ELECTRICAL STRUCTURE OF THE CRUST BENEATH THE CENTRAL-NORTHERN SEGMENT OF THE LONGMEN SHAN FAULT ZONE AND ITS GEODYNAMIC MODEL

doi:10.3969/j.issn.0253-4967.2015.13

Abstract

Abstract:

Magnetotelluric (MT) exploration with station spacing of about 3km in 2012 was undertaken across the Longmenshan region from Maoxian to Mianzhu in Sichuan, China to investigate the crustal structure of this region, and a 2-D resistivity model with 70km in length and 50km in depth is acquired. By jointly interpreting and comparing with a seismic reflection profile at the same sites, we obtained the crustal structure of central-northern segment of Longmenshan Fault zone as follows: 1)Sichuan Basin has a very stable electrical structure, beneath which is the rugged Yangtze middle and lower crust; the three main faults in Longmenshan show low resistivity or middle to low-resistivity, and its structural features are tilted to the northwest by a steep to gentle slowing downward, with the steepest part greater than 60° at the shallow, and the gentlest part less than 30° at the deep. 2)the upper crust beneath Longmenshan Fault zone is thrusting over the rigid basement of Yangtze block, forming not only the three main faults, but also numerous blind secondary faults as well due to frequent seismicity; there is an is obvious electrical gradient zone beneath the Yingxiu-Beichuan Fault, which is steep and inclined to the northwest, and where Wenchuan earthquake aftershocks are concentrated. A large concealed basement fault may be developed on the footwall of the Anxian-Guanxian Fault, which might be the seismogenic fault of Wenchuan earthquake. Seismic energy transmitted along the secondary faults, leading to severe damage to Yingxiu-Beichuan Fault. 3)The subduction of the mid and lower crust beneath the eastern margin of Tibetan plateau causes the high-resistivity mid-lower crust of the Yangtze block embedded in the mid and lower crust between the Longmenshan thrust zone and the eastern margin of Tibetan, forming a "crocodile mouth structural frame". The uplift of Longmenshan is a result of brittle deformation under overthrusting in the upper crust and high-conductivity crustal mass flow in the middle and lower crust. We propose a "conveyor-like" geodynamic model that the crustal mass flow is not only blocked by rigid Yangtze block, but also flows downward into the upper mantle beneath Yangtze block. The "conveyer" drives the brittle thrusting deformation in the hanging wall of the fault zone.

Key words: Longmenshan Fault zone, magnetotellurics, seismic reflection, joint interpretation, geodynamics

摘要：

2012年在四川龙门山断裂带的茂县—绵竹段进行了点距约3km、横跨断裂带的大地电磁探测, 精细处理并反演获得长70km, 深50km的2维电性剖面。通过与同位置的反射地震资料对比进行综合解释, 刻画出龙门山断裂带中北段的地壳结构: 1)四川盆地上覆地层为低阻, 电性结构稳定并叠置于坚固的扬子中下地壳之上;龙门山3条主断裂均表现为低阻—中低阻, 其构造形态都沿NW向倾斜, 并由陡变缓向下延伸, 浅部最陡处 > 60°, 深部最缓处 < 30°。2)龙门山断裂带上地壳整体逆冲推覆于扬子板块的刚性基底之上, 不仅形成由陡趋缓的3条主断裂, 而且多期次的地震活动造成隐伏次级断裂发育;映秀-北川断裂之下具有明显NW倾斜且陡立的电性梯度带, 2008年汶川地震余震在该区域内集中分布, 其中安县-灌县断裂下盘发育大型隐伏的基底断裂, 可能为发震断裂, 地震能量沿隐伏次级断裂向上传递导致映秀-北川断裂遭破坏最为严重。3)青藏高原东缘的中下地壳下插, 使高阻的扬子中下地壳嵌于龙门山逆冲推覆带和青藏高原东缘中下地壳之间, 形成 "鳄鱼口"样式的构造格架。龙门山的隆升是由上地壳的逆冲推覆脆性变形和中下地壳的壳内高导物质流的韧性变形共同作用的结果。同时提出, 由于中下地壳物质流在龙门山不仅受阻于刚性的扬子块体, 而且下插于扬子板块上地幔, 形成一种可能的类似 "传送带"的动力模式, 带动了其上盘发生持续的逆冲推覆脆性变形。

关键词: 龙门山断裂带, 大地电磁, 反射地震, 联合解释, 地球动力学

CLC Number:

P631.3⁺25

PENG Miao, JIANG Mei, TAN Han-dong, LI Qing-qing, ZHANG Li-shu, XU Le-hong, ZHANG Fu-bin, TANG Lu-te. ELECTRICAL STRUCTURE OF THE CRUST BENEATH THE CENTRAL-NORTHERN SEGMENT OF THE LONGMEN SHAN FAULT ZONE AND ITS GEODYNAMIC MODEL[J]. SEISMOLOGY AND GEOLOGY, 2015, 37(1): 162-175.

彭淼, 姜枚, 谭捍东, 李庆庆, 张立树, 许乐红, 张福彬, 唐路特. 龙门山断裂带中北段的地壳电性结构及其动力学模型[J]. 地震地质, 2015, 37(1): 162-175.

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