Shallow crustal structure of the junction of the grabens of Chapala, Tepic-Zacoalco and Colima, Mexico

Abstract

Gravity data is used to infer the shallow crustal structure of the Tepic graben adjacent to the City of Guadalajara. This depression does not constitute a symmetric graben with steep master faults but rather a complex transition zone. Blocks detached from the granitic sierras are downfaulted and tilted to the northeast along the front of these sierras. Farther to the north, the granitic basement deepens gradually. However, it steepens at the limit between the Sierra Madre Occidental and the Jalisco Block. North- west of our study area, the limit is located north of the Plan de Barrancas fault. At the east the limit is located closer to the southward prolongation of the Plan de Barrancas fault. The change in position occurs near La Primavera caldera. The La Primav- era caldera, Tequila volcano, and vents of cinder and lava cones are emplaced along NW-SE weakness zones in the batholith. The crust of the Sierra Madre Occidental adjacent to the limit features NE-SW lineaments perpendicular to those of the neighbouring Jalisco Block (JB).