Left-lateral transtension along the Tierra Colorada deformation zone, northern margin of the Xolapa magmatic arc of southern Mexico

Journal of South American Earth Sciences, Vol. 5, No. 3/4, pp. 237-249, 1992 Printed in Great Britain

0895-9811/92 $5.00 + .00 © 1992 Pergamon Press Lid & Earth Sciences & Resources Institute

Left-lateral transtension along the Tierra Colorada deformation zone, northern margin of the Xolapa magmatic arc of southern Mexico U. RILLER, L. RATSCHBACHER*, a n d W. FRISCH Institut fiir Geologie und Pal~ontolgie, Universit~t, Sigwartstrasse 10, D-7400 Tiibingen, Germany

(Received October 1991; Revision Accepted March 1992 ) Abstract--Structural analysis of steeply NNW-dipping tectonites along the northern margin of the Xolapa magmatic arc, southern Mexico, reveals progressive deformation involving ductile and brittle deformation mechanisms. Ductile deformation detached Cretaceous cover rocks from the Xolapa basement along a crustal-scale mylonite zone with normal fault geometry. Normal faults dissected the mylonite zone into blocks which rotated a minimum of 35° to the north. Stress tensors calculated from fault-striae data show subhorizontal, roughly N/S-trending principal extension. Deformation resulted from differential uplift of the Xolapa magmatic arc with respect to its northern hinterland (Mixteca terrane). The oblique normal fault geometry of the mylonites conforms with strike-slip and dip-slip movements along the faults. Leftlateral transtension commenced ductilely between 90 Ma (age of deformed cover rocks) and 34 Ma (U/Pb zircon age of an undeformed pluton cutting the mylonite zone) and continued brittlely into the late Tertiary (tilted Miocene volcanic rocks). We argue that deformation resulted from the interaction of a left-lateral strike-slip regime established during formation of the Caribbean, and an extensional collapse of the Xolapa magmatic arc resulting from a change in steady-state plate-boundary conditions in the early Tertiary. Resumen--E1 an~ilisis estructural de las tectonitas fuertemente buzantes hacia el norte, que afloran a lo largo del margen norte del arco magmdtico de Xolapa (sur de M6xico), revel6 la existencia de una deformaci6n progresiva que involucra mecanismos de deformaci6n di~ctil y fr~gil. La deformaci6n dtlctil caus6 el despegue de la cubierta de rocas eretAcico-terciarias del basamento de Xolapa, a lo large de una zona de milonita de escala cortical y con geometrta de falla normal. Fallas normales separaron la zona de milonita en bloques, los cuales rotaron un minimo de 35° hacia el norte. Los tensores de esfuerzos calculados a partir de estrtas sobre los pianos de fallas muestran una extensi6n principal en direcci6n aproximada norte-sur. La deformaci6n result6 del levantamiento diferencial del arco magm~itico de Xolapa respecto a su 'hinterland' (terreno Mixteca). La geometria de falla normal oblicua de las milonitas coincide con los movimientos de desplazamiento de rumbo y de desplazamiento de inclinaci6n a lo largo de las fallas. La transtension sinestral se inici6 entre los 90 Ma (edad de las rocas deformadas de la cubierta) y los 34 Ma (edad de un plutbn no deformado que corta a traves de la zona de milonita) y continu6 con una deformaci6n de tipo fragil en el Terciario tardio (rocas volc~inicas basculadas del Mioceno). Se argumenta que la deformaci6n es producto de la interacci6n de un r6gimen de desplazamiento de rumbo sinestral, originado durante la formacibn del Caribe, con un celapso extensional del arco magmdtico de Xolapa, causado pot un cambio en las condiciones de estabilidad del limite de placas, ocurrido durante el Terciario temprano.

