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‐arc junction.