No abstract

Vertical seismic profile (VSP) data from two drill sites on the Cascadia margin show low-velocity zones, indicative of free gas, beneath a bottom-simulating reflector (BSR). Offshore Oregon, at Ocean Drilling Program (ODP) Site 892, velocities drop from an average of 1750 m/s above the BSR to less than 1250 m/s below it. Sonic logs confirm that seismic velocity in the sediments adjacent to the borehole is less than that of water for at least 50 m beneath the depth of the BSR at this site. Similarly, at ODP Site 889 offshore Vancouver, velocities range from 1700 to 1900 m/s in the 100 m above the BSR and drop abruptly to 1520 m/s in the 15 m just beneath it. The low velocities observed beneath the BSR are strong evidence for the presence of l%-5% free gas (by volume). The BSR at these two sites results from the contact between gas-free sediments containing a small quantity of hydrate above the BSR and a low-velocity free-gas zone beneath it. Although the BSR is associated with the base of the hydrate stability field, hydrate appears to account for relatively little of the velocity contrast that produces the BSR. Velocity above the BSR at Site 889 is only about 100 m/s greater than that expected for sediments of similar porosity. Sediments above the BSR at Site 892 appear to have normal velocity for their porosity and may contain little hydrate.

The Oregon margin near 45øN spans a regional transition in structural style from seaward vergence in the south to landward vergence in the north. This variation probably reflects a regional change in both sediment type and rate of deposition that affects the potential for overpressure in the sediments. Structural style within the survey area shows a gradual northward transition from seaward to landward vergence and to lower slopes within the landward vergent area, suggesting a northward decrease in basal shear stress. Superimposed on this gradational variation are abrupt changes in structural style that correlate with NW striking strike-slip faults in the Cascadia Basin.Because sediments thicken toward the east, translation along the strike-slip faults results in juxtaposition of sediments with different physical properties and loading histories. In addition, the faults themselves may act as fluid conduits, resulting in stepwise changes in pore pressure on the d6collement and concomitant change in structural style across the faults. Although the Oregon-Washington margin is dominated by landward vergence, landward vergence has not been adequately explained by theoretical models or replicated in experimental models because of a simple omission in the boundary conditions of the underlying conceptual model. Landward vergence requires not only low basal shear stress but also an arcward dipping d6collement (and to a lesser degree, a relatively strong wedge). In order for landward vergence to predominate, these three factors must combine in such a way that the backward verging thrust planes are favored. IntroductionAs sediments are incorporated into an accretionary prism, the structural style of the developing prism records a complex interplay of changing sediment and pore fluid properties with tectonic stress. Because the physical properties are not measurable on a regional scale, the processes controlling deformation and fluid flow at convergent margins have been inferred from variation in structural style. The importance of faults as fluid conduits and the role of overpressure in faulting [e.g., Hubbert and Rubey, 1959] have focused particular attention on pore pressure conditions at the d6collement. The d6collement is a primary control on the morphology and structural style of the accretionary prism; the strength, or effective stress, at the d•collement affects the taper and length of the prism, as well Paper number 95TC02320. 0278-7407/95/95TC-02320510.00 as fault geometry and spacing [e.g., Davis et al., 1983; and Platt, 1988]. The stratigraphic position of the d6collement also controls the relative thickness of the accreted and underthrust sections. Beneath the prism, rapid loading due to tectonic thickening can produce high pore fluid pressures on the d6collement; however, this process often begins seaward of the deformation front, where high sedimentation rates or fluid expulsion from beneath the prism produces high pore fluid pressures and lower effective stress along the potential d6collement horizons [Westbr...

Conical Seamount, the edifice drilled at Ocean Drilling Program (ODP) Sites 778 through 780, is a large seamount on the outer half of the 200-km-wide Mariana forearc in the western Pacific. It is forming by the protrusion of cold, unconsolidated serpentine mudflows and debris flows and by vertical tectonic activity. Dredging on the flanks of this seamount recovered rocks and serpentine muds that are similar to sedimentary serpentinite deposits found in subaerially exposed convergent margin terranes worldwide. The dredged samples were formed by different mechanisms from those previously proposed for sedimentary serpentinite deposits. The dredged samples from Conical Seamount are primarily serpentinized harzburgite. However, serpentinized dunite, metamorphosed gabbro, and basalts have been retrieved from similar seamounts on the Mariana forearc. SeaMARC II imagery and bathymetry of Conical Seamount revealed sinuous flow forms on the flanks of the seamount. Conical Seamount also has both concentric ridges and radial fractures indicative of tumescence. Alvin submersible studies showed these flows to be composed of unconsolidated serpentine muds, containing clasts of serpentinized ultramafic and metamorphosed mafic rocks and authigenic carbonate and silicate minerals. Near the summit of one of the seamounts, chimney structures less than 150 yr old and composed of carbonate and silicate were sampled using Alvin. During sampling of a silicate chimney, cold fluids seeped from numerous orifices in the chimney. The fluids associated with the chimney are unique in composition among fluids collected in the oceans and point to a deep source, probably the subducted Pacific lithospheric slab. Small limpets and gastropods and bacterial mats were collected from the chimneys. Faulting of the forearc region partially controls the distribution of the Mariana forearc serpentinite seamounts. Seismic-reflection profiles show that Conical Seamount is located at the intersection of at least two fault zones. The extent of the exposure of serpentinized ultramafic rocks on the Mariana forearc and the pervasive normal faulting of the region argue for considerable extension. Dredged samples from the 2-km-high wall of a deep graben adjacent to Conical Seamount include a variety of mafic rock types of arc tholeiite, boninite, alkalic basalts, and basalts of mid-ocean ridge (MORB) composition. The presence of MORBs in this part of the forearc suggests either exposure of a fragment of entrapped or accreted oceanic plate or a period of rifting with associated magma generation in the forearc. The serpentine mud volcanoes and associated egress of fluids generated at great depth in the forearc provide a mechanism for the flux of fluids from the subducted oceanic lithosphere through the outer forearc region. The escape of circulating fluids would alleviate problems of mass balance of constituents derived from subduction in the convergent margin environment. In particular, the disparity between the observed volatile effluent from arc volcanoes and the ...

No abstract