Geodetic imaging of late postseismic lower crustal flow in Tibet

Abstract

Integral to understanding past and present seismotectonic activity in orogenic zones is whether any part of the lower crust is weak enough to flow, since a weak zone at depth influences the style of deformation that occurs. Thick crust such as found beneath the Tibetan Plateau and the Altiplano–Puna Plateau in the Andes leads to elevated temperatures at depth, which potentially allows the lower crust to flow on human time scales, as well as on the time scales of plateau formation and evolution. Here we use the late postseismic response of the crust to large earthquakes as a tool for investigating rheological structure beneath the Tibetan Plateau. Eighteen years (1992–2010) of InSAR data on the southern Tibetan Plateau show a late postseismic relaxation response to two large earthquakes in 1951 and 1952. The relaxation occurs throughout a thick viscoelastic lower crust and possibly extends into the upper mantle. The lower crust has viscosity 6–10×1019 Pas, with an associated Maxwell relaxation time of 63 years. Flow within a thin mid-crustal channel can be ruled out. These late-stage postseismic observations represent the first clear evidence of present-day lower crustal flow, and are consistent with geophysical evidence for fluid-weakened lower crust across much of the Tibetan Plateau, as required by models of plateau formation. Active deformation of continental plateaus in other parts of the world, notably the Altiplano–Puna Plateau in South America, may also be strongly influenced by a weak lower crust.