Listric faulting and continental rifting in western Mexico—A paleomagnetic and structural study

Abstract

The western Trans-Mexican Volcanic Belt is characterized by several large-scale tectonic depressions which recently have been interpreted in terms of active rifting and independent motion of the Jalisco Block. In this paper we report results for the northwestern part of the Jalisco Block, within the Ameca Tectonic Depression. The trend of the central part of the Depression changes, where it is bounded by the Ameca River Fault. There the fault, which is characterized by a steep, high scarp and a pronounced surficial curvature, bounds the Pleistocene Amatlán de Cañas volcanic field which consists of small eroded volcanic structures and mafic lava flows, intercalated with conglomeratic talus debris. The sinuous pattern of outcrop of the fault indicates that the fault plane dips toward the volcanic field. The conglomeratic units are abruptly truncated at the fault contact and generally dip 12 to 20° toward the fault plane. Paleomagnetic results for the young volcanic units give a mean direction which is interpreted to be the result of 12 to 20° of tilting toward the fault. Regional listric faulting (“block sliding”) of the entire block occupied by the volcanic field is postulated to explain the structural, stratigraphic and paleomagnetic patterns observed. Listric faulting occurs within a transtensional tectonic environment related to the southwesterly motion of the Jalisco Block as it rifts away from mainland Mexico.