Latest Cretaceous to Miocene deformation events in the eastern

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ABSTRACT

The Cretaceous-Cenozoic major lithologic units and structures of the Sierra Madre del Sur are well known. The Laramide orogeny is generally considered as the cause of the contractile structures, but the details about the migration, kinematics, and intensity of deformation are poorly known. Further-more, the deformation events responsible for the post-Laramide strike-slip and normal faults have not been identifi ed. In this paper, we document the migration of the deforma-tion events that occurred in southern Mexico from Maastrichtian to Miocene time. We identify different groups of structures repre-senting three successive deformation events, based on the geometry, age, and kinematics of tectonic structures. Deformation migrated from west to east. The fi rst event, correspond-ing to the Laramide orogeny, occurred dur-ing Late Cretaceous time in the Guerrero-Morelos Platform and ended in the middle Eocene in the east within the Veracruz basin. The Oaxacan fault system, which bounds the Acatlan-Oaxacan block to the east, records

Laramide shortening. From six structural sections, we interpret the juxtaposition of the Oaxacan complex against the mylonite belt of the Sierra de Juarez, with subsequent uplift of the eastern border of the Oaxacan complex and, fi nally, the gravitational over-riding of the sedimentary cover in a radial centripetal arrangement. The second event produced strike-slip faulting during NE-SW horizontal shortening from Eocene to Oli-gocene time. The third event produced nor-mal and strike-slip faults, indicating NE-SW horizontal extension during Oligocene-Mio-cene time. Major structures produced during these three deformation events are roughly distributed in an arcuate pattern bounding the block formed by the Acatlan and Oaxa-can complexes. Based on this pattern and the relatively less deformed Mesozoic rocks within the Acatlan-Oaxacan block, we inter-pret that most of the deformation resulted from the impingement of this block on thin-ner crustal domains adjoining the block.

Keywords: southern Mexico, Laramide orog-eny, Oaxacan shear zone, strike-slip faulting.

INTRODUCTION

The Cenozoic evolution of southern Mexico involves the movement of the Farallon, North America, and Cocos plates (Engebretson et al.,

1985). Although the relative motion of the tec-tonic plates is well known (e.g., DeMets and Stein, 1990; DeMets et al., 1990), their asso-ciated deformation within the continent has not been well understood. Only the Laramide orogeny has been recognized as the cause of the contractile structures (De Cerna et al., 1980). The deformation events responsible for post-Laramide strike-slip and normal faults have not been inferred. The details about the migration, kinematics, and intensity of deformation of these events remain unknown.

This paper describes the structural features located in the eastern part of the Sierra Madre del Sur, grouped by ages. The knowledge of the geometry and kinematics of the groups of struc-tures is necessary to enhance the understanding of the relationship between the subduction and the continental deformation in southern Mexico.

The basement of southern Mexico consists of metamorphic complexes with ages span-ning from Proterozoic to Cretaceous that form a mosaic of blocks bounded by large faults. Those blocks contain a wide diversity of rocks and structures. The younger structures form a group with folds of reverse transport direction or refolded, strike-slip faults, which are left lateral regardless of their orientation, groups of struc-tures with incompatible kinematics superposed in the same zone, and reactivated structures.

Knowledge of the distribution and charac-teristics of Cenozoic rocks and tectonic struc-

GSA Bulletin; January/February 2006; v. 118; no. 1/2; p. 238–252; doi: 10.1130/B25730.1; 7 fi gures; 1 table.

Latest Cretaceous to Miocene deformation events in the eastern Sierra Madre del Sur, Mexico, inferred from the geometry and

age of major structures

A.F. Nieto-Samaniego†

S.A. Alaniz-Alvarez‡

Universidad Nacional Autónoma de México, Campus Juriquilla, Centro de Geociencias, Apartado postal 1-742, 76001 Querétaro, Qro., México

G. Silva-Romo§

Universidad Nacional Autónoma de México, Facultad de Ingeniería, Ciudad Universitaria, Delegación Coyoacán, 04510 México D.F., México

M.H. Eguiza-Castro#

Universidad Nacional Autónoma de México, Campus Juriquilla, Centro de Geociencias, Apartado postal 1-742, 76001 Querétaro, Qro., México

C.C. Mendoza-Rosales††

Universidad Nacional Autónoma de México, Facultad de Ingeniería, Ciudad Universitaria, Delegación Coyoacán, 04510 México D.F., México

†E-mail: [email protected].‡E-mail: [email protected].§E-mail: [email protected].#E-mail: [email protected].††E-mail: [email protected].

238

LATEST CRETACEOUS TO MIOCENE DEFORMATION IN SOUTHERN MEXICO

Geological Society of America Bulletin, January/February 2006 239

tures in southern Mexico has been signifi cantly improved during the last fi fteen years. Most information comes from recently produced maps (Delgado-Argote, 1989; Campa-Uranga et al., 1998; Rivera et al., 1998; Gonzalez-Ramos et al., 2000; Sanchez-Rojas et al., 2000; Martinez-Amador et al., 2001) and from specifi c studies on major structures (Riller et al., 1992; Tolson et al., 1993; Alaniz-Alvarez et al., 1994; Alaniz-Alvarez and Nieto-Samaniego, 1997; Weber et al., 1997; Eguiza-Castro, 2001; Alaniz-Alvarez et al., 2002; Elias-Herrera and Ortega-Gutiér-rez, 2002). Two studies attempted to integrate the post-Jurassic tectonic evolution of southern Mexico by analyzing tectonic structures inland. One of them (Meschede et al., 1997) infers stress transmission within the continent from the Pacifi c margin. The other study assumes that the Oaxacan and Acatlan complexes form a thick and more rigid block that rotated and was indented into the surrounding crust (Cerca et al., 2004).

