Glacially-derived overpressure in the northeastern Alaskan subduction zone: combined tomographic and morphometric analysis of shallow sediments on the Yakutat shelf and slope, Gulf of Alaska

The Pamplona zone fold and thrust belt is the offshore expression of convergence and shallow subduction of the Yakutat microplate beneath North America in the northeastern Alaska subduction zone. The combination of convergent tectonics and glaciomarine sedimentary processes create patterns of deformation and deposition resulting in a shallow sedimentary sequence with varying compaction, fluid pressure, and fault activity. We propose that velocity variations observed in our tomographic analysis represent long-lived fluid overpressure due to loading by ice sheets and sediments. Regions with bathymetric and stratigraphic evidence of recent ice sheets and associated sedimentation should be collocated with evidence of overpressure (seismic low velocity zones) in the shallow sediments. Here, we compare a velocity model with shelf seismic stratigraphic facies and modern seafloor morphology. To document glacially derived morphology we use high resolution bathymetry to identify channel and gully networks on the western Yakutat shelf-slope then analyze cross-channel shape indices across the study area. We use channel shape index measurements as a proxy of recent ice-proximal sedimentation based on previously published results that proposed a close correlation. Profiles taken at many locations were fitted with a power function and assigned a shape - U-shape channels likely formed proximal to recent ice advances. Detailed velocity models were created by a combination of streamer tomography and pre-stack depth migration velocities with seismic data including: a 2008 R/V Langseth dataset from the St. Elias Erosion and Tectonics Project (STEEP); and a 2004 high-resolution R/V Ewing dataset. Velocity-porosity-permeability relationships developed using IODP Expedition 341 drilling data inform interpretation and physical properties analyses of the shallow sediments. Initial results from a 35 km profile extending SE seaward of the Bering glacier and subparallel to the Bering trough suggest a spatial relationship between the extent of U-shaped profiles and low-velocity shallow sediments. Towards the SE end of the model we observe a large overlap of U-shaped indices, and a shallow low-velocity zone in the mapped extent of the last glacial maximum suggestive of overpressure due to loading by ice sheet activity.