DY072

BODC profile

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 describe division of earth's surface into several plates that move independently over the planet. Each plate acts as a rigid solid shell, called Lithosphere, floating over material below that flows slowly, called Asthenosphere. Most of the geological activities occur at plate boundaries as solid lithosphere moves independently, producing giant earthquakes, volcanoes, great mountains like the Himalayas, 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, LAB is defined as an isotherm (a surface of constant temperature), which is 1300C, melting point of mantle rock. Rock above this isotherm is sufficiently cool to behave rigidly, but rock lying below this isotherm is sufficiently hot so it deforms and flows. Beneath oceans in this model, the base of this isotherm, hence LAB, is controlled by cooling of lithosphere as it moves away from spreading centres where it could be 2-6km thick at age 0 and thicken to 100km by the time it reaches 120Ma toward continents or subduction zones. Beneath continents, lithosphere is older and thicker, 100-250km depending upon age. Other models suggest that the LAB could be a boundary between dry, depleted mantle above hydrated and fertile mantle (Hirth and Kohlstedt 1996; Karato 2012). Since continents have gone through complex geological history, the precise depth of LAB is poorly defined. Here we focus on oceanic lithosphere where different models can be tested and verified. These results can then be used to understand the nature of continental lithosphere.

The most direct evidence of the lithosphere base has come from surface wave studies where lithosphere is associated with high S-wave velocity above low velocity, high attenuation asthenosphere (Priestley and McKenzie 2006; Eaton et al. 2009) with gradual decrease in velocity. 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, therefore uncertainty could be more than 20 km (Tan and Helmberger 2007).

In the Passive Imaging of the Lithosphere-Asthenosphere Boundary (PiLAB) phase 1 our goal is to provide in situ passive seismic and electromagnetic constraints on the structure of the Lithosphere-Asthenosphere system on young seafloor (<40Ma) as part of an experiment to characterize the Atlantic lithosphere. The goal of the experiment is to use complementary information from different geophysical techniques (active and passive seismic, electromagnetic, shipboard geophysics, heatflow) to constrain at high resolution properties of lithosphere and asthenosphere. This will allow us to address questions about the evolution of oceanic lithosphere-is it thermally controlled or is it a compositional boundary, or does its rheological behavior transition with age between the two? For the asthenosphere we will determine whether its relative weakness, high conductivity and low seismic velocity are caused by increased melt, hydration or is purely thermally controlled.

For the first phase of the experiment, we will deploy 30 broadband ocean bottom seismometers to image the crust and upper mantle down to 300 km using passive seismic techniques and 3 Magnetotelluric instruments to measure resisitivity. We will use surface and body waves to determine the isotropic and anisotropic structure of the crust and upper mantle, we will use converted phases to determine the character and depth of the LAB and other upper mantle discontinuities. We will use shear wave splitting to determine azimuthal anisotropy across the region. We will construct 1-D resistivity profiles at 0, 25 and 40 Ma seafloor.