Mobility of continental mantle: Evidence from postseismic geodetic observations following the 1992 Landers earthquake

The crust around the rupture zone of the 1992 Landers earthquake has contin ued to deform in the years following the earthquake at rates similar to 3 t imes greater than pre-earthquake rates. We use a combination of Global Posi tioning System (GPS) and synthetic aperture radar (InSAR) data collected du ring a similar to 3-year epoch following the earthquake in order to investi gate postseismic mechanisms responsible for the high transient velocities. In order to maximize the potential signal from viscoelastic relaxation we e valuate and model postseismic relaxation following the first few months of documented accelerated deformation. The combination of GPS and InSAR data a llows us to establish viscoelastic relaxation of the lower crust and upper mantle as the dominant postseismic process and to discriminate among possib le viscoelastic models. The data particularly require the presence of a hig hly ductile uppermost mantle beneath the central Mojave Domain, with temper ature between the wet and dry basalt solidus. This is consistent with indep endent seismic and geochemical inferences of a regionally warm uppermost ma ntle. Further consideration of seismic velocity variations in conjunction w ith faulting patterns within the Mojave Desert suggests that the primary fa ulting characteristics of the Mojave Desert, namely, the pervasive late Cen ozoic deformation within the Eastern California Shear Zone versus the near absence of faults in the Western Mojave Domain, are controlled by the rheol ogy of the uppermost mantle.