Evidence for the superposition of tectonic systems in the northern Songliao Block, NE China, revealed by a 3‐D electrical resistivity model

Abstract

The creation and evolution of the Songliao Block, Northeast China, is believed to be related to the closure of the Paleo-Asian and Mongol-Okhotsk oceans, and to the subduction of the Paleo-Pacific Ocean. Geochemical and geophysical data indicate a complex tectonic history that may have left signatures of the superposition of separate tectonic events. However, the relationship between the current lithospheric structure of the Songliao Block and those tectonic events is not clear. In order to shed light on this question, a three-dimensional electrical resistivity model of the crust and mantle was generated from 138 magnetotelluric sites. The results show a heterogenous lithosphere with anomalous low-resistivity zones (∼10 Ω·m). The position of upper-crustal features (<10 km) correlates with a network of fault systems and rift basins. Based on estimates of high crustal temperatures, mid-lower crustal features (15–35 km), some of which appear near seismic reflectors, are attributed to partial melts (∼5%) containing moderate-high water contents (4.5 wt. %). A deep (>45 km) feature is interpreted as an asthenospheric upwelling near the center of the Songliao Block, where seismic evidence indicates a thin lithosphere. It is proposed that the collision, convergence, and suturing of blocks caused by the closure of the Paleo-Asian Ocean and the bidirectional convergence between the Mongol-Okhotsk and Paleo-Pacific Ocean may have produced weak tectonic zones throughout the lithosphere. They provided a structural basis for the later upward movement of fluids and melt, likely due to hydrous upwellings caused by the subduction of the Paleo-Pacific system.