Geology Reference
In-Depth Information
Large-scale changes in sea level, the size of gla-
ciers, and the discharge of rivers at these time
scales leave strong imprints on the landscape.
These climate cycles are often the “drivers” that
generate the geomorphic markers, in the form
of moraines or marine and fluvial terraces, that
are so important in the field of tectonic geomor-
phology. At time scales of a million years or
more, numerous climatic cycles can create a
more “time-averaged” landscape.
These different time scales also often create a
natural segregation of the focus of tectonic-
geomorphic studies. At Holocene time scales,
cause and effect can be examined rather directly.
For example, if faulting causes the land surface to
tilt in a given direction, the timing and rate of
diversion of rivers in response to that tilt can be
measured. Present rates of crustal deformation
can be compared against the sediment load of
rivers. Commonly, geomorphic markers, such as
river terraces, are almost untouched by erosion
and can function as pristine recorders of Holocene
deformation. The strength of Holocene studies is
that the record is often most complete, the dating
is most precise, rates of many competing processes
can be directly measured, and their interactions
can be examined. At least three potential disad-
vantages can limit Holocene studies. First, the
rates of tectonic processes may be sufficiently
slow or the occurrence of deformational events,
such as earthquakes, may be sufficiently rare that
the tectonic signal is obscure. Second, rates of
geomorphic processes during Holocene (intergla-
cial) times may not be very representative of
long-term rates. Third, the time it takes for geo-
morphic systems to respond to a change imposed
by tectonic forcing can be longer than the
Holocene. Thus, the geomorphic system may be
in a state of transition with respect to tectonic
perturbations imposed upon it.
As described in subsequent chapters, the
limited precision of most dating techniques
means that, as one delves beyond the Holocene
and farther into the past, it becomes progressively
more challenging to associate specific events in
time. Thus, it can be difficult to define direct
responses to individual forcing events. On the
other hand, major climate changes often have
created robust geomorphic markers, such as
river terraces, that persist as recognizable
features and provide a lengthy time framework
within which to document patterns of deforma-
tion. At short time scales, tectonic forcing is com-
monly unsteady, because it results from discrete
events, such as earthquakes, that are widely sep-
arated in time. At time scales of more than 10
4
y r ,
this unsteadiness is commonly smoothed out,
and average rates of deformation can be defined.
Because many geomorphic markers persist in
the landscape at time scales of single glacial-
interglacial cycles, this is an ideal interval within
which to document past rates of tectonic forcing.
At still longer time scales, erosion has typically
removed most markers that are more than about
10
5
yr old. Yet, it is at this time scale that large-
scale landscape responses to sustained tec-
tonic forcing can be clearly seen. Typically,
landscapes must be treated at a coarse spatial
scale in order to examine the products of tec-
tonic-geomorphic interactions, such as the topo-
graphic characteristics of a collisional mountain
belt or the broad swath of deformation that
occurs as a continent passes over a hot spot.
We conclude with a chapter on numerical
modeling of tectonically active landscapes. In
contrast to efforts aimed at directly measuring
tectonic-geomorphic processes, the interactions
between deformation and surface processes can
also be studied theoretically. If we could write
numerical rules that represent phenomena such
as the displacement of the crust due to faulting or
the erosion and redistribution of mass due to sur-
face processes, we could proceed to investigate
interactions among these processes. Consider, for
example, displacement on a normal fault that
bounds the front of a mountain range. The topo-
graphic offset will change the local gradient of
any river crossing the fault, and a numerical rule
for river incision could predict how that reach of
the river would respond and how that response
would be propagated upstream.
Rather complex models for landscape evolu-
tion in different tectonic environments have
recently been formulated. It is not our intent to
describe or compare these in detail. Instead, we
describe several of the basic building blocks that
could go into a numerical model, and we illus-
trate some of the predictions of these models.
