1. Introduction the Location of the Study Area Between Two Extensive

Journal of South American Earth Sciences 13 (2000) 443#457 www.elsevier.nl/locate/jsames

Stratigraphic assessment of the Arcelia#Teloloapan area, southern Mexico: implications for southern Mexico$s post-Neocomian tectonic evolution E. Cabral-Canoa,*, H.R. Langb, C.G.A. Harrisonc

aInstituto de Geof ´%sica, Universidad Nacional A ut ´onoma de Me´ xico, Ciudad Universitaria, M e´ xico , D F 04510, M exico bJet P ropulsion L aboratory, California I nstitute of Technology, 4 800 Oak Grove D rive, P asadena, CA 91109, USA cMarine Geology and Geophysics, R osenstiel School of Marine and A tmospheric Science, University of Miami, 4 600 R ickenbacker Cswy, M iami, FL 33149, USA

Abstract

Stratigraphic assessment of the &Tierra Caliente Metamorphic Complex+ (TCMC) between Arcelia and Teloloapan in southern Mexico, based on photo interpretation of Landsat Thematic Mapper images and field mapping at the 1:100,000 scale, tests different tectonic evolution scenarios that bear directly on the evolution of the southern North American p late margin. The regional geology, emphasizing the stratigraphy of a portion of the TCMC within the area between Arcelia and Teloloapan is p resented. Stratigraphic relationships with units in adjacent areas are also described. The b ase of the stratigraphic section is a chlorite grade metamorphic sequence that includes the Taxco Schist, the Roca Verde Taxco Viejo Formation, and the Almoloya Phyllite Formation. These metamorphic units, as thick as 2.7 km, are covered disconformably b y a sedimentary sequence, 2.9 km thick, composed of the Cretaceous marine Pochote, Morelos, and Mexcala Formations, as well as undifferentiated Tertiary continental red b eds and volcanic r ocks. The geology may be explained as the evolution of Mesozoic volcanic and sedimentary environments developed upon attenuated continental crust. Our results do not support accretion of the Guerrero terrane during Laramide (Late Cretaceous#Paleogene) time. q 2000 Elsevier Science Ltd. All rights r eserved. Keywords: Tectonic evolution; Arcelia; Teloloapan; Stratigraphy

1. Introduction

The location of the study area between two extensive carbonate p latforms (Huetamo area and the Guerrero# Morelos platform located between Mexico City and Chil- pancingo; Fig. 1) of similar mid-Cretaceous age and depositional environment has resulted in the formulation of contrasting evolutionary schemes for southern Mexico. Some interpretations explain the non-continuity of the two carbonate p latforms as the result of deposition controlled by topography (e.g. de Cserna et al., 1978). Others assert that the metamorphic rocks of the &Tierra Caliente Metamorphic Complex+ (TCMC), upon which the carbonates sit, are allochthonous, resulting from the tectonic accretion of an island arc (e.g. Campa and Ramirez, 1979; Tardy et al., 1991), with consequent dissimilar stratigraphic records and geologic evolution from the rest of cratonic Mexico. These contrasting tectonic scenarios can only be tested by * Corresponding author. Tel.: 152-5-622-4027; fax: 152-5-550-2486. E-mail address: [email protected] (E. Cabral-Cano). a tectonostratigraphic analysis carried out in the surround- ings of the proposed tectonostratigraphic terrane boundary. Ortega-Gutierrez (1981, p . 194) considered the TCMC in southern Mexico to be a p rovisional designation, #until better geochronology and mapping establish their true geological relationships.$ A compilation and comparison of published maps (Cabral-Cano, 1995) reveals contradic- tory stratigraphic affinity. This is a direct consequence of the diverse criteria used to define mapping units and the lack of clear contact definitions and lithologic characterizations. A tectonostratigraphic assessment on the metamorphic rocks of the Tierra Caliente complex in the vicinity of the alleged terrane boundary was p recluded by the absence of a reliable cartographic b ase. Thus, new mapping and stratigraphic analyses serve as the basis to test opposing tectonic scenarios and derive important constraints on the tectonic evolution of the southern North American p late margin.

2. Approach

The approach used for this study was that described b y Lang et al. (1987) and Lang and Paylor (1994) in which

444 E. Cabral-Cano et al. / Journal of South American E arth Sciences 13 (2000) 443%457 Fig. 1. Location of the Tierra Caliente Metamorphic Complex (TCMC) and other metamorphic complexes in southern Mexico, and Cretaceous carbonate deposits. Study area is boxed. Modified from Ortega-Gutierrez et al. (1992).

Fig. 2. Location of the Teloloapan#Arcelia study area in southern Mexico (boxed). Major r oads and towns in the region are shown for r eference. Landsat Thematic Mapper satellite imagery was used for photointerpretion of the area. The background is a greyscale version of a principal component image

(RGB ? PC1, PC2, and PC3) generated from a Landsat TM Image which shows an example of digital enhancement of the imagery to better discriminate lithologies Cd1u,eP to contrasting spectral cehda frraocmtera istL icasn dosfa tthT e MdifIf emreanget rw ohckic units, wwsh iacnh are expressed as dlief fnehreanntc sehmaednets ooff grey on tehriys image. Compare iwnaitteh Fig. 3 for correspondence to lithostratigraphic units.

E. Cabral-Cano et al. / Journal of South American E arth Sciences 13 (2000) 443#457 445 geologic mapping and structural/stratigraphic analyses using p hotogeology and spectral interpretations of Landsat Thematic Mapper (TM) images are guided b y p ublished mapping and field work. This approach makes field work more efficient b y remotely identifying localities where key stratigraphic and structural r elationships are well exposed and are most accessible. Three digital TM scenes acquired in the winter of 1985#1986 under essentially cloud-free conditions provided image b ase maps. Color composites using different b and combinations, p rincipal component, decorrelation stretch and edge enhanced images (Moik, 1980; Gillespie et al., 1986) were registered to Universal Transverse Mercator (UTM) geographical coordinates and photographically enlarged to 1:250,000; 1:100,000, and 1:50,000 scales. Fig. 2 is an example of the images used.

