Submarine Volcanic Cones in the Central Aleutian Arc: Relationship to Arc Rifting and Oblique Plate Convergence

Plate convergence along the 2200km Aleutian Arc varies from orthogonal at the Alaskan Peninsula to fully strike-slip on the west end of the arc. Deformation response of the upper plate to oblique convergence appears to accelerate markedly between Adak (177W) and Amchitka Pass (180W). On a regional scale, this deformation appears to be concentrated at the boundaries of crustal blocks, with clockwise rotation and westward translation [Geist et al., Tectonics 7, 327-341, 1988]. In the block rotation model, extensional rift structures develop between the blocks in arc-normal orientation. Summit basins develop at the northern, trailing edge of the blocks in arc-parallel orientation. These summit basins are located near or within the volcanic front. Thus structures in the upper plate driven by oblique convergence are predicted to interact with arc volcanism. We report on multibeam mapping in 2003-2004 and ROV Jason II dives in 2004. The data reveal locations and patterns of fault structures, volcanic cones, and lithologies in several locations critical to understanding the arc's response to oblique convergence. A large submarine volcano, named Amchixtam Chaxsxii in the Unangan language, was mapped next to Semisopochnoi Island. Additional small cones are identified on the flank of Tanaga Volcano, and near Bobrof Volcano on possible fault structures. The largest extensional `block boundary' is located at Amchitka Pass; in this area the seafloor is offset by a network of faults. Small volcanic cones are clustered at these faults. Some show signs of erosion and mass wasting; others, especially deeper ones, are intact. Surfaces are dominated by `a`a flows and spatter, and have light sediment cover and moderately fresh lavas. Our mapping focused on specific sites that were chosen to be representative, and suggests that (1) small, probably monogenetic cones are common; (2) the cones occur preferentially in areas of extensional faulting in the volcanic front; (3) these cones are present largely because of oblique convergence and arc deformation. Geochemical analyses will test their relationship to nearby subaerial arc volcanoes.