Seismic Structure of the Eastern Aleutian Arc from Wide-Angle Measurements

by M M Fliedner (Stanford University) and Aleutian Working Group (University of Delaware, LDEO, University of Texas at Austin, WHOI, and Stanford University)

The three-component land recordings of the 1994 MCS shots along the Aleutian Arc (see talk by S. McGeary and poster by N. Bangs in this session) provide information about the P- and S-wave velocity structure of the eastern Aleutian arc. Refracted rays are bottoming in front of the volcanic line between 10 and 60 km depth. We show results from recordings between Atka Island and Cape Sarichef on Unimak Island, i.e. the part of the arc that is built on previously oceanic crust.

Main refractors are found at P-velocities of 6.5 km/s at about 10 km depth, 7.2 km/s at 30 km depth, 7.8 km/s at 45 km depth, and 8.2 km/s at 60 km depth. The strongest wide-angle reflection can be associated with the 7.8-km/s refractor, localising it in the seismic mantle. We observe weaker reflections in even greater depth (down to at least 60 km). There is no clear trend in the velocity structure along the arc, but the ratio of P- to S-wave velocity seems to increase from west to east.

The seismic crust-mantle boundary is conventionally defined by a P-velocity of 7.6 km/s in absence of a clear step-like increase from crustal to mantle velocities. If the 45-km reflector defines the base of the petrologic arc, then there is a layer of about 5 km thickness at the base of the arc crust that is part of the seismic mantle, consisting presumably of ultramafic cumulates. This makes the arc considerably thicker than previous models. The thick section of high velocities in the lower crust of the arc that suggest gabbroic composition poses a problem for the concept of island arcs forming continental crust by accretion: only the upper half of the Aleutian arc has velocities of average continental crust (6.45 km/s). Most or all of the arc's lower crust has to be assimilated to the mantle before the arc becomes continental. Seismic recordings from the continental part of the Aleutian Arc could reveal this transition.