(Table 2) Mineralogy and carbon, oxygen, and hydrogen isotope ratios for secondary mineral samples from igneous rocks from ODP Hole 128-794D

The basaltic rocks of Hole 794D drilled during Leg 128 are strongly altered. Microprobe analyses and XRD spectra on small quantities of matter extracted from thin sections show that primary minerals and glassy zones of the groundmass are totally or partially replaced by clay minerals with chlorite/saponite mixed-layer composition whatever the rock sample considered. This mixed-layer was also identified in veins and vesicles where it crystallizes in spheroidal aggregates. The largest veins and vesicles are filled by a zoned deposit: the chlorite/saponite mixed-layer always occupies the central part and is rimmed by pure saponite. Calcite crystallizes in secondary fractures which crosscut the clayey veins and vesicles. Chemographic analysis based on the M+-4Si-3R2+ projection shows that the chemical composition of the saponite component in the mixed-layer is identical to that of the free saponite. This indicates that the clay mineral crystallization was controlled by the chemical composition of the alteration fluids. From petrographic evidence, it is suggested that both chlorite/saponite mixed-layer and free saponite belong to the same hydrothermal event and are produced by a temperature decrease. This is supported by the stable isotopic data. The isotopic data show very little variation: d18O saponite ranges from 13.1 per mil to 13.5 per mil, and dD saponite from -73.6 per mil to -70.0 per mil. d18O calcite varies from +19.7 per mil to +21.9 per mil vs SMOW and d13C from -3.2 per mil to +0.4 per mil vs. PDB. These values are consistent with seawater alteration of the basalt. The formation of saponite took place at 150°-180°C and the formation of calcite at about 65°C.

Geochemistry and isotopic composition of volcanic rocks of ODP Hole 128-794D

The Yamato Basin basement in the Sea of Japan was drilled below the sediment pile during Legs 127 and 128. Two superposed volcanic complexes are distinguished. The upper complex consists of continental tholeiite sills dated around 20-18 Ma and attributed to the rifting stage of the backarc basin. The lower complex consists of backarc basin basalts probably intruded below the upper complex during the spreading stage. Trace-element compositions and Sr and Nd isotopic signatures may be explained by mixing of at least two end members with a very small addition of crustal and subducted sediment component. Thus, upwelling of mantle diapir occurred during the rifting stage. Contribution of the depleted mantle increased in the spreading stage. The Neogene magmatic history of the Japan Sea is reviewed in the light of the ODP new data.

Structural features in ODP Holes 128-794D and 128-799B

Numerous structural features occur in the Leg 128 cores from the Japan Sea. They include (1) gravity-induced structures such as slump folds, (2) dewatering structures comprising several sets of veins, and (3) larger faults and veins developed in the volcanic basement of the Yamato Basin as well as in the sedimentary rocks of the Oki Ridge and Kita-Yamato Trough. Gravity-induced structures, mainly slumps and associated faults, suggest the existence of paleoslopes and the dominance of gravitational tectonics during the early and middle Miocene, at the Pliocene/Pleistocene boundary, and during the Quaternary. Several types of mud-filled veins having various shapes were observed. These are especially abundant in the middle Miocene siliceous claystones and porcellanites from the Kita-Yamato Trough. They have been interpreted as dewatering conduits that formed preferentially in highly porous, water-saturated diatomaceous muds on a slope, because of episodic loss of sediment strength, collapse of the sediment framework, and consequent fluid migration. The central part of the vein serves once as a fluid conduit, whereas the transition between conduit-controlled and intergranular flow occurs at the branching extremities, with concentration of fines. The likely trigger responsible for the strength loss is seismic activity. Development of these veins, spatially and chronologically linked to small normal microfaults, implies an extensional regime having layer-parallel extension and a local bedding-parallel shear couple, probably the result of gravitational gliding. The brittle fractures found in Yamato Basin basement Hole 794D cores comprise joints, faults, and veins filled with chlorite-saponite, saponite, and calcite. They suggest a likely transpressive to transtensional regime around the early Miocene/ middle Miocene boundary, with a north-northeast-south-southwest compression alternating with a west-northwest-eastsoutheast extension. The faults from Site 799 cores on the Yamato Rise exhibit a prominent early Miocene-middle Miocene extensional environment, a late Miocene-early Pliocene phase of normal and strike-slip faulting, and a final phase that began during the latest Pliocene. Site 798, on the Oki Ridge, reveals faults that recorded a consistent extensional tectonic regime from Pliocene to the Holocene. These data support the pull-apart kinematic model for early Miocene-middle Miocene time, as regarding the stress regime deduced from the Yamato Basin basement fractures. The recent compression known in the eastern margin of the Japan Sea was not documented by compressive structures at any site. The late Miocene-early Pliocene faulting phase corresponds to a major and general reorganization of the stress distribution in the arc area. Evidence for rapid and main subsidence and synsedimentary extension of the Yamato Basin and Yamato Rise areas between 20 and 15 Ma, and the concomitant rotation of southwest Japan, raise the question of links between this opening and the Shimanto Belt collision in southwest Japan, between the arc and the Philippine Sea Plate.