From Bureau of Economic Geology, The
University of Texas at Austin (www.beg.utexas.edu).
For more information, please contact the author.
Bureau Seminar, October 30, 2009
Extension of continental crust at the margin of the eastern Grand Banks, Newfoundland
Harm J.A. Van Avendonk , Institute for Geophysics, Jackson School of Geosciences, UT-Austin
Seismic and gravity observations from the rifted margin of the eastern Grand Banks, Newfoundland, support a new model for extension of the continental crust from the shelf edge to ODP Site 1277, where mantle rocks are exhumed. We find that the largest decrease in crustal thickness, from about 28 km to 6 km, occurs beneath the continental slope of the Grand Banks over a distance of just 20 km. This rapid decrease in crustal thickness coincides with anomalously high seismic velocities (7.0-7.2 km•s-1) in the lower crust of the shelf edge. The thin crust of the continent-ocean transition (COT) in this area has a smooth basement surface, void of upper crustal blocks and prerift sediments.
We compare our geophysical results with a geodynamical model that represents rifting of a relatively hot continental lithosphere and with another numerical model that represents rifting of a cold lithosphere. Both geodynamic models suggest that crustal thinning beneath the continental slope was achieved by extensional faulting in the upper crust and ductile shear zones in the middle crust. The geodynamic models provide an explanation for the formation of distinct continental slopes at rifted margins: Beneath the continental shelf of the Grand Banks, the Moho and the strong lower crust show a dip of 50º without visible internal deformation.
A comparison of our numerical models with the seismic refraction data suggests a tectonic coupling between the strong lower crust and mantle in the final stages of rifting, which led to strain localization just seaward of the modern continental slope. The cold extension model extends the continental crust to a thickness smaller than 10 km over a width of 50 km in the distal margin, similar to what we inferred at the eastern Grand Banks. This model requires about 23 Myr of extension before continental breakup, and it predicts a small degree of melting in the mantle (13%). These numbers are applicable to Newfoundland-Iberia rift.