Seismic reflectivity anomalies in sediments at the eastern flank of the Juan de Fuca Ridge: Evidence for fluid migration?

A high-resolution seismic survey was carried out in the vicinity of Ocean D rilling Program (ODP) Leg 168 drill sites at the eastern flank of the Juan de Fuca Ridge. Three seismic systems with different source frequencies up t o 4000 Hz were used simultaneously along each seismic line. The data sets w ere combined to provide the best possible resolution at any given depth lev el. An integrated interpretation of narrow zones of low reflection amplitud es is presented, which are related to basement highs but are independent of seafloor topography. The effect is most pronounced above a buried basement ridge in the vicinity of ODP Sites 1030 and 1031, where higher porosity va lues are present. Higher porosities can reduce seismic impedance contrasts and may therefore cause the observed low reflection amplitudes. In addition , Biot-Stoll's [Biot, 1956a, b; Stoll, 1989] model suggests that attenuatio n coefficients are porosity dependent and reflection amplitudes are further decreased at higher seismic frequencies when porosity increases. Since zon es of higher porosities are potential pathways for fluids, they may be asso ciated with hydrothermally driven fluid discharge, which was previously. pr edicted above the buried basement ridge. It is therefore proposed that the observed seismic amplitude reduction in the vicinity of ODP Sites 1030 and 1031 indicates zones of upward fluid migration.