Monterey Bay Aquarium Research Institute

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

August 28, 2001: Day #9

A lovely crab cruising on the bottom.

Log Entry: This is the final update from the Mendocino Transform Zone expedition. Today’s dive was the final dive of our seven dive series, and dedicated to the geochemistry of the Gorda Escarpment. Our first dive confirmed that the western part of the Mendocino Transform Zone, the Mendocino Ridge, is likely to be a tectonic melange where slices of ocean crust of different origin have been stacked together as a result of the strike-slip motion of the fault. The evidence for this comes from the very brecciated fault rocks collected,. The geochemistry results thus far show poor correlation between the units.

We have proposed that the eastern part of the Mendocino Transform Fault, the Gorda Escarpment, is a more coherent structure where slices of intact pieces of crust have been uplifted in a transpressional setting. The accretion of crust from the Gorda Plate onto the Pacific Plate may in fact be related to the reorientation of the Mendocino Transform to be more parallel to the regional plate motion. The geochemistry shows a good correlation between different units. We hoped to find evidence for this on today’s dive, and we did indeed find a sheeted dyke complex (see picture). Sheeted dykes form feeder channels for magma when new ocean crust is being formed at spreading ridges (see also day 2 update). The fact that we observed a whole intact sheeted dyke complex with possible gabbro contact directly below (which would have been the source of magma at a spreading ridge) strongly suggests that the crust at the Gorda Ridge was uplifted as a coherent structure.

We got a good variety of samples for geochemical analysis, and were very happy with today’s dive. One of the more interesting outcrops included a zone of hydrothermal alteration where we collected both rocks and push cores. We ended the dive in an ancient beach horizon, with pockets of black sand from eroded manganese and glass particles. This is evidence that portions of the Gorda Escarpment were uplifted above sea level at some point in the past. Unfortunately we had to cut today’s dive short because the wind was blowing at almost 40 knots, and we could not keep Tiburon in the water at those wind speeds. Overall, we have had a great trip with lots of exciting geology.

- Jenni Kela

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