The goal of the present study is to understand the way in which the upper crust accommodated deformation in the study area. We present sig-

nifi cant improvements in the timing of the struc-tural features in order to enhance our under-standing of their causes. From the published information about major structures, as well our fi eld data, we integrate a map with the geometry, kinematics, and age of the main structures at a scale of 1: 2,000,000 (Figs. 1C and 2). With this map and based on the stratigraphic record and some key structural characteristics, we formed groups of major structures that defi ne the main deformation events from Maastrichtian to Mio-cene time. Our strategy consisted of: (1) identi-fying the major structures, (2) determining their kinematics, and (3) grouping them by ages. All structures of each group must be “kinemati-cally compatible,” which means that they could have formed or activated under the same homo-geneous deformation event. We highlight the restrictions for tectonic interpretations, which are imposed by the ages and kinematics of the major structures. Most importantly, we present detailed structural cross sections through the Oaxacan fault system (Fig. 1C), the major tec-tonic limit of the region, which controlled most of the deformation.

GEOLOGIC SETTING

The Sierra Madre del Sur is a physiographic province (Raisz, 1959) composed of distinct crustal blocks with different lithologies bounded by major faults. The basements of these blocks constitute outcrops of the lower and middle crust. The Sierra Madre del Sur was divided into the Guerrero, Mixteca, Oaxacan, Xolapa, Juarez, and Maya terranes (Fig. 1; Campa and Coney, 1983). It is notable that many of the structures and the styles of deformation seem unrelated to the terrane divisions. In order to show how the deformation was expressed in different ways and migrated in time, we used lithostratigraphic units grouped in paleogeographic elements.

Oaxacan Complex

The Oaxacan complex outcrops in a north-south belt from 50 km north of Puerto Angel to Tehuacan. In the northern part, this complex is in the hanging wall of the Oaxacan fault (Fig. 2). It is made up of metapelite, quartz feld-spathic gneiss, calcsilicate rocks, metagabbro,

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Figure 1. (A) Location of the study area. (B) The Sierra Madre del Sur is located between the dotted line and the Pacifi c Coast. Tectonostrati-graphic terranes proposed for southern Mexico after Campa and Coney (1983): Ju—Juarez terrane, Gr—Guerrero terrane, Ma—Maya terrane, Mt—Mixteca terrane, Ox—Oaxaca terrane, Xo—Xolapa terrane. (C) Location of the paleogeographic units mentioned in the text and the major structures obtained from the maps of Figure 3; TF—inferred trace of the Tamazulapan fault.

NIETO-SAMANIEGO et al.

240 Geological Society of America Bulletin, January/February 2006



Figure 2 (on this and previous page). Geologic map of the study area, modifi ed from Ortega-Gutiérrez et al. (1992).



and marble, which are intruded by anorthosite, charnockite, and garnetiferous orthogneiss. The protolith ages obtained on igneous rocks vary between 1300 (U-Pb zircon, Solari et al., 2003) and 1012 Ma (U-Pb zircon, Keppie et al., 2003). The cover of the Oaxacan complex is composed of sedimentary rocks: (1) lower Paleozoic (Tiñu Formation) and upper Paleozoic (Santiago, Ixcaltepec, Yododeñe Formations, Robinson and Pantoja-Alor, 1968; and Matzitzi Forma-tion; Carrillo-Martinez and Martinez, 1983; the two later formations are continental); (2) Juras-sic red beds (Etlaltongo Formation; Schlaepfer, 1970); (3) Lower Cretaceous marine sandstone, shale, and calcareous sediments (Tlaxiaco and Zapotitlan Formations) with local evaporites (San Juan Teita Formation); (4) Upper Creta-ceous limestone (Teposcolula Formation) and clastic deposits of Campanian to Maastrichtian age (Yucunama Formation) (e.g., Lopez-Ticha, 1985); and (5) Cenozoic rocks, which are con-tinental and include conglomerate (Tamazula-pan Conglomerate) and sandstone (Yanhuitlan Formation) intruded by hypabyssal mafi c bod-ies, and volcanic rocks, which are mostly pyro-clastic (Llano de Lobos Formation) (Ferrusquia, 1970; Martiny et al., 2000; Moran-Zenteno et al., 2000).

Acatlan Complex

The Acatlan complex (Fig. 2) is made up of three groups of Ordovician-Devonian metamor-phic rocks (Petlancingo Group, Piaxtla Group, and Tecomate Formation), which were formed in trench-forearc, oceanic lithosphere, and vol-canic arc environments, respectively (Ortega-Gutiérrez et al., 1999; Meza-Figueroa et al., 2003). Overlying the Acatlan complex are con-tinental (Matzitzi Formation) and marine (Los Arcos and Olinala Formations) sedimentary deposits of Late Devonian–Early Permian age and strata from the Tlaxiaco basin described in the following (Lopez-Ticha, 1985). The Acatlan and the Oaxacan complexes have the same Cre-taceous and Cenozoic cover.

The outcrops of Acatlan and Oaxacan com-plexes outlined in Figure 1 form the larger block of pre-Mesozoic metamorphic rocks in southern Mexico and are bounded by major faults. Elias-Herrera and Ortega-Gutiérrez (2002) proposed the joining of the Oaxacan and Acatlan com-plexes in the Permian, and Lopez-Ticha (1985) inferred that the common cover was not initiated until Early Cretaceous time. Since the Creta-ceous, the stratigraphic record and the structures indicate that both complexes formed a basement unit with similar sedimentation and deformation conditions. In order to analyze Maastrichtian–Miocene deformation in this work, we grouped

the Oaxacan and Acatlan complexes into a sin-gle unit, which we refer to as the Acatlan-Oaxa-can block.

