Plate-scale potential-energy distributions and the fragmentation of ageing plates

Abstract

Geoid anomalies associated with mid-ocean ridge systems and a number of continental margins imply that, on the scale of individual plates, the old ocean lithosphere represents a gravitational potential energy sink. Since lateral variations in potential energy contribute to deviatoric stresses in the lithosphere, the changing potential-energy distributions in individual plates associated with the growth and ageing of the oceanic lithosphere may be expected to result in changes in the intraplate stress field. Analytical models for simple plate geometries using lithospheric density models consistent with small positive ( +6 m) geoid anomalies across continental margins show that the growth of oceanic lithosphere over a period of 200 Ma may contribute to a decline in the mean plate potential energy U ̄ P of about −1 × 10 12 N m −1 and thus contribute a mean extensional stress difference ( σ zz − σ xx ) in continental lithosphere of up to about 8 MPa (averaged over a 125 km thick lithosphere). These estimates are sensitive to the assumed mean continental potential energy U ̄ c , about which there is some uncertainty. For higher U ̄ c , approaching that of the mid-ocean ridges ( U MOR), the net decline in U ̄ P may be as much as −1.7 × 10 12 N m −1, whereas for significantly lower U ̄ c , approaching that of old ocean lithosphere, plate growth may increase U ̄ P transiently by up to 2.7 × 10 12 N m −1, leading to compression in the continents. In the African and Antarctic plates the ageing of the ocean lithosphere since the late Jurassic is estimated to have contributed to a decline in U ̄ P of about −0.6 × 10 12 and −0.95 × 10 12 N m −1 respectively, contributing a mean stress difference of about 5 MPa and 7.5 MPa in the respective continents. Whereas the predicted stress changes associated with ageing of the oceanic lithosphere are significantly smaller than most laboratory-based estimates of the extensional strength of the lithosphere, they may provide an important contribution to the stress fields that eventually lead to the fragmentation of ageing plates.