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
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:
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
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.
- The deforming
plate boundaries are very narrow in comparison to the lateral
dimensions of the individual plates.
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
average elevation of the surface of the earth,
length-scales of deformation in active mountain belts,
gross chemical composition of the continental crust, and
normal thermal regime in the lithosphere.
File translated from TEX by TTH, version 2.25.
On 7 Oct 2000, 11:25.