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Produced by the NASA Center for Aerospace Information (CASI)
,

This report was prepared for the Jet Propulsion Laboratory,

California Institute of Technology, sponsored by the

National Aeronautics and Space Administration.

SIR-A Imagery in Geologic Studies of the

Sierra Madre Oriental, Northeastern Mexico.

Part 1 (Regional Stratigraphy):

The Use of Morphostratigraphic Units in Remote Sensing Mapping

(NASA-CR-X 175457) SIR-A IFIGFFY IV GEOICGIC

STUDIES OF THE SIxRRA MALFE CEIENTAL,

NORTHEASTERN MEXICO. PART 1 ( RIG TONAL

STRATIGHAPHY): IHB USE OF

MCRPHOSTRATIGRAFHIC UNITS IN FFMOTE SENSING

V

Jose F. Longoria and Oscar H. Jimenez

Programs in Geoscier;ces

The University of Texas at Dallas

Box 688

Richardson, Texas 75080

:r.

•^ ^. 1

t

N65-19497

Unclas

G3/43 14304

ABSTRACT

SIR-A imaging was used in geological studies of sedimentary terrains in

the Sierra Madre Oriental, northeastern Mexico. Geological features such as

regional strike and dip, bedding, folding and faulting were readily detected

on the image. The recognition of morphostructural units in the imagery,

coupled with field verification, enab'led geological mapping of the region at

the scale of 1:250 000. Structural profiling lead to the elaboration of a

morphostructural map allowing the recognition of en echelon folds and +^ld

trends which were used to postulate the tectonic setting of the region.

INTRODUCTION

Spaceborne radar imagery is an important remote sensing technique for

regional geological studies. Recent studios using imagery from SIR-A

(Elachi et al., 1982) have centered in thr detection of gross geological

features such as the detection bCtwe.en metamorphic and sedimentary terrains

s— —_ 
(e.g. Sabins, 1983).

The objective of the present study was to test the SIR-A imagery in

geologic interpretations of sedimentary terranes such as the Sierra Madre

Oriental of northeastern Mexico. This area was choosen because it has: 1)

strong relief contrast, 2) dense vegetation, 3) complex geologic setting, 4)

lithologic homogeneity, i.e. it is formed of sedimentary rocks displaying

enormous thickness, and 5) importance in understanding the geologic, history

of the Gulf of Mexico and Caribbean region.

Location of the Studied Area

The study area lies approximately between parallels 24 bO'and 25 30',

and meridians 99 30'and 101 00' covering the southeast corner of the State

of Coahuila and the south-central part of the State of Nuevo Leon, in

northeastern Mexico. Part of this area was covered by swath 22 of the SIR-A 

imagery (Figure 1). Physiographically, it is located in the Transverse

Ranges of the Sierra Madre Oriental Province. and corresponds to the place

where the Sierra Madre Oriental abruptly bends towards the west. It lies in

the Victoria Segment of the Sierra Madre Oriental geologic province of

northeastern Mexico. The paleogeographic reconstruction of northeastern

Mexico has been a difficult task. Early workers (Humphrey, 1956) believed

that the present day physiographic provinces correspond to paleogeographic

elements. Accordingly, the Sierra Madre Oriental would be related to the

Mesozoic Mexican Geosyncline, and the Coahuila Folded Belt to the Jurassic

Sabinas Basin. As pointed out by Weide (1968) and Longoria (1984) the

existence of those paleogeographic elements throughout the Mesozoic needs to

be documented.

GEOLOGIC INTERPRETATION OF THE SIR-A IMAGES OF NE MEXICO

Geomorphologic Description of the Image
The geological interpretation of the SIR-A images from northeast Mexico

was based on the visual analysis of morphological, stratigraphic, and

tectonic features. Due to the particular geological setting of the region

under study, mainly formed of homogenous piles of sedimentary rocks, special

emphasis was given to the recognition of stratigraphic units which could be

mapped at the image scale. The direct relationship betwen textural

differences and topographic expression of lithologies was used as the

fundamental basis for the diagnosis of morphostratigraphic units. The

concept of image texture introduced by Ford (1980, 1982) is herein followed.

The region studied is characterized by elongated subparallel, narrow

ridges which are separated by narrow, dip valleys in the northwest and wide,

shallow valleys in the southeast. The ridges correspond to both anticlinal

and synclinal structures, varying from fan-shaped to recumbent. The

contrast of the image is high and mainly enhances topographic features;

however, stratification and structural trends are also highlighted. The

brightest points corresponding to the steep slopes of eroded asymmetric

anticlines whose limbs, in the form of alligned ridges, faced the incident

radar beam. The brightness observed in the southwest corner of the area is

due, in part, to the scattered gypsiferous outcrops (grids H3, H4, I5) and

clearly to cultural features such as towns, electric plants and high voltage

cables. The dark areas on the image correspond mainly to the slopes that

face away from the incident radar beam. Some of the darkest areas

correspond to valleys with thick alluvial cover (grids J1,K1,L1,M1,K3,L3,M3

and M4), which probably are associated to ground water reservoirs. Where

the alluvium sediments are thin, probably few meters, it is possible to

follow the bedding below it.

