Upper crustal structure of southern 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 is used to investigate the upper crustal structure of southcentral Alaska. The data consist of two intersecting refraction lines: the 135-km Chugach profile which follows the E-W strike of the Chugach Mountains and the 126-km Cordova Peak profile which follows the N-S regional dip. The four shots of the Chugach profile and the five shots of the Cordova Peak profile were recorded on 120 portable seismic instruments spaced at 1-km intervals. Interpretation of data from the Chugach terrane indicates that near-surface unconsolidated sediment and glacial ice overlie rocks of unusually high average compressional velocities (5.4-6.9 km/s) in the upper 10 km of crust. A thick unit correlated with a metasedimentary and metavolcanic flysch sequence has velocities of 5.4-5.9 km/s. It is underlain by mafic to ultramafic metavolcanic rocks (6.0-6.4 km/s) correlated with the terrane basement. Mid-crustal layers beneath the Chugach terrane contain two velocity reversals (6.5 and 6.7 km/s) attributed to off-scraped oceanic sedimentary rocks which are underlain by mafic to ultramafic oceanic volcanic crust (7.0-7.2 km/s). Interpretation of data from the Prince William terrane indicates systematically lower velocities in Prince William terrane rocks as compared to Chugach terrane rocks at comparable depths. The upper 10 km of crust, having average compressional velocities of 3.0-6.2 km/s, is correlated with clastic sedimentary and volcanic rocks which are overlain by younger terrigenous sedimentary rocks. A 2-km thick layer at 10-12 km depth is correlated with mafic to ultramafic Prince William terrane basement rocks. The difference in velocity structure between the Chugach and Prince William terranes suggests that the Contact fault zone is a terrane boundary which extends to a depth of at least 10-12 km. Deep structure beneath the two terranes is not well constrained by the seismic refraction data. Potential field data support the interpretation that a thick low-velocity zone occurs at a 12-15 km depth and may contain subducted continental rocks of the Yakutat terrane, which is currently accreting to and being thrust beneath the North American continent along the Gulf of Alaska margin.