| dc.description.abstract | The Tibetan Plateau, resulting from the active Eurasian-India collision, presents a major scientific challenge in understanding its growth and propagation. One key region is the Longmen Shan mountain belt in western Sichuan, which forms the steepest margin of the plateau and has been active as demonstrated by the Mw 7.9 Wenchuan (2008) and Mw 6.6 Lushan (2013) earthquakes. Tectonic history of the Longmen Shan belt and the neighboring Songpan-Garze terrane, however, began in the Triassic Indosinian orogenesis, which complicates the geologic records. But the major thickening of Tibet was formed in Himalayan orogenesis. Therefore, quantitative constraints on the pre-Tertiary tectonic evolution of the region are crucial in delineating Himalayan geodynamics.
In this study, 50 samples were collected in eastern margin of Tibetan Plateau along several transects in NW-SE direction, perpendicular to the structural grain of the Longmen Shan and its border with the neighboring Songpan-Garze terrane. The raman spectroscopy of carbonaceous material (RSCM) geothermometer is applied to the samples to obtain their peak metamorphic states. The distribution of the peak temperatures from RSCM analyses (RSCM-T) is not correlated to later igneous intrusions, ruling out significant contact metamorphism overprint. And there is no apparent geothermal gradient between RSCM-T and stratigraphy, showing that the strata were affected by peak thermal event after basin diagenesis. Together with existing data, it is confirmed that its distribution is controlled by tectonic activities. Furthermore, through the constraints from thermochronometers data, it can be certain that peak thermal metamorphism documented as RSCM-T occurred during the Indosinian orogeny, and its distribution was affected by the different mechanism of wedge growth (frontal/basal accretion). The difference of regional exhumation rate in the Himalayan orogeny is still not enough to change this situation.
The evolution history of eastern Tibet Plateau inferred from the result shows that the Triassic flysch was been accreted due to the closure of Paleotethys. Some of the flysch rocks were underplated under the rear part of the orogenic wedge at around 13-19 km depth, at the same time acquired their peak thermal metamorphic state in lower-middle Triassic. The stacking of the basal accreted duplex system led the thrust slices been exhumed within the wedge at depth of 10-14 km during early Jurassic. Finally, during the Cenozoic Himalayan orogeny, due to the substantial shortening and uplift of the Tibet Plateau, the metamorphosed flysch rocks were exhumed to the surface.
At the boundary between Songpan-Garze and Longmen Shan: the oblique subduction of the Paleotethys, causing the accretionary wedge to thicken and transport to east, forming an eastward thrust decollement at the boundary. And the eastern migration of the decollement, caused the passive margin sediments which has been tectonic buried to be exhumed by basal accretion and exposed to the surface along the decollement. This decollement was eventually inherited by the Wenchuan Fault. Therefore, the Silurian to Permian passive margin sediment west of the Wenchuan fault has high degree of metamorphism (RSCM-T> 400°C), having continuous RSCM-T with Triassic flysch. | en_US |