Seafloor fluid vents imaged by 3D seismic data on the upper Amazon deep-sea fan

Description

Submarine gas hydrate systems are increasingly recognised to be associated with seafloor features recording the venting of gas-rich fluids to the deep ocean. The Amazon deep-sea fan contains a gas hydrate system characterised by a discontinuous bottom simulating reflection (BSR) forming elongate patches that follow the crests of thrust-folds within an upper slope (500-2250 m) compressional belt related to the gravitational collapse of the fan. Multibeam hydroacoustic data have previously shown the upper slope to be associated with dozens of water column gas flares, most rising from high backscatter seafloor mounds from two of which gas hydrates have been sampled. Here we present examples of seafloor features observed using bathymetry derived from extracts of a commercial 3D seismic volume (10 m grid resolution). In places, the data reveal fields of sub-circular morphological features up to tens of meters in relief and hundreds of meters in width. The features include a variety of positive and negative morphologies, including mounds, depressions, and mounds within depressions. The morphology of the mounds allows them to be interpreted as small mud volcanoes, some with crater-like depressions at their summits; some mud volcanoes sit within larger depressions interpreted to represent calderas due to localised subsidence. A few unfilled depressions are observed and may be calderas or large pockmarks. Seismic profiles show the seafloor features to be underlain by chaotic vertical columns consistent with plumbing systems, which pass through the BSR patches to root in subjacent thrust-folds. Some seafloor features, or groups of features, are associated with 'pluming' BSRs that indicate the rise of warm fluids; however, only a few coincide with water column gas flares previously identified on multibeam imagery, possibly reflecting episodic activity. The surprising variety of relatively small extrusive morphologies observed on the upper Amazon fan underscores the potential of high resolution seafloor datasets to provide new insights into the processes and products of deep-sea fluid venting.

Abstract

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