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Synthesis and tectonic interpretation of the westernmost Paleozoic Variscan
orogen in southern Mexico: From rifted Rheic margin to active
Pacific margin
J. Duncan Keppie a,⁎, Jaroslav Dostal b , J. Brendan Murphy c , R. Damian Nance d
a
Depto. de Geología Regional, Instituto de Geología, Universidad Nacional Autónoma de México, México, Mexico
b
Department of Geology, St. Mary's University, Halifax, Nova Scotia, Canada B3H 3C3
c
Department of Earth Sciences, St. Francis Xavier University, Antigonish, Nova Scotia, Canada B2G 2W5
d
Department of Geological Sciences, 316 Clippinger Laboratories, Ohio University, Athens, Ohio 45701, USA
Received 8 June 2007; received in revised form 26 October 2007; accepted 27 January 2008
Abstract
Paleozoic rocks in southern Mexico occur in two terranes, Oaxaquia (Oaxacan Complex) and Mixteca (Acatlán Complex) that appear to
record: (1) Ordovician rifting on the southern margin of the Rheic Ocean, (2) passive drifting with Amazonia during the Silurian, (3) DevonianPermian subduction beneath southern Mexico producing an arc complex that was partially removed by subduction erosion, subjected to HP
metamorphism and Mississippian extrusion into the upper plate, followed by reestablishment of a Permian arc.
In the Oaxaquia terrane, the 920–1300 Ma basement is unconformably overlain by a ∼ 200 m uppermost Cambrian–lowest Ordovician shelf
sequence containing Gondwanan fauna (Tiñu Formation), unconformably overlain by 650 m of shallow marine-continental Carboniferous
sedimentary rocks containing a Midcontinent (USA) fauna.
In the Mixteca terrane, the low-grade Paleozoic sequence is composed of: (a) a ?Cambrian–Ordovician clastic sequence intruded by ca. 480–
440 Ma bimodal, rift-related igneous rocks; and (b) a latest Devonian-Permian shallow marine sequence (N 906 m) consisting of metapsammites,
metapelites and tholeiitic mafic volcanic rocks. High pressure (HP) metamorphic rocks in the Mixteca terrane consists of: (i) a Cambro-Ordovician
rift-shelf intruded by bimodal rift-related intrusions that are similar to the low-grade rocks; (ii) periarc ultramafic rocks, and (iii) arc and MORB
rocks. The Ordovician granitoids contain concordant inherited zircons that range in age from ca. 900 to 1300 Ma, indicating a source in the
Oaxacan Complex. Concordant ages of detrital zircons in both the low- and high-grade Cambro-Ordovician metasedimentary rocks indicate a
provenance in local Ordovician plutons and/or ca. 1 Ga Oaxacan basement, and distal northwestern Gondwana sources with a unique source in the
900–750 Ma Goiás magmatic arc within the Brasiliano orogen. These data combined with the rift-related nature of the Cambro-Ordovician rocks
are most consistent with an origin along the southern margin of the Rheic Ocean. Latest Devonian-Permian deposition was synchronous with
Mississippian extrusion of the HP rocks into the upper plate during extensional deformation. The HP Cambro-Orodivician rift-shelf rocks are
inferred to have originated in the forearc region of the upper plate that was removed by subduction erosion, carried down the subduction channel,
and then extruded into the upper plate in the middle of the Mixteca terrane. The presence of arc-related rocks in the HP assemblage suggests that
the arc complex was also removed, whereas the MORB rocks may have been derived from the subducting slab. Detrital zircons in the
Carboniferous rocks of both the Mixteca and Oaxaquia terranes contain Devonian detrital zircons and volcanic clasts that are inferred to have
come from the removed Devonian arc on the western margin of the Mixteca terrane and/or from exhumed HP rocks. During the Permian, arcrelated intrusions in both the Mixteca and Oaxaquia terranes were accompanied by dextral transtensional deformation and deposition of clastic
rocks containing Permian detrital zircons and carbonates in periarc, pull-apart basins. Empirical relationships between the dip of the Benioff zone
and the widths of arc and forearc indicate that the Permian trench lay beneath the eastern edge of the Mesozoic Guerrero terrane.
© 2008 Elsevier B.V. All rights reserved.
Keywords: Paleozoic; Southern Mexico; Acatlán Complex; Tectonics; Rheic Ocean; Paleo-Pacific Ocean
⁎ Corresponding author. Tel.: +52 555 622 4290.
E-mail address: [email protected] (J.D. Keppie).
0040-1951/$ - see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.tecto.2008.01.012
Please cite this article as: Keppie, J.D., et al., Synthesis and tectonic interpretation of the westernmost Paleozoic Variscan orogen in southern Mexico: From rifted
Rheic margin to active Pacific margin, Tectonophysics (2008), doi:10.1016/j.tecto.2008.01.012
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J.D. Keppie et al. / Tectonophysics xx (2008) xxx–xxx
1. Introduction
Paleozoic rocks in southern Mexico are restricted to the
Mixteca terrane and small outliers in the Oaxaquia terrane
(Fig. 1). Knowledge of the Paleozoic geology of southern
Mexico, especially in the Mixteca terrane (Fig. 2), during the
twentieth century was hampered by generally poor preservation
of fossils and a lack of reliable age dates. However, the last decade
has seen a rapid increase in such data supplemented by
geochemistry (Ortega-Gutiérrez et al., 1999; Elías-Herrera and
Ortega-Gutiérrez, 2002; Vachard and Flores de Dios, 2002;
Keppie et al., 2004a,b; Sánchez-Zavala et al., 2004; Vachard et al.,
2004; Talavera-Mendoza et al., 2005; Keppie et al., 2006; Nance
et al., 2006; Murphy et al., 2006a,b; Miller et al., 2007; Nance
et al., 2007; Middleton et al., 2007; Elías-Herrera et al., 2007;
Landing et al., 2007; Vega-Granillo et al., 2007; Keppie et al.,
2008; Morales-Gámez et al., 2008; Ramos-Arias et al., 2008;
Hinojosa-Prieto et al., in review; Grodzicki et al., in press). This
has led to considerable revision of the geological history, which
has recently been reviewed by Nance et al. (2006, 2007).
