** Finite Deformation Calculations for Viscous Flow
Problems**

**Introduction**

Basil is a finite element program which calculates quantities which describe stress and strain in non-linear viscous materials, for strains up to the of order 100%. The calculations describe very viscous Earth materials which undergo irreversible large-strain deformation at high temperature and over long time periods, under the influence of body forces and surface tractions. Sybil is the post-processing program that permits basil solutions to be examined in detail using an interactive graphical user interface.

The program permits a spatially variable Newtonian
or non-Newtonian viscosity in a 2-D geometry with boundary
conditions on traction and/or velocity. It is also possible
to include a single fault or discontinuity in the problem in a dynamically
self consistent way. The 2-D deformation field represents
either plane-strain deformation, or it permits a specified distribution
of normal stress in the third direction. The latter
is referred to as the thin viscous sheet formulation when the normal force
is due to gravity acting on variations of the layer thickness.
Plane-stress calculations are a specific case of the thin viscous sheet
formulation.

The programs basil and sybil have been developed mainly at Monash University since 1988, and before that at ANU and Harvard. The present set of programs has been developed mainly by Greg Houseman, Terence Barr and Lynn Evans. Important contributions by others, particularly by Philip England during the early stages of development are gratefully acknowledged. Comments on, or questions about, basil and sybil should be sent to greg@earth.leeds.ac.uk