Tectonic Synthesis and Implications of Japan Sea ODP Drilling

Abstract

ABSTRACTthat the basin volcanism results from seafloor spreading or a continental rifting process. The borehole seismometer experiment at Site 794 indicates the crust is 12 km thick with the lower crust twice as thick as that of the Japan Basin. Eight discrete stages of basin subsidence history were derived from the comparative study of stratigraphy of the drilling sites and the surrounding offshore and onshore geological sections.Analyses of these new data, together with more detailed identification of magnetic anomaly lineations in the Japan Basin, have yielded a tectonic scenario for the evolution of the Japan Sea. The entire Japan Sea was extending from 32 Ma to 10 Ma, during which major opening associated with vigorous basin volcanism occurred about 28 to 18 Ma. The Japan Sea started its opening by crustal thinning within the proto-Japan arc. As thinning proceeded, breakup of the lithosphere was initiated at the major strike-slip fault forming the eastern margin of the juvenile sea, triggering seafloor spreading. Subsequent rifting and seafloor spreading process propagated southwestward during a period when the southern part of the Japan Sea was still undergoing crustal thinning, which ceased about 18 Ma and left oceanic crust in its wake. This crust now forms the eastern floor of the Japan Basin. Thinned arc crusts and relic pieces of the upper crust of the proto-Japan arc were left in the southern part of the Japan Sea as deep sea basins and rises and ridges, respectively. Crustal thinning and propagating seafloor spreading of thinned crust are identified as the most significant processes for the opening of the Japan Sea.

Age relations and petrology of alkalic igneous rocks from Oki Dozen, Southwest Japan.

“…1) consist of four islands, three of which form Oki Dozen, and the fourth Oki Dogo. The Oki Islands sit astride a topo graphic high, which according to Tamaki et al (1992), is a crustal fragment left behind after Miocene back-arc spreading that created the Japan Sea. Oki Dogo has received the most attention in the literature, as it is floored by some of the oldest rocks in Japan (Lower Proterozoic gneiss: Tanaka and Hoshino, 1987;Yamashita and Yanagi, 1994), and some Pliocene to Holocene basalts, which are unfractionated from primary liquids, have been used to examine back-arc magmatic processes in the Japan Sea (Nakamura et al, 1985(Nakamura et al, , 1989(Nakamura et al, , 1990).…”

Re-Os dating of the Kuroko ores from the Wanibuchi Mine, Shimane Prefecture, southwestern Japan.

“…The scenario for the formation of the Japan Sea deduced by magnetic anomaly lineations, 40 Ar/ 39 Ar data for the seafloor basalts, and crustal structure by seismic studies is as follows (e.g., Tamaki et al, 1992): (1) The Japan Sea started opening by crustal stretching within the proto-Japan arc (ca. 35 Ma); (2) seafloor spreading began 28 Ma ago in the northeastern Japan Sea area and subsequent rifting and seafloor spreading propagated southwestward until about 18 Ma; (3) during the latter period, the southwestern part of the Japan Sea was still undergoing crustal thinning, but seafloor spreading had not yet occurred in the area; (4) rapid rotation of southwest Japan took place around 15 Ma in association with a second phase of extention and subsidence in the back-arc area.…”

A Two-Dimensional Post-Stack Seismic Inversion for Acoustic Impedance of Gas and Hydrate Bearing Deep-Water Sediments Within the Continental Slope of the Ulleung Basin, East Sea, Korea

“…1). The deep back-arc basins were developed commencing from the early Oligocene (Tamaki et al 1992) through the late Miocene (Tamaki and Honza 1985) between the PrecambrianPaleozoic craton of the Korean Peninsula to the west and the Cenozoic Japanese Island arc to the east.…”