neotectonic evolution of the isthmus of tehuantepec (southeastern mexico)

TECTONOPHYSICS

Tectonophysics 287 (1998) 77-96

Neotectonic evolution of the Isthmus of Tehuantepec (southeastern Mexico)

Eric Barrier a,* , Luis Velasquillo b*c, Mario Chavez ‘, Roland Gaulon b

” Ddpartement de Giotectonique, URA 1759 CNRS, Universite’ I? et M. Curie, 4 place Jussieu, 75252, Paris Cedex 05, France ’ Institut de Physique du Globe, 4 place Jussieu. 75252, Paris Cedex 05, France

L’ Institute de Ingenien’a, UNAM, Apto. Postal 70-472, Coyoacan 04510, Mexico, D.R, Mexico

Received 26 September 1996: accepted 20 August 1997

Abstract

The Isthmus of Tehuantepec of southeastern Mexico is located near the triple junction of the North American, Cocos and Caribbean plates. A neotectonic study, including fault tectonic analysis and study of sub-surface data, was performed in order to understand the tectonic evolution of this complex zone. The Plio-Quaternary fault pattern is described and the brittle structures are interpreted in terms of palaeostress orientations. We propose a model of tectonic evolution of the Isthmus of Tehuantepec since the Late Miocene (“6 Ma) characterized by extensional tectonics. The present structure of the isthmus results from the superimposition of three distinct types of tectonism: (1) a tilting of the eastern Isthmus during the Late Miocene-Early Pliocene, along a major N-S-trending normal fault zone (the Isthmus Fault Zone), as a consequence of the deformation of the subducting slab of the Cocos plate along the subducted part of the Tehuantepec Fracture Zone; (2) the subsidence of the southern isthmus in relation to N-S extension, associated with the eastward displacement of the western Caribbean plate with respect to the southern North-American plate along the Polochic-Motagua fault system; and (3) extensions that develop in the northern half of the Isthmus related to the evolution of the passive margin of the Gulf of Mexico. 0 1998 Elsevier Science B.V. All rights reserved.

Keywords: seismotectonics; North-Central America; fault analysis; triple junction; Tehuantepec

1. Introduction

The Isthmus of Tehuantepec in southern Mex- ico is located near the triple junction of the North American, Cocos and Caribbean plates, where the Middle American Trench and the Polochic-Motagua left-lateral strike-slip system are supposed to merge in the Tehuantepec Gulf area (Fig. 1). In addition, the Tehuantepec Isthmus lies in continuation of the Tehuantepec Fracture Zone (TFZ), an inactive lin-

* Corresponding author. E-mail: [email protected]

ear bathymetric feature of the Cocos plate, which is being subducted beneath the North American margin (Fig. 1.). The seismicity in the Isthmus area is anomalously high while the activity along the subduction zone in the same area is lower compared with the other segments of the Middle American subduction zone located to the east and to the west (Fig. 2).

At the longitude of the Isthmus of Tehuantepec, the width of the North American continent decreases to about 200 km (Fig. l), and its elevation drops from about 2000 m, at the Chiapas and the Trans-Mexican

OO40-1951/98/$19.00 0 1998 Elsevier Science B.V. All rights reserved.

PII SOO40- 195 1(97)00233-3

78 E. Barrier et al. ITectonophysics 287 (1998) 77-96

- 20’N

1 1O’N

Fig. 1. Geodynamical framework of the triple junction region of the North American, Caribbean and Cocos plates. Contour interval: loo0

m; box: study area and location of Figs. 4 and 7. Abbreviations: PMF = Polochic-Motagua Fault system; MAT = Middle American

Trench; TMVB = Tram-Mexican Volcanic Belt: arrows indicate relative plate motions in cm/yr (from Minster and Jordan, 1978); solid

triangles represent active volcanoes.

Volcanic Belt, to 200 m (Fig. 3). The central Isthmus is occupied by low hills that gradually rise to form a small coastal range toward the Pacific Coastal Plain (Fig. 3, cross-sections A-A’ and B-B’). Two plains form the extremities of the Isthmus: the wider one in the north (about 100 km wide) merges into the plain of the Gulf of Mexico. In the south, the Tehuantepec Coastal Plain is less than 30 km wide and merges eastward with the narrow Pacific Coastal Plain (Fig. 3).

The origin of the remarkable structure of the Isthmus is still controversial. Did it result from a single tectonic event, and of what age? What are the relationships between the Isthmus tectonics and the regional geodynamical evolution? In this paper, we first present a tectonic analysis mainly based on a fault tectonic investigation, including palaeostress reconstructions and an analysis of subsurface data

(seismic profiles, well logs). Then we propose a model of the tectonic evolution of the Isthmus of Tehuantepec since the Late Miocene. We present three distinct tectonic patterns and we discuss the geodynamical implications of our results.

2. The Isthmus of Tehuantepec in the regional tectonic setting

The Isthmus of Tehuantepec and adjacent areas reveal a complex structural pattern. Late Cenozoic faulting, the objective of our study, is superimposed on several compressional events. After a complex Mesozoic tectonic evolution, probably related to collisions of continental blocks with the Mexican margin (Carfantan, 1986), the first major phase of deformation is the Late Cretaceous-early Cenozoic Laramide orogeny. This event originates from the

Fig. 2. Seismic activity in the Isthmus of Tehuantepec area. (A) Epicentres with magnitudes greater than 5.0 from ISC for the years 1970-1996; dots = shallow seismicity (~70 km); triangles = intermediate seismicity (70 to 250 km). (B) Focal mechanism solutions

from me CMT Harvard catalog (from 1977 to Present). Shallow events (~70 km) are shown by dotted sectors.

