Geology Reference
In-Depth Information
define many terms and review basic concepts
and models throughout the topic, we assume
that our audience understands many commonly
used geomorphological and structural terms,
such as
base level
or
conjugate faults
. For those
readers without such a background, occasional
reference to a basic geomorphological or struc-
tural text will probably be helpful.
It is beyond the scope of this topic to cover
the contributions of each of the subdisciplines
of tectonic geomorphology. Instead, we focus
on some of the key tools, approaches, and
concepts that have served to advance tectonic
geomorphological studies during the past
few decades. Several building blocks underpin
many such studies, including a knowledge of
how the Earth deforms both during earth-
quakes and between them, what sorts of
features can be used to track deformation in the
past, and what types of techniques are useful
for dating features of interest.
Initially, we introduce geomorphic markers:
landscape features, such as marine or fluvial
terraces, that can be used to track deformation.
Prior to deformation, the surfaces of these
markers represent planar or linear features with
a known geometry that can be predictably
tracked across the landscape. Faulting or
folding can subsequently deform such markers.
Documentation of any departure from their
unperturbed shape can serve to define the
magnitude of deformation. Consequently, the
recognition and measurement of such displaced
or deformed markers is critical to many
tectonic-geomorphic studies. These deformed
markers are the raw data that cry out for
interpretation.
In order to delineate rates of deformation,
both the timing and amount of deformation must
be defined. Fortunately, dozens of techniques
have been used successfully to define the age
of displaced features. In a chapter on dating, we
present examples of some of the more com-
monly used techniques, and we try to convey
a sense of the situations in which use of each
technique would be appropriate. For each tech-
nique, we provide its conceptual underpinnings,
discuss some of its limitations, and describe
what data are actually collected in the field.
The oft-used term “tectonic processes” is a
grab-bag expression that encompasses all types
of deformation, including the motion of tectonic
plates, slip on individual faults, ductile defor-
mation, and isostatic processes. We concern
ourselves here primarily with those processes
that are most relevant to relatively localized
deformation of the Earth's surface. In this topic,
we have generally chosen to ignore deformation
and surface processes related to volcanism. Many
of the concepts that are developed here are
descriptive of landscape responses to volcanic
processes, but the way in which material is
added to the system (sometimes from above or
only from below), as well as the time and spatial
scales, are commonly different between volcanic
and non-volcanic settings.
Because the accumulated movements of
individual faults and folds have built many
landscapes, we describe some current concepts
from seismology and structural geology con-
cerning coseismic rupture, the scaling of fault
slip, fault-related folding, and geometries of
deformation in different tectonic settings (com-
pressional, extensional, and strike-slip envi-
ronments). This deformation generates the
fundamental topographic morphology on which
erosive forces act. Much has been learned about
ongoing deformation of the Earth's surface
through geodetic studies: detailed surveys that
delineate regional to local crustal displacements,
often at time scales of a few years. During the
past two decades, the increasingly widespread
application of new techniques, such as the
Global Positioning System (GPS) and radar
interferometry, has yielded spectacular new
images of current crustal motions. In a chapter
on geodesy, we describe these techniques and
some of the insights gained from them. In many
ways, it is in the context of this greatly improved
understanding of modern deformation that we
now can make the largest strides in interpreting
past deformation and landscape evolution.
Knowledge of current rates of crustal deforma-
tion leads to a host of provocative (and socially
relevant) questions: Have these rates been
steady over time? Do individual faults rupture
at regular intervals and produce earthquakes of
similar magnitude through time? Do groups of