INTRODUCTION

spatial origins a r e i m p l i e d n o t only r e l a t i v e to e a c h

DIFFICULTIES IN UNRAVELLING t h e p l a t e tectonic e v o l u t i o n of c o n t i n e n t a l c r u s t in s o u t h e r n Mexico a r i s e f r o m t h e e x t e n s i v e c o v e r of u p p e r T e r t i a r y a n d Q u a t e r n a r y volcanic rocks and from the internal c o m p l e x i t y of t h e b a s e m e n t a s s e m b l a g e s . D i s t i n c t lithology, s t y l e of d e f o r m a t i o n , a n d r a d i o m e t r i c a g e ( r a n g i n g f r o m P r e c a m b r i a n to Eocene t i m e s ) indic a t e d i f f e r e n t origins for t h e b a s e m e n t c o m p l e x e s t h r o u g h t i m e . P a l e o m a g n e t i c d a t a i m p l y t h a t , duri n g t h e Mesozoic, S o u t h A m e r i c a o v e r l a p p e d w i t h p a r t s of s o u t h e r n Mexico u n t i l t h e G u l f of Mexico o p e n e d in t h e m i d - J u r a s s i c (Gose a n d SanchezB a r r e d a , 1981; P i n d e l l et al., 1988). T h e r e f o r e , basem e n t c o m p l e x e s older t h a n m i d - J u r a s s i c could not h a v e o r i g i n a t e d in t h e i r p r e s e n t position. D i f f e r e n t *Address all correspondence and reprint requests to: Dr. Lothar Ratechbacher: telephone [49] (7071) 295240; telefax [49] (7071) 296990; E-mail E P I F R 0 1 @ M A I L S E R V . ZDV.UNI-TUEBINGEN.DE

o t h e r b u t also r e l a t i v e to c r a t o n i c N o r t h A m e r i c a (e.g., M c C a b e et al., 1988; F a n g et al., 1989; B a l l a r d et al., 1989). S e v e r a l m o d e l s h a v e b e e n p r o p o s e d for tectonics of t h e M e x i c a n c o n t i n e n t a l m a r g i n . T h e s e involve e i t h e r l e f t - l a t e r a l or r i g h t - l a t e r a l s t r i k e - s l i p tectonics f r o m l a t e Mesozoic t i m e onward. A u t h o r s s u g g e s t i n g r i g h t - l a t e r a l m o v e m e n t s b a s e t h e s e on t h e oblique s u b d u c t i o n b e n e a t h w e s t e r n N o r t h A m e r i c a ( K a r i g et al., 1978; Beck, 1986). I n this scenario, t h e tectonic h i s t o r y of s o u t h e r n Mexico is s i m i l a r to t h a t of t h e N o r t h A m e r i c a n C o r d i l l e r a (Coney, 1989). R i g h t - l a t e r a l s t r i k e - s l i p h a s b e e n o c c u r r i n g a l o n g t h e c o n t i n e n t a l m a r g i n of w e s t e r n N o r t h A m e r i c a a t l e a s t since t h e L a r a m i d e o r o g e n y (80-40 Ma, E n g e b r e t s o n et al., 1985; Coney, 1989). Left-lateral displacements caused the eastward m o v e m e n t of t h e C h o r t i s a n d N i c a r a g u a blocks as a r e s u l t of t h e o p e n i n g of t h e C a r i b b e a n (Malfait a n d D i n k e l m a n , 1972; Ross a n d Scotese, 1988; PindeU et al., 1988; K a r i g et al., 1978). P a l e o m a g n e t i c d a t a for

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Left-lateral transtension along the Tierra Colorada deformation zone, Mexico southern Mexico, which have been summarized by Urrutia-Fucugauchi et al. (1987), suggest displacements of continental blocks along left-lateralstrikeslip faults. However, fieldevidence for such faults in southern Mexico is scarce (e.g., Robinson et al., 1989a,b, 1990; Rifler et al., 1991; Ratschbacher et al., 1991). The assemblage of basement complexes in southern Mexico has also been interpreted as a consequence of terrane accretion. According to Campa and Coney (1983), 80% of the Mexican continental crust is composed of terranes of suspect origin. Suspect terranes are fault-bounded crustal entitiesof regional extent with uncertain paleogeographic or paleotectonic position (in the case of southern Mexico, uncertain relative to cratonic North America) (Coney et al., 1980; Coney, 1989). The south Mexican terranes are thought to have accreted to the continental margin of Mexico by Late Cretaceousearly Tertiary times (Campa and Coney, 1983; Coney, 1983; Campa, 1984). The purpose of this paper is to contribute to the discussion of the tectonic history of southern Mexico by a structural and kinematic analysis of the Tierra Colorada deformation zone, which represents a section of the margin between the Xolapa and Mixteca complexes.

