中亚造山带东段软流圈分布特征——基于长周期大地电磁探测的结果

中亚造山带东段经历了古亚洲洋、蒙古—鄂霍茨克洋和古太平洋构造体制的多重作用,多期次的构造活动不仅改造了地形地貌、岩石圈结构,同时也改造了软流圈分布,因此对软流圈结构研究具有重要的科学意义.为了揭示复杂多重构造体系下软流圈的分布特征,完成了中亚造山带东段约1800km长大地电磁测深剖面,并对数据进行非线性共轭梯度反演,获得二维电性结构模型.研究发现:中亚造山带东段岩石圈内部存在多处低阻异常,地表多对应第四纪火山群或古缝合带,表明这些低阻异常与软流圈上侵有关.软流圈呈现高导特征,南部略薄,电阻率值为10～30Ωm,北部厚度较大,电阻率值为10～0.1Ωm,这种电性结构特征体现了中亚造山带东段软流圈南北两侧厚度、部分熔融程度的不均一性.而已有的层析成像结果显示中亚造山带软流圈东西向则呈现东厚西薄的特征.结合区域地质,提出软流圈与岩石圈一样在碰撞造山过程中厚度、部分熔融程度也会随之变化的动力学认识.中亚造山带东段软流圈主要经历了古亚洲洋构造体系、蒙古—鄂霍茨克构造体系和太平洋构造体系三阶段的构造事件影响,因此中亚造山带东段软流圈的南北向差异,推断为古亚洲洋闭合早于鄂霍茨克洋闭合的时限差异所致,东西向差异则主要受太平洋构造体系的影响.

Characteristics of the asthenosphere structure beneath the eastern segment of the Central Asia orogenic belt inferred from a long-period magnetotelluric survey

The eastern section of the Central Asian orogenic belt experienced multiple functions of the Paleo-Asian Ocean, the Mongolia-Okhotsk Ocean, and the Paleo-Pacific tectonic system.The multi-phase tectonic activities have not only reshaped the topography and lithosphere structure,but also changed the geometry of the asthenosphere beneath this belt. To reveal the characteristics of the asthenosphere under the complex background system, we acquired an electromagnetic sounding profile about 1800 km long in the eastern part of the Central Asia orogenic belt. Non-linear conjugate gradient inversion was performed to the data to establish a two-dimensional electrical structure model. The results show that there are many low resistivity anomalies inside the lithosphere beneath this region, which, on the surface, mostly correspond to the Quaternary volcanic groups or ancient suture zones.It implies that these low resistance anomalies are related to asthenosphere uplifting invasion. The asthenosphere exhibits high conductivity and is slightly thin in the south with a resistivity value of 10 to 30 Ωm and thicker in the north with a resistivity value of 10 to 0.1 Ωm. This electrical structure evidences the heterogeneity of the thickness, and partial melting of the asthenosphere on the north and south sides in the eastern Central Asian orogenic belt.The existing tomographic results show that the thickness of the asthenosphere in this orogenic zone is thicker in the east and thinner in the west. Combining with regional geology, we propose that like the lithosphere, the thickness and partial melting of the asthenosphere also changed in the process of collision orogenesis. The asthenosphere in the eastern Central Asia Orogenic belt has undergone three stages of tectonic events:the Paleo-Asian oceanic tectonic system, the Mongolian-Ihotsk tectonic system, and the Pacific tectonic system. Therefore, we estimate that the north-south difference of the asthenosphere beneath the eastern Central Asian orogenic belt was likely caused by that the closure of the Paleo-Asian Ocean was earlier than the Okhotsk Ocean. And The east-west variation was related with the influence of the Pacific tectonic system.