Displacement partitioning and arc-parallel extension in the Aleutian volcanic island arc

Abstract

Convergence between the Pacific and North American plates along the Aleutian volcanic island arc is generally right-oblique; the obliquity increases from 0° in the east to nearly 90° in the west where the plate boundary becomes a transform fault zone. Preliminary results of a structural geology study of Attu Island (the most westerly U.S. island in the Aleutian chain and part of the Near Islands group) shows that the deformational structures are not directly related to the relative plate-convergence-rate vector, but to the plate-boundary-normal and -parallel components of this vector. The plate-boundary-(sub)normal component is responsible for conjugate sets of arc-(sub)parallel thrust faults and the plate-boundary-parallel component resulted in arc-parallel dextral strike-slip faults and their conjugates. However, the most penetrative structures measured in outcrop are conjugate sets of strike-slip faults and arc-perpendicular normal faults and fractures (veins and dikes) which have formed by arc-parallel extension. This extension is most likely the result of the increase of obliquity from east to west and thus the increase of the arc-parallel component of the convergence-rate vector along the Aleutian arc. These structures require that Attu Island migrated westward, possibly for as many as 1150 km.