Nance et al. (in review) have compiled histograms of detrital
zircon ages to give a broad overview of potential sources that
they argued were of dominantly local and Gondwanan
provenence. Here we try to refine these conclusions by focusing
on concordant data (c.f. Anderson, 2005) (Figs. 3 and 4) from
which emerges a suite of 900–750 Ma detrital zircons that has a
unique source in the Goiás magmatic arc within the Brasiliano
orogen of Amazonia (Pimental et al., 2000). We separate the
zircon populations according to the age of the unit (Ordovician,
Carboniferous and Permian). During the Ordovician both the
Iapetus and Rheic oceans were in existence and the characteristics of these zircons test whether they had Gondwanan or
Laurentian provenance. On the other hand, during the
Carboniferous and Permian, the location of southern Mexico
in Pangea implies that both continents may have contributed to
the zircon populations. The earliest evidence of a southern
source for flysch deposits in the Ouachitan orogen occurs in the
Mississippian (ca. 330 Ma: Arbenz, 1989). At about the same
time, faunal interchange between Laurentia and Oaxaquia is
documented with the appearance of Laurentian (Midcontinent
USA) fauna in the Mississippian rocks of southern Mexico
(Navarro-Santillan et al., 2002). Deformation associated with
Laurentia–Gondwana collision started in the Pennsylvanian
(Arbenz, 1989).
This paper, then, presents a tectonic interpretation of the
Paleozoic geological record of southern Mexico, which
Fig. 1. Terrane map of southern Mexico showing the locations of the Oaxaquia, Mixteca and Guerrero terranes and Permian arc intrusions (modified from Keppie et al.,
2003c).
Please cite this article as: Keppie, J.D., et al., Synthesis and tectonic interpretation of the westernmost Paleozoic Variscan orogen in southern Mexico: From rifted
Rheic margin to active Pacific margin, Tectonophysics (2008), doi:10.1016/j.tecto.2008.01.012
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J.D. Keppie et al. / Tectonophysics xx (2008) xxx–xxx
3
Fig. 2. Geological map of the Mixteca and western Oaxaquia terranes showing the locations of Paleozoic rocks and Ordovician plutons detailed in Figs. 3, 4 and 6
(modified after Keppie et al., 2006).
indicates that they formed on, and adjacent to, Oaxaquia on the
northwestern margin of Gondwana (Amazonia) in two tectonic
environments: (i) a Cambrian–early Silurian rift-shelf setting on
the southern margin of the Rheic Ocean, and (ii) a latest
Devonian-Permian active margin setting of the eastern Pacific
Ocean (Fig. 5). However, southern Mexico, although it
represents the western end of the Variscan–Alleghanian orogen,
was not involved in the collisional stage typical of the rest of the
orogen.
2. Review of the geological records of the Mixteca and
Oaxaquia terranes
The Mixteca terrane (Acatlán Complex) is juxtaposed on
its eastern side against the ca. 920–1300 Ma Oaxacan
Complex of the Oaxaquia terrane (Figs. 1 and 2) along a
Permian dextral flower structure where syntectonic migmatites
have yielded an age of 276 ± 1 Ma (Caltepec fault zone of
Elías-Herrera and Ortega-Gutiérrez, 2002). These ∼ 1 Ga
rocks belong to the Middle American microcontinent, which
lay along the northwestern border of Gondwana (i.e.
Amazonia: Keppie, 2004).
2.1. Oaxaquia terrane
The Oaxaquia terrane consists mainly of granulite facies
rocks of the Oaxacan Complex, in which ages range from ca.
920 Ma to 1300 Ma. Important igneous and metamorphic
events in this time interval occurred at ca. 1200–1300 Ma
(intrusion of continental tholeiites: Keppie and Dostal, 2007),
ca. 1160–1130 Ma (intrusion of bimodal igneous plutons), ca.
1106 Ma (Olmecan tectonothermal event, including migmatization), ca. 1004–978 Ma (Zapotecan orogeny, including
granulite facies metamorphism and intrusion of an anothosite–
mangerite–charnockite–granite suite), and ca. 920 Ma (intrusion of a calc-alkaline pluton)(Keppie et al., 2001, 2003a;
Solari et al., 2003; Ortega-Obregón et al., 2003). In small
outliers in southern Mexico, these Precambrian rocks are
unconformably overlain by a thin (ca. 200 m) shallow marine,
shelf sequence consisting of carbonates and clastic rocks (Tiñu
Formation) containing latest Cambrian–earliest Ordovician
fossils with Gondwanan affinity (Robison and Pantoja-Alor,
1968; Landing et al., 2007). The clastic rocks contain detrital
zircons exclusively derived from the underlying Oaxacan
Complex (Gillis et al., 2005) and show a geochemistry typical
Please cite this article as: Keppie, J.D., et al., Synthesis and tectonic interpretation of the westernmost Paleozoic Variscan orogen in southern Mexico: From rifted
Rheic margin to active Pacific margin, Tectonophysics (2008), doi:10.1016/j.tecto.2008.01.012
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J.D. Keppie et al. / Tectonophysics xx (2008) xxx–xxx
Fig. 3. Concordant detrital zircon ages from (?Cambrian–)Ordovician rocks of the Mixteca terrane, southern Mexico (see Fig. 2 for locations). 206Pb/238U ages b1 Ga,
and 207Pb/206Pb ages N1 Ga. Shaded area indicates unique source for detrital zircons. Data sources: Ixcamilpa and Mimilulco (Talavera-Mendoza et al., 2005); Asis
(Murphy et al., 2006a,b); Orgonal (Vega-Granillo et al., 2007); Otate, Las Minas, and Mal Paso (Keppie et al., 2008); Amate (Morales-Gámez et al., 2008); Calaveras
and Epazote (Hinojosa-Prieto et al., in review); Huerta (Keppie et al., 2006).
