Article

Late Cenozoic Evolution of the Central Longmen Shan, Eastern Tibet: Insight from (U – Th)/He Thermochronometry

Tectonics (Impact Factor: 3.32). 10/2009; 28(5):TC5009. 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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Available from: Raphaël Pik, Jan 22, 2014
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    • "50 Ma, with eastward thrusting, dextral shear and drastic uplift (Yin and Harrison, 2000;Tapponnier et al., 2001). In the central segment of the Longmen Shan thrust belt where the Pengguan complex exposed, the thermochronology study byGodard et al. (2009)shows that the major rapid exhumation started at 8–11 Ma, and an earlier rapid exhumation event was determined about 30– 25 Ma (Wang et al., 2012). Furthermore, the YBF is considered to have a major role in response to the rapid exhumation, forming the newly seismogenic background for the 2008 M w 7.9 Wenchuan earthquake. "
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    ABSTRACT: In order to understand the mechanism for occurrence of large earthquakes in the Longmen Shan region, we indirectly estimated the flow stress and pore fluid pressure in the brittle–ductile transition zone by studying exhumed granitic rocks which experienced Mesozoic ductile deformation, and constructed rheological profiles for the brittle regime and transition zone. The samples were collected from a small pluton that includes granites and deformed granitic rocks overthrust at an outcrop along the southern segment of the Yingxiu-Beichuan fault. The outcrop profile consists of granitic gneiss, protomylonite and mylonite. We observed that heterogeneous ductile deformation occurred in the brittle–ductile transition zone, and that the flow stress ranges from 15 to 80 MPa. Moreover, the fault in that zone experienced a temporary brittle deformation, which might indicate a high strain rate during the co-seismic process and early post-seismic creep. Secondary fluid inclusions were found and measured to define the possible range of the capture temperature and fluid pressure. Sublithostatic pore fluid pressure was determined at the capture temperatures of 330–350 �C during the process of filling and/or healing of microcracks. According to constructed rheological profiles and related mechanisms, high, sublithostatic pore fluid pressure is likely to significantly weaken the fault and to be related to inception of a brittle fault slip above the brittle–ductile transition zone. A high strain rate driven by the coseismic slip in the brittle regime may lead to a brittle fault slip in the brittle–ductile transition zone, and then plastic deformation in the transition zone may resume gradually during post-seismic creep. The focal depth of the 2008 Mw 7.9 Wenchuan earthquake may be controlled by a velocity weakening to velocity strengthening transition in frictional slip (brittle regime) of granite around a temperature of 350 �C.
    Full-text · Article · Mar 2016
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    • "In the eastern margin of the Tibetan Plateau, thermochronology analysis of zircon and apatite indicated that the denudation rates reached 1e2 mm/y for the Longmen Shan region during the Late Cenozoic (Kirby et al., 2002). Later, long-term denudation rates for the Pengguan Massif and Longmen Shan front measured by low- temperature thermochronology were estimated to be ~0.5e0.7 mm/y (Kirby, 2008; Godard et al., 2009). Recently, shortterm denudation rates in the earthquake region were estimated to vary between 0.2 and 0.3 mm/y, which are measured from concentrations of 10 Be in quartz extracted from river sand prior to the Wenchuan earthquake (Ouimet et al., 2009; Ouimet, 2010). "
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    ABSTRACT: Identifying and understanding the respective influence of climate and tectonic processes becomes a frontier topic in tectonically active mountain ranges. In the eastern Tibetan Plateau, earthquake occurred frequently and usually led to formation of many dammed lakes. In this study, a last deglacial section of lacustrine sediments at Xinmocun in eastern Tibet is selected for conventional X-ray fluorescence (XRF) and Scanning XRF (SXRF) elemental analysis. The high correlation of major and trace element abundances between the Xinmocun lacustrine samples and the loess-soil samples from the Chinese Loess Plateau (CLP) supports the previous view of the eolian origin of the Xinmocun lacustrine sediments. Analysis of major element abundances, ratios and trace element ratios indicates that the dust provenance of the Xinmocun lacustrine sediments is different from that of the CLP, and is similar to that of the loess at Ganzi and Hongyuan nearby the study area. The significant variations of most geochemical elements and their close coupling with grain-size variations can’t be reasonably explained by the changes in transport dynamics of eolian dust, and is possibly caused by the intermittent changes in available dust provenance. Frequent earthquakes usually triggered abundant landslides and provided large amounts of dust for the study area. Accordingly, like the >16 μm fraction of the Xinmocun grain-size record, many element abundances and their ratios, such as SiO2/Al2O3, TiO2/Al2O3, CaO/Al2O3, Sr/Al2O3, Rb/Sr, and Na2O/Al2O3, can be regarded as sensitive indicators of earthquake events in the tectonically active regions. SXRF measurements at the U-channel surface can provide some cost-effective indicators of seismic events, such as Si/Al, Ti/Al, Ca/Al, Sr/Al, Zr/Rb, and Rb/Sr. On the other hand, significant variations in most element abundances and their ratios of the Xinmocun lacustrine sediments correlate closely with those of its grain-size record. They all have no increasing or decreasing trends with time. These suggest that tectonic activities characterized by seismic events possibly had a major role on the landscape erosion in the eastern Tibetan Plateau while the climatic influence seems minor, which needs to be examined in the future.
    Full-text · Article · Aug 2015 · Quaternary International
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    • "Since Late Triassic, the block experienced extensive crustal deformation following the convergence between the Indian and Eurasian plates (e.g. Wang and Shi, 1982; Xiao et al., 2007) and is considered as a window to continental collision, uplift and plateau growth (Maddox, 1984; Kirby et al., 2002; Godard et al., 2009; Li et al., 2012). This region has been in the focus of several investigations based on geological and geophysical techniques. "
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    ABSTRACT: Following the 2008 Mw 7.9 Wenchuan and 2013 Ms 7.0 Lushan earthquakes, the Longmenshan thrust-fault belt and the Songpan–Ganzi terrane have been the focus of several investigations. Here we use the H–k stacking technique and neighborhood algorithm to investigate the seismic structure of this area. Based on the presence of felsic lower crust and the Mesozoic crustal architecture of the Songpan–Ganzi and Longmenshan area, we exclude the model on the eastward flow of the middle and lower crust assigned as the cause for the crustal thickening in previous studies. In contrast, the E–W trending cumulative compression induced by the continued northward motion of the Indian plate and India-Asian collision are identified as the dominant factors leading to the crustal thickening in the Longmenshan thrust-fault region as well as the Songpan–Ganzi terrane. Particularly, the 2008 Mw 7.9 Wenchuan and 2013 Ms 7.0 Lushan earthquakes likely indicate the cumulative offsets and the E–W continuing compression.
    Full-text · Article · Dec 2014 · Journal of Asian Earth Sciences
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