Structural geology and tectonic evolution of the Santa Rosa area, SW Mexico State, Mexico

Abstract

A three part sequence of metamorphosed volcanic and sedimentary rocks is exposed between Tejupilco and Zacazonapan, in the SW of the State of Mexico. The lowermost portion, of uncertain age and up to amphibolite facies metamorphic grade, includes the Tizapa gneiss and overlying schists and phyllites. Overlying these rocks in thrust contact is a tectonically imbricated rock package consisting of graywackes of the Jurassic Acuitlapan Formation and limestones with enclosed stratabound Pb-Zn ores (e.g . Santa Rosa) of the Early Cretaceous Amatepec Formation. These two units have been folded twice, arc locally mylonitic, and have undergone greenschist facies metamorphism. Transposed bedding is parallel to an SI foliation which dips predominantly 20°-30° SW and contains a stretching lineation plunging 10°_20° WSW. SI has been subsequently folded about an axis plunging 20° W, with local development of a pressure-solution spaced cleavage S2' Poles to S2 describe a partial great circle girdle as a result of fanning around fold hinges and a third deformation penetrative only on a regional scale. Above this imbricated package are rocks of the Late Cretaceous Xochipala Formation with volcanics of intermediate composition at the base grading upward through tuffs to graywackes and limestones. These rocks have been folded once about the same W plunging axis as the underlying package and the only evidence of metamorphism is slight growth of muscovite or chlorite flakes with a weak preferred orientation slightly oblique to S2 of the underlying package. Calcite c-axis fabrics of rocks from the Acuitlapan, Amatepec, and Xochipala limestones have been studied. The Amatepec rocks show strong crystallographic preferred orientation and a monoclinic symmetry which indicates a top-to-NE sense of shear, compatible with that suggested by asymmetric porphyroClasts in mylonitic portions of thc graywackes; the deformation mechanism responsible for the calcite fabric development is e-twinning, with r-slip. The Xochipala rocks exhibit no such crystallographic preferred orientation. These differences in the structural evolution of the three packages, at this and other localities in the region, suggest a complex sequence of arc construction with repeted back-arc development and collapse for this portion of Mexico during the Mesozoic.