Crustal rheology of southern Tibet constrained from lake-induced viscoelastic deformation

Abstract

We probe the rheology of the Tibetan lithosphere using the rebound that accompanied climate-driven lake level variations. At the modern decadal time scale, we used deformation around Siling Tso measured from InSAR. At the millennial time scale, we use Holocene paleoshorelines around Siling Tso and Zhari Nam Tso. We use chronological constraints from the literature and Digital Elevation Models to constrain their ages and geometry. We observe a small post-highstand subsidence of the area near the center of mass of the paleolake-load and a low-amplitude short-wavelength outer bulge. In the context of a model consisting of an elastic lid over a viscous channel with a rigid base, these observations preclude the existence of a thick low viscosity channel and require a thin elastic lid. Based on Monte Carlo inversion, we constrain the range of possible equivalent elastic thickness of the lid (<5 km), the viscosity (2×1018–1020 Pa.s) and thickness of the crustal channel (<10–20 km). By contrast, the modern data requires a stiffer lid with equivalent elastic thickness >20 km and a >20 km thick channel with lower crustal viscosity (<5×1018 Pa.s). The different rheologies inferred at these different time-scales could be explained by a Burgers body rheology of the middle and lower crust, with a short-term viscosity of 1018 Pa.s and long-term viscosity of 1020 Pa.s, or even better by vertical variations of viscosity. To illustrate the latter claim, we show that the observations at the decadal and Holocene time scales can be reconciled by assuming a low viscosity zone (1018 Pa.s) at mid-crustal depth (between ∼10 and 30 km depth) embedded in a higher viscosity crust (>1020 Pa.s). In both cases, the interferences in space of the deformation signals induced by the lakes geometry, and in time through the viscoelastic response to the lake level variations results in limited distortion of the paleo-shorelines. While the elastic lid in the upper crust needs in any case to be thin (<10 km), the low amplitude distortion requires significant viscoelastic support from the lower crust and upper mantle; this explains the relatively high effective elastic thickness (>20 km) inferred in some previous studies of Holocene paleoshorelines. In the longer term, the effective elastic thickness of the lithosphere must drop asymptotically to the value of the elastic lid in the upper crust (<10 km); this explains the low effective elastic thickness derived from gravity studies.