Investigators:	Hans G. Avé Lallemant
Collaborator:	John S. Oldow, University of Idaho
Funding Source:	NSF

GPS instrument set up near World War II made infamous Engineering Hill, looking across Massacre Bay, Attu Island, Aleutian volcanic island arc

Volcanic island arcs are often thought to be stationary elements of the plate upon which they are built. Structural analysis of faults and fractures on four Aleutian islands (Unalaska, Atka, Adak, and Attu) indicates, however, that the Aleutian arc has been migrating westward since its presumed Middle Eocene birth. GPS measurements on five Aleutian islands (the four mentioned above plus Shemya) suggest that the island arc is presently still migrating westward. Most rocks on the islands have undergone penetrative, brittle deformation. Faults of all scales are ubiquitous. Four sets of faults have been recognized. Set #1 consists of two conjugate sets of thrust faults indicating arc-(sub)perpendicular shortening. Set #2 consists of two conjugate sets of normal faults indicating arc-parallel extension. Set #3 consists of two sets of conjugate strike-slip faults, the acute bisector of which is parallel to the major extensile strain axis (X) and parallel to the island arc indicating arc-parallel extension as well. Set #4 consists of arc-parallel dextral strike slip faults with locally a conjugate sinistral set. Fractures, veins, and dikes are locally abundant; they often are oriented perpendicular to the arc, also suggesting arc-parallel stretching. The brittle structures on the Aleutian Islands indicate that the plate-convergence rate vector (between the Pacific and North American plates) is partitioned into arc-(sub)perpendicular and arc-parallel components and that much of the arc parallel stretching is related to the curvature and the increase toward the west of the obliquity of convergence. Finite strains were estimated in several places, but these measurements will only yield minimum displacement rates. Preliminary results of the GPS strain study shows that Unalaska is moving 3 mm/year westwards parallel to the arc and Attu Island moves 34 mm/year toward the NW parallel to the arc. The GPS measurements on Adak indicate plate-convergence-rate-parallel migration. It is expected that this project will continue for five more years.