GEOLOGIC EVOLUTION OF THE GULF OF CALIFORNIA NEAR MULEGÉ, BAJA CALIFORNIA SUR: RESULTS FROM BAJA BASINS NSF-REU (RESEARCH EXPERIENCE FOR UNDERGRADUATES)

This study examines the stratigraphy and structure of volcanic and volcaniclastic rocks in the Gulf of California rift ten km east of Mulegé, BCS, Mexico. The volcanic rocks had not been divided and nothing was previously known about this region’s structure. A 390 km² mapped area is divided into two distinct regions by a N-S, steeply dipping fault: (1) West of the fault, an >820 m thick, volcaniclastic red bed sedimentary formation. The red beds include an eastern section of polymict volcaniclastic debris flow deposit and massive sandstones, interfingering westward with fluvially deposited conglomerates, fine-grained sandstones and siltstones. The red beds are interpreted to be a clastic wedge shed into a half graben separated from an uplifted arc to the east by a west-dipping normal fault. Block-and-ash-flow tuffs are interstratified with proximal (eastern) parts of the red bed graben fill, suggesting collapse of lava domes plumbed up the graben-bounding fault to the east. The red beds were then cut by a NE to ENE-striking dike swarm. (2) East of the fault lies a lava shield complex. Each lava shield has lavas and flow breccias that dip radially away from the center, with primary dips of 20-30°, and a radius of 1.2 – 2.5 km. The composition of the lavas on the east side of the fault is similar to that of the dikes on the west side of the fault¹. The base of the lava shield complex is exposed in one area, where it rests on the proximal red bed sequence. Based on this, we infer that the N-S fault that divides the map area has a >800 m down-to-the-east normal component of slip, and that lava shields on the west were eroded away, leaving the feeder dikes exposed. Finally, planation occurred across a broad surface, and widespread mesa-forming aphyric lavas were deposited on the surface. ⁴⁰Ar/³⁹Ar geochronology in progress on the block-and-ash-flow tuffs, the dikes, the lava shields and the mesa-forming lavas will test this model.

¹ Acuña et al., this volume