Investigation of the Deformation along an Oblique Convergent Margin, Aleutian Trench



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Deformation of the Aleutian Arc is widely interpreted to occur due to oblique convergence between the North American and Pacific plates. Due to the obliquity between the relative plate motion and the strike of the margin, the strain within the accretionary prism and forearc is interpreted to be partitioned resulting in the formation of five crustal blocks and a forearc sliver. The aim of this study is to better understand the geometry of these features and their potential kinematic role in accommodating oblique convergence. Forearc partitioning, rotation and translation of the forearc sliver plate are evaluated using: 1) data from 6 GPS stations collected between 2004 and 2015, 2) 1,798 earthquake events (focal mechanisms and depth) that occurred between January 1976 to April 2015, 3) plate convergence directions and velocities, and 4) trends of Holocene magmatic features. My results show that the boundary of blocks including what I interpret as a forearc sliver correlate to the angle of obliquity between the North American and the Pacific plates. Strain-partitioning structures in the Aleutian arc develop where the angle of obliquity is ≥ 20. The boundary between blocks undergoing vertical axis rotation and the forearc sliver corresponds to an angle of obliquity equal to 70 (170E). This position in the arc also correlates to a change in the azimuth of Holocene volcanic features where they trend subparallel to the strike of the arc. Four GPS stations are located within the arc. Velocities of these stations increase toward the west. These GPS stations’ velocities cannot represent the blocks’ rotations by themselves but they can represent the movement direction of the block limits. To find the pivot axis of the arc, three GPS stations were chosen as fixed locations. My results show that the Aleutian Arc is undergoing clockwise rotation with respect to Station AB02 in the central portion of the Aleutian at 170W, which is the pivot point of the localities where these stations exist. This indicates that the angle of obliquity west of 170W is increasing. Earthquake depths show that the angle of subduction is moderate in the east and shallow in the west. Arc-parallel and arc-normal convergence rates studies correlate with convergence obliquity along the arc.



Aleutian Arc, Oblique convergence, Forearc, Forearc sliver, Forearc partitioning, Strain-partitioning