Leg 169 will drill in two areas in the northeast Pacific; Middle Valley at the northern end of the Juan de Fuca Ridge and Escanaba Trough at the southern end of the Gorda Ridge (Fig. 1). Both sites are sediment-covered seafloor spreading centers that contain both active hydrothermal systems and massive sulfide deposits formed from recently active hydrothermal venting.
Middle Valley forms one leg of a Ridge-Transform-Transform unstable triple junction with the Sovanco fracture zone and the Nootka fault (Fig. 1A; Davis and Villinger, 1992). Middle Valley is a medium-rate spreading center (58 mm/yr), but the proximity to the cold Explorer plate results in a reduced magma supply and a slow-spreading ridge morphology with a deep and wide axial trough. A ridge jump is in progress and current magmatic activity is mostly confined to the West Valley spreading center. Proximity of the Middle Valley spreading center to an abundant supply of terrigenous sediment during the Pleistocene low stand of sea level has resulted in burial of the spreading center by 200 to >1000 m of turbiditic and hemipelagic sediment, with sediment thickness increasing to the north. Two main areas are targeted for drilling (Fig. 1B), the Dead Dog vent field in the area of active venting (Site 858) and the Bent Hill area (Site 856).
Dead Dog Vent Field
The principle center of hydrothermal activity in Middle Valley is the Dead Dog vent field (Fig. 4). Contoured heat flow values show a concentric high which is coincident with a side scan acoustic anomaly that outlines the vent field. The vent field contains at least 20 active vents with temperatures ranging up to 276°C (Ames et al., 1993). Active vents occur predominantly on top of 5-15 m high sediment-covered mounds a few tens of meters in diameter. The vent fluid composition indicates significant interaction of hydrothermal fluid with sediment and the resultant chimneys are predominantly composed of anhydrite with only minor Mg-rich phyllosilicates and sulfide minerals. Available data from piston cores and ODP Hole 858B suggest that subsurface deposition of anhydrite, Mg-rich smectite, and sulfide minerals contribute to the growth of the mounds. Surface deposition of collapsed chimney debris may also contribute to the growth of the mounds, but appears to be of relatively minor importance. Following collapse of the unstable chimney structures, the anhydrite dissolves in the cold seawater. The uppermost sediment recovered from Hole 858B appears to have formed in this manner, however, this layer is only a few meters thick and does not account for the bulk of the anhydrite, Mg-smectite, and sulfide that occurs at greater depth within the mound. Because the high temperature hydrothermal fluid is strongly depleted in both Mg and SO4, the abundance of these minerals in the subsurface requires that cold seawater (with abundant Mg and SO4) is drawn into the subsurface by the vigorous upflow at the active vent sites.
Seismic profiles across the vent field show it is located about 2 km east of prominent basement fault (Fig. 5; Rohr and Schmidt, 1994). Sediment thickness over the fault block in the area surrounding the vent field is approximately 450 m and overlies a sill-sediment complex that forms the transition to oceanic crust (Davis, Mottl, Fisher, et al., 1992). However, hard acoustic reflectors that occur only immediately beneath the vent field were confirmed by drilling to be the top of a volcanic edifice at only 250 m depth (Fig. 6). The presence of more permeable volcanic basement penetrating up into the sediment cover acts as a conduit to focus flow of hydrothermal fluid to the seafloor (Davis and Fisher, 1994).
Bent Hill
Bent Hill is one of a string of small topographic highs that run parallel to the eastern rift bounding normal fault scarp (Fig. 1B). These bathymetric highs include volcanic cones to the south where sediment cover thins and uplifted sediment hills to the north. These features lie close to a normal fault that offsets basement reflectors (herein referred to as the Site 856 fault), but near surface sediment layering appears to be continuous across this fault. The transition from essentially nonmagnetic oceanic crust that typifies the center of Middle Valley to crust with normal levels of magnetization passes through this area and probably marks the boundary between normal extrusive basalt and the sill-sediment complex that forms the upper oceanic crust in center of Middle Valley (Currie and Davis, 1994). Bent Hill is a roughly circular feature 400 m in diameter that has been recently uplifted approximately 50 m (Fig. 7). It is bounded on the west by a steep scarp that parallels the rift bounding faults and exposes semiconsolidated turbiditic sediment. A very primitive olivine-rich sill, which is peterogenetically distinct from the diabases and basalts recovered by drilling elsewhere in Middle Valley, was recovered at the base of the two drill holes that penetrated Bent Hill (Fig. 8). Bright, reverse polarity seismic reflections that are limited in extent to the area under Bent Hill are interpreted to be generated at the interface between the base of these sills and the underlying sediments (Rohr and Schmidt, 1994).
A ridge of massive sulfide that rises 35 m above the surrounding turbidite fill of the valley is located approximate 100 m south of the southern edge of Bent Hill and is referred to here as the Bent Hill deposit (Figs.Fig. 7, Fig. 8). The massive sulfide mound is extensively weathered to iron oxyhydroxides and partially buried by sediment. Massive sulfide extends a minimum distance of 60 m N-S and 90 m E-W. Hole 856H penetrated 94 m of massive sulfide (Fig. 8) before the hole had to be abandoned due to inflow of heavy sulfide sand from the upper weathered section of the borehole wall. A strong magnetic anomaly across this mound is related to late stage hydrothermal alteration of pyrrhotite to pyrite and magnetite and has been modeled to suggest that mineralization continues at least another 30 m below the level drilled and possibly much deeper (Tivey, 1994).
