GEOLOGY OF MIDDLE VALLEY

Middle Valley, Juan de Fuca Ridge, is an intermediate rate (58 mm/yr) spreading center that forms the ridge component of a ridge-transform-transform unstable triple junction (Fig. F1) (Davis and Villinger, 1992). Reduced magma supply along the Juan de Fuca Ridge has resulted in a ridge morphology that more closely resembles a slow-spreading center (Davis and Villinger, 1992). The ridge is buried by 200 to >1000 m of turbiditic sands and hemipelagic sediments supplied during the Pleistocene glacial low stand of sea level.

Two areas of venting at Middle Valley, the area of active venting (AAV) and Bent Hill, were drilled during Leg 169. This paper deals only with the Bent Hill locality because these cores recovered not only massive sulfide but also the underlying sulfide feeder system. Both the AAV and Bent Hill are located between the north-south trending rift-bounding faults located ~15 km apart. Bent Hill lies ~3 km west of the eastern bounding fault and is one of several linear mounds that parallel the eastern normal fault scarp (Mottl, Davis, Fisher, and Slack, 1994). A ridge-parallel normal fault underlies the western edge of the BHMS and the hills that lie farther south (Goodfellow and Franklin, 1993; Davis et al., 1992). The BHMS deposit lies ~100 m south of Bent Hill (Fig. F2), is ~35 m high, extensively weathered to iron oxyhydroxides, and partially buried by sediment.

The second sulfide mound (Ore Drilling Program Mound [ODP Mound]) is ~330 m south of the BHMS mound and lies along the same north-south trending fault that underlies Bent Hill. ODP Mound is actually two adjacent 12-m mounds. It is believed that ODP Mound is younger than the BHMS because of the lack of sediment cover and limited oxidation of the sulfide phases (Fouquet, Zierenberg, and Miller, et al., 1998). One 264°C hydrothermal vent is present on the northern flank of the deposit (Davis and Villinger, 1992).

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