Crustal-Scale Seismic Reflection Profiling Constrains How the Paleo-Asian Ocean Was Closed

The Central Asian Orogenic Belt (CAOB) is the most significant accretionary orogenic belt since the Phanerozoic and the most ideal site for studying continental growth evolution processes. A 460-km-long high-resolution crustal-scale seismic reflection study was conducted across the eastern CAOB in North-Central China to constrain the closure mode and location of the Paleo-Asian Ocean, that is, the previous ocean of the CAOB. The resultant seismic reflection profile revealed opposite-dipping reflectors in the northern and southern parts of the profile, which converge at the profile center to form an inverted U-shaped reflector pattern near the crust-mantle transition zone beneath the Solonker Suture. The dipping reflectors represent bidirectional fossil subduction zones sloping to the north and south, and the convergence reflector pattern represents the ocean closure location. Integration of these results with available geological data facilitated model construction whereby Paleo-Asian Ocean closure was accomplished by divergent subduction of the Paleo-Asian oceanic plate, with northward subduction beneath the southern margin of the Mongolian Block and southward subduction beneath the northern margin of the North China Craton. The oceanic lithosphere contracted and deformed, yielding the observed inverted U-shaped reflector pattern, representing Paleo-Asian Ocean closure. This subsurface location lies beneath the Solonker Suture surface exposure, suggesting that this suture marks the ocean closure location, rather than the previously proposed Hegenshan-Heihe Suture to the north or Xar Moron Suture to the south. Our study suggests that divergently dipping subduction and associated accretion and magmatism may constitute the primary continental growth mode for accretionary-type orogens.