239

metamorphic mylonites and ultramylonites, 1-2 k m thick, originated mainly from acidic volcanic rocks (lapilli-bearingrhyolite, as seen in weakly deformed rocks; Rifler, 1991). Foliation dips N N W and lineation trends W N W (Fig. 3a). The high-grade rocks of the footwall in the Xolapa complex exhibit similar orientations of foliation and lineation. Marble of Albian/Cenomanian age (Morelos Formation; Fries, 1960) was also affected by ductile deformation. All three units -- the Xolapa complex, the mylonite zone, and the marble unit -- were intruded by the Tierra Colorada pluton (B6hnel et al., 1988) at 34 M a (concordant U/Pb zircon age; U. Herrmann and B. Nelson, pets. commun., 1991). The pluton shows no ductile deformation. Therefore, mylonitization must have occurred between 90 M a (Albian/Cenomanian, deposition age of the Morelos Formation) and 34 Ma. A probably Oligocene conglomerate (Balsas Formation) and Miocene volcanic rocks (Papagayo Formation) cover the older rocks unconformably (De Cserna, 1965) and are tilted to the north (Fig. 2).

DUCTILE DEFORMATION

The mylonites and ultramylonites preserve a strong foliationand a stretching lineation defined by the preferred orientation of phyllosilicates (Mg-rich GEOLOGIC SETTING chlorite and muscovite) and the linear arrangement of porphyroclasts (K-feldspar and epidote). PorphyTectonostratigraphic terranes of southern Mexi- roclasts are surrounded by a ductile matrix of chloco are shown in Fig. 1 (after C a m p a and Coney, rite,muscovite, and quartz. Grain-size reduction in 1983). The Mixteca terrane consists of a multiply quartz by dynamic recrystallization indicates dedeformed metamorphic core of Paleozoic age termed formation temperatures exceeding 250°C (Voll, the Acatl~n complex, which is overlain by sedimen- 1980). Plagioclase is partly altered to epidote and tary sequences, topped by thick Late Cretaceous deformed below the temperature of feldspar plas'flysch' (Mexcala Formation; Ortega-Guti6rrez, ticity(500°C to 550°C; Voll, 1980). K-feldspar is sur1978, 1981; Moran-Zenteno, 1984). The Mixteca rounded by muscovite, suggesting muscovite formaterrane is bordered to the east by the Oaxaca and tion by feldspar dissolution and release of silica and Juarez basement terranes (Ruiz et al., 1988; Yafiez et alkalis into the fluid phase (O'Hara, 1988, 1990). al., 1991). To the west the composite Guerrero This reaction is not important below 300°C (Shade, terrane is made up of Jurassic to mid-Cretaceous 1974). The temperature estimates from deformation volcano-sedimentary sequences (Campa and Coney, 1983). All these terranes border the Xolapa complex, which extends 600 k m parallel to the Pacific N N coast. The Xolapa complex forms a late Mesozoic/ Tertiary magrnatic arc composed of migmatite, gneiss, amphibolite, and high-grade metasedimen/'~o~oo° o [shearband] \ tary rocks (predominantly Early and mid-Cretacous isotopic ages) intruded by voluminous plutonic bodies (mainly Eocene isotopic ages; OrtegaGuti6rrez, 1981; Moran-Zenteno et al., 1990; Robinson et a/., 1989a,b, 1990; Herrmann et al., 1991). A continuous profile across the Tierra Colorada deformation zone, which marks the northern edge of the Xolapa complex, crops out at La Venta Vieja (south of Tierra Colorada, Fig. 2; Robinson et al., Fig. 3. Poles to foliationand shear bands, and trend of stretching 1989a; Ratschbacher et al., 1991), where the Rio lineation (equal area, lower hemisphere projection) measured Papagayo cuts across the entire deformation zone, along the (a) Tierra Colorada and (b) Cerro el Macho deformation creating excellent exposures. Greenschist facies zones.

