John Diebold (1), Lincoln Hollister (2)
(1) L-DEO, Palisades, NY 10964, USA, (914) 365-8524, (914) 365-8168,
johnd@ldeo.columbia.edu
(2) Dept. Geol. & Geoph. Sci., Princeton University, Princeton,
NJ 08544, USA (609) 258-4106, (609) 258-5275, linc@princeton.edu
Abstract:
R/V EWING acquired 1700 km of deep penetration MCS data across
the accreted terranes and principal terrane boundaries of SE Alaska. Narrow
Fjordlike waterways and rough, shallow bottom produce artifacts which were
partly removed by prestack velocity filtering and statics corrections based
on a model obtained from brute stacks. In Dixon Entrance we
have 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 degrees)
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. 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, suggesting their creation by oblique extension
(transtension). The strike of the Dixon Entrance Moho arch is subparallel
to and centered between theadjacent terrane boundaries, and similar arches
are observed, centered within adjacent terranes to the east. This suggests
that post-accretion crustal extension may be controlled by accretionary
fabrics.
