Initial faulting age of the Longmen Shan thrust fault belt: Paleo-earthquake information from Scientific Drilling (WFSD)

The Longmen Shan thrust fault belt has got much more attention after the 2008 Wenchuan earthquake, but there is still no accordant cognition about its formation age. The frequently fault activities of the Longmen Shan thrust fault belt have triggered several strong ancient seismic activities in the geological history, and induced unconsolidated soft-sediment deformed. Soft-sediment deformation structures formed during or shortly after deposition are important indicators of past seismic activity. These structures are a direct response to processes of fluid escape during liquefaction and fluidization related to past seismic activity in the area, suggest they could play an important role in analyzing the distribution and intensity of ancient tectonic activity. Many layers of conglomerate with peculiar shapes of breccias occur in the Xujiahe Formation sediments in the WFSD-1 drilling core. The peculiar conglomerate layers spaced at irregular intervals, which can be classified into 4 groups, from top to bottom, the depths are: 759.03-812.48 m, 932.8-978 m, 991.88-1025.25 m and 1097.4-1156.51 m. The breccias in the peculiar conglomerate are mostly black calcareous fine-grained siltstone, with the sizes varying from a few millimeters to dozens of centimeters, mostly are 1-5 cm. The cementing material is fine-grained quartz sandstone (particle diameter: 0.05-0.2 mm). The content of the calcareous siltstone breccia in sandstone is about 5-60%. The breccias vary in their morphology and pattern, such as embay structure, small irregular flame structure, liquefied droplet and homogeneous layer. Those are typical liquefied deformation features caused by earthquake without remote transport. The original rock is alternating layers of black calcium siltstone and yellow-grey fine-grained sandstone, formed below epicontinental sea wave base. Strong earthquake triggered the fine-grained sandstone liquefied, then traversed and flowed into the soft sedimentary siltstone layer made it disconnected as breccias with irregular shapes. In situ liquefied breccias are mixed sizes without sorting, makes a clear distinction with waves washed gravel. Viewed from the Longmen Shan region, soft-sediment deformations in middle-north segment of the western Sichuan foreland basin can be widely tracked, as a component of the lowest layer in the Xujiahe Formation or the marker bed of the Xiaotangzi Formation. The four groups of liquefied breccia in WFSD-1 drilling core can be regarded as the products of four strong paleo-earthquakes associating with the intracontinental subduction of the Songpan-Ganzi terrane and the Yangtze block in the early Late Triassic in the Longmen Shan region, which equivalent to the initial activities of Wenchuan-Maoxian fault accompanying with strong earthquakes. Therefore, in WFSD-1 drilling core, the soft-sediment deformations in the bottom of the Xujiahe Formatoin are the most powerful proof of that the Wenchuan-Maoxian fault began faulting in the early Late Triassic.