Bulk composition and clay mineralogy at DSDP Legs 56 and 57

Altogether 513 samples from sediments of Cretaceous to Pleistocene age from DSDP Legs 56 and 57 were examined by x-ray methods. The main constituents are clay minerals, quartz, feldspar, opaline silica, and volcanic glass. The sediment composition reflects the position of the sites in relation to the main source area, the Japanese Island Arc. For example, relatively coarse-grained material rich in quartz and feldspar was deposited closest to the islands, whereas finer-grained material rich in clay minerals (mainly smectite and illite, with lesser amounts of kaolinite and chlorite) was deposited farther seaward. Vertical fluctuations in the composition of the sediments show the same trend in all sites and are caused mainly by a fluctuating contribution of biogenic silica with time. A trend reversal in the chlorite/kaolinite ratio at Site 438 supports the conclusion that the subsidence of the Oyashio ancient landmass took place during the middle Miocene. That ratio also indicates a northwest drift in the position of Site 436 by sea floor spreading. Oscillations of the illite/smectite ratio during the Pleistocene at Site 436 show the variations of climate during this period. During early diagenesis potassium is fixed in smectite. With increasing depth of burial a smectite-illite mixed layer is formed, with increasing illite layering. At Sites 434, 440, and 441, stepwise changes confirm intensive tectonic process at the midslope terrace and the lower inner slope of the Japan Trench.

Paleoelevations in the Pacific Ocean 35 million years ago

Data from deep sea drilling, linear magnetic anomalies and bathymetric measurements together with age and morphometric characteristics of seamounts have been used to construct a paleobathymetric map of the oceans 35 million years ago. A brief analysis of these results is presented.

Permeability measurements on sheared and unsheared samples from different IODP Holes of Leg 316 and from IODP Hole 315-C0001E and ODP Hole 190-1174B

At subduction zones, the permeability of major fault zones influences pore pressure generation, controls fluid flow pathways and rates, and affects fault slip behavior and mechanical strength by mediating effective normal stress. Therefore, there is a need for detailed and systematic permeability measurements of natural materials from fault systems, particularly measurements that allow direct comparison between the permeability of sheared and unsheared samples from the same host rock or sediment. We conducted laboratory experiments to compare the permeability of sheared and uniaxially consolidated (unsheared) marine sediments sampled during IODP Expedition 316 and ODP Leg 190 to the Nankai Trough offshore Japan. These samples were retrieved from: (1) The décollement zone and incoming trench fill offshore Shikoku Island (the Muroto transect); (2) Slope sediments sampled offshore SW Honshu (the Kumano transect) ~ 25 km landward of the trench, including material overriden by a major out-of-sequence thrust fault, termed the "megasplay"; and (3) A region of diffuse thrust faulting near the toe of the accretionary prism along the Kumano transect. Our results show that shearing reduces fault-normal permeability by up to 1 order of magnitude, and this reduction is largest for shallow (< 500 mbsf) samples. Shearing-induced permeability reduction is smaller in samples from greater depth, where pre-existing fabric from compaction and lithification may be better developed. Our results indicate that localized shearing in fault zones should result in heterogeneous permeability in the uppermost few kilometers in accretionary prisms, which favors both the trapping of fluids beneath and within major faults, and the channeling of flow parallel to fault structure. These low permeabilities promote the development of elevated pore fluid pressures during accretion and underthrusting, and will also facilitate dynamic hydrologic processes within shear zones including dilatancy hardening and thermal pressurization.