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龙门山山脉,即2008年5月12日汶川(M=7.9)毁灭性地震的发生地,界定了喜马拉雅造山带的东缘,并显现出比青藏高原任何地方都大的地形起伏。然而,在这次地震之前,大地测量与地质调查工作都没有测到横跨该山系前缘有明显的缩短变形(Shenet al,2005;Meade,2007;Chenet al,2000),从而引起了关于该山岳带地形的形成与发展过程的激烈争论。现已提出两个端元模型:(1)脆性地壳增厚,即具有大量滑动的逆冲断层切穿岩石层并引起隆起(Ta-pponnieret al,2001);(2)地壳流动,即青藏高原下地壳的低粘度物质向外挤出,抬升了喜马拉雅山东北部的地壳(Roydenet al,1997;Burchfiel,2004;Bird,1991)。在此,我们使用平衡地质剖面来说明该山系前缘的地壳缩短、构造起伏与地形地貌是密切相关的。这表明,地壳缩短是位于青藏高原东缘的龙门山的隆起及其地形形成的主要驱动力。致使沿该山系前缘大型逆冲断层发生破裂,并造成数万人死亡和大范围破坏的2008年汶川地震(M=7.9)就是这种地壳缩短过程的作用表现。
The Longmenshan Range, the site of the devastating earthquake that hit Wenchuan (M = 7.9) on May 12, 2008, delimited the eastern edge of the Himalayan orogenic belt and showed much larger relief than anywhere else on the Tibetan Plateau. However, no significant shortening of deformation across the front of the mountain range was detected by both geodetic and geological surveys prior to this earthquake (Shenet al, 2005; Meade, 2007; Chenet al, 2000) The fierce debate over the formation and development of the mountainous terrain. Two endmember models have been proposed: (1) brittle crust thickening, ie, thrust faults with a large amount of slip cut through the rock and cause uplift (Ta-pponnieret al, 2001); (2) crustal flow, Low-viscosity material in the lower crust is extruded outwards, lifting the crust in the northeastern Himalayas (Roydenet al, 1997; Burchfiel, 2004; Bird, 1991). Here, we use the balanced geological profile to illustrate the crustal shortening of the front of the mountain system. The structural fluctuation is closely related to the topography. This shows that crustal shortening is the main driving force for the uplift of the Longmen Mountains and the formation of its topography on the eastern edge of the Qinghai-Tibet Plateau. The 2008 Wenchuan earthquake (M = 7.9), which led to the rupture of a large thrust fault along the front of the mountain system and caused tens of thousands of deaths and widespread damage, is the result of this crustal shortening process.