Presentation on theme: "Conjugate Passive Rifted Margins Separated by Rapid Strain Rate 1 – The Southern Margin Preliminary Insights to the Crust and Upper Mantle Structure of."— Presentation transcript:
Conjugate Passive Rifted Margins Separated by Rapid Strain Rate 1 – The Southern Margin Preliminary Insights to the Crust and Upper Mantle Structure of the Northern Seychelles Continental Margin. V. Sansom 1, J.S. Collier 1, T.A. Minshull 2, R.B. Whitmarsh 2, J.M. Kendall 3, C. Lane 2, T. Ryberg 4, G. Rumpker 4. 1 Dept. of Earth Science and Engineering, Imperial College, RSM Building, Prince Consort Road, London, SW7 2BP, UK. 2 School of Ocean and Earth Sciences, Southampton University, Southampton Oceanography Centre, European Way, Southampton, SO14 3ZH, UK. 3 School of Earth Sciences, Leeds University, The University, Leeds, LS2 4DW, UK. 4 GeoForschungsZentrum, Potsdam, Telegrafenberg, Potsdam, 14473 Germany. Corresponding Author : email@example.com 4. Preliminary MCS Interpretation Over 1000 km of multichannel seismic data (MCS) were collected in the area (red lines in Figures 3) using a 96 channel, 2.4 km long streamer with a 6290 in 3 / 60 s pop air gun array for the longer main wide-angle line, and a 3890 in 3 / 30 s pop for the other lines. An interpreted brute stack of the main 1. Introduction Marine geophysical profiles were collected over the conjugate Seychelles-Laxmi Ridge pair of rifted conjugate margins during Charles Darwin cruise 144 in Jan/Feb 2003 (Figure 1). The aim was to obtain data from an initially fast spreading rift (half rate 59 mm/yr ) to constrain modelling of strain rate and its effect on passive margin structure. The margins were restored using seafloor spreading isochrons and a single transect was designed across the entire reconstructed rift avoiding fracture zones and seamounts. Figure 2 shows the reconstruction from the Royer et al. (2003) pole, rotating the Indian Margin to the Seychelles to Anomaly 27. In this poster we present results from the Seychelles margin. 2. Background and Underway Data The Seychelles Islands are situated in the NW Indian Ocean (Figure 1) and consist principally of pre-Cambrian granite surrounded by a carbonate platform. The transect extends north of Mahé, the main island, over the shallow water plateau and into the deep-water Eastern Somali Basin (1.5-5°S, 55-58°E – Figure 3). The line extends beyond magnetic anomaly A27 onto undisputed oceanic crust. Several prominent sea-mounts were imaged using swath bathymetry within the continent ocean transition zone. Three of these were dredged (1, 2 and 3 on Figure 2) and yielded basalts erupted from shallow marine or sub-aerial environments (see Figure 12). Figure 1: Seychelles – Laxmi rifted conjugate continental margins. Seychelles: This Poster Laxmi Ridge: Poster T12A -0442 60° 0° T12A- 0441 3. Wide-Angle Data and Forward Modelling Wide-angle data were obtained along the line with 32 OBS/H from GEOMAR, 21 land seismometers from Leeds/Potsdam and a tuned 12 gun bolt airgun array (Figure 4). Good quality data were obtained with arrivals observed to the maximum 60 s and 360 km range (Figure 5). Forward modelling with rayinvr has been used to make a preliminary interpretation. Figure 6 is the velocity Figure 7: Line 9 wide-angle seismic data recorded on Mahé. Arrivals seen out to 360 km. rayinvr forward modelling shows a possible mantle reflection (blue) arriving behind mantle diving rays (green). Figure 9: Wide-angle data from OBH49, mid line 9. Crustal diving rays are easily identified. Modelling shows ‘normal’ oceanic crust is present. Figure 8: Wide-angle data recorded at Denis Island (near the edge of the carbonate platform). A clear moho reflection (PmP, green) is observed. Figure 4: Wide-angle station locations and quality. We have a good spread of high quality data. Also labelled are the three stations shown for rayinvr