malone2002

Journal of South American Earth Sciences 15 (2002) 511–524
www.elsevier.com/locate/jsames
Deformational history of part of the Acatlán Complex: Late Ordovician –
Early Silurian and Early Permian orogenesis in southern Mexico
J.R. Malonea, R.D. Nancea,*, J.D. Keppieb, J. Dostalc
b
a
Department of Geological Sciences, Ohio University, Athens, OH 45701-2979, USA
Instituto de Geologı́a, Universidad Nacional Autónoma de México, Mexico DF 04510, Mexico
c
Department of Geology, St Mary’s University, Halifax, Nova Scotia, Canada B3H 3C3
Received 1 July 2001; accepted 1 December 2001
Abstract
The Paleozoic Acatlán Complex of southern Mexico comprises polydeformed metasedimentary, granitoid, and mafic– ultramafic rocks
variously interpreted as recording the closure of the Iapetus, Rheic, and Ouachitan Oceans. The complex is tectonically juxtaposed on its
eastern margin against Grenville-age gneisses (Oaxacan Complex) that are unconformably overlain by Lower Paleozoic strata containing
fossils of Gondwanan affinity. A thick siliciclastic unit (Chazumba and Cosoltepec Formations) at the base of the complex is considered part
of a Lower Paleozoic accretionary prism with a provenance that isotopically resembles the Oaxacan Complex. This unit is tectonically
overridden by a locally eclogitic mafic – ultramafic unit interpreted as a westward-obducted ophiolite, the emplacement of which was
synchronous with mylonitic granitoid intrusion at ca. 440 Ma. Both units are unconformably overlain by a deformed volcano-sedimentary
sequence (Tecomate Formation) attributed to a volcanic arc of presumed Devonian age. Deformed granitoids in contact with this sequence
have been dated at ca. 371 (La Noria granite) and 287 Ma (Totoltepec pluton).
Three phases of penetrative deformation (D1 – 3) affect the Cosoltepec Formation; the last two correlate with two penetrative deformational
phases that affect the Tecomate Formation. D1 is of unknown kinematics but predates deposition of the Tecomate Formation and likely
records obduction at ca. 440 Ma (Acatecan orogeny). A folded foliation in the Totoltepec pluton appears to record both deformational phases
in the Tecomate Formation, bracketing D2 and D3 between 287 Ma and the deposition of the nonconformably overlying Leonardian Matzitzi
Formation.
D2 records north– south dextral transpression and south-vergent thrusting and is attributed to the collision of Gondwana and southern
Laurentia (Ouachitan orogeny) at ca. 290 Ma, the kinematics being consistent with the northward motion of Mexico that is required by most
continental reconstructions for the final assembly of Pangea. D3, which produced broadly north– south, upright folds, is also attributed to this
collision and likely followed D2 closely in the latest Paleozoic. q 2002 Elsevier Science Ltd. All rights reserved.
Keywords: Acatlán Complex; Paleozoic orogenesis; Totoltepec pluton
1. Introduction
The Acatlán Complex of southern Mexico has been
interpreted as the vestige of a Lower Paleozoic ocean that
closed during the Late Ordovician– Early Silurian (Ortega-Gutiérrez, 1993; Ortega-Gutiérrez et al., 1999). Comprising a repeatedly deformed assemblage of Paleozoic
metasedimentary rocks, granitoid bodies, and metamorphosed mafic –ultramafic units that form the basement of the
Mixteco terrane, the complex is tectonically juxtaposed
against Grenville-age (ca. 1 Ga) gneisses of the granulite
* Corresponding author. Tel.: þ 1-740-593-1107; fax: þ1-740-593-0486.
E-mail address: [email protected] (R.D. Nance).
facies Oaxacan Complex at its eastern margin along the
north– south Caltepec fault zone (Fig. 1). Lower Paleozoic
rocks that nonconformably overlie the Oaxacan Complex
are unmetamorphosed and contain fossil taxa of Gondwanan
affinity (Robison and Pantoja-Alor, 1968).
On the basis of its tectonostratigraphic similarity to parts
of the Appalachian orogen and its close proximity to the
Grenville-age gneisses of the Oaxacan Complex, the
Acatlán Complex has been interpreted in terms of
Laurentia – Gondwana collision (Yañez et al., 1991; Ortega-Gutiérrez et al., 1999; Keppie and Ramos, 1999).
However, the paleogeographic position of the Acatlán
Complex during the Paleozoic is uncertain, and several
models have been proposed to account for its tectonic
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J.R. Malone et al. / Journal of South American Earth Sciences 15 (2002) 511–524
Fig. 1. Location and tectonostratigraphic setting of the Acatlán Complex in southern Mexico (from Ortega-Gutiérrez et al., 1999). Index map shows Cenozoic
volcanic rocks of the Sierra Madre Occidental (SMOCC) and Trans-Mexican volcanic belt (TMVB).
evolution. For example, Yañez et al. (1991) related the
complex to the Acadian belt of the Appalachians,
suggesting that both were deformed in a latest Silurian –
Middle Devonian collision (Mixtecan orogeny) between
eastern Laurentia and northwestern South America. According to Yañez et al. (1991), the Acatlán Complex was then
transported southward with Gondwana to a position near
present-day Colombia and recollided with southern Laurentia during the late Carboniferous before moving to its
present position following the breakup of Pangea.
On the basis of new age data interpreted to indicate an
earliest Silurian date for its deformation, Ortega-Gutiérrez
et al. (1999) proposed that the Acatlán Complex represents a
vestige of the Iapetus suture formed during a Late
Ordovician – Early Silurian collision (Acatecan orogeny)
between eastern Laurentia and Oaxaquia, a crustal fragment
of Grenville-age in present-day Mexico (Ortega-Gutiérrez
et al., 1995). In this context, Oaxaquia represents either a
microcontinent or part of the Columbian margin of
Gondwana. In contrast, Keppie and Ramos (1999) proposed
that the Acatlán Complex lay adjacent to northwestern
South America throughout the Paleozoic on the southern
margin of the Rheic Ocean and was not involved in
continent – continent collision until the Permo-Carboniferous assembly of Pangea.