E. Barrier et al. /Tecronophyics 287 (1998) 77-96 79

ZO’N

20’N

80 E. Barrier et al. /Tectonophysics 287 (1998) 77-96

TEHUANTEPEC GULF OF TEHUhTEPEC

COASTAL RANGE GULF CATEMACO VOLCANO COASTAL PLAIN N

-I

A

TEHUANTEPEC GULF OF TEHUANTEPEC COASTAL PLAIN

COASTAL PLAIN

PACIFIC COASTAL PLAIN CHIAPAS GULF

COASTAL PLAIN

ISTHMUS OF TEHUANTEPEC

2

Fig. 3. Morphqlogy of southeastern Mexico. 3-D perspective view to the southwest centred on the Isthmus region; contours at 500 m and

1000 m.

large transcurrent movements and terrane accretions that developed along the southern North~,Ame& can margin in relation with the formation and the migration of the Chortis block (Malfait ‘and Dinkel- man, 1972; Campa and Coney, 1983; Wadge and Burke, 1983; Carfantan, 1986; ,Pindell et al., 1988; Ratschbacher et al,., 1991; Schaaf et al., 1995). Dur- ing this phase, the ‘Sierra de Juarez’ formations

were thrust northeastwards over the stable Mesozoic platform (Figs. 4 and 5).

The next, and last, major regional tectonic event is the Late Miocene orogeny of Chiapas (Carfantan, 1986). This compressive event resulted in the Chia- pas fold and thrust belt that extends in a WNW-ESE direction from the Mexico-Guatemala border in the east, to the Gulf of Mexico in the West, through

E. Barrier et al. / Tectonophysics 287 (I 998) 77-96 81

Normal faults

Laramlde (Paleocene) thrust front (from Carfantan. 1986)

Main anticline axes from the Chlapas told and thrust belt

I I , 1

Fig. 4. Geological sketch of the Isthmus of Tehuantepec area. Location on Fig. 1, M = h4R = Matias Romero, T = Tehuantepec, SC = Salina Cruz. Cross. -sections A-A’ and B-B’ are in Fig. 5; C-C’ in Fig. 6; D-D’ in Fig. 8.

Minatitlan, C = Coatzacoalcos, A = Acayucan,

the Isthmus of Tehuantepec (Figs. 3-5). This belt is made up of Mesozoic to Tertiary sedimentary sequences lying on a Palaeozoic basement, the Chiapas batholith, constituting the internal part of the belt (Figs. 4 and 5). The sedimentary cover is folded above an evaporitic decollement layer. To the northwest, the fold belt plunges beneath the Gulf Coastal Plain where the folds are sealed by several thousand metres of Neogene to Quatemary marine sediments as shown in the schematic cross-section of Fig. 6. This orogeny is presumed to originate from the displacement of the western Caribbean plate along the Polochic-Motagua fault system (Carfantan, 1986).

The Chiapas belt as well as the Laramide oro-

Permo-Triassic gramtic batholiths of Chiapas and Miitequita (Mx)

Mesozoic sequences of the late Miocene fold and thrust belt (Chlapas orogeny)

Cenozoic sequences of the late Miocene fold and thrust beil (Chiapas orogeny)

Unconformable post-erogenic Neogene to Quaternary sediments

Undifferentiated late Miocene intrusives and extrusives

Undifferentiated formations of the Senonian and Paleogene orogenies (Sierra Madre del SW and Sierra de Juarez)

Pliocene to Quaternary volcanlcs 01 the Catemaco volcanic complex

Quaternary alluvium and gravels

gen, trending NW-SE, are oblique to the axis of the Isthmus which trends N-S (Figs. 3 and 4). Both pass through the Isthmus where they are dissected (Figs. 4 and 5), showing that the Isthmus of Tehuan- tepee is younger than these major regional tectonic events. The deformation of the Isthmus is clearly superimposed on these pre-late Miocene orogens. Palaeomagnetic results (Molina-Garza et al., 1994) indicate that no significant rotation or displacement of the Tehuantepec region relative to North Amer- ica has occurred since 13 Ma, and corroborate the geological observations.

The Tehuantepec area is seismically active (Guz- man-Speziale et al., 1989; Ponce et al., 1992). Most

82

ssw

E. Barrier et al. /Tectonophysics 287 (1998) 77-96

NNE

PACIFIC COASTAL PWN CHIAPAS GULF COASTAL PLAIN

I 1 1 1 1 I E

ISTHMUS AXIS CHIAPAS

Paleocene to late Miocene sequences of the Chiapas fold and thrust beit

Permo-Triassic granitic batholiths of Chiapas and Mixtequita sl

‘,

‘I3 I

Undifferentiated formations of the Senonian and Paleogene orogenies

Late Permian to late Jurassic continental deposits of the Todos Santos formation

Paleogene thrust

Unconformable post-erogenic pliocene to quaternaty sediments

Undifferentiated late Miocene intrusives and extrusives

late Jurassic and Cretaceous sequence of the Chiapas fold and thrust belt

Fig. 5. Geological cross-section of the Isthmus of Tehuantepec region. Location on Fig. 4.

