Plate Tectonics is the foundation of modern earth sciences, and provides basic framework for the origin of continents, ocean basins and mountain ranges. Plate Tectonics describes the division of the surface area of the earth into several plates that move independently over the surface of the planet. Each plate acts as an essentially rigid solid shell, which is called Lithosphere, and floats over the material below that flows slowly, called Asthenosphere. Most of the geological activities occur at plate boundaries as the solid lithosphere moves independently, producing giant earthquakes such as 2004 Sumatra and 2011 Japan, volcanoes, great mountains such as the Himalayas, the Andes. The base of the lithosphere, Lithosphere Asthenosphere Boundary (LAB), is the lower boundary of the plate. There are many definitions of the LAB, depending upon the method used. In one model, the LAB is defined as an isotherm (a surface of constant temperature), which is ~1300° C, melting point of mantle rocks. The rocks above this isotherm are sufficiently cool to behave rigidly, whereas rocks lying below this isotherm are sufficiently hot so they deform readily and flow. Beneath the oceans under this model, the base of this isotherm, and hence LAB, is controlled by cooling of the lithosphere as it moves away from spreading centres where it could be 2-6 km thick at zero age and thicken to ~100 km by the time it reaches 120 Ma toward continents or subduction zones. The precise depth-age curve of this isotherm depends on the thermal model used. Beneath the continents, the lithosphere is older and hence thicker, 100-250 km depending upon the age of the lithosphere. However, other models suggest that the LAB could be a boundary between dry, depleted mantle above a hydrated and more fertile mantle (Hirth and Kohlstedt, EPSL, 1996; Karato, EPSL, 2012). Since continents have gone through a complex geological history, the precise depth of the LAB is rather poorly defined. Therefore, here we focus on the oceanic lithosphere where different models of evolution of the lithosphere can be tested and verified. These results can then be used to understand the nature of the continental lithosphere as well. The most direct evidence of the base of the lithosphere has come from surface wave studies where the lithosphere is associated with a high S-wave velocity above a low velocity and high attenuation asthenosphere (Priestley and McKenzie, EPSL, 2006; Eton et al., Lithos, 2009) with a gradual decrease in the velocity. One of the limitations of surface wave tomography is that surface wave alone cannot distinguish a change in mantle velocity that occurs instantaneously in depth from a change that occurs over tens of kilometres. For example Eton et al. (2009) have shown that a typical fundamental mode surface wave data can be fitted by a sharp LAB at 160 km or a transition zone from 125 to 225 km. Body wave tomography can be used to estimate lithosphere thickness, but since the waves travel vertically, there is a trade-off between velocity and thickness, and therefore uncertainty could be more than 20 km (Tan and Helmberger, JGR, 2007). Recently, the mode-converted waves (P to S (Ps) and S to P (Sp)) from a sharp boundary can be used to determine the depth of the boundary and its velocity gradient. All these phases are primarily sensitive to changes in shear-wave velocity structure (either shear-wave velocity or impedance), and hence provide better resolution than surface waves. Rychert et al. (Nature, 2005) inverted both Ps and Sp waveforms and found that a velocity drop across a LAB of 5-8% in <11 km zone is required. Similar studies have been carried out by several authors such as Kawakatsu et al. (Science, 2009), Rychert and Shearer (Science, 2009), Schmerr (Science, 2012), but these results do not fit with the conventional understanding of the LAB (Priestley and McKenzie, EPSL 2006; Hirth and Kohlstedt, JGR, 1996; Faul and Jackson, EPSL, 2005; Karato, EPSL, 2012), which have led to heated debates about the plate tectonics and geodynamical processes. The recent image of an interface at 80-100 km depth beneath continents where the LAB should be at 150-200 km depth (Fisher et al. An. Rev. Earth Planet. Sci, 2010; Eton et al., 2009) enhances the debate further. There are two main problems with these studies: (1) Since these authors use long period body waves, the vertical resolution is >10 km. (2) Since earthquake data are used, the region of study is limited by stations on land, and there is no systematic study with appropriate lateral resolution. The magnetotelluric method has also been used to image the base of the lithosphere but its resolution is similar to the surface wave (Evans et al. Nature, 2005). We propose to address these issues using a seismic reflection method, where the vertical and lateral resolutions could be on the scale of hundreds of metres, several orders of magnitude better.