of a rift-shelf origin (Murphy et al., 2005). These rocks are
overlain by Carboniferous clastic and shallow marine rocks
(Mississippian Santiago and Pennsylvanian Ixtaltepec formations: 200 m and 425 m thick, respectively) that contain a
Midcontinental (USA) fauna (Navarro-Santillan et al., 2002)
indicating proximity to Laurentia during in the initial stages
of Pangea amalgamation. The Ixtaltepec Formation contains
clasts of felsic volcanic rocks (Gillis et al., 2005). Whereas
most of the detrital zircon ages from these two units indicate
derivation from the Oaxacan Complex, there are a few
Ordovician (447 ± 17 Ma single grain TIMS analysis and
several single grain LA-ICPMS analyses in the range ca.
470 to 475 ± 4 Ma); and Devono-Carboniferous (340 and
358 Ma—single grain TIMS analyses and 339–405 Ma LAICPMS analyses: Gillis et al., 2005) ages (Fig. 4).
2.2. Mixteca terrane
The geological history of the Acatlán Complex has undergone rapid revision in recent years largely due to a rapidly
improving geochronological and fossil database. As these
changes have been recently reviewed by Nance et al. (2006,
2007) only a summary of the revised geological history is
presented here (Fig. 5). The description is subdivided into lowgrade and high pressure (HP) rocks.
2.2.1. Low-grade rocks
The low-grade sedimentary rocks underlie N60% of the
Acatlán Complex (Fig. 2). They are predominantly arkoses,
quartzites, psammites and pelites that were previously
included in the Cosoltepec Formation (Ortega-Gutiérrez
et al., 1999), which has since been shown to be a composite
unit (e.g. Morales-Gámez et al., 2008; Ramos-Arias et al.,
2008; Hinojosa-Prieto et al., in review; Grodzicki et al., in
press). These clastic units may be subdivided into three age
categories based upon the ages of the youngest detrital
zircons and cross-cutting intrusions (Table 1): (1) younger
than 900–650 Ma and pre-460–440 Ma; (2) Ordovician and
pre-465–440 Ma; and (3) Devonian or younger, probably
Carboniferous.
Please cite this article as: Keppie, J.D., et al., Synthesis and tectonic interpretation of the westernmost Paleozoic Variscan orogen in southern Mexico: From rifted
Rheic margin to active Pacific margin, Tectonophysics (2008), doi:10.1016/j.tecto.2008.01.012
ARTICLE IN PRESS
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5
Fig. 4. Concordant detrital zircon ages from Carboniferous and Permian rocks of southern Mexico showing potential sources (see Fig. 2 for locations). 206Pb/238U ages b1 Ga,
and 207Pb/206Pb ages N1 Ga. Underlined ages are from the Ixtaltepec Formation. Bold numbers indicate TIMS analyses. Shaded areas indicate significant age ranges
discussed in the text. Data sources: Olinala and Cosoltepec (Talavera-Mendoza et al., 2005); Canoas and Coatlaco (Grodzicki et al., in press); Ojo de Agua (Keppie et al.,
2008); Salada (Morales-Gámez et al., 2008); Tecomate (Sánchez-Zavala et al., 2004); Iaxtaltepec and Santiago formation (Gillis et al., 2005).
2.3. (?Cambrian–)Ordovician–earliest Silurian
Deposition of a the (?Cambrian–)Ordovician sequence was
accompanied by intrusion of a 480–440 Ma bimodal igneous
suite consisting of tholeiites (Keppie et al., 2008) and calcalkaline megacrystic granitoids (Miller et al., 2007). The close
association of these felsic rocks with tholeiitic mafic rocks
indicates a rift origin (Keppie et al., 2008). Inherited zircons in
the granitoid rocks indicate a source in a ca. 920–1300 Ga
basement, probably the Oaxacan Complex (Fig. 6).
The absence of Ordovician detrital zircons in clastic rocks of
the first category (Fig. 3) suggests that the strata were deposited
either before, or synchronously with buried, Ordovician
intrusions. Deposition of the rocks of the second category
appears to be synchronous with these intrusions. Detrital zircon
populations of both clastic suites (categories 1 and 2) indicate a
wide range of concordant ages (490–2100 Ma: Fig. 3).
However, those ranging from 900 Ma to 750 Ma are unusual,
and the only known source that could provide such zircons is
the Goiás magmatic arc of Amazonia (Pimental et al., 2000).
Such ages are absent in Laurentia. The Goiás magmatic arc
occurs within the Brasiliano orogen, which formed as a result of
collision between the Sao Francisco and Amazonian cratons,
and could also have provided the associated ca. 750–560 Ma
detrital zircons. A drainage system originating in the Brasiliano
orogen may have passed across Oaxaquia (which probably
provided the ubiquitous 920–1300 Ma detrital zircons).
