Crustal structure of the active margin, south central Alaska: An interpretation of seismic refraction data from the Trans‐Alaska crustal transect

Abstract

Seismic refraction and wide‐angle reflection data from the U.S. Geological Survey's Trans‐Alaska Crustal Transect are used to investigate the upper crustal structure of southcentral Alaska and to develop a model using two‐dimensional asymptotic ray theory. The data considered here are from the N‐S 126‐km Cordova Peak refraction profile, which was positioned to cross the Chugach and Prince William terranes. These two tectonostratigraphic terranes form part of a large accretionary complex related to the Alaska subduction zone. Interpretation of data from the Cordova Peak profile indicates systematically higher velocities in Chugach terrane rocks as compared with Prince William terrane rocks at comparable depths. Unconsolidated sediment and glacial ice overlie Chugach terrane rocks Of unusually high compressional velocities (4.7–7.0 km/s) in the upper 10 km of crust. A northward thickening metasedimentary and metavolcanic flysch sequence, having an average velocity of 6.0 km/s, is underlain by metavolcanic mafic basement rocks having an average velocity of 6.6 km/s. The upper 10 km of the Prince William terrane have compressional velocities ranging from 3.7 to 6.2 km/s and are correlated with clastic sedimentary and volcanic rocks which are overlain by younger terrigenous sedimentary rocks and underlain by mafic to ultramafic Prince William terrane basement rocks. The upper crustal velocity structures of the Chugach and Prince William terranes are distinct to at least 10 km depth, which suggests that the Contact fault zone is a boundary which extends to at least 10 km. Midcrustal layers beneath the Chugach terrane contain two velocity reversals which may indicate the presence of off‐scraped oceanic sediment, overpressurized zones, shear zones, or some combination of these possibilities. Data from energy sampling midcrustal layers beneath the Prince William terrane do not provide similar evidence for low‐high velocity zones in that area.