Most of the region under consideration is densely vegetated with the
exception of the SE part, whereby arid conditions predominte. The ridges

and mountainous areas are covered by coniferous and mixed deciduous forests.

The wide valleys are cultivated or otherwise densely covered with grassy

vegetation. Although tonal variations such as in grids G3, G4, H3, and H4

are due to the presence of vegetation, the underlying rock terrain is still

well displayed in the image.

In general the ridges have slopes varying between 25 and 40 though in 

some steep areas, slopes can range from 50 to 70 degrees. Slopes with more

than 35 degrees are present over more than 65 percent of the area. The

valleys have gently undulating floors but in several places they form deep

narrow intermontane valleys. The drainage pattern is not obvious on the

image. An intricate network of streams,' however, can be observed on the

slopes of the high ridges. Smaller streams and meandering rivers are seen

in the deep valleys. All of them are tributaries to the trellis drainage

system developed in a series of arcuate, roughly parallel anticlines and

synclines, resulting in a characteristic denudation. Where erosion is

well-advanced breached anticlines are developed.

Geologic Features Observed on the SIR-A Image

The examination of the SIR-A images was focused on the mapping of features

such as: a) Bed attitude, b) Lithologic type, c) Faults, d) Folds, e)

Morphostructural trends, and f) Structural profiling.

a) Bed Attitude.- Regional strike and dip of bedding planes were hard

to obtain from visual examination of the image. It seems like the

observation of bed attitude greatly depends upon the incidence of the beam

angle at which the image was taken. Furthermore, the geologic complexity of

the region under study makes it even harder to obtain regional strike and

dip directions. However, morphological trends were used to infer the
regional attitude of strata. When the illumination is perpendicular to

bedding trends, as in grids I2, I3 excellent attitudes were obtained. On

the other hand, the lack of well-defined bedding attitudes such as in grids

B4 and B5 helped to interpret structural deformations such as twisted and

doubly plunging structures. These structures were later confirmed during

field verification.

b) Lithologic Ty pe.- Visual analysis of the SIR-A images permits the

recognition of Morphostratigraphic Units herein defined as lithological

packages which mantain their textural expresion in the images throughout the

region. Their visual identification on the images is vital in that they are

the remote sensing mapable units comparable with lithostratigraphic units

(formations) recognized on detailed aereal photographs or detailed ground

mapping. The Morphostratigraphic Units are herein regrarded as the main

basis for remote sensing geology in sedimentary terranes. Their recognition

in this image resulted in the elaboration of a remote sensing geologic map

of the region at the scale of 1:250 000 (Figure 5). During the present

study three morphostratigraphic units were mapped throughout the area

(Figure 2).

c) Faults.- High angle reverse, strike-slip and thurst faults were

recognized in the region (Figure 3). The geometric arrangement of the

morphostratigraphic units as well as their morphostructural relationship

were used to distinguish the three kind of faults on the image. Thrust

faults occur in the eastern part of the image and were the more difficult to

recognize. Their morphology is characterized by narrow ridges separated by

close, V-shaped, deep valleys. Reverse faults are associated to strike-slip

faulting and were detected on the image because of their abrupt topograhic

break in relation to the surrounding ranges. The down thrown block forms
wide open valleys. Strike-slip faults are readily identified on the image

by the horizontal disruption of the morphostructural ranges involved in the

faulting.

b) Folds.- The majority of the ranges in the area correspond to folds

giving an excellent opportunity to study folding in sedimentary terranes.

These folds include wide open, symmetric, fan-shaped and recumbent types

(Figure 4). Geometry of folds and fold variations, including en echelon,

periclinal, plunging , and bifurcated folds were easily recognized using the

geometric arrangement of the morphostratigraphic units.

e) Morphostructural trends.- As stated above, the majority of the

individual ranges correspond to folds (Figures 3 and 4). The general trend

of mountain ranges define morphostructural zones, i.e., areas within the

region unified by the general morphologic expression of the structures.

Contiguous morphostructural zones can be separated from one another by their

fold trend and thier boundaries may be delineated by changes in fold trends

or they may be bounded by faults. The morphostructural zones recognized in

the area are shown in Figure 3.

f) Tectonic Profiling.- Scale and resolution of SIR-A imagery permit

generation of direct structural profiles. In the studied region the

profiling was fundamental in understanding the tectonics of northeastern

Mexico. Image profiling was compared with field profiling during field

verification (Figure 4).

Stratigraphy

Four morphostratigraphic units were established for the Mesozoic of the

region under-study (Figure 2). Their geographic distributiot^ was traced on

the image resulting in a remote sensing geologic map of the area (Figure 5). 

It is important to note that in some valleys the continuity of the bedding
was followed in the SIR-A image underneath thinner alluvial cover (grid D2).

Morphostratigraphic Unit 0.- The alluvium cover is widely exposed in

the region and was considered as a separate mappable unit. Unit 0 consists

of continental sediments derived from fluvial, lagoonal and probably playa

lake sedimentation during early Quaternary and Recent time.