Although these latter ages overlap those of the Grenville
orogen, the 980–920 Ma ages are limited to Oaxaquia (Keppie
et al., 2003a: Fig. 3). The Maya terrane may also have
contributed 540–560 Ma zircons, which have been recovered in
ejects from the Chicxulub impact crater (e.g., Krogh et al.,
1993). The interpretation that the zircon populations were
derived from sources in Amazonia and Oaxaquia obviates the
requirement of other sources for some of the suites (c.f.
Talavera-Mendoza et al., 2005; Vega-Granillo et al., 2007).
Please cite this article as: Keppie, J.D., et al., Synthesis and tectonic interpretation of the westernmost Paleozoic Variscan orogen in southern Mexico: From rifted
Rheic margin to active Pacific margin, Tectonophysics (2008), doi:10.1016/j.tecto.2008.01.012
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J.D. Keppie et al. / Tectonophysics xx (2008) xxx–xxx
Fig. 5. Paleozoic geological history of the Oaxaquia and Mixteca terranes.
The mafic rocks associated with these Ordovician sedimentary rocks generally occur either as part of a dike swarm or are
spatially associated with megacrystic granitoids plutons. The
mafic rocks have a differentiated tholeiitic, within-plate
chemistry, some of which indicate crustal contamination
indicative of an origin in a rifted continental tectonic setting
(Keppie et al., 2008). Only one grabbroic dike has been dated
directly: 442 ±± 1 Ma (Keppie et al., 2008). The associated
megacrystic granitoids have ages ranging from 478 ± 5 Ma
(Talavera-Mendoza et al., 2005) to ca. 440 Ma (Keppie et al.,
2008; Morales-Gámez et al., 2008) and have a crustal signature.
Their close spatial and temporal association with the rift-related
tholeiites suggests the mafic magmas probably triggered
melting of a crustal source. Inherited zircons in these
megacrystic granitoids have ages ranging from ca. 910 Ma to
ca. 1250 Ma (Table 1) consistent with a source in the Oaxacan
Complex (Talavera-Mendoza et al., 2005; Miller et al., 2007;
Morales-Gámez et al., 2008).
2.4. Late Paleozoic
Late Paleozoic events include deposition of latest Devonian
(Strunian)–Late Permian, shallow marine carbonate and clastic
rocks of the Patlanoaya Group and the Tecomate and Olinalá
formations (Vachard et al., 2000; Vachard and Flores de Dios,
2002; Vachard et al., 2004; Keppie et al., 2004b; Ramos-Arias
et al., 2008), psammites and pelites of the Salada, Cuatlaco/
Canoas and type Cosoltepec units (The term Cosoltepec is here
restricted to the type locality: Talavera-Mendoza et al., 2005;
Morales-Gámez et al., 2008; Grodzicki et al., in press), and
alluvial fan deposits of the Leonardian Matzitzi Formation
(Weber, 1997). The Salada Unit is an along-strike continuation
of the type Cosoltepec Formation (Fig. 2). The Salada,
Cosoltepec (s.s.) and Cuatlaco units all contain mafic lenses
of tholeiitic, within-plate affinity (Keppie et al., 2007;
Grodzicki et al., in press). Compared with the Ordovician
units, concordant detrital zircons in the Salada, Cosoltepec (s.s)
and Cuatlaco units still show a strong preponderance of ca.
560–750 Ma and ca. 920–1250 Ma ages with relatively few
ages in the ca. 775–900 Ma range (Fig. 4) indicating these strata
are derived either directly from Amazonia and Oaxaquia or are
recycled from older sediments which had Amazonian/Oaxaquian provenance. The ca. 440–480 Ma detrital zircons in these
units indicate a local Mixtecan source, whereas units with
detrital zircon ages of ca. 1.3–2.8 Ga suggest a source in the
Amazon craton (Fig. 4). However, the appearance of detrital
zircons with ages of ca. 350–390 Ma indicates a new Devonian
source: such zircons also appear in the Carboniferous rocks of
Oaxaquia. There are no igneous rocks of this age in either
Mexico or in the Ouachita orogen, and only a few occur in
massifs within the Andes (e.g. 394 ± 23 Ma and 370 ± 20 Ma in
the eastern Cordillera: Aleman and Ramos, 2000). Another
potential source is in a postulated Devonian arc on the western
margin of the Mixteca terrane as discussed below.
In addition to those in the older rocks, detrital zircons and
granitic pebbles in the Permian Tecomate and Olinalá formations have concordant ages between ca. 325 and 290 Ma
(Fig. 4). A source for these is provided by the Permo-Triassic
arc that runs along the backbone of Mexico (Torres et al., 1999),
with local representatives in the Mixtecan and Oaxaquia
terranes (Fig. 1)(Totoltepec pluton dated at ca. 288 Ma:
Yánez et al., 1991; Keppie et al., 2004b, and La Carbonera
stock dated at 275 ± 4 Ma: Solari et al., 2001). The present width
of the arc is 100–120 km (depending on whether a small
Please cite this article as: Keppie, J.D., et al., Synthesis and tectonic interpretation of the westernmost Paleozoic Variscan orogen in southern Mexico: From rifted
Rheic margin to active Pacific margin, Tectonophysics (2008), doi:10.1016/j.tecto.2008.01.012
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undated stock intruding the Patlanoaya Group is included or
not). Using the empirical relationships between the widths of
the arc and the dip of the Benioff zone (Fig. 7) one may estimate
the width of the forearc to have been 180 ± 40 km, a conclusion
that places the Permian trench near the present eastern edge of
the Mesozoic Guerrero terrane (Fig. 1). The Guerrero terrane
was thrust over Mesozoic platformal carbonates during the
Laramide orogeny, consistent with the inference that the
Acatlán Complex underlies the Mesozoic platformal carbonates
west of the Cenozoic Papalutla thrust (Centeno-Garcia et al.,
in press).
