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Sub-lithospheric small-scale convection (SSC) and delamination of oceanic lithosphere have been proposed already 30 years ago to be responsible for high heat flow at and flattening of oceanic seafloor, relative to a half-space cooling model (Parsons and McKenzie, 1978). More recently, the process of small-scale convection has been quantified in terms of their onset time and vigour. Here, we will give a brief overview of the major achievements in this field and will present some of our recent results on the volcanic and seismic consequences of oceanic SSC.
Different origins for intra-plate volcanic chains in oceanic basins have been proposed. The most obvious and popular one is the presence of a stationary mantle plume. But a lot of the Pacific volcanic chains (e.g. Puka-puka and Line Islands) lack a clear linear age progression, which makes a hotspot origin less likely. Another mechanism could be SSC, and we elaborated this model numerically using 3-D thermo-chemical convection models. Results provide us with quantitative relationships between lithospheric age, mantle rheology, and melting relations. These new insights allow us to further constrain the origin of intra-plate volcanism. We will show that volcanism by SSC can explain many of the characteristics of the Puka Puka ridges, and the volcano groups associated with the Cook-Austral, Line and Marshall Islands.
Below the lithosphere, SSC stirs the mantle. This might have consequences for the observed seismic anisotropy of (e.g.) the Pacific plate. The amplitude of seismic anisotropy in the old Pacific is small compared to the young Pacific (Nishimura & Forsyth, 1989; Becker et al., 2003). This suggests that the build-up anisotropy is somehow destroyed during aging of the plate. SSC could be a cause for destruction or a change of fabric. We investigated this possibility with a series of numerical models. From these, we conclude that SSC in combination with non-Newtonian rheology is easily capable to reduce the amplitude of Pacific azimuthal anisotropy by a factor 2 or more, which fits the observations well.
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