These contrasting tectonic models are based largely on
the pioneering regional mapping of the Acatlán Complex by
Ortega-Gutiérrez (1975) and are currently being tested as
aspects of the complex are reexamined in more detail.
Recent geochronological studies have focused on the ages
of the main tectonothermal events. For example,
Ortega-Gutiérrez et al. (1999) obtained an age of ca.
440 Ma for the Acatecan collision (originally thought to be
Early Devonian) by dating a granitoid body that they
interpret as syntectonic. Similarly, by interpreting the
emplacement of a younger granitoid body as synchronous
with the Mixtecan orogeny, Sánchez Zavala et al. (2000)
date this event at ca. 370 Ma (Yañez et al., 1991).
This paper examines the deformational and kinematic
history of the Acatlán Complex by focusing on the structural
J.R. Malone et al. / Journal of South American Earth Sciences 15 (2002) 511–524
513
Fig. 2. Simplified geological map and cross-section of the northern part of the Acatlán Complex. Outlined region shows study area and its division into
structural subareas (modified from Ortega-Gutiérrez et al., 1999).
history of two of its principal metasedimentary units, the
Cosoltepec Formation (deposited prior to the proposed Late
Ordovician –Early Silurian collision) and the Tecomate
Formation (deposited following this event). Our results
show that the complex has experienced three periods of
penetrative deformation, one of which is assigned to the
Late Ordovician –Early Silurian Acatecan event, whereas
the other two are of Early Permian age and synchronous
with the Ouachitan orogeny of the southern United States
and an unnamed orogenic belt in northwestern South
America.
2. Acatlán complex
Understanding of the geology of the Acatlán Complex
(Fig. 2) is based largely on the doctoral work of
Ortega-Gutiérrez (1975), who identified its principal
lithologic units and recognized within it two major thrust
slices unconformably overlain by a sequence of deformed
metasedimentary rocks. Ortega-Gutiérrez (1993) later
assigned the rocks of the lower and upper plates to the
Petlalcingo and Acateco subgroups, respectively, and
interpreted the lower as an obducted accretionary prism
and the upper plate as an ophiolitic sequence.
2.1. Petlalcingo subgroup
The lower plate of the thrust nappe, or the Petlalcingo
subgroup (Fig. 3), comprises a thick sequence of siliciclastic
rocks derived from a Grenville-age provenance that
isotopically resembles the Oaxacan Complex (Yañez et al.,
1991). The subgroup is divided into two formations: a lower
Chazumba Formation of alternating psammitic and pelitic
rocks and an upper Cosoltepec Formation dominated by
quartzose phyllites. Metamorphosed to the amphibolite
facies, the Chazumba Formation is dominated by quartzrich biotite schists that contain occasional garnet, staurolite,
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J.R. Malone et al. / Journal of South American Earth Sciences 15 (2002) 511–524
Fig. 3. Simplified tectonostratigraphic column for the Acatlán Complex (modified from Ortega-Gutiérrez et al., 1999).
and sillimanite and, at the highest grade, are transitional
with the underlying Magdalena migmatite.
The Chazumba Formation is overlain by the Cosoltepec
Formation, which makes up more than 90% of the exposed
Acatlán Complex. The Cosoltepec Formation is not thought
to have experienced more than greenschist facies metamorphism and is dominated by phyllitic quartz –chlorite –
phengite schists with occasional retrogressed biotite and
abundant quartz veins. Other components locally include
pillowed greenstones, metachert, massive quartzite, serpentinite, and manganiferous rocks (Ortega-Gutiérrez, 1993).
The nature of the contact between the Cosoltepec and
Chazumba Formations is uncertain, though its association
with amphibolite bodies suggests it may be tectonic.
On the basis of its siliciclastic composition, extreme
thickness, and the presence of inferred ocean floor
fragments, Ortega-Gutiérrez et al. (1999) interpreted the
Petlalcingo subgroup as the parautochthonous trench and
forearc deposits of a convergent continental margin. The
Magdalena migmatite, formerly thought to form the base of
the lower plate (Fig. 3), has recently yielded a concordant
U – Pb crystallization age of 170 ^ 2 Ma and is now
attributed to Jurassic extension (Powell et al., 1999) coeval
with the posttectonic emplacement of the granitic San
Miguel dikes at 173 ^ 0.3 Ma (Rb/Sr whole-rock isochron;
Ruiz-Castellanos, 1979).
