Hydrothermal Circulation in the Oceanic Crust: Eastern Flank of the Juan de Fuca Ridge

Abstract

Ocean Drilling Program Leg 168 investigated three hydrothermal regimes located on the sediment-covered eastern flank of the Juan de Fuca Ridge. The three regimes are (1) a transition from very young (<0.8 Ma) unsedimented igneous crust to sediment-buried crust (0.8-1.2 Ma), (2) an older (~3.5 Ma) area with large topographic relief (>300 m) of igneous crust under a thick (250-600 m) sediment cover, and (3) an intermediate age (1.4-2.6 Ma) area of low relief igneous crust buried continuously over tens of kilometers by sediments averaging about 300 m thick. Ten sites were cored and roughly 1500 m of sediment and basalt was recovered. A total of four CORKs were emplaced, with thermistor strings and osmotic water samplers, in Holes 1024C, 1025C, 1026B, and 1027C. Temperature, pressure, and water chemistry data will be recorded from these holes over the next 3 to 5 years. The first direct BACKGROUND

Thermally Significant Fluid Seepage Through Thick Sediment on the Juan de Fuca Plate Entering the Cascadia Subduction Zone

“…The observed basement overpressure at Site 1026 (18 kPa) does not drive detectable seepage through the 229 m thick mud‐dominated sediment column immediately northwest of Nitinat Fan (Figure 5b). For Site 1026, we estimate a range of hydraulic impedance values based on the lithostratigraphy at the site and porosity‐depth and permeability‐porosity relationships (Shipboard Scientific Party, 1997b; Spinelli et al., 2004). The estimated ∼13 kPa overpressure at Sites 1030 and 1031 drives vertical seepage at 0.19–0.32 cm yr −1 through a 41–47 m thick column of silt turbidites and hemipelagic mud (Figure 5b) (Giambalvo et al., 2002; Shipboard Scientific Party, 1997c; Spinelli et al., 2004).…”

“…The basal hemipelagic muds, predominantly silt and silty clay, are characterized in the ODP Leg 168 drilling transect and at Deep Sea Drilling Project (DSDP) Site 174 (Figure 1a) (Shipboard Scientific Party, 1973, 1997a; Underwood et al., 2005). At the eastern end of the Leg 168 drilling transect (3.58 Ma lithosphere), the basal hemipelagic muds are up to ∼100 m thick in basement troughs (Shipboard Scientific Party, 1997b; Underwood et al., 2005). At Site 174 (∼7.1 Ma lithosphere), they are ∼625 m thick (Shipboard Scientific Party, 1973).…”

Characterizing borehole fluid flow and formation permeability in the ocean crust using linked analytic models and Markov chain Monte Carlo analysis

“…The area is characterized by thick sediments overlying relatively young crust, due to high sedimentation rates and the infilling of turbidites above abyssal hill topography. Hole 1026B was drilled in 1996 on ODP Leg 168 as part of an east‐west crustal age transect [ Davis et al ., ]. Although the hole was initially drilled through 247 m of sediments and 52 m into volcanic rocks, upper basement was unstable.…”

“…Although the hole was initially drilled through 247 m of sediments and 52 m into volcanic rocks, upper basement was unstable. A “liner” of drillpipe was drilled into the rubble at the bottom of the hole to keep the hole from collapsing before installation of a long‐term borehole observatory (CORK) [ Davis et al ., ]. A thermal log collected 13 days later using an autonomous temperature probe lowered by wireline showed that warm formation fluid was moving rapidly up the hole from depth, indicating that the natural formation overpressure had overcome the differential pressure induced by drilling and other operations [ Fisher et al ., ] (Figure ).…”

Observation of the 2009 Samoa tsunami by the NEPTUNE-Canada cabled observatory: Test data for an operational regional tsunami forecast model

“…The tsunami network measurements began on September 26, 2009 with the installment of six bottom pressure recorders (BPRs) extending from the coast to central Cascadia Basin (Figure 1). At the core of the network is a triangular “antenna‐like” array of BPRs at depths of roughly 2600 m centered on Ocean Drilling Program (ODP) Circulation Obviation Retrofit Kit (CORK) observatory site 1026b [ Davis et al ., 1997]. This tsunami detection array (see Figure 1, Inset B) was intended to consist of a central CORK station and three branches terminated by BPRs 1027‐NW, 1027‐S, and 1027‐NE, each located 12.5 km from the CORK site.…”