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Left-lateral transtension along the Tierra Colorada deformation zone, Mexico textures are consistent with the metamorphic assemblage, which contains actinolite,albite,chlorite, clinozoisite, and epidote (low-grade metamorphism; Winkler, 1979). Most of the ductile deformation in the Xolapa orthogneisses along the footwall of the mylonite zone was accommodated by dynamic recrystallization of quartz (Fig. 4a). Elongate quartz crystals have sutured grain boundaries and undulatory extinction. Macroscopic and microscopic structures (thin sections were cut perpendicular to foliation and parallel to lineation) reveal the kinematic history of the mylonites, the adjacent marble, and the gneisses of the Xolapa complex. They indicate tectonic transport parallel to the stretching lineation, with predominantly top-to-NW flow. The most commonly observed structures in the ultramylonite are small-scale shear zones, bookshelf sliding structures (Ramsay and Huber, 1987), and epidote 8-clasts (Figs. 4b-4d; e.g., Passchier and Simpson, 1986). Shear bands (e.g., White et al., 1980) form angles between 30 ° and 35 ° with the foliationand are aligned in single sets (in contrast to conjugate sets: Platt and Vissers, 1980). Metarhyolite boudins contain synthetically rotated quartz-o-clasts and ecc-fabrics (extensional crenulation cleavage: Platt and Vissers, 1980; Simpson and Schmid, 1983; Fig. 4e). The meta-rhyolite developed asymmetric boudins by the interaction of foliation and ecc-planes. In the Xolapa gneisses, elongated quartz grains define S-surfaces, and mica decorates C-surfaces (type II mylonite-fabric; Berth6 et al., 1979; Lister and Snoke, 1984). Both S-C fabrics and synthetic,ally rotated K-feldspar (Fig. 4a) indicate top-toNW transport. In the marble (Morelos Formation), subhorizontal ductile shear zones and S-C structures (Fig. 4f) also demonstrate top-to-NW tectonic transport. The predominance of asymmetrical structures indicates a history of non-coaxial deformation during mylonitization (e.g., Choukroune et al., 1987). However, symmetrical structures occur on all scales. Clasts show foliation-normal fractures, are pulled apart without rotation, and have symmetrical strain shadows (Fig. 4g). East of the working area, another mylonite zone occurs at the top of the Cerro el Macho. In this zone, foliation and stretching lineation of mylonitized migmatite and garnet-mica schist of the Xolapa basement dip and plunge to the northwest (Fig. 3b). Flat-lying and SE-dipping shear bands (Fig. 3b) imply SE-directed displacement. Symmetrical structures may have formed as a result of extreme deformation, volume loss during deformation, or the presence of a coaxial component in the overall deformation. Flattened quartz clasts occur within weakly deformed meta-rhyolite boudins and may have undergone pressure solution or coaxial shortening. Flattening strain (Fig. 5), measured on quartz-phyllosilicate lenses (probably replacing feldspar; Rf/d~ method), is interpreted as the

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result of volume loss, probably in an open system during diffusional flow (e.g., O'Hara, 1990). Strain calculations using mesoscopic lapilli in massive meta-rhyolite indicate plane and constrictional geometry.

BRITTLE DEFORMATION Mylonitic structures are overprinted by faults formed during brittle-ductile and brittle deformation. We collected fault-striae data at five localities (stations 1 to 5, for location see Fig. 2) to study the state of stress associated with late-stage deformation in three dimensions. We use the usual terminology to analyze finite displacements along faults in terms of 'stress' or 'paleostress.' Fault-striae orientations indicate subhorizontal, approximately N-trending extension and subvertical or approximately E-trending compression. Data clustering suggests superposition of homogeneous data sets formed during successive deformation events. Therefore, we present a more rigorous geometric and mathematical analysis below (Fig. 6a-e). The three techniques used for stress-tensor calculation are outlined as follows: 1) The 'direct inversion method' (Angelier and Goguel, 1979; Angelier, 1979) performs a least square minimization of the angular discordance between the calculated orientation of the maximum shear stress and the measured striae. 2) The 'grid search method' (Gephart and Forsyth, 1984; Hardcastle, 1989) identifies all possible

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tensors t h a t fit all or portions of the data. ]'he method tests thousands of tensors against the fault data. The orientation, of principal stresseets rotated stepwise. For each tensor position, the stress ratio is varied in increments. For each tensor configuration, it is determined whether the Mohr-Coulomb yield criterion, related by: -- C + POn) has been satisfied. Parameters chosen in this study are C = 0, because we assume pre-existing faults, and la = 0.4, which is close to n a t u r a l observation (e.g., Zoback et al., 1.987) as opposed to 0.6 to 0.9 from laboratory experiments (Byerlee, 1978). 3) The 'right dihedra method' (Angelier and Mechler, 1977) calculates compression and extension right dihedra for each fault, superposes the dihedras for all faults, and derives areas of maxim u m compression and extension (containing 01 and o3, respectively). Conditioned least square fitting is used to derive orthogonalized loci of ol to o3 (Caputo and Caputo, 1988).

