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

Tectonics (Impact Factor: 3.99). 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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    • "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.
    Journal of Asian Earth Sciences 12/2014; 96:226–236. DOI:10.1016/j.jseaes.2014.09.026 · 2.83 Impact Factor
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    • "Zircon grains (prismatic to round-shaped, 0 to 2 pyramids, with an equivalent spherical radius ranging from 35 to 60 μm) were then measured, and loaded in Pt capsules for He extraction at CRPG. They were outgassed at 1500 • C for 20 min, and analyzed for He concentrations with a VG603 noble gas mass spectrometer (Pik et al., 2003; Godard et al., 2009). After total helium extraction, Pt capsules containing zircon grains were retrieved for U, Th, and Sm content measurements at SARM (Nancy, France). "
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    ABSTRACT: thermal inheritance detrital thermochronology margin inversion inverse modeling Pyrenees The extent to which heat recorded in orogens reflects thermal conditions inherited from previous rift-related processes is still debated and poorly documented. As a case study, we examine the Mauléon basin in the north-western Pyrenees that experienced both extreme crustal thinning and tectonic inversion within a period of ∼30 Myrs. To constrain the time–temperature history of the basin in such a scenario, we provide new detrital zircon fission-track and (U–Th–Sm)/He thermochronology data. The role of rift-related processes in subsequent collision is captured by inverse modeling of our thermochronological data, using relationships between zircon (U–Th–Sm)/He ages and uranium content, combined with thermo-kinematic models of a rift-orogen cycle. We show that the basin recorded significant heating at about 100 Ma characterized by high geothermal gradients (∼80 • C/km). Our thermo-kinematic modeling and geological constraints support the view that subcontinental lithospheric mantle was exhumed at that time below the Mauléon basin. Such a high geothermal gradient lasted 30 Myr after onset of convergence at ∼83 Ma and was relaxed during the collision phase from ∼50 Ma. This study suggests that heat needed for ductile shortening during convergence, is primarily inherited from extension rather than being only related to tectonic and/or sedimentary burial. This should have strong implications on tectonic reconstructions in many collision belts that resulted from inversion of hyper-extended rift basins.
    Earth and Planetary Science Letters 10/2014; 408:296-306. DOI:10.1016/j.epsl.2014.10.014 · 4.72 Impact Factor
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    • "In the LMS, this type of thermal history (heating to sufficiently high temperatures to anneal the damage that affects He diffusion just before the ultimate cooling episode) may explain the weak or non-existent zircon date–eU correlations for samples from the Xuelongbao transect (Godard et al., 2009) and the WMF hanging wall (Tian et al., 2013), which otherwise fulfill the basic requirements for showing a correlation (Fig. 3). Given that both datasets show reproducible dates of ∼10 Ma with little or no significant variation, it is likely that these samples resided at temperatures well above the ZFT PAZ prior to rapid cooling through the zircon He PRZ at ∼10 Ma. "
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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. DOI:10.1016/j.epsl.2014.06.050 · 4.72 Impact Factor
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