TERTIARY TECTONICS OF THE SEA-OF-OKHOTSK, RUSSIA - FAR-FIELD EFFECTS OF THE INDIA-EURASIA COLLISION

A new structural map of the western Sea of Okhotsk, based on grids of multichannel seismic data, provides several new insights into the Tert iary deformation of Asia, especially in regard to the extension of ''e xtrusion'' tectonics, linked to the India-Eurasia collision, into nort heastern Asia. The sedimentary basins in this offshore region are the result of two regional shear systems. In the west, the north trending Sakhalin-Hokkaido dextral fault system transects Sakhalin Island. Acce ssory structures closely related to its dextral shear include northeas t trending normal faults as well as northwest trending en echelon fold s and thrusts. The accessory normal faults are predominantly Eocene to lower Miocene and indicate transtensional shear, whereas the folds an d thrusts are mostly younger and indicate late Miocene and Pliocene tr anspression. In the north, an east-west trending sinistral shear zone enters the sea at its northwest corner; this sinistral shear system li nks farther west to extensional faults of the early Tertiary Baikal ri ft. Extending northeastward from the terminus of this sinistral shear zone are a series of long, predominantly northwest dipping listric nor mal faults that form a large ''lazy-S'' shaped pull-apart basin, the S hantar-Liziansky basin (SLB). Normal faults in this 500 x 140 km basin accommodate 15 to 20% extension. Extension and related sinistral shea r appear to be largely Eocene to Oligocene in age, with lesser later a ctivity. To the northeast of SLB, a further extension of the sinistral shear zone appears to bend northeastward, transecting Pustorets and P enzhina basins and following the course of the older, Mesozoic Mongol- Okhotsk-Chukotsk active margin and suture; it possibly reaches as far as the Arctic. As suggested by previous workers, the Baikal rift and i ts associated sinistral shear zone may be interpreted as by-products o f the India-Eurasia collision in Eocene time; mapping results included here show that this diffuse sinistral shear system extends much farth er to the northeast. The sinistral shear zone is transected by the dex tral Sakhalin-Hokkaido fault zone in the Kashevarov region of central SLR, where a fan of north to northwest striking late Tertiary transpre ssional fault splays marks the northern termination of the dextral fau lt zone. Older SLB normal faults are locally deformed, and half graben s are partly inverted, by the slightly younger dextral splays. The two regional fault systems are essentially conjugate, and the pattern of intersection closely resembles that predicted by recent modelling stud ies of India/Eurasia collision tectonics.