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Tony Watts
Abstract: The Oxford View
Plate flexure is a phenomenon that describes how the lithosphere responds to long-term geological 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 characterized by Te values of up to 80 km and higher. Rheological considerations suggest that Te reflects the integrated brittle and ductile strength of the lithosphere. Te differs, therefore, from the seismogenic layer thickness, Ts, which is indicative of the depth to which faulting extends in the uppermost brittle layer. Despite differences in their time-scales, Te and Ts are similar in the oceans where loading reduces the initial mechanical thickness to a thickness that coincides with that of the brittle layer. They differ, however, in continents which, unlike oceans, are characterized by a multi-layer rheology. As a result, Te >> Ts in cratons and some rifts. Most rifts, however, are characterized by low Te which has been variously attributed to a young thermal age of the rifted lithosphere, thinning and heating at the time of rifting, and yielding due to post-rift sediment loading. Irrespective of their origin, the Wilson Cycle makes it likely that the low values of rifts will be inherited by foreland basins, which in turn helps explain why similarities between Te and Ts extend beyond rifts into other tectonic regions such as orogenic belts and, occasionally, the cratons themselves.
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