Crustal variations and terrane boundaries in southern Mexico asimaged by magnetotelluric transfer functions

A. Jordinga,*, L. Ferrarib, J. Arzatec, H. JoÈdickea

aInstitut fuÈr Geophysik, WestfaÈlische Wilhelms-UniversitaÈt, MuÈnster, Corrensstrasse 24, 48149 MuÈnster, GermanybInstituto de GeologõÂa, Universidad National AutoÂnoma de MeÂxico, Apdo. Postal 70-296, 04510 Mexico D.F., Mexico

cUnidad de Ciencias de la Tierra, Instituto de GeofõÂsica, Universidad National AutoÂnoma de MeÂxico, Campus Juriquilla, Apdo.

Postal 1-742, 76001 QuereÂtaro, Qro., Mexico

Received 13 September 1999; accepted 3 July 2000

Abstract

To investigate the complex crustal structure of southern Mexico magnetotelluric (MT) data were acquired at 75 stations along

two north±south pro®les, 200±250 km apart, crossing several terrane boundaries whose interpretation has been controversial in

the literature. Resulting sounding curves can be grouped, according to their shapes, into 10 different sets. Each of them may

represent a crustal unit with a characteristic depth dependent electrical conductivity distribution. Often, the spatial extent of

such a crustal unit matches well with a geological terrane. However, we also observed transition zones or sharp contrasts in

crustal electrical properties which do not correspond to any de®ned terrane boundary. Transition zones are associated with low

angle crustal structures (regional thrust faults or mylonitic zones) and/or with areas of plutonism, which are adding hetero-

geneities to the crust. In other cases our data suggest that terrane boundaries proposed on the basis of surface geology are

probably shallow structures, which may not continue at depth. The comparison of apparent resistivity pro®les with the available

geologic information indicates that the MT method is capable of clearly de®ning zones affected by recent and active volcanism

and tectonics as well as the part of the crust with Paleozoic and Precambrian metamorphic rock assemblages. q 2000 Elsevier

Science B.V. All rights reserved.

Keywords: magnetotelluric surveys; continental crust; southern Mexico; tectonostratigraphic terranes; electrical crustal units; apparent

resistivity

1. Introduction

The terrane concept refers to geological entities of

regional extent characterized by a coherent strati-

graphic sequence and bounded by major tectonic

discontinuities (Coney, 1989). The ®rst application

of terrane analysis in Mexico by Campa and Coney

(1983) led to the de®nition of 14 terranes (Fig. 1).

According to their work most of western and southern

Mexico would have been aggregated by exotic blocks

formed in oceanic environment and successively

accreted to the North America plate in Mesozoic

times. Since then new terranes were proposed and

the nature and boundaries of others were rede®ned

(e.g. Grajales-Nishimura, 1988; Ratschbacher et al.,

1991; Sedlock et al., 1993; Lang et al., 1996).

However, the substance of the Campa and Coney

(1983) classi®cation has remained.

The location of terrane boundaries as well as their

Tectonophysics 327 (2000) 1±13

0040-1951/00/$ - see front matter q 2000 Elsevier Science B.V. All rights reserved.

PII: S0040-1951(00)00166-9

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* Corresponding author. Fax: 149-251-83-36100.

E-mail address: jording@earth.uni-muenster.de (A. Jording).

A. Jording, et al. / Tectonophysics 327 (2000) 1±132

Fig. 1. Map of southern Mexico with terrane boundaries according to Campa and Coney (1983) with MT pro®les A-A 0, B-B 0.

kinematics and their geometry at depth has been espe-

cially controversial in southern Mexico. The bound-

aries of the Guerrero, Mixteca, Oaxaca, JuaÂrez and

Xolapa terranes have been modi®ed (e.g. in Sedlock

et al., 1993) and the mere existence of some of them

has been questioned (Ratschbacher et al., 1991; Herr-

mann et al., 1994; Lang et al., 1996). Part of the

discussion probably arose because terrane de®nition

often is based on relatively scarce surface geologic

information and because published borehole and seis-

mic re¯ection data are not available.

