T72 Tectonics and Sedimentation of the Central Gulf of California, Mexico, and Surrounding Regions

Sunday, 22 September 2024
1:30 PM - 5:30 PM
303A

MIOCENE TECTONIC AND STRATIGRAPHIC EVOLUTION OF THE CENTRAL BAJA CALIFORNIA PENINSULA AND RAPID MARINE FLOODING INTO THE GULF OF CALIFORNIA AT CA. 6.3 MA

Paper number: T72-15
Program: Technical Sessions
Time: 4:55 PM – 5:15 PM

Authors

Abstract

The central Baja California peninsula contains a record of crustal deformation, sedimentation, magmatism, and geomorphic changes driven by tectonic evolution of the Pacific-North America plate boundary over the past 30 Myr. Here we summarize results from two collaborative studies: the Baja Basins program that trained undergraduate students in field-based research, and the Baja GeoGenomics project that is testing hypotheses for N-S genomic divergence in modern plants and animals along the peninsula. This abstract focuses on Miocene evolution of the central Baja California region as determined from geologic mapping, stratigraphic analysis, Ar/Ar and U-Pb ages of volcanic rocks, and U-Pb ages of detrital zircon (DZ) from sandstones and tuffs. We divide the Comondú Group into: (1) ~33–23 Ma sandstone and conglomerate including the El Salto Formation at the base; (2) ~23–19(±2) Ma tuff breccias and lahar deposits; and (3) ~19(±2)–11 Ma intermediate lavas, breccias, and pyroclastic rocks. The Comondú Group is interpreted to record westward migration of arc magmatism in response to steepening of the Farallon slab. The change to a dextral-oblique plate boundary ca. 12.3 Ma led to the end of subduction and a change to slab-window magmatism. Upper Miocene (12–6 Ma) rocks include tholeiitic basalts, bajaites, thick tuff breccias and dacite dome complexes, rift-transition bimodal volcanics near Santa Rosalía, and >10-Ma marine deposits (San Ignacio Formation) on the western peninsula below 200 m elevation. Age data define a magmatic gap and regional unconformity from 8.8 to 6.4 Ma, when plate-boundary dextral strain was becoming localized into the present-day Gulf of California prior to marine incursion. The Boleo Formation in the Santa Rosalía basin contains marine-deltaic gravels and sands that pass laterally into distal gypsum facies. DZ ages from the basal Boleo limestone yield a maximum depositional age of 6.35 ± 0.21 Ma, nearly 1 Myr younger than a previous age estimate. This dates earliest marine flooding into the central Gulf of California that resulted from linking of strike-slip faults and rapid subsidence in pull-apart basins during onset of sea floor spreading and related hydrothermal activity. Marine incursion was geologically instantaneous at ca. 6.3 Ma along ~1,000 km of the plate boundary, with unknown but potentially large impacts on Earth-life evolution.

Geological Society of America Abstracts with Programs. Vol. 56, No. 5

doi: 10.1130/abs/2024AM-402328

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