Morphostratigraphic Unit 1.- Morphostratigraphic Unit 1 is recognized

on the image by its characteristic medium "granular" texture, high contrast

and low definition. Lithologically it is predominantly evaporitic,

consisting of an impure gypsum mass in the lower part and sandstone, sandy

conglomerate and shale in the upper part. The lower part of the unit is

characterized by an earthy appearance of whitish coloration, easily

erodable. The upper part is formed of a flyschoidal interbedding of

sandstone, black and varicolored shale and minor amounts of conglomeratic

sandstone. Locally the sandy conglomerate is more extended (grids H3,I3 and

H4) while in others places (grids A1,A2,B1,B2 and B3) the shale is the main

constituent. Unit 1 is the oldest in the study area and is assigned to the

Upper Jurassic , it is transitionally overlain by unit 2.

Morphostratigraphic unit 1 is widely exposed in Morphostructural Zone I and

is distributed in the topographic lows formed in the breached anticlines.

Morphostratigraphic Unit 2.- On the image, this unit displays: 1) high

contrast, 2) fine to coarse granularity , and 3) high definition. It is

formed of a homogenous pile of carbonate rocks, varying from this. to

thick-bedded limestone, locally black chert nodules and layers are abundant

alternating with thin-bedded dark shale. Unit 2 generally forms elongated

ridges with steep slopes displaying well-developed bedding planes. Unit 2

is observed in the whole region of study. It ranges from Upper Jurassic to

Upper cretaceous and is the predominant lithology observed in
morphostructural zones III and IV, where it torms elongated, semiparallel

anticlinal ridges corresponding to topographic highs. Although there is a

pitching of the structures that form the ridges, producing depressions and

culminations, the ridge-forming rocks in Zone IV (I1, I2, I3, H2, H3, and

H4) are formed by the lower part of Unit 2. The E-W distribution of unit 2

reflects a regional tilting and bending of the ridge forming structures.

Consequently, the youngest ridge-forming rocks in Zone III, and neighboring

areas of Zone IV, belong to the upper part of the unit 2, exposing rocks in

the western parts of Zone IV that correspond to the lower part of the same

unit.

Morphostratigraphic Unit 3.- Unit 3 is formed of a rhythmic alternation

of thin to medium-bedded, varicolored shale and limestone with areraceous

layers. It is a predominantly terrigenous unit easily erodable which was

deformed contemporaneously with the previous units. It crops out on the

flanks of the ridge-forming limestone of unit 2, and in the middle of the

synclinal valleys where it has not been covered by alluvium or else by

stream denudation. It was identified in zones III and IV. In general, Unit

3 displays low to medium contrast, low definition and fine granularity.

Morphostructure

Five morphostructural zones were defined on the image (Figure 3).

Their characterization was based on: 1) Topographic expression of

individual ranges, 2) Fold trends, 3) Fold distribution, and 3)

Morphostratigraphic character of the lithologies.

Morphostructural Zone I.- This Zone is located in the southern part of

the image (Figure 3). It contains poorly defined synclines and anticlines

surrounded by low, wide valleys composed of evaporites. The Cerro del

Potosi is the more prominent structure in this zone, with more than 1000 m
of relief (Figure 4); however, it is important to notice that on the SIR-A

image this structure lacks topographic expression.

Morphostrucuural Zone II.- This Zone covers the northeastern part of

the image (Figure 3). Several SSE-NNW trending reverse faults and east

verging thrusts are found in this zone. (profile K-K', Figure 4). Bedding

planes are well defined on the slopes of the ranges (see grids I2). The

Cerro Rayon is the more prominent structure in this zone. In general

structures in this zone display an imbricated arrangement.

Morphostructural Zone III.- Zone III covers the central part of the

image (Figure 3). The structures in this zone have an ESE-WNW general

trend. They are recumbent anticlines and synclines most of which show a

characteristic twisting becoming more pronounced towards the eastern end of

the structures (see profiles G-G', F-F', and E-E', Figure 4). Sierra La

Marta, Sierra La Ventana and Sierra Los Pilares are typical examples of the

structures from Zone III.

Morphostructural Zone IV.- This zone occupies the western half of the

image. The strucuures observed have an E-W general trend, framing

subparallel, en echelon patterns (profiles A-A', C-C', D-D', Figure 4).

They vary from recumbent, twisted anticlies such as Sierra de Tapanguillo,

Sierra E1 Arenal, Sierra Chapultepec, Sierra La Esmeralda, and Sierra E1

Pinal Alto to doubly plunging.anticlines with bifutrcated ends such as

p

Sierra San Lucas, Sierra La Nieve, Sierra La Marta, Sierra San Antonio, and

Sierra Zapaliname,

Morphostructural Zone V.- This Zone is located in the southern and

western portions of the image (Figure 3). Zone V includes areas with

alluvium cover represented by wide, low and undulating valley floors. The

darkest parts of this zone seem to correspond to places where the alluvial
cover is the thicker. The high contrast, coarse granularity, high

definition and finelly mottled appearance is also persistent in the imagery

response.

