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The Sierra San Francisco (SSF) is a Neogene volcanic range along the topographic crest of the Baja California peninsula in northern Baja California Sur, Mexico. The SSF is ~55 km long (NW-SE) and ~30 km wide and its highest peaks exceed 1500 m elevation. The SSF has a long history of volcanism and has been eroded by deep, rugged, radially-draining canyons. The development of SSF topography is intimately associated with the volcanic evolution of the range. The SSF is a large and complex dacitic adakite dome complex largely built of a thick, up to 800 m, stratigraphic succession of dacitic tuff breccias with minor interbedded basaltic andesite lavas. These deposits overlie rare exposures of aeolian sandstone of unknown age. The tuff breccias represent block-and-ash-flows and lahars generated from steep-sided peleean dacite and andesite domes, with three radiometric dates of 11-10 Ma. This intermediate sequence is unconformably capped by widespread bajaite mafic lavas, 5.5-4.5 Ma. SSF topography evolved dramatically since the late Miocene: 1) From 11-10 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. The duration of this volcanism is unknown. 2) From 10-5 Ma, deep erosion of the pyroclastic strata formed a range-wide radial drainage network, with channel depths of up to 130 m or more. 3) 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. 4) 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. At Santa Martha, an area in the central SSF with the highest concentration of domes, 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. Comparable volcanic history and topographic development are likely to have occurred in a dome field of similar age and size at Santa Agueda, 60 km SE of Santa Martha. 

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. These are overlain by volcaniclastic units and dacite domes that erupted between ~11-10 Ma. Volcaniclastic deposits comprise a section up to 800 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. Low interbedded mafic lavas are chemically similar to syn-subduction lavas (>15 Ma) SE of the SSF, suggesting a typical subduction supraslab mantle source during waning, late Miocene Farallon plate subduction. ~11-10 Ma dacite 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. Further analysis will elucidate the timing of slab window development and the post-subduction mantle processes that drove the chemical evolution of SSF magmas. 

Central Baja California (BC) experienced tectonism and volcanism that shaped the landscape from the Miocene to Recent. One important feature is the San Ignacio trough (SIT) that hosted a marine seaway or embayment and acted as a physical barrier to animal and plant migration. This barrier may be responsible for a well-known break in the DNA, N and S of this region. Central BC has also hosted contemporary voluminous and chemically diverse volcanism. Radiometric ages provide important constraints on the origins and longevity of critical topographic features. The Baja GeoGenomics research group is investigating the nature and timing of Pliocene marine and tidal deposits in the NE-oriented, low-lying SIT, located W of the peninsular divide. These new data reveal that the Sierra San Francisco, a highland volcanic area immediately N of the SIT, is a series of volcanoes constructed of dacitic and andesitic Peleean domes with voluminous lahar and pyroclastic flow deposits. These calcalkaline rocks were previously thought to be subduction-related magmatism and part of the early to middle Miocene (~24–12 Ma) Comondú Group. However, zircon U-Pb and 40Ar/39Ar dates yield ages of 11-9 Ma. These data indicate the Sierra San Francisco erupted post-subduction and is not part of the lithologically similar but older Comondú Group. Within the SIT, 12km NE of San Ignacio at 200 m asl, newly mapped marine tidal deposits, informally called the San Regis beds, indicate that the SIT has been significantly uplifted. Mafic scoria interbedded in tidal deposits yield a groundmass 40Ar/39Ar age of about 4.2 ± 0.1 Ma. San Regis tidal beds are unconformably overlain by a rhyolite ash-flow tuff from the Quaternary La Reforma caldera situated to the E, on the Gulf of California coast. The highly mobile ash cloud flowed W into the SIT at least as far as the San Regis beds locality NE of San Ignacio. The tuff yielded a preliminary U-Pb zircon age of 1.09 ± 0.04 Ma and an 40Ar/39Ar anorthoclase age of 1.11± 0.01 Ma. These dates indicate that the ash-flow was one of the latest erupted from the caldera and its distribution was in part controlled by the SIT. In BC genetic diversity along the peninsula appears to change at the latitude of the SIT. Tidal and volcanic deposits suggest this topographic low persisted for over 4Ma and remains a distinctive feature in the topography today.