The Mongol-Okhotsk Ocean: No Relationship to the Paleo-Tethys

The Mongol-Okhotsk Ocean (MOO) closed when the Amuria-North China Block (NCB) and the southern margin of Siberia collided in Late Jurassic to Early Cretaceous times. As such, the MOO could be considered as a branch of the Paleo-Tethys, given the position of the latter between NCB and Siberia in the reconstructions of the classical series of papers by Sengor et al. in 1984-1987. However, there are two enigmas that result from this idea. One is that the Mongol-Okhotsk suture must have terminated in Mongolia, because no evidence exist for any westward prolongation of the MOO near the Tarim block, which in the Mesozoic was already part of Eurasia. A connection between the MOO and the Paleo-Tethys is therefore unlikely. The other enigma is that the Mongol-Okhotsk suture runs east-west, whereas tomographic images of the subducted Mongol-Okhotsk slab in the deeper mantle appear to have a north-south strike. No sensible explanation can be constructed for a rotation of some 90 degrees of this slab. There is a solution, however, to these enigmas if we consider that the MOO did exist east of an initially north-south paleo-Pacific margin of continental fragments of Eurasia and was subducting westward during the Triassic-Early Jurassic. This would readily explain the tomographic slab orientation. As the Siberian and NCB approached each other and were overriding the MOO, they rotated towards each other in opposite senses (Siberia clockwise and NCB counterclockwise). The subduction-related volcanics above the subduction zone, following the outline around the core of the Tuva-Mongol Belt in the Altaids between NCB and Siberia, formed a tightening orocline. This large-scale oroclinal bending of the crust above a disappearing ocean is reminiscent of the similarly tightening orocline in Kazakhstan, which closed earlier, in the late Paleozoic, by subduction of the Junggar-Balkhash Ocean.