Plate Tectonics is the foundation of modern earth sciences, and provides basic framework for the origin of continents, ocean basins and mountain ranges. Plate Tectonics describes the division of the surface area of the earth into several plates that move independently over the surface of the planet. Each plate acts as an essentially rigid solid shell, which is called Lithosphere, and floats over the material below that flows slowly, called Asthenosphere. Most of the geological activities occur at plate boundaries as the solid lithosphere moves independently, producing giant earthquakes such as 2004 Sumatra and 2011 Japan, volcanoes, great mountains such as the Himalayas, the Andes. The base of the lithosphere, Lithosphere Asthenosphere Boundary (LAB), is the lower boundary of the plate.

There are many definitions of the LAB, depending upon the method used. In one model, the LAB is defined as an isotherm (a surface of constant temperature), which is ~1300° C, melting point of mantle rocks. The rocks above this isotherm are sufficiently cool to behave rigidly, whereas rocks lying below this isotherm are sufficiently hot so they deform readily and flow. Beneath the oceans under this model, the base of this isotherm, and hence LAB, is controlled by cooling of the lithosphere as it moves away from spreading centres where it could be 2-6 km thick at zero age and thicken to ~100 km by the time it reaches 120 Ma toward continents or subduction zones. The precise depth-age curve of this isotherm depends on the thermal model used. Beneath the continents, the lithosphere is older and hence thicker, 100-250 km depending upon the age of the lithosphere. However, other models suggest that the LAB could be a boundary between dry, depleted mantle above a hydrated and more fertile mantle (Hirth and Kohlstedt, EPSL, 1996; Karato, EPSL, 2012). Since continents have gone through a complex geological history, the precise depth of the LAB is rather poorly defined. Therefore, here we focus on the oceanic lithosphere where different models of evolution of the lithosphere can be tested and verified. These results can then be used to understand the nature of the continental lithosphere as well.

The most direct evidence of the base of the lithosphere has come from surface wave studies where the lithosphere is associated with a high S-wave velocity above a low velocity and high attenuation asthenosphere (Priestley and McKenzie, EPSL, 2006; Eton et al., Lithos, 2009) with a gradual decrease in the velocity. One of the limitations of surface wave tomography is that surface wave alone cannot distinguish a change in mantle velocity that occurs instantaneously in depth from a change that occurs over tens of kilometres. For example Eton et al. (2009) have shown that a typical fundamental mode surface wave data can be fitted by a sharp LAB at 160 km or a transition zone from 125 to 225 km. Body wave tomography can be used to estimate lithosphere thickness, but since the waves travel vertically, there is a trade-off between velocity and thickness, and therefore uncertainty could be more than 20 km (Tan and Helmberger, JGR, 2007).

Recently, the mode-converted waves (P to S (Ps) and S to P (Sp)) from a sharp boundary can be used to determine the depth of the boundary and its velocity gradient. All these phases are primarily sensitive to changes in shear-wave velocity structure (either shear-wave velocity or impedance), and hence provide better resolution than surface waves. Rychert et al. (Nature, 2005) inverted both Ps and Sp waveforms and found that a velocity drop across a LAB of 5-8% in <11 km zone is required. Similar studies have been carried out by several authors such as Kawakatsu et al. (Science, 2009), Rychert and Shearer (Science, 2009), Schmerr (Science, 2012), but these results do not fit with the conventional understanding of the LAB (Priestley and McKenzie, EPSL 2006; Hirth and Kohlstedt, JGR, 1996; Faul and Jackson, EPSL, 2005; Karato, EPSL, 2012), which have led to heated debates about the plate tectonics and geodynamical processes. The recent image of an interface at 80-100 km depth beneath continents where the LAB should be at 150-200 km depth (Fisher et al. An. Rev. Earth Planet. Sci, 2010; Eton et al., 2009) enhances the debate further.

There are two main problems with these studies: (1) Since these authors use long period body waves, the vertical resolution is >10 km. (2) Since earthquake data are used, the region of study is limited by stations on land, and there is no systematic study with appropriate lateral resolution. The magnetotelluric method has also been used to image the base of the lithosphere but its resolution is similar to the surface wave (Evans et al. Nature, 2005). We propose to address these issues using a seismic reflection method, where the vertical and lateral resolutions could be on the scale of hundreds of metres, several orders of magnitude better.

