Evolving Neogene Sediment Delivery to and Dispersal in the Aleutian-Alaska Subduction Zone

The Aleutian-Alaska subduction zone is the most tectonically active region in North America and contains pronounced along-strike variations in seismicity, volcanism, and incoming plate properties. Evaluation of the continental margin in the Kodiak Island region has argued for a fundamental change from erosion to accretion within the Neogene related to glacigenic sediment input from the north. Here we review recent geophysical and scientific drilling results to evaluate this hypothesis. Reanalysis of trench wedge sediment thickness coupled with recently acquired geophysical data reveal pronounced along-strike variation in trench sedimentation. Integrated Ocean Drilling Program Exp 341 recovered Miocene-to-recent strata from the Surveyor Fan that is subducting/accreting along the Aleutian Trench. Age control from Site U1417 allows for regional isopach mapping of trench and fan strata and a temporal evaluation of sediment delivery to the trench. The onset of major tidewater glaciation at 2.7 Ma represents a fundamental regional change in sedimentation. Prior to this, pre-Surveyor fan accumulation consisted of pelagic facies interspersed with episodic gravity flow deposition whose deep-water dispersal was dictated by seamounts with a relatively minor flux to the trench. The onset of glacigenic delivery from the coastal St. Elias orogen to the deepwater resulted in the formation of the Surveyor channel, which created the modern turbidite-dominated Surveyor fan and initiated direct sediment delivery to the Aleutian trench. Sediment accumulation on the fan and in the trench doubled after 2.7 Ma. The lack of avulsion of the Surveyor channel since 2.7 Ma may be a consequence of an underfilled trench that has maintained steep topographic gradients on the fan. 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 cross-shelf glacial input sources varied from the St. Elias orogen. The Pleistocene transition in sedimentation in the Aleutian trench to rapidly accumulating fine-grain facies has the potential to alter the physical properties of accreting and subducting sediments and the seismogenic interface between them.