Plate Flexure and the Strength of Planetary Lithospheres

Tony Watts
Oxford University

ABSTRACT

Plate flexure is a phenomenon that describes how the lithosphere responds to
long-term (> 105 a) loads. By comparing the flexure in the vicinity of ice,
volcano, and sediment loads to predictions based on elastic plate models it
has been possible to estimate the elastic thickness of the lithosphere, Te.
In the oceans, Te is the range 2-50 km and is determined mainly by plate and
load age. Continents, in contrast, are associated with Te values of  > 80
km. There is no simple relationship between continental Te and plate and
load age, although old (>1.5 Ga) cratonic lithosphere appears to have a
higher Te than younger (< 1.5 Ga) accreted terrains. Rheological
considerations suggest that Te is determined mainly by crustal composition,
geothermal gradient, and plate curvature and that it reflects the integrated
strength of the lithosphere. When considered as a whole, the terrestrial
planets show a wide range of Te. Mars and the Moon have the highest values,
Earth and Venus the lowest. The Earth has the smallest values overall and is
the only planet characterized by plate tectonics, the driving force of which
is believed to be some form of mantle convection. Thus, Te may be a key
controlling parameter in plate tectonics with low values encouraging
convection and high ones inhibiting it.