TRANSPRESSIONAL STRAIN PARTITIONING IN THE ALEUTIAN ARC: COMPATIBILITY OF GPS VELOCITIES AND NONPLANE INCREMENTIAL STRAIN DERIVED FROM EARTHQUAKE FOCAL MECHANISMS

Oblique plate convergence is commonly partitioned into boundary normal and parallel components resulting in displacement of the frontal portion of the overriding plate with respect to the backarc region along arc-parallel strike-slip faults. In the Aleutian Islands, the east to west increase in relative plate motion between the North American and Pacific plates from 65 mm/yr to 75 mm/yr is accompanied by a change from normal convergence to boundary parallel displacement. The convergence obliquity increases from near zero to ~80° along strike together with an observed increase in velocities measured by GPS geodesy between 1996 and 2000. GPS velocities have a strong arc-parallel orientation and systematically increase around the curved arc from 4 mm/yr in the east (Unalaska), to 7-10 mm/yr in the center (Atka and Adak), and 25-31 mm/yr in the west (Shemya and Attu). In all cases, GPS velocities record a small arc-normal component of displacement. The orientation of the incremental shortening axis derived from earthquake focal mechanisms on the Aleutian megathrust has an obliquity that varies systematically from zero (normal to the plate boundary) in the east to ~35° in the west. The variation in incremental strain axes do not document displacement field partitioning but rather are a consequence of nonplane strain (transpression). Existence of displacement partitioning can only be assessed by direct measurement. Unfortunately, GPS velocities record permanent and recoverable strain, necessitating determination of the elastic strain component in the velocity field. The incremental shortening axis related to megathrust deformation constrains the azimuth of the elastic component of the GPS velocity field and yields minimum arc-parallel displacement components of 3 mm/yr in the east, 5-6 mm/yr in the central Aleutian chain, and 23-30 mm/yr in the west. GPS velocities along the Aleutian chain record a lateral variation in displacement field partitioning efficiency of 35-45% in the west, 30% in the east, and 20% in the central part of the arc. Some of the variation is attributable to known or suspected arc-parallel transcurrent faults between the Aleutian Islands and the plate boundary, but in the center of the arc may reflect increased plate-boundary coupling related to subduction of the Amlia fracture zone.