Rapid and punctuated Late Holocene recession of Siling Co, central Tibet

This work was supported by a National Science Foundation grant from the Tectonics program (EAR-0911587) to E.K. and K.P.F. Additional support to E.W. was provided by grants from the Chinese Academy of Sciences (XDB03010500). X.S. also thanks the support from the Earth Observatory of Singapore, Nanyang Technological University through its funding from the National Research Foundation Singapore and the Singapore Ministry of Education under the Research Centers of Excellence initiative.Variations in the strength of the Asian monsoon during Holocene time are thought to have been associated with widespread changes in precipitation across much of Tibet. Local records of monsoon strength from cave deposits, ice cores, and lake sediments typically rely on proxy data that relate isotopic variations to changes in precipitation. Lake expansion and contraction in response to changing water balance are likewise inferred from sedimentologic, isotopic and paleobiologic proxies, but relatively few direct records of changes in lake volume from preserved shorelines exist. Here we utilize relict shoreline deposits and associated alluvial fan features around Siling Co, the largest lake in central Tibet, to reconstruct centennial-to-millennial-scale variations in lake area and volume over the Holocene. Mapping and surveying of lacustrine shorelines coupled with optically stimulated luminescence dating of associated deposits indicate protracted occupation of a highstand elevation from >8 ka to 4 ka, followed by rapid recession that was likely punctuated by several stillstands of centennial-scale duration. Calculation of the changes in lake surface area and past hydrologic indices of the Siling Co basin suggests the effective moisture during the early Holocene highstand was approximately three times greater than today. In contrast to other lakes in central and western Tibet, our results suggest that Siling Co did not begin to recede synchronously with decreasing solar insolation at ca. 9–8 ka. Rather, initial recession of Siling Co appears to correspond to a time period of enhanced aridity and weakened monsoon in both Africa and Asia at ca. 4.2 ka. Our results add to a growing body of literature that suggest a period of relatively severe aridity on the Tibetan Plateau at this time. We suggest that subsequent punctuated recession of Siling Co was punctuated by similar periods of abrupt climate change during the Late Holocene.Publisher PDFPeer reviewe

Flexural bending of southern Tibet in a retro-foreland setting.

The highest elevation of the Tibetan Plateau, lying 5,700m above sea level, occurs within the part of the Lhasa block immediately north of the India-Tibet suture zone (Yarlung Zangbo suture zone, YZSZ), being 700m higher than the maximum elevation of more northern parts of the plateau. Various mechanisms have been proposed to explain this differentially higher topography and the rock uplift that led to it, invoking crustal compression or extension. Here we present the results of structural investigations along the length of the high elevation belt and suture zone, which rather indicate flexural bending of the southern margin of the Lhasa block (Gangdese magmatic belt) and occurrence of an adjacent foreland basin (Kailas Basin), both elements resulting from supra-crustal loading of the Lhasa block by the Zangbo Complex (Indian plate rocks) via the Great Counter Thrust. Hence we interpret the differential elevation of the southern margin of the plateau as due originally to uplift of a forebulge in a retro foreland setting modified by subsequent processes. Identification of this flexural deformation has implications for early evolution of the India-Tibet continental collision zone, implying an initial (Late Oligocene) symmetrical architecture that subsequently transitioned into the present asymmetrical wedge architecture

Holocene slip rate along the Gyaring Co Fault, central Tibet

Although geodetic measurements of interseismic deformation in interior Tibet suggest slow strain accumulation, active slip along the right-lateral Gyaring Co Fault is suggested to be between 8 and 21 mm/yr. Reliable geologic constraints on the slip rate along this fault are sparse. Here we document 12 ± 2 m of right-lateral displacement of lacustrine shorelines across the Gyaring Co Fault. Optically stimulated luminescence ages of the shorelines are tightly clustered between 4.1 and 4.4 ka. These data require an average slip rate of 2.2–3.2 mm/yr along the central Gyaring Co Fault during the latter half of the Holocene. Consideration of seismic cycle effects allows the possibility of slightly higher average slip rates, up to 2.2–4.5 mm/yr. Overall, our results suggest that the slip rate along the Gyaring Co Fault is similar to other strike-slip faults in interior Tibet, supporting the notion that active deformation in this region is distributed among numerous, slowly moving faults.Keywords: Active faulting, Slip rate, Gyaring Co, Lacustrine shoreline

