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 deposi- tional 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 0895-981 1/00/$ - see front matter q 2000 Elsevier Science Ltd. All rights reserved. PII: S0895 -98 11(00)00035 -3 444 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 occa- sionally r etain primary structure and texture. The discon- formable 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.

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