Geochronologic, petrologic, and structural data related to large morphologic features between the Sierra Madre occidental and the Mexican Volcanic Belt.

Abstract

This area is at the junction of the Sierra Madre occidental and the TMVB is centred on the Rio Grande de Santiago (RGS), 10 km NW of Guadalajara. On the basis of stratigraphy and K/Ar chronology, the rocks can be divided into seven units: 1) an Upper Oligocene plutonic sequence; 2) a Lower-to-Middle Miocene andesite, calcic latite and acid ignimbrite volcanic sequence; 3) a late Miocene bimodal calcic latite-acid ignimbrite volcanic suite; 4) Pliocene lacustrine sediments (tuffs and basalts); 5) Pleistocene basalts of Mesa de Santa Rosa and other Pleistocene volcanosedimentary products; 6) alkaline basalts confined to the margins of the RGS; and 7) lahars and laharic breccias exposed on the walls of the RGS. Units 1 and 2 can be correlated with the Sierra Madre occidental volcanoplutonic sequence and 3 is probably related to the proto-MVB. Unit 4 represents a period of basaltic volcanism and internal drainage prior to the cutting of the gorge of the RGS, and 5-7 are contemporaneous with the MVB.-J.M.H.