Station I, Mylonite Zone We separated homogeneous subsets from the heterogeneous total data (Fig 6a) for stress-tensor calculation. These subsets correspond to groups for which we suspected different ages in the field and which form distinct, generally conjugate sets in the total population. Chlorite mineralization along reverse faults (Fig. 6a-l) indicates formation during brittle-ductile deformation. We grouped these reverse faults with steeply northward-dipping normal faults as a conjugate set (set 1). This set is thought to have formed during NW-trending, subhorizontal extension (03 subhorizontal, ol -> 02 -> 03, principal stresses) soon after mylonitization, when the mylonite zone had a shallow dip. A second dominant fault population (set 2; Fig. 6a-2) dips shallowly southeastward. Foliation was dragged along the faults, indicating down-dip shear (Fig. 4h). Fault zones are characterized by disruption of the dragfolds and accompanying cataclasites. They form a conjugate set with NW-dipping faults, which reactivated or overprinted the mylonitic foliation at a slightly higher angle. The plunge of the calculated extension direction is shallower t h a n t h a t of set 1 (see above) and steeper t h a n t h a t of younger extension directions (see below). This indicates rotation around a subhorizontal axis trending subparallel to the strike of the mylonitic foliation (about N70°E). A third subset (set 3; Fig. 6a-3) characterized by calcite fibers formed coevally with or slightly later t h a n set 2 and is dominated by strike slip-faults. A final set (Fig. 6a-4) of steep, conjugate normal faults shows principal extension trending subhorizontally northeast.

Stations 2 a n d 3, Marble (Morelos FormationJ We divided fault-striae data collected along the Rio Papagayo (station 2; Fig. 6b) into two subsets iFig. 6b-1, 6b-2,. Note that stress orientations vf both subsets can be brought into coincidence by rotation of set 1 (Fig. 6b-1) around a subhorizontal N70°E-trending axis. Populations near Omitlan !station 3; Fig. 6c~ contain reverse, ~mrmal, and strike-slip faults. Strike-slip and reverse faults arc' overprinted by normal faults (subsets of Fig. 6c- l, 6c2) The orientation of o3 of set 1 (Fig. 6c-1; can be brought into a subhorizontal position by rotation around a subhorizontal N70°E-trending axis. Faultstriae data of sets formed early in the deformation history have apparently rotated during progressive deformation.

Stations 4 a n d 5, Tierra Colorada P l u t o n a n d Calc-Silicate R o c k s o f the Contact A u r e o l e Fault-striae populations of both stations 4 and 5 (Fig. 6d,e) indicate t h a t strike-slip faulting dominated late Tertiary (post 34 Ma) deformation in the study area. Constrictional stress geometry (R = 0.6) in the pluton implies a transition from a strike-slip (Fig. 6e) to an extensional setting (Fig. 6d) north of the main deformation zone. The different stress orientations indicated by the two stations imply either a spatial or a temporal change in the state of stress. Similar mineralization (talc, epidote, chlorite) along the faults of both stations make a spatial change more likely.

Fig. 6. Fault-striae populations [see Fig. 2 for locaiitms: a) mylonitezone, b) marble alongthe Rio Papagayo,c) marble near Omitlan, d) Tierra Coloradapluten, e) cale-silicate rocks of the pluton aureole] and principal paleostress orientations (ol >- o2 -> %). The first plot containsall data, while the following plots are subsets separated from the heterogeneoustotal data and represent populationsproven or assumedto have formedin a time sequence during progressive deformation (see text). Faults are drawn as great circles (on equal-area, lower hemisphere projection), and arrows show sense of movementof the hanging-wallblocks. Qualityof slip-sense determination along fault plane is expressed in the arrowhead: filled,certain; double-shafted, reliable; single-shafted, unreliable; lacking, very poor. Numbers (lower left) give fluctuation (F, average angle between the measured striae and the orientation of calculated shear stress), stress ratio [R = (o2-os)(o 1-o3)- 1], and number ofmeasurementsused for stress-tensor calculation out of total data. Lowerhemisphere, equal-area projections (lowerright) showcharacteristicconjugatesets in their present orientation (solid lines) and their assumed orientation during formation (dottedlines). Rotationsof up to 50° about a N70°Etrending axis, parallel to the strike of mylonitic foliation, probablyoccurred.