The latest Devonian–Carboniferous deposition was also
synchronous with extensional deformation on listric normal
shear zones (down-to the-east) that have yielded 40Ar/39Ar
muscovite plateau ages of 347 ± 2 Ma (Ramos-Arias et al.,
2008). This greenschist facies deformation appears to have
continued through ca. 330 Ma as indicated by cleavage
development in other areas of the Acatlán Complex (e.g.
Hinojosa-Prieto et al., in review). Furthermore, the Permian
deposition coincided with a period of dextral transtensional
deformation on N–S vertical shear zones (Morales-Gámez
et al., 2006), during which local pull-apart basins and overthrust
regions formed, depending on the location of releasing or
7
restraining bends in the transcurrent faults. In the latter case,
small clastic wedges up to Triassic in age were deposited in
front of the advancing thrusts (Malone et al., 2002; Keppie
et al., 2006).
2.4.1. High pressure rocks
A belt of high pressure (HP) rocks that strikes N–S occurs in
the middle of the Acatlán Complex and is ≤15 km wide and
70 km in length. Although Vega-Granillo et al. (2007) argue for
three periods of HP metamorphism in the Ordovician and
Silurian in this belt, presently existing direct dating of the
eclogite and blueschist metamorphism has only yielded
Late Devonian–Mississippian dates (Middleton et al., 2007;
Elías-Herrera et al., 2007; Keppie et al., 2008). This belt is
bounded on either side by similar low-grade rocks. (Fig. 2). The
HP rocks of the Acatlán Complex were originally inferred to be
a nappe that was thrust over the low-grade rocks (OrtegaGutiérrez et al., 1999 and references therein). However recent
reexamination of the main central band reveals that contacts are
generally steeply dipping dextral transcurrent shear zones/faults
of Permian age. In contrast, in the Patlanoaya–Asis area
(Fig. 8), the eastern contact of the HP rocks is a moderately eastdipping, listric normal shear zone with E-vergent kinematic
Table 1
Pb/238U zircon age constraints for psammitic–pelitic rocks in the Acatlán Complex
206
Unit
Youngest detrital Zircon (206Pb/238U)
Upper Paleozoic
Permo-Carboniferous
Tecomate Fm
Tecomate Fm (type)
Olinalá Fm
292 ± 3 Ma
460 ± 5 Ma (peak)
300 ± 12 Ma
Devono-Carboniferous
Cosoltepec Fm (ss)
Salada Unit
Ojo de Agua Unit
Mimilulco
Cuatlaco Unit
Canoas Unit
Early Paleozoic
Ordovician
Huerta Unit
El Epazote Unit
Las Calaveras Unit
Mal Paso Unit
Las Minas Unit
Ordovician or older
Amate Unit
Otate Unit
High pressure units
Asis Unit
Santa Cruz Orgonal
Mimilulco
Ixcamilpa
Cross-cutting intrusion
486 ± 15 Ma
452 ± 11 Ma
482 ± 25 Ma
(mean of 3)
496 ± 25 Ma
902 ± 14 Ma
563 ± 49 Ma
705 ± 8 Ma
764 ± 24 Ma
898 ± 26 Ma
475 ± 3 Ma
Reference
Late Penns
Keppie et al., 2004b
Sánchez-Zavala et al., 2004
Talavera-Mendoza et al., 2005
Vachard et al., 2004
M. Permian
~399 Ma (mean of 4)
352 ± 3 Ma
466 ± 25 Ma (peak)
~389 Ma (mean of 7)
357 ± 35 Ma (mean of 8)
462 ± 15 Ma (?interbedded with Cuatlaco)
455 ± 4 Ma
Overlying unit
Permian
Strunian
464 ± 4 Ma
467 ± 16 Ma
461 ± 2 Ma
Ord. Granite
Talavera-Mendoza et al., 2005
Morales-Gámez et al., 2008
Keppie et al., in press
Talavera-Mendoza et al., 2005
Grodzicki et al., in press
Grodzicki et al., in press
Keppie et al., 2006
Morales-Gámez et al., 2008
Hinojosa-Prieto et al., in review
Miller et al., 2007
Keppie et al., 2008
Miller et al., 2007
Keppie et al., 2008
452 ± 6 Ma
447 ± 3 Ma
442 ± 1 Ma
461 ± 2 Ma
Morales-Gámez et al., 2008
Morales-Gámez et al., 2008
Keppie et al., 2008
Keppie et al., 2008
Miller et al., 2007
470–420 Ma
440 ± 15 Ma (mean)
461 ± 9 Ma (peak)
441 ± 11 Ma
Murphy et al., 2006a,b
Vega-Granillo et al., 2007
Talavera-Mendoza et al., 2005
Talavera-Mendoza et al., 2005
Please cite this article as: Keppie, J.D., et al., Synthesis and tectonic interpretation of the westernmost Paleozoic Variscan orogen in southern Mexico: From rifted
Rheic margin to active Pacific margin, Tectonophysics (2008), doi:10.1016/j.tecto.2008.01.012
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J.D. Keppie et al. / Tectonophysics xx (2008) xxx–xxx
Fig. 6. Concordant ages of inherited zircons in Ordovician megacrystic granitoids within low-grade and high pressure suites of the Acatlán Complex (see Fig. 2 for
locations). 206Pb/238U ages b0.9 Ga, and 207Pb/206Pb ages N0.9 Ga. Data sources: Teticic, Ocotitlan, Esperanza, Piaxtla and Tecomate (Talavera-Mendoza et al.,
2005); Palo Liso, La Noria, and Los Hornos (Miller et al., 2007); Huerta and Amate (Morales-Gámez et al., 2008).
indicators (Ramos-Arias et al., 2008). The western boundary of
the HP rocks is a steeply dipping dextral fault (Barley, 2006).