of this activity (Fig. 2) is related (1) to the subduction gion of the Isthmus marks a zone of transition from of the Cocos plate at the Middle American Trench, shallow subduction dip angles (15”) beneath southern and (2) to deformation within the lithospheric slab Mexico, to steeper angles (45”) beneath Chiapas and along the subducted TFZ, that generates shallow and Central America (Molnar and Sykes, 1969; Ponce et intermediate events (Ponce et al., 1992). The events al., 1992). Both the change in dip of the slab and belonging to the first category are shallow reverse the occurrence of intermediate earthquakes appear to fault events located along the interplate thrust zone, correlate with the TFZ. According to Ponce et al. whereas those of the second category are shallow to (1992), this particular seismicity pattern is related to intermediate (~250 km) events i&&d in the subducted lithosphere of the Cocos plate and showing mainly normal fault plane solutions (Fig. 2). The re-

a warping of the subducting slab across the TFZ.

Fig. 6. Schematic cross-section of the coastal plain of the Gulf of Mexico drawn from seismic profiles. The Encanto Formation is Late

Miocene-Early Pliocene, the Conception and Paraje Solo-Filisola formations are middle Pliocene. Location in Fig. 4.

E. Barrier et al. /Tectonophysics 287 (1998) 77-96 83

3. Palaeostress reconstruction using fault-slip for the values of the @ ratio given in Table 1, they data are only indicative.

Palaeostress reconstructions based on analyses of fault-slip data have allowed us to characterize tectonic mechanisms in terms of palaeostress tensors. The method of fault-slip analysis consists in deter- mining the best fitting reduced palaeostress tensor for a set of fault-slip data (Angelier, 1984, 1990). It per- mits reconstruction of palaeostress axes (maximum oI, intermediate cr2 and minimum a3 stress axes) and of the ratio @ between principal stress magnitudes [@ = (02 - ~s)/(cri - as), with 0 < 4 < 11, know- ing the orientations and senses of slips on faults acting during the same tectonic event. In the case of polyphase tectonics a relative chronology of stress tensors has been established from the identification of geometrical relationships between tectonic features in the field: (1) superimposition of striae on a single fault plane, and (2) observation of cross-cut- ting and offset relationships between successive fault slips, incompatible in terms of a single stress tensor.

The measured tectonic features are mainly faults with slickenside lineations. A total of 44 sites was analysed, and 60 stress tensors have been computed. Site locations are shown in Fig. 7 and the diagrams of fault populations are in Appendix A. The results of palaeostress tensor determinations are given in Table 1. The quality estimator (Ang) corresponds to the average angle between the actual slip on the fault plane and the computed shear stress vector (Table 1). In 95% of the stress tensor computations presented herein, these angle values are below 20” (6 1% below loo). These low values show the consistency between measured and computed values for each site. In this study, the computed values of the @ ratio mainly range from 0 to 0.5. The significance of the @ values is questionable in the methods of inversion of fault slip data. This ratio is mainly controlled by the geometry of fault populations, especially by the faults oblique to the stress axes. Thus, for instance, the conjugate fault systems do not provide reliable values of @. In fact, the @ ratio is well determined only in special cases, mainly where the fault popula- tion displays a large variety of both fault strikes and dips. In this study, we focus on the determination of the directions of the principal stress axes, which is reliable, and we also use conjugate faults systems; as

The sites are distributed in various formations (Figs. 4 and 7, Table 1): Neogene-Quaternary formations (volcanics, shales, clays, sands, conglomerates and gravels) in the northern Isthmus, carbonate and detrital (sandstones and conglomerates) Mesozoic and Paleocene formations in the central Isthmus, and mainly Late Miocene intrusives and extrusives in the southern Isthmus. Some sites have also been stud- ied in metamorphic and ophiolitic formations of the Sierra de Juarez in the southern Isthmus. Many sites are practically monophase and reveal simple conjugate patterns of neoformed faults, particularly in the Neogene-Quatemary sediments of the northern Isth- mus (sites 6a, 10, 12, 13, 18a, 19, 21, 24 and 31a, in Appendix A). In these sites, the direction of principal stress axes is well determined. Other sites, mainly located in volcanic formations, folded sedimentary sequences or metamorphics, display polyphase faulting with common inherited faults, and/or more complicated fault patterns including reactivated joints, cleavages, bedding planes or other various tectonic features showing a large variety of fault plane orientations (see for instance sites 1, 2b, 9, 17, 18b, 3 lb, 36b, 37a and 42b in Appendix A).

Particular attention was focused on polyphase or superimposed deformations; these deformations result from different tectonic events (see sites 18a and b, 31a and b, 43a and b in Appendix A) or from permutations of principal stress axes during a single tectonic event (see sites 6a and b, 13a and d, 13b and c in Appendix A). Some sites contain either few fault measurements (sites 3, lOc, 16, 26 and 27 in Appendix A) or/and poorly reliable fault populations (sites 5, 11, 26, 32, 38 and 39 in Appendix A). In these cases the computed stress tensors are poorly constrained. Despite these drawbacks, these results have been presented because they concern key areas where exposures are rare and poor. In these cases the values of the palaeostress tensors only indicate the prevailing conditions.