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L~. RILLER, L. RATSCHBACHER, a n d W. FRISCH

EVIDENCE FOR SUBHORIZONTAL EXTENSION Several criteria indicate large-scale subhorizontal extension along the northern margin of the Xolapa complex. Orientation and sense of displacement indicate normal fault geometry along the Tierra Colorada mylonite zone. Stress tensors calculated from fault-striae populations imply a similar strain history for the late stage of progressive deformation. A component of coaxial extension led to subvertical thinning subparallel to layering. The change in deformation mechanism with time - - at first ductile, then ductile-brittle, and finally brittle - - indicates that the deforming units moved from deeper toward shallower crustal levels. Decompression during deformation is indicated by the metamorphic assemblage. The stratigraphic succession is still preserved, which it would not be in a thrust setting. The juxtaposition of lower (orthogneisses) and upper crustal rocks (low-grade, fossil-bearing marble) separated by mylonites and resulting in a steep metamorphic gradient implies attenuation, at least of the upper crust. The thickness of the Tierra Colorada mylonite zone is similar to other shear zones in extensional environments (e.g., Basin and Range Province; Rehrig, 1986). To explain the great thickness of mylonite zones (> 1 km) in such a setting, we suggest the distribution of deformation across a broad deformation zone in thermally weakened, rheologically soft crust. The coexistence of both types of deformation regimes (non-coaxial and coaxial) has also been reported from detachment faults in metamorphic core complexes in the Basin and Range Province (Rehrig, 1986; Malavieille and Taboada, 1991). Plutons similar to the Tierra Colorada granodiorite are common along the northern margin of the Xolapa complex (Salinas-Prieto, 1984). At other deformation zones along the Xolapa-Mixteca mar-

(a)

"'):...

/

a = 2s-

/~.~.:::.~-~::.?::~:i'.'~ ~ i ~ ' : : i : : : ' ~ ' ~ ...... i : : ~ i ' # . . ' ~

;ii::::iil..iiiii=:i ~.~,

gin, we observed evidence of hydrothermai act~wty ~Ayutla, Juchatengo; Fig. 1~ Ratschbacher ~.~ a[.. 1991L We speculate that extensiona] deformation controlled magmatic activity.

U P L I F T MODEL Progressive deformation along the Tierra Colorada mylonite zone, from ductile to ductile-brittle to brittle, records the uplift of the deforming units through the upper crust. We propose the following uplift model. Ductile deformation caused the leftlateral transtensive, NW-directed detachment of cover rocks (Morelos Formation) from basement rocks of the Xolapa complex (Fig. 7a). Differential uplift of the Xolapa magmatic arc with respect to the Mixteca hinterland caused a progressive steepening of the mylonite zone and a passive rotation of early structures. We assume a rotation axis lying subhorizontally and subparallel to the WSW strike of the mylonitic foliation. Rotation axis (N70°E) and rotation sense (south-side up) are consistent with axis and sense derived from the stress-tensor calculations (Fig. 6 and discussion above). After ductile deformation, brittle-ductile normal faults began to form (Fig. 7a). Ongoing extension caused rotation of fault blocks bounded by these normal faults (Fig. 7b). Extension of the upper crust created normal faults dipping initially at approximately 65 ° (Angelier and Colletta, 1983; Jackson and White, 1989). Assuming a similar initial dip of faults transecting the mylonitic foliation and that all normal faults in the study area are planar, fault block rotation can be modeled according to the domino model (Wernicke and Burchfiel, 1982; Davison, 1989). Because the normal faults (major set Fig. 6a-2) dip at approximately 30 °, they must have been rotated toward the north by roughly 350 about a subhorizontal axis. We calculated 80% N-

(b)

"--..

a +6=

"'~

"