The geological record is as follows (Fig. 5:
(1) Post-700 Ma deposition of immature rift to shelf
psammites and pelites derived from a continental source
with a TDM age of 1.86–1.68 Ga (Murphy et al., 2006a,b;
Vega-Granillo et al., 2007). Whereas the Asis/Santa Cruz
Orgonal metasedimentary rocks yielded detrital zircons
with ages of 705 ± 8 Ma, 764 ± 24 Ma, 888 ± 18 Ma, 900–
1200 Ma, and 1220–1500 Ma (Fig. 3), those along strike
to the north at Mimilulco show an additional population
between ca. 800 and 900 Ma (Fig. 3). The latter are
similar to those occurring in blueschists in the western
Acatlán Complex, which also contains an Ordovician
population (460–490 Ma: Talavera-Mendoza et al., 2005;
Vega-Granillo et al., 2007)(Fig. 3).
(2) Ordovician intrusion of bimodal, megacrystic granitoids
(∼470–420 Ma U–Pb SHRIMP data: Murphy et al.,
2006a,b; 440 ± 15 Ma: Vega-Granillo et al., 2007) and
continental rift tholeiites (SHRIMP age of 442 ± 2 Ma
from zircon cores: Elías-Herrera et al., 2004) from a 0.8–
1.1 Ga mantle source (TDM age: Murphy et al., 2006a,b).
(3) Latest Devonian–Mississippian eclogite facies metamorphism at ∼ 750 °C at ≥ 14 kb was accompanied by
polyphase deformation. A concordant TIMS zircon age of
346 ± 3 Ma (Middleton et al., 2007) provides a younger
limit for this event. A similar, 353 ± 1 Ma SHRIMP age
has also been recorded by Elías-Herrera et al. (2007).
Glaucophane from the blueschists along strike to the
north have yielded a 342 ± 4 Ma age (Elías-Herrera et al.,
2007) and records cooling from peak metamorphic
conditions of ∼ 495–550 °C and ∼ 11.5–13 kb (Elías-
Herrera et al., 2006). This was followed rapidly by
decompression melting producing migmatites (347–
330 Ma zircon SHRIMP ages: Middleton et al., 2007),
retrograde metamorphism, and polyphase deformation
with westward extrusion fabrics (Middleton et al., 2007).
The ∼20 my older ages of 372 ± 8 Ma (zircon LA-ICPMS
mean U/Pb 206Pb/238 U age) and 374 ± 4 Ma (40Ar/39Ar
phengite age from a foliated leucogranite dike at Santa
Cruz Orgonal may indicate slightly earlier decompression
melting (Vega-Granillo et al., 2007) The retrograde
effects are constrained between ∼ 350 and 335 Ma by a
Fig. 7. Present width of the Permian arc in the Acatlán and Oaxaquia terranes
plotted on a graph showing the empirical relationships between the dip of the
Benioff Zone and the widths of arcs and forearc (modified after Tatsumi and
Eggins, 1995).
Please cite this article as: Keppie, J.D., et al., Synthesis and tectonic interpretation of the westernmost Paleozoic Variscan orogen in southern Mexico: From rifted
Rheic margin to active Pacific margin, Tectonophysics (2008), doi:10.1016/j.tecto.2008.01.012
ARTICLE IN PRESS
J.D. Keppie et al. / Tectonophysics xx (2008) xxx–xxx
9
Fig. 8. Cross-section of the Patlanaoaya area (modified from Keppie et al., 2003c).
351 ± 2 Ma 40Ar/39Ar phengite plateau age (Middleton
et al., 2007), the 347 ± 2 to 335 ± 2 Ma phengite ages of
Vega-Granillo et al. (2007), and the ∼341 Ma phengite
age recorded by Elías-Herrera et al. (2007).
2.4.2. Comparison of low-grade and HP rocks
The geochemistry of the HP mafic rocks of the Asis area is
almost identical to that in the overlying Ordovician rift tholeiites
(Fig. 9a and b). In addition, the chemistry and detrital zircon age
populations of the HP sedimentary rocks of the Asis area are
comparable to those of the Cambro-Ordovician Tiñu Formation
of Oaxaquia (Murphy et al., 2005) (Figs. 8c and 6).