3. Results

3.1. Overview

The study area in Guerrero State, Mexico (Fig. 1), encom- passes approximately 1600 km2 of greenschist facies metamorphic rocks k nown as the TCMC. In 1981, Ortega- Gutierrez defined the TCMC as a complex of low-grade metamorphic rocks, including calc-alkaline andesites, ignimbrites, tuffaceous shales, sandstones, and limestones exposed #mainly in the southern slopes of the Balsas River Basin and beyond the southern limits of the Transmexican Volcanic Belt$ (1981, p . 194) (see Fig. 1). The earliest formal lithostratigraphic description in the study area was that of Fries (1960), with complementary work b y de Cserna (1965). In a later stratigraphic review, Ontiveros-Tarango (1973) reported a thrust of Roca Verde Taxco Viejo green- stone over mid-Cretaceous Morelos limestone west of Telo- loapan. This fault was later interpreted by Campa and Coney (1983) and Centeno-Garc´ %a et al. (1993) as a tectonostratigraphic terrane boundary. Mapping of this area has also been published b y Campa et al. (1974), de Cserna (1978), Campa and Ramirez (1979) and INEGI (1981a,b). Lithostratigraphic units used here are summarized in the composite column (Fig. 4). The metamorphic sequence in the Teloloapan&Arcelia area contains three pre-Aptian low- grade, greenschist facies metamorphosed units that occasionally r etain primary structure and texture. The disconformable sedimentary cover comprises four units that span Aptian through early Tertiary times. The western limit of these metamorphic rocks is defined by the Arcelia fault zone, a NW-trending system of high-angle faults that j uxta- pose Tierra Caliente metamorphic rocks on the east against Tertiary volcanic and clastic rocks on the west. The eastern limit is the Teloloapan thrust fault that carried the metamorphic rocks onto Cretaceous limestones.

3.2. Taxco Schist Formation

The Taxco Schist Formation is composed primarily of fine- and coarse-grained mica and/or chlorite pelitic schists and phyllites (Fries, 1960). Outcrops are restricted to the lowermost topographic areas, in the north-central portion of the study area (Fig. 3). The schists have a well-developed cleavage, which is folded to centimeter-scale chevron folds, and crenulation cleavage in the case of the fine-grained rocks. The b ase of the Taxco Schist is not exposed in the study area, but near Zacazonapan (90 k m northwest of the study area) it rests in a fault contact with Permian#Early Triassic mylonitic granite of continental affinity (Elias-Herrera and Sanchez-Zavala, 1990). The upper contact with the Roca Verde Taxco Viejo is p oorly exposed near La Parota Lidice (Fig. 3). All outcrops of the Taxco Schist we visited are deeply weathered and thus inappropriate for radiometric dating. The age can b e constrained only as p re-Aptian#

Albian $ that is, older than the base of the overlying Roca Verde Taxco Viejo (94.4#82.8 my; Fig. 5) and More- los Formations. The Roca Verde Taxco Viejo and the Taxco Schist Formations are composed of distinct lithostratigraphic units (Fig. 3). Besides their lithological differences, the Taxco Schist presents high drainage density and a low resis- tance to erosion, in contrast with the more resistant geomorphic expression of the Roca Verde Taxco Viejo. Within the study area, the Taxco Schist outcrops are consis- tently topographically below outcrops of the Roca Verde Taxco Viejo, suggesting a lower stratigraphic position for the former. The minimum exposed thickness of the Taxco Schist in the study area is approximately 900 m. This was calculated in the vicinity of La Parota Lidice b y measuring the relief between the lowest p art of the schist exposure along the Sultepec River and the highest point of the upper contact with the Roca Verde Taxco Viejo. This estimate is crude, because it is likely that this sequence is duplicated by small thrust faults that are difficult to detect at the 1:50,000 scale mapping. We correlated rocks that we mapped as the Taxco Schist with rocks mapped b y Fries (1960) in the Taxco type area, 33 km northeast of the study area, and with the metamorphic sequence mapped b y Elias-Herrera and Sanchez-Zavala (1990) in the Tejupilco area, 68 km northeast of the study area. These correlations are based on the similarities in their lithologies and stratigraphic p osition, the lateral continuity of exposures that can be mapped on the TM images, and mapping by de Cserna and Fries (1981) and El#´ as-Herrera (1993). The micaceous, pelitic composition of the Taxco Schist implies a pelitic protolith of probable continental margin origin. Lithologies reported in adjacent areas b y Elias- Herrera and Sanchez-Zavala (1990) include fine- and medium-grained sericite and quartz schist, as well as chlorite schist, graphite schist, quartzite, and b lack slate.

3.3. R oca Verde Taxco Viejo Formation

Fries (1960) assigned foliated, low-grade metamorphosed 464 h Sciences 13t

2000) 443#457( Fig. 3. Geologic map and cross sections of the Arcelia#Teloloapan area. Geology east of Pachivia is modified after Barros (1996).