Xolapa Complex

The Xolapa complex consists of a belt ~600 km long and ~50 km wide of metamor-phic and plutonic rocks emplaced along the southern margin of Mexico. The larger bound-aries are the Pacifi c Coast to the south and a major tectonic structure to the north (Fig. 1). The Xolapa complex is composed of mid-crustal, arc-related gneisses intruded by unde-formed Cenozoic calcalkaline granitoids. A wide range of ages has been determined for the plutonism: 160–136 Ma (U/Pb zircon, Ducea et al., 2004); 144–128 Ma (Rb/Sr whole rock, Moran-Zenteno, 1992); and 66–46 Ma (U/Pb zircon; Herrmann et al., 1994). The metamor-phic complex was intruded by Paleocene-Mio-cene granitic rocks with no penetrative defor-mation (Ortega-Gutiérrez, 1980; Schaaf et al., 1995). Mylonitic rocks located on the northern edge of the Xolapa complex represent a major shear zone with activity between Albian time and 34 Ma (U/Pb zircon) in Tierra Colorada (Riller et al., 1992) and between 29 (U/Pb zircon) and 23 Ma (K-Ar hornblende) in the Huatulco area (Tolson, 1998). No strata cover the crystalline rocks in the Xolapa complex.

Guerrero-Morelos Platform

The Guerrero-Morelos platform is the west-ernmost paleogeographic element of the study area (Fig. 1). The stratigraphic record here can be divided into three lithological units: (1) In the eastern part, the lower unit is the Zicapa Formation, which is made up of red beds with intercalated beds of marine limestone. Its con-tact with the overlying limestone is transitional. (2) In the western part, the Zicapa Formation is absent and the lower unit is the Huitzuco Anhy-drite (DeCserna et al., 1980). The Zicapa For-mation and Huitzuco Anhydrite do not have fos-sils; their ages are inferred to be Aptian-Albian because they underlie the Morelos Formation, which contains fossils indicative of Albian age. (3) The most characteristic rocks of the Guer-rero-Morelos platform comprise a thick succes-sion (>800 m) of Albian-Maastrichtian marine strata (Morelos, Cuautla, and Mexcala Forma-tions), which overlie the Zicapa Formation and Huitzuco Anhydrite (DeCerna et al., 1980; Her-nandez-Romano et al., 1997). This marine suc-cession is made up of shallow marine limestone that grades up to Turonian-Campanian pelagic limestone and siliciclastic rocks. These strata are as young as Maastrichtian in the eastern

part of the platform (Hernandez-Romano et al., 1997; Aguilera-Franco, 2003). An unconformity is present between this sequence and the over-lying Tetelcingo Formation, which is made up of volcanic rocks of Maastrichtian age (Ortega-Gutiérrez, 1980) and continental red beds of Paleocene-Eocene age. There are no outcrops of basement within the Guerrero-Morelos plat-form; to the east and south outcrops of the Acat-lan complex are exposed (Campa-Uranga et al., 1998; Gonzalez-Ramos et al., 2000).

Tlaxiaco Basin

Lopez-Ticha (1985) proposed that a major basin formed above rocks of the Acatlan-Oa xacan block, the Tlaxiaco basin (Fig. 1). The Tlaxiaco basin was developed mainly during the Jurassic with deposition of shallow-marine shelf sediments, starting with continental conglomer-ates (Cualac Conglomerate), paludal deposits (Tecocoyuncan and Chimeco Formations) that progressively become deep-marine sediments (Cidaris Limestone and Sabinal Formation). These sediments are limited to the east by the Tamazulapan fault (Caltepec fault in Elias-He rrera and Ortega-Gutiérrez, 2002). During Cretaceous time, calcareous sedimentation dominated and extended to the east of the Tama-zulapan fault (Tlaxiaco Formation). Anhydrites of the San Juan Teita Formation are interbedded in the limestones, between the Tlaxiaco Forma-tion (Neocomian) and the Teposcolula Forma-tion (Albian). The western part of the Tlaxiaco basin remained active until the Cenomanian and the eastern part until the Maastrichtian (Yucu-nama Formation; Lopez-Ticha, 1985; Gonzalez-Ramos et al., 2000). Eocene-Miocene volcanic rocks and continental strata overlie the Tlaxiaco basin (Martiny et al., 2000).

Zongolica Basin

Located in the northeastern part of the study area, the Zongolica basin consists of marine deposits of Kimmeridgian-Tithonian to early Eocene age (Fig. 1C). In the Zongolica basin, the older rocks are Upper Jurassic–Lower Cre-taceous volcanosedimentary deposits (Chivi-llas Formation; Carrasco, 1978), composed of marine basalts, sandstone, conglomerate, and shale. The volcanosedimentay rocks crop out along the footwall of the Oaxacan fault (Fig. 2). Overlying the volcanosedimentary rocks, the Cretaceous of the Zongolica basin is com-posed of limestone, mudstone, and sandstone (Ta maulipas Superior, Maltrata, and Tecam-alucan lower member Formations), overlain by fl ysch-type sediments from the upper member of the Tecamalucan Formation (Alzaga-Ruiz



and Santamaria-Orozco, 1987). These forma-tions change laterally to the east into reef facies of the Cordoba platform described next.

Cordoba Platform

The Cordoba platform (Fig. 1C) consists of more than 5000 m of Cretaceous calcareous deposits, mainly reef limestones (Orizaba, Guz-mantla, and Atoyac Formations). This sequence overlies Jurassic continental strata, which con-sist of red conglomerate and sandstone (Todos Santos Formation). The Xonamanca Formation, in the base of the platform, consists of lithic sandstone, mudstone, and minor volcanic rocks deposited in a shallow-marine environment. These units are correlated with the Zongolica basin located to the west. Some Upper Creta-ceous basinal sediments are interbedded in the platform (Maltrata, Guzmantla pelagic member, San Felipe, and Mendez Formations; Gonzalez-Alvarado, 1976).

