The ancient eastward-dipping subduction zone reactivated during the Mesozoic between the Alxa and Ordos Blocks: Electrical evidence from an extended MT profile

Abstract

This paper investigates the characteristics of the electrical structure and key geological issues of the Alxa and Ordos blocks using a lithospheric resistivity model obtained from a west-east trending magnetotelluric sounding profile. The electrical model shows that the lithosphere of the Alxa Block is high resistivity, except for two crustal low-resistivity bodies. The crust of the Helanshan Orogenic Belt is characterized by medium to high resistivity surrounded by two low-resistivity bodies on both sides. The upper crust of the Ordos Block exhibits discontinuous high resistivity. Still, the lower crust and upper mantle show distinctly low resistivity, which connects with the shallow low-resistivity body above it. The lithosphere of the Lüliang Uplift and the Shanxi Graben is generally characterized by high resistivity, with only localized low-resistivity anomaly found near Moho and in the shallow part of the Shanxi Graben. We propose that the Alxa Block has a stable cratonic lithosphere and that the crustal low-resistivity body within it is a remnant of the southward subducted oceanic crust of the Paleo-Asian Ocean. The low-resistivity body in the lower crust of the Trans-North China Orogen is also interpreted as a remnant of oceanic crust; the upper mantle low-resistivity body is proposed to be a partial melt resulting from the upwelling of asthenosphere caused by the combination of westward subduction of the Paleo-Pacific Plate and northeastward growth of the Tibetan Plateau. This low-resistivity anomaly spreads to the shallow crust, with its lateral distribution to the east and west being blocked by the Lüliang Orogenic Belt and the Alxa Block, respectively, then consequently upwelling along the Yinchuan Graben during the Mesozoic and Cenozoic. We propose that the Alxa Block has been subducted to the Ordos Block along the Helan Mountain in the Early-Middle Paleozoic, which may have formed an ancient suture zone. This ancient suture zone was reactivated in the Mesozoic and Cenozoic by the Paleo-Pacific Ocean subduction and northeastward growth of the Tibetan Plateau, becoming a material upwelling channel and ablating the roots of the Helan Mountain.