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Geological Society, London, Special Publications; 2007; v. 282; p. 47-61;
DOI: 10.1144/SP282.3
© 2007 Geological Society of London

The Iberia-Newfoundland continental extensional system (geological and geophysical constraints)

Structure of the Flemish Cap margin, Newfoundland: insights into mantle and crustal processes during continental breakup

J. R. Hopper1, T. Funck2 & B. E. Tucholke3

1 Department of Geology and Geophysics, Texas A&M University, College Station, TX 77843, USA (e-mail: hopper{at}geo.tamu.edu)
2 Geological Survey of Denmark and Greenland, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark
3 Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA

Seismic reflection and refraction data from the Flemish Cap margin off Newfoundland reveal the large-scale structure of a magma-starved rifted margin. There is little evidence for significant extensional deformation of the Flemish Cap, consistent with the hypothesis that it behaved as a microplate throughout the Mesozoic. The seismic data highlight important asymmetries at a variety of scales that developed during the final stages of continental breakup and the onset of oceanic sea-floor spreading. In strong contrast to the conjugate Galicia Bank margin, Flemish Cap shows: (1) an abrupt necking profile in continental crust, thinning from 30 km thick to 3 km thick over a distance of 80 km, and a narrow, less than 20 km-wide, zone of extremely thin continental crust; (2) no clear evidence for horizontal detachment structures beneath continental crust similar to the ‘S’ reflection; and (3) evidence for at least a 60 km-wide zone of anomalously thin oceanic crust that began accreting to the margin shortly after continental crustal separation. The oceanic crust averages only 3–4 km thick and in places is as thin as 1.3 km thick, although seismic layer 3 is missing where this occurs. The data suggest that there are large spatial and temporal variations in the available melt supply following continental breakup as oceanic sea-floor spreading becomes established. In addition, wide-angle data show that anomalously slow mantle P-wave velocities appear approximately where continental crust has thinned to 6–8 km thick, indicating that low-degree serpentinization begins where the entire crust has become embrittled.