2.2. Acateco subgroup
The upper plate of the thrust nappe, or the Acateco
subgroup (Fig. 3), consists of high-grade, mafic – ultramafic
and interlayered pelitic and siliceous metasedimentary
rocks (Xayacatlán Formation) that are structurally overlain
by high-pressure metagranitoids and migmatites (Esperanza
granitoids). The Xayacatlán Formation includes micaceous
schists, gneisses, porphyroblastic amphibolites, sepentinites, and related ultramafics that are commonly mylonitic
and locally preserve relict eclogite facies mineral assemblages (Ortega-Gutiérrez, 1975, 1993. The sheet-like
mylonitic Esperanza granitoids comprise megacrystic Kfeldspar augen gneiss, migmatite, schist, and minor
amphibolite. On the basis of a mineralogy that includes
high-silica phengite, grossular-rich garnet, pseudomorphs of
zoisite or epidote þ phengite þ albite ^ garnet after plagioclase, and relict rutile, the granitoids are considered to have
experienced eclogite facies metamorphism and are interpreted to be syntectonic with respect to the emplacement of
the thrust nappe at 440 ^ 14 Ma (U –Pb zircon lower
J.R. Malone et al. / Journal of South American Earth Sciences 15 (2002) 511–524
515
Fig. 4. Geochemistry of the Totoltepec pluton. (a) Jensen cation plot (Jensen, 1976) for rock classification. Fields: PK ¼ peridotitic komatiite; BK ¼ komatiitic
basalt; HFT ¼ high Fe tholeiitic basalt; HMT ¼ high Mg tholeiite basalt; TA ¼ tholeiitic andesite; TD ¼ tholeiitic dacite; TR ¼ tholeiitic rhyolite;
CB ¼ calc-alkaline basalt; CA ¼ calc-alkaline andesite; CD ¼ calc-alkaline dacite; CR ¼ calc-alkaline rhyolite. (b) Chondrite-normalized REE abundances
for felsic and mafic components of the Totoltepec pluton. Normalizing values after Sun (1982). (c) Mantle-normalized trace element abundances for felsic and
mafic components of the Totoltepec pluton. Normalizing values after Sun and McDonough (1989). (d) Variations of Nb (ppm) versus Y (ppm) in the felsic
rocks of the Totoltepec pluton (Pearce et al., 1984). VAG ¼ volcanic arc granites; syn-COLG ¼ syn-collision granites; WPG ¼ within-plate granites;
ORG ¼ ocean ridge granites.
intercept age; Ortega-Gutiérrez et al., 1999). The relative
displacement of the upper plate, according to its outcrop
pattern, exceeds 200 km and is attributed to westwardvergent thrusting during the Late Ordovician – Early Silurian Acatecan orogeny. The upper intercept age of the
metagranitoids (1161 ^ 30 Ma), as well as their peraluminous composition, depleted mantle model age ðTDM ¼ 1:50
GaÞ; high initial strontium ratio ð87 Sr=86 Sr ¼ 0:7189Þ; and
negative epsilon neodymium value (e Ndð0Þ ¼ 210:0;
Yañez et al., 1991), suggest a Precambrian source
(Ortega-Gutiérrez et al., 1999). The Acatecan subgroup is
inferred by these authors to represent an obducted slice of
oceanic and continental lithosphere.
2.3. Tecomate Formation
Both the Petlacingo and Acateco subgroups were
exhumed before the deposition of the uppermost unit of
the Acatlán Complex, the volcano-sedimentary Tecomate
Formation. On the basis of poorly preserved fossils, this unit
is thought to be of Devonian age (Ortega-Gutiérrez, 1993)
and represents the earliest overstep across the upper and
lower plates of the thrust nappe (Fig. 3). The formation is
mildly metamorphosed but strongly deformed and consists
of thinly bedded pelitic and psammitic sedimentary rocks,
occasional marbles and pebble conglomerates, and volcaniclastic units of basaltic – andesite and less common felsic
composition (Sánchez Zavala et al., 2000). Metagranitic
clasts, common in the conglomerates and thought to be
derived from the ca. 440 Ma Esperanza granitoids (Yañez
et al., 1991), provide a maximum depositional age for the
Formation. A minimum depositional age of ca. 370 Ma is
provided by the deformed La Noria granite, which is
reported to intrude the formation (Ortega-Gutiérrez et al.,
1999). Zircons from this granite have yielded upper and
lower intercept ages of 1116 ^ 44 and 371 ^ 34 Ma
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J.R. Malone et al. / Journal of South American Earth Sciences 15 (2002) 511–524
Table 1
Representative analyses of rocks of the Totoltepec pluton
Sample
SiO2 (wt%)
TiO2
Al2O3
Fe2O3
MnO
MgO
CaO
Na2O
K2 O
P2O5
LOI
Total
Cr (ppm)
Ni
Co
V
Cu
Pb
Zn
Rb
Cs
Ba
Sr
Ga
Nb
Hf
Zr
Y
Th
U
La
Ce
Pr
Nd
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
F-1
F-2
F-3
M-1
M-2
M-3
74.40
0.12
14.89
0.92
0.04
0.37
1.26
5.73
2.25
0.06
0.48
66.97
0.31
16.43
3.36
0.06
1.46
2.74
4.97
1.32
0.11
2.19
73.79
0.10
16.32
0.87
0.02
0.45
2.77
5.41
0.74
0.04
0.20
56.76
0.75
17.06
8.75
0.12
4.87
6.43
2.73
1.06
0.09
1.34
48.72
0.78
17.74
10.86
0.18
7.79
8.39
1.60
2.30
0.05
1.41
56.15
0.66
14.28
8.09
0.13
4.29
5.48
4.42
0.94
0.08
5.28
100.51
99.93
100.71
99.96
99.82
99.80
9
26
20
4
9
58
12
26
77
3.14
1907
316
17
2.41
1.52
61
22.79
2.31
0.68
6.09
14.11
2.04
9.55
2.71
0.72
3.06
0.55
3.68
0.84
2.44
0.38
2.38
0.38
12
69
52
3.49
1461
554
19
1.22
0.91
32
14.99
0.70
0.22
3.11
7.50
1.10
5.34
1.69
0.55
2.07
0.36
2.46
0.55
1.60
0.24
1.60
0.24
20
8
34
51
2.04
550
752
17
2.46
1.69
6.9
22.25
2.59
0.81
6.90
16.43
2.36
9.68
2.62
0.73
3.03
0.53
3.50
0.86
2.28
0.35
2.40
0.38
55
13
32
180
72
11
92
29
1.37
1669
248
18
1.98
0.84
29
6.80
1.00
0.22
2.52
5.89
0.80
3.32
0.86
0.18
0.90
0.15
1.05
0.24
0.64
0.10
0.69
0.12
140
35
43
212
99
10
55
37
1274
281
15
2.27
1.16
44
7.87
0.45
0.22
4.57
9.95
1.38
6.12
1.44
0.43
1.32
0.22
1.39
0.31
0.84
0.13
0.90
0.13
33
10
32
129
56
9
136
3.9
1.11
703
530
13
0.38
0.74
22.5
1.20
0.02
0.10
1.32
2.47
0.27
1.09
0.20
0.18
0.19
0.03
0.16
0.04
0.11
0.02
0.12
0.02
F ¼ felsic samples; M ¼ mafic samples.
