Crust structure revealed from the deep seismic reflection profile across Solonker suture zone in North China: a preliminary interpretation

The Solonker suture zone represents an important tectonic boundary between the North China Craton (NCC) and the Mongolia composite terrane. It has been considered as the final collision suture formed in the late Paleozoic, after a long accretionary history in both its southern and northern sides. However, existing models related to the tectonic framework and evolution of this suture zone are highly variable. The newly completed deep seismic reflection profile, SinoProbe-02, spanning ca. 480 km from Erenhot to Beijing, provides a new look into this orogenic belt. The seismic reflection profile can be divided into three segments. The southern segment is about 120 km, covering the Yanshan fold and thrust belt in the southern side of the Shangyi-Chongli-Gubeikou-Pingquan Fault zone (SCGP). Except one small Mesozoic basin and its boundary faults, transparent image with minor discontinuous reflections dominates the deep structure of the crystal basement area of the NCC. The middle segment of the profile covers the region between the SCGP and Sonid Youqi that is located in the center of the Solonker suture zone. This segment is characterized by the northward dipping reflection structures in the lower crust and transparent granite bodies in the upper crust. The northward dipping reflection structures may represent the accreted wedges formed along the north margin of the NCC during Paleozoic. In some places, these structures are truncated by transparent columnar bodies that often connect with the granite outcrops. The middle segment passes through the Ondor Sum ophiolite mélange, but no structure is observed in the mantle to suggest the possible subduction. Together with geological data, we speculate that this area had experienced considerable crust thickening, uplift and lithosphere delamination, and had a new Moho. The SCGP zone may represent the tectonic transition from the NCC to its northern accreted marginal region. The contraction structures which have likely formed in the collision stage (latest Paleozoic to early Mesozoic) can be traced from the wedge-shape crust piece of the NCC insetting the lower crust of the SCGP zone. This deep structural feature is also common in the transitional zones from rigid block to mobile belt in western China. The northern segment covers ca. 120 km of the profile from Sonid Youqi to Erenhot, showing a more complex deep structure. The eastward extending Solonker suture is superimposed by the Mesozoic extension structures. There are a few reflection structures dipping southward, while others dipping northward. Some transparent bodies that occurred along the boundary faults of the Mesozoic basins may represent magma activities. The major boundaries revealed by the seismic profile so far are in first-order agreement with the geological observations.