Electrically Anisotropic Crust From Three‐Dimensional Magnetotelluric Modeling in the Western Junggar, NW China

Abstract

An island arc environment related to intraoceanic subduction or ridge subduction from the Late Carboniferous to the Early Permian has been proposed for the formation of Western Junggar, NW China, situated in the southwest of Central Asian Orogenic Belt. However, the details and consequences of the subduction remain controversial. To further identify the relics of the Late Carboniferous subduction at depth, three magnetotelluric profiles were deployed. Phase tensors, real induction arrows, and three‐dimensional (3‐D) isotropic resistivity models indicate the presence of a 3‐D anisotropic resistivity structure in the crust. Consequently, a 3‐D anisotropic resistivity model was constructed by forward modeling. The 3‐D anisotropic resistivity model contains two azimuthally anisotropic middle‐upper crust anomalies with minimum resistivity striking N90°E at depths of 5 to 20 km, and an azimuthally anisotropic lower crust anomaly with minimum resistivity striking N20°E at depths of 20 to 46 km. The electrically anisotropic anomalies are closely related to relatively high shear‐wave velocities with significant negative radial anisotropy. The anisotropies in the middle‐to‐upper crust and the lower crust are related to the oceanic ridge (N90°E) subduction with a slab window in the Late Carboniferous and the remnant oceanic slab, respectively. The magnetotelluric observations support a NW70° subduction with an intraoceanic ridge‐transform system in the Late Carboniferous and a remnant oceanic slab trapped in Western Junggar.

The Role of Multiple Trapped Oceanic Basins in Continental Growth: Seismic Evidence From the Southern Altaids

“…As a result, the remnant ocean basins in eastern and western Junggar were preserved in the complicated multiple convergence process (Figure 5b). Our geophysical profile might have captured the final crushed situation of these intra‐arc oceanic basins (Figure 5c), which is consistent with geophysical works (Y. Liu et al., 2019; Wang et al., 2006; Wu et al., 2018; Y. X. Xu et al., 2020; see details in Text S4 in Supporting Information ) and other petrological works in the Junggar (W. Liu et al., 2013; Q. Q. Xu et al., 2013).…”

“…Our geophysical profile might have captured the final crushed situation of these intra-arc oceanic basins (Figure 5c), which is consistent with geophysical works (Y. Liu et al, 2019;Wang et al, 2006;Wu et al, 2018;Y. X. Xu et al, 2020; see details in Text S4 in Supporting Information S1) and other petrological works in the Junggar (W. Liu et al, 2013;Q.…”

Lithospheric Conductors Shed Light on the Non‐Uniform Destruction of North China Craton

“…The electrical structure around C4 derived from isotropic inversion of MT data resembles the typical feature caused by macro-anisotropy (J. Liu, Cai, Pearson, & Scott, 2019;Y. Liu, Junge, et al, 2019;Wannamaker, 2005) and the results of the 3-D numerical experiments with anisotropy in Löwer and Junge (2017).…”

“…Seismic tomographic images show a flat stagnant slab in the mantle transition zone (MTZ) beneath ENCC and its front edge has reached as far as beneath the NSGL (J. Huang & Zhao, 2006), which has been supposed to be the subducted Pacific plate rather than the Paleo‐Pacific plate that should have already sunk into the lower mantle (X. Liu et al., 2017). The subducted Pacific slab in the MTZ induced regional mantle upwelling and the generation of melts and/or fluids, contributing to Cenozoic destruction of the NCC (Chen et al., 2014; J. Liu, Cai, et al., 2019; Y. Liu, Junge, et al., 2019; R. Zhu et al., 2011). Furthermore, this region has exceptionally high surface heat flow (>80 $$\text{mW}/{\mathrm{m}}^{2}$$), dense lithospheric mantle (3.37–3.43 $$\,\mathrm{g}/{\text{cm}}^{3}$$) and thin thermal lithosphere (<80 km), all suggesting the original Archean lithospheric mantle underwent significant modification in Phanerozoic (G. Jiang et al., 2019; Q. Liu et al., 2016; J. Zheng et al., 2020).…”

“…The subducted Pacific slab in the MTZ induced regional mantle upwelling and the generation of melts and/ or fluids, contributing to Cenozoic destruction of the NCC (Chen et al, 2014;J. Liu, Cai, et al, 2019;Y. Liu, Junge, et al, 2019;R.…”