FEEDING A SUBDUCTION ZONE FROM THE SIDE: NEOGENE SEDIMENT DELIVERY TO AND DISPERSAL ALONG THE ALEUTIAN-ALASKA SUBDUCTION ZONE

The 2-D view of sediment dispersal from the arc across to the trench overlooks the pronounced lateral input of mass into many subduction zones. The Aleutian-Alaska subduction zone is the most seismically active region in North America and contains pronounced along-strike variations in seismicity, volcanism, megathrust locking, and incoming plate properties. Evaluation of the continental margin in the Kodiak Island region argues for a fundamental change from erosion to accretion within the Neogene related to glacigenic sediment input from the north rather than across the arc/prism. We review geophysical and scientific drilling results to evaluate this hypothesis as a contribution to the Landscapes to Seascapes component of SZ4D. Reanalysis of seismic reflection profiles reveals pronounced along-strike variation in trench sedimentation. Age control from Integrated Ocean Drilling Program Expedition 341 Sites U1417 and U1418 allows for regional isopach mapping of trench and fan strata. The onset of tidewater glaciation at ~2.7 Ma represents a fundamental regional change in sedimentation. Prior to this, pre-Surveyor fan accumulation consisted of pelagic and episodic gravity flow deposition whose deep-water dispersal was dictated by seamounts with a relatively minor flux to the trench. Onset of glacigenic delivery from the coastal St. Elias orogen resulted in the formation of the deepwater Surveyor channel, creating the modern turbidite-dominated Surveyor fan delivering sediment to the Aleutian trench. Cross-trench/prism pre-stack depth migrated seismic reflection data reveal the décollement off Kodiak Island sits on the contact between these two depositional units. Transition to longer duration glacial conditions starting ~1.2 Ma resulted in increased flux through Surveyor Channel but also significant increases in along-strike flux down the Aleutian Trench as glacial cross-shelf troughs routed sediment from the Chugach-St. Elias orogen. 2 km+ thick trench fill is sourced from the Bering Glacier-Prince William Sound region and has likely accumulated in < 0.5 Ma. The rapid Pleistocene transition in sedimentation in the Aleutian trench to rapidly accumulating fine-grain facies appears to alter the physical properties of accreting and subducting sediments and the seismogenic interface between them.