Abstract

R. S. White writes: A central result of the deep seismic reflection profile across the Goban Spur rifted continental margin presented by Peddy et al. 1989 is that, except where the water bottom multiple obscures the image, highly reflective layeredlower crust is found across the entire margin. The profile is of high quality and, together with a similar line across the conjugate Canadian margin (Keen et al. 1989), provides one of the best images yet of the structure across both sides of a continental rift immediately prior to formation of a new ocean basin (Fig. 1).

The reflectivity pattern of the crust beneath the continental shelf (labelled Goban Spur on Fig. 1) is broadly similar to that in the 50 km wide thinned zone (called 'transitional crust' by Peddy et al., fig. 1), with transparent upper crust and layered lower crust. From this observation Peddy et al. suggest that the layering in the thinned crust represents the attenuated remnants after extension of an originally much thicker package of reflectors. It follows from this interpretation that all the reflectors were emplaced prior to rifting. Peddy et al. go on to infer that crustal extension occurred by pure shear from the similarity on the continental shelf and beneath the rift zone in the ratio of the thickness of the transparent upper crust to that of the layered lower crust.

I suggest that, given the amount of stretching that has occurred beneath the transitional crust of the rift zone, the adiabatic decompression

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