GEOPHYSICAL INVESTIGATION OF A CONJUGATE PAIR OF RIFTED MARGINS FORMED AT HIGH EXTENSION RATE: LAXMI RIDGE – NORTHERN SEYCHELLES BANK, WESTERN INDIAN OCEAN Collier, J S 1, Minshull, T A 3, Whitmarsh, R B 3,Kendall, J-M 2, Lane, C I 3, Sansom, V 1, Rumpker, G 4, Ryberg, T 4 1 Dept. of Earth Science and Engineering, Imperial College, RSM Building, Prince Consort Road, London, SW7 2BP United Kingdom; 2 School of Earth Sciences, Leeds University, The University, Leeds, LS2 4DW United Kingdom; 3 School of Ocean and Earth Sciences, Southampton University, Southampton Oceanography Centre, European Way, Southampton, SO14 3ZH United Kingdom; 4 GeoForschungsZentrum, Potsdam, Telegrafenberg, Potsdam, Germany Figure 1 (a) inset: Satellite gravity field for the NW Indian Ocean 12 showing the Seychelles (S) and Laxmi Ridge (LR) margins in their present position. Laxmi Ridge is marked by a well-defined gravity low (blue/green). The two roughly triangular regions were reconstructed in (b), red line = cruise track. (b) Reconstruction of the conjugate survey areas at A27 time, using a Chron 27ny pole 13, showing magnetic anomalies and depth contours (1000, 2000m: GEBCO 2003). The white line at the join roughly follows the Chron 27ny picks of Miles et al. 14. Offshore, symbols and lines mark OBS/H deployments and shooting tracks; red = Seychelles margin, black = Laxmi Ridge margin. Black arrow = spreading direction at A27 - A26 time 13. Hollow blue arrows = fast direction for teleseismic split S-wave arrivals at land stations. Introduction: Although the majority of numerical models of lithospheric extension recognise the importance of strain rate [1-4], thermal conditions [5-7] and temperature-dependent rheology [8, 9] there is no consensus on which, if any, is the dominant factor. Our objective was to test the role of extensional strain rate in the development of rifted continental margins. Geophysical profiles and swath bathymetry were acquired in January-February 2003 by RRS Charles Darwin across a pair of conjugate rifted margins in the NW Indian Ocean (Fig. 1a) which are thought to have formed under much higher strain rates (59 mm/a half- rate) than typical Atlantic margins. A single transect was designed across the pair of margins avoiding fracture zones and seamounts (Fig. 1b). Laxmi Ridge margin: Here, we collected wide-angle and multichannel seismic (MCS) reflection data along a 480 km profile from anomaly A27 to the continental rise north of the Gop Rift. 32 OBS recorded shots from a 6920 cu.in airgun source, fired every 60s (Fig. 2). Coincident MCS profiles were recorded with a 2.4km 96-channel streamer fired every 30s (Fig. 5). The sediment thickness is 2-3 km. The crustal thickness is ~7 km at anomaly A27, and reaches ~15 km at the northern end of the profile. Lower crustal velocities reach km/s on the seaward flank of Laxmi Ridge (Fig. 6). Seaward-dipping intrabasement reflections are seen for over 30 km south of the crest of Laxmi Ridge. The northern edge of Laxmi Ridge abuts the Gop Rift within which high-amplitude, linear, SSE- NNW trending magnetic anomalies have been mapped whose origin is currently unresolved. NE Seychelles margin:The Seychelles islands consist principally of pre-Cambrian granite surrounded by a carbonate platform. Wide-angle and MCS data were obtained along a 300 km NNE line extending from the main island Mahé to undisputed oceanic crust (beyond anomaly A27) of the eastern Somali Basin (Fig. 1). An additional 800 km of MCS data, including Line 13 (Fig. 4), were also collected in the area. 32 OBS were deployed along the main line and 21 land seismometers were installed on the islands for the wide-angle work (Fig. 3). On MCS profiles, top oceanic basement and Moho are imaged at 6.5 s and 8 s twt, respectively, indicating that the early oceanic crust of this margin is anomalously thin. This idea is supported by wide-angle modelling results (Fig. 6). Seaward-dipping reflectors are also evident. Three seamounts were dredged and yielded basalts erupted in shallow marine or subaerial environments. Figure 4 : Example of an MCS reflection profile (unmigrated and fk-dip filtered) from the Seychelles margin (Line 13; Fig.1b). SDRS = seaward dipping reflector sequence, individual reflector lengths reach 6 km, lateral extent of the wedge is approximately 30 km (CDPs ), and the estimated wedge thickness is 3.5 km. Possible sill intrusions (black arrows) are indicated. White arrows indicate Moho reflections. The grey arrow marks the position of magnetic anomaly A27. Sediments are thinner than over the conjugate Laxmi Ridge margin (Fig. 5). Figure 5 :Example of an MCS profile (unmigrated) from the Laxmi Ridge margin and Gop Rift (Line 5; Fig.1b). SDRS = seaward dipping reflector sequence, individual reflector lengths reach 12 km, lateral extent of the wedge is > 35 km (CDPs ), and the estimated wedge thickness is 5 km. The profile shows a relatively thick (up to 2.6 s) sequence of Indus basin sediments. The Gop Rift extends between CDPs 3000 and 9500 and magnetic anomaly A27 is positioned approximately 145 km to the south of CDP Summary: The seaward parts of both margins are similar in the