In the post-erogenic deposits of the Isthmus area, mainly one single type of palaeostress tensor has been determined corresponding to extension (al sub- vertical, 02 and 03 sub-horizontal). In the folded sequences of Chiapas, the NE-SW-trending Late Miocene compression dominates. Palaeostress re-

84

Table 1

E. Barrier et al. /Tectonophysics 287 (1998) 77-96

Reduced stress tensors computed using fault-slip data sets through the direct inversion method (Angelier, 1984, 1990)

Site q axis ua axis as axis Ratio @ N Ang Formation

1 352 85 229 03 139 04 0.5 6 21

2a 171 86 042 03 312 03 0.3 I 18

2b 359 69 135 15 229 14 0.4 5 15

3 141 16 23101 321 13 0.2 4 11

4 249 79 122 07 031 09 0.3 11 9

5 344 19 127 09 218 07 0.1 10 11

6a 039 87 13000 220 03 0.4 17 10

6b 235 83 048 07 138 01 0.4 9 6

6c 065 II 182 06 273 11 0.3 I IO

7 00481 18209 272 00 0.0 6 3

8 012 65 262 25 170 03 0.2 6 20

9 092 62 333 15 237 24 0.0 9 9

IOa Ill 84 287 06 017 00 0.3 16 6

lob 223 78 331 04 062 12 0.4 I 5

1oc 235 13 019 14 Ill 10 0.4 5 12

11 042 68 240 21 148 06 0.3 6 6

12 11087 340 02 250 02 0.6 11 13

13a 201 86 023 04 293 00 0.3 19 11

13b 199 80 044 09 313 04 0.5 12 7

l3c 151 80 326 10 056 01 0.3 22 11

13d 11081 286 09 016 01 0.5 26 9

14 216 69 062 19 329 08 0.1 12 II

15 203 80 308 03 039 10 0.2 11 9

16 353 80 11405 205 08 0.2 4 1

17 027 65 1.56 16 251 19 0.0 5 5

l8a 11665 281 25 014 06 0.5 8 19

l8b 281 19 043 06 134 09 0.5 9 4

19 109 70 217 07 309 19 0.0 5 5

20 087 84 272 06 18200 0.4 5 IO

21 153 88 256 00 346 02 0.0 16 6

22 102 85 298 05 208 02 0.6 8 15

23a 328 87 21102 121 03 0.4 8 5

23b 113 64 214 25 008 08 0.2 9 8

24 107 69 295 21 204 03 0.3 21 23

25a 218 88 005 01 095 01 0.4 5 6

25b 259 76 112 12 020 08 0.3 5 3

26 121 81 00104 210 08 0.5 4 4

21 067 13 322 05 231 16 0.6 4 3

28 252 83 106 06 015 04 0.1 8 6

29 180 81 005 09 215 01 0.1 12 6

30 076 72 302 13 209 13 0.1 28 10

31a 306 80 094 09 184 05 0.2 14 11

31b 268 71 039 12 132 14 0.5 I 14

32 012 14 291 12 199 IO 0.3 8 5 33 105 84 292 06 202 01 0.3 16 6

34 291 79 117 I1 027 01 0.3 10 I

35 298 62 090 25 185 12 0.3 19 13

36a 066 80 241 IO 331 01 0.4 8 14

36b 005 86 110 01 201 04 0.6 5 7

3la 333 84 175 06 085 02 0.4 16 22

37b 314 16 095 11 187 08 0.4 7 6 38 086 40 252 10 342 02 0.5 11 3

39 215 86 087 02 35103 0.8 6 8

Volt-Q

Volt.-Q

Vole.-Q

Volt.-Q

Volt.-Q

Vole.-Q

N-Q N-Q

: _

a N-Q N-Q -

N-Q N-Q N-Q N-Q Q

N-Q N-Q N-Q N-Q N-Q

Mes.

N-Q

N-Q Mes.

Mes.

Mes.

Volt.-M

Volt.-M

Volt.-M

Mes.

Vole.-M

Volt.-M

Vole.-M _

Volt.-M -

Mes.

Volt.-M


Table I (continued)

Site 0, axis 02 axis 03 axis Ratio @ N Ang Formation

40

41

42a

42b

43a 43b

44

225 71 107 06 016 I I 0.4 I1 12 Volt-M

333 68 214 11 121 19 0.4 6 12 Volt-M

165 82 028 06 298 06 0.1 9 6 Volt-M

069 67 271 22 178 08 0.6 6 3

113 87 282 03 012 01 0.3 11 6 Mes. 195 67 047 20 313 11 0.4 6 15 _

271 86 011 01 101 04 0.1 9 5 Volt.-M

Sites are located on Fig. 7, fault populations are shown in Appendix A. All angles in degrees. Principal stress axes (T,, q, 03 are defined

by trends and plunges. @ = (~2 - oa)/(at - ~73). N = number of faults used for computation. Ang = average angle between observed

stria and computed shear stress. Formation: Volt-Q = Quatemary volcanics; Q = Quatemary deposits; N-Q = Neogene to Quarternary

marine deposits; Mes = Mesozoic formations; Volt.-M = Late Miocene extrusives and intrusives.

constructions concerning this compressional event are beyond the scope of this paper and are not presented. All chronological relationships between extensional and compressional structures indicate that extension postdates this compression. In the Sierra de Juarez metamorphics and ophiolites, where fault tectonic analysis is more critical because of the highly polyphase deformations, a qualitative ap- proach is used: where observations are available, the youngest brittle deformations are extensional features. Generally speaking, our tectonic analysis highlights the extensional character of the post-Late Miocene tectonics in the Isthmus area. However, extensional tectonism is not homogeneous in time and space. The next sections present the three distinct extensional tectonic events that serve to explain the present structure of the Isthmus of Tehuantepec.