The magnetotelluric (MT) method involves the

measurement of orthogonal components of natural

electric and magnetic ®elds, which by inductive

coupling contain information about the electrical

conductivity distribution at crustal and upper mantle

depths. This electromagnetic method is particularly

well suited for studying regional structures and may

help in de®ning crustal variations and terrane bound-

aries based on lateral changes of conductivity. We

present the results of a total of 75 long period MT

soundings at stations aligned in two north±south

trending, coast-to-coast pro®les, which cross some

of the most debated terranes in southern Mexico.

2. Crustal structure of terranes in southern Mexico

In the studied region (Fig. 1), the cratonic block

represented by the Paleozoic and Precambrian

complexes of the Mixteca and Oaxaca terranes is

bounded to the west and to the east by volcanic and

sedimentary assemblages of Mesozoic age belonging

to the Guerrero and JuaÂrez terranes, respectively (the

original terrane names of Campa and Coney, 1983, are

used throughout the text). These four terranes are in

turn bounded to the south by the high grade meta-

morphic rocks of the Xolapa terrane, which is

intruded by Tertiary plutons. Terrane boundaries are

obscured to the north by the Trans-Mexican Volcanic

Belt (TMVB), the volcanic arc formed as a result of

the subduction of the Cocos and Rivera plates since

the late Miocene (Ferrari et al., 1999). North of the

TMVB the Late Jurassic to Cretaceous carbonate and

shallow marine clastic rocks of the Sierra Madre

Oriental fold-thrust belt and the late Tertiary and

Quaternary sediments of the coastal plain of the

Gulf of Mexico mostly cover the Precambrian and

Paleozoic basement of the Sierra Madre and Coahuila

terranes.

In recent years, several works shed light into the

structure of the basement of this region. One of the

main advances of the past decade was the recognition

that much of eastern and southern Mexico is underlain

by a Grenvillian-type basement which was part of an

ancient microcontinent named ªOaxaquiaº (Ortega-

GutieÂrrez et al., 1995; Lawlor et al., 1999). This

Precambrian basement is thought to underlie the

Mixteca, Oaxaca, Maya, Sierra Madre and Coahuila

terranes. In addition, lower crust xenoliths and Nd

model ages of magmatic rocks permit the inference

of a similar Precambrian basement to the west of the

Mixteca terrane at least up to longitude 100830 0

(Centeno-Garcia et al., 1993; Elias-Herrera and

Ortega-Gutierrez, 1998; and references therein). To

the east of the Oaxaca terrane Carfantan (1986) specu-

lated on the existence of an oceanic basin and a volca-

nic arc during Jurassic and Cretaceous times

(Cuicatecan basin). However, this hypothesis has

never been convincingly demonstrated (e.g.

Delgado-Argote et al., 1992) and is not con®rmed

by published paleo-plate reconstructions for this

period (Pindell, 1994; Meschede and Frisch, 1998).

According to recent ®eld works, between Oaxaca

and CuicatlaÂn there is neither evidence of an oceanic

basin nor has a volcanic arc of Late Jurassic±Early

Cretaceous age been evidenced but rather an intracon-

tinental basin with terrigenous and carbonate sedi-

mentation which covered unconformably both the

Oaxaca and the JuaÂrez terrane (Ferrari and Solari,

2000).

The southernmost slice of southern Mexico is occu-

pied by the Xolapa terrane (Fig. 1). Several works

have shown that the continental margin of southern

Mexico has been truncated and uplifted during mid- to

late Tertiary times. Particularly, geochronologic,

paleotectonic and geobarometric studies demon-

strated that the Xolapa terrane represents portions of

lower continental crust exhumed and deformed by

left-lateral shear since the Oligocene as a result of

the eastward migration of the North America±

Cocos±Chortis triple junction (Herrmann et al.,

1994; Schaaf et al., 1995; MoraÂn-Zenteno et al.,

1996; Tolson, 1998). The boundary of the Xolapa

terrane with the Mixteca and Oaxaca terranes is a

dominantly left-lateral mylonitic zone, named the

A. Jording, et al. / Tectonophysics 327 (2000) 1±13 3

Tierra Colorada, Juchatengo, and Chacalapa fault

zone (Ortega-GutieÂrrez et al., 1990; Ratschbacher et

al., 1991; Meschede et al., 1996).