ADVANCED INDUSTRY TECHNOLOGY

Although we have been able to image down to 50-60 km using modified technology from industry (Singh et al., Nature Geoscience, 2008; Singh et al., Nature Geoscience, 2011; Singh et al., EPSL, 2012), imaging down to the base of the lithosphere at 80-100 km depth remains a great challenge. Recently, Schlumberger has revolutionised industry by developing a new streamer, called IsoMetrix (ww.slb.com/IsoMetrix), which does not contain only hydrophones but also 3-component accelerometers capable of recording seismic energy down to 2 Hz. It has several major advantages over the conventional streamers used in industry. Since the reflections from the sea surface recorded on hydrophones and vertical components have opposite signs (-1 and +1, respectively), they can be added together to remove the sea surface reflection effects, which enhances the frequency bandwidth, particularly the lower frequencies crucial for ultra-deep imaging, and remove the multiples. Since horizontal components record energy from third dimensions, the effect of seafloor scattering could be reduced significantly. This new technology was launched in June 2012. We propose to use this new technology to image the lithosphere down to the base (120 km depth) during transits from Europe to Brazil and Africa.

These reflection data will be complemented by coincident refraction (tomography) along the same profile to determine velocity structure of the crust and upper mantle. We also propose to deploy broadband seismometers for one year along the profile to determine the large-scale velocity structure and use receiver function techniques to image LAB at 10 km scale along the profile. Magnetotelluric measurements along the profile will provide information about the resistivity structures. These combined data will be interpreted to develop a model of the formation and evolution of oceanic lithosphere.

SCIENTIFIC OBJECTIVES

The main objectives of this project are:

1.Image the oceanic lithosphere and the LAB systematically, from zero to 100 Ma of age, using a combination of seismic reflection, refraction, receiver function, surface wave and magnetotelluric methods across the Atlantic Ocean along a 2D profile providing images of the LAB at different scales and distinguish among different models of the LAB, once and for all.

2.Image the possible presence of melt lenses in the mantle beneath the spreading centres, and develop a model of melt generation and migration towards the surface

3.Quantify aging of the oceanic crust and upper mantle as it moves away from the spreading centre.

4.Image possible deep penetrating faults and their distribution as the lithosphere cools and subsides with age away from the ridge axis.

5.Quantify and model the hydration process of the oceanic lithosphere with age.

STUDY AREA

The equatorial Atlantic Ocean was chosen for this study because the oceanic fracture zones in this region maintain their azimuth over 2000 km and thus enable a straight 2 D profile to be shot along one segment of the oceanic lithosphere. However, the recent United Nation Law of Sea has allowed the countries to claim sea up to 350 miles from their continental self (instead of the earlier 200 miles Exclusive Economy Zone), and hence permits from surrounding Sub-Saharan African countries are required prior to data acquisition. Given the multidisciplinary nature of the project involving several different experiments by international partner, we decided to acquire in the international waters.

Secondly, the equatorial Atlantic also hosts largest fracture zones and transform faults on the Earth, namely Chain, Romanche and St Paul Fracture Zones, with a total age contrast of 85 Ma in only a distance of 500 km, providing an unique opportunity to study oceanic lithospheres of contrasting ages. The age contrast across the Chain Fracture Zone is 15 Ma and that across the Romanche Fracture Zone is 45 Ma and 35 Ma across the St Paul Fracture Zone. These fractures zones are seismically active, including Mw=7.1 1994 Romanche earthquake. These fracture zones have also been responsible for the un-mixing of the water between the southern and northern oceans, and hindering the migration of ecosystem from south to north across the Equator.

Bathymetry map showing the TransAtlanticIlab profile (white) with age of the lithosphere (light blue lines). The numbers indicate the age of lithosphere in million years. The red dashed line marks the position of the Mid-Atlantic Ridge, where the age of lithosphere is zero. The yellow lines mark the position of the transform faults (solid) and fracture zones (dashed). Black dashed lines mark the Greenwich meridian and the Equator.

SCIENCE OUTLINE

Scientific context

Advanced industry technology

Scientific objectives

Study area