Upper crustal structure of the accreted Chugach Terrane, Alaska

Abstract

We have developed a model for the upper crustal structure of the Chugach terrane of southcentral Alaska from U.S. Geological Survey seismic refraction data using two‐dimensional asymptotic ray tracing. The refraction profile, acquired as part of the Trans‐Alaska Crustal Transect program, extends approximately 135 km in an east‐west direction along the local strike of the Chugach Mountains. The refraction survey consists of four shots, recorded by 120 portable seismic instruments spaced at 1‐km intervals. We observe a velocity‐depth profile which has unusually high velocities at shallow depths and at least two velocity reversals. The average velocity increases from 4.0 km/s at the surface to 6.9 km/s at a depth of 9 km. Near‐surface velocities in Cretaceous Valdez Group rocks increase eastward following the metamorphic gradient observed by Hudson and Plafker (1982). Lower layers may be correlated with rocks mapped to the south of the Contact fault zone, a suture boundary between the Chugach and Prince William terranes, and units not having surface expression. The first velocity reversal occurs at a depth of 9 km. This 1‐ to 2‐km zone has velocities which vary laterally from 6.0 to 6.9 km/s and pinches out to the east in the profile. This unit may represent a zone comprising Tertiary intrusives and country rock. It is followed at a depth of 10 km by a layer with velocities of 7.2–7.23 km/s. The second velocity reversal occurs at a depth of 12 km. Velocities within this layer range from 6.6 to 6.7 km/s and may correlate with underplated oceanic crust. Higher velocities of 7.35–7.4 km/s are attributed to a thin unit at a 14‐km depth associated with ultramafic and mafic rocks. The deepest unit is modelled as a 5‐km layer having velocities of 7.2–7.25 km/s. This layer may be underplated Prince William Terrane rocks, oceanic crust, or some portion of the Yakutat Block. The refraction data alone do not resolve deeper structure.