CONCLUSIONS

This study shows that spaceborne radar imagery can be used for geologic

mapping of complex sedimentary terrains whereby abrupt topography and dense

vegetation impede making direct geologic observations. As in any other

remote sensing technique, visual analysis of the SIR-A imagery should be

followed by field verification. Examination of the SIR-A images from the

Sierra Madre Oriental lead to postulate the following: 1) the continuity of

ridges can be traced under a thin alluvial cover. 2) Bedding can be

detected in the slopes of the elongated ridges where the incident radar beam

is close to perpendicuclar. 3) the stratification combined with the

topographic expression of the flanks of the ridges and hills can be used to

define morphostratigraphic units, i.e., packages of rocks that can be

distinguished from other units above and below by means of their visual

textural expression. The definition of these morphostratigraphic units

permits geological mapping. The use of morphostratigraphic units instead of

formations in remote sensing geologic mapping avoids the complexities of

lithostratigraphic nomenclature which in this region has hindered the

paleogeographic and tectonic interpretations. 4) Major morphological and

structural elements are•indentified allowing the recognition of

morphostructural zones, i. e., zones within the studied area whose general

morphologic expression is the result; of common structural behavior. Five

zone were thus defined. 5) Structural profiles can be generated from the

images by interpreting the geometric arrangement of the morphostratigraphic

units. Several previously unrecognized tectonic patterns in the region
including en echellon folding and strike-slip and associated reverse faults

were so detected. We interpret the existence of these structures as the

result of transpression associated to active sea floor spreading in the Gulf

of Mexico in the late Jurassic. Consequently„ transpressional tectonics is

invoked to explain the observed structural arrangement in NE Mexico.

Circumstantial evidence shows that the required subsurface (basement)

transcurrent faults are likely to exist in the subsurface of this region.

This tectonic model is discussed in a separate paper.

REFERENCES CITED

Elachi, C., Brur!n, W,. F,; Cimino, J. B.; Dixon, T.; Evans, D. L.; Ford, J.

P.; Saunders, R. S.; Breed, C.; Masursky, H.; McCauley, J. F.;

Schaber, G.; Dellwing, L.; England, A.; MacDonald, H.; Martin-Kaye,

P. and Sabins, F., 1982, Shuttle Imaging Radar Experiment: Science,

v. 218, P. 3, p.996-1003.

Ford, J. P., 19@0, Seasat orbital radar imagery for geologic mapping:

Tennessee-Kentucky-Virginia: American Association of Petroleum

Geologists Bulletin, v. 64, no. 12, p. 2064-2094.

Ford, J. P., 1982, Analysis of the SIR-A and SEASAT SAR images of the

Kentucky-Virginia. In Cimino, J. A. and Elachi, C. D., eds.,

Shuttle Imaging Radar-A (SIR-A) experiment: National Aeronautics and

Space Administration, Jet Propulsion Laboratory, JPL Publication 82-77,

p. 4-10/4-18.

Humphrey, W. E., 1956, Tectonic framework of northeast Mexico: Gulf Coast

Association of Geological Societies Transactions, v. 6, p. 25-35.

Longoria, J.- F. , 1984, Stratigraphic studies in the Jurassic of

northeastern Mexico: Evidence for the origin of the Sabinas Gulf. In

Ventress, P. S., Bebout, D. G., Perkins, B. F. and Moore, C. H.
(eds.) The Jurassic of the Gulf Rim. Gulf Coast Section Society of

Economic Paleontologists and and Mineralogists, Foundation Third annual

Research Conference, march, 1984, p. 171-193.

Sabins, F. F., 1983, Geologic interpretation. of the Space Shuttle Radar

Images of Indonecia: American Association of Petroleum Geologists

Bulletin, v. 67, n. 11, p. 2076-2099.

Weidie, A. E., 1968, The myth of the Mesozoic Mexican Geosyncline [Abs.]:

Geological Society of American, Abstract with Programs, p. 314.

ACKNOWLEDGMENTS

The present investigation was carried out at the University of Texas at

Dallas (U.T.D. Contribution 458). The research for this report was founded

by the Jet Propulsion Laboratory (JPL Contract 956430). We express our

gatitude to the Programs in Geosciences, U.T.D., for the facilities made

available to us for the preparation of this research. Our appreciation is

here expressed to Tim Dixon (J. P. L.) for critically reviewing the

manuscript. Special thanks are due to Mr. Manuel Valenzuela (U.T.D.) for

his help in drafting the figures.
FIGURE CAPTIONS

Figure 1. Index map of Mexico showing the location of the si.uay area

and the track of SIR-A swath 22 (parallel lines).

Figures 2. Diagnostic features of the morphostratlgraphic units

recongized.

Figure 3. Structural elements and morphostructural interpretation of

the SIR-A image of the Sierra Madre Oriental, northeast Mexico.

Figure 4. Structural profiles of the Sierra Madre Oriental as

interpreted from the morphostrati graphic units recognized on the image (see

Figure 3 for location of profiles).

