The San Emigdio (SE) area in the southern San
Joaquin Valley and the San Juan Bautista (SJB) area in
the northern Gabilan Range lie approximately 295 km
apart on opposite sides of the San Andreas fault. The
geochemistry of the lower Miocene volcanic rocks from
these two areas was studied ·to determine: 1) whether
they could be correlated across the fault, and 2> their
petrogenesis. The Miocene volcanic rocks from the SE and SJB
areas are the same age (approximately 22.5 Ma). Close
similarities in major- and trace-element, and isotopic
composition strongly suggest that the two areas were in
close proximity at the time of eruption and have since
been offset by movement along the San Andreas fault. The geochemical data suggest that basalt located in the SE area is not compositionally primitive and
probably interacted with the crust. Trace-element and
isotopic data from the SE-SJB intermediate to felsic
volcanic rocks represent melted crustal rocks, perhaps
greywacke from the Franciscan Formation. The SE-SJB volcanic rocks fit well with a group of Neogene volcanic rocks in California that correlate
closely with the temporal and spatial position of the
Mendocino triple junction. Geochemical comparisons with
other volcanic rocks of this group support a similar
origin of triple-junction magmatism. One possible model
involves decoupling of the subducted slab and consequent
upwelling of mantle material. The rising mantle induces
melting, which produces basalt. The basalt then induces
melting of crustal rocks to produce the more siliceous
volcanic rocks.
Description:
Includes bibliographical references (pages 85-88)
California State University, Northridge. Department of Geology.