presence of upper crustal seaward-dipping reflector sequences and indications of coincident modest underplating in the lower crust. This suggests that both margins formed in a magma-rich (volcanic?) environment. Basalts dredged from three seamounts on the NE Seychelles margin appear to substantiate this interpretation. Although the seismostratigraphy of the post-rift sediment successions over the Gop Rift and the oceanward side of the Laxmi Ridge appear similar, the crustal structures under these two regions appear quite different. This suggests that the Gop Rift may be a pull-apart basin that formed at a late stage of continental break-up. Laxmi Ridge is underlain by thinned continental crust up to 8 km thick under our transect. In broad terms the crustal structure along the reconstructed transect is asymmetrical with a region of broad extension across the Laxmi Ridge margin and a relatively abrupt transition from continental crust to oceanic crust over the Seychelles margin. Work in progress will analyse and date the dredge samples, interpret the MCS profiles and refine the seismic crustal structure. Finally the crustal structure will be simulated using numerical models of extension. References: 1 England, P., Constraints on extension of continental lithosphere, J. Geophys. Res. 88, , 1983.; 2 Kusznir, N.J. & R.G. Park, The extensional strength of continental lithosphere. Geol. Soc. spec. pub. 28, pp.35-52, 1987.; 3 Bassi, G., Relative importance of strain rate and rheology for the mode of continental extension, Geophys J Int 122, , 1995.; 4 Bown, J.W. & R.S. White, Effect of finite extension rate on melt generation at continental rifts, J Geophys Res 100, , 1995.; 5 White, R.S. & D.P. McKenzie, Magmatism at rift zones: the generation of volcanic continental margins and flood basalts, J. Geophys. Res., 94, 7685, 1989.; 6 Hopper, J.R. & W.R. Buck, The effect of lower crustal flow on continental extension and passive margin formation, J Geophys. Res. 101, 20175, 1996.; 7 Buck, R., Modes of continental lithospheric extension, J. Geophys. Res., 96, , 1991.; 8 Boutilier, R.R. & C.E. Keen, Geodynamic models of fault-controlled extension, Tectonics 13, , 1994.; 9 Hopper, J.R. & W.R. Buck, Styles of extensional decoupling, Geology 26, , 1998.; 10 Miles, P. R., M. Munschy, and J. Ségoufin, Structure and early evolution of the Arabian Sea and East Somali Basin, Geophys. J. Int., 15, , 1998.; 11 Royer. J.-Y., and 6 others, Paleogene plate tectonic evolution of the Arabian and Eastern Somali Basins, Spec. Publ. Geol. Soc. London, 195, 7-23, 2003.; 12 Sandwell, D.T. and Smith, W.H.F. Marine gravity anomaly from Geosat and ERS 1 satellite altimetry. Journal of Geophysical Research, B, Solid Earth and Planets, 102(5): 10,039-10,054, 1997.; 13 Royer. J.-Y., and 6 others, Paleogene plate tectonic evolution of the Arabian and Eastern Somali Basins, Spec. Publ. Geol. Soc. London, 195, 7-23, 2003.; 14 Miles, P.R., Munschy, M. and Segoufin, J. Structure and early evolution of the Arabian Sea and East Somali Basin. Geophysical Journal International, 134: , Zelt, C.A. and R.B. Smith, Seismic traveltime inversion for 2-D crustal velocity structure. Geophys. J. Int. 108, Acknowledgements: We thank the Master and crew of cruises RRS Charles Darwin 134B, 144 and 149 for their assistance in collecting the data presented here. We also thank Bramley Murton for collecting rocks at Dredge Site #3 and Ernst Flueh for providing access to the Geomar pool of OBS. The Seychelles National Oil Company (SNOC) and Patrick Joseph provided invaluable help and advice in the Seychelles. Figure 2 (top). Ocean bottom hydrophone (OBS03) record section from the Laxmi Ridge margin (Fig. 1b). For clarity, only every third trace is plotted. The section shows clear sedimentary, crustal and mantle arrivals, both reflections and refractions (filter 5-25 Hz, amplitude proportional to offset). Mode conversions from the top of the basement are also observed. The offset at which the mantle reflection (PmP) is first seen varies from ~ 15 km at the southern (oceanic) end of the Laxmi profile, to ~ 70 km at the northern (continental) end [not shown], reflecting the variation in crustal thickness across the margin. Figure 3 (bottom): Vertical geophone record section from station MSEY(04) on Mahé, Seychelles (Fig. 1b). The record shows PmP arrivals between 80 and ~170 km, and the mantle refraction (Pn) as a first arrival only beyond 160 km (filter 5-25 Hz, no amplitude scaling) because of the thick continental crust beneath the Seychelles Plateau. A sub- Moho reflection from around 60 km depth (white inverted triangles) is also imaged. Figure 6. Preliminary P-wave velocity models of the conjugate margins. Contours are every 0.25 km/s, and bold lines are model boundaries. The models were obtained by fitting observed travel times of sediment and crustal arrivals with the ray-tracing inversion program Rayinvr 15. Approximately half the instruments have been modelled. Additional constraints on basement structure were taken from the coincident MCS reflection profiles. Data from the wide-angle experiment MCS profiles Preliminary wide-angle models A27 LR S Fig. 2 Fig. 3