Extension and mantle upwelling within the San Andreas fault zone, San Francisco Bay area, California

Abstract

Upper mantle and lower crustal xenoliths in Tertiary alkali olivine basalts located along the trace of the Calaveras Fault system near Coyote Lake reservoir, northern California, provide an opportunity to study physical conditions of crystallization, composition, and rheology of magmatic mantle rocks beneath the San Andreas fault system (SAF). The episode of mafic alkaline volcanism in the Calaveras Fault at the area of Coyote Lake reservoir can be related to stepping‐fault interaction. The right‐stepping en echelon fault geometry along with right‐lateral motion of the SAF and the spatial and temporal distribution of the Coyote Lake volcanics suggest that fault activity was synchronous to the volcanic event. Upper mantle xenoliths are Cr‐diopside spinel lherzolites with minor dunites both having porphyroclastic textures. Lower crustal xenoliths are two‐pyroxene metagabbros and andesine megacrysts. Two‐pyroxene and olivine‐spinel (sp‐lherzolites only) geothermometric estimates for both spinel lherzolites and two pyroxene metagabbros range between 885° and 1077° C and 915° and 1000° C, respectively. Pressures are bracketed between 12 and 16.8 kbars and 9 and 10.5 kbars for spinel lherzolites and two‐pyroxene metagabbros, respectively. These thermobarometric data agree with mantle temperatures projected along the elevated surface heat flow geotherms estimated for the SAF system and also with lithologies inferred from geophysical studies in this area. Furthermore, these thermobarometric estimates along with xenolith chemistry and the presence of alkaline volcanism within the fault suggest that this segment of the Calaveras Fault may have experienced near‐fault normal extension during the Pliocene producing mantle upwelling and decompressional melting at relatively shallow depths.