NEW APATITE FISSION TRACK AND (U-TH)/HE DATA FROM THE MAGADAN BATHOLITH: IMPLICATIONS FOR THE NATURE AND TIMING OF THE FORMATION OF THE SEA OF OKHOTSK

The Sea of Okhotsk is one of the many behind-the-arc basins fringing the Pacific margin of Asia. Outstanding questions regarding the timing and nature of formation of the Sea of Okhotsk still remain. The predominance of tectonic models suggest a micro-continental block or oceanic plateau collided with the Asian continent in the Late Cretaceous causing a seaward jump in the position of the subduction zone and subduction-related magmatism to a trend similar to the present day Kuril-Kamchatka Arc (Sengor and Natal'in, 1996). More recently, satellite-based gravity data across the Sea of Okhotsk has revealed a striking corrugated basement topography leading to the suggestion that it may represent a region of continental extension that has since subsided below sea level (Mann, 1998). The interpretation of this corrugated basement topography as horsts and grabens is supported by seismic data available from the marine basins (Worral, 1997) as well as onshore in the Yama-Tauy basin system.

Preliminary apatite fission track and (U-Th)/He data show shallow crustal levels of the Magadan batholith underwent protracted post magmatic cooling in the late Cretaceous. In contrast,, structurally deeper samples exhibit concordant apatite (U-Th)/He and fission track ages (44-38 Ma) and are characterized by uniformly long fission track lengths ( >13.9 microns). These thermochronogical data indicate rapid cooling in the late Eocene to Earliest Oligocene that we attribute to exhumation of the Magadan Batholith in response crustal extension along the northern margin of the Sea of Okhotsk.

These new data in concert with land-based constraints on the style of deformation and the available gravity and seismic data from on- and offshore, provide compelling evidence for a Late Eocene-Early Oligocene extensional origin for the northern Sea of Okhotsk.