Monterey Bay Aquarium Research Institute

Mendocino Fracture Zone Cruise - 2001
August 20 - 30, 2001
Over 300 km off the California-Oregon Coast

August 22, 2001: Day #3

A look at the bottom from today's dive.

Log Entry: ROV Tiburon began its’ way down to the seafloor this morning at 0635 local time. The wind was blowing at 25 knots, which slowed us down a little bit. When wind speeds start getting above 25 knots, chances of Tiburon being lowered down start diminishing. We were lucky to be able to dive. Tiburon reached sediment-covered seafloor at 0925 am at the depth of 3,154 meters in the Escanaba Trough. At places soft, white, carbonate ooze covered the sediment-water interface. The dive continued south towards the Mendocino Ridge along the sedimented bottom. Several straight, parallel channels could be seen on the sonar image. We crossed a couple of these. The channels lacked major features, although scours and ripples could be observed locally. During the first four hours that Tiburon spent tracking the seafloor we did not encounter any bedrock. The biology, however, showed a little more variability. We captured an interesting looking sponge to bring back up on the ship. In addition to this sponge we saw brittle stars, sea stars, worm burrows, more sponges, crabs and anemones (see framegrab) amongst other creatures. We also encountered white ‘balls’ and ‘mats’ of carbonate ooze in several locations. A pushcore of this material was brought back on the ship for further study.

After four hours we were still at the same depth (around 3,100 meters). Our contour map on ArcView showed that we should have been moving up the uplifted slope of the Mendocino Ridge. We were lost. It was decided that Tiburon would fly south across the water at the depth of 2,800 meters until we hit the Mendocino Ridge. This lost us three hours of dive time. But at least we were back on track and finally hit bedrock at 1720 local time. We expected from last years’ dives that gabbroic bedrock would be exposed at this depth.

One of our aims is to follow a complete vertical crustal section that we believe is exposed as a coherent structure along the fault scarp. Oceanic crust formed at spreading centers (see figure) consists of mantle rock/gabbro at the bottom. A sheeted dykes complex can be found on top of gabbros. These dykes form when hot magma is intruded into oceanic crust from magma chambers underlying the spreading ridge. On top of the sheeted dyke complex extrusive volcanics, such as pillow lavas and glassy hyaloclastites, can be found. Finally, marine sediments, such as carbonates and clays, will be deposited on the top.

We did not discover a complete crustal section exposed but a sediment covered slope. The slope did expose angular blocks of igneous rocks, and we collected several of these for geochemical analysis (see framegrab below). We will have to wait until tomorrow to saw the rocks to see what rock types we encountered. We suspect that we are looking at fault breccia, and the same sediment cover that is present in the Escanaba Trough. This mélange of broken rocks randomly mixed in with sediments is more typical of fracture zones than the coherent slabs we found last year in the Gorda Escarpment.

Tiburon was back on board the Western Flyer at 2150 local time.


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