Crustal strength in central Tibet determined from Holocene shoreline deflection around Siling Co

Abstract

Editor: A. YinKeywords: crustal strength Tibetan Plateau lake shoreline effective elastic thickness viscosity Controversial end member models for the growth and evolution of the Tibetan Plateau demand quantitative constraints of the lithospheric rheology. Direct determinations of bulk crustal rheology, however, remain relatively sparse. Here we use the flexural rebound of lacustrine shorelines developed during the Lingtong highstand around Siling Co, in central Tibet, to place bounds on the effective elastic thickness (T e ) and viscosity of Tibetan crust. Shoreline features associated with the Lingtong highstand complex ∼60 m above present lake level are deflected from horizontal by 2-4 m over wavelengths of ∼200 km. Optically stimulated luminescence dating of aggradational shoreline deposits indicates that these lake levels were reached at 6-4 ka. Assuming that surface loads were entirely supported by an elastic layer overlying an inviscid fluid, the range and spatial distribution of variations in shoreline elevation are consistent with deflections predicted by a uniform elastic plate with thickness, T e of 20-30 km. If viscoelastic relaxation in response to lake withdrawal is complete, our data suggest an average viscosity ≤10 19 Pa s. These results imply that the apparent viscosity of the lower crust inferred over millennial timescales is comparable with that estimated from post-seismic relaxation over decadal timescale.Published by Elsevier B.V.

Dominant Controls of Downdip Afterslip and Viscous Relaxation on the Postseismic Displacements Following the Mw7.9 Gorkha, Nepal, Earthquake

“…Even though the WRMS misfit of the model with W e = 20 km is slightly smaller than that with W e = 30 km, the multiple‐mechanism models with W e = 20 km perform worse than those with Tibet's elastic thickness at 30 km. In addition, the 30 km elastic thickness agrees with the upper bound inferences constrained from Holocene shoreline deflections around Siling Co (Figure ), central Tibet, which is located in the northeast corner of our study region (Shi et al, ). Therefore, we assign the elastic thickness of Tibet to be 30 km, = 6 × 10 19 Pa s and = 6 × 10 18 Pa s. Although the optimal viscous model cannot explain the near‐field observations in Nepal, its prediction exhibits good agreement with observations in the southern Tibetan Plateau with a very small misfit of 2.7 mm.…”

“…Some postseismic deformation studies also favor an elastic thickness of no more than 20 km in the northern Tibetan Plateau (e.g., Ryder et al, ; Wen et al, ). However, the “long‐term” estimate of T e in Siling Co (Figure ), central Tibet is constrained to be 20–30 km from millennial lake loading deformation (Shi et al, ). Such thicker results are comparable to values of 20–40 km deduced from gravity anomalies and topography (e.g., Chen et al, ).…”

Evaluating the size and extent of paleolakes in central Tibet during the late Pleistocene

“…Calculation of expected shoreline elevations considering the crustal rebound induced by the lake unloading of the hypothesized East Qiangtang Lake (EQL). (a and b) The map pattern and contours (10 m interval) of the crustal rebound with crustal elastic thickness ( T e ) of 20 km and 30 km [ Shi et al ., ] for central Tibet, respectively. Numbers within the contours denote the magnitude of the rebound in meters.…”

“…The modern Siling Co sits in a broad depression that would have been near the center of the hypothesized EQL (Figure ). Within the Siling Co basin, Holocene shorelines are recognized up to 50 m above the 2010 lake level (4543 m) [ Meng et al ., ; Shi et al ., , ]. The highest of these, referred to as the Lingtong shoreline, sits at 4594 m (Figure ) and represents a period of time from ~6–4 ka when Siling Co was at its Holocene highstand [ Shi et al ., ].…”

“…Within the Siling Co basin, Holocene shorelines are recognized up to 50 m above the 2010 lake level (4543 m) [ Meng et al ., ; Shi et al ., , ]. The highest of these, referred to as the Lingtong shoreline, sits at 4594 m (Figure ) and represents a period of time from ~6–4 ka when Siling Co was at its Holocene highstand [ Shi et al ., ]. Remnants of even higher shorelines have been observed above this level, ranging up to ~4640 m in elevation [ Li et al ., ; Meng et al ., ].…”