4. The Isthmus Fault Zone

Viniegra (1971) first proposed that a major left- lateral fault follows the axis of the Isthmus. Several authors (Dillon and Vedder, 1973; Anderson and Schmidt, 1978; Carfantan, 1986; Guzman-Speziale et al., 1989) suggested that this fault is an inherited feature reactivated during Neogene time. Carfantan (1986) reported that a normal fault system separates the granitic batholiths of Chiapas and Mixtequita (Figs. 4 and 5), both belonging to the basement of the Chiapas fold and thrust belt. Seismic profiles (PEMEX, unpublished data), crossing the Isthmus just north of the Mixtequita granites, confirm that N-S- to NNW-SSE-trending normal faults cut not

only the granitic batholith, but also the whole folded sedimentary cover (Figs. 4 and 5).

Palaeostresses related to such normal faulting are also indicated by the analysis of fault-slip data sets measured in Mesozoic and Cenozoic formations of the central Isthmus (Fig. 7, Appendix A). Several sites show N-S-trending normal faults consistent with E-W to WNW-ESE extension (see sites 23, 25, 26 and 29; Fig. 7). According to tectonic criteria (superposition of slickensides on a single fault plane, fault intersections, systematic post-folding normal faulting), this extension is clearly younger than the NW-SE-trending thrusts and folds related to the major NE-SW compression of the Late Miocene Chia- pas orogeny. In site 23 (Fig. 7; Appendix A), relative chronological observations indicate that this extension is older than the N-S- to NNE-SSW-trending extension that predominates in the central Isthmus.

Geological evidence in the central Isthmus suggests vertical movements along a N-S- to NNW- SSE-trending fault zone. The present morphology of this fracture zone, where no steep topography has been observed, supports the idea that normal faulting is no longer active in the area where the inferred vertical displacement is maximum. In addition, neither shallow seismicity (Ponce et al., 1992) nor surface rupture have been detected. A minimum vertical offset of 2 km may be estimated from the displacement of the top of the granitic batholith (Fig. 4) as shown by the cross-section B-B’ on Fig. 5. This value is probably a minimum taking into account the effects of erosion on the Mixtequita granite. The northward extension of this main fracture zone into the


Fig. 7. Palaeostress reconstruction related to the post-Late Miocene extension. Arrows indicate directions of extension (us) reconstructed

from analysis of fault slip data sets. Fault populations and computed stress tensors are shown in Appendix A; results are listed in Table I (numbers correspond to those in Table I and Appendix A. Short lines with a black dot: maximum horizontal stress orientation (SH)

based on borehole elongations (from Suter, 1991). The rose diagrams indicate the frequency of os orientations for the northern (27

tensors) and southern (21 tensors) Isthmus areas.

coastal plain of the Gulf of Mexico may be indicated by the Quatemary Catemaco volcanic complex that stands on the Gulf Coastal plain in a direct line with the Isthmus Fault Zone (Figs. 3 and 4). The southern extension of this fracture zone, along the western boundary of the Tehuantepec Coastal Plain, is marked by NNE-SSW normal faults (Fig. 4). Brit- tle tectonic analysis of several sites (42, 43; Fig. 7)

performed in formations as young as Late Miocene in age, shows NW-SE to WNW-ESE extension in good agreement with the NNE-SSW-trending steep high cliffs related to normal fault scarps, bordering the plain to the west (Figs. 3, 4 and 8). In sites 42 and 43 some normal faults striking NE-SW bear oblique slickenside lineations with N-S trends (normal-dextral). These observations suggest that an


Fig. 8. Normal faults and tilted blocks in the Neogene marine sediments of the northern Isthmus near Acayucan (site 13, location Fig. 7).

View looking northeastward.

earlier NW-SE to WNW-ESE extension developed conjugate systems of normal faults and that a later extension, roughly trending N-S, reactivated these faults as oblique-slip normal faults.

The Late Cenozoic evolution of the Isthmus of Tehuantepec includes a N-S normal faulting episode that occurred during the Late Miocene-Pliocene. We infer that a major crustal fault zone cuts the Isthmus of Tehuantepec in a roughly N-S direction, downdropping the eastern part of the Isthmus and tilting the whole western Chiapas Range toward the west. This tilting may explain the morphology of the western Chiapas Massif which is characterized by a gentle northwestward plunging of the major fold axis (Fig. 3). It is difficult to state accurately the age of the faulting because few recent deposits exist in the central Isthmus which has been emerging since at least the Late Miocene. Nevertheless, it is clear that a N-S-trending fault zone follows the axis of the Isthmus, and cuts the Late Miocene

Chiapas fold and thrust belt (Fig. 4). In the southern Isthmus, WNW-ESE extension is recorded in Late Miocene volcanic rocks. In the northern Isthmus, the earliest post-erogenic transgression occurred during the middle Pliocene (Akers, 1979). We interpret this transgression as resulting from the subsidence associated with the downdropping of the eastern block. These data indicate that the fault zone was active during Late Miocene-Early Pliocene time, and possibly later in the Pliocene. The main effect of the normal faulting has been a significant reduction of elevation, particularly in the central Isthmus, that originated the marine transgression in the northern Isthmus during the middle Pliocene and explains the present strike of the Isthmus.

