TECTONICS OF THE CHAPALA GRABEN, WESTERN MEXICO

Intense and widespread Late Cenozoic tectonic and volcanic activity in central Mexico has been associated to the subduction of the Cocos and Rivera plates beneath North America plate. The western-central part of Mexico is segmented by the Tepic-Zacoalco, Colima, and Chapala grabens they form the so called Guadalajara triple junction. These structural systems outline continental crustal blocks as the Michoacan and Jalisco blocks. Geometry and sense of slip of a total of 125 striated faults, corresponding to Late Miocene-Pliocene, were measured along the 115 km long and up to ~37 km wide Chapala graben. The paleostress regime responsible for the observed deformation was computed by fault slip data inversion. The trend of the measured mesofaults displays a dominant E-W direction in Chapala graben. Although some faults shows a small left-lateral component of motion most of them shows pitches higher than 45° and inclinations ranging between 45° a and 75°. On the other hand, paleomagnetic data have shown counterclockwise rotations to the northern and in eastern Chapala graben suggesting that a left-lateral transtension played a principal role in their early phases of formation. These structural and paleomagnetic data in the Chapala graben can be explained if a left-lateral component, in a transtensive framework, associated to the SE-motion of the Michoacan block is considered.