Lateral Rheology Differences in the Lithosphere and Dynamics as Revealed by Magnetotelluric Imaging at the Northern Tibetan Plateau

Abstract

Differences in topography and surface deformation in the northern Tibetan Plateau may reflect variable rheology in the lithosphere. Whether middle to lower crustal flow exists below the region between the eastern Kunlun Shan range and the Qaidam Basin remains debated. We provide electrical resistivity images of three new magnetotelluric (MT) profiles across the Qiman Tagh and the western Qaidam Basin. The new models reveal low‐resistivity anomalies below the Qaidam Basin in the lower crust and generally high‐resistivity crust in the Qiman Tagh range. A low‐resistivity anomaly at the uppermost‐mantle level is imaged below the Qiman Tagh range in the profile close to the Altyn Tagh Fault. This anomaly is possibly connected with partial melting in the lower portion of the thickened Songpan‐Ganzi crust close to the southern end of the study area; the minimum requirements for the low‐resistivity anomaly below the Qiman Tagh are 0.05 wt% water content or 0.02 melt fraction, which may greatly decrease the effective viscosity in the uppermost‐mantle rocks and allow flowing. Consequently, we propose that the crustal flow is not mature below the Kunlun Shan range south of the Qaidam Basin because no continuous and stable low‐resistivity anomaly was imaged at this level. The uppermost‐mantle low‐resistivity anomaly below the Qiman Tagh is a passage that allows the continuous northward transfer of materials in the interior of the plateau. This low‐resistivity channel shallows northward and reaches the lower crust of the southwestern Qaidam Basin.

Bifurcated Crustal Channel Flow and Seismogenic Structures of Intraplate Earthquakes in Western Yunnan, China as Revealed by Three‐Dimensional Magnetotelluric Imaging

“…The MT time series were processed by the SSMT2000 (http://www.phoenix-geophysics.com) for the V5‐2000 data (e.g., Zhang et al, 2016; Zhao et al, 2012) and the Metronix software Mapros (https://www.geo-metronix.de) for the GMS‐07e data (e.g., Borah et al, 2015; Xiao et al, 2018). The V5‐2000 time series were transformed into the frequency domain where the frequency‐dependent impedance tensors were calculated by the remote reference processing with statistically robust algorithms (Gamble et al, 1979; Jones et al, 1989).…”