Figure 5. Remote sensing.'(SIR-A) geologic map of part of the Sierra

Madre Oriental, northeastern Mexico.
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SIR-A in Geologic Studies of the

Sicrra Madre Oric.,tal, Northeastern Mexico.

Part 2, Tectonic Transpression:

An Alternative Model

Jose F. Longoria and Oscar H. Jimenez

Programs in Geosciences

Box 688

Richardson, Texas 75080

ABSTRACT

The visual analysis of the Shuttle Imaging Radar (SIR-A) of northeast

Mexico revealed the existence of several geological features including: 1)

a well-developed pattern of en echelon folds, 2) juxtaposition of

tectonostratigraphic domains, 3) fold structures varying from fan-shaped,

asymmetric to recumbent doubly plunging anticlines, 4) anticlinal-synclinal

trends associated to regional plunging and tilting of the structures, and 5)

strike-slip faults oblique to the fold trends. These structures are

interpreted as the result of transpressional stress related to a complex,

anastomosed wrench-faulting system in the basement reactivated several times

since the Late Jurassic. This transpressional orogenic belt in northeastern

Mexico is linked to the spreading phase in the Gulf of Mexico.

INTRODUCTION

In spite of the fact that the first tectonic interpretations of
northeast Mexico date back to the begining of this century and also that

excellent field studies have been done since the thirties, no attempts have

been done to conciliate the diffrence in geologic interpretations and relate

the geology of this region to the dynamic processes taking place in the Gulf

of Mexico during the Mesozoic.

Recent geologic studies in northeastern Mexico using Space Shuttle

Columbia Imaging Radar (SIR-A Experiment) permited a re-evaluation of the

tectonic significance of the structures observed in the Saltillo-Galeana

Folded Belt. Visual analysis of the SIR-A images was combined with field

verification. The extension of the observed geologic features outside the

SIR-A image was accomplished using LANDSAT images. The SIR-A images studied

were covered by Swath 22 (Figure 1). The geologic interpretation of these

images is presented in a separate paper (Longoria and Jimenez, 1984). Our

purpose in this paper is to present a transpressive tectonic model as a

mechanism to account for the deformation observed in northeastern Mexico.

TECTONIC SETTING OF NE MEXICO

The area under study lies in the Victoria Segment of the Sierra Madre

Oriental geologic province of northeastern Mexico (Figure 1). Three general

tectonic styles have been postulated to explain the tectonic evolution of

northeastern Mexico: 1) simple "compresional stresses" with vertical

movements of the basement of unknown mechanism (Bose, 1923; Humphrey,

1956); 2) sliding of a detached cover above a decollement zone by a gravity

induced mechanism (Avenius, 1982); and 3) large-scale thrusting (Tady et

al., 1975) as consequence of an "alpine-type tectonics" in the region.

Several combinations of the three main styles are identified: a)

compressional stress and salt movement (Wall et al., 1961), b) compressional

stress and decollement (Cserna,1956), c) compressional stress on a
pre-Mesozoic transformally displaced basement (Alfonso, 1976), d) thrusting

and nappe-forming over transcurrent faults (Tardy, 1980), and e) decollement

associated to strike- slip movement (Padilla, 1982).

BASES FOR TECTONIC TRANSPRESSION IN NE MEXICO: A DISCUSSION

The identification of a transpressive system in northeastern Mexicu is

linked to transcurrent faults in the basement. This tectonic model will be

analyzed in light of four kinds of geological observations:

1. Theoretical Considerations.- The basis of wrench-tectonics was

extensively discussed by Moody and Hill (1956), providing thr; theoretical

grounds for understanding the deformation of a sedimentary pile above a

horizontally displaced basement. The concept of wrench tectonics has been

applied in different parts of the world at both regional and continental

scales (Woodckok and Robertson, 1982). Experimental models have also

demonstrated the possible mechanism of the wrench faulting (Closs,1955;

Wilcox et al., 1973). Recently; the concepts of tectonic transpression and

oblique-slip mobile zones have been introduced by Harland (1971) and

Ballance and Reading (1980), respectively. The reader is referred to those

papers for details on the theoretical principles of transpressive tectonics

and oblique-slip mobile zones and their realtionto wrench-tectonics.

2. The Saltillo-Galeana Folded Belt.- The Saltillo-Galeana Orogenic

Belt covers a surface of about 5,000 km2. It displays considerable

morphological and structural variations (Figure 2). The geology of this

area was interpreted from the SIR-A image couple with field verification.

The stratigraphic and structural aspects of the area were presented in a

separate paper (Longoria and Jimenez, 1984). Among the significant regional

structural features observed are the following (Figure 2): 1) the en

echelon folding, 2) the doubly plunging folds, displaying an ellipsoidal
patern, and 3) strike-slip faults. Local structures include: 1) the

rotations of the fold axes as displayed by the Sierra La Nieve, Sierra La

Esmeralda and Sierra Zapaliname, 2) bending of the structures such as in

Sierra E1 Cedral, Sierra E1 Pinal Alto and Sierra La Borrada, and 3)

branching of the fold structures such as In Sierra La Nieve and Sierra San

Antonio.

