Ductile deformations of opposite vergence in the eastern part of the

Transcription

Journal of South American Earth Sciences 13 (2000) 389±402
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, UniversiteÂ 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
390
J.C. Salinas-Prieto et al. / Journal of South American Earth Sciences 13 (2000) 389±402
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.
391
392
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.
393
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.
394
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.
395
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).
396
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.
397
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.
398
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.
399
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
400
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.
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
401
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-CientIÂ®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.
402
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 estratigraÂ®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 UniversiteÂ 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 UniversiteÂ 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.

Ductile deformations of opposite vergence in the eastern part of the

Transcription

Similar documents

How to Tie a Woggle

RÉSoNANSS: a regional contribution to

Real Time Control of Hand Prosthesis Using Surface EMG

PrimeFilm 1800 AFL Low-Cost 35mm Slide/Film Scanning Through

Menetriers Disease : Often Heard But Seldom Seen !

IC-910 SDR Mod

The new RS-35M, June, 2010 - Stu Martin, [email protected]

ECO (1) Bed Frame Instructions

BALTICA 10

MODEL 324 ADDENDUM Please make the ,following changes in