Build-up and depositional dynamics of an arc front volcaniclastic complex : the Miocene Tepoztlán Formation (Transmexican Volcanic Belt, Central Mexico)

Abstract

Volcanic terrains such as magmatic arcs are thought to display the most complex surface environments on Earth. Ancient volcaniclastics are notoriously difficult to interpret as they describe the interplay between a single or several volcanoes and the environment. The Early Miocene Tepoztla´n Formation at the southern edge of the Transmexican Volcanic Belt belongs to the few remnants of this ancestral magmatic arc, and therefore is thought to represent an example of the initial phase of evolution of the Transmexican Volcanic Belt. Based on geological mapping, detailed logging of lithostratigraphic sections, palaeocurrent data of sedimentary features and anisotropy of magnetic susceptibility, mapping of two-dimensional panels from outcrop to field scale, and geochronological data in an area of ca 1000 km2, three periods in the evolution of the Tepoztla´n Formation were distinguished, which lasted around 4 Myr and are representative of a volcanic cycle (edifice growth phases followed by collapse) in a magmatic arc setting. The volcaniclastic sediments accumulated in proximal to medial distances on partly coalescing aprons, similar to volcanic ring plains, around at least three different stratovolcanoes. These volcanoes resulted from various eruptions separated by repose periods. During the first phase of the evolution of the Tepoztla´n Formation (22Æ8 to 22Æ2 Ma), deposition was dominated by fluvial sediments in a braided river setting. Pyroclastic material from small, andesitic–dacitic composite volcanoes in the near vicinity was mostly eroded and reworked by fluvial processes, resulting in sediments ranging from cross-bedded sand to an aggradational series of river gravels. The second phase (22Æ2 to 21Æ3 Ma) was characterized by periods of strong volcanic activity, resulting in voluminous accumulations of lava and tuff, which temporarily overloaded and buried the original fluvial system with its detritus. Continuous build-up of at least three major volcanic centres further accentuated the topography and, in the third phase (21Æ3 to 18Æ8 Ma), mass flow processes, represented by an increase of debris flow deposits, became dominant, marking a period of edifice destruction and flank failures.