Crustal structure along the EDGE transect beneath the Kodiak shelf off Alaska derived from OBH seismic refraction data

Along the EDGE Alaska transect over the Kodiak shelf off Alaska, a deep-crustal seismic refraction experiment was carried out in 1994 using an airgun source with a high shot density and closely spaced ocean-bottom hydrophones (OBHs). These data clearly define a gently dipping plate boundary that gradually increases from 2.5° at the trench to 5° 250 km landwards of the trench beneath the shelf. A landward increase of the velocities in the subducting oceanic crust was found. The sedimentary upper plate beneath the continental slope can be divided into a young Neogene accretionary prism beneath the lower slope, and a more layered structure below the middle and upper slope, where a body with elevated velocities (4.2–5.1 km s−1) in the lower part is interpreted as well-consolidated sediment of possibly late Eocene age, acting as the backstop. Across the inner shelf between Kodiak Island and Kenai Peninsula, near-surface high velocities (> 6.0 km s−1) document a strong uplift (> 10 km) of the Mesozoic and Palaeocene accretionary complex, and furthermore suggest its seaward extension beneath the outer shelf. The most striking feature along the EDGE transect is the thick low-velocity zone (LVZ) that coincides with the arched reflectors in the lower crust beneath the Kodiak Island-Kenai Peninsula axis on the EDGE seismic reflection data. This LVZ is interpreted as indicating underplated low-velocity rocks, which contribute the same volume of new material as the frontal accretion to the continent, the growth of which is rapid, totalling 4200 km2 across the Kodiak shelf since Eocene times. A calculation of the material balance shows that an underplating of underthrust sediment alone may not account for the huge volume of the underplated low-velocity rocks. We speculate that the underplating of continental fragment, or more likely seamounts or plateau, may be involved in continental growth