Collision and extrusion at the Kuril-Japan arc junction

Abstract

Hokkaido (Japan) is a region where collision tectonics is ongoing at an arc-arc junction and where lower crust is presently exposed as a result of the collision of the forearc sliver of the Kuril Arc with the Northeast Japan Arc. Deep-seated ductile deformation in metamorphic rocks has already been described, but more shallow deformation is not sufficiently well observed to clarify a total history of the collision. We focus on the shallow deformation in the Cape Erimo area in central Hokkaido and discuss in detail the process of collision at this arc-arc junction. Four phases of deformation are discriminated in the study area as follows: (1) ENE-WSW extension immediately after sediment deposition, (2) NW trending dextral strike slip, (3) NE-SW compression in association with the formation of conjugate sets of strike-slip faults, and (4) E-W compression represented by the formation of low-angle reverse faults. The first and the second phases of faulting may be a manifestation of the Oligo-Miocene dextral transform plate boundary between the Eurasia and Okhotsk Plates. A conjugate system of strike-slip faults formed in the third phase, newly clarified in this study, by NE trending horizontal compression nearly normal to the NW trending dextral strike-slip fault zone of the previous phase. Development of the strike-slip faults, especially the predominance of sinistral strike-slip faults in the southern half of the arc-arc junction, must be explained by extrusion in association with collision of the forearc sliver of the Kuril Arc. Such extrusion may be a common tectonic feature even in the case of a small-scale collision at an arc-are junction.