摘要
青藏高原东缘的地壳结构是两种主流青藏高原隆升模式争辩的焦点之一.中下地壳流曾经被认为是高原东缘隆升的主要构造驱动力,但是中上地壳之间低阻低速层的发现及其与2008 MS8.0汶川地震良好的对应关系表明,高原东缘具有向东刚性挤出的可能性.然而大部分关于龙门山断裂的数值模拟仍建立在下地壳流的基础上,仅将低阻低速层作为断裂的延续或是弱化地壳物性参数的软弱层,而非能够控制块体滑动的"解耦层",也没有考虑到刚性块体变形中的断裂相互作用.本文建立了包含相互平行的龙门山断裂与龙日坝断裂的刚性上地壳模型,用极薄的低阻低速层作为块体滑动的解耦带,采用速率相关的非线性摩擦接触有限元方法,基于R最小策略控制时间步长,计算了在仅有侧向挤压力作用下,低阻低速层对青藏高原东缘的刚性块体变形和断裂活动的作用.计算结果显示,低阻低速层控制了刚性块体的垂直变形和水平变形分布特征.在侧向挤压力的持续作用下,在低阻低速层控制下的巴颜喀拉块体能够快速隆升,而缺乏低阻低速层的四川盆地隆升速度和隆升量均极小,隆升差异集中在龙门山断裂附近,使其发生应力积累乃至破裂.龙日坝断裂被两侧的刚性次级块体挟持着一起向南东方向运动,但该断裂的走滑运动分解了绝大部分施加在块体边界上的走滑量,使得相邻的龙门山次级块体的走滑分量遽然减少,也使得龙门山断裂表现出以逆冲为主,兼有少量走滑的运动性质.本文所得的这些计算结果显示了在缺乏中下地壳流,仅在低阻低速层解耦下刚性块体隆升过程及相关断裂活动,提供了青藏高原东缘刚性块体挤出的可行性,为青藏高原东缘隆升机制的研究讨论提供了重要依据.
The crustal structure of the eastern Tibetan plateau plays a key role in studying the uplift mechanism of Tibetan.Ductile flow in middle-lower crust was considered to be the main driving force in the uplifting of eastern Tibetan plateau.However,both the discovery of the low resistivity and low velocity layer between the mid-upper crust and its correspondence with the 2008 Ms8.0 Wenchuan earthquake show the possibility of southeastward splaying of coherent lithospheric blocks in eastern Tibetan plateau.Unfortunately,popular FEM models of eastern Tibetan plateau and Longmenshan fault zone in recent years were established with viscous flow of the crust.And the low resistivity and low velocity layer was set as a fault plane or a factor weakening the crust in these models.Few researchers have addressed the decoupling problem of the low resistivity and low velocity layer.The interaction of faults was ignored in previous FEM models either,for continuous thickening and deformation.In this paper,we present a coherent block model with elasto-plastic material,containing Longmenshan fault and Longriba fault which parallel to each other,setting a thin soft layer under the upper crust as the decoupling layer.A non-linear rate-independent finite element method（including the frictional contact） was carried out using R-minimum strategy to limit the step size,to simulate the block deformation of upper crust and fault behavior of the eastern Tibetan plateau.The results show that the low resistivity and low velocity layer controls the distribution of horizontal and vertical deformation on surface.The continuous lateral extrusion pressure causes quick and huge uplift of the Bayan Har block which is under the control of decoupling layer,while little uplift of the Sichuan Basin which is not controlled by decoupling layer.The difference of uplifts makes stress accumulating on the Longmenshan fault,which is the border of the two blocks,and results in large earthquake on the fault,as Wenchuan earthquake.The Longriba fault moves southe
出处
《地球物理学报》
SCIE
EI
CAS
CSCD
北大核心
2012年第3期863-874,共12页
Chinese Journal of Geophysics
基金
国家自然科学基金(41004037和40821160550)
中国地震局地质研究所基本科研业务专项(IGCEA0905)共同资助
关键词
青藏高原东缘
低阻低速层
有限元
龙门山断裂
龙日坝断裂
The eastern Tibetan plateau
Low resistivity and low velocity layer
Finite-element method
Longmenshan thrust belt
Longriba fault
