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

Tectonics (Impact Factor: 3.99). 10/2009; 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: Cooling History of a Crustal Section in Eastern Tibet (Well HC1) Constrained by Thermochronology
    ACTA GEOLOGICA SINICA (English Edition). 07/2013; 87(12):57-58.
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    ABSTRACT: The devastating 2008 Mw 7.9 Wenchuan earthquake, China, demonstrates that the central and northern parts of the Longmen Shan are currently active. Evidence for active faulting and folding in the southern Longmen Shan, however, remains poorly documented. In this paper, we define the structural geometry, fault kinematics, and seismic hazard of the Qiongxi thrust fault system (QTF) along the southern Longmen Shan range front by integrating deep and shallow seismic‐reflection data and geomorphic observations. The QTF is a 50 km long, north–south‐trending set of faults and associated folds that exhibit geomorphic evidence of Quaternary surface deformation. Geomorphic observations and seismic‐reflection data reveal that these faults dip steeply to the east and merge at depth with a blind, west‐dipping thrust ramp. The trend and reverse sense of slip along the QTF indicates that the structure accommodates east–west crustal shortening. Based on uplift of stratigraphic horizons across the fault zone, we define a Late Pliocene–to–Early Pleistocene fault slip rate of 0.2–0.3 mm/yr and a Middle Pleistocene–to–present rate of 0.4–1.2 mm/yr on the west‐dipping thrust ramp. This ramp soles to a basal detachment in the Triassic section at a depth of 4.5–5.5 km. To the west, this detachment steps down onto a blind, northwest‐dipping thrust termed the Range Front thrust. A rupture of the QTF in combination with the Range Front thrust could generate an Mw 7.8 earthquake with average displacement of 5.7 m. This type of earthquake source poses significant hazards to the adjacent, highly populated Sichuan basin.
    Bulletin of the Seismological Society of America 08/2013; 103(4):2369-2385. · 1.96 Impact Factor
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    ABSTRACT: Zircon (U-Th)/He (zircon He) dates from the Longmen Shan (LMS, on the eastern margin of the Tibetan Plateau) show a distinctive compositional dependence consistent with a strong effect of radiation damage on He diffusion. Using a new model accounting for the evolution of damage and diffusivity as a function of time and temperature, we use these data, together with constraints from other low-T thermochronometers, to interpret the Precambrian to Neogene thermal and exhumation histories of LMS basement rocks. For most samples, several features of the inverse correlations between single-grain zircon He dates and effective uranium (eU) concentrations, combined with geologic constraints, require near-surface exposure in the Precambrian, followed by burial and heating to temperatures less than about 200 °C over hundreds of Ma, and a final episode of cooling (exhumation) to surface temperatures after ∼30 Ma. In contrast, samples from the hanging wall of the Wenchuan–Maowen thrust fault in the LMS show weak or no date–eU correlations, requiring exhumation from greater depths than corresponding footwall rocks. Our modeling focuses particularly on maximum temperatures prior to Cenozoic exhumation, as well as the timing of the Cenozoic rapid cooling event, as these thermal history segments are most pertinent to debates about the timing and kinematics of recent exhumation in the LMS. Models for one sample near the front of the range in the central LMS (LME-18) require rapid Cenozoic cooling from ∼180 °C to less than ∼50 °C from ∼30–25 Ma. Model results from a more hinterland transect in the central LMS (Wenchuan) require a later rapid cooling event from ∼190 °C to the surface, beginning at ∼15 Ma. Finally, our models for samples from the southern LMS (WMF footwall transect) require rapid cooling from ∼200 °C to the surface beginning at ∼12 Ma. Taken together, these reinterpretations of previously published results lead to a cohesive burial and exhumation history for samples from a large area of the orogen and require large-magnitude exhumation in hinterland regions of the LMS more than 10 Ma after exhumation in the frontal part of the range.
    Earth and Planetary Science Letters 10/2014; 403:328–339. · 4.72 Impact Factor


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