Crustal Structure of Cretaceous Accretionary Orogenic Belts of North-Eastern Asia Based on Data from the 2DV Geophysical Transect

North-Eastern Asia consists of the Siberian craton and its deformed passive margin and broad accretionary mosaics of terrains with diverse affinities (carbonate platform, island arcs, micro-continents, etc) intruded and overlapped by several belts of Jura-Cretaceous plutonic and volcanic rocks. Three main orogenic belts can be distinguished in the region: 1. Late Jurassic to Lower Cretaceous Yano-Kolyma belt, 2. Cretaceous Oloi- Chukotka belt, and 3. Cretaceous Okhotsk-Koryak belt. Geophysical transect 2DV extended from Magadan to Pevek and has imaged the deep structure of the crust beneath these main belts. The interpretation of this new seismic data allows the following insights: (1) The Oloi-Chukotka orogenic belt represents a fold belt with Archean to Early Proterozoic? crust near its margins transitioning to possible Paleozoic age (rifted) crust beneath its central part and accreted Precambrian basement beneath the Omolon and Ckukotka blocks. (2) Thickness of the crust increases across the paleo continental margins from 35-39 km (Uda-Murgal island arc and inner zone of Okhotsk-Chukotka volcanic belt-OCVB) up to 55-58 km (near the junction of OCVB with Yano-Kolyma orogenic belt and under margin of Omolon massif near Oloi volcanic arc). Thickening of the lower crust can be seen as imbrication. Seismic "transparency" and increasing velocities up to 5-7 km/sec characterize the middle crust to the Moho beneath the main igneous/ore-bearing zones of these orogenic belts, suggesting that fluid-magmatic processes were important in creating present-day crustal composition/structures (3) The South-Anyui suture was imaged and is interpreted as structures related to thrusting of Chukotka onto weakly deformed Oloi terrain from north to south. (4) Isotopic signatures of magmatic rocks shift to more depleted values towards Cretaceous paleo continental margins (beneath OCVB), in agreement with the rejuvenated nature of the lower crust beneath extensional provinces superimposed on accretionary belts, as supported by xenolith studies.