Part 1

Chapter 1
The tectonic agenda

The Earth is a hot, dense planet in a cold, sparse Universe. At the most basic level the processes that govern the Earth's behaviour are simple physical consequences of this reality, combined with the constraints imposed by the materials that make up the Earth. In particular, heat transfer operates to disperse this thermal energy fluctuation while self-gravitation serves to hold the mass-anomaly together. In order to understand why the Earth is the way it is, and how it evolves in time, it is necessary to understand the consequences of these two, simple, physical processes. The intricate structure of the Earth, at all scales, is a direct consequences of these physical processes, and why it is made of the materials that it is. Moreover, these processes are responsible for the great diversity and beauty of our planet that provides the basic motivation for much of the Earth Science community - a motivation that should not be forgotten.

1  A useful description?

Dynamicists are concerned with changes (equivalently, motions) accompanying the passage of time in dynamical systems. The system of concern to the geodynamicist is the Earth and our geological perspective focuses our interest primarily on the lithosphere (on which we reside) and, less directly, the asthenosphere. Consequently, geodynamics is primarily concerned with the basic physical processes that modulate the time evolution of the lithosphere; with one of its principal aims being the description of the behaviour of the lithosphere in the modern Earth.

In order to develop an appropriate geodynamic description we need to understand what constitutes utility. To be useful, the dynamical description must reduce some aspect (optimistically, all aspects) of the behaviour of a complex system to a few general statements. In geodynamics, in which our concern is primarily with the motion of the lithosphere and the forces that drive the motion, we may distinguish between descriptions that are concerned solely with the motion or kinematics without regard to the forces or, alternatively, mechanical descriptions concerned with the interaction between forces and motions.

The kinematic description is far less formidable than the mechanical description and has received considerably more attention as illustrated by plate tectonics. Plate tectonics attempts nothing more than a description of the motion of the lithosphere, and as such is purely kinematic. The fundamental tenets of the plate tectonic description are:

  1. The lithosphere is composed of only a few rigid plates that deform only at their boundaries. That is, plate motions can be described in terms of translation and rotation.
  2. The deforming plate boundaries are very narrow in comparison to the lateral dimensions of the individual plates.
Plate tectonics can only provide a useful description if the number of plates remains small; if large numbers of plates are needed to describe the behaviour of the lithosphere, then the description becomes sufficiently complicated to be rendered useless. Does plate tectonics provide a useful description of the behaviour of the lithosphere? Our prejudice concerning the answer to this question - only in part - provides the key to the way in which we present the subject matter. Let us explain! Since every seismic event represents a deformation of the lithosphere, a logical consequence of the plate tectonic description is that a plate boundary should be drawn through the focal point of every seismic event (or, earthquake). In the ocean basins seismicity is restricted to very narrow zones along the mid ocean ridges, subduction zones and transform zones, that are separated by large, essentially aseismic regions. The lack of seismicity over large regions within the ocean basins implies that, to a first approximation, they do behave as rigid, or elastic, plates. In contrast, seismicity in the continents is widely distributed, even in relatively inactive continents such as Australia, with very large areas of intense, distributed seismicity. The most notable regions of distributed seismicity in the modern earth are the Basin and Range Province in the western USA, the Himalaya - Tibet region in Asia and in the Aegean Sea (which is floored by thin continental crust) in Europe. In these locations, active deformation is taking place over vast areas, which are equally as large as the surrounding aseismic regions. In order to adequately account for such deformation the plate tectonic description would need to invoke hundreds of thousands of individual plates each with lateral dimensions of only a few kilometers.

Clearly, while plate tectonics may provide a useful description of the ocean basins it usefulness for many continental regions is doubtful. Indeed, an alternative description of the continents as essentially continuous ductile media (rather than elastic plates) and which therefore effectively ignores the existence of any discontinuities in the strain field such as faults and, more importantly, plate boundaries is useful for many purposes and will be the description we will emphasize here.

2  The tectonic agenda

At the very outset it is useful to establish our agenda in relation to other Earth science studies, such as structural geology, petrology, sedimentology, geomorphology and seismology. The overlap with these fields is substantial, but significant differences remain. We will be concerned primarily with those processes that give rise to the large-scale, regular (or so-called first-order) features that characterize the modern Earth. For example, we will be explicitly interested in understanding the controls on