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Trans-Atlantic Imaging of Lithosphere Asthenosphere Boundary

Final Report Summary - TRANSATLANTICILAB (Trans-Atlantic Imaging of Lithosphere Asthenosphere Boundary)

The objectives of the TransAtlanticILAB project were to image lithosphere-asthenosphere boundary (LAB) starting from the ridge axis at zero age up to 75 Ma oceanic lithosphere in the equatorial Atlantic Ocean continuously using a combination of seismic reflection and refraction methods. Four experiments were performed in the same study area. Over 2750 km of seismic reflection data were acquired using a 12 km long multi-sensor streamer and a large air gun source in partnership with industry, allowing to image the LAB down to 100 km depth on hundreds of meter scale. These results show that the LAB is thermally controlled and hence deepens with age and consists of a melt channel whose thickness decreases with age. Although the upper boundary of the channel deepens with age, following the 1200-1300° C isotherms defining a freezing front, the lower boundary of the channels is nearly flat at 80-90 km depth, possibly representing the base of shear deformation, separating the melt channel with the underlying asthenosphere. The P-wave velocity contrast at these boundaries are ~ 8%, requiring ~1.4% of melt in the channel. The presence of melt in the channel would reduce the viscosity, and hence would decouple the tectonically driven lithosphere with the convecting mantle below.

Coincident wide-angle seismic refraction data along these profiles using ocean bottom seismometers have also provided the image of LAB down to 70 km depth and show that the LAB is influenced by nearby transform faults and ridge-transform intersections. Beneath the transform faults, our results show deep hydration leading to serpentinization, shear mylonitization and low temperature (920° C) water-induced melting, thinning the lithosphere significantly. We also found that seismicity extends down to 34 km beneath the transform fault in the region of 2016 super-shear earthquake rupture, suggesting that the brittle deformations occur down to 600° C isotherm whereas the semi-brittle deformations extend down to 900° C in the mylonite zone. The combined analyses of deep seismic reflection and refraction data show that although the depth of the LAB increases smoothly with age, it varies significantly across fracture zones and other small-scale heterogeneities, indicating that the ridge segmentation and its evolution play important role in the evolution of oceanic lithosphere. We also find that the LAB thickens rapidly near the ridge axis due to deep hydrothermal circulations.

Our results have also allowed to characterise the nature of oceanic crust. We find that the crust beneath fracture zones and transform faults are not thin, consisting mainly of mantle peridotite, but are thick involving magmatic rocks, similar to normal oceanic crust. We find that the crustal thickness increases with age, suggesting that the mantle was hotter and has been cooling slowly. We also find a periodicity in the crustal thickness on the scale of a million year, implying the pulsing of the melt at the ridge axis. Our result has allowed to provide the very first continuous image of the Moho from 0 to 75 Ma for slow spreading crust. Using advanced analysis technique, we have discovered seismic layering the lower crust, revealing that the lower oceanic crust is formed by in-situ melt sill injection in the lower crust, and the magmatism plays more important in crustal accretion than previously realised.

Eleven papers have been published in peer reviewed journals and 13 papers are under consideration for publication in peer reviewed journals, including Science and Nature journals. Five PhD students, thirteen post-doctoral workers and three geophysicists from more than 8 countries have been trained on this project. Schlumberger provided their best technology to this project, reflecting the synergy between academia and industry. This project acted as catalysts for funding for another ERC project for passive seismic and magnetotelluric study, funding from UK NERC, Germany and US National Science Foundation, allowing to study the LAB using different techniques, providing group breaking discoveries.