THERMAL-RHEOLOGIC EVOLUTION OF THE UPPER-MANTLE AND THE DEVELOPMENT OF THE SAN-ANDREAS FAULT SYSTEM

The evolution of the San Andreas fault system differs from that of many other major fault zones in that it can be directly linked to processes in and properties of the underlying mantle. This fault system serves as the plate boundary between the North American and Pacific plates. It has progressively formed since approximately 30 Ma in response to a fundamental change in plate boundary structure: subduction replaced by transform motion with the northward migration of a triple junction. As a result of triple junction migration, major adjustments to lithospheric structure occur and cause the growth and maturation of the fault system. The geodynamic processes which have driven the development of the San Andreas system are primarily associated with the thermal and rheologic evolution of the uppermost mantle in the vicinity of the plate boundary. The emplacement of asthenospheric mantle at shallow levels beneath the North America crust after triple junction passage has led to crustal partial melting and volcanism, development of a well-defined plate-bounding mantle shear zone, and a sequence of events which produced the observed pattern of crustal faults and terranes. As a result of a complex three-dimensional thermal structure, plate boundary deformation (within the lithospheric mantle) is localized to a narrow zone. High strain rates and cooling-induced strengthening of the plate boundary zone lead to changes in grain size and ultimately to changes in deformation processes. The overall result of this is the development of a well-defined and relatively narrow plate boundary within the mantle lithosphere which is initially offset from the crustal fault zone. The mismatch between crustal and mantle parts of the plate boundary leads to the development of additional faults in the system, within the North American plate, which eventually mature to become the primary plate boundary structure in the crust. This is seen in a discrete jump in the location of the crustal plate boundary. All aspects of the evolution of the crustal plate boundary can be linked to the rheologic character of the underlying mantle lithosphere which in turn is largely a consequence of the plate tectonic evolution and the conversion of asthenosphere to lithosphere at shallow levels along the plate boundary.