Geology Reference
In-Depth Information
provide a robust argument for pulsed, rather than
steady, rock uplift during the Cenozoic. Similar
approaches may provide insights on numerous
other ranges for which their topographic develop-
ment and rock uplift history are poorly known.
broader and more reliable dating of events
and processes within landscapes than was
previously possible. Digital topography allows
rapid characterization of the land surface. We
have seen that the regional overviews that can
be readily attained with DEMs provide a key
basis for comparing different areas and for
quantifying some of their geomorphically and
geophysically important characteristics.
Although the crust is heterogeneous and
can deform in some unexpected ways, we find
some predictable patterns of deformation and
landscape evolution. The sinuosity of mountain
fronts has been shown to be a good indicator of
fault activity. Drainage systems not only help
delineate present-day structural geometries, but
their persistence as geomorphic entities means
that they also record aspects of earlier history.
Moreover, we recognize that they can record
major events of river capture, pulsed uplift of
mountain ranges, and large scale strain that
deforms catchments.
We have seen that the concept of a dynamic
equilibrium or long-term steady state in which
rock uplift is balanced by erosion within a
deforming landscape is easy to articulate, but
challenging to test. It is likely rare that such
equilibrium exists at human time scales. The
problems lie in part in the mismatch between
the time and length scales over which the avail-
able geological tools operate. When one com-
pares the periodicities of climate change
(thousands of years), the effect that climate
exerts on rates of erosion, and the shifting loci
of deformation within an orogen, it seems
apparent that the most useful definition of a
steady state is one that spans sufficient time
(>100 kyr) to average among the fluctuating
forcing functions of landscape evolution.
In addition to these temporal challenges,
plentiful challenges are related to the spatial
distribution of data in long-term landscapes. For
a given mountain range, direct and unambiguous
geomorphic or structural measures of rock uplift
and of erosion rates are typically available from
only a few, if any, sites, and it is difficult to know
whether these data reliably represent the rates
in the remainder of the mountain range. The
interpretation of raw data also often relies on a
Summary
Interpretations of landscapes that have evolved
over millions of years provide a significant
challenge to tectonic geomorphology owing to
the likely superimposition of numerous climatic,
tectonic, and geomorphic events over that time
period. The explosion of thermochronological,
isotopic, geomorphic, and topographic tech-
niques, however, has provided new tools for
investigating landscapes at even long time
scales. Any pristine feature, such as a fault scarp,
in a long-lived landscape relates only to the
most recent phases of its evolution. Such
features, however, can provide a useful template
and should provoke these questions: If the
processes that created this feature were repeated
innumerable times, would it generate this large-
scale landscape? If not, why not? Because plate
motions tend to be rather steady over millions
of years, the ultimate driving forces and input of
energy into a deforming region may remain
approximately steady through time. On the one
hand, this steadiness could lead to predictable
patterns of deformation that simply become
structurally amplified through time and are only
obscured through erosional modification. On
the other hand, as rocks deform and rotate,
initial geometries may be overprinted by later
ones, and simple correlation of structures with
the plate motions may become increasingly
obscured. When interpreting terrains with
prolonged and complex histories, the challenge
becomes one of discerning the signature of
earlier tectonic events in the now-degraded
geomorphological record, and of utilizing
structural, stratigraphic, and chronological data
to reconstruct successions of past deformational
and erosional episodes.
In recent years, several new tools or data sets
have assisted studies of long-term tectonic
landscapes. New dating techniques permit far