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. 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; the Iberian margin has been the target of three ODP drilling legs; and ODP Leg 210 is scheduled to drill the Newfoundland margin in July 2003. Data from the Summer 2000 SCREECH survey (Studies of Continental Rifting and Extension on the Eastern Canadian SHelf) reveal fundamental asymmetries between the Newfoundland and Iberian margins. Specifically, whereas exhumed continental mantle exists on the Iberian margin between unequivocal continental and oceanic crust, velocities and reflectivity characteristics along SCREECH Transect 2 (the primary drilling transect) 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 Atlantic oceanic crust. The average upper-crustal Poisson’s ratio of 0.28 is too low for serpentinized peridotite (given the P-wave velocities of ~5.0 km/s) and higher than the Poisson’s ratio of 0.24 calculated for continental crust. Seismic reflection data also reveal marked differences between exhumed mantle on the Iberian margin and crust on and around 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. 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. The observed compositional asymmetry of the margins suggests that asymmetric processes controlled continental breakup, at least during the final stages of rifting and initial seafloor spreading. We tentatively propose ridge migration or extremely asymmetric magmatic accretion to account for the asymmetric distribution of oceanic crust.
- Publication:
-
EGS - AGU - EUG Joint Assembly
- Pub Date:
- April 2003
- Bibcode:
- 2003EAEJA.....8170S