THE INFLUENCE OF A STRATIFIED RHEOLOGY ON THE FLEXURAL RESPONSE OF THE LITHOSPHERE TO (UN)LOADING BY EXTENSIONAL FAULTING

We present a two-layered finite difference model for the flexural resp onse of the lithosphere to extensional faulting. The model allows for three modes of flexure: (1) fully coupled, with the upper crust and ma ntle welded together by the lower crust; (2) fully decoupled, with the upper crust and mantle behaving as independent layers; and (3) partly decoupled, signifying that the response of the upper crust to small-w avelength loads is superimposed on the response of the entire lithosph ere to long-wavelength loads. Which of these modes of flexure is to be expected depends on the rheology and especially the thermal state of the lithosphere. Coupled behaviour is related to a cold and strong lit hosphere. The Baikal Rift Zone provides a typical example for this mod e of flexure. A fully decoupled lithosphere is an exceptional case, re lated to anomalous high temperatures in the lower crust, and is observ ed in the Basin and Range province. The most common case is a partly d ecoupled lithosphere, with the degree of decoupling depending on the t hickness and viscosity of the lower crust. This is inferred, for examp le, for the Bay of Biscay margin.