Abstract
Palaeoshorelines around four large lakes in central Tibet record a latest-Pleistocene-to-Holocene high stand during which the lakes were filled 150–200m more deeply than they are at present. GPS measurements of shoreline elevations around Zhari Namtso show that they are horizontal to within ±2m at the 2-σ level. Measurements of height made by combining Shuttle Radar Topographic Mission elevations with Google Earth imagery of shorelines around Zhari Namtso, Tangra Yumtso, Taro Tso, and Ngangla Ringtso show that all the palaeoshorelines are horizontal within measurement uncertainty. Support of the lake loads by elastic stresses can explain the horizontality of the shorelines only if the equivalent elastic thickness of the crust exceeds 15–25km. The observations are more plausibly explained by support of the lake loads through viscous stresses in the middle to lower crust. This support requires that the viscosity of the middle to lower crust is at least 1019–1020Pas. These values are consistent with estimates from studies of post-seismic relaxation after large earthquakes of the region and are higher, by two orders of magnitude, than would permit significant lateral flux of material through a channel in the middle to lower crust.
Highlights
Palaeoshorelines around four large lakes in central Tibet record a latest-Pleistocene-to-Holocene high stand during which the lakes were filled 150–200 m more deeply than they are at present
We present an analysis of the response of the Tibetan crust to loading by palaeolakes, on the time scale of kiloyears, which supports the inferences drawn from post-seismic relaxation on the time scale of years
The shorelines of the four palaeolakes are horizontal because their loads were supported by a combination of elastic stresses in the upper crust and viscous stresses in the middle and lower crust, the fraction of parameter space in which these two sources of support are comparable in magnitude is small (Fig. 9)
Summary
Different from deformation of the lower lithosphere and deduction, from surface observations, of forces acting on the lithosphere as a whole will be difficult or impossible (e.g. Molnar, 1988). Different from deformation of the lower lithosphere and deduction, from surface observations, of forces acting on the lithosphere as a whole will be difficult or impossible Estimates based on post-seismic deformation following large earthquakes suggest that the viscosity remains relatively high (≳1018 Pa s) throughout the crust in Tibet (Hilley et al, 2005, 2009; Ryder et al, 2011; Wen et al, 2012; Yamasaki and Houseman, 2012). We present an analysis of the response of the Tibetan crust to loading by palaeolakes, on the time scale of kiloyears, which supports the inferences drawn from post-seismic relaxation on the time scale of years
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