Geology Reference
In-Depth Information
displacements; or steady, incremental folding,
tilting, or rotation. Both modes can trigger
geomorphic responses as deformation modulates
base level, differential uplift, and tilting. Abrupt
changes in base level due to seismic events
commonly initiate waves of erosion, such as
represented by knickpoints, or waves of
deposition that propagate outward from the
active fault. Perhaps surprisingly, inherent
thresholds related to geomorphic processes can
also create propagating waves of erosion in
response to steady changes in base level.
Because river systems are the most widespread
geomorphic features that are strongly affected by
small changes in slope, much analysis of Holocene
landscapes focuses on fluvial responses to defor-
mation. Moreover, aggradation or erosion by riv-
ers sets the local base level for adjacent hillslopes,
such that fluvial behavior typically modulates the
entire landscape response to tectonism.
River planform patterns may provide insight
on the distribution and nature of deformation on
both historic and prehistoric time scales. Given
their sensitivity to subtle changes in gradient, riv-
ers commonly respond rapidly to tectonically
imposed changes. Whereas tilting of fluvial ter-
races of a few degrees can usually be revealed
only through careful surveying, changes in river
patterns along a river's course can be readily
observed from maps, aerial photographs, and
satellite images. Because rivers respond to
numerous controls (water and sediment dis-
charge, slope, roughness, sediment caliber),
observed planform changes commonly cannot
be uniquely attributed to a tectonic cause.
Nonetheless, the changes in fluvial planform pat-
terns serve to pinpoint areas where deformation
may be occurring. Moreover, the type of the plan-
form change, such as from meandering to straight
or from lower to higher sinuosity, suggests what
the nature of the deformation may be: increasing
slope, lateral tilting, or changes in erosion rates.
Three different aspects of geomorphic studies
have significantly improved our understanding
of tectonically active, Holocene landscapes. First,
field studies have focused on detailed measure-
ments that provide insights on interactions
within geomorphic systems, so that we now
have much better understanding of why rivers
narrow or steepen in response to differential
uplift or what modulates rates of knickpoint
migration. Second, the recent proliferation of
digital elevation models has provided new tools
for studying modern fluvial systems. Not only
can channel slopes and contributing areas be
more reliably obtained, but also specific fluvial
indices, such as channel steepness, channel
concavity, and slope-area relationships, can be
readily calculated. Such indices enable holistic
exploration of landscapes in search of both
common characteristics and anomalies. These
indices also serve to pinpoint rivers that are
being perturbed by active deformation because
their indices depart from normative values in
non-deforming sites. Third, advances in dating
techniques have underpinned increasingly
successful attempts to quantify rates of tectonic
and geomorphic processes and interactions.
Clearly, geomorphic systems other than rivers
will also initiate responses to tectonic deforma-
tion at Holocene time scales. As fluvial knick-
points migrate past the toes of hillslopes,
steepened hillslope gradients will initiate changes
in the flux of material off the hillside into the
channels and may also influence the rates,
efficiency, or nature of the dominant hillslope
processes that modify the landscape. In many
rapidly eroding orogens (erosion at more than
several mm/yr), the entire landscape is predicted,
on average, to have experienced multiple land-
slides during the Holocene, and many tens of
meters of bedrock should be removed during
that time. Hence, such landscapes have the
potential to approach steady-state forms during
the Holocene. Most active orogens, however, are
eroding at slower rates (<2 mm/yr). As a conse-
quence, most non-fluvial landscape responses
may be difficult to discern at Holocene time
scales. In more slowly deforming orogens, the
scales and rates of tectonic deformation, base-
level lowering, and tilting are sufficiently small
that hillslopes may not be pushed over a threshold
that would provoke a dramatic or readily
recognizable response. It is only as one examines
longer time scales (>10 kyr) that such landscape
responses become clear. In the following chapter,
deformation and landscape responses at interme-
diate time scales (12 ka to 300 ka) are examined.