modelling, OBH49, Denis and Mahe. Figure 6: A basic starting velocity model for the rayinvr modelling to make a preliminary assessment of the arrivals present in the data. Figure 14: An uninterpreted and interpreted brute stack of line 9 MCS. Moho nature changes from complex (continent side) to smooth (ocean side) across the margin. Crust thins from 14 km to 6 km towards the ocean end. A narrow package of seaward dipping reflectors is observed as well as an unidentified block that dredge material indicates to be oceanic. Figure 11: Zoom in on Line 9 to show uninterpreted continental side of margin. Figure 13: Zoom in on Line 9 to show uninterpreted oceanic side of margin. Figure 10: Zoom in on Line 19 to show close up of Seaward Dipping Reflectors (SDRs). Line 9 Figure 3: Bathymetric map of area. Sea mount dredge sites are indicated. Red lines are seismic survey lines. model used for the forward modelling taken from previous work (Matthews and Davis, 1966). From Mahé, one of the southern most stations (Figure 7), we can see mantle diving rays and mantle reflections out to maximum offset (360 km). Data from a station near the edge of the carbonate platform (Denis), show clear moho reflections (Figure 8). Normal oceanic crust can be identified on data from OBH49 towards the north (Figure 9). Figure 5: Representation of data picked so far along model line. Arrivals are seen out to 360 km with a possible shadow zone which supports the idea of underplating in the region (e.g. 280-320 km, Figure 7). Moho SouthNorth SouthNorth Figure 2: Magnetic reconstruction to Anomlay 27 from Royer et al (2003). The main profile (red) is shown. Survey line line 9 is shown in Figure 14. The Moho is widely imaged beneath the transect due to the sediment starved nature of the margin. Its character changes from complex and layered on the continent side (Figure 11) to a more flat and smooth form towards the ocean (Figure 13). Based on preliminary depth conversion using likely crustal velocities, the crust appears to thin from 14 km to 6 km over a horizontal distance of 8 km. That is, before the Moho image is lost under the rough platform edge. A narrow package of seaward dipping reflectors is evident under sediments off the edge of the platform with 15-20° dip and 16 km length (Figure 10). The observed block at CDP 10000 in Figure 14 corresponds to dredge 1. Figure 12 shows hand-specimen and thin-section photographs that indicate an intergrown felspar trachytic texture that suggests excess volcanism during the development of the margin. Dredge 1 OBH 49 OBH 60 Figure 11 Figure 10 Figure 13 moho sub moho moho Figure 12 References : 1.DeMets, C. and J. Y. Royer, A new high-resolution model for India-Capricorn motion since 20 Ma: Implications for the chronology and magnitude of distributed crustal deformation in the Central Indian Basin, CURRENT SCIENCE, 85 (3): 339-345, 2003 2.Matthews, D.H. and D. Davies, Geophysical studies of the Seychelles bank, Philosophical Transactions of the Royal Society of London Series A Mathematical and Physical Sciences. 259; 1099, Pages 227-239. 1966. 3.Zelt, C. A. and R. B. Smith, Seismic traveltime inversion for 2-D crustal velocity structure, Geophysical Journal International, 108, 16-34, 1992 Figure 12: Hand Specimen (1) & Thin Sections (2-4). Showing trachytic texture, oxide exsolution (black groundmass) and ocassional pyroxene phenocrysts. Dredge 1 Samples 1234 50μm70μm880μm xp pp xp 5. Conclusions and Future Work Figure 15: Underway data profile plot across reconstructed conjugate pair. Asymmetry between the Laxmi Ridge - Seychelles Plateau conjugate pair can be seen from gravity, magnetic and bathymetry data in Figure 15. Future work will involve the reduction of the wide-angle data from both margins, to establish the nature of this assymetry or otherwise. South North
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