Bimodal Miocene magmatism at the northeastern end of the Eurasian margin in response to horizontal propagating tear of slabs due to the simultaneous opening of the Japan and Kuril basins

From the Late Cretaceous to the Quaternary, the northeastern end of the Eurasian margin experienced a complicated tectono-magmatic history including the subduction of the Izanagi-Pacific ridge in the Eocene time, the opening of the Japan and Kuril basins and the associated trench migration in the Oligocene to Miocene time, the possible collision of the Eurasian plate and the North American (Okhotsk) plate around the Oligocene to Miocene time, and subduction zone magmatism in all periods. In central Hokkaido (Japan), Eocene-Miocene plutonic bodies are distributed along the north-south orientated Hidaka Magmatic Zone (HMZ). We report new zircon U-Pb ages and geochemical data from plutonic rocks in the HMZ, which reveal Miocene compositionally bimodal magmatism; the felsic magmatism present is characterized by island-arc geochemical signatures. Trace element compositions of the Miocene mafic-intermediate plutonic rocks of the HMZ appear as a mixture between typical N-MORB and island-arc compositions. Trace element profiles from HMZ plutonic rocks are similar, albeit with less pronounced arc signatures, to the Miocene volcanic rocks formed along the Paleo-Japan Trench. Together, these data suggest the coexistence and mixing of N-MORB-type primitive magma, with the parental magmas of the HMZ mafic rocks, implying petrogenesis of a different nature than typical subduction zone magmatism. The cause of the north-south orientation of the Miocene plutono-volcanic rocks from central Hokkaido to its northern extension into Russia (Sakhalin) is probably along some kind of tectonic/structural boundary. However, the inferred paleo-position of the HMZ is very close to the trench and far from the volcanic front at that time and the existence of N-MORB-type primitive magma cannot be explained by subduction magmatism. The newly proposed possible geodynamic setting in this study that can reasonably explain the distribution and geochemical signature of these rocks is the simultaneous opening of the Japan and Kuril basins at different rates. In the Japan Trench, the Pacific plate was subducted at a relatively shallow angle and the magmatic arc forcibly moved eastward due to the opening of the Japan Basin. In the Kuril Trench, the rollback corresponding to the steep subduction of the plate and the associated opening of the Kuril Trench occurred simultaneously in a short period of time. If the Paleo-Kuril Trench retreated rapidly relative to that of the Paleo-Japan Trench, a horizontal propagating tear that cuts the slab horizontally is estimated to have occurred at the bend of both trenches, together with a (vertical) slab tearing and blocky opening of the subducting oceanic plate on the Paleo-Japan Trench side. At the western margin of the Kuril Basin, the N-MORB magma and hot asthenosphere inflow induced remelting of the mantle already contaminated to various degrees at the subduction zone.