Veracruz Basin

The Veracruz basin is located to the east of the Cordoba platform (Fig. 1C). This basin contains Tertiary strata in open folds associated with strike-slip faults. Most of the stratigraphic information comes from wells and seismic lines. In the western part of the Veracruz basin, the Tertiary strata overlie a sequence of Upper Cre-taceous limestones and shaly limestones belong-

ing to the Cordoba platform, which are folded and faulted by thrusts. This area was interpreted as the tectonic front of the Laramide orogeny (Mossman and Viniegra, 1976). Some Paleo-cene and lower Eocene rocks appear involved in the imbricated structure (see schematic cross section in Figure 2 of Mossman and Viniegra, 1976). The oldest strata unconformably overly-ing the tectonic front are upper Eocene. They consist of a lower conglomerate of limestone clasts, green shale, marl, and sandy shales (Chapopote Formation). The Oligocene to middle Miocene section consists of green shale and sandy shales (Horcones, La Laja, Deposito, Encanto, and Lower Concepcion Formations) culminating with the presence of volcanic clasts in the shales (Upper Concepcion Formation). The youngest strata consist of continental shale and sandstone with volcanic clasts, tuff, and conglomerate (De la Fuente-Navarro, 1959). For a complete description of the stratigraphy of the Veracruz basin, see Meneses-Rocha and Velasco-Vasquez (1999).

THE MAIN STRUCTURES OF SOUTHERN MEXICO

Major Structures and Kinematic Incompatibilities

Figure 1C shows the main structures of the eastern Sierra Madre del Sur, obtained from the maps referenced in Figure 3. The structures

form a complex pattern in which structures have incompatible displacements. A group of folds and thrust faults provides evidence of a shortening event related to the Laramide orogeny. A second group was formed by strike-slip faults that are not kinematically compatible with the Laramide shortening and could not have been produced under a simple transpressional or transtensional regime. The last group consists of major normal faults that are incompatible with the other struc-tures and that record a stretching event.

Considering that the groups could not be formed under a single deformational event given the kinematic compatibility criterion used here, and taking into account their wide distri-bution in the area, we separate structural groups by ages in order to look for events that may explain each group of structures. This exercise leads to the identifi cation of compatible groups of structures and reconstruction of the sequence of deformation events, from Maastrichtian to Miocene time (Fig. 4).

Groups of Structures

Laramide Structures (Late Cretaceous–Middle Eocene)

Migration of the deformation. Rocks affected by major thrust faults and folds appear throughout the study area (Fig. 4B). In the eastern half of the area, the youngest rocks affected by thrust faults and folds are Late Cretaceous in age (Eguiza-Castro, 2001; Martinez-Amador et al., 2001), but subsurface data from the nearby Veracruz basin indicate that the youngest rocks affected by shortening deformation are early Eocene in age. In the Veracruz basin, the thrust faults imbricate Cretaceous to early Eocene units, and the tec-tonic front underlies an unconformity that spans the middle Eocene (Mossman and Viniegra, 1976). The existence and the distribution of the unconformity are well established by seismic profi les and petroleum exploratory and produc-tion wells (Mossman and Viniegra, 1976). An important characteristic is that the unconformity is restricted to the zone affected by shortening deformation and is absent to the east of the tec-tonic front. Mossman and Viniegra (1976) inter-preted this to mean that the unconformity devel-oped under marine conditions. This information indicates the Laramide shortening in this region spans the middle Eocene.

Within the Acatlan-Oaxacan block, the old-est units unaffected by shortening deformation are continental sedimentary rocks intruded by a hypabyssal mafi c body with a K-Ar hornblende age of ca. 40 Ma (Martiny et al., 2000). In the western part of the study area (Guerrero-More-los platform, Figs. 1B and 2), the youngest unit affected by intense shortening deformation is

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Figure 3. Location of the maps used in this work. 1—Rivera et al. (1998), 2—Campa-Uran ga et al. (1998), 3—Martinez-Amador et al. (2001), 4—Gonzalez-Ramos et al. (2000), 5—San-chez-Rojas et al. (2000), 6—Tolson et al. (1993), 7—Delgado-Argote (1989), 8—Lopez-Ticha (1985), 9—Ortega-Gutiérrez et al. (1992), 10—Eguiza-Castro (2001), 11—Silva Romo (2005, personal commun.), 12—Alaniz-Alvarez et al. (1994).



Figure 4 (on this and following page). The major structures of southern Mexico grouped by age and considering kinematic compatibil-ity. Convergent arrows indicate direction of maximum horizontal shortening, and divergent arrows the maximum horizontal extension. (A) Map showing major structures of the study area. (B) Maastrichtian to Paleocene contractile structures, arrows in the western part indicate the regional shortening direction. (C) Late Eocene–early Oligocene strike-slip faults. (D) Oligocene-Miocene normal and strike-slip faults; little arrows indicate local strain directions.

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the Mexcala Formation. The age of this unit has been established by fossils as Coniacian in the west (Tetelcingo region) and Maastrichtian in the east near Texmalac (Fig. 4A) (Alencaster, 1980). Alaniz-Alvarez et al. (2002) reported basic dikes without shortening deformation with an 40Ar/39Ar hornblende age of ca. 52 Ma in Taxco, and Ortega-Gutiérrez (1980) reported that Tetelcingo Formation volcanic rocks of Maastrichtian age (66 ± 2.3, K-Ar biotite; 68.8 ± 2.3, K-Ar basalt whole rock) discordantly over-lie the deformed Mexcala Formation.

The available data agree with the migration of contractile deformation to the east as postu-lated by DeCserna et al. (1980). Deformation took place during the Santonian-Campanian (ca. 85 to ca. 71 Ma) at Tetelcingo, in the Paleocene at Texmalac, before Bartonian (40 Ma) across the Acatlan-Oaxacan block, and in the middle Eocene in the Veracruz basin far to the east (Fig. 4B).

Style and kinematics of deformation. Major structures that we classifi ed as hav-ing been produced by the shortening defor-mation phase (Fig. 4B) fulfi ll the following characteristics: (1) they are contractile struc-tures, and (2) appear in most of the published maps, although in Veracruz basin, the young-est deformed units are in the subsurface. The probable direction of shortening was E-W in the Guerrero-Morelos platform and NE-SW in the Zongolica fold-and-thrust belt, including the eastern edge of the Acatlan-Oaxacan block. The transport direction toward the east and northeast is inferred from the thrusting blocks on reverse faults, the tendency of the axial plane of folds to dip toward the west, and from stretching lineations and kinematic indicators measured in the Oaxacan fault system (Fig. 1, and section 3 in Fig. 5A).