Seismic refraction data and gravimetric studies

indicate that the crustal thickness reaches a maxi-

mum of ,45 km beneath the cratonic block formed

by the Oaxaca and Mixteca terranes (Valdez et al.,

1986; GEOLIMEX Working Group, 1994; Urrutia-

Fucugauchi and Flores-RuõÂz, 1996) and in the

western part of the Sierra Madre terrane (Steinhart

and Meyer, 1961). It decreases by over 10 km in the

central part of the Guerrero terrane (Zihuatanejo

sub-terrane) and in the southern JuaÂrez terrane

(Ortega-GutieÂrrez et al., 1990; GEOLIMEX Work-

ing Group, 1994; Urrutia-Fucugauchi and Flores-

RuõÂz, 1996; Garcia-Perez and Urrutia-Fucugauchi,

1997). Based on these data, it can be inferred that

the western part of the Guerrero terrane as well as

the JuaÂrez terrane are constituted by a thinned conti-

nental crust.

3. Magnetotelluric measurements and transferfunctions

Our MT measurements were carried out along two

pro®les A-A 0 and B-B 0 (Fig. 1). On line A-A 0, which

is part of the multidisciplinary project of the GEOLI-

MEX transect (GEOLIMEX Working Group, 1994),

the MT sites had a separation of 10±12 km and on line

B-B 0 of 10±15 km. The period range covers 0.25±

4096 s and the recording time was 2±3 days to ensure

suf®cient data quality at long periods. The MT study

was, primarily, aimed at the detection of conductivity

signatures related to the subducting Cocos plate

together with a general comprehension of the conduc-

tivity distribution of the deep subsurface. The ®rst

results were presented by Arzate et al. (1995) and

Pareja-LoÂpez et al. (2000), but here we discuss our

observation that the various terranes and their bound-

aries, as crossed by the two pro®les (Fig. 1), are distin-

guishable by means of basic MT transfer functions,

which describe amplitude and phase relations between

magnetic and electric ®elds. These transfer functions

were calculated using Fourier transforms of time

variations of the horizontal electric ®eld E ���Ex;Ey� (in (mV/km)) and the horizontal magnetic

induction B � �Bx;By� (in (nT)) through the linear

bivariate equations:

Ex � ZxxBx 1 ZxyBy

Ey � ZyxBx 1 ZyyBy

with elements of Z denoting the transfer tensor which

for different periods, depending on the penetration

depth of the electromagnetic ®elds (ªskin depthª),

characterizes the change of conductivity versus

depth at a speci®c site. Estimation of Z has been

performed using well-known multiple regression

techniques (e.g. Sims et al., 1971), and can be written

as, e.g.:

Zxy � �BxBpx��ExEp

x �2 �ExBpx��BxEp

x ��BxBp

x��ByEpx �2 �ByBp

x��BxEpx �

where �BxBpx� denotes Gaussian summation.

In case of low energy in the magnetic channels or of

correlated arti®cial noise in the time series the data

quality of the tensor elements had to be improved.

Data selection after careful visual inspection of time

series was the most powerful tool for data improve-

ment but multiple coherency based rejection or

weighting of spectra (Jenkins and Watts, 1968) were

also applied. As the last step, the more illustrative

apparent resistivity and phase curves (ªsounding

curvesº) were derived from off-diagonal tensor

elements at logarithmically equidistant periods T

using the expressions:

raij�T� � 0:2T uZiju2; ij � xy; yx

wij�T� � arctanIm Zij

Re Zij

!

4. Rotation to strike direction and grouping of data

For further analysis, the transfer tensors had to be

rotated to the electrical strike direction. Standard

dimensionality criteria (e.g. Swift, 1967; Bahr,

1988) suggested that two-dimensional conductivity

structure predominates so that rotation to the principal

axes will result in coordinate systems of decoupled

pairs of off-diagonal E- and B-components with the

orientation of the E-components parallel (E-polariza-

tion) or perpendicular (B-polarization) to strike. Due

A. Jording, et al. / Tectonophysics 327 (2000) 1±134

to the electrical boundary conditions, the E-polariza-

tion curves are continuous when crossing lateral

boundaries while B-polarization curves can change

discontinuously. Two different techniques to deter-

mine rotation angles have been examined, which are

Swift's (1967) and Bahr's (1988, 1991) methods.

