Richard G Davy1, Timothy A Minshull1, Gaye Bayrakci1, Jon M Bull1 1 University of Southampton, Southampton, United Kingdom

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4. Gravity ModellingOne way to check the validitiy of the velocity model is to calculate and compare the free-air gravity anomaly with the observed anomaly along WE-1. The velocity model is padded east, west and below, using existing models and extrapolation. The extended model is converted to density using the Nafe-Drake equation (Ludwig et al. (1970)). Gravity due to each grid node in the model is approximated as an infinite cuboid, and is summed at observation points along the WE-1 profile (Fig 3). A linear regional trend is observed and subsequently removed from the data. The resulting calculated free-air gravity anomaly shows a good first order approximation of the observed anomaly, but highlights slight misfit at the Peridotite Ridge western end of WE-1.

Figure 2: Top left: sediment velocity model, bottom left: preferred TOMO2D model, top right: residual travel times through the sedi-ment velocity model, bottom right: residual travel times through the preferred TOMO2D model. Location of the Peridotite Ridge and S-reflector are indicated.

Figure 3 Above: Extended velocity model for gravity calculation. A is the WE-1 TOMO2D model, B is the RayInvr model of ISE-1 (Zelt et al. (2003)), C is an extrapolation of mantle velocities to match the depth of the ISE-1 velocity model, D is an extrapolation of the western end of the WE-1 velocity model. Below: The match between the observed and calculated free-air gravity anomalies.

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Continental Hyper-extension and Mantle Exhumation at the Ocean-continent Transition, West Iberia :

New Insights from Wide Angle Seismic

1. IntroductionRifting and the subsequent breakup of continental lithosphere has given rise to the magma-poor Iberia-Newfoundland rifted margin in the North Atlantic Ocean. Hyper-extension of continental crust is observed at the deep Galicia margin (west of Spain, Fig. 1) and has been accommodated by the rotation of continental fault blocks, which are underlain by the S-reflector, an interpreted detachment fault, which has exhumed serpentinised mantle peridotite. West of these features is the enigmatic Peridotite Ridge (PR) which has been suggested to delimit the western extent of the ocean-continent transition. An outstand-ing question at this margin is of the nature of basement west of the PR and where unequivocal oceanic crust begins, with little existing data to constrain this boundary.

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Figure 1: A) Atlantic Ocean and the relative locations of the conjugate Iberia-Newfoundland rift margins. B) Satellite magnetics (EMAG2, Maus et al. (2009)) over study region. White dots indicate location of the C34 isochron, black dashed indicates inferred sea-ward limit of the OCT at Iberia, solid black lines indicate previous seismic lines, white line indicates new 2D seismic profile WE-1. C) Galicia-3D survey design, large black dots indicate OBS/H from new study, red triangles indicate location of previous ISE-1 OBH.

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2. Galicia-3D / WE-1New 3D multichannel seismic covering an area of 80 km (east-west) by 20 km (north-south) (the 3D Box,) was collected by the R/V Marcus G. Langseth during June - August of 2013 (Fig. 1). Seventy two ocean bottom seismometers and hydrophones (OBS/H), arranged in four east-west profiles also record wide angle seismic arrivals. Here we focus on Western extension 1 (WE-1), an additional 2D seismic profile extending 90 km west of the 3D Box, with the aim of identifying the onset of oceanic crust west of the Peridotite Ridge (Fig 1 & 2). This profile consisted of 32 OBS/H, 6 westward of the Peridotite Ridge, and is co-incident with the western end of ISE-1 (Sawyer et al. 1997) (Fig 1). An additional eight OBH from the ISE-1 seismic line are uti-lised in our dataset.

3. Seismic TomographySeismic velocity models along WE-1 are created using RayInvr, the forward ray tracing algorithm of Zelt and Smith (1992), and TOMO2D, the tomographic inversion algorithm of Korenaga et al. (2000). An initial sediment velocity model (determined through forward modelling , Fig 2), 9,517 first arrival picks, 1,187 S-reflection picks and an arbitrarily defined floating reflec-tor are input into the TOMO2D algorithm and inverted over 10 iterations. The final model exhibits a smooth reduction in travel time misfit and χ2 value over the 10 iterations to 49 ms and 0.98, respectively (Fig 2). The final model clearly resolves the Peridotite Ridge and the S-reflector. Observed and calculated seismic travel times can be seen in Fig 4.

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Figure 5: 1D velocity profiles east and west of the Peridotite Ridge. Dark gray show velocity bounds of Atlantic Oceanic crust, light gray is velocity bounds of transitional crust at the SIAP and Newfoundland.

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6. Velocity StructureWest of the Peridotite Ridge, the top of acoustic basement corresponds to velocities between 3.5 - 4.2 km/s (Fig 5 & 6). We observe seismic velocities, in a top basement layer 2.5 – 3.0 km thick, increase rapidly from ~4.0 km/s to 7.2 – 7.5 km/s, with vertical veloci-ty gradients of 1.1 – 1.5 s-1. Below this, seismic velocities increase from 7.2 – 7.5 km/s to 7.5 – 8.2 km/s, with vertical velocity gradi-ents of > 0.3 s-1. This velocity structure is consistent with decreasing serpentinisation of mantle material with depth, which is also observed east of the Peridotite Ridge, below the S-reflector, and in the Southern Iberian Abyssal Plain.

