Ongoing Asthenospheric Upwelling and Delamination-style Downwelling beneath the NE China: Evidence from High-Resolution MT Profiles

The Central Asian Orogenic Belt (CAOB) in NE China was formed by the amalgamation of several micro-continental blocks during the progressive closure of the Paleo-Asian Ocean in the Neoproterozoic to the Early Mesozoic. This region was also widely affected by the subsequent westward subduction of the Paleo-Pacific Plate (PPP) and the west-to-east closure of the Mongolia-Okhotsk Ocean, and huge volumes of granites were emplaced in many tectonic settings during the Mesozoic. To study the deep lithospheric structure of the western part of the NE China, two parallel high-resolution magnetotelluric (MT) sounding profiles were deployed with a dense station spacing (3.5 km average).

Both profiles gave models with comparable structures. They revealed a high resistivity layer in upper and mid crust beneath the Greater Xing'an Range and regions east to it and a low resistivity layer in the lower crust and upper mantle. The low resistivity layer was interpreted as a region with partial melt and indicates the occurrence of delamination induced by the subduction of PPP in Mesozoic. The upper mantle resistors beneath the Greater Xing'an Range and the Songliao Block and Liaoyuan Terrane respectively, are interpreted as delamination-style downwelling. West to the Greater Xing'an Range, upper crust low resistivity layer was interpreted as combined result of aqueous fluids and sulphides and carbonates. However, the lower crust and upper mantle low resistivity body was interpreted as partial melt since very high heat flow data (~84.5 mW/m²) was acquired at this area. Moreover, it is connected with a northwest tilting upper mantle conductor, which was interpreted as asthenosphere upwelling.

The asthenosphere upwelling might be triggered by the closure of the Mongolia-Okhotsk Ocean in the Middle Jurassic, since the upper mantle conductor comes from northwest. Whereas the downwelling might be the combined effect of both subductions of the PPP and Mongolia-Okhotsk Ocean.