Following the observations by Campbell (1958), Tanner (1961), Wilcox et

al. (1973) and Odonne and Vialon (1983), the plunge culminations and

depretions, recumbent folds, and thrusting of the folds of the

Saltillo-Galeana Belt are interpreted as due to an advanced stage of the

development of horizontal movements in the basement. They also interpreted

"en echelon folding" as the result of deformation of a stratified cover

above a wrench-fault system in its basement. As noted by Odonne and Vialon

(1983) the geometry and orientation of the folds depends on the magnitud of

the wrenching.

Consequently, we postulate herein that the structures observed in

northeastern Mexico, typified by the Saltillo-Galeana Belt (figure 2), are

the result of deep, ancient transcurrent faults present in the subsurface

(basement) of the region, perhaps since the late Paleozoic but reactivated

during the Jurassic and Cretaceous. As discussed below our working

hypothesis conciliates the difference in tectonic styles, as well as the

diffences in fold vergences and stress directions much debated among

previous authors (Figure 5).

3. Regional pre-late Jurassic Fault Trends.- Major pre-Late Jurassic

lineaments described by several authors fron North America have been

compliled in Figure 3. Most of these correspond to Paleozoic transcurrent

faults. Unfortunately, very little is known on the few isolated Paleozoic
and Precambrian outcrops in northeastern Mexico, and much less is even known

on the dynamics of the basement in this region. Recently, Anderson and

Schmidt (1983) postulated the extension of the Mojave-Sonora Megashear

(left-l a teral transcurrent fault) into northeast Mexico. Although the

timing of the dislocation has been controversial, it is likely that a major

horizontal translation of Early Jurassic terranes in north-central Mexico

took place in the Middle Jurassic (Longoria, 1984a). This mid-Jurassic

transcurrence invoked by Longoria (1984a) in east-central Mexico is herein

considered as the southern boundary of the transpressive system in the

subsurface of northeastern Mexico. From paleomagentic studes by Gose et al.

(1982), it becomes evident that large portions of northeastern Mexico (the

Sierra Madre Oriental) have rotated 130 degrees in a counterclockwise

direction in the early Mesozoic.

In surounding regions, major lineaments of continental extent now

related to transcurrent movements have been postulated by Hills (1962),

Sales (1968) and Walper (1977). Sales (1968) Texas Lineament is of

funadmental siginficance in NE Mexico since this ESE-WNW trending destral

fault parallels a major left lateral transcurrence in the Victoria Segment

of the Sierra Madre Oriental (Figure 4). Walper (1977) indicated the likely

existence of such transcurrences in northern Mexico parallel to the Wichita

Lineament. King (1975) postulated several prominent Paleozoic

discontinuities of undetermined character associated to transcurrent faults

and rift trends in eastern United States. He related these faults to the

late Proterozoic. From the examples given by Molinar and Tapponnier (1975)

it is clear that transcurrent faults are associated to the effect of

contiental collision since this type of faults are common features in those

settings. Accordingly, transcurrent faults form after the direct
compressive stress is transmited far into the contiental masses, then

becoming inactive ("latent"), until another major tectonic evdnt takes place

breaking the static equilibrium.

4. Plate Tectonic Reconstructinn of Mexico and the Geologic Evolution

of the Gulf of Mexico and Caribbean Region.- The geologic history of NE

Mexico is directly linked to the plate tectonic history of the Gulf of

Mexico-Caribbean region and undoubtedly was a part of this region since

Paleozoic time. Any plate tectonic reconstruction of this region will

certainly involve large scale horizontal despacernents of continental blocks

such as the present day basement of northeast Mexico. Attempts to unravel

the plate tectonic history of the Gulf of Mexico and the Carribbean by

Walper and Rowet (1982), Pindell and Dewey (1982) and Anderson and Schmidt

(1983) are well-known to invoke the existence of large scale horizontal

despacement, via transcurrent faults or megashears, providing circunstancial

evidence to assume that the basement of northeastern Mexico is bounded by an

intricate system of transcurrent faults some of which, logically, may have

been active since the Precambrian.

PROPOSED TECTONIC MODEL IN NE MEXICO

It is suggested herein that the deformation of the Mesozoic sedimentary

succession found in northeastern Mexico is the result of tectonic

transpression active perhaps since the Late Jurassic. This transpressive

regime is directly related to a wrench-faulting system in the basement. The

proposed wrench-fault system in northeastern Mexico (Figure 4) is bounded on

the north by the Texas Lineament, and on the south by the Walper Lineament

Emegashear proposed by Longoria (1984a), herein named after Jack L. Walper

who first suggested the presence of this lineament], These first order

transcurrent faults trend ESE-WNW (N60W). Second order faults include the

le
San Marcos and La labia faults, trending SE-NW (N40W) and intersecting the

first order wrench-faults at a 20 degree angle. The Torreon-Monterrey

Fractrue described by Cserna (1971) is considered as a thrid order fault,

trending in an F-W (N90W) direction and intersects the San Marcos fault at a

50 degree an q le. Fourth order transcurrent faults include the Galeana and

Chihuahua faults (N05W9), trending almost in a N-S direction, and

intersecting first order faults at a 90 degree angle. These faults have

been verified in the ground at a few localities (Figure 4), their

interelation is inferred from the regional (large-scale) deformational

patterns they produce. Their assumed geometric arrangement is interpreted

from theoretical models.