,

Fig. 7. Fault block rotation model, a) Mylonitization due to differential uplift of the Xolapa complex with respect to its northern hinterland started along a deformation zone initially inclined 0°
Left-lateral transtension along the Tierra Colorada deformation zone, Mexico trending extension during the brittle faulting history, based on the domino model. The back-tilted mylonitic foliation dips 25 ° to the north. The oblique, ductile detachment along the Tierra Colorada mylonite zone may have begun at an angle of < 2 5 ° and rotated due to uplift of the Xolapa complex into the 25 ° dip position until faultblock rotation commenced (Fig. 7a). Differential uplift most probably also affected the Cerro el Macho mylonite zone. Back-tilting toward the south (a m i n i m u m of 20 ° is required; Fig. 3b) around the N70°E axis transforms the present thrust fault into a normal fault geometry. The Cerro el Macho zone is considered to be conjugate to the crustal-scale Tierra Colorada mylonite zone.

D I S C U S S I O N AND CONCLUSIONS The deformation and kinematic history of the Tierra Colorada zone reveals differential uplift, subhorizontal extension, and left-lateral strike-slip along the northern margin of the Xolapa complex between the Late Cretaceous and early Tertiary. To evaluate the tectonic history in light of a regional plate tectonic framework, boundary conditions influencing southern Mexico must be considered. Left-lateral movements along the Mexican continental margin fit Caribbean tectonics. A forerunner of the Motagua-Polochic transcurrent fault zone, a section of the North American-Caribbean plate boundary with a left-lateral sense of displacement, originated as a result of the eastward movement of the Farallon and Caribbean plates starting in Eocene times (Figs. 8 and 9a; Malfait and Dinkelman, 1972; Pindell et al. 1988; Pindell and Barrett, 1990). Kinematics and age of deformation along the Tierra Colorada mylonite zone are in accordance with Caribbean geodynamics. First, the slip-sense along the mylonite zone shows a left-lateral component. Second, the time interval of ductile deformation (9034 Ma) falls into the period of eastward motion of the Chortis block and the opening of the Cayman trough (mid-Eocene and Oligocene; e.g., Pindell and Barrett, 1990). We suggest that Cretaceous-Tertiary magmatism along the former Mexican continental margin formed a thermally weakened crust (the Xolapa and Chortis magmatic arcs) between two rigid blocks (the Farallon plate and the consolidated Mixteca and Oaxaca terranes; Fig. 9b) and, therefore, was particularly susceptible to deformation. Oblique plate convergence (dashed arrows show tangential components in Fig. 8) resulted in left-lateral motion, which may have been distributed across the northern margin of the Xolapa complex, forming, for example, the Tierra Colorada mylonite zone (Fig. 10). In our model (Ratschbacher et al., 1991), leftlateral Caribbean-related strike-slip motion is distributed throughout a broad deformation zone, wider than generated by fault zones restricted to the trench, and typical of deformation in continental

245

crust. Stress directions implied by left-lateraloblique plate convergence (Fig. 8) correlate well with stress directions calculated from fault-striae data along the Tierra Colorada deformation zone (Fig. 6; see Ratschbacher et al., 1991 for additional data on stress directions in southern Mexico). Thus, the m a x i m u m compressive stress direction along the plate boundary conforms with the plate-motion direction(e.g.,Zoback et al.,1989). The transtensional deformation observed during parts of the deformation history of the Tierra Colorada deformation zone requires a component of N-S extension in addition to simple strike-slip. W e have suggested two, possibly interacting,causes (Ratschbacher et al., 1991): 1) Extensional collapse of the Xolapa magmatic arc due to extensive magrnatic accretion or due to a change from steady-state subduction along the southern Mexican continental margin. We speculate that the initiation of the collapse may be due to changes in the plate boundary forces caused by removal of continental blocks (e.g., Chortis block) south of the Xolapa complex. 2) Back-arc extension, possibly indicated by the formation of the 'flysch' basin (Mexcala Formation) of the Mixteca terrane. Left-lateral strike-slip in southern Mexico may also constrain the location of the ancient ridge between the Kula and Farallon plates. According to Engebretson et al. (1985), two extreme possibilities exist: the 'northern' option puts the ridge off California, the 'southern' option puts it off Guatemala. If the Kula plate (southern option) subducted beneath southern Mexico by the end of the Cretaceous, dex-

100-85 Ma 87 mm/a 41°E

85-75 Ma 72 mm/a N23

114-75 Ma

75-40Me

75-50 Ma 100-160 mm/a /

40-0 Ma

Fig. 8. FaraUon-North America plate-convergence vectors after (a) Engebretson et al. (1985) and (b) Pindell et al. (1988). Length of vectors (solidarrows) corresponds to convergence velocity for the indicated time interval. Dashed arrows show tangential components which are responsible for trench-paralleltranslation. The strong tangential component between about 75-40 M a m a y have been responsible for left-lateraltranstension within southernMexico.

