Stratigraphy and Tectonics of the Guadalajara Region and Triple-Junction Area, Western Mexico

Abstract

In the region between Guadalajara and the triple junction of the Tepic-Zacoalco, Chapala, and Colima rifts, the late Miocene to Quaternary volcanics of the Mexican Volcanic Belt (MVB) largely conceal the boundaries between the basement domains of the Sierra Madre Occidental (SMO) to the north and the Jalisco and Michoacan blocks to the south. Integrating previous work with new geologic mapping and isotopic age determinations, we propose a comprehensive regional stratigraphy for Guadalajara and the triple-junction area and attempt to define the boundaries between these basement domains. In the study area, the silicic succession of the SMO is restricted to the north of the Santa Rosa-Cinco Minas fault and the northern boundary faults of Lake Chapala. The succession of the MVB began with widespread mafic volcanism, and lacustrine sedimentation occurred at 11 to 8 Ma in several tectonic basins developed along the Tepic-Zacoalco and the Chapala rifts, suggesting that the extensional reactivation of the block boundaries took place earlier than previously suggested. Since latest Miocene time, volcanism has been dominated by silicic products in the north (Guadalajara area) and by more intermediate to mafic products in the south (Chapala and Zacoalco areas). We also present a new interpretation of the structural geometry of the Zacoalco half-graben, in which the Bola del Viejo fault is considered the main detachment structure responsible for the NE tilting of the Sierra de San Marcos and Sierra de Tapalpa blocks. Furthermore, because of the almost identical tilting of the Sierra de San Marcos block and other blocks to the SE, we propose that the San Marcos fault has a planar surface. Finally, we have confirmed that motion along the Santa Rosa fault was normal in post-late Miocene time and that it probably has been inactive since the Middle Pleistocene.