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

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

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

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

Weakly coupled lithospheric extension in southern Tibet

AbstractWest–east extension is a prominent tectonic feature of southern and central Tibet despite ongoing north–south (N–S) convergence between India and Eurasia. Knowledge of deep structure beneath the N–S trending rifts is key to evaluating models proposed for their origin, including gravitational collapse, oblique convergence along the arcuate plate boundary, and mantle upwelling. We model direct S and Moho-reflected SsPmp phases at teleseismic distances to constrain variations in crustal thickness across the major rifts crossed by a ∼900-km long, W–E broadband array in the Lhasa Terrane. Crustal thicknesses are ∼70–80 km. However, Moho depth decreases by ∼10 km within a horizontal distance of 100 km west of the Yadong–Gulu rift (YGR) and Nyainquentanghla mountains (NQTL). This Moho uplift, taken with deep, extensional focal mechanisms and reduced seismic velocity in the upper mantle, suggests that asthenospheric upwelling has significantly contributed to the pattern of extension across the YGR and NQTL. The ∼100-km separation between surface rift and Moho uplift is likely enabled by partial decoupling across a ductile middle crust

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

A challenge to the concept of slip-lines in extrusion tectonics

Wide-open V-shaped conjugate strike-slip faults in Asia are typically related to extrusion tectonics. However, the tectonic model based on the slip-line theory of plasticity has some critical problems associated with it. The conjugate sets of slip-lines in plane deformation, according to the theory of plasticity should be normal to each another but, in reality, the angles between the conjugate strike-slip faults, which are regarded as slip-lines in extrusion tectonics in the eastern Mediterranean, Tibet-middle Asia, China and the Indochina Peninsular regions, are always more than 90° (on average ∼110°) in the direction of contraction. Another problem is that the slip-line theory fails to explain how, in some cases, e.g., in the Anatolian area in the eastern Mediterranean, the extrusion rate is much higher than the indent rate. The two major problems are easy to solve in terms of the Maximum-Effective-Moment (MEM) Criterion that predicts that orientations of the shear zones are theoretically at an angle of 54.7° and practically at angles of 55°±10° with the σ1- or contractional direction. The orientations of the strike-slip faults that accommodate extrusion tectonics are, therefore, fundamentally controlled by the MEM Criterion. When extrusion is along the MEM-orientations, the extruding rate is normally higher than the indenting rate

GPS Strain Rate and Seismic Activity Before the Ludian Earthquake (Ms 6.5), Northeast Yunnan, China: New Implications for Eastward Chuan-Dian Block Extrusion

We characterized E-W shortening crustal deformation using GPS measurements and an earthquake focal mechanism solution to estimate conditions before the Ms 6.5 Ludian earthquake of 3 August 2014 (16:30, UTC+8). A series of NW-SE striking transpressive faults are distributed outside the Chuan-Dian Block. They accommodated the E-W shortening and N-S stretching in this region resulting from Chuan-Dian Block outward expansion. The displacement velocity field about the eastward, fault-normal slip rate and the vertical component are also examined. The results show that they share a similar rate change at the Anninghe, Zemuhe, Puxionghe, Jiaojihe, and Baogunao-Xiaohe faults. Among these faults, the maximum thrusting slip rate of the Baogunao-Xiaohe fault is less than 4 mm yr-1. The slip rate change may represent a transition from lateral translation into a vertical thrusting movement. Furthermore, counterclockwise rotation also plays an important role in absorbing the E-W shortening. Relative to the inner zone, the outer Chuan-Dian Block together with the Daliang Shan fault zone represent the development of a new deformation area. A prediction could be made that the Daliang Shan fault zone will eventually replace the east boundary fault role of the Anninghe-Zemuhe-North Xiaojiang fault due to Chuan-Dian Block continuous outward expansion