E. Cabral-Cano et al. / Journal of South American E arth Sciences 13 (2000) 443#457 447 Fig. 4. Composite stratigraphic column for the study area. Stratigraphic locations of the whole r ock K/Ar dated samples (see also Fig. 5) shown as sample numbers, with ages in p arentheses. andesitic tuffs, b reccias, lavas, and associated sandstones quartz are common. In the vicinity of Agua Colorada (Fig. west of Taxco Viejo to the Roca Verde Taxco Viejo Forma- 3), sparse marble beds (3#6 m thick) occur in the metatuff tion. Probable equivalents in the study area are porphyritic sequence. andesite metalavas, p hyllitic andesitic metatuffs showing The Roca Verde Taxco Viejo is exposed in two major cleavage, and monomictic metaconglomerates with andesite areas: between Acapetlahuaya and the Vicente Guerrero clasts. Metaconglomerates locally grade into metasand- reservoir, and between Villa de Ayala and Teloloapan stones. All of these r ocks show greenschist facies chloritiza- (Fig. 3). The base is exposed (poorly) only in the north- tion of the matrix. Epidote, sericite, biotite, plagioclase, and central portion of the area (Fig. 3). The upper contact with

448 E. Cabral-Cano et al. / Journal of South American E arth Sciences 13 (2000) 443#457 Fig. 5. Reported radiometric dates for the study area and vicinity. Lightly shaded b oxes are previously reported ages (see text). Darkly shaded b oxes are new whole rock K/Ar dated samples. the Almoloya Phyllite is exposed along Highway 51 east of Las Ceibitas where meta-andesite intervals (3#5 m thick) interfinger with the base of the Almoloya Phyllite. This contact indicates volcanic activity during the deposition of basal Almoloya beds. The margins of these thin flows appear chilled, b ased on the presence of 1#3 cm-thick dark cryptocrystalline b ands. In the eastern portion of the area, the upper contact of the Roca Verde Taxco Viejo is discordant with the overlying Morelos Formation and equivalent Pochote strata as exposed on the road from Pochote to La Yerbabuena and southeast of Acatlan de la Cruz. The metavolcanic member of the Roca Verde Taxco Viejo is approximately 620 m thick near the Neblinas Radio Station. The 1330 m thickness for metatuff and metacon- glomerate members was measured near Agua Colorado.

3.4. Almoloya Phyllite

The Almoloya Phyllite is an informal lithostratigraphic unit that consists of monotonous well-foliated, black p hyl- lite that weathers to light brown. This unit also includes intervals of b lack chert nodules and irregular beds 3#4 m thick. The b est exposures of the Almoloya Phyllite are along Highway 51, near the village of Almoloya, south of the Vicente Guerrero Reservoir, and along the Copaltepec# Tejupilco r oad near the 29 km marker. Along the western margin of the study area, the Arcelia fault (Fig. 3) j uxta- poses the Almoloya Phyllite against undifferentiated Tertiary red beds and volcanic and intrusive rocks (Jansma et al., 1991 ; Jansma and Lang, 1996). West of Las Ceibas, the basal contact with the Roca Verde Taxco Viejo is a concordant depositional surface. In other areas, such as in the vicinity of Almoloya, this relationship is not so clearly exposed because an undeter- mined amount of slip has occurred along this surface. The upper contact was not clearly observed in the field but is inferred to b e disconformable with overlying Cretaceous sediments of the Pochote Formation. Alternatively, the lack of a clear exposure of this contact may b e evidence that the Almoloya Phyllite is a lateral equivalent of the laminated mudstones of the Pochote Formation. If this is the case, then the Almoloya Phyllite r epresents a deeper, hemipelagic facies of the Pochote. The Almoloya Phyllite is not exposed east of Tehuixtla (Fig. 3). There, the Roca Verde is found in contact with either the Morelos or Pochote Formations. This suggests a disconformable upper contact. The extent of the Almoloya Phyllite exposures may have been controlled by: (1) r elief on the Roca Verde; and/or (2) uplift and subsequent erosion of the Almoloya Phyllite p rior to deposition of the Morelos Formation. The absence of the Almoloya Phyllite west of Zacatlan- cillo#Tehuixtla (Fig. 3) and the presence of a conglomeratic member in the uppermost p art of the Roca Verde Taxco Viejo suggests that b oth the phyllite and part of the Roca Verde may have been removed b y erosion. The presence of the phyllites may indicate substantial relief during or after deposition of the Roca Verde Taxco Viejo, if the p hyllites are distal turbidities derived from the volcanic centers of the Roca Verde Taxco Viejo metavolcanic rocks. W e measured

E. Cabral-Cano et al. / Journal of South American E arth Sciences 13 (2000) 443#457 449 a thickness of 800 m for the Almoloya Phyllite east of Arcelia. The only reported fossils from the rocks that we mapped as the Almoloya Phyllite are r adiolaria (Da ´vila-Alcocer and Guerrero-Su´ astegui, 1990) in the vicinity of Arcelia. D ´avila-Alcocer and Guerrero-Su´ astegui (1990) suggested that the fossil assemblage is Albian#Cenomanian age, possibly Cenomanian. However, b ased on ranges of the genera as reported by Moore (1954) and the zonation scheme of Sanfilippo and Riedel (1985), the co-occurrence of P odobursa sp., A canthocircus sp., and Archaeodictyomi- tra sp. suggest a Valanginian#Aptian age. Alternatively, using the zonation of Pessagno (1971), an assemblage consisting of Crucella messinae Pessagno, P odobursa sp., Acanthocircus sp., Paronaella sp., Praeconocaryomma sp., andA rchaeodictyomitra sp. is Tithonian#Cenomanian. This is b ased on Pessagno$s (1971) original description of C. messinae from the Great Valley Sequence of the California Coast Ranges, where the base of the r ange is not defined. Therefore, fauna in the Almoloya Phyllite record a latest Jurassic to early Late Cretaceous age.

