Sedimentation patterns, folding, and fluid upflow above a buried basement ridge: Results from 2‐D and 3‐D seismic surveys at the eastern Juan de Fuca Ridge flank

Abstract

During R/V Sonne Cruise SO 149, new two‐dimensional (2‐D) and 3‐D multifrequency seismic data were collected at the eastern flank of the Juan de Fuca Ridge, off the west coast of North America. Two‐dimensional overview lines indicate that turbidity currents moving southward are partly trapped between a buried basement elevation called First Ridge and another elongated basement obstruction further to the west. As a consequence, sediments form a local topographic high and a distinct wedge‐shaped section in between the two ridge segments. On the basis of these seismic observations a simple conceptual model is developed to explain local sediment accumulation patterns at the flanks and on top of First Ridge. This model is consistent with seismic, coring, and drilling results that provide quantitative constraints for the remaining seafloor relief above a fully buried basement high as well as sediment porosity values on top of First Ridge that are higher than those in adjacent troughs. In general, higher porosity values may explain lower seismic reflection amplitudes, but the general trend of porosity increase above First Ridge due to vertical grain size sorting within a turbidity current has to be distinguished from additional, overprinting effects that confine the major portion of the observed reflection amplitude decrease to narrow, vertical zones. At the location of such zones, seismic data indicate that pronounced basement peaks are associated with distinct folding of the sediment layers above. It is thus suggested that the folding and the local seismic amplitude decrease are related. Folding and the associated extensional strain field forming throughout such folds can explain locally high layer inclination and increased porosity, which both affect seismic reflection amplitudes. Folding and associated fracturing may cause locally increased permeability and have the potential to focus hydrothermally driven fluid upflow and to develop the plumbing system required to explain the observed fluid seepage through a fine‐grained sediment seal.