Seismic velocity structure of the rifted margin of the eastern Grand Banks of Newfoundland, Canada

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Read Full Article (file size: 15993278 bytes) Cited by JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 111, B11404, doi:10.1029/2005JB004156, 2006 Seismic velocity structure of the rifted margin of the eastern Grand Banks of Newfoundland, Canada Harm J. A. Van Avendonk University of Texas Institute for Geophysics, Jackson School of Geosciences, Austin, Texas, USA W. Steven Holbrook Department of Geology and Geophysics, University of Wyoming, Laramie, Wyoming, USA Gregory T. Nunes ExxonMobil, Houston, Texas, USA Donna J. Shillington National Oceanography Centre, Southampton, UK Brian E. Tucholke Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts, USA Keith E. Louden Department of Oceanography, Dalhousie University, Halifax, Nova Scotia, Canada Hans Christian Larsen Integrated Ocean Drilling Program, Sapporo, Japan John R. Hopper Department of Geology and Geophysics, Texas A&M University, College Station, Texas, USA Abstract We present a compressional seismic velocity profile of the crust of the eastern margin of the Grand Banks of Newfoundland, Canada. This velocity model was obtained by a tomographic inversion of wide-angle data recorded on a linear array of 24 ocean bottom seismometers (OBSs). At the landward side, we imaged a crustal thickness of 27 km in Flemish Pass and beneath Beothuk Knoll, which is thinner than the 35-km-thick crust of the central Grand Banks. We therefore assume that the eastern rim of the Grand Banks stretched uniformly by 25%. Farther seaward, the continental crust tapers rapidly beneath the continental slope to ∼6 km thickness. In the distal margin we find a 60-km-wide zone with seismic velocities between 5.0 and 6.5 km s⁻¹ that thins to the southeast from 6 to 2 km, which we interpret as highly extended continental crust. Contrary to other seismic studies of the margins of the Grand Banks, we find seismic velocities of 8 km s⁻¹ and higher beneath this thin crustal layer in the continent-ocean transition. We conclude that mantle was locally emplaced at shallow levels without significant hydration from seawater or serpentinized mantle was removed along a décollement in the final stages of continental rifting. The outer edge of highly extended continental crust borders a 25-km-wide zone where seismic velocities increase gradually from 6.3 km s⁻¹ just below the top of acoustic basement to 7.7 km s⁻¹ at 5 km below basement. We interpret this area as a relatively narrow zone of exhumed and serpentinized continental mantle. Seaward, we imaged a thin and laterally heterogeneous layer with a seismic velocity that increases sharply from 5.0 km s⁻¹ in basement ridges to 7.0 km s⁻¹ at its base, overlying mantle velocities between 7.8 and 8.2 km s⁻¹. We interpret this area as unroofed mantle and very thin oceanic crust that formed at an incipient, magma-starved, ultraslow spreading ridge. A comparison of the conjugate rifted margins of the eastern Grand Banks and the Iberia Abyssal Plain show that they exhibit a similar seaward progression from continental crust to mantle to oceanic crust. This indicates that before continental breakup, rifting exhumed progressively deeper sections of the continental lithosphere on both conjugate margins. A comparison between the continent-ocean transition of the Grand Banks and Flemish Cap shows that the final phase of continental rifting and the formation of the first oceanic crust required more time at the Grand Banks margin than at the southeastern margin of Flemish Cap. Received 8 November 2005; accepted 14 August 2006; published 17 November 2006. Keywords: continental rifting; marine geophysics; passive margin. Index Terms: 3025 Marine Geology and Geophysics: Marine seismics (0935, 7294); 8105 Tectonophysics: Continental margins: divergent (1212, 8124); 8109 Tectonophysics: Continental tectonics: extensional (0905); 8150 Tectonophysics: Plate boundary: general (3040); 8180 Tectonophysics: Tomography (6982, 7270). Read Full Article (file size: 15993278 bytes) Cited by Citation: Van Avendonk, H. J. A., W. S. Holbrook, G. T. Nunes, D. J. Shillington, B. E. Tucholke, K. E. Louden, H. C. Larsen, and J. R. Hopper (2006), Seismic velocity structure of the rifted margin of the eastern Grand Banks of Newfoundland, Canada, J. Geophys. Res., 111, B11404, doi:10.1029/2005JB004156. Copyright 2006 by the American Geophysical Union.

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