3.5. Pochote Formation

The Pochote Formation is an informal lithostratigraphic unit. This name was informally used by Baro-Santos (1959) and PEMEX (1979) for a sequence of laminated mudstone and shale near Campo Morado (Burckhardt, 1930) and El Pochote (Campa, 1978). In the study area, the Pochote Formation consists of dark gray to black, thinly bedded, (.1cm) and/or laminated (,1cm) lime mudstone, interbedded with dark gray shale in centimeter-thick b eds. The unit locally shows intense penetrative deformation, evidenced b y axial plane foliation and b oudinage. The b est exposures are between Villa de Ayala and Las Ceibitas, as well as near El Pochote and north of La Yerbabuena (Fig. 3). The Pochote#Roca Verde Taxco Viejo contact is discordant, based on exposures along the road from El Pochote to La Yerbabuena and southeast of Acatlan de la Cruz (Fig. 3). In these exposures, the contact is a sharp depositional inter- face. PEMEX (1979) reported the existence of conglomerate b eds at the base of the Pochote where it sits disconformably on meta-andesites and meta-tuffs of the Roca Verde Taxco Viejo Formation in the vicinity of El Pochote. Exposures of the contact of the Pochote with the Almoloya Phyllite were not found, due to deep weathering. The Pochote and Morelos Formations, locally with b asal conglomerate, r est disconformably on the Roca Verde Taxco Viejo, Taxco Schist and Almoloya Phyllite. The upper contact of the Pochote Formation with the Mexcala Formation was mapped north of Los Aguajes. The Morelos#Pochote formational contact was not found exposed. The gradual thickening of limestone b eds and decrease in shale intercalations near La Yerbabuena suggest that Morelos and Pochote strata may be a facies change. The compositional variation may b e r esponsible for the differ- ential response to Late Cretaceous#Eartly Tertiary deformation. The total thickness of the Pochote could not b e measured because no locality was found where b oth upper and lower contacts are exposed. The minimum thickness is 420 m in the El Pochote area, which is a tentative estimate because thrust faults probably exist that are too small to be recognized at the 1:50,000 mapping scale. Reported thicknesses of similar lithologies measured near Ixtapan del Oro (Parga, 1976) and Ixcatepec (Gutierrez, 1975) are 2000 and 1600 m, respectively. It is likely that these reported thicknesses also include fault duplications. Biostratigraphic data for the Pochote are b etter than underlying units. Burckhardt (1930) found D ufrenoya aff. furcata Sow. near Campo Morado. This fossil indicates a Late Aptian age (Moore, 1957). Campa (1978) and Campa and Ramirez (1979) reported Parahoplites sp., Hamites sp., Calcisphaerula innomiata, and Stomiosphaera sphaerica in the vicinity of El Pochote and Los Aguajes. These fossils also indicate Late Aptian age. PEMEX (1979) reported the presence of Hamites sp. at the base of the unit, which dates it as Late Aptian#Turonian according to Moore (1957), although PEMEX (1979) assigned an Aptian age because higher strata contain unidentified Albian#Cenomanian microfaunas. De Cserna and Fries (1981) noted the presence of poorly preserved and unidentified calpionelids and globi- gerinids that they claim suggest an Albian#Cenomanian age for beds of the Pochote Formation. The even laminations observed in dark, carbonaceous Pochote limestones indicate that they were deposited b elow wave b ase, in stagnant waters with low oxygen content and containing few biotur- bating organisms.

3.6. Morelos Formation

The Morelos Formation, defined by Fries (1960), includes miliolid-rich, Albian limestones and dolostones. Similar strata of the same age crop out extensively in Morelos, northern Guerrero, and eastern Michoacan states. The original definition included an evaporitic member that has not been encountered in the study area. The rocks that are mapped as Morelos include reefal facies that crop out east of the study area (de Cserna and Fries, 1981; Gonzalez- Pacheco, 1991). Strata that we assign to the Morelos Formation include massive wackestone, p ackestone, and grainstone that contain miliolids, b ioclasts, and intraclasts, as well as r udist banks, occasionally with chert. Strata are commonly dolo- mitized. Based on Wilson (1975), this suite of rocks indicates deposition in environments that span toe-of-the-slope to open platform facies. Limestones of the Morelos Forma- tion h ave traditionally b een assigned an Albian#Cenoma- nian age (Fries, 1960; Bonet, 1971; Ontiveros-Tarango, 1973). Equivalent age carbonates exist elsewhere in Mexico, such as those of the El Abra Formation in

450 E. Cabral-Cano et al. / Journal of South American Earth Sciences 13 (2000) 443#457 northeastern Mexico (Wilson and Ward, 1993; Basa $nez- Loyola et al., 1993) and throughout the Cretaceous Tethys domain (Simo et al., 1993). The Morelos Formation is b est exposed near the eastern margin of the area, around Telo- loapan and Acatempan (Fig. 3), and in thin faulted slices near Simatel and Tenanguillo. It also defines most of the north-trending Chilacachapa range. In the vicinity of La Yerbabuena, small remnants of thin laminated algal bound- stone and calcareous conglomerate b eds crop out at the base of the unit, resting discordantly on Roca Verde metapyro- clastics. The Morelos Formation rests disconformably on the Roca Verde Taxco Viejo Formation. For example, in the vicinity of Pipicantla (Fig. 3) the Teloloapan thrust fault exposes imbricate fault slices that include the Roca Verde#Morelos contact. There, a conglomerate at the b ase of the Morelos Formation contains Roca Verde Taxco Viejo clasts. The upper contact of the Morelos with clastic b eds of the Mexcala Formation is sharp but apparently conformable. The Morelos Formation shows a marked increase in thickness between its westernmost exposures at Acatempan, where we determined a 380 m thickness, and its eastern exposures near Chilacachapa, where Gonzalez-Pacheco (1991) calculated a 1000 m thickness. The variable thickness of the Morelos Formation is partly the result of post- depositional erosion, as in the Teloloapan area, as well as original depositional variations resulting from the existence of a r eefal facies along the western margin of the Chilaca- chapa range (de Cserna, 1978).

