Mechanics: Force and Rheology

Abstract

In this chapter we discuss the forces involved in geodynamic processes. Knowing a bit about forces is a great tool for the field geologist to test field observations. Let us consider an example: A field geologist finds folds and thrusts in a Precambrian terrain that he or she interprets to have formed as the consequence of crustal shortening. Strain analysis shows that 80% shortening occurred and the geometry of shortening indicates that this resulted in fourfold thickening of the crust. He therefore further infers (using the principle of isostasy) that - at the time - a mountain range of some 15 km elevation existed above the metamorphic terrain. This interpretation is consistent with the field observations, but it has no independent test. In this particular example we could argue that we have no knowledge of any present day mountain range that is this high and that, therefore, this interpretation is unlikely. However, in many less obvious examples there are no direct analogies and the resulting model is - albeit perfectly imaginable and fully consistent with field observations - wrong. One way to provide an independent test of such models is to make a rough estimate of the forces involved. In the next chapters we want to perform such estimates. In order to do so, it is necessary to commence with a brief repetition of the basics of stress and strain. For more details on the basics the interested reader is referred to a range of excellent text books on the subject.