A second mound of massive sulfide occurs approximately 300 m further south and is referred to here as the Sunnyside Up deposit. The morphology, degree of oxidation, and sediment cover indicate that this deposit is younger than the Bent Hill deposit. A single 264°C hydrothermal vent is present on the north flank of this deposit. Contoured heat flow values for the Bent Hill area show high values centered around this active vent. The composition of the vent fluid is similar to those from the Dead Dog vent field, but this vent has lower salinity and only half as much dissolved Ca.
The Gorda Ridge spreading center is located offshore of Oregon and northern California and is bounded by the Mendocino Fracture Zone on the south and the Blanco Fracture Zone on the north (Fig. 1C). A small offset in the spreading axis at 41°40'N latitude marks the northern boundary of Escanaba Trough, which forms the southernmost part of Gorda Ridge. Escanaba Trough is opening at a total rate of approximately 24 mm/yr and has a morphology consistent with the slow-spreading rate. The axial valley, which is at a depth of 3300 m, increases in width from about 5 km at the north end to more than 15 km near the intersection with the Mendocino Fracture Zone.
South of 41°17'N latitude, the axial valley of Escanaba Trough is filled with several hundred meters of turbiditic sediment (Figs. 9, 10). The sedimentary cover thickens southward and is a kilometer or more in thickness near the Mendocino Fracture Zone. Turbiditic sediment enters the trough at the southern end and is channeled northward by the axial valley walls (Vallier et al., 1973; Normark et al., 1994). Sedimentation was relatively rapid (up to 5 m/1000 yr) during low stands of sea level in the Pleistocene, and the entire sediment fill of the trough was probably deposited within the last 100,000 years (Normark et al., 1994; Davis and Becker, 1994a).
Seismic reflection surveys show that the floor of Escanaba Trough is generally a smooth, flat plain underlain by continuous and relatively undisturbed turbidites (Fig. 11; Davis and Becker; 1994a; Morton and Fox, 1994). However, local areas along the axis of spreading have irregular seafloor topography characterized by circular hills 0.5 to 1.2 km in diameter that are uplifted 50 to 120 m above the surrounding seafloor. The sediment cover in these areas is described as moderately to highly disturbed based on the discontinuity or absence of seismic reflectors (10; Morton and Fox, 1994). Morton et al. (1994) mapped the distribution of the topographically rough, seismically disturbed zones, which typically are 3 to 6 km wide oval-shaped areas aligned along the spreading axis. The strongly disturbed zones are also areas of high heat flow (Fig. 11; Davis and Becker, 1994a).
The areas of sediment disruption are sites of recent axial rift igneous activity. The geologic and geophysical evidence suggests that axial rift igneous activity at these sites is manifested by the intrusion of dikes, sills, and laccoliths into the sediment with less abundant volcanic flows (Morton and Fox, 1994; Zierenberg et al., 1994). Sulfide mineralization has been sampled by dredging, sediment coring, or submersible at four igneous centers within the sediment-covered part of Escanaba Trough. The northern Escabana Trough study area (NESCA) (Figs. 10, 12) contains several large massive sulfide deposits including an area of active hydrothermal venting. The dominant morphologic features in the NESCA are the Southwest (SW) Hill and the Central Hill (Figs. 10, 12). The SW Hill is an elongated sediment hill that has been uplifted by 120 m above the surrounding turbidite plane. The steep sides of the hill expose semiconsolidated turbiditic sediment. Massive sulfide deposits occur at the base of the scarp that bounds the uplifted sediment hill. SW Hill is interpreted to have formed by uplift of sediment over a laccolithic sill; high permeability fault zones that accommodated the uplift provided pathways for flow of hydrothermal fluid to the seafloor (Denlinger and Holmes, 1994).
A large exposure of volcanic rock occurs east of the crest of the Central Hill (Fig. 12). The elevated area east of the Central Hill is covered by glassy basalt pillows 1 to 2 m in diameter. Lava tubes drape the north flank of the hill indicating flow to the north. These lava tubes fed sheet flow basalts that ponded within the central depression of the spreading center. The area of Central Hill west of the outcropping pillow basalt is interpreted to have been uplifted by intrusion of basalt into the sediment. The western, sediment-covered part of the Central Hill contains the most extensive sulfide deposits observed in Escanaba Trough. The massive sulfide deposits on the west and southeast flanks of the Central Hill are actively venting hydrothermal fluid, and the area on the northern flank shows indications of very recent hydrothermal activity, suggesting that these deposits are all part of the same hydrothermal system. An extensive area of massive sulfide is exposed on the north slope of the Central Hill. Massive sulfide extends more than 270 m from north to south and more than 100 m from east to west, but the western edge of the deposit has not been defined with certainty. Within this area there is nearly continuous outcrop of massive sulfide with few sediment-covered areas. The best explored and most hydrothermally active area of sulfide mineralization on the Central Hill extends west from the northern end of the sediment covered hill top (Fig. 12). This is not an area of continuous sulfide outcrop, but rather a region of abundant, closely spaced sulfide mounds. The mounds are typically 20 to 60 m in diameter and 5 to 10 m high. Two mounds were observed actively discharging high-temperature hydrothermal fluid; one near the eastern margin of the sulfide area was venting 217°C fluid, and one on the western edge of the explored area was venting 108°C fluid (Fig. 12). Even though these mounds are 275 m apart, the major-element composition of the end-member fluid at each vent is identical (Campbell et al., 1994), a result that is consistent with the hypothesis that this large mineralized area is a single hydrothermal system hydrologically interconnected at depth.
To 169 Proposed Operational Sites and Drilling Plan/Strategy