Swift's angle aS which is simply de®ned by the

minimization of the squared sum of the diagonal

elements of the transfer tensor Z, mainly re¯ects

upper crustal structures. For most of our MT sites

aS turned out to be signi®cantly stable and almost

frequency independent. Problems will occur in the

presence of static shift distortion, caused by galvanic

effects due to charge accumulations at boundaries of

near surface heterogeneities (Bahr, 1988; Groom and

Bailey, 1989). In such cases aS will likewise be

distorted. To achieve rotation angles not being

affected by static shift distortion, Bahr's (1988)

phase based angle aB was used. In the majority of

our cases, some frequency dependency of aB over

the whole MT period range appeared to be a

characteristic complicating the adoption of a mean

or representative angle needed for two-dimensional

analysis. Generally, differences between aS and aB

are taken as an indication of the presence of static

shift distortions in the data set.

Rotation of all transfer functions to principal axes

following Swift's criterion readily revealed that the

sites may be grouped, mainly according to the shapes

of the B-polarization curves. These shapes suggest the

existence of 10 different electrical crustal units, 4 on

pro®le A-A 0 and 6 on pro®le B-B 0, indicating a char-

acteristic depth dependent conductivity distribution for

each. Corroborating this result, Bahr's angles used for

rotation led to the same grouping. Thus, the grouping

appears to be strikingly robust and, in particular, largely

unaffected by static shift distortions. Fig. 2 gives an

example from pro®le A-A 0 of grouped stations consid-

ered to represent the ªSouth Mayaº electrical crustal

unit. The similarity of the shapes of the phase curves

as well as of the apparent resistivity curves is evident.

At sites CHIL and SEBA, the shift of the resistivity

curves to higher values in the B-polarization may be

interpreted as static distortion in this case (cf. Jones,

1987, 1988). Fig. 3 gives another example of grouped

stations for pro®le B-B 0 representing the electrical crus-

tal unit ªGuerreroº. The curves are more widely scat-

tered in their amplitudes but similarities are still quite

obvious. The electrical crustal units are named here

according to the tectonostratigraphic terranes.

In a compilation of all our data, curves of apparent

resistivity for E- and B-polarization along pro®les A-

A 0 and B-B 0 are presented in form of ªpseudosec-

tionsº, i.e., in form of isolines of apparent resistivity

in a diagram of period T versus pro®le length (Figs. 4

and 5). The various electrical crustal units resulting

from different curve types are marked. In addition,

apparent resistivity curves of representative sites

from each crustal unit are shown on top of the pseu-

dosections. Such pseudosections may easily be

converted into a simpli®ed apparent resistivity versus

depth section, however, we expressly want to restrict

ourselves to the most basic MT data presentation. As a

rule, short MT periods provide information about

upper crustal resistivities while long periods probe

the whole crust and the uppermost mantle in case of

moderate to high resistivities. In case of near surface

good conducting layers, like in the Veracruz sedimen-

tary basin, the penetration depth of the longest periods

is restricted to a few kilometers because of shielding

effects. Different types of apparent resistivity curves

re¯ect different conductivity distributions of larger

depths rather than small scale near surface anomalies.

The grouping of sites is quite clear in the B-polar-

ization pseudosection but becomes, in part, visible

also in the laterally more homogeneous E-polarization

curves. For this presentation, the niveau of apparent

resistivity curves has been smoothed in order to avoid

local unsteadiness of the isolines. This smoothing

means a mere shift correction of a resistivity curve

at some sites without changing its shape and, thus,

not affecting the grouping criteria.

5. Geological interpretation of results

Fig. 6 displays the boundaries of grouped data as

deduced from our MT measurements. In general, the

distribution of these electrical conductivity crustal

units matches well with geological terranes as de®ned

by Campa and Coney (1983), but the boundaries

appear to be shifted in many cases.

