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Ductile deformations of opposite vergence in the eastern part of the Guerrero Terrane (SW Mexico)
Journal of South American Earth Sciences 13 (2000) 389±402
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Ductile deformations of opposite vergence in the eastern part of the Guerrero Terrane (SW Mexico) J.C. Salinas-Prieto a,b,*, O. Monod b, M. Faure b,c a
Escuela Regional de Ciencias de la Tierra, Universidad Autonoma de Guerrero, Apartado Postal 197, Taxco (Gro) 40200, Mexico b Institut de Sciences de la Terre, Universite d'OrleÂans, CNRS-UMR 6530, OrleÂans Cedex 2, F-45067 France c Institut Universitaire de France, France
Abstract The Teloloapan volcanic arc in SW Mexico represents the easternmost unit of the Guerrero Terrane. It is overthrust by the Arcelia volcanic unit and is thrust over the Guerrero±Morelos carbonate platform. These major structures result from two closely related tectonic events: ®rst, an eastward verging, ductile deformation (D1) characterized by an axial-plane schistosity (S1) supporting an E±W trending mineral stretching lineation (L1) and associated with synschistose isoclinal, curvilinear folds (F1). Numerous kinematic indicators such as asymmetrical pressure-shadows, porphyroclast systems, and micro-shear bands (S±C structures) indicate a top-to-the-east shear along L1. This ®rst deformation was followed by another ductile event (D2) that produced a crenulation cleavage (S2) associated with westward overturned folds (F2), hence showing that the vergence of D2 is opposite to that of D1. Regionally, both D1 and D2 deformations have been identi®ed east and west of the Teloloapan unit, in the Arcelia volcanic rocks as well as in the Mexcala ¯ysch of Late Cretaceous age overlying the Guerrero± Morelos platform. This implies that all three units were deformed and thrust simultaneously, during the Late Cretaceous or Paleocene, prior to the deposition of the overlying, undeformed Eocene red beds of the Balsas group. q 2000 Elsevier Science B.V.. All rights reserved. Keywords: Ductile deformations; Microstructures; Laramide orogeny; Guerrero (Mexico)
1. Introduction The Paci®c margin of North America from Alaska to Central America, including the Caribbean islands, is currently considered a mosaic of exotic terranes that have been accreted to the North American continent during Mesozoic or early Tertiary times (Coney et al., 1980; Coney and Campa, 1987). These units have usually been described as volcano-sedimentary series lacking basement rocks. In Mexico, the Guerrero Terrane as de®ned by Campa and Coney (1983) is the largest representative of these units (Fig. 1A). From west to east, it comprises the Zihuatanejo, the Huetamo, the Arcelia and the Teloloapan units (Fig. 1B). The Teloloapan unit forms the eastern boundary of the Guerrero Terrane and was chosen for the present study because it is the unit that best shows the tectonic relationships with another terrane, here represented by the Guerrero±Morelos platform carbonates (ªMixteca terraneº). Numerous interpretations have been proposed * Corresponding author. Escuela Regional de Ciencias de la Tierra, Universidad Autonoma de Guerrero, Apartado Postal 197, Taxco (Gro) 40200, Mexico. E-mail address: [email protected] (J.C. Salinas-Prieto).
for the structural evolution of the Guerrero Terrane (Campa and Ramirez, 1979; de Cserna, 1983; Tolson, 1993). This paper, however, is the ®rst to present a systematic account of microstructural data in the central and eastern part of the Guerrero Terrane. In the ®rst regional descriptions of the area, all the schistose rocks were considered as Palaeozoic or PreCambrian regional basement (de Cserna, 1965; de Cserna et al., 1978). The existence of a Mesozoic volcanic arc was ®rst proposed by Campa et al. (1974) and Campa and Ramirez (1979), in whose interpretation an island arc was created upon an eastward facing subduction zone. Coney (1983) and Tardy et al. (1990) proposed a westward facing subduction zone under the Teloloapan arc. Recently Ramirez et al. (1991) associated the Zihuatanejo unit with the Huetamo unit, and the Arcelia unit with the Teloloapan unit, and suggested that both associations were accreted to the continent during the Late Cretaceous. In contrast Monod and Faure (1991), considered that the Teloloapan arc has a continental basement and that the ductile tectonics resulted in the closure of the Arcelia marginal basin during the Late Cretaceous and early Paleocene. It is currently believed that several distinct tectonic events affected the Teloloapan unit: during the Triassic
0895-9811/00/$ - see front matter q 2000 Elsevier Science B.V.. All rights reserved. PII: S 0895-981 1(00)00031-6
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Fig. 1. (A) Location of the Guerrero Terrane in SW Mexico. (from Campa and Coney, 1983). (B) Location of the Teloloapan arc within the Guerrero Terrane. The rectangle shows the studied region.
(de Cserna et al., 1978), the Late Jurassic (Campa et al., 1974; Campa and Ramirez, 1979), or the mid-Cretaceous (Tardy et al., 1986, 1992; Tolson, 1993), prior to the undisputed Laramide event (Late Cretaceous±Palaeocene).
In contrast to these interpretations, we suggest that the microstructures indicate a probably single period of ductile deformation during the Late Cretaceous or Palaeocene. The aim of this paper is a systematic microstructural
Fig. 2. Structural map (modi®ed from Ramirez et al., 1990) of the central part of the Teloloapan arc showing the direction and dip of S0/S1 planes, the direction of L1 lineation (black arrows indicate the shear direction), and F1 largest folds (around Teloloapan). On the cross-section (A±A 0 , same scale) are also indicated some S2 cleavage and F2 folds. In the lower part, the diagrams (equal area, lower hemisphere) illustrate the scattering of the F1 fold axes, mainly in the sedimentary cover of the Teloloapan arc. This contrasts with the steady E±W direction (N80±1008) of the L1 stretching lineation measured in the same formations.
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Fig. 3. Generalized lithostratigraphy of the Teloloapan arc and its sedimentary cover (modi®ed from Guerrero et al., 1990).
description and kinematic analysis of the Teloloapan volcanic rocks and sedimentary cover along a W±E section through the central part of the unit. The relative succession of tectonic events and their respective modes of deformation and
kinematics are established. A close comparison with deformation in the neighboring units (Arcelia unit and Guerrero± Morelos platform) sheds light on a possible mechanism of continental accretion in SW Mexico.
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Fig. 4. Micritic limestones with F1 isoclinal folding near Zacatlancillo. The F1 folds present curved hinges. The foliation surfaces, S1, are subparallel to the bedding planes, S0, and also to the F1 axial plane.
2. Geological setting of the Teloloapan unit The Teloloapan unit is the ®rst of the volcanic units situated west of the Guerrero±Morelos platform carbonates (Fig. 1). West of Teloloapan, andesitic±basaltic lava ¯ows and pillow lavas of calc-alkaline af®nity predominate (Talavera et al., 1990, 1992, 2000; Talavera, 1993) with intercalations of breccias, pyroclastic rocks, and rare siliceous sediments (Fig. 3). This magmatic complex overlies the thick Tejupilco schistose series northwest of the studied area and is conformably overlain by a calcareous sedimentary cover. Insuf®cient data have generated a controversy concerning the stratigraphic position of the Teloloapan volcanic complex. On the basis of unreliable radiometric data de Cserna (1983) ®rst considered the Tejupilco schists to be basement rocks of Permo-Triassic age. Subsequently Campa et al. (1974) correlated these schists
with Late Jurassic strata dated with ammonites which in fact belong to the Teloloapan volcanic arc. The sedimentary cover of the volcanic rocks consists of argillaceous silts and volcaniclastics of Tithonian(?) to Aptian age (Burckhard, 1927; Campa et al., 1974; Campa and Ramirez, 1979), followed by limestones and ¯ysch. In spite of noticeable changes in the limestone facies (Fig. 3), this disposition may be recognized from Tejupilco in the north to the Rio Balsas south of the studied area. East of Telolopapan, the upper volcaniclastic beds are intercalated with calcareous reef deposits containing rudists of Aptian age (Guerrero et al., 1991). A good marker is the uppermost limestone horizon, which yielded ammonites of latest Albian age (Monod et al., 2000). Above this calcareous sequence comes a thick ¯ysch deposit which is equated with the Mexcala ¯ysch (Late Cretaceous) that overlies the Guerrero±Morelos carbonates. In spite of
Fig. 5. Volcanoclastic material, 2 km NW of Acapetlahuaya. The stretching lineation (L1) is clearly expressed on a nearly horizontal S1 plane (seen from above) by the preferential orientation of phyllosilicates and elongated volcanic fragments. North is to the left.
