The Beginning of a Wilson Cycle in an Accretionary Orogen: The Mongol–Okhotsk Ocean Opened Assisted by a Devonian Mantle Plume.

The opening of oceans within accretionary orogens is important for understanding the Wilson cycle. The Mongol–Okhotsk Ocean (MOO) began opening within the early Paleozoic accretionary collage of the Central Asian Orogenic Belt (CAOB), representing a world‐class example to constrain the geodynamic history of ocean opening in accretionary orogens, but the kinematics and mechanisms associated to this process are highly debated. We report on a newly‐discovered bimodal volcanic suite and associated volcanic‐sediments that comprise part of the Altay‐Sayan Rift System, which indicate a widespread Early Devonian extensional event within the CAOB. This extension regime is attributed to a Devonian mantle plume, which is thought to have impinged upon and weakened the lithosphere of the Early Paleozoic collage, and drove the opening of the MOO. Opening of the MOO suggests continent breakup in accretionary orogens tends to focus along intervening weak orogenic lithosphere between the rigid microcontinents. Plain Language Summary: The opening of oceans within subduction related accretionary orogens is a process that is common within the geologic record. The tectonic mechanisms involved in this process, however, are not well understood. The Mongol–Okhotsk Ocean began opening within the Early Paleozoic accretionary collage of the Central Asian Orogenic Belt, providing an opportunity to constrain the geodynamic history of ocean opening in accretionary orogens, but the kinematics and mechanisms related to this process are highly debated. We report on newly‐discovered bimodal volcanic suite (410–415 Ma) and associated volcanic‐sedimentary rocks in Northwest Mongolia, which comprise part of the Altay‐Sayan Rift System (also termed Altay‐Sayan Large Igneous Province). This giant rift system represents a widespread Early Devonian extensional event associated with a mantle plume. The ascent of this mantle plume is thought to have impacted and weakened the lithosphere of northern Early Paleozoic collage, ultimately leading to the opening of the Mongol‐Okhotsk Ocean between microcontinents that had earlier accreted to Siberia Craton. Our model suggests continent breakup in accretionary orogens tends to focus along weak orogenic lithosphere between the rigid microcontinents. Key Points: The 415–410 Ma bimodal volcanic suite and associated volcanic‐sediments document a widespread extension regime related to a mantle plumeThe Devonian mantle plume weakened structures within the accretionary orogen and led to the opening of the Mongol‐Okhotsk OceanContinent breakup in accretionary orogens tends to happen along weak orogenic lithosphere between rigid microcontinents [ABSTRACT FROM AUTHOR]