VOLCANOLOGY AND STRUCTURE OF THE MULEGE-BAHIA CONCEPCION REGION, GULF OF CALIFORNIA, BAJA CALIFORNIA SUR, MEXICO: RESULTS FROM THE BAJA BASIN RESEARCH EXPERIENCE FOR UNDERGRADUATES (REU) AND INTERNATIONAL RESEARCH EXPERIENCE FOR STUDENTS (IRES)

The Baja Basins NSF-REU and NSF-IRES volcanology projects from 2017-2022 focused on stratigraphic and structural relations across a 40 x 60 km area from Mulegé south to the Bahia Concepción Peninsula and west to the Sierra la Giganta. Ten-day field campaigns each winter were followed by three-week lab sessions each summer.

The region is divided into three structural blocks, described from west to east. Block 1 lies in the footwall of an active, west-dipping normal fault zone, the Bahía Concepción fault zone (BCFZ), and this block forms the Concepción peninsula (40 X 14 km). Block 1 exposes Cretaceous granitic basement overlain by east-dipping Oligocene to middle Miocene strata and Miocene intrusions. Block 2 is in the hanging wall of the BCFZ, and is dropped below sea level adjacent to the fault (6 km wide Bahía Concepción) but is well exposed along the rest of its 24 km width, and is a half graben filled with 10-12 Ma lavas. Block 3 is separated from block 2 by a N-S steeply-dipping oblique-slip fault that uplifts lower Miocene strata on the west. These are beveled and capped by shallowly west-dipping Pliocene lavas that form the crest of the Sierra la Giganta.

Preliminary regional correlations are based on petrography, geochemistry, and ⁴⁰Ar/³⁹Ar geochronology. The stratigraphic units in the BCFZ footwall block (block 1) include, from base to top: 1) Oligocene forearc aeolian and fluvial sandstones and silicic ignimbrites; 2) lower Miocene adakitic block-and-ash-flow tuffs and debris deposits cut by 16 Ma intermediate dikes; 3) middle Miocene alkalic welded silicic ignimbrites and alkalic mafic to intermediate lavas, and local Mn veined basalt lavas that interfinger with alluvial fan deposits; 4) sheet-like basaltic trachyandesite lavas; 5) 11 Ma mounded intermediate lavas. The half-graben fill (block 2) exposes only the 11-12 Ma mounded lavas. Block 3 contains adakite block-and-ash-flow tuffs that are correlative to stratigraphic unit 2 from the BCFZ footwall block (block 1) and are similarly cut by 16 Ma dikes.

In summary, this work documents an overall regional progression from Oligocene forearc ignimbrites to lower Miocene arc block-and-ash-flow tuffs, reflecting slab rollback, followed by middle to upper Miocene ignimbrites and lavas that record the transition to Gulf of California rifting.