The boundaries of tectonic plates cover ~15% of Earth’s surface and constitute areas of significant natural hazards, including earthquakes. As ~40% of Earth’s population lives along plate boundaries, it is critical to understand what aspects of lithospheric deformation control earthquakes in interplate settings. This talk will address how mantle-crust interactions may affect the rheology of the deep sections of strike-slip plate boundaries.

I will present geological data from three field areas: 1) the San Andreas fault system in the USA, 2) the Baja California shear zone in Mexico, and 3) the Bogota Peninsula shear zone in New Caledonia. Analysis of upper mantle and lower crust xenoliths from the San Andreas fault and the Baja California shear zone, indicates that differential stress remains constant, and low, with depth. This result is not consistent with the typical lithospheric strength profiles constructed from deformation experiments. Data from the Bogota Peninsula shear zone, which comprises the exhumed mantle section of an oceanic transform zone, indicate spatial and temporal variations in stress. These variations are interpreted to be the result of imposed localization, rather than intrinsic localization produced by strain weakening. Imposed localization is induced by the mechanical interaction between the upper, “brittle” part of the oceanic lithosphere, and the underlying viscously deforming upper mantle, during earthquake rupture. The results from the three study areas indicate that crust and lithospheric mantle act together as an integrated system, and allow us to build a picture of earthquake-related deformation in the upper mantle during the seismic cycle.