A Study Of Lithospheric Electrical Structure And Deep Thermal State Across The Altyn Tagh Fault On The Northern Margin Of The Tibetan Plateau

The active sinistral strike-slip Altyn Tagh Fault forms the northern margin of theTibetan Plateau and plays a significant role in accommodating the convergencebetween the colliding Indian and Eurasian plates. To better understand the tectonicstructures along the fault system, new magnetotelluric (MT) data were collected alongtwo profiles that cross the central and eastern segments of the fault. Time series dataprocessing used robust algorithms to give high quality responses. Dimensionalityanalysis showed that a2D approach is generally valid for the western profile andnorthern section of the eastern profile. MT data were then rotated to correspondingregional strike direction and2D inversions were conducted.3D inversions were alsoconducted on full dataset to further constrain2D models and explore off-profileconductive features. In the inversion results, the mid-lower crust of the northernTibetan Plateau is imaged to be conductive, which is indicating a weak layer thatprobably caused by partial melt and/or aqueous fluids, while the Precambrianbasement of the Tarim Block is imaged to be quite resistive. The lithospheric structurebeneath the Altyn Tagh Range is characterized by a large-scale south-dippingconductor caused by the subduction of the Tarim Block, which indicates a lithosphericstructure. This conductor is interpreted as a region with high fluid content formedthrough the thrust induced upwelling of mantle sourced fluids. Serpentinizationprocess is suggested to be active and results in the formation of a seismically slow andelectrically resistive region. On the eastern profile the fault only appears as a crustalstructure. This difference suggests accommodation of strike slip motion by thrustingwithin the Qilian Block. Locally distributed east-dipping conductors from3Dinversion model for eastern profile indicate eastward thrust of the Qaidam Blockbeneath the Qilian Block. The Localized deformations thus do not support theexistence of crustal flow beneath the eastern segment of the Altyn Tagh Fault. Aconstraining method was further proposed to determine the upper mantle thermalstructure by using electrical resistivity model on the basis of Hashin-Shtrikmanbounds. This method was proved to be valid by comparing the constraining resultswith distribution of surface heatflow.