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Velocity model derived
by simultaneous traveltime inversion of wide-angle and
reflection seismic data. The unconstrained portion of the
model is omitted. The ocean bottom receiver locations are
indicated along the seafloor as yellow dots. The PmP
reflection points are indicated by black dots along the Moho
(thick white line). The black contour is the
sediment-basement boundary. The range of P-wave velocities
in the three sediment layers is indicated along with
representative velocities along the top and bottom of the
crust and beneath the Moho. Large red dots are reflection
points of picks of the S reflector from the MCS data that
were inverted simultaneously with the wide-angle data as
reflections from the Moho. Small pink dots are reflection
points of picks from the MCS data modeled as "floating"
reflectors, i.e., reflectors not associated with a layer
velocity discontinuity. The yellow contours labeled 7.0 and
7.6 are isovelocity contours (km/s) from a minimum-structure
model derived by tomographic inversion of the first-arrival
traveltimes. The coast line is indicated by the arrow at the
top of the model. OC - oceanic crust; PR - Peridotite Ridge;
GB - Galicia Bank; GIB - Galicia Interior
Basin.
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Abstract
We have analyzed wide-angle and MCS data from the Iberia
margin along a 335 km dip profile over the Galicia Interior
Basin (GIB), Galicia Bank (GB), S reflector, and Peridotite
Ridge. The MCS data provide a detailed image of the margin
sediments and basement. The wide-angle data constrain the
sub-sedimentary velocity structure. Zero-offset reflection
times from the sediments, basement and S reflector were
inverted simultaneously with the deeper
refraction/wide-angle phases to account for the complex
shallow structure on the deeper raypaths. A
minimum-parameter, preferred final model satisfying all the
data was obtained which includes subjective features
considered geologically reasonable. The S reflector appears
to be the Moho just 3-5 km beneath the seafloor and its
seaward dip suggests it is a detachment fault. To
objectively assess this model, the first arrival and Moho
reflection data were inverted using a minimum-structure
tomographic approach. The isovelocity contours representing
the Moho in the final tomographic model obtained from the
first arrivals agree well with the Moho in the preferred
final model. The flattest and smoothest Moho obtained from a
tomographic inversion of the zero-offset S reflector and
wide-angle reflections also agrees well with the Moho in the
preferred final model. Landward of the S reflector, the
correlation between crustal thickness, water depth and
sediment thickness is typical for a rifted margin. Crustal
thickness variations suggest the margin extended in two
phases spatially separated by about 100 km, with stretching
factors of 2.5-3 and nearly infinite implied for the early
and late (preceding seafloor spreading) stages of rifting
beneath the GIB and GB basin. A checkerboard resolution test
using first arrival tomography shows that lateral velocity
resolution is 5-15 km in the sediments and basement to ~8 km
depth, and resolution is about 40-50 km at the Moho landward
of the S reflector.
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