Controls on bending-related faulting offshore of the Alaska Peninsula

Clarke, Jacob W (2022) Controls on bending-related faulting offshore of the Alaska Peninsula. Masters thesis, Northern Arizona University.

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Abstract

Bathymetric, seismic reflection, and modelling studies have demonstrated that subducting oceanic plates experience extensive normal faulting as they bend and subduct at the trench axis. However, the relative importance of pre-existing (usually abyssal-hill) faults, plate curvature and other factors in controlling the extent and style of bending-related faulting remain debated. The subduction zone off the Alaska Peninsula is an ideal place to investigate controls on outer-rise faulting. Pre-existing abyssal-hill fabric in the study area, inferred to be parallel to magnetic anomalies, show a range of orientations, from east-west trends west of a remnant triple junction at 158W and north-south oriented trends to the east. The orientation of the trench itself varies smoothly over this region, and plate curvature gradually increases to the west. Previous studies with limited data suggest that the style and magnitude of bending faulting also varies along this subduction zone. We analyze new multibeam bathymetry data collected as a part of the Alaska Amphibious Community Seismic Experiment (AACSE) to characterize bending faulting between longitudes of 161W and 156ºW. We also used a compilation of legacy and recently acquired seismic reflection data to constrain patterns of sediment thickness on the incoming plate, since sediment has the potential to mask bending faulting. Finally, we estimated along-strike changes in the degree of slab bending by calculating the bending from the seafloor and the top of oceanic crust, which was estimated using the sediment thickness map. Comparisons between trends of outer-rise faults and magnetic anomalies imply that orientations of faults are strongly influenced by pre-existing structures. We observe a decrease in summed scarp heights from west to east, with the highest amount of faulting in the Shumagin Gap region. Sediment cover increases from west to east, which has the potential to mask bending faulting outboard of the Semidi segment where we observe little to no seafloor fault expressions. However, the increase in sediment is likely due to the existence of the Eocene-to-Oligocene aged Zodiac Fan. Thus, most of these sediments are much older than bending faults and would be expected to be cut by extensional faults, and sediment masking is unlikely to be the cause for the decrease in summed faulting we see from west to east. Therefore, westward increase in the summed scarp heights on bending faults at the seafloor likely reflects an increase in the amount of faulting. The dip angle of the slab also varies along strike, increasing from a minimum bending angle of ~1.2 between longitudes 156-157W to ~2.4 between longitudes 160-161W in the west suggesting that the westward increase in the degree of slab bending contributes to the westward increase in summed faulting. In summary, bending faulting is influenced by both pre-existing structures and slab bending; pre-existing abyssal-hill faults are the primary control on the orientations, and the bending angle of the slab is the primary control on the summed amount of faulting.

Item Type:	Thesis (Masters)
Publisher’s Statement:	© Copyright is held by the author. Digital access to this material is made possible by the Cline Library, Northern Arizona University. Further transmission, reproduction or presentation of protected items is prohibited except with permission of the author.
Keywords:	Alaska Peninsula; Bending-related faulting; Subduction zone; Plate dynamics; Outer-rise faulting; Abyssal-Hill faults;
Subjects:	Q Science > QE Geology
NAU Depositing Author Academic Status:	Student
Department/Unit:	Graduate College > Theses and Dissertations College of the Environment, Forestry, and Natural Sciences > School of Earth Sciences and Environmental Sustainability
Date Deposited:	15 Jul 2022 17:32
Last Modified:	25 May 2023 08:30
URI:	https://openknowledge.nau.edu/id/eprint/5859

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