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Escanaba Trough is the southern sediment-filled portion of the Gorda mid-ocean Ridge. It extends northward from the Mendocino Fracture Zone about 130 kilometers, to the southernmost right-lateral offset on Gorda Ridge, at about 41°35’N. The spreading rate along this part of the Gorda Ridge is about 2.3 centimeters per year. The valley floor is 3-5 kilometers wide at the northern end and widens to about 18 kilometers near the Mendocino Fracture Zone. South of about 41°8’N, the axial valley is filled by as much as 500 meters of sandy turbidites. These turbidites derived from the Columbia River drainage during floods produced by outbursts from glacial lakes, (including lake Missoula) during the Late Pleistocene (Zuffa et al., 2000). The sediment fill is pierced by a series of discrete volcanic centers that uplifted the sediment over sills and erupted some lava flows on the seafloor (Morton et al., 1987). The sediments are also disrupted by faulting within the trough, indicating deformation and extension continued after deposition of most of the sediment. Uplifted sediment hills above some of the sills have been used to estimate the timing and amount of vertical motion using the thickness of specific turbidite units (Normark, and Serra, 2001). The volcanic centers have been mapped in some detail using towed photographic systems and observations from submersibles (Ross et al., 1994). The lavas recovered have been studied to evaluate assimilation of sediment during shallow crustal storage and fractional crystallization (Davis et al., 1994, 1998). These uplifted hills are also the site of extensive hydrothermal mineralization, although active vents are restricted to a single region near 41°N named NESCA (Morton et al., 1994). NESCA and SESCA are acronyms for Northern and Southern ESCAnaba, two regions where extensive study has been conducted. A single volume (Morton et al., 1994), published as U.S. Geological Survey Bulletin 2022 in 1994, summarizes nearly all previous work. Subsequent to these studies, the Ocean Drilling Program drilled a series of holes at the NESCA site to investigate hydrothermal circulation and massive sulfide formation (Fouquet et al., 1998). Numerous papers on the results of the drilling program can be found in the ODP Initial Reports volume 169.
The new high-resolution bathymetry shows the faults that offset the sediment fill in Escanaba Trough in great detail. In addition, it is evident that there are two types of sills present: the small deep sills that uplift sediment hills, as described by Morton et al.(1987, 1994) and a second type that are shaped like large lobate flows. These flow-shaped sills are apparently much shallower in the sediment and have uplifted large regions by 30-50 meters. Recent remotely operated vehicle (ROV) dives on the tops of several of these sills shows extensive hydrothermal deposits and outcrops around the margins of the sill where the sediment has slumped.
Escanaba Trough perspective views
|From the north||From the south-southeast|
|Northern end of Escanaba Trough showing volcanic cones, from the east||Southern Escanaba Trough from the southeast|
|Northern Escanaba Trough from the northeast||Southern Escanaba Trough showing faults in sediment, from the southeast|
|Index Map||Axial Volcano||Gorda Ridge||Cleft Segment||Taney Seamounts|
|Guide, Gumdrop, Pioneer Seamounts||Mendocino Fracture Zone||President Jackson Seamount||Davidson Seamount||Vance Seamounts|