Structural and thermal characters of the Longmen Shan (Sichuan, China)

Abstract : The Longmen Shan (Sichuan, China) is characterised by an unusual morphology which results from a Triassic prism tectonics and a more recent Neogene thick-skin thrusting. Among its specific features is the high elevation of the internal zones in continuity with the Songpan Garze fold-and-thrust belt (SG), which is associated with an abrupt 20 km Moho offset between the Sichuan Basin and the Tibetan plateau as revealed by the analysis of teleseismic data acquired by a dense seismic network. The jump in crustal thickness is located at the apex of the Wenchuan shear zone (WSZ) and marks the western boundary of the metamorphosed units of SG characterized by temperatures varying from 590 degrees C down to 300 degrees C as commonly observed in mature accretionary wedges. Both the structural style marked by intense shortening in tight kink folds and geophysical data suggest the presence of an horizontal discontinuity at similar to 15 km depth over a thick crust (similar to 63 km).;The Longmen Shan east of the WSZ is characterized by thick-skin detachment which thrusts the internal sedimentary units and the Proterozoic basement over the series of the Sichuan Basin deposited on a thinner crust of similar to 44 km thick. The major front is the Beichuan Fault Zone (BFZ) which brought the internal zones onto Triassic and Jurassic series with lower temperatures (less than 400 degrees C). Locally, temperatures of similar to 425 degrees C are found just below the klippes.;These results are in agreement with an original contact of the SG zone represented by an accretionary wedge of sediments thrusted over the margin of the continental crust of the Yangtze craton in the early stage (Indosinian) of the evolution. The present-day slow E-W component of the convergence, added to the difference in crustal thickness caused the Yangtze crust to indent the Songpan Tibetan crust which was softened by a high thermal regime. As a response the edge of the Tibet crust was inflated to the bottom (up to 70 km), whereas to the top, the crystalline massif were exhumed and pushed the deformation eastward as emerging and blind thrusts. This configuration reflects a moderate shortening of the crust which behaves as a soft thick material abutting the resistant and cold Yangtze crust.
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A. Robert, M. Pubellier, J. de Sigoyer, J. Vergne, A. Lahfid, et al.. Structural and thermal characters of the Longmen Shan (Sichuan, China). Tectonophysics, Elsevier, 2010, 491 (1-4), pp.165-173. ⟨10.1016/j.tecto.2010.03.018⟩. ⟨hal-00534849⟩



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