We propose to link these tectonics to the subduction of the Tehuantepec Fracture Zone (TFZ) beneath the North American plate. This fracture zone is outlined on the Cocos plate (Fig. 1) by its mor- phologic expression: the Tehuantepec Ridge (Mam-


merickx and Klitgord, 1982; Klitgord and Mammer- i&x, 1982; Schilt and Karig, 1982). Its intersection with the Middle American Trench marks the strong change in dip of the subducted slab of the Cocos plate (Molnar and Sykes, 1969; Burbach et al., 1984; Guzman-Speziale et al., 1989; Ponce et al., 1992). The TFZ separates two distinct provinces of the Co- cos plate of different ages. In the subducted slab, the TFZ is reactivated as shown by the high seismicity of the downgoing slab (Ponce et al., 1992). The dip change is attributable to the difference in age of the subducting Cocos plate along the Middle American Trench (Burbach et al., 1984; Ponce et al., 1992) which reaches 15 Ma across the TFZ (Couch and Woodcock, 1981). To explain the coincidence between the TFZ and the Isthmus Fault Zone, we suggest that, when the oceanic lithosphere located east of the TFZ began to sink in Late Miocene- Pliocene time, the upper plate ruptured in response to the change in isostatic equilibrium. This rupture resulted in the N-S-trending normal faulting of the Isthmus Fault Zone. During the Pliocene, the eastern part of the Isthmus collapsed, drawn down by the Cocos lithosphere, until a new isostatic steady state was reached.

5. Extensional tectonics of the northern Isthmus

The northern half of the Isthmus consists of a vast plain, with well developed marshes and large meandering rivers which are features that are char- acteristic of low-lying areas. In this region, seismic profiles indicate that the thickness of the Plio- Pleistocene sedimentary sequences below the plain reach several thousands of metres (Figs. 6 and 9). Subsurface stratigraphic data from calcareous nan- nofossils and planktic foraminifera (Akers, 1979; Herrera-Andagua, 1983) show that a large part of this sequence is middle-Late Pliocene in age (Fig. 6). These formations unconformably overlie the whole sequence of the Late Miocene Laramide fold and thrust belt, including the Mesozoic and Cenozoic formations up to the Middle Miocene (Figs. 4 and 6). In the northern Isthmus, including the Catemaco volcano area, normal faulting dominates (Figs. 4 and 7). The normal faults shown in Fig. 4 are drawn from a study of seismic profiles (PEMEX, unpublished report) and from field investigations. They cut

the Plio-Pleistocene sedimentary sequence (Fig. 9). Two main sets of normal faults, trending NW-SE and NE-SW, coexist in the northern Isthmus. These normal fault sets respectively developed west and east of the Isthmus axis. The study of the best exposures located in the plain of the northern Isthmus, especially along the new roadcuts between Acayuan and Minatitlan, shows that this extensional event is associated with the tilting of blocks along normal faults (Fig. 8). This observation is corroborated by the seismic profile of Fig. 9, illustrating the deformation pattern of the thick Plio-Pleistocene marine sedimentary sequence of the region. This tectonic, stratigraphic and morphological evidence shows that the northern Isthmus area has subsided strongly since the Pliocene, appreciably contributing to the narrowing of the continent at the Isthmus of Tehuantepec.

Most sites in this part of the Isthmus only show extensional brittle structures. Our results from the analysis of fault-slip data sets mainly in Plio- Pleistocene formations (Fig. 7, Appendix A) indicate the coexistence of three main directions of extension: NNE-SSW, NE-SW and NW-SE (sites l-6, 9-l 1, 13-19 in Fig. 7), in good agreement with the large-scale normal faults observed on seismic profiles (Figs. 6 and 9). The faults are exemplified by numerous conjugate systems of normal faults, particularly common in the Neogene to Quaternary marine sediments (sites lOa, 13a, b, c, d, 18b in Fig. 7). In the volcanics (sites l-6), where pre-existing fractures are widespread (cooling and tectonic joints), neoformed conjugate systems are rare. These particular sites display more complicated fault patterns, including oblique-slip on preexisting planes (sites l-

6). In several sites of the northern part of the Isth-

mus (sites 2, 6 and 13), two perpendicular directions of extension (NE-SW and NW-SE) coexist. Both are recorded in marine Pliocene formations as well as in the Quaternary volcanic rocks and sediments of the Catemaco volcano, suggesting that the corresponding deformations may be contemporaneous. The absence of definite chronological relationships, the spatial correlation between these patterns (site 2 and 13), and the perpendicularity of the principal stress axes support the hypothesis of alternation between principal axes a2 and (~3 during a single extensional event ((~1 axis remaining vertical). Such

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a palaeostress pattern characterized by switches between a2 and a3 axes, is typical of regional extensional tectonism.