246

U RILLER, i., RATSCHBACHER, a n d W. FRISCH

\ \

consolidated

-4-

(b)

terranes

(r~id)

-

North A m e ca

,

Mixteca

4 ,,

l

" -] i

~:.

. /.::i):~::i:=::i:!~:::~:..

,,

Choa!S-"(--N.,~

:. !.:. ,

~ ~ " ~ - ::

"

Block

,

!" < ~::i::::.ii!:.:/i!:7;: ~:i::!"::% , . ..~L

iiiS,*

iiH i

Farallon Plate

(a)

Fig. 9. a) Plate-tectonic reconstruction of the Caribbean during the Eocene, after Ross and Scotese (1988). The eastward movement of the Chortis (Ch) and Nicaragua (Ni) blocks along a forerunner of the Motagua-Polochic transcurrent fault may have induced distributed continental deformation in southern Mexico. Inset shows tectonic interpretation of the Tierra Colorada mylonite zone. (b) Sketch of Eocene block geometry along southern Mexico. We suggest that the Mixteca (M) and Oaxaca (O) terranes ("consolidated terranes") and the Farallon plate were rigid while the intervening domain (Xolapa and Chortis magmatic arcs~ underwent deformation.

Convergence vector = N60o_80OE

N

Basement Sedimentary cover Cretaceous/Tertiary plutonic rocks ~ql

r

Chortis and Xolapa magmatic arcs

y

Consolidated terranes

Fig. 10. Schematic illustration of left-lateral transtension along the northern boundary of the Xolapa terrane during the early Tertiary. The interaction of strike-slip faulting related to the formation of the Caribbean with subhorizontal extension in the Xolapa magmatic arc resulted in oblique normal faulting.

Left-lateral transtension along the Tierra Colorada deformation zone, Mexico

247

tral displacement along southern Mexico would be expected. W e suggest that the ridge was located north of the working area (i.e.,we opt for a modified 'northern' option; see also B~hnel et al., 1989), causing left-lateralmotion in southern Mexico.

B6hnel, H., Negendank, J. F. W., and Urrutia-Fucugauchi, J., 1988. Palaeomagnetism and ore petrology of three CretaceousTertiary batholiths of southern Mexico. Neues Jahrbuch far Geologieund Paliiontologie,Monatshefte 1988.97-127.

Acknowledgments--The work was supported financially by Deutsche Forschungsgemeinschaft (Me 915/4-1), and logistically and scientifically by UNAM colleagues H. B6hnel, D. MoranZenteno, and J. Urrutia-Fucugauchi. Our Pale)stress Analysis Laboratory benefited from programs supplied by K. Hardcastle and B. Simmer. We thank B. K. Nelson (Seattle) and P. O~hea (T(ibingen) for pre-submission reviews, Dr. Verma for a review for the Journal, and G. Gastil and M. Suter for discussions on South Mexican geology and help with the literature. The abstract was translated by Ana Leyla Chinchilla Chaves and Lolita CamposBejarano.

Campa, M. F., and Coney, P. J., 1983. Tectono-stratigraphic terranes and mineral resource distributionsin Mexico. Canadian Journal ofEarth Sciences 20, 1040-1051.

Campa, M. F., 1984. Interpretaci6n tectSnica continental de la region Mixteca. Boletln de ia Sociedad Geoldgiea Mexicana 45, 1-4.

Caputo, M., and Caputo, R., 1988. Structural analysis: N e w analyticalapproach and applications. Annales Tectonicae 2, 84-89. Choukroune, P., Gapais, D., and Merle, O., 1987. Shear criteria and structural symmetry. Journal of Structural Geology 9, 525530. Coney, P. J., 1983. U n modelo tectonico de M~xico y sus relaclones con America del norte, America del Sur y el Caribe. Revista del lnstituto Mexicano del Petroleo 15, 6-15.

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