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

Abstract

The study area is at the juncture of the Sierra Madre Occidental and Mexican Volcanic Belt centered on the Río Grande de Santiago, 10 km northwest of Guadalajara. On the basis of stratigraphy and K-Ar chronology, the rocks of the area can be divided into seven units: Unit I is an upper Oligocene plutonic sequence; Unit II is a lower-to-middle Miocene andesite, calcic latite, acid ignimbrite volcanic sequence; Unit III is a late Miocene bimodal calcic latite-acid ignimbrite volcanic suite; Unit IV is composed of Pliocene lacustrine sediments (tuffs and basalts); Unit V includes the Pleistone basalts of Mesa de Santa Rosa and other Pleistocene volcano-sedimentary products; Unit VI consists of alkaline basalts confined to the margina of the Río Grande de Santiago (RGS); and Unit VII comprises lahars and laharic breccias exposed on the walls of the RGS. Units I and II can be correlated with the Sierra Madre Occidental volcano-plutonic sequence. Unit III is probably related to the proto-Mexican Volcanic Belt. Unit IV represents a period of basaltic volcanism and internal drainage prior to the cutting of the gorge of the RGS, and Units V-VII are contemporaneous with the Mexican Volcanic Belt (sensu strictu). Structural data from the study area delineate a large right-lateral strike-slip fault system, which is expressed as a complex array of left-handed en-échelon riedels and conjugate antithetic faults. The fractures largely control the course of the river and produce spectacular, horizontally striáted vertical cliffs. Triangulation surveys of various monuments at Santa Rosa dam, for the period 1964-1981, indicate continuous movement of both flanks in a direction parallel to the maximum elongation axis of the strain ellipsoid. In a different riedel, recent
displacements of terrains in a corn field at the base of the river are consistent with a right-lateral sense of motion and strongly suggest that the fault system is tectonically active. Rose diagrama are in good agreement with the fracture pattérns for other areas similarly defonned. Geologic observations indicaté a minimum displacement of 4 km. Similar rose diagrama ínade independéntly in western Nayarit and southwestern Jalisco show conjugate pairs consistent with a large left-lateral northwest-oriented fault, passing west of the area through the RGS in Nayarit and. continuing southeasterly along the Valle de San Marcos, west of Laguna de Chapala. This model can then explain the presence of large north saouth oriented fold axes in Neogene ignimbrites in Nayarit, while the sector of the fault studied here appears to be a right-lateral conjugate fault of a larger left-lateral system. Several lines of seismic, gravimetric and paleomagnetic data support the postulated mechanism.