By comparing the mean individual Swift's angle of

each single MT site with the dominant trend of regio-

nal tectonic structures, correspondence to surface

geology becomes obvious: At many MT sites the

A. Jording, et al. / Tectonophysics 327 (2000) 1±13 5

orientation of the E-polarization component parallels

near surface geological structures or major tectonic

elements. In particular, MT strike directions are

related to the azimuths of the geological terrane

boundaries. On the whole, the directions of the E-

polarization components may coincide with the strike

of the subducting plate. We have further tried to corre-

late geologic structure and apparent resistivities along

the pro®les.

Pro®le A-A 0 crosses the Xolapa (XO), Oaxaca (O),

JuaÂrez (J) and Maya (M) terranes as de®ned by Campa

and Coney (1983). Along this transect we observed

four groups of sounding curves (Figs. 4 and 6). The

ªXolapaº electrical crustal unit, which is almost iden-

tical with the Xolapa terrane, is characterized by a

high apparent resistivity in the B polarization (Fig.

4), and is separated from the ªOaxacaº crustal unit

by a wide transition zone observed between stations

CERR and AGUA (Fig. 6). A transition zone is under-

stood as a region where one or two sounding curves do

not ®t with the preceding or following set of sites.

Here it corresponds to the Juchatengo mylonitic

zone (Meschede et al., 1996) and the Honduras bath-

olith (Grajales-Nishimura, 1988). The existence of the

transition zone suggests that the Juchatengo mylonitic

zone, which bounds the Xolapa terrane to the north is

not vertical, but, rather, dips at a relatively low angle

to the north also at depth. On the other hand, the

Honduras batholith constitutes a crustal unit with

electric properties different from the Xolapa and

A. Jording, et al. / Tectonophysics 327 (2000) 1±136

Fig. 2. Example of grouping data after Swift's rotation criterion for pro®le A-A 0 (South Maya electrical unit): sounding curves for E- and

B-polarization. See Fig. 1 for location of sites.

Oaxaca terranes, and could also be responsible for the

observed transition zone.

Although slightly disturbed and showing larger

than normal scatter of rotation angles, MT soundings

obtained at stations within most of the Oaxaca and

JuaÂrez terrane can be grouped together. The sites

within these terranes are characterized by high appar-

ent resistivities at short periods (Fig. 4) which could

correlate, at least in part, with the presence of the

Precambrian Oaxacan complex in the upper crust.

High resisitivity is in accordance with the dry granu-

lite facies of this rock assemblage. Our data suggest

that no real crustal boundary exists between these two

terranes along the pro®le. The wide mylonitic zone of

the Sierra de JuaÂrez (Alaniz-Alvarez et al., 1994,

1996) thought to be the boundary between the two

terranes, ends at the latitude of Oaxaca and is not

crossed by our pro®le (Fig. 6). The pre-Mesozoic

basement of the JuaÂrez terrane along this part of the

pro®le is poorly known (Sedlock et al., 1993) but

metamorphic rocks of possible Paleozoic ages are

reported at a number of locations (Consejo de Recur-

sos Minerales, 1998; Martinez-Reyes, 2000). With

respect to the relative similarity of MT soundings

described above, it may not be excluded that the

Oaxaca and the JuaÂrez terranes could share a similar

pre-Mesozoic basement at depth.

A clear change in MT curves is observed some tens

A. Jording, et al. / Tectonophysics 327 (2000) 1±13 7

Fig. 3. Example of grouping data after Swift's rotation criterion for pro®le B-B 0 (Guerrero electrical unit): sounding curves for E- and

B-polarization. See Fig. 1 for location of sites.

of kilometers to the south of the proposed boundary

between JuaÂrez and Maya terranes (sites MACU and

ESPE). In the surface geology the boundary between

these two terranes is the low angle southwest-dipping

Vista Hermosa fault system, where the Mesozoic sedi-

mentary cover of the JuaÂrez terrane is thrust over the

lower crustal Paleozoic metasediments of the Maya

terrane (Ortega-GutieÂrrez et al., 1990). In the pseudo-

section of Fig. 4 the mid- to lower crustal assemblage

of the southern part of the Maya terrane (South Maya

electrical crustal unit) is seen as a homogeneous block

with a high apparent resistivity in the B polarization.

