EXTENSION AND MAGMATISM AT THE GOBAN SPUR RIFTED CONTINENTAL MARGIN:

1. EXTENSION AND MAGMATISM AT THE GOBAN SPUR RIFTED CONTINENTAL MARGIN: GEOPHYSICAL AND GEOCHEMICAL APPROACHES T. A. Minshull1, A. D. Bullock1, S. M. Dean1, T. J. Henstock1, B. J. Murton1, R. N. Taylor1 and R. S. White2 1Southampton Oceanography Centre, University of Southampton, European Way, Southampton, SO14 3ZH United Kingdom 2Department of Earth Sciences, University of Cambridge, Bullard Laboratories, Madingley Road, Cambridge CB3 0EZ, United Kingdom. Introduction:The presence of a broad zone of exhumed mantle at the West Iberia rifted margin is now well documented (e.g., Whitmarsh et al., 2001), but it remains unclear how widespread such margins are. The UK western approaches margin at Goban Spur (Figure 1) represents another generally amagmatic rifted margin where extensive geophysical and borehole datasets exist. Here the timing of rifting is constrained by the recovery of syn-rift and early post-rift sediments at DSDP sites 549 and 550 of late Hauterivian to late Albian age (de Graciansky et al., 1985), which suggests a rift duration of 14-22 m.y. This poster presents results from a wide-angle seismic profile acquired across the margin in 2000, combined with modelling of coincident potential field data. Geophysical observations:A wide-angle seismic profile was acquired roughly coincident with the seaward end of the BIRPS WAM reflection profile (Peddy et al., 1989), extending from magnetic anomaly 34 to the foot of the continental slope. Our new data were supplemented at the landward end by data from an existing coincident wide-angle profile (Horsefield et al., 1994). Traveltime modelling of these data shows the presence of a 70 km region between thinned continental crust and anomaly 34 where velocities of 7.2-7.6 km/s are found 4-6 km below basement, and Poisson’s ratio in the upper 1 km of basement is high (Figure 2). In this region, the high Poisson’s ratios indicate the presence of serpentinised mantle at top basement (Figure 3) and the high velocities at depth are also best explained by the presence of partially serpentinised mantle. The velocity model also satisfies gravity observations (Figure 4). However, in contrast to the region of exhumed mantle off West Iberia, here the region of exhumed mantle must have relatively high magnetisations to explain the observed magnetic anomalies (Figure 4). These high magnetisations may be due to magnetite formation due to prolonged interaction of sparsely sedimented serpentinite with seawater. Figure 1:Location map for seismic experiment. Coloured grid represents water depth contoured at 200 m intervals, compiled from swath bathymetric data acquired in 1999 and 2000 and extensive single-beam echosounder data. Thin black line marks the BIRPS WAM seismic reflection profile and thick line marks the wide-angle seismic profile presented here. Also marked are ocean bottom hydrophone (OBH) locations, sonobuoy locations, DSDP drill sites and a dredge site where basement rocks were recovered in 2001. Figure 2:Upper panel shows P wave velocity model based on traveltime modelling of OBH and sonobuoy data. Velocities are contoured every 0.25 km/s. Black triangles mark OBH locations and white triangles mark sonobuoys. Arrows mark locations of DSDP Sites 551 (close to the line) and 550 (50 km to the south). Bold numbers indicate Poisson’s ratio at the top of crystalline basement and thick black lines mark regions where wide-angle reflections are observed from the top of unaltered mantle. Lower panel shows interpreted geological structure based on geophysical observations and drillling. Figure 4:Gravity and magnetic models along the wide-angle seismic profile. (a) Misfit between observed and predicted gravity anomaly. Dashed line marks misfit resulting from direct conversion of seismic velocities to densities, while solid line marks misfit corresponding to the model shown in (c). (b) Solid line marks predicted gravity, crosses mark shipboard observations and dotted line marks satellite-derived gravity anomaly. (c) Best fit gravity model. (d) Solid line marks predicted magnetic anomaly from model in (e), crossed mark shipboard magnetic measurements and dashed line marks anomaly extracted from the grid of Verhoef et al. (1996). (e) Magnetic model with magnetised layers shaded vertically for normal polarity and horizontally for reversed polarity. Figure 3:Poisson’s ratio as a function of P-wave velocity for laboratory samples of serpentinite (solid symbols), basalts (open symbols) and gabbro (grey symbols). Data are compiled from the literature by Bullock and Minshull (2004). The range of values determined from 