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Fig. 6. Typical F1 isoclinal folds in the Mexcala ¯ysch; roadcut, 4 km south from Pachivia.
numerous tectonic disturbances (Fig. 2), normal stratigraphic relationships could be recognized between all formations within the Teloloapan unit (Ramirez et al., 1990). An angular unconformity separates the Teloloapan arc unit, the Arcelia unit, and the Guerrero Morelos platform from the almost undeformed red beds of the Balsas Group (Eocene) above. The two major thrusts bounding the Teloloapan unit are the Pachivia thrust and the Arcelia thrust (Fig. 2). Their orientation is roughly N±S over 130 km, with a westward dip of 50±708. No tectonic window was found through the Teloloapan unit. Other thrusts with a similar geometry may be seen within the Teloloapan unit near Zacatlancillo, El Najanjo, and Ahuacatitlan (Fig. 2). These regional structures are accompanied by widespread microstructures exhibiting contrasted folding styles and involving two schistosities and a stretching lineation. The analysis of these microstructures is necessary to establish the mechanisms that have led to the present structure of the Guerrero Terrane.
3. D1 deformation in the Teloloapan unit and adjacent units 3.1. General features of the D1 deformation The ®rst and strongest tectonic event (D1) is characterized by a planar and highly penetrative ductile cleavage (S1) that is best expressed in the volcaniclastic formations, in the micritic limestones, and in the ¯ysch facies. The strike of S1 is homogeneous (NE±SW), with a generally low dip angle that occasionally may reach 508 to the NW (Fig. 2). Typically, S1 is the axial plane of isoclinal folds (F1) that are abundant in the calcareous and volcaniclastic facies, so that on the outcrop, the S1 plane and the S0 bedding plane are mostly subparallel (Figs. 6±8). A conspicuous mineral and stretching lineation (L1) is regularly present on the S1 surface (Fig. 5). This lineation is clearly de®ned by the orientation of phyllosilicates (white mica and green chlorites), or actinote or calcite and quartz. L1 is also indicated by elongated fragments in limestone breccias, deformed
Fig. 7. Sketch of some typical F1 folds in ®ne, black calcarenites near Zacatlancillo. (A) Oblique section of a sheath fold showing the stretching lineation L1. (B) Curviplanar F1 fold of S0 showing that the direction of the stretching lineation L1 remains constant (N1008). See section A±A 0 in Fig. 2 for location of samples.
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Fig. 8. Sketches showing some ®eld examples of fold limbs with ªpinch and swellº structures (boudinage) and isolated hinges in alternating limestone and shale, SE of Acapetlahuaya (top) and south of El Pochote (bottom). Note that the S2 cleavage is sub-perpendicular to the S0/S1 plane. See section A±A 0 on Fig. 2 for location of samples.
pebbles in conglomerates, or deformed pillow lavas and rudist fragments. Synschistose folds (F1) range in amplitude from several meters down to millimeter scale and are most abundant in the volcaniclastic material. Although F1 fold axes remain nearly horizontal in most places, their direction appears highly dispersed, as shown in Fig. 2. This results from the abundance of curvilinear F1 folds that have been
measured along random sections in the ®eld. On the outcrop, isoclinal and curvilinear folds are most abundant in the sedimentary sequence (Figs. 4 and 6). Undisputable sheath folds are rare but, in some areas such as northeast of Zacatlancillo, a few circular sections (Fig. 7) typical of sheath folds may be seen (Quinquis et al., 1978; Faure and Malavieille, 1980; Lacassin and Mattauer, 1985).
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Thus, the F1 fold axes and L1 lineations present a contrasting disposition in the sedimentary cover of the Teloloapan arc volcanics. The strongly dispersed directions of the fold axes re¯ect the general curvilinear nature of the F1 folds at any scale, except for sheath folds which have their axes subparallel to L1. In contrast, the very stable WSW±ENE trend of the L1 stretching lineation demonstrates that it cannot be an earlier lineation that would have otherwise been dispersed by F1, but it must be cogenetic with the F1 folds. This disposition implies that F1 and L1 belong to the same progressive ductile deformation process (D1). Moreover, west of El Pochote near Los Aguajes, boudinage and isolated hinges are frequently observed in alternating black micrites and shales (Fig. 8) on the ¯anks of the F1 folds. The axes of the boudins (N160) are nearly perpendicular to L1 and indicate that stretching was greatest along the direction of L1 (Malavieille et al., 1984). Although weakly asymmetric, the relationship between S0 and S1 generally suggests an eastward overturning of the F1 folds (Fig. 6), as derived from the polarity of the Teloloapan sedimentary sequence. As we shall see, stronger evidence of the shear sense is given by the shape fabric of clasts in the volcanic and detritical formations. Differences in the rheological properties of each formation include differences in ductility (Gapais and Le Corre, 1981), which have resulted in the frequent detachment of the sedimentary cover from the volcanic arc, as evidenced by several thrusts such as those observed at El Najanjo, El Pochote, Zacatlancillo, or Ahuacatitlan (Fig. 2). In order to delineate the regional extent of D1 deformation, additional structural data were measured in the tectonic
units adjacent to the Teloloapan volcanic arc (Arcelia unit and Guerrero Morelos platform Fig. 1). To the west, the Arcelia unit is composed of ultrabasic rocks and basic volcanics of tholeiitic af®nities, followed by an important siliceous sedimentary cover (DaÂvila and Guerrero, 1990). According to recent radiolarian determinations by K. Ishida (in Salinas, 1994), an Early Cretaceous age may be attributed to the sedimentary cover of the Arcelia unit. The deformation of this unit, as seen near the Vicente Guerrero dam, is characterized by an axial planar subhorizontal S1 schistosity in the sedimentary rocks. A stretching lineation L1 (N70±N908) is well expressed in the volcanic rocks by streched pillow lavas and in the volcaniclastic rocks by abundant elongated clasts in the S1 plane. Numerous curvilinear isoclinal folds, F1, with subhorizontal axial planes are present in the sedimentary sequence. The ¯attening and oblique preferred orientation of the radiolaria, as well as the other kinematic criteria, indicate a top-to-the-east displacement of the unit (Salinas, 1994). We assume that this deformation is associated with thrusting of the Arcelia unit onto the Teloloapan unit. To the east, the Teloloapan unit is thrust upon the Guerrero±Morelos platform carbonates (ªMixteca Terraneº). This latter unit consists of argillaceous limestones of Aptian to Albian age (Campa and Ramirez, 1979), followed by thick reef limestone of Albian to Cenomanian age. It is normally succeeded by a ¯ysch formation of Turonian to Coniacian age (Mexcala ¯ysch; Fries, 1960; de Cserna, 1965). The eastward imbrication of the Mexcala ¯ysch with the platform carbonates may be seen clearly between Taxco and Teloloapan. Near Taxco, the Mexcala ¯ysch is thrust over volcaniclastic rocks and greenschists that are
Fig. 9. Some typical rotational criteria observed in XZ section. (A) Asymmetric pressure shadow of quartz and calcite around pyrite in a lava ¯ow, east of Almoloya. (B) Polycristalline aggregate of ªsigma typeº of feldspar and calcite in volcaniclastics, east of Teloloapan. (C) Dynamic recrystallization fabric in calcareous sandstone west of El Naranjo. (D) Pull-apart structure in a quartz crystal from a lava ¯ow, west of Zacatlancillo. The shear sense is top to the east in each case. Legend: pi pyrite; cl chlorite; cal calcite; feld feldspar; Q quartz.
