TECTONIC IMPLICATIONS OF LA MONTAÑA GROUP: A NEW LOWER CRETACEOUS STRATIGRAPHIC UNIT IN THE PUZZLING TRUNCATED CONTINENTAL MARGIN OF SOUTHERN MEXICO

The contact relationships between the Paleozoic metamorphic Acatlán Complex (AC) and the Mesozoic-Paleocene mid-crustal arc-related rocks of the Xolapa Complex (XC), southern Mexico, currently represented by the Chacalapa and La Venta faults, remain as a poorly known and controversial issue. Our new geologic, geochronologic and palynologic data allow a different interpretation. Along the N-S transect extending across such boundary from Malinaltepec to San Luis Acatlán, Guerrero, a thick (≥2 km) meta-sedimentary succession (here termed La Montaña group, LMG) composed of shale, sandstone, limestone and andesitic volcanic and volcaniclastic rocks is gradually metamorphosed into schists, gneisses and migmatites of the XC. The LMG was formerly mapped as the southernmost sector of the Acatlán Complex, or interpreted as the southerly marine facies of the Middle Jurassic Tecocoyunca Group.

Two quartzite samples of the LMG were analyzed by LA-ICPMS U-Pb on separated zircons, and the results of about 100 core crystals per sample show ages ranging from ca. 145 to 2830 Ma and ca. 178 to 2440 Ma. The youngest ages at ca. 145 Ma (Tithonian-Berriasian boundary) suggest a maximum Berriasian age for the deposition of the LMG, age also supported by palynomorphs in black shales (spores and pollen, Cicatricosisporites and Complexiopollis; dinoflagellate cysts, Cyclopsiella and Kallosphaeridium). The presence of the pollen Complexiopollis found in some beds could extend the stratigraphic age range up to the Cenomanian. Major distribution peaks in the complex detrital zircon patterns suggest that their main sources are the Permian-Triassic continental arc of SE Mexico, Acatlán Complex, Pan-African basement of the Maya terrane, and Grenvillian Oaxacan Complex.

Correlations of the interbedded volcanic rocks in the LMG with the Early Cretaceous Chapolapa, Taxco-Taxco Viejo and Tejupilco continental arc successions suggest that the autochthonous XC may represent the mid-crustal levels of this arc in the Guerrero region. In the context of the southern Mexico-Chortís Block interactions, our data support those Cretaceous tectonic models in which an ocean marginal basin between those blocks was closed in the Barremian-Aptian by sinistral oblique convergence associated with a NE-dipping subduction zone.