Décollement initiation at the Cascadia Subduction Zone from Full-Waveform Inversion

Abstract:

At the Cascadia subduction zone where the incoming Juan de Fuca plate is covered by 3-4 km thick sediment, the stratigraphic level of décollement varies significantly along strike. This results in changes in the thickness of the subducting sedimentary column from a minimum (0-0.5km) offshore Washington to a maximum (1.4-1.7 km) offshore central Oregon. To investigate the sediment properties associated with décollement initiation and the conditions that give rise to different décollement depths, we conducted 2-D elastic full waveform inversion on multichannel seismic data along two transects offshore central Oregon and Washington. We inverted refracted and reflected energy on shot gathers downward extrapolated to close to seafloor and resolved the fine-scale velocity structure of the incoming sediments within 40 km seaward of the deformation front. Offshore central Oregon, our model shows that a ~400 m thick low-velocity interval initiates ~7 km seaward of the deformation front beneath the stratigraphic boundary between Astoria Fan sediments and abyssal plain turbidites. This low velocity interval is likely associated with anomalously high porosity that developed due to poor drainage beneath a thin layer of low permeability. The sediment section beneath this low velocity interval is delayed in its consolidation process. Further landward, décollement develops within this interval with along-strike variations in depth of a few hundred meters. In contrast, offshore Washington, we do not observe low velocity intervals in the incoming sediment section near the deformation front and the décollement is only ~200 m above the basement. This suggests that the incoming sediment section offshore Washington is well drained. Our results suggest that the presence of a low permeability layer at the base of Astoria Fan sediments may play an important role in forming a shallow décollement offshore central Oregon.