Seismic Evidence for Compositional Asymmetry on the Newfoundland-Iberia Nonvolcanic Rifted Margin Pair: New Results From SCREECH Transect 2
Abstract
Many questions remain concerning the contribution of pure and simple shear to rifting processes, the symmetry of the resulting continental margins, and the transition from rifting to initial seafloor spreading. Nonvolcanic continental margins provide excellent localities to investigate continental rifting and initial seafloor spreading. The Newfoundland-Iberia margin is one of the best-studied nonvolcanic margin pairs in the world; relatively new seismic data sets are available on both margins, and the Iberian margin has been the target of three ODP drilling legs. Additionally, ODP Leg 210 is scheduled to visit the Newfoundland margin in July 2003. The newest dataset collected on the Newfoundland margin is SCREECH (Studies of Continental Rifting and Extension on the Eastern Canadian SHelf). Work on this dataset has revealed fundamental asymmetries between the Newfoundland and Iberian margins. Specifically, geophysical and drilling investigations have discovered exhumed continental mantle on the Iberian margin between unequivocal continental and oceanic crust. In contrast, new wide-angle velocities and reflectivity characteristics from prestack time and depth migrated MCS data along SCREECH Transect 2 (the central of three primary transects) suggest that all crust seaward of continental crust on Newfoundland around Transect 2 is slow-spreading oceanic crust. The velocity gradient structure obtained by amplitude modeling resembles 1D velocity profiles from Mid-Atlantic oceanic crust. The average upper-crustal Poisson's ratio of 0.28 is too low for serpentinized peridotite (for the modeled P-wave velocities) and higher than the Poisson's ratio of 0.24 calculated for continental crust on Transect 2. Seismic reflection data also reveal marked differences between exhumed mantle on the Iberian margin and crust on Transect 2. Exhumed mantle on the Iberian margin appears unreflective in the upper 0.5 seconds and riddled with intracrustal reflections below. In contrast, most basement seaward of continental crust on SCREECH Transect 2 is represented by a reflective upper 0.25 seconds, and, in most places, is seismically transparent below. However, there are several places on Transect 2 where the top of basement is marked by a simple reflection (rather than a zone of reflectivity); these sections of crust also show the most intra-crustal reflectivity. We tentatively conclude that these differences are the result of heterogeneous processes at work in the creation of slow-spreading oceanic crust, where reflective crust represents magmatic periods during the creation of oceanic crust and unreflective crust represents amagmatic periods. Additionally, the density difference between oceanic crust and serpentinized peridotite can explain the 1-km discrepancy in seafloor and basement depths observed on the Newfoundland and Iberian margins.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2002
- Bibcode:
- 2002AGUFM.T52C1210S
- Keywords:
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- 3040 Plate tectonics (8150;
- 8155;
- 8157;
- 8158);
- 7220 Oceanic crust;
- 8105 Continental margins and sedimentary basins;
- 8109 Continental tectonics: extensional (0905)