V020-0025
Late Miocene-Pliocene Volcanism in the Sierra de San Francisco, Central Baja California Peninsula, Mexico
Thursday, 10 December 2020
Poster
Sam Grandy1, Brian P Hausback1, Scott E K Bennett2, Rebecca J Dorsey3, MIchael Darin4, Greer Dolby5, Michael G Sawlan6, Benjamin Wilder7, Genaro Martinez Gutierrez8 and Yahil Emmanuel Hernandez Salgado9, (1)California State University Sacramento, Sacramento, CA, United States, (2)U.S. Geological Survey, Moffett Field, CA, United States, (3)University of Oregon, Department of Earth Sciences, Eugene, OR, United States, (4)Nevada Bureau of Mines and Geology, Reno, United States, (5)Arizona State University, Tempe, AZ, United States, (6)US Geological Survey, Menlo Park, CA, United States, (7)University of Arizona, Desert Laboratory on Tumamoc Hill, Tucson, AZ, United States, (8)Universidad Autonoma de Baja California Sur, Department of Earth Sciences, La Paz, Mexico, (9)Universidad Autónoma de Baja California Sur, Department of Earth Sciences, La Paz, BS, Mexico
Abstract:
Volcanic deposits of the Sierra de San Francisco (SSF), the northeastern-most mountain range in Baja California Sur, record post-subduction magmatism related to the opening of a slab window. Reconnaissance mapping shows that the bulk of the 1400 m-high SSF is constructed of andesitic and dacitic intrusives, peléean domes, lavas, and radially-dipping block-and-ash flow and lahar deposits of the Comondú Group that constitute the proximal and medial facies of a latest Oligocene-Miocene aged (24-12 Ma) arc stratovolcanic center. The Comondú Group is overlain by a series of ~11-3 Ma Mg-rich basaltic andesites (bajaites), the eruptions of which occur in multiple distinct pulses. These magmas formed lavas, domes and scoria cones that are regionally distributed, typically along NNW trending, E-side down normal faults and locally adjacent to older Comondú vents. Lavas flowed radially away from the center of the range and were locally inset into post-Comondú paleo valleys. Scoria cones are now largely eroded away, revealing feeder dikes that typically align with the NNW-trending faults.
Gradual stalling of Farallon-Pacific spreading centers from ~12-8 Ma stopped subduction and resulted in detachment of the downgoing slab, which opened a slab window beneath Baja California. Extensional forces initiated the NNW-trending normal faults in the Comondú Group during this time and may be related to hot asthenospheric upwelling through the slab window or early Gulf of California extension. Melting of the metasomatized supraslab mantle wedge may have led to the generation of the Mg-rich andesites.
Eruption of these Mg-rich lavas may have been coeval with subsidence to the southeast of the SSF that led to deposition of newly mapped ~3-4 Ma tidal deposits on the distal flank of the range, 10 km east of San Ignacio. Substantial post-Pliocene incision through the inset lavas may indicate more recent uplift. The drivers of magmatism and subsidence in this area are still poorly understood. Further analysis will help to elucidate the mantle processes associated with tearing, detachment, and foundering of the subducted slab that may be responsible for magmatism and episodic subsidence and uplift of the mid-peninsula.
