Geology Reference
In-Depth Information
creates lakes within mountains, or avulsion of
river channels, which occurs due to aggradation
on floodplains. If the relationship of a suite of
markers to known and dated climatic variations
can be established, the markers themselves can
be indirectly dated through correlation to the
climatic record. For example, climate change
often affects the water discharge in a river and
its sediment load. Consider the situation in
which, owing to a climatically induced increase
in discharge, a river begins to incise its bed.
After sufficient incision, the river may be flanked
by fluvial terraces (geomorphic markers) that
now represent a geomorphic response to the cli-
mate change. If we know the age of the climate
change that caused the change in discharge and
sediment load, then the age of the terrace itself
can be deduced from the climatic record. With
these terraces as time markers in the landscape,
it becomes possible to measure both the amount
and rate of deformation since they were created.
Therefore, it is very useful to understand those
aspects of the climatic record that are most
likely to be relevant to the creation of geomorphic
markers. In the absence of climatic calibration,
or for markers whose origin may not reasonably
be attributed to climatic cycles, direct dating
of a marker is required. A variety of dating
approaches is discussed in Chapter 3.
Ephemeral features, such as small levees or
even tire tracks, that could wash away in the next
storm can provide markers that are adequate to
calibrate coseismic offsets of a recent earthquake
(McGill and Rubin, 1999). Long-lived geomorphic
features, however, are required in order to
document deformation over many thousands of
years. But, erosion is continually modifying the
geometry of such markers, and ongoing tectonic
and climatic changes are overprinting the land-
scape with new features. Consequently, a tectonic
geomorphologist benefits both from a knowledge
of a marker's potential for long-term preservation
in the landscape and from an ability to recognize
useful fragments of older markers within the
complexly intertwined geomorphic elements of
most natural landscapes.
The sensitivity (amplitude of the response)
and response time of geomorphic systems to
changes in the variables that control them vary
markedly. The discharge of rivers, for example,
will respond almost instantaneously to changes
in precipitation, whereas glaciers often take
several years to translate increases in snowfall
into an advance of the snout of the glacier.
In general, the response time of a geomorphic
system to changes in the climatic forcing of
the system increases dramatically with the
scale of the system, and inversely with respect
to the efficiency of the processes involved.
When the variables that control a geomorphic
system change, the highly sensitive compo-
nents of an integrated system, such as the
cross-sectional area of a river or the elevation
of its bed, can usually achieve a new equilib-
rium rapidly. In contrast, larger and less sensi-
tive components, such as an entire drainage
basin (Fig. 1.3), can take many millennia to
come into equilibrium with any new controls
on the system.
Planar geomorphic markers
To be useful as a reference frame against
which to measure displacement, the initial,
pre-deformational geometry of a geomorphic
feature must be defined. Because erosion and/
or deposition continually modify old geomor-
phic markers, a map of a presently exposed
marker often reveals only fragments of for-
merly continuous surfaces or features. The
task of anyone wishing to use such markers to
calibrate deformation is to create a reliable
reconstruction of the undeformed geometry
of the marker. Because modern analogs com-
monly provide the best model to use in recon-
structing the shape of older, presently
fragmented features, we will describe coastal,
lacustrine, fluvial, and several terrestrial mark-
ers that have been extensively used to define
tectonic deformation.
Marine terraces, beaches, and shorelines
Along coastal regions of many parts of the world,
bench-like features, or
marine terraces
, have
been created by the interaction of the ocean
with the adjacent landmass. Marine terraces