The shifting of the conductivity boundary to the south

with respect to the surface geology is consistent with

the southwest dip of the Vista Hermosa fault system

shown by Ortega-GutieÂrrez et al. (1990). The obser-

vation of the pseudosections of Fig. 4 also indicates

that this fault system extends to great depths. This

con®rms the results of the GEOLIMEX experiment,

in which the Vista Hermosa fault appears to be the

sole crustal structure clearly observed in the refraction

seismic sections (Spranger, 1994).

Another sharp electrical boundary is observed in

the northernmost part of the pro®le near the city of

Tuxtepec, between sites SEBA and OTAT. Due to the

young sedimentary cover of the Gulf of Mexico

A. Jording, et al. / Tectonophysics 327 (2000) 1±138

Fig. 4. Pro®le A-A 0: Sounding curves (E-, B-polarization) of representative sites (top); isolines of apparent resistivities for B-polarization

(ªpseudosectionº) (middle); isolines of apparent resistivities for E-polarization (bottom).

coastal plain, no surface geological structures are

revealed at this location. The boundary might corre-

spond to the abrupt increase of the thickness of

Tertiary sediments related to the Gulf of Mexico

extensional basin which is characterized by a signi®-

cant decrease in crustal thickness and an increase in

heat ¯ow. Because of this electrical boundary we

divide the Maya terrane into a more resistive southern

and a conductive northern part (South Maya and

ªNorth Mayaº).

Pro®le B-B 0 crosses the Xolapa (XO), Guerrero

(G), Mixteca (MI), Sierra Madre (SM), and Coahuila

(COA) terranes as well as the TMVB. Our results

show that a separate group of MT curves exists in

the southernmost part of the pro®le (sites ACAP-

TIER), followed to the north by a transition zone

(sites CAJE, ACAH) which matches with the border

of the Xolapa terrane as introduced by Campa and

Coney (1983). This transition zone coincides broadly

with the Oligocene Tierra Colorada pluton, which is

bounded to the south by a north dipping mylonitic

zone (Riller et al., 1992; Meschede et al., 1996). Like-

wise to pro®le A-A 0 the transition may originate from

the northward inclination of the mylonitic zone and/or

by the irregular shape and crosscutting of the intrusive

bodies, which make the crust heterogeneous from the

electrical point of view.

A boundary between the Guerrero and the Mixteca

A. Jording, et al. / Tectonophysics 327 (2000) 1±13 9

Fig. 5. Pro®le B-B 0: Sounding curves (E-, B-polarization) of representative sites (top); isolines of apparent resistivities for B-polarization

(ªpseudosectionº) (middle); isolines of apparent resistivities for E-polarization (bottom).

A. Jording, et al. / Tectonophysics 327 (2000) 1±1310

Fig. 6. Comparison between MT electrical crustal unit boundaries, terrane boundaries of Campa and Coney (1983) and main geologic structures

presently thought to represent terrane boundaries, mainly based on Riller et al. (1992), Suter et al. (1997) and Ortega-GutieÂrrez et al. (1992).

JMZ� Juchatengo mylonitic zone, VHFS�Vista Hermosa fault system, SJMC� Sierra de JuaÂrez mylonitic complex; LCT� Lobo-Cienega

thrust and IHF� Ixtlapala-Huiznopala fault.

terranes is not clearly detected along our pro®le. As

summarized in Sedlock et al. (1993) its exact location

is uncertain. Furthermore, Lang et al. (1996) argue

that no terrane boundary should exist in this area.

We note that a zone of high apparent resistivity

north of station PLAO could roughly correspond to

the Permian to early Triassic granitic rocks overlain

by the pre-Jurassic Taxco schists (Elias-Herrera and

Sanchez-Zavala, 1990). Therefore, the weak bound-

ary seen in short periods between sites PLAO and

BUEN could be associated with a lateral crustal varia-

tion inside the Guerrero terrane.

The electrical boundaries between stations ANDR

and ZAPA and stations ATOT and MONI match with

the boundaries of the TMVB, characterized by recent

volcanism and tectonics. Laboratory experiments and

numerical models indicates that less than a 5% of

partial melting is suf®cient to induce a drastic increase

in conductivity if the melt fraction is interconnected

(Partzsch et al., 2000). In our cases MT soundings

likely map the extent of an anomalous region where

a thermal anomaly is associated with the volcanism

(Fig. 5). Here, conductivity due to melts or

hydrothermal ¯uids in the crust is enhanced by the

active and recent extensional faulting which affect

the upper crust (e.g. Suter et al., 1995).

