Geological and structural study of the Chapala rift, State of Jalisco, Mexico

Abstract

The regional structure and the morphology south and west of Guadalajara arc dominated by a Neogene system of three rifts intersecting in a continental triple junction situated about 50 km SSW of the city. This system controls Mexican Volcanic Belt (M.V.B.) Volcanism. Field studies define the regional stratigraphy. The oldest rocks outcropping in Chapala Rift arc a thick succession of heterogeneous breccias unconformably covered by a sequence of extensive andesitic lava f1ows dated between 12 and 8.8 m.y. This sequence forms a broad plateau dissected by the faults associated with rifting. locally it is overlapped by lacustrine sediments of Late Miocene/Early Pliocene age. Rifting started during the volcanic hiatus after the end of the Late Miocene volcanic cycle. In Mid-Pliocene the essentially effusive volcanism of M.V .B. begins and continues with decreasing intensity into the Quaternary. The opening of Chapala Rift began at the end of the Late Miocene along N90°E trending lineaments, presenting at first left-lateral and then normal motion. Almost all the N90° lineaments of Chapala Rift have been inactive since the Pleistocene and the region is now being deformed by lineaments of orientation ranging from N135°E to Nl80°E. These lineaments control the other two branches of the triple system. The Chapala Rift can thus be considered as an aulacogen of the incipient crustal fracturation process in west-central Mexico. In contrast, extensional deformations along and E-W directed axis are now affecting the southern Citala Graben, whose western end may be regarded as the site of a new triple point.