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Fig. 10. Sketch showing the effect of pressure-solution in cataclastic carbonate rocks. The shear sense is ambiguous.
well known as the ªTaxco schistsº (Fries, 1960) and that have been correlated with the Teloloapan arc volcanics (Campa and Ramirez, 1979). Ten kilometers farther south, near Taxco Viejo, the Taxco schists are thrust upon the ¯ysch and the carbonates (Elias Herrera and Sanchez Zavalla, 1992). A subhorizontal schistosity, S1, and numerous isoclinal folds with dispersed directions are the prominent microstructures in the Mexcala ¯ysch and in the Taxco schists (Fig. 15). On the S1 surfaces, a stretching lineation, L1, is directed N70±N908, and kinematic indicators along L1 are consistent with a top-to-the-east displacement of both formations (Salinas, 1990, 1994). 3.2. Kinematics of the D1 deformation For the purpose of kinematic analysis, we assume that the plane of maximum ¯attening (XY) is the S1 schistosity
plane, and that the direction of maximum extension (X) is parallel to the L1 stretching direction (Flinn, 1965). In the XZ plane, (normal to S1 and containing L1), fossil fragments and clasts with pressure-shadows exhibit the strongest elongation, on the outcrops as well as in thin section. In the ®eld, a top-to-the-east shear is evidenced by the easterly dip of the C surfaces, whereas the S1 plane remains subhorizontal. In thin sections parallel to the XZ plane of the ellipsoid of ®nite deformation, many criteria are available to determine the sense of shear (cf. Simpson and Schmidt, 1983; Passchier and Simpson, 1986; Hanmer and Passcher, 1991), such as asymmetrical pressure-shadows of calcite, quartz, or chlorite (Fig. 9A), sigmoidal S/C fabrics (Fig. 9B), elongated shape fabrics of quartz grain in sandstone beds (Fig. 9C), or pull-apart fractures in quartz grains or pyroxenes (Fig. 9D). In the calcareous formations, however, numerous dissolution surfaces show apparent displacement
Fig. 11. D2 deformation. Sketch of volcaniclastic material near Almoloya, showing a D2 fold affecting S1 surfaces and the lineation L1. Development of a crenulation cleavage, S2, and intersection lineation, L1. The geometry of the folds and the tension gashes (dark grey) indicate a westward vergence for D2.
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Fig. 12. D2 deformation: The S1 foliation surfaces are folded into F2 folds with SW vergence. Note the S2 cleavage and the intersection lineation. Tuffs near Almoloya.
features that are not signi®cant as they are only the result of the loss of volume. In the Teloloapan unit, most of the shear criteria indicate a top-to-the-east displacement parallel to L1. A comparison of the most evolved ®gures of the pressure-shadows ¯anking pyrite cubes in the Teloloapan unit (Fig. 9A) with the computerized models of Etchecopar and Malavieille (1987) indicate comparable shapes for a simple shear of ®nal magnitude g 6 This value is compatible with the minimum shear strain required to obtain curvilinear folds and sheath folds according to theoretical models of Cobbold and Quinquis (1980). 3.3. Mechanisms of the D1 deformation Several deformation mechanisms have been identi®ed in thin section. In calcareous sediments, the most noticeable is the process of dissolution and transport-solution (pressure-
solution). This process is coeval with the neogenesis of phyllosilicates in the volcaniclastic rocks, in the lavas, and also in the limestones. Another process is the dynamic recrystallization of quartz and calcite, which is conspicuous in ®ne-grained calcareous sandstones owing to the shape of the grains (Fig. 9C). The development of phyllosilicates is best seen in pressure-shadow areas and within the S1 planes in the volcanic and volcaniclastic rocks. In calcareous thin sections, the abundance of insoluble minerals (oxides) in the schistosity planes suggests that the dominant process of deformation has been dissolution and transport-solution. Calcitic veins of various generations are strongly deformed in the oldest, whereas the more recent ones are not. The abundance of these veins suggests that the loss of volume by carbonate dissolution is somewhat compensated, and thus the change in volume during deformation may not be important. Pressure-solution
Fig. 13. D2 deformation. F2 fold with SW vergence. Development of S2 foliation. The F1 isoclinal folds are folded. Limestones SW of Acapetlahuaya.
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Fig. 14. D2 deformation. (A) In black shales W of El Pochote (PAE 1104), the D1 asymmetric pressure-shadows are distorted by micro-shear bands corresponding to the D2 deformation and indicating a westward displacement. (B) Some rotational criteria associated with D2 in sandstone west of El Naranjo (JS 12). Legend: cal calcite; Q quartz; cl chlorite; pi pyrite. Shear is to the west.
frequently alters the shape of the pressure-shadows and other shear criteria, thus obscuring the interpretation of the sense of shear (Fig. 10). For Groshong (1988), such mechanisms of deformation are typical of low temperatures, as the original textures of the rocks are preserved. The metamorphic paragenesis in the metabasites include chlorite, epidote, actinote, white mica, and sphene, as previously described regionally (Campa et al., 1974; de Cserna et al., 1978; Campa and Ramirez, 1979; Talavera, 1993). This mineral assemblage belongs to the lower greenschist facies and constrains the thermodynamic conditions of the D1 deformation between 250±4008 with low pressures.
3.4. D2 deformation in the Teloloapan unit and adjacent units The S0/S1 planar surfaces are deformed by a later tectonic event (D2), which is de®ned by a crenulation cleavage (S2) associated with large, asymmetric folds (F2) that are widespread in the area from Arcelia to Taxco (Salinas, 1994) (Figs. 2 and 15). The strike of S2 is fairly constant (N140±N1708), with dips from 208E to vertical. The F2 folds often present subhorizontal axes (N1708) and, surprisingly, are overturned westward, which is the opposite of D1 (Fig. 11). This is clearly visible, for instance, in volcaniclastic silts at the entrance to the village of
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Fig. 15. Diagrams A and B (equal area, lower hemisphere) show the scattering of F1 fold axes and the steady direction of L1 stretching lineation in the Arcelia unit (A) and in the Mexcala ¯ysch of the Guerrero±Morelos platform (B). As observed in the Teloloapan unit, the direction of the F2 folds axes is constant (NW±SE) and the vergence of D2 is towards the SW in both units.
Almoloya (Fig. 12). It is not unusual to observe isoclinal F1 folds that are refolded by the larger F2 folds (Fig. 13). In thin section, the S0/S1 surface is folded asymmetrically, with a stretched ¯ank and occasional microshear bands
(Fig. 14A). The angle between the shear bands and the S0/S1 surface is about 308, dipping westward. Microfolds and shear bands clearly overprint the D1 pressure-shadows (Fig. 14A). The sense of motion associated with D2 may be
Fig. 16. Two idealized structural sketches of Teloloapan unit showing the general geometry of the D1 structures (A) and the superimposed D2 structures (B) with opposite vergence. The numbers indicate the places where the actual structures may be situated in this interpretation.
