Late Cenozoic evolution of the central Longmen Shan, eastern Tibet: Insight from (U-Th)/He thermochronometry

Tectonics (Impact Factor: 3.49). 01/2009; DOI: 10.1029/2008TC002407

ABSTRACT This article presents (U-Th)/He thermochronological data from the Longmen Shan belt, eastern Tibet. Located between the Songpan-Garze terrane and the Yangtze craton, this mountain range is one of the steepest margins of the Tibetan Plateau and an important area for the comprehension of the mechanisms that control the dynamics of such plateau borders in terms of spatial distribution of deformation or timing of topographic building. We describe several age-elevation transects and perform forward modeling of our data to derive quantitative information on the exhumation of the range. A major phase of exhumation started at 8–11 Ma, with an average rate of ∼0.65 mm a−1. Comparison of zircon and apatite ages indicates that the eastern part of the range may have experienced a significant decrease in exhumation since 2–3 Ma. We use the distribution of finite exhumation across the major faults of the area to quantify their dip-slip throw rate over the last 10 Ma. The Beichuan Fault, which was activated during the 2008 Sichuan earthquake, is the major active structure of the Longmen Shan since the late Miocene, with an average thrusting slip rate between 0.4 and 1 mm a−1. Conversely, over the same time period, only minor dip-slip activity occurred on the Wenchuan Fault Zone. This distribution in space and time of exhumation and deformation is discussed and compared to the different proposed models for the geodynamical evolution of the eastern Tibetan margin. It also provides an important long-term perspective to put in context the destructive 2008 Sichuan earthquake that struck the central Longmen Shan.

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    ABSTRACT: GPS field and seismic data show that the southeastern margin of the Tibetan plateau is tectonically and seismically active. This activity is due to the southeastward extrusion of the Chuandian fragment, a large crustal block rotating clockwise around the northeastern syntaxis of the Himalayas. The eastern boundary fault of this fragment is defined by the left-lateral Xianshuihe-Xiaojiang fault, which abruptly truncates the Sichuan basin of the Yangtze block. Our paper presents evidence indicating that the Sichuan basin experienced right-lateral shear along its margin, including the Longmen Shan fault belt, as shown by the presence of a large number of interference deformation features, including S-shaped and Z-shaped folds and faults, aligned in an en echelon pattern. This study hypothesizes that the Sichuan basin experienced counterclockwise rotation, dragged by the left-lateral movement along the Xianshuihe fault, and it is this rotation that was the underlying cause of the 12 May 2008 Wenchuan Ms 7.9 earthquake. During the rotation, the Sichuan basin decoupled along a sub-horizontal decollement fault zone that developed along Triassic gypsum and coal bearing rocks, below which the Paleozoic rocks experienced much more intense deformation than the overlying Mesozoic rocks, suggesting that the lower part of the basin experienced a larger scale rotation relative to the uppermost part of the basin. Based on thermal data from the western margin of the Sichuan basin and from along the Xianshuihe fault, the counterclockwise bending/rotation of the Sichuan basin initiated in late Cenozoic time (~13 Ma).
    Tectonics. 03/2014;
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    ABSTRACT: [1] Recent studies of the northeastern part of the Tibetan Plateau have called attention to two emerging views of how the Tibetan Plateau has grown. First, deformation in northern Tibet began essentially at the time of collision with India, not 10–20 Myr later as might be expected if the locus of activity migrated northward as India penetrated the rest of Eurasia. Thus, the north-south dimensions of the Tibetan Plateau were set mainly by differences in lithospheric strength, with strong lithosphere beneath India and the Tarim and Qaidam basins steadily encroaching on one another as the region between them, the present-day Tibetan Plateau, deformed, and its north-south dimension became narrower. Second, abundant evidence calls for acceleration of deformation, including the formation of new faults, in northeastern Tibet since ~15 Ma and a less precisely dated change in orientation of crustal shortening since ~20 Ma. This reorientation of crustal shortening and roughly concurrent outward growth of high terrain, which swings from NNE-SSW in northern Tibet to more NE-SW and even ENE-WSW in the easternmost part of northeastern Tibet, are likely to be, in part, a consequence of crustal thickening within the high Tibetan Plateau reaching a limit, and the locus of continued shortening then migrating to the northeastern and eastern flanks. These changes in rates and orientation also could result from removal of some or all mantle lithosphere and increased gravitational potential energy per unit area and from a weakening of crustal material so that it could flow in response to pressure gradients set by evolving differences in elevation.
    Tectonics. 09/2013; 32(5):1358-1370.
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    ABSTRACT: The 2013 Mw 6.6 Lushan earthquake occurred in the Longmen Shan fold-and-thrust belt, Sichuan Province, China, near the five-year anniversary of the devastating 2008 Mw 7.8 Wenchuan earthquake. To define the fault that generated the 2013 earthquake and its relationship with the Beichuan fault, which ruptured in the Wenchuan earthquake, we construct several cross sections and a 3D structural model. The sections and models reveal that the main-shock of the Lushan earthquake occurred on a portion of the Range Front blind thrust (RFBT) and that the structural geometry of this fault varies along strike. The Lushan main-shock occurred at a location along the strike of the fault where the geologic shortening and total fault slip are greatest. A lateral ramp of the RFBT appears to coincide with the northern limit of aftershocks from the Lushan earthquake, leading to a 75 km seismic gap between the Wenchuan earthquake and the 2013 earthquake sequence. Although both the Wenchuan and Lushan earthquakes occurred within the Longmen Shan fold-and-thrust belt, different faults generated the two events. Based on this structural characterization and analysis of the aftershocks of the Wenchuan and Lushan earthquakes, we suggest that the Lushan earthquake may have been triggered by the 2008 rupture but is best considered as an independent event rather than an aftershock of the Wenchuan earthquake. The RFBT that generated the Lushan earthquake is linked to a detachment that extends into the Sichuan basin along the Triassic evaporite layer. The coulomb stress change simulation suggests that other faults linked to this detachment may have been loaded by the 2008 and 2013 earthquake, posing the risk of future earthquakes along the Longmen Shan and in the densely populated Sichuan basin.
    Earth and Planetary Science Letters 01/2014; 390:275–286. · 4.72 Impact Factor


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