Strike-slip partitioned transpression of the San Andreas fault system: a lithospheric-scale approach

This paper is part of the special publication Continental transpressional and transtensional tectonics (eds R.E. Holdsworth, R.A. Strachan and J.F. Dewey). Physical and numerical experiments on transpression indicate that strike-slip partitioning is facilitated by low angle of convergence, such as occurs on the modern San Andreas fault system. Cross- sections across the San Andreas fault overestimate the amount of margin- normal contraction, because they do not include the effect of horizontal stretching caused by the wrench component of deformation. When this correction is made assuming a strike-slip partitioned transpression model, contraction estimation from cross-sections is consistent with both the normal and tangential movements imposed by plate motion: there is no San Andreas discrepancy. Seismic anisotropy (shear-wave splitting) and teleseismic analyses suggest that the upper mantle in the San Andreas region is thick and largely undeformed to the SW of the San Andreas fault, but thin and intensely sheared in a zone 50-100 km wide beneath the NE region of the San Andreas fault system. Seismic anisotropy is best explained by a transpression zone of vertical foliation (and possibly horizontal lineation) which penetrates the asthenospheric mantle. Using a lithospheric-scale approach, the transfer of the displacement field from the penetratively deformed mantle to the strike-slip partitioned upper crust necessitates zones of accommodation in the mid- to lower crust, corresponding to the flat-lying detachments that underlie the San Andreas fault system. The mechanics of the San Andreas fault are best considered in connection with the strength of the various lithospheric layers involved in this system.