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inferred from rare s-type porphyroclasts (Fig. 14B) and also from the shape of tension gashes in limestones and in volcaniclastic rocks, as seen near Almoloya (Fig. 11) and Zacatlancillo. In both cases, the motion is westward, in agreement with the vergence of the F2 folds. The absence of insoluble minerals in the microshear bands suggests that pressure-solution was not the dominant mechanism of D2, and hence that the rock volume did not change signi®cantly. For Platt and Vissers (1980), the mechanism of deformation related to such conditions is mainly of a cataclastic and crystalloplastic nature, with reduction of grain size at low temperature. Regionally, in the Arcelia unit, the presence F1 folds that are refolded by larger, westward-overturned folds clearly denotes the D2 deformation event. Associated with the F2 folds, a crenulation cleavage (S2) dips weakly (108) eastward (Fig. 15). Thus, as in the Teloloapan unit, the initial eastward thrusting of the Arcelia unit was followed by a westward D2 backthrusting. Farther east, in the region of Taxco, the S0/S1 surfaces in the Taxco schists, as well as in the Late Cretaceous Mexcala ¯ysch, are also deformed into large, asymmetric folds verging westward, as exempli®ed in Arroyo Hondo near Taxco Viejo and also north of Taxco City (Loma Linda) (Salinas, 1994). Thus, the Guerrero± Morelos carbonate and ¯ysch unit has also undergone the same D1 and D2 ductile deformations, of opposite vergence, as the Teloloapan and Arcelia units. In a wider area within the Guerrero Terrane, a completely different structural picture is provided by the Huetamo and Zihuatanejo units (Fig. 1) in which no ductile tectonics is recorded in the Lower Cretaceous volcaniclastic and sedimentary formations, but only broadly open folds (Salinas, 1994). 4. Discussion and conclusions The timing of the D1 and D2 deformations is constrained by the youngest ages found in the deformed units (AlboCenomanian in Arcelia and Teloloapan units and Late Cretaceous in the Mexcala ¯ysch), and the earliest undeformed red beds of the overlying Balsas group (Eocene; de Cserna, 1965). The D1 microstuctures observed in the Teloloapan unit are consistent with a single ductile shear. The asymmetric shape of porphyroclasts denotes a noncoaxial eastward-verging regime on the outcrop that is coherent with other criteria. We assume that D1 deformation results from the thrusting of both magmatic units onto the carbonate platform, as suggested by the common D1 deformations recorded in the Arcelia unit, in the Teloloapan unit, and in the Mexcala ¯ysch of the Guerrero±Morelos unit. Differences in ductility and in strength of deformation have produced the F1 folds with curviplanar axis and the L1 folded stretching lineations depicted in the Teloloapan unit. These microstructures are not interpreted as successive events but in term of a progressive ductile shear. The
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synkinematic mineral associations are indicative of incipient greenschist facies conditions. As seen above, the second deformation (D2) de®ned in the Teloloapan unit is also present in the Arcelia volcanosedimentary sequence and in the Mexcala ¯ysch, and this suggests that all three units were deformed simultaneously (Salinas et al., 1992). D2 is interpreted as a non-coaxial ductile deformation that occurred soon after D1 but with the opposite vergence (Fig. 16). On a broader scale, the D1 deformation is the most penetrative one and it may be related to the closure of a marginal basin such as the Arcelia unit (Monod and Faure, 1991; Monod et al., 1994) during the Late Cretaceous and Paleocene. This led to the obduction of the Arcelia basic and ultrabasic rocks on top of the Teloloapan unit and the accretion of both units to the American platform. Several interpretations may be proposed for D2 (Fig. 16): After the thick piling of tectonic units produced by D1 on the gently westward-dipping surface of the Guerrero Morelos platform, the D2 deformation may result from a backthrusting of the nappe pile, in a direction opposite to that of the initial D1 thrusting. Another possibility considers the F2 folds as to result from a westward compressive event that is well recorded in the neighboring province of Puebla, but not yet interpreted. In any case, the overlying continental red beds of the (Eocene) Balsas group post-date both ductile tectonic events. Acknowledgements This work was supported by CNRS-UMR 6530, OrleÂans University and Universitad Autonoma de Guerrero-Escuela Regional de Ciercias du la Tierra. The authors are grateful to Gustavo Tolson and to an anonymous reviewer for many useful remarks which helped to improve this manuscript. References Burckhard, C., 1927. CefalopoÂdos del Jurasico Medio de Oaxaca y Guerrero. Inst. Geol., Univ. Nac. Mexico Bull. 47. Campa, M.F., Campos, M., Flores, R., Oviedo, R., 1974. La secuencia mesozoica vulcano-sedimentaria metamor®zada de Ixtapan de la Sal, MeÂxico-Teloloapan Guerrero. Soc. Geol. Mexicana Bull. 35, 7±28. Campa, M.F., Ramirez, J., 1979. La evolucioÂn geoloÂgica y la metalogeÂnesis del noroccidente de Guerrero. Univ. Autonom. Guerrero, Serie TeÂcnico-CientI®ca 1, 1±84. Campa, M.F., Coney, P.J., 1983. Tectonostratigraphic terranes and mineral resources distribution in Mexico. Can. J. Earth Sci. 20, 1040±1051. Coney, P.J., Jones, D.L., Monger, J.W., 1980. Cordilleran suspect terranes. Nature 288, 329±333. Coney, P.J., 1983. Un modelo tectoÂnico de MeÂxico y sus relaciones con AmeÂrica del Norte, AmeÂrica del Sur y el Caribe. Revista Inst. Mexicana Petrol. 25 (1), 6±15. Coney, P.J., Campa, M.F., 1987. Lithotectonic Terrane Map of Mexico. US Geological Survey Map MF 1874-D. Cobbold, P.R., Quinquis, H., 1980. Development of sheath folds in shear regimes. J. Struct. Geol. 2, 119±126.
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DaÂvila, V.M., Guerrero, M., 1990. Una edad basada en radiolarios para la secuencia volcanico-sedimentaria al oriente de Arcelia. Estado. de Guerrero. Abstracts, X Convencion Sociedad Geologica Mexicana, p. 83. de Cserna, Z., 1965. Reconocimiento geoloÂgico de la Sierra Madre del Sur de MeÂxico entre Chilpancingo y Acapulco, Estado de Guerrero. Inst. Geol., Univ. Nac. Mexico Bull. 62, 1±77. de Cserna, Z., Palacios, N., Pantoja, A., 1978. Relaciones de facies de las rocas CretaÂcicas en el noroeste de Guerrero y en areas colindantes de MeÂxico y MichoacaÂn. Inst. Geol., Univ. Nac. Mexico Bull. 2, 8±18. de Cserna, Z., 1983. ResuÂmen de la GeologIÂa de la Hoja Tejupilco. Estados de Guerrero, MeÂxico y MichoacaÂn: Texto Explicativo de la Hoja Tejupilco 14Q-g(9). Instituto de GeologIÂa, Universidad Nacional AutoÂnoma de MeÂxico, Carta GeoloÂgica de MeÂxico 18, scala. 1:100,000. Elias Herrera, M., Sanchez Zavalla, J.L., 1992Z. Tectonic implications of a mylonitic granite in the lower structural levels of the Tierra Caliente Complex (Guerrero Terrane), southern Mexico. Inst. Geol., Univ. Nac. AutoÂnom de MeÂxico, Revista 9, 113±125. Etchecopar, A., Malavieille, J., 1987. Computer models of pressure shadows: a method for strain measurement and shear-sense determination. J. Struct. Geol. 9 (5±6), 667±677. Faure, M., Malavieille, J., 1980. Les plis en fourreau du substratum de la nappe des schistes lustreÂs de Corse. Signi®cation cineÂmatique. Comptes Rendus Acad. Sci. (Paris) 290, 1349±1352. Flinn, D., 1965. On the symmetry principle and the deformation ellipsoid. Geol. Mag. 102, 36±45. Fries, C., 1960. Geologia del Estado de Morelos y Partes Adjacentes de MeÂxico y Guerrero, Region Central Meridional de MeÂxico. Inst. Geol., Univ. Nac. AutoÂnom. MeÂxico, Bull. 60, 236. Gapais, D., Le Corre, C., 1981. Processus de deÂformation aÁ basse tempeÂrature dans les argilo-siltites et des quartzites: effets de la lithologie et des conditions thermiques. Revue Geol. Dynam. Geog. Phys. 23, 203±210. Groshong Jr, R.H., 1988. Low-temperature deformation mechanisms and their interpretation. Geol. Soc. Am. Bull. 100, 1329±1360. Guerrero, M., Ramirez, J., Talavera, O., 1990. Estudio estratigra®co del arco volcaÂnico CretaÂcico inferior de Teloloapan. Guerrero. Abstracts, X Convencion Sociedad Geologica Mexicana, p. 67. Guerrero, M., Ramirez, J., Talavera, O., Campa, M.F., 1991. El desarrollo carbonatado del CretaÂcico inferior asociado al arco de Teloloapan, Noroccidente del Estado de Guerrero. Abstracts, ConvencioÂn sobre la EvolucioÂn Geologica de MeÂxico, Sociedad Mexicana de Mineralogia and Instituto de Geologia, Universidad Nacional AutoÂnoma de MeÂxico, p. 67. Hanmer, S., Passchier, C., 1991. Shear sense indicators: a review. Geol. Surv. Can., Paper 90-17, 1±72. Lacassin, R., Mattauer, M., 1985. Kilometre-scale sheath folds at Mattmark and implications for transport direction in the Alps. Nature 316, 739± 742. Malavieille, J., Lacassin, R., Mattauer, M., 1984. Signi®cation tectonique des lineÂations d'allongement dans les Alpes occidentales. Soc. GeÂol. France Bull. 26, 895±906. Monod, O., Busnardo, R., 2000. Late Albian ammonites from the carbonate cover of the Teloloapan arc volcanic rocks (Guenro State, Mexico) J. S. Am. Earth Sci. 13, 377±388 Monod, O., Faure, M., 1991. La tectoÂnica laramidica del sudoeste de MeÂxico: Cierre de una cuenca intra-arco (Arcelia) abierta en el Albiano-Cenomaniano en el arco continental Teloloapan-Zihuatanejo, Estado de Guerrero. Abstracts, ConvencioÂn sobre la EvolucioÂn Geologica de MeÂxico, Sociedad Mexicana de Mineralogia and Instituto de Geologia, Universidad Nacional AutoÂnoma de MeÂxico, pp. 117±118.
