THE VELOCITY DESCRIPTION OF DEFORMATION .1. THEORY

There are several advantages in using velocities to describe the defor mation of the crust and lithosphere induced by extension, compression, isostasy and other processes. (1) The method is very general. Any def ormation style can be described in this way. Simple examples are: pure shear, inclined or vertical simple shear, bulk rotation; Airy or flex ural isostasy and compaction. (2) Velocity fields can be combined to m odel more complex deformation. (3) The syn-tectonic evolution of param eters such as temperature, pressure, hydrocarbon maturity and porosity are easily modelled once the deformation velocities are specified. (4 ) The syn-tectonic evolution of sedimentary architectures can be model led using a general tectono-sedimentary forward modelling equation whi ch incorporates the deformation velocities. These advantages are illus trated by a computer model which simulates elastic sediments progradin g into an active half-graben formed by extension above a listric fault . Processes included in this model are compaction of the sediments, ha nging wall deformation by inclined simple shear and simultaneous footw all deformation. To avoid space problems, the velocity field for the h anging wall block above the non-rigid footwall must obey a simple cont act condition.