Time on hole: 2 days, 16 hr, 48 min Position: 48°41.958'N, 126°52.098'W Bottom felt (rig floor; m; drill-pipe measurement): 1322.0 Distance between rig floor and sea level (m): 10.8 Water depth (drill-pipe measurement from sea level; m): 1311.2 Total depth (rig floor; m): 1667.8 Penetration (m): 345.8 Number of cores (including cores with no recovery): 44 Total length of cored section (m): 325.8 Total core recovered (m): 223.14 Core recovery (%): 68 Oldest sediment cored: Depth (mbsf): 346.0 Nature: glauconitic clayey silt Earliest age: late Pliocene Time on hole: 1 day, 1 hr, 36 min Position: 48°41.850'N, 126°52.392'W Bottom felt (rig floor; m; drill-pipe measurement): 1327.0 Distance between rig floor and sea level (m): 10.8 Water depth (drill-pipe measurement from sea level; m): 1316.2 Total depth (rig floor; m): 1713.5 Penetration (m): 386.5 Number of cores (including cores with no recovery): 21 Total length of cored section (m): 189.5 Total core recovered (m): 48.15 Core recovery (%): 25

Leg 146 was directed toward investigation of tectonic dewatering in the accretionary wedge at the Cascadia continental margin. At five sites, two principal thematic objectives were pursued: (1) the mechanisms and geological consequences of diffuse flow and channelled flow within the wedge; and (2) the cause of bottom-simulating seismic reflectors (BSRs) and their relationship to the occurrence of gas hydrate and free gas. Site 888, located in Cascadia Basin near the northern edge of Nitinat Fan, provides a reference section (with particular importance to porosity, temperature, and pore-water geochemistry) for the other sites located on the accretionary wedge. Diffuse fluid outflow from the wedge and the nature of a well-defined BSR were investigated at Sites 889 and 890 off Vancouver Island. Flow through fault zones was investigated at Sites 891 and 892 off Oregon. Site 891 examined the frontal thrust fault that connects to the master décollement. Site 892 exhibited a BSR displaced toward the surface along a hydologically active fault in the Pliocene section of the wedge. A wide variety of downhole determinations were successfully completed on Leg 146 to characterize the fluid regime on the Cascadia margin and complement detailed laboratory analyses. In addition, the first long-term observatories -instrumented borehole seals (CORKs) -were deployed on a modern accretionary prism at Sites 889 and 892. INTRODUCTION Accretionary complexes evolve through a complex interplay of sedimentation, structural evolution, diagenesis, and fluid flow. These processes are particularly active near the toe of an accretionary wedge, where high porosity sediments overlying oceanic basement are initially deformed. Convergent motion in this region results in sediment compaction, over-pressuring and expulsion of pore fluids, and development of fault zones to structurally accommodate shortening and perhaps to function as aquifers. The movement of fluids to the surface perturbs normally diffusive geochemical systems and may result in localized cementation, formation of gas hydrates, and acceleration of geochemical fluxes to the water column. Leg 146 Preliminary Report Page 8 Leg 146 was directed toward examining the role and behavior of fluids in these accretionary processes. By necessity, a single drilling leg can investigate only some aspects of the fluid regime; Leg 146 had as its primary foci: 1. The relative importance of dispersed and focused fluid flow and the associated geochemical fluxes from an active accretionary wedge; 2. The nature of gas-hydrate zones and the physical and chemical factors which support them in a convergent-margin setting.Because the fluids are derived largely from the sedimentary section, investigation of fluid processes required that we measure the fluid chemistry, the temperature and pressure, the physical properties of the sediments involved, determine the nature and history of diagenetic events, and establish the structural evolution associated with accretion and fluid discharge. The Cascadia margin repr...