STRESS ESTIMATES FROM THE MANTLE BELOW THE CALAVERAS FAULT

Mantle xenoliths from the Coyote Lake basalt, located on the Calaveras fault, record the fabrics developed in the lithospheric mantle directly beneath the San Andreas fault system. These spinel peridotite xenoliths are interpreted to originate from depths of 37.8-42.5 km. The lattice preferred orientation (LPO) of olivine in the xenoliths shows very strong point distributions, suggesting high-temperature conditions of deformation and consistent with the interpretation that they constitute the mantle continuation of the upper crustal fault.

We used experimentally derived paleopiezometric relationships for olivine in the mantle xenoliths to constrain the stresses below the Calaveras fault system. Phases were differentiated using false-color images prepared from Al, Ca and Fe X-ray maps, which provide unequivocal discrimination among olivine, orthopyroxene, and clinopyroxene. Olivine grain outlines were traced to produce grain boundary maps, which were analyzed using ImageSXM software, available from the University of Liverpool, to determine grain size. The average grain size (~ 3 mm) was basically consistent for the six xenolith samples analyzed. The dynamically recrystallized grain size suggests stresses of ~15 MPa. The flow law for dry olivine at 1100° C, 1800 MPa, (based on compositional analyses of xenolith minerals) yields strain rates of 10^-11-10^-12 1/sec, while wet olivine suggests much faster strain rates (~10^-9 1/sec). This stress estimate suggests that significant strength exists in the lithospheric mantle below major strike-slip faults.