四川龙门山断裂带高精度地壳/岩石圈黏度结构及其动力学意义

岩石圈黏度是大陆动力学研究中一个重要参数,但是岩石圈黏度,尤其是横向小尺度(<100 km)黏度结构的确定是一个挑战.本文根据电阻率和黏度与它们控制因素的相似关系,直接把一条跨过青藏高原东缘和四川龙门山断裂带的大地电磁(MT)探测的电阻率剖面转换成黏度结构作为输入,在GPS速度和地表地形数据的约束下,利用地球动力学数值模拟获得了该剖面的二维地壳/岩石圈黏度结构.本文推断的黏度与前人获得的区域尺度的黏度值一致,但揭示出了更多的细节.本文的黏度结构揭示出研究区域内的地壳/岩石圈黏度存在较大的空间变化范围(约5量级),黏度值分布在1.48×10^17~8.44×10^22 Pa·s之间;龙门山断裂带下的黏度存在强烈的小尺度横向变化,其中、下地壳的黏度分别为1.99×10^18~8.21×10^20 Pa·s(平均1.17×10^20 Pa·s)和4.09×10^19~7.08×10^20 Pa·s(平均1.77×10^20 Pa·s).基于该黏度结构的地球动力学模型表明驱动青藏高原中-下地壳物质流动的可能是热-化学浮力,以及上地壳和中-下地壳可能处于解耦状态.本文获得的黏度结构可以为龙门山断裂带地震成因和机制、岩石圈小尺度变形和构造应力状态的深入研究提供重要的帮助.

High-resolution crustal/lithospheric viscosity of the Longmenshan fault zone,Sichuan province and its geodynamic implications

The crustal/lithospheric rheology is an important factor in the study of lithospheric dynamics. However, estimation of this parameter remains challenging, especially for small-scale (<100 km) lateral variations in viscosity. In this study, a resistivity profile from magnetotelluric (MT) survey, which crosses the eastern margin of the Tibetan Plateau and the Longmenshan fault zone in Sichuan province, is directly converted into a viscosity structure as an input based on the similarity between resistivity and viscosity, then geodynamic modeling is used to infer the two-dimensional crustal/lithospheric viscosity along this MT profile under the constraints of GPS-derived crustal velocity and surface topography. Our inference is consistent with previous regional-scale estimates but reveals more details. The viscosity structure exhibits large spatial variations (~5 orders) in the study area and has the viscosity of 1.48×10¹⁷~8.44×10²² Pa·s. The viscosity of the Longmenshan fault zone has strong small-scale lateral variations, and its values in the middle and lower crust are 1.99×10¹⁸~8.21×10²⁰ Pa·s (1.17×10²⁰ Pa·s on average) and 4.09×10¹⁹~7.08×10²⁰ Pa·s (1.77×10²⁰ Pa·s on average), respectively. Our viscosity-based geodynamic model implies probably the middle-lower crustal flow is driven by thermal-chemical buoyancy and the decoupling of the upper crust from the middle-lower crust below the Tibetan Plateau. The viscosity structure presented in this paper may provide an important aid for further research of seismogenic mechanism, fine-scale lithospheric deformation and tectonic stress state of the Longmenshan fault zone.