Lithospheric flexure
Plate tectonics is based on the assumption that the Earth's
outermost layer, or lithosphere, behaves rigidly on long (i.e. > 106
a) geological time-scales. The main evidence for its rigidity has
come from studies of the way that it responds to surface loads
such as ice-sheets, sediments, and volcanoes. In
most cases, the response takes the form of a simple bending, or
flexure, of the crust. By comparing
observations of flexure to calculations based on simple elastic
plate models it is possible to constrain the long-term strength of
the lithosphere.
The elastic thickness of the oceanic lithosphere
Oceanic flexure results show that the elastic
thickness (which is determined by the flexural rigidity) increases
with age of the lithosphere at the time of loading. Therefore, as
the oceanic lithosphere ages and cools, it becomes more rigid in
the way that it responds to
surface loads. As the figure to the right shows, the depth to which
materials behave elastically on long time-scales is 2-3 times less
than the seismic thickness. This has been interpreted as indicating
that there must be some form of stress relaxation in the
lithosphere after a load is emplaced on it but, we know little about
the mechanism by which this is accomplished. We can say,
however, that once a volcano has been on the lithosphere for
longer than about 1 Ma there is little subsequent relaxation. Most
estimates of the elastic thickness from oceanic regions can be
explained by the depth to the 300-600 degree isotherm based on
plate cooling models. There are some regions (e.g. French
Polynesia), however, which are difficult to explain by these
isotherms and these regions have been the subject of much study
in the past few years.
Collaborator: Andrew Goodwillie
The long-term strength of extended continental
lithosphere
One of the more controversial topics in flexure studies is the question of the long-term strength of extended
continental lithosphere. Some workers have proposed (based on
gravity modelling studies) that extended continental crust is weak
during rifting and has remained so since then. Why extended
continental lithosphere is weak is difficult to explain because
oceanic lithosphere would in all likelihood increase its strength
following a heating event. Some workers have suggested that
rifting results in a catastrophic reduction in the strength of the
mantle while others have speculated that the mantle recovers its
strength but, is somehow de-coupled from the strong uppermost
crust. We have been carrying studies to determine the elastic
thickness at rift-type basins that takes into account the possibility
of strength during rifting. Preliminary results at the Gabon
margin, offshore West Africa show it to be highly segmented as
regards its long-term strength with strong zones that abut weak
ones.
Collaborator: Jonathan Stewart
The elastic thickness of continental lithosphere
Although it has been suggested that the elastic thickness of
continental lithosphere shows a similar dependence with age, as
does the oceanic lithosphere, it is difficult to explain continental
elastic thickness estimates by a single set of thermal parameters.
A majority of continental elastic thickness estimates, especially
those from extended regions, are relatively low and in the range
of 0-20 km. Only in the cool cratonic interiors does the continental
lithosphere appear capable of showing any degree of long-term
strength. We have been using high-resolution gravity anomaly
and topography data sets and forward and "inverse" modelling
techniques to estimate the elastic thickness of the continental
lithosphere. Results in South America (summarised in the figure
below) show that the elastic thickness varies spatially from high
values over Brazilian and Guyana shields to low values over the
sub-Andean basins. We have speculate that the low values correspond to regions that had been rifted during a previous extensional event.
Collaborators: Jonathan Stewart, Simon Lamb, Roxby Hartley,
Derek Fairhead (Leeds)
Relevant publications
tony@earth.ox.ac.uk
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