(Yañez et al., 1991), the latter of which is interpreted as the
date of its crystallization (Ortega-Gutiérrez et al., 1999).
Originally thought to represent a postorogenic, molasselike unit deposited in the wake of thrust nappe emplacement
(Ortega-Gutiérrez, 1993), the Tecomate Formation has
recently been interpreted as a turbiditic volcanic arc
sequence deposited in advance of an arc-continent collision
(Mixtecan Orogeny) in the Late Devonian that was
responsible for its deformation (Sánchez Zavala et al.,
2000). Early Mississippian sedimentary rocks (e.g. San
Salvador Patlanoaya Formation) that are reported to overlie
the Tecomate Formation unconformably (Esquivel-Macı́as
et al., 2000; Vechard et al., 2000) are folded but
unmetamorphosed.
2.4. Totoltepec pluton
The Tecomate Formation is tectonically overridden by
the foliated Totoltepec pluton along the pluson’s mylonitic
southern margin. Zircons from the more felsic component of
this body have yielded a concordant U – Pb zircon age of
287 ^ 2 Ma (Yañez et al., 1991). The pluton grades upward
(to the north) from a basal gabbro/diorite through trondhjemite/tonalite to a mafic marginal phase at the top. Foliation
and compositional banding generally dip steeply north. The
pluton is nonconformably overlain by deformed, but
unmetamorphosed, clastic rocks of the Early Permian
(Leonardian, 280 ^ 4– 269 ^ 7 Ma; Okulitch, 1999) Matzitzi Formation (Silva-Romo and Mendoza-Rosales, 2000).
This formation also dips steeply north and is unconformably
overlain by gently dipping Jurassic clastic rocks that contain
leaves of Bennettita leau.
Samples collected from the Totoltepec pluton as part of
this study exhibit typical calc-alkaline geochemical signatures (Fig. 4a) accompanied by relatively flat rare earth
element (REE) patterns with only minor enrichments of
light REE and (La/Yb)n , 2– 3 (Fig. 4b). Representative
samples of both mafic and felsic rocks were analyzed by Xray fluorescence for major and some trace elements (Rb, Sr,
Ba, Zr, Nb, Y, Cr, Ni) and by inductively coupled plasmamass spectrometry (ICP-MS) for the REE, Nb and Th, at the
Ontario Geological Survey in Sudbury (Table 1). The ICPMS method, described by Ayer and Davis (1997), indicates
a precision and accuracy of 2– 10% for trace elements. All
rocks were affected by secondary processes including
amphibolite grade metamorphism and hydrothermal
activity, which may have modified their composition.
Hence, the samples were petrographically and chemically
screened. Strongly altered samples, including those with
high LOI values, were discarded. The remaining samples
are compositionally similar to modern rock suites, and thus,
the concentrations of most major elements, as well as the
high field strength elements (HFSE), REE, and transition
elements are believed to reflect the primary magmatic
distribution. These samples are employed for petrogenetic
considerations and to discriminate the tectonic setting.
Mantle-normalized trace element patterns (Fig. 4c)
display small negative Nb anomalies and distinct enrichment of Ba. Felsic rocks have SiO2 in the range of 66–
75 wt% and low K2O but high Na2O. Their REE patterns are
rather flat with low (La/Yb)n , 1– 2 and (La)n , 10– 22.
Mantle-normalized trace element patterns are also relatively
flat with Nb depletion and distinct enrichment of Ba and Rb.
On the Nb – Y discrimination diagram (Fig. 4d), the felsic
rocks plot in the arc field. Thus, the pluton appears to be one
of a series of Permo-Triassic granitoid bodies that extend
the length of Mexico and have been interpreted as a
J.R. Malone et al. / Journal of South American Earth Sciences 15 (2002) 511–524
517
(Weber et al., 1997). But whereas the structural geometry of
the Cosoltopec Formation reveals three main phases of
penetrative deformation, that of the Tecomate Formation
reveals only two. According to their structural style and
overprinting relations, these phases are designated DC1 –
DC3 and DT1 –DT2, respectively. To clarify the relationship
between these phases and highlight the east – west variations
displayed in their structural orientation, the field area has
been divided into three structural subareas (Fig. 2).
Fig. 5. Equal-area stereographic projection of structures within the
Cosoltepec Formation. (a) DC2 structures; (b) DC3 structures.
primitive arc developed above an east-dipping subduction
zone along the western margin of Pangea (Torres et al.,
1999).
3. Deformational history
To clarify the deformational history of the Acatlán
Complex, the structural geometry of the Cosoltepec and
Tecomate Formations have been examined in detail east of
Acatlán de Osorio (Malone, 2000), where east –west road
and river sections provide excellent access to the complex
along the cross-section AB (Fig. 2). Because deposition of
the Cosoltepec Formation predates the emplacement the
Acatecan thrust nappe, whereas that of the Tecomate
Formation does not, a comparison of their individual
structural records provides a unique opportunity to unravel
some of the details of the region’s deformational history. To
assist in constraining the timing of the deformation, a brief
examination also was made of the structure of the
Totoltepec pluton, which is in tectonic contact with both
formations.
3.1. Structural geometry
The metasedimentary rocks of both the Cosoltepec and
Tecomate Formations are known to be strongly deformed
3.1.1. Cosoltepec Formation
East of Acatlán, the Cosoltepec Formation comprises a
monotonous sequence of strongly veined and polydeformed
phyllites with thin psammitic interlayers that are thought to
record original bedding (SC0). A prominent, and possibly
composite, bedding-parallel foliation forms the principal
fabric in outcrop and is defined by the orientation of
greenschist facies phyllosilicates. Matrix quartz and coarser
quartz grains in veins that subparallel the foliation are
strongly annealed with granoblastic textures. Minor plagioclase grains in psammitic layers show similar textures.