3. 7. Mexcala Formation

The Mexcala Formation, which was defined by Fries (1960) in the Balsas River region north of the study area, consists of a b asal member of thinly b edded limestones or limey siltstones grading into an upper member composed of a sequence of shale and sandstone and minor conglomeratic beds. In the eastern p art of the study area, on both flanks of the Chilacachapa range, the Mexcala consists of black shale and wackestone or p ackestone, intercalated within other- wise monotonous dark gray shale with minor fine sandstone beds. Shale beds may be thin (,2#3 cm) or thick (up to 60 cm). Axial p lane cleavage is conspicuous in more thickly bedded intervals. In the Pachivia region, the Mexcala Formation is exposed in an elongate, north-trending, asymmetric, east-verging synclinorium (Fig. 3; A-A0 and C-C0). On the eastern flank of the synclinorium, the Mexcala is in sharp depositional contact with Morelos limestone. The western limb, in the vicinity of Pipicantla and Tenanguillo, is cut b y a west- dipping thrust fault involving older Morelos strata as an imbricate, as well as thin slices of the Roca Verde Taxco Viejo. De Cserna (1978) mapped exposures in the Pachivia area as the Xochipala Formation, based on the apparent absence of sandstone b eds and the occurrence of thin limestone beds and volcanic flows. We found no volcanic flows in the Mexcala Formation in the Pachivia area, although we did find probable Tertiary volcanic rocks near Chapa, which are flat lying, probably of subaereal origin, and discordantly above the steeply dipping Mexcala beds. East of the Chila- cachapa r ange, the Mexcala Formation is in thrust contact with overlying Morelos strata that form most of the range. The minimum thickness of the Mexcala Formation in the Pachivia area is 2000 m. However, many small thrust faults and folds have probably increased the apparent thickness of this unit. Thickness estimates elsewhere are 1300 m near Iguala (Gonzalez-Pacheco, 1991) and approximately 2500 m near Mitepec, 100 k m east of the study area (Lang et al., 1996). PEMEX (1979) reported that foraminifera found in the Mexcala include: M arginotruncana sp., Globotruncana rioensis, Praeglobotruncana rioensis, Globotruncana rosseta, Globotruncana sp., R otalipora sp., Quinquelocu- lina sp, P yrgo sp., c.f.; ammonites include P eroniceras a.f. tricarinatum, Peroniceras c.f. P. subtricarinatun sturn, P eroniceras c.f. P . subtricarinatum Drescher, P eroniceras sp., B arroisiceras c.f. P . alstadenense, and Barroi- siceras sp. According to PEMEX (1979), these faunas indicate a Turonian#Senonian, possibly Maastrichtian age for the Mexcala Formation. However, according to Moore (1957), the ammonite fauna is Coniacian. The ages of the reported foraminifera do not overlap; therefore we assume that the sampling represents the entire Mexcala section. The presence of M arginotruncana sp., Globotruncana sp., and Rotalipora sp. suggest an Albian to Maastrichtian age (Caron, 1985).

4. Discussion

4.1. Age of the metamorphic rocks

Sparse radiometric dating (Fig. 5) and limited biostratigraphic evidence have allowed a wide range of interpretations regarding the age of the Taxco Schist and Roca Verde Taxco Viejo in the study area and vicinity. De Cserna et al. (1974) published a Grenvillian Pb-a age ?1020 ^ 110 Ma? f(1or9 7t4he) Tpuabxcliosh Sedch aistG near iTllaixanco P. bT-ahisa gdeat e?1 was challenged by Campa and Ramirez (1979), who asserted that zircons dated by de Cserna et al. (1974) were not authigenic and that the sample was not from the Taxco Schist but rather from the Roca Verde Taxco Viejo. Thus, this date may show that, during Roca Verde time, there was a Grenvillian-age sedimentary source terrain. Urrutia-Fucugauchi and Valencio (1986) reported 108 ^ 5 and 125 ^ 5 Ma (Barremian# Albian) whole-rock K/Ar ages from schist and metaande- site, r espectively, collected near Ixtapan de la Sal. These ages were attributed to the regional tectonic event that metamorphosed the rocks. W e correlate these r ocks with the Taxco Schist and Roca Verde Taxco Viejo, respectively, based on the similar lithologies and stratigraphic position and the lateral continuity of the Taxco Schist from our study