The Oaxacan fault system has been reac-tivated during multiple deformation phases (Alaniz-Alvarez et al., 1996; Alaniz-Alvarez and Nieto-Samaniego, 1997). In Figure 5, we present cross sections showing the kine-matics of the main structures as well as the structural style. Three main observations are noted. (1) Along the segment between Oaxa-can and Cuicatlan (sections 6 and 5), a Jurassic mylonite belt is present in the footwall of the Oaxacan fault system with mineral lineations trending N-S, and the reverse faults have mod-erate SW dips. (2) From Cuicatlan to Calipam (sections 4 and 3), the mylonite belt disap-pears toward the north because it is covered by the Cretaceous folded sequence (Fig. 2). Flanking the Cretaceous sequence is a zone of high-angle thrust faults, which form imbricate slices of metamorphic rocks (gneiss, schist, and phyllite) with 1000–1067 Ma U/Pb zir-

con core sensitive high-resolution ion micro-probe (SHRIMP) ages (Angeles-Moreno et al., 2004), thrust over the Mesozoic sedimentary rocks. Considering the attitude of foliations and lineations, the vergence of folds, as well as the measured kinematic indicators, we infer an ENE-WSW shortening direction and trans-port direction to the ENE (Fig. 5A, section 3). (3) From Calipam to near Tehuacan (sections 2 and 1), the metamorphic rocks do not crop out, only volcanosedimentary Jurassic and Cretaceous calcareous units are present. In sec-tion 2, the volcanosedimentary unit is strongly deformed, with many folds and thrusts and a persistent foliation, whereas near Tehuacan, in the type locality of the volcanosedimen-tary sequence (Chivillas Formation; Carrasco, 1978), the sequence is not folded or thrusted, only gently tilted, and lacks foliation. (4) The fold-axis directions in the Lower Cretaceous rocks change from N-NW to E-NE, to form an arcuate pattern of folds and thrust faults around the basement outcrops (Figs. 2 and 4). In sec-tion 1, the Cretaceous limestone and shale have recumbent folds with vergence to the NNE and overlie a volcanosedimentary sequence that shows small or no deformation. As sketched in section 2, there is a clear diminishing of defor-mation intensity toward the lower levels. This style of deformation suggests thin-skinned deformation, with the Cretaceous limestone and shale overriding the volcanosedimentary Upper Jurassic–Lower Cretaceous rocks.

These observations represent a sequence of deformational events linked to the Laramide orogeny. The shortening event migrated from west to east, changing from Santonian-Cam-panian (ca. 85–71 Ma) around ~100°W longi-tude to post-Maastrichtian, pre-Bartonian (ca. 65–41 Ma) at ~98°W longitude, and ending in the Veracruz basin near ~96°W longitude in the middle Eocene (49–37 Ma). During the migration of deformation, the Acatlan-Oaxa-can block impinged against the mylonite belt located along its eastern border and overrode the mylonite. Overthrusting by the basement rocks only occurred along the block’s eastern border, where its trend is N-S and is absent in the northern boundary, which is oriented NE-SE to WNW-ESE (Figs. 2 and 5). The arc formed by the folds and thrust faults, and the thin-skinned style of the deformed Cretaceous rocks, strongly suggest the uplift of the Acat-lan-Oaxacan block, or at least the formation of an elevated border. This situation was envi-sioned in the classic explanation for the origin of the thrust faults and folds in the Cordoba platform, which is located immediately east of the Zongolica basin (Fig. 1C) (Gonzalez-Alvarado, 1976).

Strike-Slip Structures (Eocene–Oligocene NE-SW Shortening)

Figure 4C shows the location of major strike-slip faults and NW-SE extension. The major structure constitutes the northern edge of the Xolapa complex (Fig. 1C). Superposed ductile and brittle structures appear along this fault zone. Three localities along the northern edge of this metamorphic belt have been studied: Tierra Colorada and Juchatengo by Riller et al. (1992) and Ratschbacher et al. (1991); and the Chacalapa–Santa Maria Huatulco zone by Tol-son (1998). In all of these locations, the ductile and ductile-brittle structures indicate a transten-sional regime, with a well-documented left-lat-eral component.

The northern edge of the Xolapa complex in Tierra Colorada region (Fig. 4) is composed of north-dipping mylonites, ultramylonites, and cataclasites with shear criteria showing top-to-the-northwest ductile fl ow (Ratschbacher et al., 1991). The structural data show that movement along the mylonite zone had normal and strike-slip left-handed components. The age of the movement is bracketed by an Albian maximum age (Morelos Formation) and a 34 Ma minimum age (U/Pb zircon, postectonic Tierra Colorada granite, Herrmann et al., 1994). In the Huatulco region, an ENE-WSW–trending L-mylonite belt (Fig. 4) recorded strike-slip left-handed move-ment that occurred between 29 and 23.7 Ma. The ages correspond to the mylonitized Huatulco intrusive and undeformed granodioritic dikes that cut the mylonite belt (Tolson, 1998).

To the north of the Xolapa complex, the available data establish the presence of strike-slip faulting in Taxco (Fig. 4C) during the late Eocene, thereby documenting a clear change in the tectonic regime after the Laramide orogeny. However, continental conglomerates of Paleo-cene-Eocene age show tilted beds, suggesting the presence of wide and open folds. These deformed strata have been reported in the Taxco (Edwards, 1955; Alaniz-Alvarez et al., 2002) and Atenango regions (Fitz-Diaz, 2001). Other faults located in the Taxco region show lateral move-ment. Those faults were active between 36.6 ± 1.0 (K-Ar in hornblende) and 33.1 ± 0.8 Ma (K/Ar in sanidine), with the maximum horizontal shortening oriented NE-SW (Alaniz-Alvarez et al., 2002) and occurring after Laramide defor-mation, which is constrained to pre-Maastricht-ian time in this region (see discussion about the Laramide age above). Similar ages (ca. 40 Ma, hornblende K-Ar) have been obtained in mafi c rocks intruded within sediments of the Yanhuit-lan Formation (Martiny et al., 2000).