Mesoscopic fold structures, most clearly outlined by the
veins, are of several generations and accompanied by
locally strong crenulation cleavages, a prominent mineral
lineation, and a variety of intersection lineations. Where
observed, contacts between the quartz– phyllitic rocks of the
Cosoltepec Formation and those of other units are sharp and
tectonic in origin.
The first phase of penetrative deformation in the
Cosoltepec Formation (DC1) produced a prominent bedding-parallel schistosity (SC1) that is the earliest recognizable fabric in outcrop. Defined by phengite –
chlorite ^ biotite, the schistosity is axial planar to rare,
small-scale, tight to isoclinal folds (FC1) and possible largescale structures of the same style. Foliation-subparallel
quartz veins that were formed either before or during the
early stages of this deformational event provide the best
means of defining the SC1 fabric in later structures because
the SC1 schistosity is often difficult to distinguish from later
foliations in outcrop. However, its presence can be clearly
demonstrated at the closures of the FC2 folds. Any lineation
that may have been associated with this deformation has
been entirely overprinted by LC2, so the kinematic
significance of the event is unknown. In all subareas, SC1
is essentially parallel to SC2, and variations in its orientation
are largely a function of younger (FC3) folding. The
mineralogy of the phyllosilicates that define SC1 suggests
that the metamorphism associated with DC1 did not exceed
the greenschist facies.
The second phase of penetrative deformation (DC2)
produced tight to isoclinal curvilinear (sheath) folds (FC2)
that possess an axial planar schistosity (SC2) defined by the
alignment of the phyllosilicates. The folds are typically
several centimeters in wavelength and deform both the SC1subparallel quartz veins and SC1, which wraps around FC2
hinges in thin sections. FC2 axes lie within the combined
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J.R. Malone et al. / Journal of South American Earth Sciences 15 (2002) 511–524
Fig. 6. Equal-area stereographic projection of structures within the
Tecomate Formation. (a) DT1 structures; (b) DT2 structures.
SC1/SC2 foliation and generally plunge NNW to NNE at
gentle to moderate angles (Fig. 5a).
Associated with DC2 is a strong mineral lineation (LC2)
that is defined by the alignment of elongate quartz grains on
SC2 foliation surfaces. LC2 plunges NE to NW at gentle to
moderate angles subparallel to the axes of FC2 folds and
slightly oblique to those of FC3, which are largely
responsible for the scatter in its orientation (Fig. 9a). The
presence of a mineral lineation and FC2 closures of
demonstrable sheath fold geometry (Fig. 10a) are consistent
with the subparallel alignment of LC2 with the near-isoclinal
FC2 axes and suggests that DC2 was associated with an
important component of simple shear. On a regional scale,
steeply dipping FC2 shear zones appear to separate the
north – south lithologic units in the vicinity of Acatlán,
whereas a north-dipping DC2 thrust forms the base of the
Totoltepec pluton along its southern margin (Fig. 2). The
phyllosilicates that define the SC2 fabric (muscovite –
biotite –chlorite – phengite) suggest that the metamorphism
associated with DC2 was of a greenschist facies grade
similar to that which accompanied DC1.
The third phase of penetrative deformation (DC3)
produced upright to inclined, open to close folds (FC3)
that plunge NNW to NE at gentle to moderate angles (Fig.
5b). With outcrop wavelengths ranging from several
centimeters to several meters, these structures refold FC2
to form Type III refolded isoclines (Fig. 10a) and possess an
Fig. 7. DT1 structures in the Tecomate Formation. (a) Folded layering in
calc mylonite within DT1 dextral shear zone (coin diameter ¼ 2.1 cm); (b)
deformed conglomerate showing top-to-the-right (south) pebble asymmetry
and asymmetric folds (coin diameter ¼ 2.7 cm).
axial planar crenulation cleavage (SC3) that is often broadly
coplanar with that of the combined SC1/SC2 foliation. FC3
axes vary only slightly in orientation across the field area,
but though they are subparallel to FC2 axes and LC2 in
subarea 1, they are slightly oblique to these structures in
subarea 3 and are aligned at a high angle to them in subarea
2 (Fig. 9a). Megascopic FC3 folds with wavelengths of
several kilometers are responsible for the regional
structure (Fig. 2) and suggest that DC3 was associated
with a significant component of east – west shortening
(Ortega-Gutiérrez, 1978).
The generally steeply dipping SC3 crenulation cleavage
associated with DC3 is a NW- to NE-striking fabric that is
often the most prominent structural feature in outcrop. The
lineation (LC3) produced by its intersection with the
combined SC1/SC2 foliation parallels the axes of FC3 folds
and is best developed at their hinges. On the basis of textural
relations, DC3 is thought to have accompanied retrograde
greenschist facies metamorphism that is evident in the
alteration of biotite to chlorite.
3.1.2. Tecomate Formation
The Tecomate Formation in the area east of Acatlán is
dominated by laminated metapelites and feldspar-bearing
J.R. Malone et al. / Journal of South American Earth Sciences 15 (2002) 511–524
Fig. 8. Equal-area stereographic projection of structures within the
Totoltepec pluton.
metapsammites with local interbeds of pebble conglomerate
and marble. Bedding is clearly discernable in places, and the
primary bedding-subparallel schistosity, which is the most
prominent structure in outcrop, is demonstrably noncomposite and defined by phyllosilicates (phengite, chlorite, and
retrograded biotite) that are less common than they are in
the Cosoltepec Formation. In comparison with the Cosoltepec Formation, widespread quartz veins are also less
common, quartz grains are only locally annealed, and the
matrix is more feldspathic, with K-feldspar and plagioclase
(An28 to An52) locally composing 5 and 20% of the mode,
respectively. Although repeatedly folded and possessing a
strong mineral lineation, the Tecomate Formation is less
deformed than the Cosoltepec Formation and has only a
single prominent crenulation cleavage and intersection
lineation. The marble horizons, which are typically 1 –
10 m in width, form distinctive marker horizons and are in
519
tectonic contact with rocks of both the Cosoltepec and
Xayacatlán Formations. In the conglomerates, white to light
pink granitic clasts, commonly 10 cm in diameter, define a
strong LS tectonite fabric. Two phases of folding (FT1 – FT2)
are recognized in the Tecomate Formation.
