Abstract
Volcanic rocks of the Sierra San Francisco (SSF), in northern Baja California Sur, Mexico, record post-subduction magmatism related to slab melting and slab window opening. The range is composed of andesitic and dacitic domes, mafic lavas, and volcaniclastic deposits (debris and block-and-ash-flow, lahar, and fluvial) that constitute the proximal to distal facies of a volcanic field with local eruptive ages that postdate the regional transition from subduction to transtension. Lowest observed volcanic units consist of interbedded and hydrothermally altered mafic lavas, tuff breccias, and andesite/dacite domes that erupted largely between ~12.1-10.7 Ma. These are overlain by volcaniclastic units and andesite/dacite domes that erupted between ~10.98-9.24 Ma. Volcaniclastic deposits comprise a section up to 600 m thick, locally flank and dip radially away from domes, and are likely associated with dome collapse. These deposits are unconformably overlain by a series of ~5.5-4.5 Ma Mg-enriched basaltic andesites (bajaites) that typically erupted along NNW-trending normal faults.
The topography of the Sierra San Francisco has evolved dramatically since the onset of volcanism in this area. From ~12-9 Ma, adakite domes erupted across the central SSF, locally along NNW faults. Thick sequences of bedded tuff breccias accumulated around the domes and are radially inclined away from source domes. From ~9-5.5 Ma, deep erosion of the pyroclastic strata formed a range-wide radial drainage network with channel depths of up to 130 m or more. From ~5.5-4.5 Ma, voluminous bajaite lavas from cinder cones and dike vents flooded the top of the range and flowed down the radial drainages with flow distances up to 12 km. Vents are strongly aligned along steep NNW normal faults. After ~4.5 Ma, erosion removed interfluves of tuff breccia not armored by younger mafic lavas. Today, the long, steep-sided, lava-capped ridges are inverted topographically.
In the center of the range, hydrothermal alteration of the volcanic deposits during and after the dome volcanism caused severe material weakening and slope failure within the volcanic center. The area is now a distinctive erosional basin, partly filled with clay-rich landslide deposits.
The ~12-9 Ma mafic lavas and intermediate lava domes and debris flow blocks display an adakitic geochemical signature, implying an origin involving late Miocene foundering and melting of the edges of the subducted Farallon plate during the opening of a slab window after the 12.3 Ma transition from subduction to transtension. Adakitic rocks of the SSF and the Santa Clara volcanic field 60 km to the SW may constrain the E-W extent of the slab window. The ~5.5-4.5 Ma bajaites display enriched REE and trace element patterns, potentially resulting from the rise of enriched subslab mantle through the slab window and interaction with supraslab mantle, previously metasomatized by slab melts. Thermal pulses associated with Gulf of California rifting may have provided the heat to generate Mg-rich magmas which ascended along rift-related faults, precluding significant crustal contamination or fractionation, and allowing magmas to retain their primitive character.