and B-axies of the folds(B 2) of the second phase were taken to indicate the orientation of the maximum com-pressive direction. The B-axes of the second phase folds (B 2) in the entire investigatedarea, and also the B-lineations (L2), have an approximate trend of N 140°E, which isnearly parallel to the Main Zagros Reverse Fault and the fold axes trend of the ZagrosActive Folded Belt.

The trend of L ~ and B 2 indicate that the maximum compressive strain directionis nearly N 140°E. Thus the maximum compressive stress orientation lies perpendicularto-the trend of L2 and B 2 and is about N 50°E; the maximum tension trend which isalso the minimum effective stress axis, is supposed to be N 140°E (Fig. 9).

Apparently, the recent compressional forces in the area have the same directionas those of the second phase (L2) (Berberian 1976). It may therefore be assumed the stress field did not change from Pyreneean to Wallachian time.


3. CONCLUSION

The structural analysis of the Hamadan metamorphic tectonites yielded threedeformational phases (excluding faulting): one phase of deformation and metamor-phism, a younger deformational phase, and three successive phases of intrusion.

1. The first major deformation and metamorphic phase of the Laramidian oro-geny folded and flattened the old bedding, and then a penetrative axial plane schistositydeveloped (S 1). The trend for B-lineation of this phase is variable. A simultaneousmetamorphism produced regional metamorphism of green schist facies.

2. After the first phase of metamorphism and deformation, a thermic flow aroseand produced the metamorphic minerals far from the later intrusive bodies (possiblyin the Upper Cretaceous period). This flow was succeeded by three successive phasesof gabbroic, granitic, and aplitic-p~gmatic intrusions (with contact metamorphism).

Fig. 9. ~. Rotation of the local principal stress directions during two difformational phases in Hamadanmetamorphics. I.B-axes, 2. stress direction, 3. rotation direction of stress from Laramidianphase to Pyreneean phase.


3. The second phase of deformation deformed and then folded the trend ofthe first phase lineation. It bent the metamorphic minerals of the first phase and theandaluite of the thermic flow phase. The effects of this second phase are found in theJurassic-Cretaceous and the Eocene formations.

The strikes and B-axes related to this phase~ which, are demonstrated by themicrotectonic study, are predominantly N140°E, parallel to the Main Zagros ReverseFault. The suggested trend for the maximum compressive stress for this phase isN 50°E.

It is beyond the scope of the present report to discuss in detail the structural fea-tures of the whole area, but the most important ones have been indicated. The detailsof the structural geology of this area remain to be worked out in future.

HAMADAN METAMORPHIC TECTONITES

REFERENCES

279

i.k*4 berian, M., 1976. Contribution to the Seismotectonics of Iran (Part I1). Geological Survey Iran, Report No. 39, 518 p., 5 maps, 259 figures.

Documents

Structural Analyses of Hamadan Metamorphic Tectonites: A Palaeotectonic Discussion. Berberian, M., and Alavi-Tehrani, N., 1977