The MONI-METZ transition zone corresponds

broadly to the extent of the Valles-San Luis PotosõÂ

carbonate platform, a tectonic nappe bounded to the

east by a basal detachment thrust (Suter et al., 1997).

The last MT boundary along the pro®le is observed

between IXTL and TLAN, close to the Sierra Madre-

Coahuila terrane boundary. This boundary may actu-

ally re¯ect the topography of the Precambrian base-

ment at depth, covered by conductive young

sediments. Proterozoic gneisses of the Huiznopala

Formation crop out at ,500 m just a few kilometers

south±west of site TLAN (Ochoa-Camarillo, 1997;

Lawlor et al., 1999) and disappear toward the north±

east due to the Ixtlapala-Huiznopala fault (Fig. 6),

which has a minimum displacement of 1600 m

(Ochoa-Camarillo, 1997). Further to the east, in fact,

rocks correlative with the Huiznopala gneisses were

cored at a depth of 2600 m in the coastal plain of the

Gulf of Mexico (Suter, 1990). These normal faults

developed in Jurassic times during the opening of the

Gulf of Mexico, and are part of the wide extensional

fault system which affect this passive margin.

6. Summary and conclusions

To some extent it is commonplace for terranes to

be intended as exotic crustal blocks separated from a

continental nucleus by sub-vertical sutures. Although

common, this is neither true for all terranes nor impli-

cit in their de®nition. Some terranes, indeed, are just

rootless nappes bounded by sub-horizontal fault

zones or complex volcano-sedimentary assemblages

separated by low angle thrust faults (e.g. Coney,

1989). Since long period MT data normally probe

the whole crust down to the uppermost mantle, the

observed changes in the shapes of the sounding

curves re¯ect lateral variations in a large depth

range rather than in the surface geology. In this

respect, our results illuminate electrical boundaries

which seem to cut through large parts of the crust

separating various electrical units, each with largely

homogeneous conductivity structures. In most cases,

these boundaries coincide with geological terrane

boundaries. Representative examples are the contact

between the lower crust assemblage of the Xolapa

terrane with the Mixteca and Oaxaca terranes

along the Tierra Colorada-Juchatengo mylonitic

zone, the contact of the mid to lower crust meta-

sediments of the Maya terrane with the sedimen-

tary cover of the JuaÂrez terrane along the Vista

Hermosa fault, and the lowering of the Precam-

brian basement of the Sierra Madre Oriental along

the Ixtlapala-Huiznopala fault (Figs. 4 and 5). In

all these cases, the resistivity contrast between

different crustal levels and the geometry of the

boundary may produce the observed variation in

the MT curves.

The main implications for terrane geology in

southern Mexico that can be deduced from the

analysis of the MT pro®les presented in this

study are the following:

(1) The Xolapa terrane is bounded to the north by a

transition zone which corresponds to a north dipping

mylonitic zone crossing large parts of the crust and

which is complicated by Tertiary intrusions.

(2) No boundary is observed between the Oaxaca

and JuaÂrez terranes, suggesting that they could share a

similar basement in the area crossed by our pro®le.

(3) The Vista Hermosa fault zone is a major crustal

structure separating the Oaxaca/JuaÂrez terrane and the

Maya terrane.

A. Jording, et al. / Tectonophysics 327 (2000) 1±13 11

(4) There are no clear indications of a boundary

between the Mixteca and the Guerrero terranes

along our pro®le.

We conclude that basic MT data without any

elaborate model calculation may be used to map

different crustal units and may help to de®ne terranes

and their boundaries in conjunction with geological

observations.

Acknowledgements

This work was supported by the German Research

Foundation (DFG) under grant number Jo 188/1

which is gratefully acknowledged. We thank F.

Ortega-Gutierrez, S. Alaniz-Alvarez, A. Nieto-Sama-

niego and D. MoraÂn-Zenteno for the useful discus-

sions on the geology of southern Mexico and two

anonymous reviewers who helped to improve the

clarity of the original manuscript.

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