The earliest phase of penetrative deformation (DT1) to
affect the Tecomate Formation produced tight to isoclinal
folds (FT1) that deform the bedding and are associated with
the formation of a bedding-subparallel greenschist facies
(phengite – biotite –chlorite) schistosity (ST1). FT1 closures
are only rarely seen in outcrop, and where observed, their
axes plunge NNW to NE at gentle to moderate angles,
parallel to those of FC2 (Fig. 6a). Where associated with
steeply dipping DT1 shear zones, however, FT1 are steeply
plunging and show a clockwise asymmetry consistent with
north– south dextral shear (Fig. 7a).
A strong mineral lineation (LT1), defined by the
alignment of elongate quartz crystals on ST1 foliation
surfaces, also plunges NNW to NE subparallel to the axes of
FT1 folds (Fig. 6a). As with LC2, the scatter in the orientation
of LT1 reflects its slight obliquity to later (FT2) folding (Fig.
9b). In the conglomerates, where it is defined by elongate
pebbles, LT1 is clearly a stretching lineation and associated
with kinematic indicators (pebble s-structures and asymmetric folds) that document thrusting from north to south
along east – west striking zones (Fig. 7b).
The second penetrative phase of deformation to affect the
Tecomate Formation (DT2) produced open to close, upright
to inclined folds (FT2) that plunge gently to moderately NW
to NE (Fig. 6b). The axes of these folds show little variation
in their orientation across the field area. Similar to those of
FC3, the axes are most oblique to LT1 in subarea 2 (Fig. 9b)
and, at a megascopic scale, play an important role in the
regional structure. The FT2 folds locally refold FT1 to form
Fig. 9. Comparative equal-area stereographic projections of (a) LC2 and FC3 and (b) LT1 and FT2 in each of the three structural subareas.
520
J.R. Malone et al. / Journal of South American Earth Sciences 15 (2002) 511–524
Type III interference patterns (Fig. 10b) and contain an axial
planar crenulation cleavage (ST2) that is generally coplanar
with ST1. The ST2 cleavage is often the most prominent
structural feature in outcrop and, similar to SC3, is
associated with the retrogression of biotite to chlorite. ST2
dips steeply NW, north, or NE and intersects ST1 at FT2
hinges to form an intersection or crenulation lineation (LT2)
that plunges broadly north, parallel to the axes of FT2 folds.
3.1.3. Totoltepec pluton
To the east of Acatlán, the trondhjemitic to hornblende
dioritic Totoltepec pluton is tectonically juxtaposed against
the Cosoltepec and Tecomate Formations along its southern
margin (Fig. 2). Two phases of penetrative deformation
(DP1 –DP2) are recognized (Fig. 8).
The first deformational phase (DP1) produced a steeply
north-dipping (608– 808N) foliation (SP1) defined by flattened quartz and feldspar grains and the alignment of altered
magmatic hornblende. Enclaves of strongly banded gneissic
hornblende gabbro in the southern part of the pluton are
concordant with this fabric and cut by granitic dikes that are
themselves foliated parallel to their margins. These features
suggest that emplacement of the pluton may have been, at
least in part, syntectonic with respect to DP1. The foliation is
locally associated with sinistral shear zones with subhorizontal mineral alignments; however, it is also accompanied
by a north-plunging mineral lineation (Lp1) associated with
asymmetric hornblende fish showing top-to-the-south shear
sense. The north-dipping deformational fabric intensifies
toward the southern margin of the pluton, becoming locally
mylonitic near the basal contact with S – C fabrics that
indicate that the plutonic rocks have been thrust south over
the Tecomate and Cosoltepec Formations in a direction
similar to the vector of DT1 vergence in the underlying
Tecomate Formation.
The second phase of penetrative deformation (DP2) takes
the form of north-plunging structures (FP2), about which SP1
is locally folded. The planar SP1 fabric also shows regional
variations in trend consistent with the presence of megascopic, north – south FP2 folds (Fig. 8).
3.2. Structural correlation
The geometries of the fold phases produced by DT1 and
DT2 in the Tecomate Formation correspond closely to those
produced by DC2 and DC3 in the Cosoltepec Formation, and
the planar fabrics that these phases generated were produced
under very similar metamorphic conditions. That these two
fabric-forming events are correlative is evident from not
only the orientational data, but also their structural styles in
outcrop. This can be demonstrated, on the one hand, by the
nearly identical orientations of LC2 and FC3 in each of the
three subareas (Fig. 9a) to those of LT1 and FT2 in the same
subareas (Fig. 9b) and, on the other hand, by the similarity
of the two fold structures illustrated in Fig. 10. Thus, Fig.
10a shows a refolded FC2 sheath fold in the Cosoltepec
Fig. 10. Comparative structural styles of DC2 and DT1, and DC3 and DT2. (a)
Type III FC3 refolded FC2 isoclinal sheath fold in Cosoltopec Formation
(coin diameter ¼ 2.5 cm). (b) Type III FT2 refolded FT1 isocline in
Tecomate Formation (coin diameter ¼ 2.7 cm).
Formation that deforms a schistosity-parallel quartz vein,
whereas Fig. 10b shows a refolded FT1 isocline in the
Tecomate Formation that is of very similar appearance but
that deforms only the bedding.
