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, peleean domes, lavas, and radially-dipping block-and-ash flow and lahar deposits of the Comondu Group that constitute the proximal and medial facies of a latest Oligocene-Miocene aged (24-12 Ma) arc stratovolcanic center. The Comondu Group is overlain by a series of approximately 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 Comondu vents. Lavas flowed radially away from the center of the range and were locally inset into post-Comondu 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 approximately 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 Comondu 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 approximately 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.