Periodic Reporting for period 4 - EURO-LAB (Experiment to Unearth the Rheological Oceanic Lithosphere-Asthenosphere Boundary)
Berichtszeitraum: 2020-10-01 bis 2022-08-31
Plate tectonics has been a fundamental tenet of Earth Science for nearly 50 years, but
fundamental questions remain, such as where is the base of the plate and what makes a plate,
“plate-like?” A better understanding of the transition from the rigid lithospheric plate to the weaker
mantle beneath – the rheological lithosphere-asthenosphere boundary (LAB) - has important
implications for the driving forces of plate tectonics, natural hazard mitigation, mantle dynamics,
the evolution of the planet, and climate change. There are many proxies used to estimate the
depth and nature of the base of tectonic plates, but to date no consensus has been reached. For
example, temperature is known to have a strong effect on the mechanical behaviour of rocks.
However, it has also been suggested that the chemical composition of the plate provides additional
strength or that melt weakens the mantle beneath the plate.
This project is designed to systematically image an oceanic plate beneath the Atlantic
from birth at ridge to 40 My old seafloor. We will deploy ocean bottom seismometers (OBS) and
magnetotelluric (MT) instruments to image the plate at a range of resolution scales
(laterally and in depth) and sensitivities to physical and chemical properties. This large, focused,
interdisciplinary effort will finally determine the processes and properties that make a plate strong
and define it. The results will be put in context through seismic imaging in other locations and also through global seismic imaging.
We imaged the lithosphere-asthenosphere system beneath the Atlantic Ocean and globally with a range of resolutions and sensitivites. We found that the thickness of ocean plates increases with age monotonically in some locations, but is more variable in others. Therefore, the thickness of the plate is determined by temperature to first order. However, the base of the plate is also sharp in many locations. Other factors also define the plate, likely melt beneath the plate, which decreases the viscosity of the asthenosphere. Therefore, the lithosphere-asthenospere system is dynamic and dictated by melt generation and migration.
fundamental questions remain, such as where is the base of the plate and what makes a plate,
“plate-like?” A better understanding of the transition from the rigid lithospheric plate to the weaker
mantle beneath – the rheological lithosphere-asthenosphere boundary (LAB) - has important
implications for the driving forces of plate tectonics, natural hazard mitigation, mantle dynamics,
the evolution of the planet, and climate change. There are many proxies used to estimate the
depth and nature of the base of tectonic plates, but to date no consensus has been reached. For
example, temperature is known to have a strong effect on the mechanical behaviour of rocks.
However, it has also been suggested that the chemical composition of the plate provides additional
strength or that melt weakens the mantle beneath the plate.
This project is designed to systematically image an oceanic plate beneath the Atlantic
from birth at ridge to 40 My old seafloor. We will deploy ocean bottom seismometers (OBS) and
magnetotelluric (MT) instruments to image the plate at a range of resolution scales
(laterally and in depth) and sensitivities to physical and chemical properties. This large, focused,
interdisciplinary effort will finally determine the processes and properties that make a plate strong
and define it. The results will be put in context through seismic imaging in other locations and also through global seismic imaging.
We imaged the lithosphere-asthenosphere system beneath the Atlantic Ocean and globally with a range of resolutions and sensitivites. We found that the thickness of ocean plates increases with age monotonically in some locations, but is more variable in others. Therefore, the thickness of the plate is determined by temperature to first order. However, the base of the plate is also sharp in many locations. Other factors also define the plate, likely melt beneath the plate, which decreases the viscosity of the asthenosphere. Therefore, the lithosphere-asthenospere system is dynamic and dictated by melt generation and migration.
We organised the use of seismic instruments from the US and France via contracts.
We deployed and recovered the seismic and MT instruments at the mid-Atlantic ridge.
We performed data quality tests and also a series of corrections to the data. We made timing corrections, solved for instrument orientation, and corrected for tilt and compliance.
We solved for sediment characteristics using P-to-S conversion from the base of the sediment. (Agius et al., 2018)
We processed gravity, swath, and magnetics. (Harmon et al., 2018)
We have our results from S-to-P imaging, surface wave tomography, body wave tomography, local seismicity, SKS splitting, and MT.
We have imaged the LAB globally with focus in several particular regions.
We have developed several cutting edge seismic techniques to enhance imaging.
We have found new and interesting results using the data that somewhat linked to the lithosphere-asthenosphere system and also relatively unrelated to the lithosphere-asthenoshere system.
We deployed and recovered the seismic and MT instruments at the mid-Atlantic ridge.
We performed data quality tests and also a series of corrections to the data. We made timing corrections, solved for instrument orientation, and corrected for tilt and compliance.
We solved for sediment characteristics using P-to-S conversion from the base of the sediment. (Agius et al., 2018)
We processed gravity, swath, and magnetics. (Harmon et al., 2018)
We have our results from S-to-P imaging, surface wave tomography, body wave tomography, local seismicity, SKS splitting, and MT.
We have imaged the LAB globally with focus in several particular regions.
We have developed several cutting edge seismic techniques to enhance imaging.
We have found new and interesting results using the data that somewhat linked to the lithosphere-asthenosphere system and also relatively unrelated to the lithosphere-asthenoshere system.
We have been able to do much more than image the tectonic plate beneath the mid-Atlantic. We also developed a method to characterise sediment (Saikia et al., 2020; Agius et al., 2018; Rychert et al., 2018; Chichester et al., 2020). We imaged the tectonic plate beneath Cascadia (Rychert et al., 2018) and also continents globally (Tharimena et al., 2017).
We were able to record and quantify the rupture of a rare M 7 event on an oceanic transform, and our research demonstrated super shear propagation of the rupture (Hicks et al., 2021). In addition, we were able to image the mantle transition zone beneath the ridge system, finding evidence for upwelling, which challenges the current paradigm of material transfer from the lower to upper mantle (Agius et al 2021). These were not the main objectives of the work but resulted in publications in Nature and and Nature Geoscience.
We put our results in a global context. We wrote a summary paper including additional in depth analyses on continental lithosphere in an invited '100 Grand Challenges in Earth Science' issue in Journal of Geophysical Research (Rychert et al., 2020).
We were able to record and quantify the rupture of a rare M 7 event on an oceanic transform, and our research demonstrated super shear propagation of the rupture (Hicks et al., 2021). In addition, we were able to image the mantle transition zone beneath the ridge system, finding evidence for upwelling, which challenges the current paradigm of material transfer from the lower to upper mantle (Agius et al 2021). These were not the main objectives of the work but resulted in publications in Nature and and Nature Geoscience.
We put our results in a global context. We wrote a summary paper including additional in depth analyses on continental lithosphere in an invited '100 Grand Challenges in Earth Science' issue in Journal of Geophysical Research (Rychert et al., 2020).