Furthermore, the chemistry of the Ordovician megacrystic
granitoids that underwent HP metamorphism is very similar to
that of the low-grade Ordovician megacrystic granitoids
(Fig. 9d and e), and their inherited zircons are very similar
(Fig. 6). The similarity of all of these suites suggests that the HP
rocks of the Asis area represent part of the (?Cambro-)
Ordovician rift-shelf sequence that was subducted (Murphy
et al., 2006a,b; Middleton et al., 2007). These data led Nance
et al. (2006, 2007) and Keppie et al. (2007) to propose that the
leading edge of the continental margin was subducted
westwards and then extruded back up the subduction channel
resulting in a nappe of HP rocks thrust eastwards (present
coordinates) over the low-grade rocks (as envisaged by OrtegaGutiérrez et al., 1999). However, it is now clear that the HP
rocks lie structurally below the low-grade rocks on their eastern
side and were extruded towards the west into the rift-shelf
sequence of the upper plate (Fig. 8). These relationships suggest
the opposite sense of polarity with the Benioff zone dipping
towards the east. The similarity of the HP and low-grade, riftshelf rocks may be interpreted in terms of subduction erosion of
the leading edge of the upper plate, as a result of which the
subducted continental margin material was subjected to HP
Fig. 9. Comparison of the chemistry of the low-grade and HP rocks: (a and b) mafic rocks; (c) sedimentary rocks; (d and e) granitoid rocks.
Please cite this article as: Keppie, J.D., et al., Synthesis and tectonic interpretation of the westernmost Paleozoic Variscan orogen in southern Mexico: From rifted
Rheic margin to active Pacific margin, Tectonophysics (2008), doi:10.1016/j.tecto.2008.01.012
ARTICLE IN PRESS
10
J.D. Keppie et al. / Tectonophysics xx (2008) xxx–xxx
metamorphism and then extruded part or all of the way to the
surface. If extrusion only occurred up subduction channel (Ernst
et al., 1997), then its location would mark a suture between the
plates. However, the central HP band lies in the center of the
Acatlán Complex with similar low-grade rocks on either side
(Fig. 2). This suggests that the HP rocks were extruded into the
rift-shelf sequence of the upper plate and that the later stages of
extrusion took place above the subduction channel into the
overlying upper plate. Such a process is in accord with
numerical models of a pre-collisional active margin undergoing
subduction erosion (Stöckhert and Gerya, 2005), where upper
crustal rocks were taken down to 70 km and returned to a depth
of 10 km in b 20 my by extrusion into the upper plate. The size
and apparent coherency of the HP rocks is also consistent with
the large slices observed in Cascadia and British Columbia
(Monger and Price, 2002; Calvert, 2003). This contrasts with
the piecemeal nature of fragments generally removed from the
Middle American margin that would result in a melange
(Ranero and von Huene, 2000).
Another component in the HP belt is the Tehuitzingo
serpentinite (Figs. 2 and 5), which contains chromite with a
periarc chemistry (Proenza et al., 2004). Other HP rocks with
igneous protoliths have volcanic arc and MORB geochemistry
(Meza-Figueroa et al., 2003). Although the protoliths ages of
these rocks is presently unknown, it would follow if the Asis
rocks underwent subduction erosion, that the arc and periarc,
HP components were likewise subjected to subduction erosion
and so might represent the vestige of a magmatic arc built on the
leading (western) edge of the Mixteca terrane. The MORB
rocks may represent fragments of the subducting crust.
Allowing for at least 20 my for the cycle of subduction erosion
and extrusion (Stöckhert and Gerya, 2005), the inferred arc
could be as old as ca. 366 Ma (i.e. Middle Devonian) or older.
The source of the 370–400 Ma detrital zircons in the Salada
Fig. 10. Paleozoic evolution of southern Mexico shown as a series of cross-sections.
Please cite this article as: Keppie, J.D., et al., Synthesis and tectonic interpretation of the westernmost Paleozoic Variscan orogen in southern Mexico: From rifted
Rheic margin to active Pacific margin, Tectonophysics (2008), doi:10.1016/j.tecto.2008.01.012
ARTICLE IN PRESS
J.D. Keppie et al. / Tectonophysics xx (2008) xxx–xxx
Unit and the Ixtaltepec Formation could then be attributed to the
inferred arc, whereas the 330–360 Ma detrital zircons may have
come from the exhumed HP rocks.
3. Tectonic implications
A series of schematic cross-sections show the sequence of
events recorded in the Paleozoic of southern Mexico (Fig. 10).
Interpretation from these sections are supplemented by
paleogeographic reconstructions for the Ordovician and late
Paleozoic (Fig. 11).
11
In the late Neoproterozoic, the Avalonian arc lay on the
margin of northwestern Gondwana (Oaxaquia and Amazonia)
(Keppie, 2004). Collision of a mid-oceanic ridge with the trench
led to diachronous extinction of the arc and the development of
a transform margin (Fig. 11a) (Keppie et al., 2003b). This
produced a seaway (Excalibur sea—new name) between
Avalonia and Oaxaquia leading to development of a distinct
Avalonian fauna in the Early-Middle Cambrian (Landing,
1996). The reestablishment of a Gondwanan fauna in Avalonia
during the Late Cambrian and lowermost Ordovician (Fortey
and Cocks, 2003; Landing et al., 2007) indicates renewed
Fig. 11. Paleogeographic reconstructions showing the location of southern Mexico through time: (a) separation of Avalonia from Oaxaquia in the Cambrian; (b) further
rifting and separation of Avalonia, Ganderia and Carolinia from Oaxaquia during the Ordovician; (c) Carboniferous reconstruction showing southern Mexico on the
active Pacific margin with subduction beneath the Acatlán and Oaxaquia terranes, which underwent Devonian–Mississippian subduction erosion and extrusion;
(d) Permian arc magmatism during oblique dextral subduction. All modified after Keppie (2004).
