Lithospheric instability in obliquely convergent margins: San Gabriel Mountains, southern California

Oblique convergence across the San Andreas Fault in southern California has lead to rapid uplift of the San Gabriel Mountains since 5 Ma. Tomographic models of this region include a sheet‐like, high velocity anomaly, extending to depths >200 km. One proposed mechanism for the formation of this feature is gravitational instability of the lithosphere, resulting in a cold thermal downwelling. We present a systematic study of the sensitivity of lithospheric deformation (lithospheric downwelling, topography, deflection of the Moho, convergent velocity profile) to the viscosity structure in 2‐D numerical models of lithospheric instability, including a pre‐existing zone of weakness. Strike‐parallel strain within a convergent region should create a local zone of weakness due to the non‐Newtonian response of the lithosphere. Such a weak zone can focus convergent motion, increasing and localizing the growth‐rate of a lithospheric instability. We find that the observed characteristics of deformation depend on the width ( x w ) and relative viscous weakening factor ( f w ), the ratio of crust to mantle‐lithospheric viscosity (η c /η m ), and the absolute viscosity of the lithosphere (η m ). For the San Gabriel Mountains, we find that a range of models is capable of reproducing the observed Moho deflection and depth extent of the downwelling within the short time since the onset of convergence. However, only a small subset of models, ( x w = 20 km, f w = 50–100, η m = 7.5 × 10 20 –1.5 × 10 21 Pa s, η c /η m > 100) reproduce both the narrow, high topography and horizontal shortening profile.