Modelling the controls on melt generation during continental extension and breakup

Abstract

Rifting is the process that leads to the formation of oceans. Rifting is the break up of
continents, leading to the formation of new oceanic floor between the two continental
plates. Although the concept of continental rifting is accepted within the scientific
community, it is still debated what controls the volume and composition of igneous
material generated at these constructive plate boundaries. Here I present the results
of dynamic modelling of rifted margins. I have explored the consequences of margin
and mantle structure on the melt generated during continental extension and breakup.
The central aim is to understand how melting affects the rifting of continents, especially
in the North Atlantic. In order to understand the enigmatic melt production observed
around the North Atlantic various tools are developed for interpreting the model output.
These are predictions of primary major element composition of the melt, rare-earth
element composition of the melt, predictions of the crystallised mid-oceanic ridge basalt
composition and the seismic velocity of the lower crust.
The thickness of the lithosphere has a very large impact on the subsequent rifting style.
Extension of a 125 km thick thermally and rheologically defined lithosphere that has no
prior thinning produces little melt during breakup. The Southeast Greenland margin
rifted above a pre-thinned lithosphere and at initial fast half spreading rates. Further-
more, to generate the thickness, chemistry and seismic velocities observed off this margin,
rifting was coincident with the arrival of a 50 km thick, 200 ?C thermal anomaly. This
thermal anomaly is not a plume, rather an exhaustible thermal layer that has drained
along the sub-lithospheric topography from a distal plume. The melts generated are high
in MgO, and depleted in TiO. They are depleted in rare-earth elements. This would
lead to high seismic velocities within the underplate being, as observed off Southeast
Greenland.

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