The case against porosity change; seismic velocity decrease at the toe of the Oregon accretionary prism

As sediments are incorporated into the first thrust sheet of the landward-vergent Oregon accretionary prism they undergo a decrease in interval velocity. This trend is opposite that observed at other accretionary prisms, where an increase in velocity normally accompanies the loss of porosity during deformation. Velocities were obtained from focusing analysis and iterative prestack depth migration of multichannel seismic data for four stratigraphically defined intervals. The shallowest interval (<500 m of sediments) shows a slight decrease in velocity toward the thrust. The two middle intervals ( approximately 1000 m of sediments) show an initial velocity increase in the hinge of the landward-vergent thrust sheet followed by a pronounced velocity decrease (300-700 m/s) toward the thrust, resulting in lower velocities in the deforming thrust sheet than in the undeformed basin sediments. Velocities increase toward the thrust in the deeper fourth interval. The lateral changes in interval velocity reflect the combined effects of vertical (overburden) and horizontal (tectonic) stresses on the deforming sediments. When the effect of changing subbottom depth is removed the same general pattern is observed, although the magnitude of velocity decrease is reduced approximately 50%. It is possible that the velocity decrease indicates a porosity increase in the upper sediments due to extensional failure during fold development; however, a more geologically reasonable explanation is that the rigidity of the sediments decreases due to loss of incipient cementation during deformation.