Monod, O., Faure, M., Salinas, J.C., 1994. Intra-arc opening and closure of a marginal sea: the case of the Guerrero terrane (SW Mexico). The Island Arc 3, 25±34. Passchier, C.W., Simpson, C., 1986. Porphyroclast systems as kinematic indicators. J. Struct. Geol. 8, 831±843. Platt, P.J., Vissers, R.L.M., 1980. Extensional structures in anisotropic rocks. J. Struct. Geol. 4, 397±410. Quinquis, H., Audren, C., Brun, J.P., Cobbold, P.R., 1978. Intense progressive shear in Ile de Groix blueschists and compatibility with subduction or obduction. Nature 273, 43±45. Ramirez, J., Talavera, O., Guerrero, M., Salinas, J.C., 1990. El Terreno Teloloapan, un arco oceanico(?) del CretaÂcico inferior. Abstracts, X Convencion Sociedad Geologica Mexicana, p. 52. Ramirez, J., Campa, M.F., Talavera, O., Guerrero, M., 1991. Caracterizacion de los arcos insulares de la Sierra Madre del Sur y sus implicaciones tectoÂnicas. Abstracts, Convencion Sociedad Geologica Mexicana, pp. 163±166. Salinas, J.C., 1990. CinemaÂtica de la deformacioÂn del arco CretaÂcico inferior de Teloloapan. Abstracts, X Convencion Sociedad Geologica Mexicana, p. 53. Salinas, J.C., Monod, O., Faure, M., Talavera, O., Guerrero, M., Ramirez, J., 1992. Nouvelles donneÂes sur les chevauchements laramides du sudouest du Mexique (Etat de Guerrero). Abstracts, 148 ReÂunion des sciences de la Terre (Toulouse, France), p. 138. Salinas, J.C., 1994. Etude Structurale du Sud-ouest Mexicain (Guerrero): Analyse Microtectonique des DeÂformations Ductiles du Tertiaire InfeÂrieur. TheÁse Universite d'OrleÂans, France, 230pp. Simpson, C., Schmidt, S., 1983. An evaluation of criteria to deduce the sense of movement in sheared rocks. Geol. Soc. Am. Bull. 94, 1281±1288. Talavera, O., 1993. Les Formations OrogeÂniques MeÂsozoIÈques de Guerrero (Mexique MeÂridional). TheÁse Universite de Grenoble, France, 462pp. Talavera, O., Ramirez, J., Lapierre, H., Monod, O., Campa, M. F., Tardy, M., 1990. The Albo-Aptian volcano-sedimentary calc-alkaline arc series of Teloloapan (S. Mexico): correlations with the contemporaneous Zihuatanejo sequence and geodynamic implications. Abstracts, 12th Geowissenschafte Latin Amerika Kolloquium. Talavera, O., Ramirez, J., Guerrero, M., Salinas, J.C., Campa, M.F., 1992. Petrology and geochemical af®nites of the Upper Jurassic±Lower Cretaceous island arc sequences of central-southern Mexico. Abstracts, 148 ReÂunion des sciences de la Terre (Toulouse, France), p. 147. Talavera, O., Lapierre, H., Ramirez, J., Guerrero, M., 2000. Low-grade progressive metamorphism in the Lower Cretaceous island-arc series of the Teloloapan Volcanic Belt, southern Mexico. J. S. Am. Earth Sci. 13, 000±000. Tardy, M., Carfantan, J.C., Rangin, C., 1986. Essai de syntheÁse sur la structure du Mexique. Soc. Geol. France Bull. 8, 1025±1031. Tardy, M., Lapierre, H., Talavera, O., Ortiz, E., Yta, M., Beck, C., 1990. Paleogeographic relations between Tethys and paleopaci®c during Mesozoic times evidenced by the tectono-magmatic features of the Late Jurassic±Early Cretaceous allochtonous arc of central-southern Mexico. Abstracts, 12 Geowissenschafte Latin Amerika Kolloquium. Tardy, M., Lapierre, H., Bourdier, J.L., Yta, M., Ortiz, E., Coulon, C., 1992. Intra-oceanic setting of the western Mexico Guerrero Terrane: implications for the Paci®c±Tethys geodynamic relationships during the Cretaceous. Inst. Geol., Univ. Nac. AutoÂnom. MeÂxico, Revista 10 (2), 118±127. Tolson, G., 1993. Structural geology and tectonic evolution of the Santa Rosa area, S.W. Mexico State, Mexico. Geo®s. Int. 32 (3), 397±413.
www.elsevier.nl/locate/jsames
Ductile deformations of opposite vergence in the eastern part of the Guerrero Terrane (SW Mexico) J.C. Salinas-Prieto a,b,*, O. Monod b, M. Faure b,c a
Escuela Regional de Ciencias de la Tierra, Universidad Autonoma de Guerrero, Apartado Postal 197, Taxco (Gro) 40200, Mexico b Institut de Sciences de la Terre, Universite d'OrleÂans, CNRS-UMR 6530, OrleÂans Cedex 2, F-45067 France c Institut Universitaire de France, France
Abstract The Teloloapan volcanic arc in SW Mexico represents the easternmost unit of the Guerrero Terrane. It is overthrust by the Arcelia volcanic unit and is thrust over the Guerrero±Morelos carbonate platform. These major structures result from two closely related tectonic events: ®rst, an eastward verging, ductile deformation (D1) characterized by an axial-plane schistosity (S1) supporting an E±W trending mineral stretching lineation (L1) and associated with synschistose isoclinal, curvilinear folds (F1). Numerous kinematic indicators such as asymmetrical pressure-shadows, porphyroclast systems, and micro-shear bands (S±C structures) indicate a top-to-the-east shear along L1. This ®rst deformation was followed by another ductile event (D2) that produced a crenulation cleavage (S2) associated with westward overturned folds (F2), hence showing that the vergence of D2 is opposite to that of D1. Regionally, both D1 and D2 deformations have been identi®ed east and west of the Teloloapan unit, in the Arcelia volcanic rocks as well as in the Mexcala ¯ysch of Late Cretaceous age overlying the Guerrero± Morelos platform. This implies that all three units were deformed and thrust simultaneously, during the Late Cretaceous or Paleocene, prior to the deposition of the overlying, undeformed Eocene red beds of the Balsas group. q 2000 Elsevier Science B.V.. All rights reserved. Keywords: Ductile deformations; Microstructures; Laramide orogeny; Guerrero (Mexico)
1. Introduction The Paci®c margin of North America from Alaska to Central America, including the Caribbean islands, is currently considered a mosaic of exotic terranes that have been accreted to the North American continent during Mesozoic or early Tertiary times (Coney et al., 1980; Coney and Campa, 1987). These units have usually been described as volcano-sedimentary series lacking basement rocks. In Mexico, the Guerrero Terrane as de®ned by Campa and Coney (1983) is the largest representative of these units (Fig. 1A). From west to east, it comprises the Zihuatanejo, the Huetamo, the Arcelia and the Teloloapan units (Fig. 1B). The Teloloapan unit forms the eastern boundary of the Guerrero Terrane and was chosen for the present study because it is the unit that best shows the tectonic relationships with another terrane, here represented by the Guerrero±Morelos platform carbonates (ªMixteca terraneº). Numerous interpretations have been proposed * Corresponding author. Escuela Regional de Ciencias de la Tierra, Universidad Autonoma de Guerrero, Apartado Postal 197, Taxco (Gro) 40200, Mexico. E-mail address: [email protected] (J.C. Salinas-Prieto).
for the structural evolution of the Guerrero Terrane (Campa and Ramirez, 1979; de Cserna, 1983; Tolson, 1993). This paper, however, is the ®rst to present a systematic account of microstructural data in the central and eastern part of the Guerrero Terrane. In the ®rst regional descriptions of the area, all the schistose rocks were considered as Palaeozoic or PreCambrian regional basement (de Cserna, 1965; de Cserna et al., 1978). The existence of a Mesozoic volcanic arc was ®rst proposed by Campa et al. (1974) and Campa and Ramirez (1979), in whose interpretation an island arc was created upon an eastward facing subduction zone. Coney (1983) and Tardy et al. (1990) proposed a westward facing subduction zone under the Teloloapan arc. Recently Ramirez et al. (1991) associated the Zihuatanejo unit with the Huetamo unit, and the Arcelia unit with the Teloloapan unit, and suggested that both associations were accreted to the continent during the Late Cretaceous. In contrast Monod and Faure (1991), considered that the Teloloapan arc has a continental basement and that the ductile tectonics resulted in the closure of the Arcelia marginal basin during the Late Cretaceous and early Paleocene. It is currently believed that several distinct tectonic events affected the Teloloapan unit: during the Triassic
0895-9811/00/$ - see front matter q 2000 Elsevier Science B.V.. All rights reserved. PII: S 0895-981 1(00)00031-6
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Fig. 1. (A) Location of the Guerrero Terrane in SW Mexico. (from Campa and Coney, 1983). (B) Location of the Teloloapan arc within the Guerrero Terrane. The rectangle shows the studied region.