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

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[subscript e]) and viscosity of Tibetan crust. Shoreline features associated with the Lingtong highstand complex ∼60 meters above present lake level are deflected from horizontal by 2–4 meters over wavelengths of ∼200 kilometers. Optically stimulated luminescence dating of aggradational shoreline deposits indicate 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[subscript e] of 20–30 km. If viscoelastic relaxation in response to lake withdrawal is complete, our data suggest an average viscosity ≤10¹⁹ 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.This is an author's peer-reviewed final manuscript, as accepted by the publisher. The published article is copyrighted by Elsevier and can be found at: http://www.journals.elsevier.com/earth-and-planetary-science-letters/,The author has provided the additional file, "Reply to Comment on “Crustal strength in central Tibet determined from Holocene shoreline deflection around Siling Co.” The published versions of the "Reply to Comment ..." and the "Comment on ... " to which the reply refers are available on the publisher's journal site as listed above.Keywords: Tibetan Plateau, Effective elastic thickness, Lake shoreline, Crustal strength, Viscosit

Evaluating the size and extent of paleolakes in central Tibetduring the late Pleistocene

Subhorizontal lake shorelines allow a geodynamic test of the size and extent of a hypothesized paleolake in central Tibet, the East Qiangtang Lake (EQL), during the last interglacial period (marine isotope stage (MIS) 5e). Reconstructions based on relict lake deposits suggest that the EQL would have been ~400 m deep and over ~66,000 km2. Models of flexural rebound driven by lake recession predict that shorelines near the EQL center, at the present-day location of Siling Co, would have rebounded 60–90 m above their initial elevation. New 36Cl chronology of the highest relict shorelines around Siling Co indicates that they reflect lake levels between 110 and 190 ka. These shorelines, however, are presently >300 m below their predicted elevations, implying a substantially smaller water load. Our results reveal that the expansion of Tibetan lakes during MIS 5e was relatively limited. Instead, individual lakes were supplied by river networks, much as they are today.Published versio

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

Subhorizontal lake shorelines allow a geodynamic test of the size and extent of a hypothesized paleolake in central Tibet, the East Qiangtang Lake (EQL), during the last interglacial period (marine isotope stage (MIS) 5e). Reconstructions based on relict lake deposits suggest that the EQL would have been ~400 m deep and over ~66,000 km2. Models of flexural rebound driven by lake recession predict that shorelines near the EQL center, at the present‐day location of Siling Co, would have rebounded 60–90 m above their initial elevation. New ³⁶Cl chronology of the highest relict shorelines around Siling Co indicates that they reflect lake levels between 110 and 190 ka. These shorelines, however, are presently >300 m below their predicted elevations, implying a substantially smaller water load. Our results reveal that the expansion of Tibetan lakes during MIS 5e was relatively limited. Instead, individual lakes were supplied by river networks, much as they are today

Uplift of the Emei Shan, western Sichuan Basin: Implication for eastward propagation of the Tibetan Plateau in Early Miocene

AbstractThe Emei Shan stands on the southeast side of the Longmen Shan. It marks the southeastern most edge of the eastern margin of the Tibetan Plateau (EMTP) and towers above the Sichuan Basin to its east, with the highest peak being 3099m. A large number of structural and thermo-chronological studies have been published focused on the Longmen Shan orogen, which is considered to be the northern part of the EMTP. However, for the southern part of the EMTP where the Emei Shan lies, its tectonics and uplift history are still poorly understood. This paper dates five granite samples from the Emei Shan batholith using the apatite fission track (AFT) method. The boundary faults of the Emei Shan and structural deformation in adjacent region are also investigated and mapped in detail. (1) In Miocene time, compressional stress from the eastward extrusion of the EMTP caused large-scale thrusting and mountain uplift along narrow Longmen Shan in northern EMTP, in contrast to southern EMTP where a broad and gentle Yaan−Emei Shan fold belt absorbed most of the stress. By Pliocene time, the southeastward extrusion of the Tibetan Plateau caused the transpressional stress and rotated the Chuandian fragment along the Xianshuihe−Xiaojiang fault belt. As a consequence, the Emei Shan region was intended northeastwards into the Sichuan Basin, bounded by two strike-slip faults on its north and southeast sides. (2) The AFT results show the Emei Shan uplifted since ∼25–20Ma ago. The threshold and main stage of uplift of the Emei Shan possibly lagged behind the Longmen Shan of approximately 5Myr. As the leading edge of fold belt, uplift of the Emei Shan indicated the eastward propagation of the EMTP was time-transgressive from Late Oligocene to Middle Miocene