Open questions ...and some answers
What is 'Ocean-plate tectonics'?
' Ocean-Plate Tectonics ' is a mode of mantle convection characterised by the autonomous relative movement of multiple discrete, mostly rigid, portions of oceanic plates at the surface, driven and maintained principally by subducted parts of these same plates that are sinking gravitationally back into Earth’s interior and deforming the mantle interior in the process.
Ocean-plate tectonics is the Earth’s primary mode of planetary cooling, with the oceanic plate being part of the overarching overturn of Earth’s mantle: The plate forms out of rising mantle material at spreading ridges; it cools the Earth’s interior as the cold thermal boundary layer to mantle convection; and its sinking portions drive not only the plate itself but also dominate global flow in the mantle. Ocean-Plate Tectonics must have emerged on Earth at least 1 Billion years ago, and dominates Earth’s dynamics today (Crameri et al., 2019, The dynamic life of an oceanic plate).
How did we find out about Ocean-plate tectonics?
History of plate tectonic research: Scientists often explain how something works, but rarely explain how we have reached that knowledge. We have written a handy overview over the past 50 years of research on plate tectonics, the major milestones reached, the latest insights gained, and also over the next steps to be taken to better understand the fascinating surface deformation of the Earth, which is only part of something much bigger (Crameri et al., 2019, Plate tectonics☆).
How does Ocean-plate tectonics start?
Lithosphere-scale thermal shear zones: The shear-heating induced localisation of lithosphere-scale deformation is suggested as a potentially important mechanism for breaking the lithosphere. We show that amongst the terrestrial planets, this type of shear localisation is expected to occur most readily on the Earth (Crameri and Kaus 2010).
Hot mantle plumes: Hot mantle plumes can crucially weaken the lithosphere regionally. Thermal erosion at the plate bottom produces critical variation in plate thickness. Laterally mobile mantle plumes further excite horizontal mantle flow below the plate that leads to drips in thickened plate portions. All these mechanisms combined can lead to plate failure and subsequent subduction initiation (Crameri and Tackley 2016).
How does on-going Ocean-plate tectonics work?
Self-consistent single-sided subduction: We have, for the first time, produced a mantle convection model that self-consistently reproduces the realistic single-sided sinking of the surface plates. A free surface and a weak crustal layer lead to nature-like single-sided subduction. The first allows the lithosphere to bend in a natural manner and the latter promotes decoupling between the colliding plates. Both factors promote asymmetry and single-sidedness of the subduction zone (Crameri et al., 2012, GRL).
Arcuate subduction trenches: We find that an arcuate shape is the natural form for trenches and slabs even in Cartesian geometries, which is contrasting the misleading 'Ping-Pong Ball Theory': The slabs and their trenches above are rather deformed by the subduction-induced toroidal mantle flow (Crameri and Tackley, 2014).
Subduction-polarity reversals: We show that a change in subduction polarity is a common feature of mantle convection. It can be induced by upper- and lower-plate age variations and has an important impact on the overall evolution of the convective system. It is, for example, shown to be a viable mechanism to reproduce new and separate subduction zones. We propose that such a polarity reversal has formed the South Sandwich subduction system after splitting from the South America subduction zone (Crameri and Tackley, 2014).
Slab-mantle dynamics: The spontaneous evolution of slab geometry is strongly affected by trench motion and subduction-induced mantle flow. This causes the slab to deform strongly in all spatial directions and through time. Fascinating dynamics of the slab like the 'slab tunnelling' and of the surrounding mantle like the 'back-slab spiral flow' can develop (Crameri and Tackley, 2014).
Subduction zone initiation: To maintain ocean-plate tectonics, new subduction zones have to be formed during its evolution. In a novel approach, we combine geologic evidence, plate reconstruction, seismic tomography, and geodynamic understanding to shine new light on one of the long-standing big unknowns of the Earth Sciences: How do new subduction zones form on Earth during the past 150 Ma? The novel platform, www.szidatabase.org, is building bridges across the different disciplines to facilitate and foster new scientific insights, and it highlights that since the last 100 Ma, subduction breeds subduction (Crameri et al., 2020, A transdisciplinary and community-driven database to unravel subduction zone initiation).
How does Ocean-plate tectonics end?
Subduction shut-off: We show that subduction can shut off after a slab break off. This can be induced by a subduction-channel closure (i.e., an increase in upper-lower plate coupling) or by a spreading centre approaching the subduction trench (Crameri and Tackley, 2014 and Crameri and Tackley, 2015).