Geology Reference
In-Depth Information
and lithologic variations, such movements exert a
fundamental control on landscape development.
Given the slope dependence of many surface
processes and the sensitivity of fluvial systems to
small variations in surface gradients, geomorphic
responses to specific tectonic perturbations
are commonly predictable. At intermediate time
scales, powerful insights are obtainable when
the growth of structures can be quantified. Not
only does this quantification reveal mean rates
of deformation and structural propagation, but it
also provides a reliable context for interpreting
geomorphic responses to deformation and test-
ing predictions concerning those responses.
Numerous difficulties, however, can thwart
successful landscape analysis at intermediate
time scales. Determination of tectonic rates
typically depends on dating of displaced geo-
morphic surfaces. In many geomorphic settings,
dating of surfaces that exceed the range
of radiocarbon dating (
>
40 kyr) is difficult or
impossible. Soil chronologies, uranium-series
dates, cosmogenic exposure ages, or lumines-
cence ages can sometimes bridge the gap
between radiocarbon and argon-argon dating.
Because each of these dating approaches may
lack accuracy, reliability is improved by (i) the
use of several techniques in conjunction with
each other whenever possible, (ii) a focus on
settings in which redundant ages can be
determined for the same surface, or (iii) use of
sites for which multiple rate calculations can be
made from a succession of offset markers.
In tectonically active coastal settings, ages of
terraces are often inferred through correlation
with a dated sea-level curve. Numerous uncertain-
ties still exist in this curve with respect to both
the magnitude of past sea-level variations and the
actual timing of those changes. The interval prior
to the last interglaciation (
>
130 ka) has only
been partially calibrated. To the extent that the
reliability of the sea-level curve and resulting
correlations can be enhanced, the accuracy of
deformation rates in coastal domains will improve.
The timing of geomorphic responses to climate
variations in terrestrial settings is even less
well understood than those along the coast.
Development of some geomorphic markers,
such as fluvial terraces, have both direct and
indirect climate controls. For example, even if
climatic conditions are conducive to terrace
building, if insufficient sediment is available
for transport within the upstream drainage,
aggradation will not occur. Nonetheless, geomor-
phic markers provide a critical basis for gauging
deformational and denudational processes
within many landscapes. Therefore, understand-
ing of the character and rates of response of
markers to climatic changes, as well as the tem-
poral lags in those responses and the potential
impact of autocyclic processes, is of paramount
importance in reconstructing tectonic and geo-
morphic histories at intermediate time scales.
The punctuated production of geomorphic
markers by climatic variations provides a means
both to delineate changes in tectonic rates and
deformation patterns through time, and to create
a robust framework in which to examine
landscape evolution. At intermediate time scales,
accumulated displacements can be sufficiently
large to allow incisive testing of conceptual
models for fault and fold propagation through
time, fault linkage, steadiness of fault-slip rates,
and stick-slip behavior in subduction zones at
multi-millennial time scales. Where rates can
be shown to change, transient landscapes can
be used to explore how the surface responds to
changing boundary conditions.
Individual folds commonly grow over an
interval of a few hundred thousand years in
tectonically active settings. During this period,
stream patterns and preserved surfaces commonly
provide unambiguous information on the amount
and geometry of deformation. If fold growth were
to persist at rates of 1 mm/yr for a million years,
1 km of displacement would occur. In most cases,
structural relief of this magnitude would cause
erosion to obliterate all but the most persistent
geomorphic markers, such that the details of fold
evolution would be difficult to reconstruct from
geomorphology alone. It is at these longer time
scales, however, that mountain ranges develop
and orogens evolve. In the succeeding chapter, we
examine landscapes that represent an integration
of geomorphic and tectonic processes at time
scales of a million years or more.