Similarities also exist between the structures associated
with DT1 and DT2 in the Tecomate Formation and those that
affect the Totoltepec pluton, in that the earlier deformation
of both units produced LS tectonite fabrics of similar
attitude and kinematics, whereas the later deformation
produced folds of similar geometry and orientation.
However, SP1 in the Totoltepec pluton dips more steeply
than ST1 in the Tecomate Formation, and the kinematics
associated with it include steep, east –west sinistral shear
zones not observed in the Tecomate Formation. The dip of
SP1 is also slightly steeper than that of the bedding in the
nonconformably overlying Matzitzi Formation and Jurassic
rocks, which implies both significant rotation of the pluton
and a rotation that took place in at least two stages: Late
Permian –Triassic and syn- to post-Jurassic. This rotation is
inferred to have occurred on a south-dipping listric normal
fault that should crop out somewhere to the north and
resurfaces along the southern margin of the pluton as a
south-vergent thrust. Restoration of this post-Leonardian
rotation produces a gently north-dipping SP1 foliation in the
pluton approximately parallel to that in the underlying
J.R. Malone et al. / Journal of South American Earth Sciences 15 (2002) 511–524
521
3.3. Timing of deformation
Fig. 11. Schematic structural evolution of the Acatlán Complex recorded in
the Cosoltepec and Tecomate Formations and the Totoltepec pluton.
Tecomate Formation. At the same time, the steeply dipping
sinistral shear zones are restored to zones with west-vergent
thrusting. The existence of such thrusts can be interpreted in
terms of dextral transpression with strain partitioning
between dextral, north– south shear zones and west-vergent
thrusts in the form of a positive flower structure, and southvergent thrusting at constraining bends. This is consistent
with observations in a nearby area, where Elı́as-Herrera and
Ortega-Gutiérrez (2000) have shown that Early Permian
dextral shear was accompanied by west-vergent thrusting.
Further work, currently in progress, is required to resolve
the kinematic inhomogeneities. However, the sequence of
structures is similar in both the Totoltepec pluton and the
Tecomate Formation, and both phases (DT1/P1 and DT2/P2)
are synchronous with or postdate intrusion of the pluton at
287 ^ 2 Ma (concordant U –Pb zircon age; Yañez et al.,
1991).
We therefore conclude that the two principle phases of
deformation recorded in the Tecomate Formation (DT1 and
DT2) correlate with the second two deformational phases
(DC2 and DC3) recorded in the Cosoltepec Formation
(Fig. 11), thereby lending support to the conclusion of
Ortega-Gutiérrez (1975) that the original relationship
between the two Formations was an unconformity. DC1
and DC2 may also correlate with two low-grade deformational events described elsewhere in the Acatlán Complex
by Weber et al. (1997), which are likewise separated by an
episode of deposition.
We further conclude that the Totoltepec pluton contains
both the DT1 and DT2 fabrics of the Tecomate Formation
(Fig. 11), DT1 having occurred perhaps during and shortly
after crystallization in a dextrally transpressive shear
regime. Hence, the Acatlán Complex witnessed three main
phases of deformation (D1 – D3), the first of which predates
the Tecomate Formation whereas the remaining two
postdate the intrusion of the Totoltepec pluton.
Of the three penetrative phases of deformation recognized in the Acatlán Complex, the first (D1) is associated
with the development of pervasive quartz veins and an early
greenschist facies schistosity (SC1), neither of which are
present in the Tecomate Formation. D1 is therefore
considered to predate the deposition of the Tecomate
Formation, a conclusion supported by the reported occurrence of deformed Esperanza granitoids in the conglomerates of this formation (Yañez et al., 1991) and the
abundance of feldspar in its psammitic lithologies. D1 is
consequently interpreted to record the tectonic emplacement of the high-grade upper plate (Acateco subgroup) onto
the Cosoltepec Formation, which is inferred to have
occurred during the syntectonic emplacement of the
Esperanza granitoids at ca. 440 Ma (Ortega-Gutiérrez
et al., 1999). Evidence of the proposed westward vergence
of this event, however, is not discernable within the
Cosoltepec Formation, though a possible resolution of its
kinematics may lie in the structural fabrics of the
metagranitoids and the Xayacatlán Formation of the upper
plate.
Constraints on the timing of D2 and D3 rest with their
relationship to the Totoltepec pluton, which, on the basis of
its structural sequence and geometry, is considered to record
both deformational events. If so, the age of D2 is tightly
constrained because it can be no older than the pluton and no
younger than the unmetamorphosed Early Permian (Leonardian) Matzitzi Formation, which overlies the pluton
nonconformably (Silva-Romo and Mendoza-Rosales,
2000). D2 is consequently considered Early Permian in
age, constrained to the narrow time interval between the
pluton’s crystallization at 287 ^ 2 Ma (Yañez et al., 1991)
and the Leonardian at 280 ^ 4– 269 ^ 7 Ma (Okulitch,
1999). Sedimentary rocks of the Matzitzi Formation also
rest nonconformably on syntectonic granitoids dated at
274 ^ 11 Ma (U – Pb zircon; Elı́as-Herrera and Ortega-Gutiérrez, 2000) that stitch the contact between the Acatlán and
Oaxacan Complexes, thereby indicating an Early Permian
age for their final tectonic juxtaposition.