Consequently, the structural style of deformation observed in

northeastern Mexico is attributed . to a transpressive regime as defined by

Harland (1971). The relation between the transpressive deformation and the

postulated wrench fault zone is evident since the transcurrent faulting is

considered as a component of transpression (Figure 4). The sinistral

transpression is regarded as a local response to the sinistral transcurrence

operating within the wrench fault system in NE Mexico. The Victoria Segment

of the Sierra Madre Oriental displays the combined effects of an early

Tertiary transpressive deformation and, in part, a wrench tectonic phase

(Figure 5). This transpressive regime in NE Mexico (Figure 4) is associated

to an oblique-slip mobile 'zone related to the Late Jurassic spreading phase

in the Gulf of Mexico (Longoria, 1984a). The propsed tectonic transpression

explains the deference in structural styles observed in northeastern Mexico,

and accounts for the different present-day physiographic provinces of the

region (Figure 5).

CONCLUSIONS
Local ^nd regional structural elements of northeastern Mexico give

evidence N r a transpressional regime related to the existence of an

intricated system of wrench faults. Circunstancial evidence also shows that

transcurrent faults are likely to nrrur in the basement of the region. The

origin of this wrench fault system is linked to Tectonic Event II (Shearing

Paase) described by Longoria (1984a) from east-central Mexico. The

transpressive tectonic model herein presented gives an adequate senario co

integrate all the previously described, apparentely incompatible,

deformational models for northern Mexico, and conciliates the differences in

fold vergence in the region (Figure 5). Lateral movements along

transcurrent faults in the basement resulted in seven tectonic elements

which account for present-aay geologic provinces recognized in northeastern

Mexico (Figure 5): 1) The Victoria Segment of the Sierra Madre Oriental,

including a) the Transverse Folded Belt, and b) the East Front, 2) The

Coahuila Folded Belt, and 3) The Sierras Tamaulipecas. Transcurrent faults

in NE Mexico resulted from the rejuvenation of ancient Paleozoic faults.

This faults resulted from: 1) The collision of the southernmost part of the

craton when Pangea was formed, 2) The brc4 up of Pangea, and 3) The origin

of the Gulf of Mexico. Active seafloor spreading in the Gulf of Mexico

(Tectonic Event III described by Longoria, 1984a) is the more likely

mechanism to break up the static equilibrium of ancient Paleozoic faults,

via transform faulting at,the spreading center, reactivating the continetal

area (basement), a somewhat, similar mechanism to that observed in the South

Atlantic between Africa and South America as discussed by Sibuet and Muscle

(1978). The La Babia and San Marcos transcurrent faults are likely to

represent a Miocene reactivation of the Late Jurassic transcurrent faults

described by Longoria (1984).
REFERENCE CITED

Alfonso Zwansinger, J., 1978, Geologia regional del sistema sedimentario Cupido:

j Boletin de la Association Mexicana de Geologos Petroleros, v. 31J, nos.

p. 1-55.

Anderson, T. H. and Schmidt, V. A., 1983, The evolution of the Middle

America and the Gulf of Mexico-Caribbean Sea Region during Mesozoic

time: Geological Society of America Bulletin, v. 94, p. 941-966.

Avenius, C. G., 1982, Tectonics of the Monterrey Salient, Sierra Madre

Oriental, northeast Mexico [Ms. Thesis]: New Orleans, University of

New Orleans, 83 p.

Ballance, P. F., and Reading, H. G., 1980, Sedimentation in oblique-slip

mobile zones: an introduction: Special Publication no. 4,

International Association of Sedimentologists, v. 4, p.1-5.

Bose, E., 1923, Vestiges of an Ancient Continent in Northeast Mexico:

American Journal of Science, 5th Series, v.6, p. 127-136, 196-214,

310-337.

Campbell, J. D., 1958, "En Echelon" Folding: Economic Geology, v. 53, p.

448-472.

Charleston, S., 1981, A summary of the structural geology and tectonics of

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81-74, p. 28-36.

Closs, E., 1955, Experimental analysis of fracture patterns: Geological

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Cserna, Z. de, 1956, Tectonica de la Sierra Madre Oriental de Mexico, entre

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Csernb, Z. de, 1971, Tectonics of northern Mexico (Mesozoic sedimentation,

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and Sundeen, D., eds., The genlogic framework of the Chihuahua tectonic

belt: West Texas Geological Society Publication 71-59, p. 99-118.

Gose, W. A., Belcher, R. C., and Scott, G. R., 1982, Paleomeagnetic

results from northeastern Mexico - Evicence for large Mesozoic

rotations: Geology, v. 10, no. 1, p. 50•:4.

