An unrecognized major collision of the Okhotomorsk Block with East Asia during the Late Cretaceous, constraints on the plate reorganization of the Northwest Pacific

Interactions at plate boundaries induce stresses that constitute critical controls on the structural evolution of intraplate regions. However, the traditional tectonic model for the East Asian margin during the Mesozoic, invoking successive episodes of paleo-Pacific oceanic subduction, does not provide an adequate context for important Late Cretaceous dynamics across East Asia, including: continental-scale orogenic processes, significant sinistral strike-slip faulting, and several others. The integration of numerous documented field relations requires a new tectonic model, as proposed here. The Okhotomorsk continental block, currently residing below the Okhotsk Sea in Northeast Asia, was located in the interior of the Izanagi Plate before the Late Cretaceous. It moved northwestward with the Izanagi Plate and collided with the South China Block at about 100 Ma. The indentation of the Okhotomorsk Block within East Asia resulted in the formation of a sinistral strike-slip fault system in South China, formation of a dextral strike-slip fault system in North China, and regional northwest-southeast shortening and orogenic uplift in East Asia. Northeast-striking mountain belts over 500 km wide extended from Southeast China to Southwest Japan and South Korea. The peak metamorphism at about 89 Ma of the Sanbagawa high-pressure metamorphic belt in Southwest Japan was probably related to the continental subduction of the Okhotomorsk Block beneath the East Asian margin. Subsequently, the north-northwestward change of motion direction of the Izanagi Plate led to the northward movement of the Okhotomorsk Block along the East Asian margin, forming a significant sinistral continental transform boundary similar to the San Andreas fault system in California. Sanbagawa metamorphic rocks in Southwest Japan were rapidly exhumed through the several-kilometer wide ductile shear zone at the lower crust and upper mantle level. Accretionary complexes successively accumulated along the East Asian margin during the Jurassic-Early Cretaceous were subdivided into narrow and subparallel belts by the upper crustal strike-slip fault system. The departure of the Okhotomorsk Block from the northeast-striking Asian margin resulted in the occurrence of an extensional setting and formation of a wide magmatic belt to the west of the margin. In the Campanian, the block collided with the Siberian margin, in Northeast Asia. At about 77 Ma, a new oceanic subduction occurred to the south of the Okhotomorsk Block, ending its long-distance northward motion. Based on the new tectonic model, the abundant Late Archean to Early Proterozoic detrital zircons in the Cretaceous sandstones in Kamchatka, Southwest Japan, and Taiwan are interpreted to have been sourced from the Okhotomorsk Block basement which possibly formed during the Late Archean and Early Proterozoic. The new model suggests a rapidly northward-moving Okhotomorsk Block at an average speed of 22.5 cm/yr during 89-77 Ma. It is hypothesized that the Okhotomorsk-East Asia collision during 100-89 Ma slowed down the northwestward motion of the Izanagi Plate, while slab pull forces produced from the subducting Izanagi Plate beneath the Siberian margin redirected the plate from northwestward to north-northwestward motion at about 90-89 Ma. (C) 2013 Elsevier B.V. All rights reserved.