Lithospheric strength and its relationship to the elastic and seismogenic layer thickness

Plate flexure is a phenomenon that describes how the lithosphere responds to long-term (> 105 yr) geological loads. By comparing the flexure in the vicinity of ice, volcano, and sediment loads to predictions based on simple plate models it has been possible to estimate the effective 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. The continents, in contrast, are characterised by Te values of up to 80 km and greater. Rheological considerations based on data from experimental rock mechanics suggest that Te reflects the integrated brittle, elastic and ductile strength of the lithosphere. Te differs, therefore, from the seismogenic layer thickness, Ts, which is indicative of the depth to which anelastic deformation occurs as unstable frictional sliding. Despite differences in their time scales, Te and Ts are similar in the oceans where loading reduces the initial mechanical thickness to values that generally coincide with the thickness of the brittle layer. They differ, however, in continents, which, unlike oceans, are characterised by a multi-layer rheology. As a result, Te ≫ Ts in cratons, many convergent zones, and some rifts. Most rifts, however, are characterised by a low Te that 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 possible for low values to 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. © 2003 Elsevier Science B.V. All rights reserved

The long-term strength of continental lithosphere: "Jelly sandwich" or "crème brûlée"?

There has been much debate recently concerning the long-term (i.e., >1 m.y.) strength of continental lithosphere. In one model, dubbed jelly sandwich, the strength resides in the crust and mantle, while in another, dubbed crème brûlée, the mantle is weak and the strength is limited to the crust. The different models have arisen because of conflicting results from elastic thickness and earthquake data. We address the problem here by first reviewing elastic thickness estimates and their relationship to the seismogenic layer thickness. We then explore, by numerical thermomechanical modeling, the implications of a weak and strong mantle for structural styles. We argue that, irrespective of the actual crustal strength, the crème brûlée model is unable to explain either the persistence of mountain ranges or the integrity of the downgoing slab in collisional. systems. We conclude that while the crème -brûlée model may apply in some tectonic settings, a more widely applicable model is the jelly sandwich