Physiography of the Eastern Mendocino Ridge, NE Pacific

Abstract

The bathymetry of the eastern 850 km of Mendocino Ridge (MR) was mapped using NOAA Ship Okeanos Explorer’s Kongsberg EM302 multibeam as part of mapping trials to support the University of New Hampshire’s U.S. Law of the Sea mapping program. The ridge was mapped from the margin to ~500 km west of the intersection of the ridge with Gorda Ridge (GR) at a spatial resolution of 40 m/sounding. The mapped section can be subdivided into two morphologic styles. East of the GR intersection, along the transform fault, MR is a single 4-18 km wide ridge with summit depths of 1200 to 1700 m deep with slopes of 10-40°. West of the intersection, along the fracture zone, MR has summit depths of >2100 m deep. A series of drag folds formed by basement ridges west of GR and north of the fracture zone reflects the differential spreading rates of GR. The almost 90° bend in the drag folds is often accompanied by large volcanic fields with summit craters in many volcanoes in water depths of 2 to 4 km. Beginning ~45 km west of GR intersection, MR evolves into a series of en echelon ridges striking 290°, some with slopes as steep as 55°. The change in morphology appears to be the effects of compression in the transform fault section between the Gorda Plate to the north and the Pacific Plate to the south, whereas extension is evident in the fracture zone section of the ridge. Mendocino Channel immediately to the north of the easternmost MR was mapped from the margin to ~105 km to the west before the channel relief was below the resolution of the multibeam system. The channel walls have as much as 50 m of relief with decreasing relief down channel. Mendocino Channel is bordered on the north by a large field of levees with bedforms and on the south by the 1200 m high wall of MR. Several large sediment failures have occurred in the levee field. The channel is straight for the first 40 km with an average channel slope of 0.7° but suddenly begins to meander when the channel slope is <0.5°. The channel has meander loops, cut-off loops and even “islands” within the channel. Two opposing tectonic hypotheses have been proposed for the transform-fault section of MR, each based on either seismic-reflection profiling or earlier multibeam bathymetry and dredges. Seismic profiling suggests the transform-fault section of the ridge is under compression and uplift. Earlier multibeam data, together with a few dredged samples suggest the ridge has a series of wave-cut terraces and dredged rounded cobbles suggest reworking at wave base; consequently, these data were interpreted to support a subsidence history to this part of the ridge. When the new multibeam bathymetry is overlain on the earlier generation of bathymetry, it becomes clear that the appearance of terraces is the result of the poor resolution of the older multibeam system and the gridding parameters used to generate the digital terrain model. The new bathymetry shows no indications of terraces on this section of MR. The rounded cobbles remain an enigma. The new data show the ridge to be a single, steep-sided ridge in the transform-fault section, suggesting compression (and uplift), as compared to multiple-ridges and pull-apart basins in the fracture-zone section, suggesting extension.

Department

Center for Coastal and Ocean Mapping

Publication Date

12-2009

Volume

90, Issue 52

Journal Title

Fall Meeting, American Geophysical Union (AGU)

Conference Date

Dec 14 - Dec 18, 2009

Publisher Place

San Francisco, CA, USA

Publisher

American Geophysical Union Publications

Document Type

Conference Proceeding

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