Nowhere west of the Peridotite Ridge are seismic velocities indicative of the velocity jump at the Moho boundary, consistent with a lack of observable PmP arrivals. Additionally there is no evidence of oceanic layer 3., and thus there is no clear evidence of oce-anic crust directly west of the Peridotite Ridge.

East of the PR, the 6.5 km/s velocity contour strongly correlates with the S-reflector (Fig 6 & 7). Exhumed mantle peridotite at the S-reflector is 45% serpentinised (Carlson and Miller (2003)). Below this, seismic velocities generally increase smoothly into mantle velocities, and 0% serpentinisation, ~2 km below this reflector. Corrugations in the velocity contour below the S-reflector have a loose correlation with the termination of normal faults, which can act as conduits for water transportation to the mantle.

Figure 6: Above: 3.0 and 6.5 km/s velocity contours overlain on a Kirchoff time-migrated reflection seismic image of WE-1. Below: Pre-ferred velocity model, illuminated east of the Peridotite Ridge by depth migrated ISE-1 reflection seismic. Black dashed lines are faults, blue dashed lines are the top of pre-rift sediment, red dashed lines are the top of crystalline crust, yellow dashed line is the S-reflector.

Figure 7: Preferred velocity model with reflected rays used to resolve the S-reflector (solid black line). Dashed lines are various horizons picked from depth migrated ISE-1, black dashed lines are faults, blue dashed lines are the top of pre-rift sediment, red dashed lines are the top of crystalline crust, yellow dashed line is the S-reflector. White boxes are OBS/H recording S-reflections.

7. Conclusions• Exhumed mantle peridotite along the S-reflector detachment fault is 45% serpentinised, decreasing to 0% at depths of~2 km below this surface. Normal faults, which bound rotated continental blocks, act as conduits to hydrate and serpentinise mantle peridotite.• Evidence of mantle at shallow depths, from receiver gathers and 1D velocity structures, as well as a lack of PmP arrivals, sug-gests continued exhumation of mantle directly westward of the Peridotite Ridge. These observations are consistent with those made over exhumed continental mantle in the Southern Iberia Abyssal Plain, (Dean et al. 2000).

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Receiver gathers from instruments recovered west of the Peridotite Ridge, and over the uni-dentified basement, reveal the presence of mantle velocities ( ~ 8 km/s) at short offsets (< 11 km) (Fig 4). Such observations indicate mantle at shallow depths, possibly ex-humed/unroofed, or underlying a thin crust. Two seismic arrivals, which resemble PmP ar-rivals, are observed at offsets of ~20 km and ~15 km on the receiver gathers of OBS 73 and 74, respectively. However, modelling of these arrivals does not return a sensible PmP reflec-tor, indicative of an oceanic crustal layer; this combined with the inconsistent offset of the ar-rivals suggests that these may be diffractions.

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Figure 4: Receiver gathers from instruments west of the Peridotite Ridge, over unidentified basement. Every fifth first arrival pick and associated uncertainty are indicated by red lines, while calculated arrivals are indicated by small boxes.

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References1. Carlson, R. L., and D. J. Miller (2003), Mantle wedge water contents estimated from seismic velocities in partially serpentinised peridotites, Geophysical Research Letters, 30(5).2. Dean, S., T. Minshull, R. Whitmarsh, and K. Louden (2000), Deep structure of the ocean‐continent transition in the southern Iberia Abyssal Plain from seismic refraction profiles: The IAM‐9 transect at 40 20′ N, Journal of Geophysical Research: Solid Earth (1978–2012), 105(B3), 5859-5885.3. Korenaga, J., W. Holbrook, G. Kent, P. Kelemen, R. Detrick, H. C. Larsen, J. Hopper, and T. Dahl‐Jensen (2000), Crustal structure of the southeast Greenland margin from joint refraction and reflection seismic tomography, Journal of Geophysical Research: Solid Earth (1978–2012), 105(B9), 21591-21614.4. Ludwig, W. J., J. E. Nafe, and C. L. Drake (1970), Seismic refraction, The sea, 4(Part 1), 53-84.5. Maus, S., U. Barckhausen, H. Berkenbosch, N. Bournas, J. Brozena, V. Childers, F. Dostaler, J. Fairhead, C. Finn, and R. Von Frese (2009), EMAG2: A 2–arc min resolution Earth Magnetic Anomaly Grid compiled from satellite, airborne, and marine magnetic measurements, Geochemistry, Geophysics, Geosystems, 10(8).6. Minshull, T. A. (2009), Geophysical characterisation of the ocean–continent transition at magma-poor rifted margins, Comptes Rendus Geoscience, 341(5), 382-393.6. Sawyer, D. S., T. Reston, M. P. Gussinye, C. A. Zelt, J. A. Austin, Y. Nakamura, J. Danobeitia, C. D., and S. P. o. E. C. 97‐05 (1997), The Iberia Seismic Experiment (ISE97): MCS reflection profiles, EOS, 78(F468).7. Zelt, C. A., K. Sain, J. V. Naumenko, and D. S. Sawyer (2003), Assessment of crustal velocity models using seismic refraction and reflection tomography, Geophysical Journal International, 153(3), 609-626.