Northwest-trending, left-lateral faults were reported in the Tilzapotla region within the Guerrero-Morelos platform (Fig. 4C). The activ-



ity of these faults was coeval with the emplace-ment of the late Eocene–Oligocene Tilzapotla caldera (35–33 Ma, K-Ar, 40Ar-39Ar, Rb-Sr; Moran-Zenteno et al., 2004). The post-Laramide counterclockwise rotations determined paleo-magnetically in the Guerrero-Morelos platform by Molina-Garza et al. (2003) are consistent with a simple shear event with NW-SE extension. The single locality reported by these authors within the Acatlan-Oaxacan block does not show sig-nifi cant rotation. With the available informa-tion, it is not possible to establish a considerable Eocene-Oligocene strike-slip event with NW-SE extension within the Acatlan-Oaxacan block.

Normal and Strike-Slip Faults (Oligocene-Miocene NE-SW Extension)

The last documented deformation event formed the structures of Figure 4D, which are compatible with NE-SW extension. The early Oligocene age of strike-slip faults in the Taxco region is well documented at ca. 33 Ma (Alaniz-Alvarez et al., 2002). Based on a K-Ar age of a tuff interbedded in the lower to middle part of the strata in the Tehuacan basin, we infer activ-ity of the Oaxacan fault under NE-SW extension before 27.1 ± 0.7 Ma (Table 1). In addition, early Miocene ages of volcanic units interbedded with basin-fi ll deposits near Oaxacan (Suchilquitongo Formation) were reported by Urrutia-Fucugau-chi and Ferrusquia-Villafranca (2001).

Within the Acatlan-Oaxacan block, thick continental deposits and volcanic rocks are late Eocene–Oligocene in age. These deposits do not show a relation with normal faults (Ferrus-quia-Villafranca, 1976; Martiny et al., 2000). In contrast, major normal faults roughly bound the eastern edge of Acatlan-Oaxacan block. We interpret this observation to indicate that exten-sional deformation was accommodated mostly around the Acatlan-Oaxacan block by reactiva-tion of major preexisting structures. Thus, the deposition of the Eocene-Oligocene strata prob-ably is linked to the high elevation of the east-ern border of the Acatlan-Oaxacan block, which formed during Laramide deformation.

DISCUSSION AND SUMMARY

Meschede et al. (1997) published a series of paleostress tensors calculated by fault-slip inver-sion in southern Mexico. In that work, there was no age control of the data sets; rather only relative striae ages were deduced using overprinting cri-teria. They calculated four groups of paleostress tensors and correlated them with plate-conver-gence vectors on the Pacifi c Coast from 150 to 20 Ma. Kinematics of tectonic plates could be ultimately responsible for inboard deformation of southern Mexico, but the direct stress transmission

mechanism proposed by Meschede et al. (1997) is not supported by the available stratigraphic and geochronological data. The stratigraphic record establishes that shortening and strike-slip events occurred at different times in different zones, but this temporal separation of deformation is evi-dent only if age control is available. The spatial distribution and ages of contractile and strike-slip structures in the study area show disagreement with the paleostresses obtained by Meschede et al. (1997). For example, a well-established strike-slip regime with NW-SE extension was active in the Huatulco zone between 29 and 23.7 Ma, and at the same time, the half-graben of Tehuacan was active at 27.1 Ma under NE-SW extension (Fig. 6).

Cerca et al. (2004) presented an analogue model simulating Laramide and post-Laramide deformation of the Sierra Madre del Sur. Their model is based on the assumption that the Acat-lan and Oaxacan complexes constitute a block (Acatlan-Oaxacan block) that was thicker and more rigid than the surrounding crust. They considered that Laramide deformation took place before the Maastrichtian and proposed that younger contractile deformations are due to other tectonic events related to Paleocene trans-lation and vertical-axis rotation of the Acatlan-Oaxacan block.

Our analysis of major structures and the strati-graphic record of the study area strongly support the existence of a more rigid block made up of the Acatlan and Oaxacan complexes and that defor-mation was accommodated mainly around it. Both features of the study area were implicitly or explicitly assumed by Cerca et al. (2004). How-ever, we disagree with the following interpreta-tions from their model, which are not supported by fi eld observations and published data.

1. What is the location of the limits of Acat-lan-Oaxacan block? The thick gray line in Fig-ure 1 shows the edge of large outcrops of the Acatlan and Oaxacan complexes. Cerca et al. (2004) proposed a similar limit for the Acatlan-Oaxacan block, except along its eastern edge. They considered the Vista Hermosa fault as the easternmost edge (Fig. 1C). We interpret the Oaxacan shear zone as the eastern limit of the Acatlan-Oaxacan block based on the following features: (1) The stratigraphy of the Cuicatecan basin, located between the Oaxacan and Vista Hermosa fault zones (Fig. 1C), is made up of Cretaceous mafi c marine volcanic rocks over-lain by a thick sequence of marine basin strata deposited prior to the Laramide shortening (e.g., Barboza and Schwab, 1994). The basin sequences indicate a relatively thinned crust for Cretaceous time. (2) The Oaxacan fault zone has been interpreted as a major crustal bound-ary, because it has been reactivated under thrust,

normal, and strike-slip regimes (Alaniz-Alva-rez et al., 1996). (3) In sections 3, 4, 5, and 6 of Figure 5 across the Oaxacan fault system, the juxtaposition of the Precambrian rocks over the mylonite belt by moderate- to high-angle thrust faults is clearly illustrated. In contrast, the Vista Hermosa fault does not involve Pre-cambrian rocks, and its trace is sinuous, which is typical of a low-angle fault; both characteris-tics strongly suggest that the fault developed in a shallow structural level. Therefore, the Vista Hermosa fault is merely the southeast front of the Zongolica fold-and-thrust belt.

