Introduction - Introducing Tectonics, Rock Structures and Mountain Belts - page 1

Introduction
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Introduction

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Meaning and scope of the terms
'tectonic' and 'structure'
The adjective ' tectonic ' merely means
'structural', i.e. applying to a struc-
ture . However, in geological usage it
has come to be applied particularly to
large-scale structures - thus ' tectonic
plate '. In order to explain rock struc-
tures, it is necessary to understand
the forces operating within and on the
crust that are responsible for creat-
ing them, and to do this, we need to
investigate the Earth-scale processes
known collectively as plate tectonics .
The term ' structure ' in everyday
usage refers normally to a building
or other artificial construction, but in
geological terms it has come to mean
a body of rock whose shape can be
defined geometrically and which has
originated by a geological process. The
most obvious and best known types of
geological structure are folds and faults ,
which have been produced by the action
of geological forces within the Earth's
crust and which can give us an insight
into the magnitude of these forces
and how they operate. Another group
of structures is typical of deep-seated
metamorphic rocks where crustal forces
have effected thoroughgoing changes
in the rock, producing new textures
through recrystallisation; the structure
produced by such changes is known
as the fabric . A fourth category of geo-
logical structures consists of igneous
intrusions. Such bodies are influenced

by the forces acting within the crust
during their emplacement and the
igneous bodies themselves may cause
structural changes in the host rocks .

maintaining the same overall movement
sense. Examples of this are discussed
in Chapters 10-12, where we look in
detail at the structure of orogenic
belts . At this larger scale, structures are
grouped into tectonic regimes, based
on their over-riding characteristics, and
linked with their plate tectonic setting,
thus: extensional regimes are associ-
ated with divergent plate boundaries
(continental rift zones); compres-
sional regimes with convergent plate
boundaries (subduction and collision
zones); and strike-slip regimes with
transform faults . These plate bound-
ary types are explained in Chapter 3.

Kinematic and dynamic models of
deformation
Kinematics is the study of movement
and dynamics the study of forces ,
and both types of model have been
used in the investigation of geologi-
cal structures. Dynamic models have
traditionally been used by structural
geologists employing the techniques of
mechanical engineering and materi-
als science. This requires a familiarity
with mathematics and physics, which
is a potential barrier to understanding
for many students. However, whereas a
background in these subjects is essen-
tial to understanding the deformation of
rock in any depth, it is not really neces-
sary to be able to grasp the essentials
of structural geology at a basic level.
The kinematic model of deforma-
tion has become in many ways more
popular as a way of understanding
geological structures. In this type of
approach, structures are analysed in
terms of the relative movement between
blocks of crust, and the causal forces
are disregarded. Thus, systems of
related folds and faults are explained
by a single set of relative movements.
Such a system can also be integrated
into the plate tectonic model by scaling
up to a higher order of magnitude but

Layout of the topic
The sequence of chapters reflects the
author's belief that an understanding
of the plate tectonic model and the
evidence for it is helpful in the under-
standing of geological structures on
the smaller scale. Thus Chapters 2-3
deal with large-scale Earth structure
and the theory of plate tectonics. These
chapters are followed by chapters 4-9
on the process of deformation and on
the various types of geological struc-
ture. Finally, Chapters 10-12 deal with
the study and understanding of oro-
genesis and orogenic belts . A brief
summary of each chapter follows.

Chapter 2: Large-scale earth
structure. This chapter describes the
most significant features of the Earth's