The age of D3 cannot be defined with the same degree of
certainty because it is not known if the Matzitzi Formation
redbeds overlying the Totoltepec pluton are affected by
these folds. A younger limit on the D3 structures in the
Chazumba Formation is provided by the San Miguel felsic
dikes, dated at 173 ^ 0.3 Ma (Ruiz-Castellanos, 1979),
which cut across the D3 folds (unpublished data by the
authors). However, the absence of any metamorphism in the
Matzitzi Formation strongly suggests that its deposition
postdates D3, the strong crenulation cleavage of which
would appear to have developed under lower greenschist
facies conditions. It is therefore considered likely that both
D2 and D3 are of Early Permian (pre-Leonardian) age, as is
suggested by a K/Ar age of 288 ^ 14 Ma reported from D3
sericite by Weber et al. (1997). If so, D2 and D3 may be the
522
J.R. Malone et al. / Journal of South American Earth Sciences 15 (2002) 511–524
Fig. 12. Permian Pangea reconstruction for North, Central, and South
America (modified from Ross and Scotese (1988) and Pindell and Barrett
(1990)) showing the transpressional deformation in the Acatlán Complex of
southern Mexico in relation to the Late Paleozoic Ouachita orogen and an
unnamed orogenic belt in northwestern South America. MSN ¼ MojaveSonora megashear; TMVB ¼ Trans-Mexican volcanic belt; present-day
coastal areas dotted.
product of a single period of deformation bracketed between
290 and 270 Ma and may reflect a gradual anticlockwise
swing in the associated stresses.
The extensive annealing of quartz fabrics in the
Cosoltepec Formation suggests that much of the Acatlán
Complex underwent a period of static metamorphism
following the development of D3. Ortega-Gutiérrez (1993)
attributed this static event to the development of a large
thermal dome cored by the Magdalena migmatite. If this is
correct, it is likely the product of a Jurassic (ca. 170 Ma)
thermal pulse (Powell et al., 1999) that is known in this part
of Mexico to be associated with the opening of the Gulf of
Mexico (Salvador, 1991).
4. Discussion
Of the three phases of penetrative deformation recorded
in the Acatlán Complex, D2 is the most tightly constrained
with respect to its timing and kinematics. The north –south
dextral transpression and south-vergent thrusting recorded
by this Early Permian event is approximately synchronous
with the dextral transpression recorded along the north–
south boundary between the Acatlán and Oaxacan Complexes at ca. 274 Ma (Elı́as-Herrera and Ortega-Gutiérrez,
2000). Permian deformation of unknown kinematics has
also been recorded in northern Mexico (López et al., 2001),
central Mexico (Ochoa-Camarillo, 1996), and Chiapas in
southernmost Mexico (Sedlock et al., 1993).
In most Permian reconstructions (Pindell, 1985; Ross and
Scotese, 1988; Pindell and Barrett, 1990; Dickinson and
Lawton, 2001), Mexico and Central America are bounded
by Late Paleozoic orogens: the Ouachita orogenic belt
bordering the southern Laurentian craton and an unnamed
belt in the northern Andes bordering the Amazon craton
(Fig. 12). These two belts show a transition from externides
adjacent to the neighboring craton to internides that face
each other. Yet they are unlikely to represent the sides of a
single Ouachita-protoAndean collisional orogen because
Late Paleozoic metamorphism in Mexico and Central
America (areas that lie between the two belts) is of
generally low-grade, in contrast to that of an orogenic root
zone. The development of separate collisional belts may
reflect the fact that much of Mexico and northern Central
America is underlain by Precambrian basement, which
likely represents a microcontinental block trapped between
Laurentia and Amazonia during the final amalgamation of
Pangea. The style of Permian deformation observed in
Mexico may be classified as externide, which is consistent
with two orogenic belts passing to the north and south of the
microcontinent. In this scenario, the dextral transpressive
deformation observed in the area east of Acatlán suggests
that the final amalgamation of Pangea involved some
northward movement of Mexican blocks relative to Pangea.
Such a motion is consistent with the requirement in current
Pangea reconstructions that Mexico and northern Central
America were displaced to the northwest. This appears
synchronous with subduction along the western margin of
Pangea, which was responsible for the Permo-Triassic arc
magmatism (exemplified by the Totoltepec pluton) that
extended from the southwestern United States through
Mexico and into northwestern South America (Torres et al.,
1999).
5. Conclusions
Three main phases of deformation (D1 – D3) are recognized in metasedimentary units of the Acatlán Complex east
of Acatlán. D1 affects only the Cosoltepec Formation and is
attributed to the Late Ordovician– Early Silurian Acatecan
orogeny, during which the eclogitic mafic – ultramafic and
metagranitoid rocks of the Xayacatlán/Esperanza plate are
thought to have overthrust the lower-grade siliciclastic
lithologies of the Cosoltepec and Chazumba Formations.
D2 and D3 are considered to be of Early Permian age and
affect the Cosoltepec Formation, the unconformably
overlying Tecomate Formation, and the granitoid Totoltepec pluton dated at ca. 287 Ma. D2 involved south-vergent
thrusting and north – south dextral shear and was likely
responsible for the tectonic juxtapositioning of the Acatlán
and Oaxacan Complexes. D3 involved significant east –west
shortening and produced the megascopic folding that is
responsible for the regional structure. No evidence is found
in this part of the Acatlán Complex of a proposed Mixtecan
orogeny during the Middle – Upper Devonian. Instead, the
J.R. Malone et al. / Journal of South American Earth Sciences 15 (2002) 511–524
kinematics of its Early Permian deformation are consistent
with the northward movement of Mexican crustal blocks
required in current continental reconstructions of the final
amalgamation of Pangea.
Acknowledgements
For his support and encouragement at all stages of this
study, we are deeply indebted to Dr Fernando OrtegaGutiérrez. Funding for this project was provided by grants
from the FIPSE North American Mobility in Higher
Education Program (P116N9600018-99) and the Ohio
University Baker Awards Committee (39-10-9668) to
RDN, and from CONACyT (25795-T) and PAPIIT
(IN136999) grants to JDK. Logistical support provided by
the Instituto de Geologı́a at the Universidad Nacional
Autónoma de México in Mexico City is gratefully acknowledged. David Fraser kindly provided unpublished structural
data on the Totoltepec pluton. Constructive reviews by
G. Draper and W. Frisch greatly improved the final
manuscript.
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