E. Cabral-Cano et al. / Journal of South American E arth Sciences 13 (2000) 443#457 451 Fig. 6. Pre-Neocomian to Early Tertiary simplified depositional and tectonic conditions for the study area and vicinity (see text for detailed explanation). area to Ixtapan de la Sal (de Cserna and Fries, 1981; de Cserna, 1983). Pb ages from Tizapa massive sulfide deposits, near Zaca- zonapan, 15 km north of Tejupilco (Fig. 2) show single- stage model ages of 156.3 Ma (Oxfordian), 128.7 Ma (Barremian), and 103.4 Ma (Albian) (JICA-MMAJ, 1991, in Sanchez-Zavala, 1993). Sanchez-Zavala (1993) recalcu- lated these dates using a two-stage model that yielded ages of 227.5 Ma (Carnian), 188.3 Ma (Pliensbachian), and 156.3 Ma (Oxfordian), r espectively. Based on the concordant relationships of these massive sulfides with the Taxco Schist, it appears that these ages are equivalent to that of the Taxco Schist p rotolith. Elias-Herrera and Sanchez-Zavala (1990) reported 40Ar/39Ar dates of 101 ^ 1 (Albian) and 93:4 ^ 0:4 Ma (Cenomanian) from rocks equivalent to the Roca Verde near Tejupilco (Fig. 5). The San Pedro Limon batholith rocks (Delgado-Argote et al., 1990) located northwest of Arcelia, have been dated by 40Ar/39Ar as 105 Ma (Albian). In the same area, Ortiz and Lapierre (1991) reported Albian (108 Ma, K/Ar on amphibole) tholeiitic dikes near Palmar Grande, and that Campanian age dikes (79 Ma, K/Ar b ulk rock) intrude pillow basalts in the Arcelia area. All of these samples are from intrusions and mafic segregations that are younger than the metamorphic rocks we mapped as the Taxco Schist and Roca Verde Taxco Viejo. Radiometric dates obtained from the metavolcanic rocks of the Roca Verde Taxco Viejo, in the vicinity of Teloloa- pan and Taxco, are incompatible with stratigraphic relationships. Talavera-Mendoza et al. (1993) reported K/Ar dates on amphiboles that indicate a 99#105 Ma age (Albian) for metavolcanic rocks from the Teloloapan area. W hole-rock K/Ar ages of six samples that we collected from the Roca Verde Taxco Viejo and Taxco Schist in the Taxco and Teloloapan areas show ages that span from Cenomanian to Campanian time (Fig. 5). W e interpret these dates as recording the cooling age of a thermal event that could not b e responsible for the original metamorphism of rocks in the Teloloapan#Arcelia region. This must b e the case because these r adiometric ages are all younger than the biostratigraphic ages for the overlying Morelos Formation that contains Roca Verde Taxco Viejo clasts in a basal

452 E. Cabral-Cano et al. / Journal of South American Earth Sciences 13 (2000) 443#457 conglomerate. The radiometric ages match very well the biostratigraphic ages reported for the Mexcala Formation that overlies the Morelos. Therefore, this Cenomanian# Campanian thermal event may date tectonic uplift of the region; probably corresponding to the initial stages of the Laramide Orogeny in this p art of Mexico. Relief associated with this event may b e responsible for the cessation of carbonate deposition of the Morelos Formation due to siliciclastic deposition of the Mexcala Formation. Biostratigraphic evidence regarding the age of the metamorphic rocks is scarce and inconclusive. As described above, r adiolaria from cherts interbedded with the p hyllites in the Almoloya Phyllite, collected near Arcelia, indicate a Valanginian#Aptian age. Volcanic activity northwest of Arcelia was responsible for the absence of carbonate deposition between the Huetamo and Chilacachapa#Taxco platforms. This discontinuity misled Campa (1978) and Campa and Ramirez (1979) to propose that metamorphic rocks in the region are allochthonous. Instead, based on the stratigraphic and depositional evidence of the rocks in the studied area, we interpret the Late Cretaceous volcanic rocks northwest of Arcelia to be the remnants of a volcanic field that was fringed by shallow carbonate deposits of the Morelos Formation on the Huetamo platform in the west and the Taxco#Chilacachapa#Teloloapan platform in the east (Fig. 6). Siliciclastic deposits, such as the Almoloya Phyllite in the study area and San Lucas Formation in the Huetamo area, as well as basinal carbonates such as the Pochote Formation in the study area, separated these platforms (Fig. 6). The p redominance of either siliciclastic or carbonate sediments was dependent on local conditions of sedi- ment availability and transport. The wide extent and uniformity of b lack shales that contain the Tithonian# Berriasian ammonite Protancyloceras sp. in the Mastlacua and Los Amoles area (Ortiz and Lapierre, 1991), and M icro- canthoceras sp. (Tithonian) and Wichmaniceras sp. (Valan- ginian) in the Ixtapan de la Sal area (Campa et al., 1974) support the idea that uniform depositional conditions may have existed in the r egion as early as Late Jurassic time. Roca Verde Taxco Viejo metavolcanic rocks disconformably underlie unmetamorphosed but locally highly tecto- nized and sheared strata of the Morelos and Pochote Formations. Following the inference of Elias-Herrera (1989) and Elias-Herrera and Sanchez-Zavala (1990) that the Roca Verde Taxco Viejo is equivalent to the Teloloa- pan#Tejupilco sequence, this implies that the p rotolith age for the Roca Verde Taxco Viejo in the Arcelia#Teloloapan area is Late Triassic through Early Cretaceous.

4.2. E nvironments of deposition

The sedimentary sequence exposed in the study area can be explained as a carbonate r eef-platform built on the surface of metamorphosed volcanic rocks of the Roca Verde Taxco Viejo Formation. Morelos strata in the eastern part of the study area r ecord basinal and platform facies that range from intertidal to calcareous slope deposits (Gonza- lez-Pacheco, 1991). The r ocks exposed elsewhere in the study area are p redominantly foreslope to shelf facies mudstones with rudist fragments, or intertidal or channel facies represented by algal mats and breccias (facies # 4#8 of Wilson, 1975). Cessation of Aptian#Albian carbonate depositional environments was controlled by contemporary topography on the volcanic rock surface, where the carbonate platform west of Teloloapan was fringed and possibly restricted b y volcanic activity of the Roca Verde Taxco Viejo Formation. This interpretation agrees with p reviously suggestions by Fries (1960), Ontiveros-Tarango (1973). de Cserna et al. (1978) believed that an Albian#Cenomanian transgression was responsible for the development of rudist reefs fringing a shallow platform located east of the study area. They recognized the lateral change of the Morelos reefal and platform facies into the deeper basinal strata that we mapped as the Pochote Formation. Our results show that rocks of the Mexcala Formation were probably deposited from turbidity currents b ordering a carbonate platform that underwent drastic changes in sedimentation conditions $ hence the sharp contact of the Morelos# Mexcala Formations.