(de Cserna et al., 1978), the Late Jurassic (Campa et al., 1974; Campa and Ramirez, 1979), or the mid-Cretaceous (Tardy et al., 1986, 1992; Tolson, 1993), prior to the undisputed Laramide event (Late Cretaceous±Palaeocene).
In contrast to these interpretations, we suggest that the microstructures indicate a probably single period of ductile deformation during the Late Cretaceous or Palaeocene. The aim of this paper is a systematic microstructural
Fig. 2. Structural map (modi®ed from Ramirez et al., 1990) of the central part of the Teloloapan arc showing the direction and dip of S0/S1 planes, the direction of L1 lineation (black arrows indicate the shear direction), and F1 largest folds (around Teloloapan). On the cross-section (A±A 0 , same scale) are also indicated some S2 cleavage and F2 folds. In the lower part, the diagrams (equal area, lower hemisphere) illustrate the scattering of the F1 fold axes, mainly in the sedimentary cover of the Teloloapan arc. This contrasts with the steady E±W direction (N80±1008) of the L1 stretching lineation measured in the same formations.
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Fig. 3. Generalized lithostratigraphy of the Teloloapan arc and its sedimentary cover (modi®ed from Guerrero et al., 1990).
description and kinematic analysis of the Teloloapan volcanic rocks and sedimentary cover along a W±E section through the central part of the unit. The relative succession of tectonic events and their respective modes of deformation and
kinematics are established. A close comparison with deformation in the neighboring units (Arcelia unit and Guerrero± Morelos platform) sheds light on a possible mechanism of continental accretion in SW Mexico.
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Fig. 4. Micritic limestones with F1 isoclinal folding near Zacatlancillo. The F1 folds present curved hinges. The foliation surfaces, S1, are subparallel to the bedding planes, S0, and also to the F1 axial plane.
2. Geological setting of the Teloloapan unit The Teloloapan unit is the ®rst of the volcanic units situated west of the Guerrero±Morelos platform carbonates (Fig. 1). West of Teloloapan, andesitic±basaltic lava ¯ows and pillow lavas of calc-alkaline af®nity predominate (Talavera et al., 1990, 1992, 2000; Talavera, 1993) with intercalations of breccias, pyroclastic rocks, and rare siliceous sediments (Fig. 3). This magmatic complex overlies the thick Tejupilco schistose series northwest of the studied area and is conformably overlain by a calcareous sedimentary cover. Insuf®cient data have generated a controversy concerning the stratigraphic position of the Teloloapan volcanic complex. On the basis of unreliable radiometric data de Cserna (1983) ®rst considered the Tejupilco schists to be basement rocks of Permo-Triassic age. Subsequently Campa et al. (1974) correlated these schists
with Late Jurassic strata dated with ammonites which in fact belong to the Teloloapan volcanic arc. The sedimentary cover of the volcanic rocks consists of argillaceous silts and volcaniclastics of Tithonian(?) to Aptian age (Burckhard, 1927; Campa et al., 1974; Campa and Ramirez, 1979), followed by limestones and ¯ysch. In spite of noticeable changes in the limestone facies (Fig. 3), this disposition may be recognized from Tejupilco in the north to the Rio Balsas south of the studied area. East of Telolopapan, the upper volcaniclastic beds are intercalated with calcareous reef deposits containing rudists of Aptian age (Guerrero et al., 1991). A good marker is the uppermost limestone horizon, which yielded ammonites of latest Albian age (Monod et al., 2000). Above this calcareous sequence comes a thick ¯ysch deposit which is equated with the Mexcala ¯ysch (Late Cretaceous) that overlies the Guerrero±Morelos carbonates. In spite of
Fig. 5. Volcanoclastic material, 2 km NW of Acapetlahuaya. The stretching lineation (L1) is clearly expressed on a nearly horizontal S1 plane (seen from above) by the preferential orientation of phyllosilicates and elongated volcanic fragments. North is to the left.
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Fig. 6. Typical F1 isoclinal folds in the Mexcala ¯ysch; roadcut, 4 km south from Pachivia.
numerous tectonic disturbances (Fig. 2), normal stratigraphic relationships could be recognized between all formations within the Teloloapan unit (Ramirez et al., 1990). An angular unconformity separates the Teloloapan arc unit, the Arcelia unit, and the Guerrero Morelos platform from the almost undeformed red beds of the Balsas Group (Eocene) above. The two major thrusts bounding the Teloloapan unit are the Pachivia thrust and the Arcelia thrust (Fig. 2). Their orientation is roughly N±S over 130 km, with a westward dip of 50±708. No tectonic window was found through the Teloloapan unit. Other thrusts with a similar geometry may be seen within the Teloloapan unit near Zacatlancillo, El Najanjo, and Ahuacatitlan (Fig. 2). These regional structures are accompanied by widespread microstructures exhibiting contrasted folding styles and involving two schistosities and a stretching lineation. The analysis of these microstructures is necessary to establish the mechanisms that have led to the present structure of the Guerrero Terrane.
3. D1 deformation in the Teloloapan unit and adjacent units 3.1. General features of the D1 deformation The ®rst and strongest tectonic event (D1) is characterized by a planar and highly penetrative ductile cleavage (S1) that is best expressed in the volcaniclastic formations, in the micritic limestones, and in the ¯ysch facies. The strike of S1 is homogeneous (NE±SW), with a generally low dip angle that occasionally may reach 508 to the NW (Fig. 2). Typically, S1 is the axial plane of isoclinal folds (F1) that are abundant in the calcareous and volcaniclastic facies, so that on the outcrop, the S1 plane and the S0 bedding plane are mostly subparallel (Figs. 6±8). A conspicuous mineral and stretching lineation (L1) is regularly present on the S1 surface (Fig. 5). This lineation is clearly de®ned by the orientation of phyllosilicates (white mica and green chlorites), or actinote or calcite and quartz. L1 is also indicated by elongated fragments in limestone breccias, deformed
Fig. 7. Sketch of some typical F1 folds in ®ne, black calcarenites near Zacatlancillo. (A) Oblique section of a sheath fold showing the stretching lineation L1. (B) Curviplanar F1 fold of S0 showing that the direction of the stretching lineation L1 remains constant (N1008). See section A±A 0 in Fig. 2 for location of samples.
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Fig. 8. Sketches showing some ®eld examples of fold limbs with ªpinch and swellº structures (boudinage) and isolated hinges in alternating limestone and shale, SE of Acapetlahuaya (top) and south of El Pochote (bottom). Note that the S2 cleavage is sub-perpendicular to the S0/S1 plane. See section A±A 0 on Fig. 2 for location of samples.
pebbles in conglomerates, or deformed pillow lavas and rudist fragments. Synschistose folds (F1) range in amplitude from several meters down to millimeter scale and are most abundant in the volcaniclastic material. Although F1 fold axes remain nearly horizontal in most places, their direction appears highly dispersed, as shown in Fig. 2. This results from the abundance of curvilinear F1 folds that have been
measured along random sections in the ®eld. On the outcrop, isoclinal and curvilinear folds are most abundant in the sedimentary sequence (Figs. 4 and 6). Undisputable sheath folds are rare but, in some areas such as northeast of Zacatlancillo, a few circular sections (Fig. 7) typical of sheath folds may be seen (Quinquis et al., 1978; Faure and Malavieille, 1980; Lacassin and Mattauer, 1985).
