Stress field of basaltic rocks of ODP Hole 127-794C

We carried out an experiment to estimate in-situ stresses at ODP Hole 794C (water depth: 2809 m) from the basaltic core samples by deformation rate analysis (DRA). Site 794 is located at the northern end of the Yamato Basin and 70 km west of the eastern Japan Sea intraplate or interplate convergent zone. Stress previously applied to a rock specimen is identified in the inelastic strain behavior of the specimen under uniaxial compression by the method used. Natural remanent magnetization of the sample was also measured to get a reference for the orientation of the horizontal stresses. The vertical, maximum, and minimum horizontal in-situ stresses estimated at a depth of 582 mbsf are 36.4, 43.1, and 31.2 MPa, respectively. The average of the largest and the least horizontal stresses is nearly equal in value to the vertical stress. This suggests that the site is in the stress field of the strike slip regime at the depth, while the stress field of the reverse fault regime has been estimated from the focal mechanism solutions of the earthquakes whose hypocenters are located near or on the convergent boundary. The directions of the largest and the least horizontal stress are estimated to be northeast-southwest and in northwest-southeast, respectively, in taking account of rotation tectonics of the Japan Sea since its formation. The directions of the largest and the least horizontal stresses are opposite to those determined from the earthquakes. These discrepancies of our results with those from earthquakes may be due mainly to the fact that the site is not in the convergent zone.

Mineral composition and geochemistry of sediments from ODP Leg 127 sites

The lithostratigraphy of Neogene hemipelagic sediments recovered from the Japan Sea during Leg 127 was revised to improve intersite consistency and to remove confusion stemming from diagenetic modification of the lithology through the opal-A to opal-CT transformation. Special emphasis was put on the presence and nature of dark-light cycles in revising the lithostratigraphy. Mineral composition analysis was conducted for samples from Sites 794, 795, and 797. In addition, major element chemical composition analysis was conducted for these same sample sets from Site 794. The result of mineral composition analysis suggests that the detrital component, which consists of such minerals as quartz, plagioclase, illite, and kaolinite plus chlorite, is diluted to various degrees by biogenic silica (opal-A) and its diagenetic equivalents (opal-CT and quartz). Smectite, on the other hand, may be a diagenetic or hydrothermal alteration product of volcanic material, although more study is necessary to confirm its origin. As a whole, vertical variation in the sediment composition is consistent with the revised lithostratigraphy and helps to characterize the redefined lithologic units quantitatively.