4.3. Tectonic interpretation

One of the most controversial aspects of the geology of southern Mexico is the nature and age of the crystalline or high-grade metamorphic r ocks that constitute the substrate upon which Late Mesozoic sediments accumulated. The reason for this controversy is that there are few outcrops of this basement (Fig. 1). According to Campa and Coney (1983), the metamorphic rocks in the study area represent the basement of the Guerrero terrane. They claimed that the region is a tectonic entity separate from similar lithologies exposed east of the study area and the alleged terrane boundary of Laramide age in the vicinity of Teloloapan. This interpretation assumes that the exposed rocks cannot be subdivided nor tied lithostratigraphically to r ocks in adjacent terranes. According to our mapping and stratigraphic assessment (Figs. 3 and 4), units correlate across the terrane boundary. The nature of the crystalline basement below the Taxco Schist, Roca Verde Taxco Viejo, and Almoloya Phyllite has been only indirectly inferred using sparse geochemical evidence. The deepest reported stratigraphic exposure in the vicinity of the study area occurs near Zacazonapan (Elias Herrera and Sanchez-Zavala, 1990). There, Permian granite is found b elow a low-grade metamorphic sequence. Granite clasts have been reported in an Aptian#Albian conglomerate cropping out near Ixcateopan#Ixtapan de la Sal (Fig. 2) (Vidal-Serratos et al., 1991) and in the Late Jurassic#Early Cretaceous Angao and San Lucas Forma- tions cropping out near Huetamo (Fig. 2) (Guerrero-Suaste- gui et al., 1992). Vidal-Serratos (1991a,b), Vidal-Serratos et al. (1991) and Elias-Herrera and Sanchez-Zavala (1990)

E. Cabral-Cano et al. / Journal of South American E arth Sciences 13 (2000) 443#457 453 Fig. 7. Locally measured stratigraphic columns (inset shows locations within the boxed area of Fig. 2) and relative distribution of mid-Cretaceous sedimentary facies in the study area. Fill patterns are as used for the geologic map in Fig. 3. See Fig. 4 for ages of the individual lithostratigraphic units. found granitic metamorphic rocks in the Zihuatanejo and the Ixcateopan areas (Fig. 2). The presence of these granitic r ocks does not support the interpretations of the geologic evolution of southern Mexico as one or several volcanic arcs built upon oceanic crust, as has b een suggested by Monod et al. (1990). According to Moran-Zenteno et al. (1991), Moran- Zenteno et al. (1992) and Herrmann et al. (1994), there is geochemical evidence from the southern Mexico Mesozoic arc that suggests the influence of an ancient continental margin. Therefore, evidence exists support- ing the idea that the p rotoliths of the metamorphic rocks exposed in the study area and in much of Guerrero and Michoacan states outside the study area (Fig. 1) were deposited upon continental b asement. W e do not reject the possibility that this basement could have been previously stretched and attenuated. This would b e expected if the continental margin of southern Mexico was within a convergent margin back-arc basin during Triassic#Jurassic time, as it is today (Engebretson et al., 1985; Coney, 1989). Although the exact location of a Jurassic#Cretaceous volcanic arc in southern Mexico is still not firmly established, Ramirez-Espinoza et al. (1991) showed that at least one volcanic arc, manifested b y arc affinity intrusive and volcanic rocks as well as Kuroko-type massive sulfide deposits such as those in the Tizapa area north of the study area, existed between Teloloapan#Tejupilco in the east and Zihuatanejo in the west. A west-dipping subduction zone in southern Mexico has been suggested b y Urrutia-Fucugauchi and Valencio (1986) and Tardy et al. (1991). Isotopic data from volcanic rocks collected in southern Mexico (Lapierre et al., 1992; Ruiz et al., 1991; Centeno-Garc´ $a et al., 1993; Tardy et al., 1994; Talavera-Mendoza et al., 1995) apparently supports this interpretation. Other evidence for a west-dipping subduction zone have been summarized b y Tardy et al. (1994, p . 69) and include: (1) the apparent lack of Late Jurassic#Early Cretaceous igneous arc rocks within the Guerrero#Morelos carbonate platform of cratonic eastern Mexico; and (2) the apparent younger Albian#Cenomanian age of the Arcelia volcaniclastic sequence (which apparently includes the Almoloya Phyllite). These observations led Ortiz and Lapierre (1991) to conclude that (3) the tholeiitic volcaniclastic strata of the Arcelia sequence are younger than the Teloloapan calc-alkaline sequence. They assert that there is a thrust contact between the Arcelia sequence and the Telo- loapan sequence, which in turn is thrust over the Guerrero# Morelos carbonate p latform. This assessment is flawed because of the erroneous age assignment of the Arcelia tholeiitic sequence. The ages are based on the use of poorly p reserved and questionably iden- tified r adiolaria (see discussion in D ´avila-Alcocer and Guer- rero-Su´ astegui, 1990). That a lack of volcanic rocks within