The third extension, trending NNE-SSW, is less common (rose diagram, Fig. 7). It has been determined in sites 10, 12, 13 and 18 (Figs. 7 and 8; Appendix A). The fault populations of sites 10, 13 and 18 (Fig. 7) indicate that NE-SW and/or NW-SE normal faults, bearing oblique slickenside lineations, have been reactivated. This suggests that (1) the first extensional tectonics developed conjugate systems of normal dip-slip faults, and (2) the second NNE-SSW extension reactivated these fault planes as oblique- slip normal faults. This extension postdates the NW- SE and NE-SW extensions. This extensional event is the last observed in the northern Isthmus. It is also characterized by alternation of oi and 03 axes (sites 6 and 13, Fig. 7). This NNE-SSW extension is in good agreement with the directions of maximum horizontal stress (SH), determined from borehole elongations (Suter, 1991) presented in Fig. 7. In the northeastern Isthmus, the SH directions range from N08O”E to NllO”E, and are almost perpendicular to the most recent a3 directions (trending NNE-SSW) computed in the same area from analysis of fault populations (Fig. 7). According to the age of the faulted formations and to the relative chronological data, a rotation of about 30” of the direction of extension occurred during the Quaternary. The new stress pattern is characterized by a dominant NNE-SSW extension.

Our data indicate that in the northwestern Isthmus and farther northwest along the coast of the Gulf of Mexico, complex extension has prevailed since the Late Miocene orogeny until Present. We attribute this extensional stress pattern of the northern Isth- mus to the flexure of the continental margin of the Gulf of Mexico, subjected to sediment loading along the continental shelf. According to this hypothesis, the bending of the continental margin would have induced the development of normal faults parallel to the flexure axis as observed east and west of the Isth- mus axis along the Gulf Coastal Plain. In the north- em Isthmus, both NW-SE and NE-SW extensions coexisted during the Pliocene, including alternations of principal axes 02 and 03. This multidirectional extension, roughly orthogonal to the shoreline, probably results from the curvature of about 90” of the

continental shelf of the Gulf of Mexico in this area (Figs. 1 and 3). At present, it seems that the NNE- SSW-trending extension dominates in the area of the northern Isthmus.

6. Subsidence of the southern Isthmus

The southern part of the Isthmus is characterized by a coastal plain, about 30 km wide at its western border, that narrows eastward and merges with the Pacific Coastal Plain of Central America (Fig. 3). Cross-sections A-A’ and B-B’ in Fig. 3 show that from north to south, following the axis of the Isthmus (approximately the trans-Isthmic road), the elevation which is very low in the northern half, increases regularly from the centre of the Isthmus up to the top of the coastal range that borders the Tehuantepec Coastal Plain. On the southern side of the coastal range, a steep slope plunges down to this plain (Figs. 3,4 and 10). Morphological evidence of recent E-W-trending normal fault scarps has been observed in the Coastal Plain as well as at its northern border (Fig. ll), indicating that these faults are still active in the southern Isthmus.

Field analyses of major and minor faults in the plain, and in the surrounding areas, show that E-W to WNW-ESE normal faults dominate (Figs. 4 and 7, Appendix A). In the plain, the E-W- to ENE- WSW-trending aligned hills, exhibiting the same formations as those found to the west in the Sierra Madre and in the Sierra de Juarez, are horsts bor- dered by normal faults (Fig. 4). In the Gulf of Tehuantepec, seismic profiles (PEMEX, unpublished reports) also display major normal faults almost parallel to the coastline (Fig. 4). Most sites in this area show extensional brittle structures with numerous striated normal faults Appendix A). Determinations of palaeostresses indicate that oi axes are vertical, whereas o3 axes are horizontal and generally trend N-S to NNE-SSW (sites 30 to 40,42 and 43; Fig. 7 and Appendix A), almost perpendicular to the trend of the plain (Velasquillo, 1994). Among these fault populations, dip-slip normal faults are common (sites 31a, 33, 34, 37b, 38, 40, Fig. 7 and Appendix A), but pure conjugate systems of normal faults are rare because of abundant inherited faults and joints. The morphology, as well as geological observations, suggest that a major fault zone separates the Tehuan-


Fig. 10. View of the northern border of the Pacific Coastal Plain east of the Isthmus area (north of site 37). The scarp outlines the

WNR-ESE-trending major normal fault bordering southward the Chiapas Massif. This normal faulting is related to the sub-meridian

extension of the Tehiantepec Gulf. View looking northward.

tepee Coastal Plain from the coastal range. The coastal range would correspond to the shoulder of a large normal fault bordering the Tehuantepec Coastal Plain to the north (Figs. 3 and 4).

The age of these extensional tectonics is no older than Late Miocene because normal fault populations were measured in plutons and associated volcanics of this age (Williams and McBimey, 1969). The relative chronology indicates that the faults are more recent than the N-S to NNW-SSE faulting of the Isthmus. Field analysis shows it to be the last tectonic event occurring in this area. This N-S to NNE-SSW active extension of the southern Isthmus resulted in (1) the subsidence of the Tehuantepec Coastal Plain and of the Tehuantepec Gulf, contributing to the N-S narrowing of the continent observed in the Isthmus area, and (2) the uplift of the shoulder related to the major normal fault bordering the Coastal Plain to the north, rejuvenating the relief bordering the

coastal plain and explaining the N-S asymmetry of the Isthmus (Fig. 3, cross-sections A-A’ and B-B’).

