North-Dipping Relict Subduction of Paleo-Asian Ocean at the Southeastern Margin of Mongolian Terrane: Study of Two Parallel Deep Seismic Profiles

Abstract:

The final closure of Paleo-Asian Ocean (PAO) and its tectonic nature have been under debate for several decades due to a lack of fine-scale lithosphere structure. The researchers have been trying to find out the extent to which surficial geology surveys can explain deep tectonic evolutionary processes in studying continental growth at the southern margin of the Mongolian Terrane. In a bid to provide a new interpretation of the deep structure with higher resolution, we studies two reprocessed deep seismic reflection profiles located at northern border of Inner Mongolia in Chagan Obo, Erenhot and Sonid Zuoqi (fig. 1), which are, the northern part (210 kilometers long) of the 630-kilometer-long deep seismic reflection profile recorded during 2009-2010, extending from Huailai at north margin of the North China Craton to the northern Sino-Mongolia border in the west; and a parallel profile (80 kilometers long) in the east near the China-Mongolia border. In this way, we have aimed to provide a new interpretation of deep structure considering the surface geological studies.

The most representative features is that, both profiles show distinguishing features of consistently north-dipping layered lower crust reflections and mantle reflections, with an estimated thickness of 3.6 kilometers to 6 kilometers between adjacent reflections under the Uliastai and Hegenshan belts, and the apparent dip angles of layered reflections is 16°~19°, respectively. And there are exhibiting arched reflections in middle and lower crusts, which might have been caused by later magmatic activities. In addition, Moho reflection has been observed to be fairly continuous and flat in most parts of these two profiles.

We interpret the mantle reflections as the relict of subducted oceanic crust, while the layered lower crust reflections have been caused by the advent of mafic and ultra-mafic magma with high wave impedance in ductile shear zones formed during the previous compression tectonic events. This indicates northwards subduction of PAO and the contribution of subducted oceanic crust to the growth of continental crust at the Uliastai Continental Margin of the southeastern margin of Mongolian Terrane.

This research is supported by The Geological Survey of China (Grant nos. DD20160207 and DD20190010)