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Thus, the F1 fold axes and L1 lineations present a contrasting disposition in the sedimentary cover of the Teloloapan arc volcanics. The strongly dispersed directions of the fold axes re¯ect the general curvilinear nature of the F1 folds at any scale, except for sheath folds which have their axes subparallel to L1. In contrast, the very stable WSW±ENE trend of the L1 stretching lineation demonstrates that it cannot be an earlier lineation that would have otherwise been dispersed by F1, but it must be cogenetic with the F1 folds. This disposition implies that F1 and L1 belong to the same progressive ductile deformation process (D1). Moreover, west of El Pochote near Los Aguajes, boudinage and isolated hinges are frequently observed in alternating black micrites and shales (Fig. 8) on the ¯anks of the F1 folds. The axes of the boudins (N160) are nearly perpendicular to L1 and indicate that stretching was greatest along the direction of L1 (Malavieille et al., 1984). Although weakly asymmetric, the relationship between S0 and S1 generally suggests an eastward overturning of the F1 folds (Fig. 6), as derived from the polarity of the Teloloapan sedimentary sequence. As we shall see, stronger evidence of the shear sense is given by the shape fabric of clasts in the volcanic and detritical formations. Differences in the rheological properties of each formation include differences in ductility (Gapais and Le Corre, 1981), which have resulted in the frequent detachment of the sedimentary cover from the volcanic arc, as evidenced by several thrusts such as those observed at El Najanjo, El Pochote, Zacatlancillo, or Ahuacatitlan (Fig. 2). In order to delineate the regional extent of D1 deformation, additional structural data were measured in the tectonic
units adjacent to the Teloloapan volcanic arc (Arcelia unit and Guerrero Morelos platform Fig. 1). To the west, the Arcelia unit is composed of ultrabasic rocks and basic volcanics of tholeiitic af®nities, followed by an important siliceous sedimentary cover (DaÂvila and Guerrero, 1990). According to recent radiolarian determinations by K. Ishida (in Salinas, 1994), an Early Cretaceous age may be attributed to the sedimentary cover of the Arcelia unit. The deformation of this unit, as seen near the Vicente Guerrero dam, is characterized by an axial planar subhorizontal S1 schistosity in the sedimentary rocks. A stretching lineation L1 (N70±N908) is well expressed in the volcanic rocks by streched pillow lavas and in the volcaniclastic rocks by abundant elongated clasts in the S1 plane. Numerous curvilinear isoclinal folds, F1, with subhorizontal axial planes are present in the sedimentary sequence. The ¯attening and oblique preferred orientation of the radiolaria, as well as the other kinematic criteria, indicate a top-to-the-east displacement of the unit (Salinas, 1994). We assume that this deformation is associated with thrusting of the Arcelia unit onto the Teloloapan unit. To the east, the Teloloapan unit is thrust upon the Guerrero±Morelos platform carbonates (ªMixteca Terraneº). This latter unit consists of argillaceous limestones of Aptian to Albian age (Campa and Ramirez, 1979), followed by thick reef limestone of Albian to Cenomanian age. It is normally succeeded by a ¯ysch formation of Turonian to Coniacian age (Mexcala ¯ysch; Fries, 1960; de Cserna, 1965). The eastward imbrication of the Mexcala ¯ysch with the platform carbonates may be seen clearly between Taxco and Teloloapan. Near Taxco, the Mexcala ¯ysch is thrust over volcaniclastic rocks and greenschists that are
Fig. 9. Some typical rotational criteria observed in XZ section. (A) Asymmetric pressure shadow of quartz and calcite around pyrite in a lava ¯ow, east of Almoloya. (B) Polycristalline aggregate of ªsigma typeº of feldspar and calcite in volcaniclastics, east of Teloloapan. (C) Dynamic recrystallization fabric in calcareous sandstone west of El Naranjo. (D) Pull-apart structure in a quartz crystal from a lava ¯ow, west of Zacatlancillo. The shear sense is top to the east in each case. Legend: pi pyrite; cl chlorite; cal calcite; feld feldspar; Q quartz.
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Fig. 10. Sketch showing the effect of pressure-solution in cataclastic carbonate rocks. The shear sense is ambiguous.
well known as the ªTaxco schistsº (Fries, 1960) and that have been correlated with the Teloloapan arc volcanics (Campa and Ramirez, 1979). Ten kilometers farther south, near Taxco Viejo, the Taxco schists are thrust upon the ¯ysch and the carbonates (Elias Herrera and Sanchez Zavalla, 1992). A subhorizontal schistosity, S1, and numerous isoclinal folds with dispersed directions are the prominent microstructures in the Mexcala ¯ysch and in the Taxco schists (Fig. 15). On the S1 surfaces, a stretching lineation, L1, is directed N70±N908, and kinematic indicators along L1 are consistent with a top-to-the-east displacement of both formations (Salinas, 1990, 1994). 3.2. Kinematics of the D1 deformation For the purpose of kinematic analysis, we assume that the plane of maximum ¯attening (XY) is the S1 schistosity
plane, and that the direction of maximum extension (X) is parallel to the L1 stretching direction (Flinn, 1965). In the XZ plane, (normal to S1 and containing L1), fossil fragments and clasts with pressure-shadows exhibit the strongest elongation, on the outcrops as well as in thin section. In the ®eld, a top-to-the-east shear is evidenced by the easterly dip of the C surfaces, whereas the S1 plane remains subhorizontal. In thin sections parallel to the XZ plane of the ellipsoid of ®nite deformation, many criteria are available to determine the sense of shear (cf. Simpson and Schmidt, 1983; Passchier and Simpson, 1986; Hanmer and Passcher, 1991), such as asymmetrical pressure-shadows of calcite, quartz, or chlorite (Fig. 9A), sigmoidal S/C fabrics (Fig. 9B), elongated shape fabrics of quartz grain in sandstone beds (Fig. 9C), or pull-apart fractures in quartz grains or pyroxenes (Fig. 9D). In the calcareous formations, however, numerous dissolution surfaces show apparent displacement
Fig. 11. D2 deformation. Sketch of volcaniclastic material near Almoloya, showing a D2 fold affecting S1 surfaces and the lineation L1. Development of a crenulation cleavage, S2, and intersection lineation, L1. The geometry of the folds and the tension gashes (dark grey) indicate a westward vergence for D2.
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Fig. 12. D2 deformation: The S1 foliation surfaces are folded into F2 folds with SW vergence. Note the S2 cleavage and the intersection lineation. Tuffs near Almoloya.
features that are not signi®cant as they are only the result of the loss of volume. In the Teloloapan unit, most of the shear criteria indicate a top-to-the-east displacement parallel to L1. A comparison of the most evolved ®gures of the pressure-shadows ¯anking pyrite cubes in the Teloloapan unit (Fig. 9A) with the computerized models of Etchecopar and Malavieille (1987) indicate comparable shapes for a simple shear of ®nal magnitude g 6 This value is compatible with the minimum shear strain required to obtain curvilinear folds and sheath folds according to theoretical models of Cobbold and Quinquis (1980). 3.3. Mechanisms of the D1 deformation Several deformation mechanisms have been identi®ed in thin section. In calcareous sediments, the most noticeable is the process of dissolution and transport-solution (pressure-
solution). This process is coeval with the neogenesis of phyllosilicates in the volcaniclastic rocks, in the lavas, and also in the limestones. Another process is the dynamic recrystallization of quartz and calcite, which is conspicuous in ®ne-grained calcareous sandstones owing to the shape of the grains (Fig. 9C). The development of phyllosilicates is best seen in pressure-shadow areas and within the S1 planes in the volcanic and volcaniclastic rocks. In calcareous thin sections, the abundance of insoluble minerals (oxides) in the schistosity planes suggests that the dominant process of deformation has been dissolution and transport-solution. Calcitic veins of various generations are strongly deformed in the oldest, whereas the more recent ones are not. The abundance of these veins suggests that the loss of volume by carbonate dissolution is somewhat compensated, and thus the change in volume during deformation may not be important. Pressure-solution
Fig. 13. D2 deformation. F2 fold with SW vergence. Development of S2 foliation. The F1 isoclinal folds are folded. Limestones SW of Acapetlahuaya.
