The 2010Mw7.2ElMayor-Cucapah earthquake provides a unique target of postseismic study as
deformation extends across several distinct geological provinces, including the coldMesozoic
arc crust of the Peninsular Ranges and newly formed, hot, extending lithosphere within the
Salton Trough. We use five years of global positioning system measurements to invert for
afterslip and constrain a 3-D finite-element model that simulates viscoelastic relaxation. We
find that afterslip cannot readily explain far-field displacements (more than 50 km from the
epicentre). These displacements are best explained by viscoelastic relaxation of a horizontally
and vertically heterogeneous lower crust and upper mantle. Lower viscosities beneath the
Salton Trough compared to the Peninsular Ranges and other surrounding regions are consistent
with inferred differences in the respective geotherms. Our inferred viscosity structure suggests
that the depth of the Lithosphere/Asthenosphere Boundary (LAB) is ∼65 km below the
Peninsular Ranges and ∼32 km beneath the Salton Trough. These depths are shallower than
the corresponding seismic LAB. This suggests that the onset of partial melting in peridotite
may control the depth to the base of the mechanical lithosphere. In contrast, the seismic LAB
may correspond to an increase in the partial melt percentage associated with the change from
a conductive to an adiabatic geotherm.