50-120 km disance in the model of Figure 2 are indicated by the large circle (upper 1 km beneath basement) and the large rectangle (depths > 1 km). Geochemical models:Significant volumes of synrift igneous rocks are present at this margin and were recovered by DSDP drilling at sites 550 and 551 (Figure 1). The composition of these rocks potentially provides an independent constraint on the rifting history, since it will depend on the variation of melt fraction with depth. Controlling parameters include the rift duration and the potential temperature of the underlying asthenosphere. We use a melting model to compute melt fractions as a function of time and depth (Figure 5), and a melt parameterisation to predict compositions (e.g., Figure 6). Application of this approach to the Goban Spur margin suggests a lower mantle temperature and slightly longer rift duration compared with the Atlantic margins further north. (Figure 7). Figure 6: Concentrations of iron2 and silca computed using the parameterisation of Niu and Batiza (1991) to relate melt fraction to melt composition. The source composition is their MPY-90 MORB pyrolite and melt fractions are calculated from the model of Bown and White (1995). While model output yields primary compositions for the generated melts, volcanics recovered from the margins have experienced variable amounts of fractional crystallisation. To account for this, modelled compositions have been regressed along liquid lines of descent to a common concentration of 8 weight percent MgO. The result are contours of Fe[8], that is particularly sensitive to mantle temperature, and Si[8], that is sensitive to rift duration Figure 5: Mean degree of melting in the melting region as a function of rift duration and mantle temperature, for a lithosphere thickness of 125 km and a maximum degree of extension of 50, predicted by the melting model of Bown and White (1995). Figure 7: To allow comparison between the real samples and predicted composition, we have recalculated the sample compositions to 8 wt. % MgO and only considered samples with between 6% and 10% MgO. These recalculated compositions are plotted in Fe[8] vs. Si[8] space and compared with the predicted values of these parameters from Figure 6. References: Bullock, A. D., and T. A. Minshull, From continental thinning to sea-floor spreading: crustal structure of the Goban Spur rifted contiental margin, southwest of the UK, Geophys. J. Int., submitted, 2004. Bown, J. W., and R. S. White, Effect of finite extension rate on melt generation at rifted continental margins, J. Geophys. Res., 100, 18011-18029, 1995. de Graciansky, P. C., and 13 others, The Goban Spur transect: geologic evolution of a sediment-starved passive continental margin, Geol. Soc. Am. Bull., 96, 58-76, 1985. Horsefield, S. J., R. B. Whitmarsh, R. S. White, and J.-C. Sibuet, Crustal structure of the Goban Spur rifted continental margin, NE Atlantic, Geophys. J. Int., 119, 1-19, 1994. Niu, Y., and R. Batiza, An empirical method for calculating melt compositions produced beneath mid-ocean ridges: application for axis and off-axis (seamounts) melting, J. Geophys. Res., 96, 21753-21777, 1991. Peddy, C., and 8 others, Crustal structure of the Goban Spur continental margin, Northeast Atlantic, from deep seismic reflection profiling, J. Geol. Soc. London, 146, 427-437, 1989. Verhoef, J., W. Roest, R. MacNab, J. Arkani-Hamed, and members of the Working Group, Magnetic anomalies of the Arctic and North Atlantic oceans and adjacent land areas, Geol. Surv. Canada, Open File 3125a, 1996. Whitmarsh, R. B., G. Manatschal, and T. A. Minshull, Evolution of magma-poor continental margins from rifting to seafloor spreading, Nature, 413, 150-154, 2001. Conclusions: 1. At the Goban Spur margin, a 70 km wide region of exhumed mantle lies between thinned continental crust and oceanic crust. 2. Based on seismic velocities, the upper 5 km of mantle is 50-100% serpentinised and the degree of serpentinisation drops to ~25% at 5-8 km depth. 3. The uppermost serpentinised mantle is strongly magnetised, perhaps due to magnetite formation by prolonged interaction with seawater. 4. The composition of synrift igneous rocks recovered by drilling is consistent with lower mantle temperatures and perhaps slower rifting compared with volcanic margins further north. Acknowledgements: We thank all who sailed with us on RRS Charles Darwin cruise 124 for their assistance with data acquisition in trying conditions. Financial support was provided by the UK Natural Environment Research Council, Amerada Hess Ltd and The Royal Society. Ocean bottom instrumentation was provided through the Geomar EU large scale facility.