Latest Jurassic-earliest Cretaceous closure of the Mongol-Okhotsk Ocean; a paleomagnetic and seismological-tomographic analysis

The Mongol-Okhotsk Ocean closed when the Amuria block, normally considered to have been part of the North China block since the early Mesozoic, and the southern margin of Siberia collided in Late Jurassic to Early Cretaceous times. The resulting suture runs WSW-ENE and is reasonably well defined to the east of longitude 100 degrees E. Because no evidence exists for any westward prolongation of the Mongol-Okhotsk Ocean suture toward the Tarim block, the cryptic termination of the suture is an enigma, compounded by the fact that a tomographically identified slab in the lower 1000 km of the mantle, interpreted as a remnant of Mongol-Okhotsk oceanic lithosphere, has a clear N-S trend, at almost right angles to the surface suture. No sensible explanation can be constructed for a rotation of some 90 degrees of this slab. There is a solution, however, to both these enigmas if we consider that the Triassic Mongol-Okhotsk Ocean existed east of an initially meridian-parallel, but later progressively more sinuous, late Paleozoic Pangea margin. This margin consisted of Siberia, Amuria, and the China continental elements. The Mongol-Okhotsk Ocean was subducting westward during the early Mesozoic and likely older times underneath this margin. This would readily explain the tomographic N-S slab orientation at depths of 2000 km and greater. Paleomagnetic inclination differences between the global apparent polar wander path in Siberian coordinates and results from the North China block show a gradually diminishing trend with time, as these cratons approached each other during the Jurassic. During this time, the paleomagnetic data of the North China block show that it underwent a slight northward motion, but with a considerable counterclockwise rotation of approximately 90 degrees . At the same time, the Mongol-Okhotsk Ocean-bordering margin of Eurasia (between Siberia and Tarim) moved southward by approximately 30 degrees and rotated 45 degrees clockwise. These continental scissoring movements caused doubly vergent subduction of the Mongol-Okhotsk Ocean. Paleomagnetic data suggest final closure of the Mongol-Okhotsk Ocean in latest Jurassic-earliest Cretaceous time. Arc-related rocks above the subduction zone follow the outline around the core of the Tuva-Mongol belt in the eastern Altaids between Amuria and Siberia, and they form a tightening, westward-convex Tuva-Mongol orocline. This large-scale oroclinal bending of the crust above a disappearing ocean is reminiscent of similarly tightening oroclines in Kazakhstan and Variscan Europe, which closed earlier by subduction in the late Paleozoic.