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Fig. 14. D2 deformation. (A) In black shales W of El Pochote (PAE 1104), the D1 asymmetric pressure-shadows are distorted by micro-shear bands corresponding to the D2 deformation and indicating a westward displacement. (B) Some rotational criteria associated with D2 in sandstone west of El Naranjo (JS 12). Legend: cal calcite; Q quartz; cl chlorite; pi pyrite. Shear is to the west.
frequently alters the shape of the pressure-shadows and other shear criteria, thus obscuring the interpretation of the sense of shear (Fig. 10). For Groshong (1988), such mechanisms of deformation are typical of low temperatures, as the original textures of the rocks are preserved. The metamorphic paragenesis in the metabasites include chlorite, epidote, actinote, white mica, and sphene, as previously described regionally (Campa et al., 1974; de Cserna et al., 1978; Campa and Ramirez, 1979; Talavera, 1993). This mineral assemblage belongs to the lower greenschist facies and constrains the thermodynamic conditions of the D1 deformation between 250±4008 with low pressures.
3.4. D2 deformation in the Teloloapan unit and adjacent units The S0/S1 planar surfaces are deformed by a later tectonic event (D2), which is de®ned by a crenulation cleavage (S2) associated with large, asymmetric folds (F2) that are widespread in the area from Arcelia to Taxco (Salinas, 1994) (Figs. 2 and 15). The strike of S2 is fairly constant (N140±N1708), with dips from 208E to vertical. The F2 folds often present subhorizontal axes (N1708) and, surprisingly, are overturned westward, which is the opposite of D1 (Fig. 11). This is clearly visible, for instance, in volcaniclastic silts at the entrance to the village of
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Fig. 15. Diagrams A and B (equal area, lower hemisphere) show the scattering of F1 fold axes and the steady direction of L1 stretching lineation in the Arcelia unit (A) and in the Mexcala ¯ysch of the Guerrero±Morelos platform (B). As observed in the Teloloapan unit, the direction of the F2 folds axes is constant (NW±SE) and the vergence of D2 is towards the SW in both units.
Almoloya (Fig. 12). It is not unusual to observe isoclinal F1 folds that are refolded by the larger F2 folds (Fig. 13). In thin section, the S0/S1 surface is folded asymmetrically, with a stretched ¯ank and occasional microshear bands
(Fig. 14A). The angle between the shear bands and the S0/S1 surface is about 308, dipping westward. Microfolds and shear bands clearly overprint the D1 pressure-shadows (Fig. 14A). The sense of motion associated with D2 may be
Fig. 16. Two idealized structural sketches of Teloloapan unit showing the general geometry of the D1 structures (A) and the superimposed D2 structures (B) with opposite vergence. The numbers indicate the places where the actual structures may be situated in this interpretation.
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inferred from rare s-type porphyroclasts (Fig. 14B) and also from the shape of tension gashes in limestones and in volcaniclastic rocks, as seen near Almoloya (Fig. 11) and Zacatlancillo. In both cases, the motion is westward, in agreement with the vergence of the F2 folds. The absence of insoluble minerals in the microshear bands suggests that pressure-solution was not the dominant mechanism of D2, and hence that the rock volume did not change signi®cantly. For Platt and Vissers (1980), the mechanism of deformation related to such conditions is mainly of a cataclastic and crystalloplastic nature, with reduction of grain size at low temperature. Regionally, in the Arcelia unit, the presence F1 folds that are refolded by larger, westward-overturned folds clearly denotes the D2 deformation event. Associated with the F2 folds, a crenulation cleavage (S2) dips weakly (108) eastward (Fig. 15). Thus, as in the Teloloapan unit, the initial eastward thrusting of the Arcelia unit was followed by a westward D2 backthrusting. Farther east, in the region of Taxco, the S0/S1 surfaces in the Taxco schists, as well as in the Late Cretaceous Mexcala ¯ysch, are also deformed into large, asymmetric folds verging westward, as exempli®ed in Arroyo Hondo near Taxco Viejo and also north of Taxco City (Loma Linda) (Salinas, 1994). Thus, the Guerrero± Morelos carbonate and ¯ysch unit has also undergone the same D1 and D2 ductile deformations, of opposite vergence, as the Teloloapan and Arcelia units. In a wider area within the Guerrero Terrane, a completely different structural picture is provided by the Huetamo and Zihuatanejo units (Fig. 1) in which no ductile tectonics is recorded in the Lower Cretaceous volcaniclastic and sedimentary formations, but only broadly open folds (Salinas, 1994). 4. Discussion and conclusions The timing of the D1 and D2 deformations is constrained by the youngest ages found in the deformed units (AlboCenomanian in Arcelia and Teloloapan units and Late Cretaceous in the Mexcala ¯ysch), and the earliest undeformed red beds of the overlying Balsas group (Eocene; de Cserna, 1965). The D1 microstuctures observed in the Teloloapan unit are consistent with a single ductile shear. The asymmetric shape of porphyroclasts denotes a noncoaxial eastward-verging regime on the outcrop that is coherent with other criteria. We assume that D1 deformation results from the thrusting of both magmatic units onto the carbonate platform, as suggested by the common D1 deformations recorded in the Arcelia unit, in the Teloloapan unit, and in the Mexcala ¯ysch of the Guerrero±Morelos unit. Differences in ductility and in strength of deformation have produced the F1 folds with curviplanar axis and the L1 folded stretching lineations depicted in the Teloloapan unit. These microstructures are not interpreted as successive events but in term of a progressive ductile shear. The
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synkinematic mineral associations are indicative of incipient greenschist facies conditions. As seen above, the second deformation (D2) de®ned in the Teloloapan unit is also present in the Arcelia volcanosedimentary sequence and in the Mexcala ¯ysch, and this suggests that all three units were deformed simultaneously (Salinas et al., 1992). D2 is interpreted as a non-coaxial ductile deformation that occurred soon after D1 but with the opposite vergence (Fig. 16). On a broader scale, the D1 deformation is the most penetrative one and it may be related to the closure of a marginal basin such as the Arcelia unit (Monod and Faure, 1991; Monod et al., 1994) during the Late Cretaceous and Paleocene. This led to the obduction of the Arcelia basic and ultrabasic rocks on top of the Teloloapan unit and the accretion of both units to the American platform. Several interpretations may be proposed for D2 (Fig. 16): After the thick piling of tectonic units produced by D1 on the gently westward-dipping surface of the Guerrero Morelos platform, the D2 deformation may result from a backthrusting of the nappe pile, in a direction opposite to that of the initial D1 thrusting. Another possibility considers the F2 folds as to result from a westward compressive event that is well recorded in the neighboring province of Puebla, but not yet interpreted. In any case, the overlying continental red beds of the (Eocene) Balsas group post-date both ductile tectonic events. Acknowledgements This work was supported by CNRS-UMR 6530, OrleÂans University and Universitad Autonoma de Guerrero-Escuela Regional de Ciercias du la Tierra. The authors are grateful to Gustavo Tolson and to an anonymous reviewer for many useful remarks which helped to improve this manuscript. References Burckhard, C., 1927. CefalopoÂdos del Jurasico Medio de Oaxaca y Guerrero. Inst. Geol., Univ. Nac. Mexico Bull. 47. Campa, M.F., Campos, M., Flores, R., Oviedo, R., 1974. La secuencia mesozoica vulcano-sedimentaria metamor®zada de Ixtapan de la Sal, MeÂxico-Teloloapan Guerrero. Soc. Geol. Mexicana Bull. 35, 7±28. Campa, M.F., Ramirez, J., 1979. La evolucioÂn geoloÂgica y la metalogeÂnesis del noroccidente de Guerrero. Univ. Autonom. Guerrero, Serie TeÂcnico-CientI®ca 1, 1±84. Campa, M.F., Coney, P.J., 1983. Tectonostratigraphic terranes and mineral resources distribution in Mexico. Can. J. Earth Sci. 20, 1040±1051. Coney, P.J., Jones, D.L., Monger, J.W., 1980. Cordilleran suspect terranes. Nature 288, 329±333. Coney, P.J., 1983. Un modelo tectoÂnico de MeÂxico y sus relaciones con AmeÂrica del Norte, AmeÂrica del Sur y el Caribe. Revista Inst. Mexicana Petrol. 25 (1), 6±15. Coney, P.J., Campa, M.F., 1987. Lithotectonic Terrane Map of Mexico. US Geological Survey Map MF 1874-D. Cobbold, P.R., Quinquis, H., 1980. Development of sheath folds in shear regimes. J. Struct. Geol. 2, 119±126.
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