Chapter 1

Introduction

In the disciplines of earth sciences where the objective is not essentially an historical or applied one, it is possible to make two main divisions, with disciplines describing geological structure and others that are concerned with the composition of natural substances. Thus, crystallography, geophysics, tectonophysics, structural geology, tectonics, geomorphology, and in large measure, sedimentology, are concerned with natural structures and their evolution, whereas mineralogy, geochemistry and petrology are concerned with the composition and chemical evolution of natural substances. In geological studies it is necessary to combine these two aspects.

Within the web of the different earth sciences, structural geology can be approached from two directions. One is concerned with the origin and history of structures. In a deformed region such as in a mountain belt, the geometry of complex deformations, principally superimposed foldings, are investigated. One has to unravel the threads then, with the aid of stratigraphical, geochronological, petrological and geochemical methods, to eluci­date the geological evolution of the region.

The physical approach (or tectonophysical which associates physical and geological considerations) deals with the detailed study of deformational mechanisms. Although the first theoretical and experimental work dates from the nineteenth century with the work of Sorby and Daubree, the development of this approach is very recent. It is founded upon the considerable progress that has been made in the experimental and theoretical study of mate­rials since the second world war. Similar studies on rock-forming minerals did not really begin until the sixties.

Beyond a geometrical description which permits deformation to be quantified, such studies can achieve results concerning the movements of rocks masses (kinematics) and the forces which produced them (dynamics). Integrated into a more comprehensive geophysical framework, these results lead to a better understan­ding of the geodynamics of the system or domain being studied. Thus in a seismic area, a better understanding of the local stress conditions and of the tectonic evolution may be gained by kinematic and dynamic analysis of faults (tectonophysics), com­bined with a study of processes at the seismic foci (seismology).

These two approaches, historical and physical, are not mu­tually exclusive. It is necessary to regard them as being comple­mentary. Analysing the history of a mountain chain requires a description of historical as well as geodynamic conditions.

A. Nicolas, Principles of Rock Deformation© D. Reidel Publishing Company, Dordrecht, Holland 1987

2 CHAPTER 1

Geometrical, kinematic and dynamic aspects

There are three approaches to structural analysis, geometri­calor structural (sensu stricto), kinematic and dynamic. When considering an object that has been deformed naturally, we are first of all obliged to make a geometrical description of it. If the object can be restored to its original shape, i.e. the shape it had before the deformation, it is possible to describe and quantify the amount of the strain that has taken place. This is finite strain analysis, that is to say, the total amount of strain that the object has undergone. This relies on structural geometrical analysis. We can also try to understand the va­rious ways by which the object under consideration passed from its initial to its final state; this is kinematic analysis. The final objective, dynamic analysis, aims to define the forces that are responsible for deformational processes. One imagines that, in the study of natural deformation, difficulties increase on passing successively from structural analysis to kinematic analy­sis and then to dynamic analysis. This is where deformation experiments are very helpful as the system of applied forces is known and it is easy to compare the initial and final states of the object being deformed. Deformation experiments also aim at understanding the rheology of the material concerned, that is how rapidly it deforms under varying applied conditions (stress, temperature, etc ... ).

Finally, we have favoured the study of large homogeneous deformation rather than heterogeneous deformation, mainly when the latter is modest as in the case of open folds. Two arguments justify this choice: the large displacements, overthrusts, stretching and shearing on the boundaries of plates or within their interiors are often localised into quite narrow zones where they are expressed by relatively homogeneous deformations of large amplitude. On the other hand it is precisely in the case of such deformations that one is now able to draw a few conclusions about the kinematics, dynamics and rheology thanks to the ap­proach developed here.