This extensional palaeostress pattern observed in the southern Isthmus and in the Gulf of Tehuante- pet may be related to the eastward migration of the Caribbean plate along the Polochic-Motagua fault system which extends for about 400 km through Central America (Fig. 1). The amount of horizontal displacement along this fault system is still controversial (Mann et al., 1990). Estimates range from a few kilometres (Erdlac and Anderson, 1982; An- derson et al., 1985) to more than 100 km (Kesler, 1971; Burkart, 1978, 1983; Deaton and Burkart, 1984; Burkart et al., 1987) on the basis of geological data for the last 6 Ma. Schwartz et al. (1979) have determined a slip rate of between 0.45 and 1.8 cm/yr for the Quaternary motion along the Motagua fault based on offsets of river terraces, whereas Min- ster and Jordan (1978) and DeMets et al. (1990)


Fig. 11. Relationship between the coastal range and the northern Tehuantepec coastal plain in the southern Isthmus of Tehuantepec. Note

the straight scarp separating the plain and the coastal range. The road on the left side is the North-South trans-Isthmic road: the town on

the lower-left comer is Asuncion-Ixtaltepec. View looking northward

have respectively estimated instantaneous rates of 1.9 cm/yr and 1.2 cm/yr. Evidence of 130 km of sinistral displacement across the Polochic faults has been presented in Burkart (1978, 1983), Burkart et al. (1987) and Deaton and Burkart (1984). They suggest that the major sinistral displacement began at around 10 Ma during the Late Miocene. These estimations fit our observations in the southern Isth- mus where the extension is not older than Late Miocene.

This hypothesis implies that the Polochic- Motagua fault system, defining the boundary between the Caribbean plate and the North Ameri- can plate, does not intersect the Middle American Trench. It ends within the Tehuantepec Gulf and the Coastal Plain and joins the Middle American Trench in a complex zone of diffuse faulting with E-W to WNW-ESE normal faults and subsidence (Fig. 12). This poorly defined triple junction is unstable and

migrates eastwards relative to North America (Del- gado-Argote and Carballido-Sanchez, 1990). Fig. 12 shows that if the western Caribbean plate moved back 130 km westwards, the Gulf of Tehuantepec would be closed. During the migration of the west- em Caribbean plate, the North American continental margin has been truncated. To the west of the triple junction, extensional tectonics has developed perpendicular to the truncated margin. This extension has resulted in a N-S stretching of the margin through large E-W normal faults.

7. Conclusions

The neotectonic evolution of the Isthmus of Tehuantepec is characterized by several extensional stress regimes. The present Isthmus structure results from the superposition of three distinct tectonic episodes that have been taken place since the Late


volcanic arc

Fig. 12. Schematic evolution showing the relationships between

the plate boundaries, the Tehuantepec Fracture Zone (TFZ) and

the tectonics of the Isthmus of Tehuantepec for the last 6 Ma.

The North American plate (NA) is held fixed. CO = Cocos

plate, CA = Caribbean plate, PMF = Polochic-Motagua fault

system. Large open arrows are directions of extension deter-

mined from our study. Circled dots and crosses indicate uplift

and subsidence, respectively.

Miocene (6 Ma). These events originate from different and independent causes. The oldest one took place during Late Miocene-Pliocene time. It resulted in the downdropping of the eastern part of the Isth- mus, along the N-S-trending normal faults of the Isthmus Fault Zone in the central Isthmus. We propose that these tectonics were consequences of the deformation of the subducting slab of the COCOS

plate along the TFZ, generated by the change of dip of the subducted slab of the Cocos plate.

Following these first extensional tectonic phases, two distinct extensional events developed simultane- ously since the Late Miocene-Pliocene, respectively in the northern and southern Isthmus areas. The multidirectional extension observed in the northern half of the Isthmus is attributed to the evolution of the passive margin of the Gulf of Mexico, whereas the N-S extension associated with the subsidence of the southern Isthmus is related to the eastwards displacement of the western Caribbean plate along the Polochic-Motagua fault system, with respect to the North American plate. The model we finally propose for the tectonics from Late Miocene to Present takes into account the constraints resulting from structural and palaeostress analyses, and it accounts for the timing of the three major tectonic episodes. To interpret the neotectonic evolution of the Isthmus of Tehuantepec it is necessary to consider the regional geodynamical context, and more particularly the evolution of (1) the Cocos plate subduction at the Middle American Trench, including the TFZ, (2) the North American-Caribbean boundary and the North American/Caribbean/Cocos triple junction, and (3) the passive continental margin of the Gulf of Mex- ico.

Acknowledgements

The authors are grateful to the Institudo de In- generia of the Universidad Autonoma de Mexico whose assistance has made the field work possible. This study was part of the CNRS-CONACYT coop- erative program in the field of Earth Sciences. We thank PEMEX for permission to publish subsurface data. IPG Paris contribution 1486.

Appendix A

Fault data sets and palaeostress reconstruction in 44 sites

of the Isthmus of Tehuantepec. Location of the sites in Fig. 4.

Fault planes as thin lines; slickenside lineation as dots on fault

planes with thin arrows indicating the sense of motion; bedding

planes as dashed lines; computed palaeostress axes obtained by the direct inversion method; ot , 02 and 03 as 5-, 4- and 3-pointed

stars, respectively (see details in Table 1). Large black arrows

indicate computed directions of extension. Schmidt’s projections,

lower hemisphere.


b

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