Analysis of Offshore-Onshore Wide-Angle Seismic Recordings from Western Alaska, Bering-Chukchi Sea

Richard M Allen (Dept. Geol. Sci., Univ. Durham, Durham, DH1 3LE, UK) T M Brocher (U.S. Geological Survey, 345 Middlefield Rd., MS 977, Menlo Park, CA 94025; tel. 415-329-4737; e-mail: brocher@andreas.wr.usgs.gov) S L Klemperer and B K Galloway (Dept. Geophys., Stanford Univ., Stanford, CA 94305)



Abstract submitted to AGU Fall Meeting, December 1995.

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The U.S. Geological Survey, in a cooperative study with Stanford University and the University of Alaska, Fairbanks, recorded nine split-spread wide-angle seismic reflection/refraction profiles in western Alaska during a deep-crustal seismic reflection survey of the Bering-Chukchi sea on the R/V Ewing (see Klemperer et al., this session; Brocher et al., this session). The wide-angle data provide useful arrivals to ranges in excess of 300 km and yield the densest wide-angle coverage in two regions: (1) in the Chukchi Sea between Point Hope and Point Lay and (2) south of the Bering Straits in a region between the northwestern tip of St. Lawrence Island and Tin City, on the Seward Peninsula. These wide-angle data complement wide-angle data recorded simultaneously in eastern Siberia on the Chukotka Peninsula. Lower crustal and upper mantle (PmP) reflections observed in the wide-angle data between Cape Lisbourne and Point Lay correlate in location and time with northward dipping reflections observed on the vertical-incidence multichannel reflection data. One-dimensional forward modeling of the refracted and reflected arrivals indicates that the crustal thickness varies from 29 km at St. Lawrence Island to 35 km in the vicinity of the Bering Strait, thinning again northward to 33 km near Point Hope. Recent global compilations suggest these thicknesses are typical of extended and/or rifted continental crust. The variation in crustal thickness, however, is more than expected from the present lack of topographic relief of the crust. This modeling suggests a lower crustal layer up to 20 km thick with velocities between 6.2 km/s and 7.3 km/s and an upper mantle velocity of 8 km/s. The high-velocity lower crust is associated with mafic cumulates inferred from crustal xenoliths (see Wirth et al., this session). The up to 5 km thick sedimentary basins along the seismic transect are evidence of significant crustal extension in the Tertiary following Mesozoic continental accretion.

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© Richard M Allen