454 E. Cabral-Cano et al. / Journal of South American Earth Sciences 13 (2000) 443#457 the Late Cretaceous carbonate platform sequence proves that the volcaniclastic sequences, such as that of Teloloa- pan#Arcelia, must have b een located far away from the North American continental margin is also incorrect. A tectonic model that includes a west-dipping subduction zone usually includes an accretionary forearc prism with associated b lueschist and related ophiolitic slivers east of the volcanic arc. This is the case in the Vizcaino Peninsula arc of Baja California (Moore, 1986). To our knowledge, no such rocks have b een mapped in southern Mexico. If the edge of a Late Jurassic#Early Cretaceous arc were located somewhere within the Teloloapan#Arteaga Zihuatanejo region, then one would expect to find ophiolites, blueschists, or accretionary complexes within the study area or east of it. The only b lueschists reported along the Pacific margin of Mexico are on Santa Margarita Island, San Benitos Islands (Cohen et al., 1963), Cedros Island (Klienast and Rangin, 1982), and the Vizcaino Peninsula (Moore, 1986) on the west coast of Baja California. Although low-temperature greenschist facies metamorphic r ocks have been mapped over large areas of southern Mexico (i.e. Arteaga, Placeres del Oro, and our study area; see Fig. 1), no b elt of Mesozoic high-pressure metamorphic rocks has been reported. We therefore interpret the study area to have b een located east of an east-dipping subduction zone and possibly within an intra-arc (or back-arc) basin during Cretaceous time. The lack of paired high-pressure/high-temperature metamorphic b elts next to the volcanic arc from which the polar- ity of subduction may be deduced precludes a definitive conclusion. Evidence necessary to substantiate the geodynamic model of Tardy et al. (1994), showing westward subduction p olarity, is circumstantial. We propose an alter- native conceptual model for the tectonic setting of southern Mexico (see Figs. 6 and 7): east-dipping subduction of an oceanic plate under continental lithosphere occurred during Cretaceous time, and the continental crust may have undergone attenuation and plutonic activity during Early Cretac- eous time. The existence of nearby granitic outcrops that probably underlie r ocks in the study area support this model.

5. Conclusions

The idea that Albian age carbonates exposed at Teloloa- pan and Chilacachapa are different entities (Guerrero-Su´ astegui et al., 1991), without a common geographic or genetic link, is a necessary conclusion if the Teloloapan thrust is interpreted as the Guerrero terrane boundary (e.g. Campa and Coney, 1983). However, the presence of r ocks of similar lithology, stratigraphic p osition, and age on both sides of the alleged western Guerrero terrane boundary does not support this interpretation. Using a simple carbonate p latform model, the sedimentary rocks mapped in the Teloloa- pan and Chilacachapa areas r ecord compatible facies. This platform was built disconformably over metamorphic rocks; the carbonates were deposited at depths controlled b y p re- existing r elief on the metamorphic surface and were fringed by contemporary volcanic activity in a back-arc b asin environment. Based on similarities of lithology, radiometric ages, protoliths, and metamorphic grade, we correlate the metamorphic rocks that we mapped with the Taxco Schist and Roca Verde Taxco Viejo exposed in the Taxco r egion (30 km east) and the Teloloapan#Tejupilco sequence as described by Elias-Herrera and Sanchez-Zavala (1993). Available evidence shows that the protolith age spans the Late Triassic to Early Cretaceous. These correlations also suggests that carbonate deposits of the Pochote#Teloloapan area and volcanics reported within the Arcelia#Otzoloapan volcanic sequence (Elias-Herrera and Sanchez-Zavala, 1990; Delgado-Argote et al., 1990; Ortiz and Lapierre, 1991) are coeval. We propose that the name TCMC b e abandoned because our mapping has established the geological relationships of units exposed in the $complex.% The study region contains a sedimentary sequence that has undergone intense strain in localized areas, such as within the Pochote Formation. However, it still retains much of its primary sedimentary structure. Moreover, there is now stratigraphic evidence that shows lateral continuity and age correlation of the metamorphic rocks in the Teloloapan#Arcelia r egion with rocks exposed east in the Guerrero#Morelos platform. We therefore propose that the Teloloapan thrust is not a terrane b oundary, b ased on:

& the lithological and radiometric similarities of the low- grade metamorphic rro acdkios mine tthriec Ts iamxciola raitnide Teloloapan# Arcelia regions; & the disconformable nature of the contact between the metamorphic mroacbklse a nnadt overlying em caornitnaec trob cketsw; aeennd & the absence of metamorphism of the Aptian#Albian tmhearina ebs sedimentary cover ihni sthme Teloloapan area. Instead, the Teloloapan thrust is one of many east-verging thrust faults that have b een mapped in the region (Lang et al., 1996). This thrust faulting probably resulted from deformation associated with the Late Cretaceous to Paleogene Laramide Orogeny.

Acknowledgements

This p aper presents the results of research conducted as part of U.S. National Aeronautics and Space Administration contracts NAGW-1678 and NAGW-2710 with the Marine Geology and Geophysics Program at the Rosenstiel School of Marine and Atmospheric Science, University of Miami, and the Jet Propulsion Laboratory at the California Institute of Technology. EC-C also received support for field work from the Gulf Coast Association of Geological Societies# Student Aid Program and the Geological Society of Amer- ica#s Student Research Grant Program, as well as a PhD

E. Cabral-Cano et al. / Journal of South American E arth Sciences 13 (2000) 443$457 455

Fellowship and PAPIIT research grants IN105596 and IN105592 from Universidad Nacional Aut´ onoma de M e´xico. Comments and discussions with Chris A. Johnson and Tony Barros were of great help during the early stages of the project. Field assistance and other logistical support from Francisco Correa-Mora, Teodoro Hernandez-Trevi% no, Esteban Hernandez-Quintero, Gerardo Cifuentes-Nava, and Ricardo Becerril-Herrera (Instituto de Geof ´&sica, Universi- dad Nacional Aut´ onoma de M e´xico) is gratefully acknowl- edged. Steve Adams (JPL) provided invaluable help with the satellite image p rocessing. We thank referees Thomas H. Anderson, Gary Prost, Charles F. Kluth, Meghan Miller, Richard Sedlock, Cynthia Dusel-Bacon, James Kellogg, and two anonymous reviewers. Their comments improved the manuscript, even though some of the r eviewers do not agree with all of our conclusions.

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