Monterey Bay Aquarium Research Institute

Gorda Ridge Cruise
August 5 - 21, 2002
Logbook

August 12, 2000: Day #8

Log Entry: Dive 193 was located near the intersection of the Escanaba Trough and the first transform fault that offsets the ridge axis to the east. The dive started in the western valley floor and found young ridge-parallel fissures cutting the sediment cover. The underlying basalt flow was exposed in some of these fissures.

The base of the scarp bounding the valley on the west side consisted of a series of small stair steps with outcrop on the steep faulted steep sections and sediment draping the flat sections. Many of the flat sections are actually back-tilted so the traverse was up steeply followed by a gentle down slope followed again by a steep slope upwards. The outcrops in the lower section are mainly altered dikes and massive lava that has been strongly sheared and fractured. Most of the rocks from this section are low-grade metamorphic rocks with chlorite alteration and some quartz veins.

As we moved upslope the steeper sections were pillow basalts or bedded sediments. The upper section of the dive, at about 2600 m depth, consisted of alternating sediments and talus of pillow fragments with little outcrop. We also encountered a zone of hydrothermally altered lava with bright yellow-orange iron oxidation. Samples from this zone have pyrite present.

The results from this dive suggest that future work here may be able to examine the pillow basalt/dike transition in the ocean crust. At other locations along the transform fault, we may also be able to find exposed ocean-crust layer-3 gabbro. Future dives will be directed at this objective.

The dive also recovered a number of non-vent fauna including two gorgonians, neither seen on previous dives, a second specimen of the "golden cockroach" worm, a small gastropod, and a large seastar with webbing between the arms. We also saw more specimens of the Taney Seamount tunicate, the large "windsock" tunicate we collected earlier, and another tunicate, similar to the Taney one, except more flattened into a disc-shape.

Sunday's dive will be directed at mapping and sampling one of the youngest flows in the Escanaba Trough. The Simrad data suggests that the eruptive fissure is neither ridge-parallel nor located near the neovolcanic zone.


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