Segmental closure of the Mongol-Okhotsk Ocean: Insight from detrital geochronology in the East Transbaikalia Basin

The Late Paleozoic–Early Mesozoic Mongol-Okhotsk Ocean extended between the Siberian and Amur–North China continents. The timing and modalities of the oceanic closure are widely discussed. It is largely accepted that the ocean closed in a scissor-like manner from southwest to northeast(in modern coordinates), though the timing of this process remains uncertain. Recent studies have shown that both western(West Transbaikalia) and eastern(Dzhagda) parts of the ocean closed almost simultaneously at the Early–Middle Jurassic boundary. However, little information on the key central part of the oceanic suture zone is available. We performed U-Pb(LA-ICP-MS) dating of detrital zircon from wellcharacterized stratigraphic sections of the central part of the Mongol-Okhotsk suture zone. These include the initial marine and final continental sequences of the East Transbaikalia Basin, deposited on the northern Argun-Idemeg terrane basement. We provide new stratigraphic ages for the marine and continental deposits. This revised chronostratigraphy allows assigning an age of ～165–155 Ma, to the collisionrelated flexure of the northern Argun-Idemeg terrane and the development of a peripheral foreland basin.This collisional process took place 5 to10 million years later than in the western and eastern parts of the ocean. We demonstrate that the northern Argun-Idemeg terrane was the last block to collide with the Siberian continent, challenging the widely supported scissor-like model of closure of the MongolOkhotsk Ocean. Different segments of the ocean closed independently, depending on the initial shape of the paleo continental margins.