Please cite this article as: Keppie, J.D., et al., Synthesis and tectonic interpretation of the westernmost Paleozoic Variscan orogen in southern Mexico: From rifted
Rheic margin to active Pacific margin, Tectonophysics (2008), doi:10.1016/j.tecto.2008.01.012
ARTICLE IN PRESS
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J.D. Keppie et al. / Tectonophysics xx (2008) xxx–xxx
proximity between Avalonia and northwestern Gondwana
(Figs. 10a and 11b). However, during the rest of the Ordovician,
faunal and paleomagnetic data indicate that Avalonia drifted
across the Iapetus Ocean to collide with eastern Laurentia
(Cocks and Torsvik, 2002), either by broadly orthogonal drift
(Murphy et al., 2006a,b) or by further transcurrent motion
(Keppie et al., 2003b). This drift was synchronous with rifting
in southern Mexico, where the presence of 480–440 Ma
bimodal rift magmatism suggests continuous active rifting on
the southern margin of the Rheic Ocean long after the separation
of Avalonia, Ganderia and Carolinia. Synchronous deposition is
indicated by the presence of Ordovician detrital zircons in some
of the units (Table 1, Fig. 10b). A modern analogue may be
found in western North America where extensional deformation
followed collision between the East Pacific Rise and the trench
leading to the development of a slab window, core complexes
and extensional Basin-and-Range tectonics (Dickinson, 2002).
The absence of a true passive margin in southern Mexico
suggests that thermal relaxation following stretching did not
take place, a situation that is consistent with transcurrent
removal of Avalonia, Ganderia and Carolinia. The presence of a
900–750 Ma detrital zircon population in these Ordovician
rocks indicates a unique source in the Goiás magmatic arc in the
Brasiliano orogen (Fig. 11b), so it is likely that associated
zircons of other ages also came from Amazonia and Oaxaquia.
Silurian rocks are absent in southern Mexico (Fig. 5).
However, in northern Mexico, a Silurian shelf sequence rests
unconformably upon Oaxaquia (ca. 1 Ga Novillo Gneiss) and
contains a Gondwanan fauna (Stewart et al., 1999). This
suggests that Oaxaquia drifted passively with Amazonia on the
southern margin of the Rheic Ocean.
During the Devonian, it is inferred that a volcanic arc
developed on top of the Ordovician rift-shelf sequence on the
western edge of the Mixteca terrane (Fig. 5). 400–370 Ma
detrital zircons and volcanic clasts in the Carboniferous rocks of
the Mixteca and Oaxaquia terranes are inferred to have been
derived from this arc because such rocks are absent in Mexico
and southern Laurentia and rare in a few Andean massifs
(Aleman and Ramos, 2000). This Devonian arc and its
underlying rift-shelf sequence is inferred to have been removed
by subduction erosion, such that it is represented only by
vestiges in the extruded HP rocks in the center of the Mixteca
terrane that were extruded during the Mississippian (Figs. 2, 10c
and d, 11c). Exhumation of these HP rocks probably provided a
source for the 330–360 Ma detrital zircons in some Carboniferous units. There appears to be an hiatus in arc magmatism
during the Late Devonian and Carboniferous, a modern
analogue for which occurs in the Cenozoic of southern Mexico
where an hiatus between 30 and 20 Ma coincides with removal
of a 260 km-wide forearc (Keppie et al., in press). The presence
of similar rocks on either side of the HP belt suggests that the
HP rocks were extruded into the upper plate. The extrusion
produced heterogeneous deformation ranging from polyphase
in the extruded rocks through polyphase-single phase in the
adjacent cover, to listric faulting and basin development near the
surface. This decrease in deformational complexity was
accompanied by a similar decrease in the metamorphic grade.
During the Permian, arc magmatism occurred throughout the
eastern Acatlán Complex and western Oaxacan Complex as part
of an arc that extends along the backbone of Mexico produced
by easterly dipping subduction with the trench lying near the
present eastern Guerrero terrane boundary (Figs. 1, 10e and
11d). That this subduction was probably oblique is suggested by
the accompanying dextral transtensional deformation. This
produced pull-apart basins in releasing fault bends with
deposition of the periarc sedimentary rocks, and clastic wedges
where constraining fault bends induced thrusting.
In conclusion, the Paleozoic rocks of southern Mexico
appear to record rifting on the southern margin of the Rheic
Ocean associated with its opening during the Ordovician
followed by passive drifting with Amazonia during the Silurian.
This was followed in the Devonian by subduction beneath
southern Mexico along the paleo-Pacific margin that produced
an arc, which, together with the underlying Cambro-Ordovician
rift-shelf sequence, was removed by subduction erosion. HP
metamorphism of these rocks was then followed by their
extrusion into the upper plate during the Mississippian. The arc
was reestablishment in the Permian with a trench estimated to
lie just west of the boundary between the Mixteca and Guerrero
terranes, which was thrust eastwards during the Laramide
orogeny. The presence of a HP belt within the upper plate rocks
of the Acatlán Complex indicates that the complex does not
mark a suture zone between terranes originating in widely
separated places on different plates (e.g. Talavera-Mendoza
et al., 2005; Vega-Granillo et al., 2007). Thus, sutures defined
by the presence of HP rocks in other orogens around the world,
where extrusion back up the subduction channel is inferred,
may require reexamination.
Acknowledgements
We would like to acknowledge a Papiit grant IN103003 and
a CONACyT grant (CB-2005-1: 24894) to JDK, an NSF grant
(EAR 0308105) and an Ohio University 1804 Award to RDN,
NSF grant (EAR-0308437) to BVM and NSERC Discovery
grants to JBM and JD.
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Rheic margin to active Pacific margin, Tectonophysics (2008), doi:10.1016/j.tecto.2008.01.012