The Longmen Shan Foreland Basin developed as a flexural foredeep during the Late Triassic Indosinian orogeny, spanning the time period c. 227±206 Ma. The basin fill can be divided into three tectonostratigraphic units overlying a basal megasequence boundary, and is superimposed on the Palaeozoic±Middle Triassic (Anisian) carbonate-dominated margin of the South China Block. The remains of the load system responsible for flexure of the South China foreland can be seen in the Songpan-Ganzi Fold Belt and Longmen Shan Thrust Belt. Early in its history the Longmen Shan Foreland Basin extended well beyond its present northwestern boundary along the trace of the Pengguan Fault, to at least the palinspastically restored position of the Beichuan Fault.The basal boundary of the foreland basin megasequence is a good candidate for a flexural forebulge unconformity, passing from conformity close to the present trace of the Beichuan Fault to a karstified surface towards the southeast. The overlying tectonostratigraphic unit shows establishment and drowning of a distal margin carbonate ramp and sponge build-up, deepening into offshore marine muds, followed by progradation of marginal marine siliciclastics, collectively reminiscent of the Alpine underfilled trinity of Sinclair (1997). Tectonostratigraphic unit 2 is marked by the severing of the basin's oceanic connection, a major lake flooding and the gradual establishment of major deltaic-paralic systems that prograded from the eroding Longmen Shan orogen. The third tectonostratigraphic unit is typified by coarse, proximal conglomerates, commonly truncating underlying rocks, which fine upwards into lacustrine shales.The foreland basin stratigraphy has been further investigated using a simple analytical model based on the deflection by supracrustal loads of a continuous elastic plate overlying a fluid substratum. Load configurations have been partly informed by field geology and constrained by maximum elevations and topographic profiles of present-day mountain belts. The closest match between model predictions and stratigraphic observations is for a relatively rigid plate with flexural rigidity on the order of 5 Â 10 23 to 5 Â 10 24 N m (equivalent elastic thickness of c. 43±54 km). The orogenic load system initially (c. 227±220 Ma) advanced rapidly (>15 mm yr À1 ) towards the South China Block in the Carnian, associated with the rapid closure of the Songpan-Ganzi ocean, before slowing to < 5 mm yr À1 during the sedimentation of the upper two tectonostratigraphic units (c. 220±206 Ma).
Abstract. From 12 to 14 August 2010, heavy rainstorms occurred in the Sichuan province in SW China in areas which were affected by the 2008 Wenchuan Earthquake, inducing catastrophic debris flows. This disaster is named as "the 8.13 debris flows". The results of the research presented in this paper show that the 8.13 debris flows are characterized by a simultaneous occurrence, rapid-onsets, destructive impacts, and disaster chain effects. They are located along the seismic fault, because the source materials mainly originate from loose deposits of landslides which were triggered by the Wenchuan Earthquake. The presence of large amounts of these loose materials on the slopes and the development of high intensity rainfall events are the main causes for the formation of these debris flows. The study of the 8.13 debris flows can provide a benchmark for the analysis of the long-term evolution of these debris flows in order to make proper engineering decisions. A flexible drainage system is proposed in this paper as a preventive measure to mitigate the increasing activity of these debris flows in the earthquakeaffected area.
The 5.12 Wenchuan Earthquake in 2008 induced hundreds of large-scale landslides. This paper systematically analyzes 112 large-scale landslides (surface area > 50000 m 2 ), which were identified by interpretation of remote sensing imagery and field investigations. The analysis suggests that the distribution of large-scale landslides is affected by the following four factors: (a) distance effect: 80% of studied large-scale landslides are located within a distance of 5 km from the seismic faults. The farther the distance to the faults, the lower the number of large-scale landslides; (b) locked segment effect: the large-scale landslides are mainly located in five concentration zones closely related with the crossing, staggering and transfer sections between one seismic fault section and the next one, as well as the end of the NE fault section. The zone with the highest concentration was the Hongbai-Chaping segment, where a great number of large-scale landslides including the two largest landslides were located. The second highest concentration of large-scale landslides was observed in the Nanba-Donghekou segment at the end of NE fault, where the Donghekou landslide and the Woqian landslide occurred; (c) Hanging wall effect: about 70% of the large-scale landslides occurred on the hanging wall of the seismic faults; and (d) direction effect: in valleys perpendicular to the seismic faults, the density of large-scale landslides on the slopes facing the seismic wave is obviously higher than that on the slopes dipping in the same direction as the direction of propagation of the seismic wave. Meanwhile, it is found that the sliding and moving directions of large-scale landslides are related to the staggering direction of the faults in each section. In Qingchuan County where the main fault activity was horizontal twisting and staggering, a considerable number of landslides showed the feature of sliding and moving in NE direction which coincides with the staggering direction of the seismic faults.
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