Breakup at the West Galicia Rifted Margin: Waveform Inversion, Depth Imaging and Structural Reconstructions

The west Galicia margin is one of the best known non-volcanic margins, with several high quality seismic surveys, including most recently a combined MCS and wide-angle data-set collected using the RV Maurice Ewing in 1997. Prestack depth migration, coupled with the results from the wide-angle data, provide not only the detailed velocity structure of the margin, but also optimum depth sections. The results allow new insights into the structure and evolution of this margin, through analysis of the structural evolution immediately before breakup. The depth and velocity sections show that landward of the exposed "Peridotite Ridge" a major detachment fault (S) cuts down from the top of rotated crustal fault blocks, flattening at the base of the crust to follow the top of a high velocity layer interpreted as partially serpentinised peridotites. Numerical modelling (Perez-Gussinye and Reston 2001) shows that the development of a mantle-penetrating fault and the serpentinisation of mantle peridotites are natural consequences of the thinning of the crust during ongoing extension: the thinning crust becomes entirely brittle, allowing water to penetrate into the mantle. Another consequence of ongoing extension appears to have been the development of multiple phases of faulting in the blocks above the S-reflector: multiple phases of faulting are expected at stretching factors greater than about 2, and can be inferred from structural reconstruction of the latest phase of extension. The relationship between detachment faulting, the emplacement of the peridotite ridge and the final breakup of the continental lithosphere however appears complex and may be analogous to that at the Iberia Abyssal Plain margin further south. Here the H detachmnent fault appears similar to S but is cut by a later west-dipping low-angle fault that follows top basement for several km before cutting to depth (Krawczyk et al., 1996). We have also addressed the nature of the S reflector further through waveform inversion, constrained by the general velocity models derived from iterative prestack depth migration and the wide-angle data. The results seem to indicate that S represents an increase in seismic velocity, perhaps overlain by a region of reduced velocity (brecciation of the overlying plate). This is completely consistent with our structural interpretation of the seismic depth images. Krawczyk, C.M., Reston, T.J., Beslier, M.-O., Boillot, G., 1996. ODP Leg 149 Sci. Results, 603-615. Perez-Gussinye, M., and Reston, T.J., 2001. J. Geophys. Res., v. 106, 3961-3976.