New seismic reflection imaging of the Andreanof Segment of the Aleutian subduction zone

We collected active-source seismic data in the Andreanof segment of the Aleutian arc to advance understanding of arc magmatism and structural evolution in this archetypal oceanic island arc. The Andreanof experiment consisted of wide-angle and multichannel reflection seismic data acquired by R/V Marcus G. Langseth in 2020. The area presents advantageous features: the arc has remained almost intact and only slightly deformed over time; the tectonic history is relatively well known; and lava geochemistry here has been extensively studied. Moreover, it is a seismic and volcanic active region; the 1957 M8.6 and 1986 M8.0 earthquakes ruptured this area. We present the preliminary processing of two composite 2D seismic reflection dip profiles that image the incoming oceanic crust, accretionary prism, forearc structure, arc platform and back-arc. Line D1 crosses the arc at Atka Island, to the west of Korovin volcano. It is partitioned into a 120-km-long section north of the arc, and a 244-km-long section south of the arc. Line D2 crosses Adak Island. It is divided into a 112-km-long section north of the arc and a 167-km-long section south of the arc. Reflection data were acquired with a 9-km streamer south of the arc and a 6-km streamer north of the arc. The source for all profiles was the 6600 cu in air gun array of the Langseth. Processing steps included bandpass frequency filtering and f-k filtering, spherical divergence correction, amplitude scaling, predictive deconvolution, multiple removal by SRME and radon transform, NMO, stacking, and migration. South of the arc, these profiles image the incoming oceanic plate, the Aleutian trench, and the forearc. They show a thin pelagic sediment layer (<0.5 s twtt) with faulting in the outer-rise with a spacing of ~2 km. The plate boundary can be imaged ~32 km from the trench, at two way travel times up to 13 s. Both profiles reveal a small accretionary prism, a prominent forearc outer high (Hawley Ridge) and a large forearc basin. The forearc basin is 2-3 s twtt thick, and seismic data image the sediment basin's internal structure and possible landslides. The northern sections of the lines show the back-arc section in the Bering Sea characterized by a thick sediment layer (>3 s twtt). We will discuss these observations and how they relate to the deformation and arc construction in the region.