Ocean-bottom seismometers deployed near the equatorial Mid-Atlantic Ridge provide new constraints on the mechanisms of plate formation and divergence.
Plate divergence at Mid Ocean Ridges where new tectonic plates are formed is generally thought to be a passive process dominated by the gravitational pull of subducting plates, however a new study published in Geology and authored by Department of Earth Sciences, Head of Department Prof. Mike Kendall and others shows that at some diverging locations this process is likely to be less passive than originally thought.
In the classic model of seafloor-spreading by Forsyth and Uyeda (1975), oceanic plate motions and the associated plate divergence that occur at mid-ocean ridges are driven by gravitational forces that pull denser, older plates down at subduction zones. These models fit fast spreading Pacific Ocean ridges well due to the system of surrounding subduction zones known as the ‘ring of fire’, but our understanding of slower spreading ridges such as the Mid-Atlantic ridge have until now been limited.
This study presents results from a year-long deployment of ocean-bottom seismometers near the equatorial Mid-Atlantic Ridge. The experiment, PI-LAB (which stands for passive imaging of the lithosphere-asthenosphere boundary) is part of a Natural Environment Research Council (NERC) project led by Dr. Kate Rychert and Dr. Nick Harmon of the University of Southampton (Kate and Nick are now at the Woods Hole Oceanographic Institution and full funding details are shared later in the post). 39 ocean-bottom seismometers and 39 ocean-bottom magnetotelluric instruments were used to detect seismic wave speed and propagation at the ridge. Based on these data the authors of the study use seismic anisotropy (how seismic wavespeed varies with direction) to better understand partial melting and mantle flow in relation to the driving forces for plate motion in slow-spreading centres.
Image showing the portion of the equatorial Mid-Atlantic Ridge between the Chain and Romanche Fracture Zones and the locations of the ocean-bottom seismometers, the deepest of which were nearly 6km below the surface.
The experiment has shown that the thickness of the lithosphere at these slow spreading ridges increases rapidly over a short distance helping to channel melt towards the ridge and to organise it in a ridge-parallel orientation. Melt removal is much less efficient than at fast spreading ridges and the organisation of the melt under the ridge weakens the boundary and focuses strain on the ridge.
The results are very different from the more widely studied fast-spreading-centres of the Pacific and overall, it suggests that ridge forces at slow spreading centres could play a more significant role in driving plate motions than previously assumed. The Atlantic Ocean has opened and closed at least three times in its history. This study shows that the Mid-Atlantic Ridge is playing an active role in opening the Atlantic.
Mike says of the study ‘This work shows the role of partially molten rock in driving the Atlantic ocean apart at the Mid-Atlantic Ridge. The PI-LAB (Passive Imaging of the Lithosphere-Asthenosphere Boundary) experiment has been incredibly fruitful, producing over 25 publications, but more importantly it has changed our view of mid-ocean ridges in place where the formation of new plates is slow.’
He continues ‘The broader project is also greatly enhanced by seismic surveys using human-made seismic sources (seismic refraction and reflection experiments). In total, the wider project included 4 cruises over nearly 20 weeks, involved 7 principal scientists from 4 countries, and over 50 support scientists, including PhD students and postdocs. Our originally conceived NERC project has grown considerably – thus showing the value of international collaboration.’
As well as NERC, funding was also leveraged from several other sources, such as: the European Reaserch Council (EURO-LAB: Experiment to Unearth the Rheological LAB, and TransAtlantic I-LAB: TransAtlantic Imaging of the LAB) and NSF in the US (CA-LAB: Central Atlantic LB).
You can read the full paper at https://doi.org/10.1130/G51550.1