Paper No. 3
Presentation Time: 9:30 AM


SAWYER, Dale, Earth Science, Rice University, 6100 Main Street, Houston, TX 77005, RESTON, Timothy, School of Geography, Earth and Environmental Sciences, University of Birmingham, Birmingham, B152TT, United Kingdom, SHILLINGTON, Donna J., Lamont-Doherty Earth Observatory of Columbia University, 61 Route 9W, Palisades, NY 10964, MINSHULL, Timothy A., National Oceanography Centre, University of Southampton, European Way, Southampton, SO14 3ZH, United Kingdom, KLAESCHEN, Dirk, Dynamik des Ozeanbodens, IFM-GEOMAR, Leibnitz Institute of Marine Sciences, Wischhofstrasse 1-3, Kiel, D-20148, Germany and MORGAN, Julia, Earth Science, Rice University, Houston, TX 77005,

In June thru September, 2013, a 3D reflection and long offset seismic experiment was conducted at the Galicia rifted margin by investigators from the US, UK, Germany, Spain, and Portugal. The 3D multichannel experiment covered 64 km by 20 km (1280 km2), using the RV Marcus Langseth. Four streamers 6 km long were deployed at 12.5 m hydrophone channel spacing. The streamers were 200 m apart. Two airgun arrays, each 3300 cu in, were fired alternately every 37.5 m, to collectively yield a 400 m wide sail line consisting of 8 CMP lines at 50 m spacing. The long offset seismic experiment included 72 short period OBS’s deployed below the 3D reflection survey box. Most of the instruments recorded all the shots from the airgun array shots. A few of the instruments were deployed twice, once to densify the instruments on a single profile and then to be moved into the full array. Finally, 6 of the OBS’s were deployed on a profile extending 90 km to the west of the 3D box in order to use combined MCS and OBS data to locate the boundary between the oceanic crust and exhumed upper mantle.

The 3D seismic box covered a variety of geologic features including the Peridotite Ridge (PR) associated with the exhumation of upper mantle rocks to the seafloor, the S reflector interpreted to be a low-angle detachment fault formed late in the rifting process, and a number of rotated fault block basins and ranges containing pre- and syn-rift sediments.

Initial observations, based only on 2D seismic dip lines (albeit 400 m apart), show the along strike variation of the PR: ~1050 m higher than adjacent basement in the South of the 3D box and much increased in height, ~2200 m high in the North. Some cross-sections of the PR show possible internal structure that may help identify the emplacement mechanism of the feature and its relationship with the boundary between rifted continental crust blocks and exhumed upper mantle rocks. To the immediate East and West of the PR there are strong negative-polarity reflectors, apparently at the boundary between post-rift sediment above and transparent sediments below. Based on the reflector polarity, the transparent sediments seem to be of lower velocity than the post-rift sediments above. This material may have sloughed off the PR as it was emplaced at the seafloor and thus may not be found in the fault bounded continental crustal blocks to the East.