Neogene strike‐slip faulting in Sakhalin and the Japan Sea opening

Abstract

We describe structural data from a 2000 km N‐S dextral strike‐slip zone extending from northern Sakhalin to the southeast corner of the Japan Sea. Satellite images, field data, and focal mechanisms of earthquakes in Sakhalin are included in the interpretation. Since Miocene time the deformation in Sakhalin has been taken up by N‐S dextral strike‐slip faults with a reverse component and associated en échelon folds. Narrow en échelon Neogene basins were formed along strike‐sup faults and were later folded in a second stage of deformation. We propose a model of basin formation along extension al faults delimitating dominos between two major strike‐slip faults, and subsequent counterclockwise rotation of the dominos in a dextral transpressional regime, basins becoming progressively oblique to the direction of maximum horizontal compression and undergoing shortening. The association of both dextral and compressional focal mechanisms of earthquakes indicates that the same transpressional regime still prevails today in Sakhalin. We present fault set measurements undertaken in Noto Peninsula and Yatsuo Basin at the southern end of the Sakhalin‐East Japan Sea strike‐slip zone. Early and middle Miocene formations recorded the same transtensional regime as observed along the west coast of NE Honshu. During the early and middle Miocene the strike‐slip regime was transpressional to the north in Sakhalin and Hokkaido, and transtensional to the south along the west coast of NE Honshu as far as Noto Peninsula and Yatsuo basin. Dextral motion accommodated the opening of the Japan Sea as a pull‐apart basin, with the Tsushima fault to the west. The opening of the Japan Sea ceased at the end of the middle Miocene when transtension started to change to E‐W compression in the Japan arc. Subduction of the Japan Sea lithosphere under the Japan arc started 1.8 Ma ago. The evolution of the stress regime from transtensional to compressional in the southern part of the strike‐slip zone is related to the inception of the subduction of the young Philippine Sea Plate lithosphere under the Japan arc during the late Miocene. Subduction related extension is a necessary condition for the opening of the Japan Sea. Two possible mechanisms can account for dextral shear in this area: (1) counterclockwise rotation of crustal blocks due to the collision of India with Asia, (2) extrusion of the Okhotsk Sea block squeezed between the North America and Eurasia plates.