Geologic Map of lower ocean crust at Pito Deep (S Central Pacific)

Gess, Michelle
The formation of ocean crust at mid-ocean ridges is one of the fundamental processes of plate tectonics and Earth evolution. However, our knowledge of how the lower gabbroic ocean crust grows is limited due to inaccessibility. Gabbroic rocks constitute > 75% of the ocean crust generated at fast spreading mid-ocean ridges, but are typically buried beneath ~1km of sediment, basalts and dikes which inhibits sampling. During January and February of 2017, a 42-day research cruise explored >35 km along a flow line of in-situ fast spread lower crust at Pito Deep (southern Pacific). There, the ridge-perpendicular exposures along the steep scarps (with up to 3 km of vertical section) of the Pito Deep canyon provide an unique opportunity to sample this lower gabbroic crust, and therefore differentiate between models for accretion and growth using petrologic, geochemical, and microstructural data. Over 400 samples were collected by the remotely operated vehicle (ROV) system JASON II using hydraulic arms and high resolution cameras to illuminate the rock exposures, with the majority of the samples being gabbroic. Post-cruise analysis of ROV dive videos (recording sample collection and outcrops along the sea floor) allows construction of the first geologic map of in situ lower ocean crust (>10 km2 of seafloor). Geologic features emphasized in the map include magmatic foliations, layering, cross cutting fractures, faults, and associated striae. This structural synthesis allows a new evaluation of the construction of lower ocean crust which was not previously possible.
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