ABSTRACT

Wenchuan earthquake is a thrust with strike-slip type, and surface ruptures are located in Beichuan fault, Pengguan fault zone, Xiaoyudong tear fault, and Leigu tear fault. The scratches of the surface rupture reveal the thrust movement occurred early and the strike-slip movement occurred lately during the earthquake. A dynamical model to illustrate possible links between surface processes and upward extrusion of lower crustal flow channel at the eastern margin of the Tibetan plateau have been studied, and the results is the material in lower crust in the Longmen Shan moving as nearly-vertical extrusion and uplift, resulting in the surface rate of tectonic movement differing according to depth rate as well as the occurrence of large shallowWenchuan earthquake.

1.INTRODUCTION

The Ms 8.0 Wenchuan Earthquake in May 12th, 2008 is one of the most disastrous earthquakes since the foundation of P.R. China, which destroyed not only the epicenter of Sichuan province but also several closed provinces. It was felt in most region of China, as well as nations outside China. This tragic event provides the opportunity to advance the subject of seismic sciences. Based on our accumulated activitiy on active tectonics in Longmen Shan area, we have undertaken several new field surveys, including international collaborative efforts after the earthquake. This paper compiles 70 sets of data from accumulated past and new surveys, detailing surface rupture and seismic disasters since our work began in the 1990s(Li et al,2000,2001,2006;Zhou et al,2006,2007;Densmore et al,2007; Li et al,2008).

2. GGOLOGICAL SETTING OF LONGMEN SHAN SEISMIC BELT

The Qinghai-Tibetan Plateau is the highest and the youngest uplift region in the world with more than 4000 m altitude. The Indo-Asian collapse is the most important tectonic event, inducing the uplift, deformation and thickening of the Qinghai-Tibetan Plateau. Concerning the relationship between this event and Cenozoic tectonic geology, two famous theories have been presented: one is the crust thickening mode(England,et.al,1990) and another one is lateral extrusion mode(Avouac, et.al,1993). The former emphasizes the shortening of the south-north direction as well as crust thickening; the later emphasizes the east-lateral extrusion mode along main strike-slip faults. The core object is the relation between uplifting process (vertical motion) and deformation process (horizontal motion) in the Qinghai-Tibetan Plateau, and the relation between the motions and Indo-Asian collapse.

The Longmen Shan thrust belt is located at the west margin of Sichuan basin and the joint between Songpan-Ganzi orogeny and Yangtze Craton. It is both the east margin of Qinghai-Tibetan Plateau and the west margin of Longmen Shan foreland basin (fig. 1). It is enclosed by Guangyuan in the north, Tianquan in the south, Dabashan orogeny in the northeast and Xianshuihe fault in the southwest, forming a northeast-southwest strike belt with 500 km long and 30 km wide. Consisting of a series of parallel imbricated thrust, it develops, from the west to the east, the Maoxian-Wenchuan, Beichuan and Pengguan faults. Along the fault planes, there are multi-classic added thrust nappe structure belts: Wenchuan-Maowen ductility fold structural belt, the Beichuan cuiductilit thrust nappe structural belt.

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