2. What is the age of Laramide contractile deformation? Cerca et al. (2004) considered Laramide deformation to be pre-Maastrichtian in age, and they did not consider the migration of the orogenic event from west to east (Fig. 6). This point is crucial for their model, because they proposed a Paleocene phase of deformation unrelated to the Laramide orogeny and associ-ated it with rotation of the Acatlan-Oaxacan block around a vertical axis. They invoked this phase of deformation to explain the reverse dis-placement structures (west-vergent folding and thrusting) in the eastern edge of the Guerrero-Morelos platform. The main question emerging is how to reconcile Paleocene counterclockwise rotation in the western edge of the Acatlan-Oaxacan block with an easternward transport direction of its eastern edge during Paleocene–middle Eocene time. We believe the well-docu-mented migration of the shortening event must be considered. The reverse vergencies can be explained by other mechanisms. In fact, there are no structural or geologic mapping studies addressing the origin of those structures.

We summarize the deformation evolution of the eastern Sierra Madre del Sur as follows (Fig. 7):

1. During Early Cretaceous time, the study area of southern Mexico was under marine conditions. During the Late Cretaceous the ces-sation of sedimentary deposition occurred in different places at different times. The end of marine sedimentation partially constrains the onset of an orogenic event that migrated east-ward with time. The migration of shortening is well constrained in time, from Santonian in the westernmost part of study area to middle Eocene in the easternmost part (Fig. 6).

2. Important evidence of the evolution of shortening deformation is obtained from the Oaxacan fault system. The presence of moder-ate- to high-angle thrust faults affecting the meta-morphic basement rocks and the sedimentary Mesozoic cover, the gravitational overriding of the uppermost part of the sedimentary sequence, and the centripetal radial pattern of folded and thrust-faulted Mesozoic rocks around the Acat-lan-Oaxacan block permit the inference of two



v

v

v

v

v

97°

17°

18°18°

Oaxaca

PuertoÁngel

Cuautla

Tehuacán

Pacific Ocean

99°

Chilpancingo

1 23

45

6

Elevation(m

abovesealevel)

Elevation(m

abovesealevel)

Elevation(m

abovesealevel)

Beds (Kacl) N = 31

N

Normal faults N = 19

N

Beds (Kacl) N = 18

N

Beds (Ksl) N = 21

N

Beds (Tth) N = 30

N

2200

1800

S N

Tth

KaclKacl

Ksl

SECTION 1

Horizontal Scale

1 205001000Meters Kilometers

Tth

Upper Cretaceous limestone-lutiteKsl

Cretaceous ( ) limestoneAlbian-Cenomanian

Tth Tertiary continental deposits

Kacl

Upper Jurassic-Lower Cretaceous volcanosedimentary strata

J-Kivs

J-Kivs

SECTION 2

Beds (Kis) N = 16

N

Faults N = 12

N

Beds (Kivs) N = 32

N

Horizontal Scale


1400

1600

1800

2000

Tth

Kis

KisKacl

Kis Lower Cretaceous shale-sandstone

Cretaceous ( ) limestoneAlbian-Cenomanian

Tth Tertiary continental depositsKacl


SW NE

J-Kivs

J-Kivs

J-Kivs

SECTION 3

Horizontal Scale1500

1900

2300

2700

JlsJls

Kis

Kis

Kis

Kis

Tth

Kis Lower Cretaceous shale-sandstoneKicg Lower Cretaceous red beds


Precambrian Oaxacan Complex (gneiss and schist)

Jurassic shale and limestoneKicg

Foliations (Met) N = 35

S-C structures, grain grooves, crystalfibers, tension gashes and asymmetricfolds indicate top-to-the-ENEtransport direction.

Lineations N = 16

N

Beds (Kis) N = 59

N

Beds (Jls) N = 31

N


N

Jls

SW NE

Ptmmet

Ptmmet




Jurassic mylonite

Kil Lower Cretaceous limestone-sandstone


SECTION 4Horizontal Scale


3000

2000

1000

0

Jmy

Jmy

Tth Kil


SW NE

J-Kivs

J-Kivs

J-Kivs

Ptmmet

Ptmmet

SECTION 5

10

Horizontal Scale


2000

0J-KivsKis

Kis

Jmy

Kil

Tth

Foliations (Jmy) N = 18

N

Lineations (Jmy) N = 16

N

SW NE

Ptmmet

Ptmmet

J-Kivs

Kis Lower Cretaceous Shale-sandstone


Jurassic mylonite


Jmy

Kil Lower Cretaceous limestone-sandstoneJ-Kivs Upper Jurassic-Lower Cretaceous volcanosedimentary strata

Ptmmet

SECTION 6Horizontal Scale


Ptmmet

Kis Lower Cretaceous Shale-sandstone

Tth

Tvr

Tertiary continental deposits

Tertiary acid pyroclastic rocks

Jurassic mylonite


Upper Jurassic-Lower Cretaceous volcanosedimentary strataJmy

0

3000

1000

2000Jmy

Kis

Kis

Tvr

TthTvr

Foliations (Jmy) N = 62

N

Lineations (Jmy) N = 14

N

SW NE

J-Kivs

J-Kivs

Ptmmet

v

v

v

v

v

97°

17°

18°18°

Oaxaca

PuertoÁngel

Cuautla

Tehuacán

Pacific Ocean

99°

Chilpancingo

1 23

45

6

Elevation(m

abovesealevel)

Elevation(m

abovesealevel)

Elevation(m

abovesealevel)

Figure 5 (on this and previous page). Structural sections along the Oaxaca fault system. Section locations coincide with some published sec-tions: Sections 6 and 5 were traced along sections C–C′ and B–B′ of Gonzalez-Ramos et al. (2000); section 4 was traced along the section B–B′ of Delgado-Argote (1989); and sections 3, 2, and 1 follow sections 4, 3, and 1 from Eguiza-Castro (2001).



