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.

Lithospheric flexure model

The elastic thickness of the oceanic lithosphere

Elasticity of Oceanic Plate vs Plate Age 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.

Segmentation of Passive Margin around Gabon, West Africa

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.

Map of Crustal Elasticity in South America

Collaborators: Jonathan Stewart, Simon Lamb, Roxby Hartley, Derek Fairhead (Leeds)

Relevant publications

  • Watts, A.B. and M. Talwani, Gravity anomalies seaward of deep- sea trenches and their tectonic implications, Geophys. J. Roy. Astro. Soc., 36, 57-92,1974
  • Watts, A.B., and J.R. Cochran, Gravity anomalies and flexure of the lithosphere along the Hawaiian-Emperor seamount chain, Geophys. J. Roy. Astro. Soc., 38, 119-141,1974
  • Watts, A.B., J.R. Cochran, G. Selzer, Gravity anomalies and flexure of the lithosphere: A three-dimensional study of the Great Meteor Seamount, N.E. Atlantic, Jour. Geophys. Res., 80, 1391-1398,1975
  • Watts, A.B. and W.B.F. Ryan, Flexure of the lithosphere and continental margin basins, Tectonophysics, 36, 25-44,1976
  • Watts, A.B., An analysis of isostasy in the world’s oceans: 1. Hawaiian-Emperor Seamount Chain, Jour. Geophys. Res., 83, 5989- 6004, 1978
  • Bodine, J.H. and A.B. Watts, On lithospheric flexure seaward of the Bonin and Mariana Trenches, Earth and Planetary Science Lett., 43, 132-148, 1979
  • Watts, A.B., J.H. Bodine and N.M. Ribe, Observations of flexure and the geological evolution of the Pacific Ocean basin, Nature, London, 283, 532-537, 1980
  • Watts, A.B., J.H. Bodine and M.S. Steckler, Observations of flexure and the state of stress in the oceanic lithosphere, J. Geophys. Res., 85, 6369-6376, 1980
  • Bodine, J.H., M.S. Steckler and A.B. Watts, Observations of flexure and the rheology of the oceanic lithosphere, J. Geophys. Res., 86, 3695- 3707, 1981
  • Watts, A.B., G.D. Karner and M.S. Steckler, Lithospheric flexure and the evolution of sedimentary basins, in: The Evolution of Sedimentary Basins, (ed.) Sr. P. Kent, M.H.P. Bott, D.P. McKenzie and C.A. Williams, Phil. Trans, Roy. Soc. Lond., 305A, 249-281, 1982
  • Karner, G.D. and A.B. Watts, Gravity anomalies and flexure of the lithosphere at mountain ranges, J. Geophys. Res. 88, 10,449- 10,477, 1983
  • Watts, A.B., U. ten Brink, P. Buhl, T. Brocher, A multi-channel seismic study of lithospheric flexure across the Hawaiian-Emperor seamount chain, Nature, 315, 6015, 105-111, 1985
  • Watts, A.B., Gravity anomalies, crustal structure and flexure of the lithosphere at the Baltimore Canyon Trough, Earth and Planet. Sci. Lettrs., 89, 221-238, 1988
  • Watts, A.B., Lithospheric Flexure due to Prograding Sediment Loads: Implications for the origin of Offlap/Onlap patterns in Sedimentary Basins, Basin Res., 2, 133-144, 1989
  • Coakley, B.J. and A.B.Watts, Tectonic controls on the development of Unconformities: The North Slope, Alaska, Tectonics, 10,101-10,130, 1991
  • Watts, A.B., “The elastic thickness of the lithosphere and the evolution of sedimentary basins”, Basin Research , 4, 169-178., 1992
  • Goodwillie, A.M. and A. B. Watts. “An altimetric and bathymetric study of elastic thickness in the Central Pacific Ocean”. Earth Planet. Sci. Lett., 118, 311-326, 1993
  • Watts, A.B. “Crustal Structure, Gravity anomalies and flexure of the Lithosphere in the vicinity of the Canary Islands”. Geophys. J. Intern., 119, 648-666, 1994
  • Hartley, R., Watts, A. B. and J. D. Fairhead. “Isostasy of Africa”, Earth Planet. Sci. Letts., 137, 1-18,1996
  • Stewart, J. and A. B. Watts, “Gravity anomalies and spatial variations of flexural rigidity at mountain ranges”. J. Geophys. Res. 102, 5327-5352,1997