Abstract
During July and August of 1997, we used the RV Maurice
Ewing to acquire multichannel seismic (MCS) reflection
profiles, ocean bottom seismograph/hydrophone (OBS/OBH)
refraction/reflection profiles, bathymetry, and magnetic
observations in the Atlantic Ocean offshore of Portugal and
Spain. The experiment consisted of MCS profiling using the
160 channel digital streamer and 8400 cu. in. airgun array
of the R/V Maurice Ewing, long-offset marine seismic
acquisition using 11 OBS's and 14 OBH's. Approximately 4000
km of MCS data, and 55 OBS/OBH datasets were acquired. The
experiment took place on the Galicia Bank, the Galicia
Interior Basin, and the transition from the Galicia margin
to the Iberia Abyssal Plain margin to the south. This work
was conducted in cooperation with scientists at the
University of Texas at Austin, the GEOMAR Marine Laboratory
in Kiel Germany, the University of Aberdeen, Scotland, the
University of Madrid and the Center for Earth Sciences of
Barcelona Spain, and the University of Aveiro in
Portugal.
We acquired data to address each of our three main
research objectives:
1. Describing the character of the S reflector in the
lower crust or upper mantle of the Galicia Bank margin
segment. We show that the S reflector is the boundary
between rocks of velocity 6.9 km/s above and 7.8 km/s below.
This suggests that S forms the Moho in this area and that
continental crustal rocks have been emplaced by faulting
over rocks of the upper mantle.
2. Mapping the variation in crust thickness and amount of
extension across the entire continental margin from
unambiguous oceanic crust to unrifted continental crust. We
show the crustal thickness and velocity structure across the
entire rifted margin at 4 locations. The rifting in the
Galicia Interior basin seems to have reached a peak
extension of beta 3.0. Extension of the crust forming the
Galicia Bank shows extension by beta 2.0.
3. Mapping the variation along-strike of the rifting
process from the Iberia Abyssal Plain to the south to the
Galicia Bank in the north. We have 9 crossings of the
transitional zone of the Galicia Bank and iberia rifted
margin segments. They show greater variation along strike
than we had envisionesd. We are now working to understand
those differences.
We have processed all the MCS data through post-stack f-k
time migration. These sections are quite good and certainly
allow first level interpretation and synthesis as well as
allowing us to choose profile segments that will benefit
from more extensive processing
The OBS/OBH are generally of excellent quality. The MCS
data however are unable to constrain the velocity structure
of the crust and uppermost mantle, or the position of the
Moho, except where it appears to be synonymous with the S
reflector. Therefore, the main role of the wide-angle data
is to constrain the sub-sedimentary velocity structure to
uppermost mantle depth. By utilizing zero-offset reflection
times picked from the MCS data with the wide-angle data in
the velocity and interface inversion, the degree of
velocity-depth ambiguity associated with the sediment
structure and above the S reflector is reduced in comparison
to the constraint provided by the wide-angle data alone.
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