Harland, W. B., 1971, Tectonic' transpression in Caledonian Spitsbergen:

Geological Magazine, v. 18, no. I t p. V-42.

Hills, J. M., 1963, Late Paleozoic tectonics ante mountain ranges, western

Texas to southern Colorado: American Association of Petroleum

Geologists Bulletin, v. 47, no-9, p. 1709-1725.

Humphrey, W. E., 1956, Tectonic framework of northeast Mexico: Gulf Coast

Association of Geological Societies Transactions, v. 6, p. 25-35.

King, P. B., 1975, Ancient southern margin of North America: Geology,

December, P. 732-734.

Longoria, J. F., 1984, Stratigraphic studies in the Jurassic of

northeastern Mexico: Evidence for the origin of the Sabinas Gulf:

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Mineralogists, Annual Research Conference Proceedings, March 1984, p.

171193.

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east-central Mexico: In Westermann, G. E. G. (ed.),

Jurassic-Cretaceous Biochronology and Paleogeography of North America.

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a continental collision: Srianre, v. 189, n.o. 4201, p. 4]q-423.

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-A regional and scale-model approach: American Association of

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Padilla y Segura, J. R., and Ramirez, C., 1975, Observaciones

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ACKNOWLEDGMENTS

The present investigation was carried out at the University of Texas at

Dallas (U.T.D. Contribution 459). Founds for the preparation of this

research were obtained from the Jet Propulsion Laboratory, NASA (JPL

Contract 956430).
FIGURE CAPTIONS

Figure 1. Index map of Mexico showing the location of the study area

and track of SIR-A swath 22 (parallel lines).

Figure 2. Structrual elements of the Saltillo-Galeana Folded Belt

(shadow area), and general trend of anticlinal folds in NE Mexico: 1)

Sierra Los Nuncios, 2) Sierra San Lucas, 3) Sierra de Arteaga, 4) Sierra Las

Nieves, 5) Sierra de Zapaliname, 6) Cerro E1 Cedral, 7) Sierra E1 Tapanque,

8) Sierra Pilar Alto, 9) Sierra La Esmeralda, 11) Sierra San Antonio, 12)

Sierra La Viga, 13) Sierra Rancho Nuevo, 14) Sierra Potrero de Abrego, 15)

Cerro el Coahuilon, 16) Sierra La VEleta, 17) Cerro E1 Banco, 18) Sierra Las

Pintas, 19) Cerro E1 Potosi, 20-21) Sierra Borrada, 22) Sierra. California,

23) Cerro Rayon, 24) Sierra La Ventana, 25) Cerro de la Silla, 26) Cerro

Loma Larga, 27) Cerro Las Mitras, 28) Cerro Topo Chico, 29) Sierra

Pichachos, 30) Sierra Potero Grande, 31) Sierra Potero Chico, 32) Sierra Las

Grutas, 33) Sierra los Muertos, 34) Sierra Catana, 35) Sierra E1 Toro, 36)

Cerro E1 Venado, 37) Sierra E1 Jabali, 38) Sierra Las Mazmorras, 39) Sierra

La Tomita, 40) Sierra Las Vallas. M) Monterrey, S) Saltillo, G) Galeana.

Figure 3. Hypothetical trace of pre-Late Jurassic lineaments,

megashears, and transcurrent faults according to different authors: 1) San

Andreas Fault, 2) Walkerlane Fault, 3) Hurricane-Seiver Lineament, 4) Texas

Lineament [Moody and Hill, 1956], 5) Park Lineament, 6) White River

Lineament, 7) Uncompaghre Lineament, 8) Zuni Lineament, 9) Walker Lineament

[Kelley, 1955 and Hills, 1968], 10, 11, 12, 13) Paleozoic Discontinuities

[King, 19751, 14) Wichita Lineament, 15) Texas Lineament [Sales, 1968], 16)

Sabinas Fault, 17) Monclova Fault, 18) Padilla Fault [Alfonso, 1976], 19)

Saltillo-Monterrey Fracture Zone [Murray, 19611, 20) Zacatecas Fracture

j Zone, 21) Torreon-Monterrey Fracture Zone [Cserna, 1971], 22) Mojave-Sonora
Megashear [Anderson and Schmidt, 19843.

Figure 4. Proposed tectonic transpressional model, and its relation to

Late Jurassic sea floor spreading in the Gulf of Mexico. Latters refer to

areas whereby field data was obtained: m) Mexicali, n) Nogales, c) Caborra,

h) Hermosillo, ch) Chihuahua, to) Torreon, b) Boquillas, d) Delicias, mo)

Monterrey, mc) Monclova, ma) Matehuala, v) Victoria, t) Tampico, sa) San

Antonio.

Figure 5. Index map of northeasten Mexico showing the location of

Mesozoic sections studied, and the relation of the wrench-faults to

present-day geologic provinces. Dotted lines indicate the assumed trace of

postulated faults. Arrows indicate the general direction of fold vergence

and local decollemant.
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