Sedimentary Processes And Deformation Styles Of Emuerhe Group In Mohe Basin,NE China

The Mohe basin is located on the south side of eastern Mongol–Okhotsk suture belt,which has recorded a large number of Mesozoic stratigraphic sequences accompanied by a large number of thrust nappe structures related to the subduction process.It is an important window for scholars to understand and identify the closing process of the eastern Mongol–Okhotsk Ocean.Based on structural analysis,petrography,geochronology,and petrochemistry studies of the Emuerhe Group in the Mohe Basin,this paper attempts to reveal the formation ages,provenance,and tectonic setting of the Emuerhe Group,and comprehensively discusses the tectonic evolution of the eastern Mongol–Okhotsk Ocean.Detrital zircon LA-ICP-MS U-Pb dating results show that the bottom-up sedimentary age limits of each formation in the Ermuerhe Group are 159 Ma,155 Ma,150 Ma and 138 Ma,respectively.Combined with previous data and regional geological background,the depositional ages of the Xiufeng Formation and Ershierzhan Formation are limited to the Late Jurassic,the Mohe Formation is estimated as the Late Jurassic to early Early Cretaceous and the Kaikukang Formation is limited to the early Early Cretaceous.Combined with the tectonic settings and the affinities of provenance areas from detrital zircons,the Xiufeng Formation and the Ershierzhan Formation share a single provenance,mainly originating from the ancient continental crust,the continental arc of the adjacent foreland region,and young igneous rocks in the south of the Mohe basin,the Erguna Block.However,the Amur Basin is also a partial provenance source of the Ershierzhan Formation.The Mohe Formation and the Kaikukang Formation have a bidirectional provenance,with the Erguna Block as the southern provenance,and the Siberian Craton and the Upper Amur Basin as the northern provenance.Based on the above geological evidence and previous research results,this paper proposes the tectonic evolution process of the eastern Mongol-Okhotsk Ocean.From Late Triassic to Late Jurassic(150 Ma),the Mongol-Okhotsk Ocean was consumed by bidirectional subduction expressed by subduction under the Siberian Craton and the Erguna Block,respectively.The intense subduction led to the compression of the continental arc,the basement uplift of the Erguna Block and the southward migration of the Upper Amur Basin depositional center.The intense subduction also caused strong calc-alkaline and subalkaline magmatism on both sides of the plate,which provided abundant clastic sediments to the Xiufeng-Ershierzhan Formations.During the Late Jurassic-Valanginian(150~135 Ma),the Mongol-Okhotsk Ocean closed gradually in a scissor pattern from west to east,ultimately resulting in the collision between the Erguna Block and the Siberian Craton at the northwest margin of the Mohe Basin.Meanwhile,the southern margin of the Siberian Craton experienced uplift and began supplying sediments into the Mohe formation.The final closure of the ocean eventually caused the Erguna block and the Siberian Craton finally closed at the northeastern margin of the Amur Basin(Mohe Basin and Upper Amur Basin),which served as the provenance for the Kaikukang Formation.The subsequent crustal shortening associated with collision between the Erguna Block and the Siberian Craton led to nappe structures,nearly N-S,E-W-trending reverse and oblique-slip faults occurred along the northwestern margin of the Mohe basin.Between 135 and 128 Ma,the Mohe basin entered the stage of tectonic setting transformation,which is the crucial period for the Mohe basin to transform from compression to extension.Since the Early Barremian(ca.128 Ma),the compressional environment was transformed into a post-orogenic extensional tectonic environment,and a series of nearly E-W and NE-SW-trending normal faults occurred,which might have been controlled by the slab rollback of the Paleo-Pacific Plate and detachment of the Mongol-Okhotsk oceanic plate after the closure of the Mongol-Okhotsk Ocean.