The Mesozoic Along-Strike Tectonometamorphic Segmentation of Longmen Shan (Eastern Tibetan Plateau)
Résumé
The Longmen Shan belt (eastern border of the Tibetan plateau) constitutes a tectonically active region as demonstrated by the occurrence of the unexpected 2008 M w 7.9 Wenchuan and 2013 M w 6.6 Lushan earthquakes in the central and southern parts of the belt, respectively. These events revealed the necessity of a better understanding of the long-term geological evolution of the belt and its effect on the present dynamics and crustal structure. New structural and thermobarometric data offer a comprehensive data set of the paleotemperatures across the belt and P-T estimates for low-grade metamorphic domains. In the central Longmen Shan, two metamorphic jumps of 150-200°C, 5-6 kbar and~50°C, 3-5 kbar acquired during the Early Mesozoic are observed across the Wenchuan and Beichuan faults, respectively, attesting to their thrusting movement and unrevealing a major decollement between the allochtonous Songpan-Garze metasedimentary cover (at T > 500°C) and the autochtonous units and the basement (T < 400°C). In the southern Longmen Shan, the only greenschist facies metamorphism is observed both in the basement (360 ± 30°C, 6 ± 2 kbar) and in the metasedimentary cover (350 ± 30°C, 3 ± 1 kbar). Peak conditions were reached at ca. 80-60 Ma in the basement and ca. 55-33 Ma in the cover, ca. 50 Ma after the greenschist facies metamorphic overprint observed in the central Longmen Shan (ca. 150-120 Ma). This along-strike metamorphic segmentation coincides well with the present fault segmentation and reveals that the central and southern Longmen Shan experienced different tectonometamorphic histories since the Mesozoic.
Domaines
Tectonique
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