TABLE 1. AGE DETERMINATION FOR THE TUFF INTERBEDDED IN THE BOTTOM OF TEHUACAN BASIN

Sample Longitude(W)

Latitude(N)

Mineral Rock type 40Ar atmospheric(%)

K(%)

40Ar radiogenic(ppm)

Age(Ma)

THE 04 97°24′2′ 18°20′40′′ muscovite Rhyolitic tuff 43 6.35 6.61 27.1 ± 0.7

Note: Mineral separation was performed by the authors in the Instituto de Geológia, Universidad Nacional Autonoma de Mexico. Analysis was performed by Activation Laboratories Ltd.

180

206

248

290

417

490543

1000

443

357

LaramideShortening

Strike-slip faulting(Taxco)

(Tierra Colorada?)

PTmmet

PmxPmx

PmxCambrian

Neoproterozoic

Mesoproterozoic

OrdovicianSilurian

DevonianCarboniferous

Permian

Jurassic

Triassic

Paleocene

MaastrichtianCampanianSantonianConiacianTuronian

Albian

Aptian

BarremianHauterivianValanginianBerriasian

Cenomanian

Oligocene

Miocene

Pliocene

Pleistocene

Holocene

Eocene

L

L

M

E

E

E

M

L

Guerrero - MorelosPlatform

Acatlan Complexand cover

Oaxacan Complexand cover

Zongolicafold-thrust belt

Jmy

Tvsc

TvscTvsc

Tvsc

(Tepeji)

Normalfaulting (Huatulco)

VeracruzBasin

Tmm

Ks

KiJ-Kivsm

KsKs

Ki

Jc

Ks

Ki

Jmmx

Ks

approximated W Longitude

96°97°98°99°100°

0.01

1.8

5.3

23.8

28.5

33.7

37

49

53

65

89

99

127

144

159

Cen

ozoi

cQ

uate

rnar

y

Mes

ozoi

c

Cre

tace

ous La

te

Ear

ly

Pal

eozo

ic

Tert

iary

Mmet

Ma

Psmx

Pimet

Pimet

Ki

Ks-paTpa

Tpa

Figure 6. Stratigraphic columns through the study area. The ages of deformation events discussed in the text are indicated: solid lines cor-respond to the Laramide shortening; dashed lines correspond to strike-slip faulting associated with NE-SW shortening; and dotted lines correspond to normal faulting associated with NE-SW extension. Tmm—Upper Eocene-Miocene marine strata (in subsurface); the other labels of stratigraphic units are the same as in the Figure 2.



steps for this deformation event: (1) an initial eastward transport and juxtaposition of Pre-cambrian basement over the mylonite belt in the Oaxacan fault zone, producing the uplift of the eastern border of the Acatlan-Oaxacan block; and (2) a gravitational detachment of the sedimentary cover with transport direction away from the Acatlan-Oaxacan block, forming an arc of folds and thrust faults around the block.

3. Shortening deformation was followed by formation of multiple strike-slip faults. The age constraints of the later event indi-cate a migration in space from west to east. The transition between shortening and strike-slip events is not well understood, but coin-cides with the Paleocene age of refolded and reverse-displacement structures located along the boundary of the Guerrero-Morelos plat-form and the Acatlan-Oaxacan block (Fig. 1). Evidence for the Eocene-Oligocene NW-SE extension has not been documented in either the Oaxacan fault system or the Acatlan-Oaxa-can block. When left-lateral strike-slip faulting took place in the Pacifi c Coast at ~97°W longi-tude, in the Oaxacan fault system near Tehua-can, the NE-SW extension was active since at least 27 Ma. The late Eocene–Oligocene event may have been a regional event with most of the strain to the west and south of the Acatlan-Oaxacan block, which acted as a rigid block after thickening in the Laramide event. NW-SE extension, NE-SW shortening, and E-W strike-slip faults at the southern edge of the Acatlan-Oaxacan block are all compatible with NE- to E-directed oblique convergence of the plates in

the Pacifi c coast against the southern edge of Mexico (Fig. 7).

4. The fi nal deformation event produced nor-mal faulting along the eastern boundary of the Acatlan-Oaxacan block and reactivated exist-ing structures as strike-slip faults in the Taxco region, indicating NE-SW extension. The age of this event is constrained by dates in Taxco at ca. 33 Ma (Alaniz-Alvarez et al., 2002) and in the Tehuacan region younger than the Paleocene.

The scheme emerging from the available data is a sequence of three major events of deforma-tion with different kinematics, which migrated from west to east. However, more chronological data are needed to establish the migration paths.

ACKNOWLEDGMENTS

We thank Mariano Cerca and Luca Ferrari for a pre-print of their work and discussions about the tec-tonics of southern Mexico. The assistance of Oscar Davalos in fi gure preparation is appreciated. The cor-rections suggested by Nancy Rigs, Tim Lawton, Paul Umhoefer, and an anonymous reviewer signifi cantly improved the manuscript. Financial support for this study was provided by grants CONACYT 41044-F and PAPIIT IN102602.

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200 km 100°W 96°

19°N

17°

Gulfof

Mexico

PacificOcean

200 km 200 km

Chortis block

Xolapa block

Maastrichtian

Migration of contractile deformation

Paleocene

early-middleEocene

Acatlan-Oaxacanblock

100°W 96°

19°N

17°

Gulfof

Mexico

PacificOcean

Migration of the Chortis block

Xolapa block

late Eocene

Migration of NW extensionstrike-slip faulting

Oligocene


100°W 96°

19°N

17°

Gulfof

Mexico

PacificOcean

Xolapa block

early Oligocene

Strike-slip faulting Normal faulting

early Oligocene-Miocene


Remains under NW extensionduring Oligocene-Miocene

Migration of horizontal NE extension

A B C

Figure 7. Tectonic evolution of southern Mexico for Maastrichtian–Miocene time. The three deformation events migrated from west to east. (A) The shortening structures outline the Acatlan-Oaxacan block. (B) Strike-slip faulting migrated from west to east along the northern boundary of the Xolapa block. (C) Strain produced both strike-slip faulting in the western region that ended in the early Oligocene (gray zone), and normal faulting in the eastern region active from the early Oligocene to the Miocene. The migration path of the Chortis block from the late Eocene to the late Oligocene was proposed by Schaaf et al. (1995).



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