CONSTRAINTS ON THE MANTLE STRENGTH OF THE SAN ANDREAS FAULT SYSTEM: IMPLICATIONS FOR RHEOLOGIC MODELS

Many large strike-slip faults continue into the lithospheric mantle, and possibly into the asthenosphere, resulting in emphasis on the question of which is the strongest of the lithospheric layers. We use mantle xenoliths entrained in the 2.5-3.6 Ma Coyote Lake basalt, located near the intersection of the Hayward and Calaveras faults in central California, to constrain mantle rheology of the San Andreas fault system. The spinel peridotite xenoliths have been extracted from 40 km depth and have strong internal fabrics which indicate that mantle shearing occurred in the continuation of the San Andreas Fault system into the lithospheric mantle. Grain size paleopiezometers suggest differential stresses of 10-18 MPa - indicating 5-9 MPa of maximum resolved shear stresses - and FTIR analyses of water content indicate dry conditions (< 7 ppm H2O). These stress estimates are significantly lower than those predicted from laboratory experiments on olivine from comparable conditions, yet are similar to the < 20 MPa maximum shear stress (<10 MPa stress drops) inferred for the upper, seismogenic portion of the fault. These results do not fit with any existing lithospheric strength profile, yet indicate that viscous flow in the lithospheric mantle is intimately linked with brittle behavior of the upper crust. We propose that the deformation behavior of these two lithospheric layers form an integrated system, in which the lithospheric mantle adjusts its strength to match the upper crustal portion of the fault. We hypothesize that a feedback mechanism exists between seismogenic behavior in the upper crust causing viscous strain localization in the upper mantle and mantle flow causing displacement loading of the crust. This “Lithospheric Feedback” model suggests that lithospheric layers cannot be treated individually, that the upper crust effectively limits the “strength” of the entire lithosphere, and that displacement due to mantle flow loads the crust during interseismic cycles.