关键词: 龙门山, 龙门山断裂带, 古地震, 软沉积物变形构造, 构造演化

Abstract:

The Longmen Shan is a mountain body composed of three main faults. The WenchuanMaoxian fault, also called the rangeback fault, constitutes the western boundary of the Longmen Shan; the YingxiuBeichuan fault is the central fault of the Longmen Shan; and the GuanxianAnxian fault, also called the rangefront fault, is the eastern boundary of the Longmen Shan. On the east of the Longmen Shan fault belt, there is a foreland basin which has been experienced different periods since the terminal of the Late Triassic. There had been rich softsediment deformation structures (SSDS) in the strata of different ages in the foreland basin from the Late Triassic to the May 12, 2008 Wenchuan earthquake (MS 8.0), including liquefaction deformation, gravitational deformation, hydroplastic deformation, and other related deformations. These deformation strata were located immediately near the three faults of the Longmen Shan, and the activities of these faults in different periods induced strong earthquakes in different periods, leading to the deformation of the sediments that had not consolidated then (seismites). The SSDS in the Upper Triassic Xiaotangzi Formation included liquefied breccia, liquefied droplet, liquefied diapir, thixotropic diapir, convolute deformation, stretched pudding, load, ballandpillow structure, pillow beds, and graded fault. In the Jurassic and Cretaceous, there were mainly coarsegrained sediments, and the main soft sediment deformation types were largescale conglomerate load structures with various shapes except liquefaction deformation found in a few horizons. In the Paleogene, there was lacustrine sediments, with fine grains, and there were large quantities of liquefaction deformation structures in the soft sediment, e.g., plunged sediment mixtures, liquefied breccia, and pillow beds. The May 12, 2008 Wenchuan earthquake (MS 8.0) induced large scale liquefaction of underground sand bed, forming a series of liquefaction deformation structures and microfeatures, e.g., liquefied dune, liquefied sheet sand, sand volcano, liquefied mound, pit landform, and chaotic accumulation. The occurrence time sequence and seismic orogenic periods of the Longmen Shan fault belt can be given with the reflection seismic results of the Longmen Shan, the paleoearthquake records and the regional structures: (1) The collision between the SongpanGarzeor orogenic belt and the Yangtze Plate occurred in the early stage of the Late Triassic, and their boundary is the current WenchuanMaoxian fault; the WenchuanMaoxian fault had thrust nappe orogenesis in the end of the Late Triassic, the mountain in the Longmen Shan region in the end of the Triassic could be called the SongpanGarze Shan, on the east of which a foreland basin was formed; the continental dynamic action period of the Late Triassic Indosinian orogenic cycle was the gestation and birth stage of the Longmen Shan. (2) The thrust activity between the YingxiuBeichuan fault and the GuanxianAnxian fault occurred in the Jurassicthe Early Cretaceous, forming high mountains and the foreland basin. (3) The Early Cretaceous Longmen Shan had been already a fault belt mountain body composed of three thrust faults, and it could be called the PaleoLongmen Shan, with mountain altitude >3500 m. (4) The activities of these three faults in the Paleogene induced the Paleogene soft sediment deformation, but the faults did not have thrust nappe orogenesis, and the sediments were finegrained lacustrine sediments, so the Paleogene was a stage with seismic activities but without orogenesis. (5) The Mesozoic Longmen Shan experienced multiple processes of instantaneous seismic orogenesis and quietperiod mountain range denudation lowering, and the current Longmen Shan is the product of multiple instantaneous seismic orogenic activities occurring during the Late Cenozoic. Different from those methods used for the Longmen Shan by numerous geoscience researchers, another idea, softsediment deformation structures, i.e., the paleoearthquake records was used in this paper, in other words, it is a new way to disscuss the tectonic evolution in the Longmen Shan by studying the SSDS.

Key words: Longmen Shan, the Longmen Shan fault belt, paleoearthquake, softsediment deformation structure, tectonic evolution