AN: T11B-04TI: Crustal extension in Dixon Entrance, SE AlaskaAU: John B. DieboldAF: L-DEO, PO Box 8000, Palisades, NY 10964-1000 USAEM: johnd@ldeo.columbia.eduAU: Lincoln S. HollisterEM: AU: Triparna DasAF: Dept of Geosciences, Princeton U., Princeton NJ 08544,    USAEM: linc@geo.princeton.eduAB: Multichannel seismic [MCS] reflection profiles, acquired in Dixon    Entrance by R/V EWING in 1994 as part of the ACCRETE project, imaged    crustal reflectors and Moho across the fundamental pair of structural    boundaries separating the Alexander, Wrangellia, and North America    terranes. The eastern margin of the Alexander terrane features strong    crustal reflectors, mostly dipping gently (ca. 12<sup>o</sup>) WSW.    Moho reflections within this zone are stronger and more continuous    than those seen in the terranes to the east. A broad Moho arch, about    3 km high, 100 km wide, and striking NNE, is defined by two E-W    profiles, 45 km apart. These two profiles are tied by a third,    arch-parallel line. Minimum Moho two-way time is 8 seconds,    corresponding to a Moho depth of approximately 26 km. It is likely    that this crustal arching is the result of the Tertiary extension    forming Queen Charlotte Basin. The overall strike of the arch is    oblique to nearby terrane boundaries, to graben and half-graben    structures previously mapped in Dixon Entrance and Hecate Strait, and    to the trend of Queen Charlotte Basin, which lies to the south. The    zone of thin crust defined by this Moho arch is apparently    discontinuous with similar zones mapped in Hecate Strait and Queen    Charlotte Basin, suggestiong their creation by oblique extension    (transtension). Terrane boundaries splay out just north of Dixon    entrance; south of Ketchikan, the Taku terrane and the Gravina belt    disappear. Also, the strike of the Wrangellia/North American boundary,    marked by the coast shear zone, rotates 35<sup>o</sup> northward,    beginning a dogleg towards the east, partially accommodating the    splaying of the terrane boundaries. The strike of the Dixon Entrance    Moho arch is subparallel to the coast shear zone and terrane    boundaries to the north, suggesting that the crust there may have been    extended within a different tectonic regime than that forming Hecate    Strait and Queen Charlotte Basin, to the south.SC: TDE: 3025DE: 8015DE: 8109MN: 1997 Fall Meeting