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In-Depth Information
(Fig. 9.19C). Given that discharge is roughly
proportional to catchment area, this relationship
is consistent with observations from stream-table
experiments in which migration rates scaled
with discharge (Fig. 8.10A). This relationship
further suggests that an erosion rule based
on stream power can adequately describe the
observed profiles. Harkins
et al.
(2007) also
used detrital
10
Be concentrations in channel
sediments upstream of knickpoints to determine
catchment-wide erosion rates. Not only are these
rates about an order of magnitude lower than
incision rates below the knickpoints, but they
also scale with the normalized steepness index,
k
sn
(Fig. 9.19D). Notably, most tributaries display
a convex upward profile downstream of the
knickpoint (Fig. 9.19B). This convexity suggests
that these tributaries are still in a transient state
of incomplete adjustment to their new base
level. Note, for example, the contrast with the
concave-up profiles above and below a knick-
point in a theoretical model for a migrating
knickpoint (Fig. 9.11). Overall, this study of the
Yellow River exemplifies the wealth of data and
insights that can be gleaned from combinations
of DEM analysis, chronological control on key
events, calibrations of erosion rates, and both
usage and tests of numerical models of rivers.
subsurface shape of the structural anomaly. The
temporal extent of the window depends on the
fault slip rate and the anomaly's length. When a
pressure ridge formed under these conditions
can be identified, an illuminating opportunity
exists to make a robust space-for-time (ergodic)
substitution and to examine how the land sur-
face responds to this transient pulse of uplift.
Along the San Andreas Fault in southern
California, the Dragon's Back pressure ridge
(Fig. 9.20) provides just such an opportunity
(Hilley and Arrowsmith, 2008). Subsurface
imaging at the Dragon's Back site (Unsworth
et al.
, 1999) defines a 2-km-long, structural
knuckle attached to the North American Plate
that juts beneath the Pacific Plate and drives
rock uplift. The slip rate on the San Andreas
Fault in this area is
∼
33 mm/yr (Sieh and Jahns,
1984), such that each kilometer of Dragon's
Back's length represents
∼
30 kyr. Recent
acquisition of high-resolution (1-m pixel) lidar
topography provides a high-resolution spatial
database with which to quantify how the
pressure ridge evolves in time and space as a
geomorphic entity.
By mapping flat-lying rock formations that
become uplifted in the pressure ridge, Hilley
and Arrowsmith (2008) show that, over the first
2 km (to the southeast), the rock-uplift rate
ranges as high as 2.3 mm/yr (Figs 9.20C and
9.21A). During the
∼
70 kyr that it takes for any
point on the Pacific block to pass across the
structural anomaly, the total rock uplift is
∼
80 m
along the crest of the fold (Figs 9.20B and 9.21B).
Dragon's Back ridge is underlain by weakly
consolidated Quaternary sediments that can
be readily eroded. Consequently, the response
time of various geomorphic processes to the
tectonic forcing is expected to be quite rapid. For
each drainage basin along the pressure ridge,
Hilley and Arrowsmith (2008) measured several
topographic metrics, including basin width and
area, channel concavity and normalized steep-
ness, relief within a radius of 50 m, density of
landslide scars, and hillslope gradients. Their
results show that local relief, normalized channel
steepness, landslide density, and hillslope gradi-
ents all broadly track the uplift rate (Fig. 9.21).
Despite the small size (
<
0.5 km
2
) of the drainage
Pressure ridges
When strike-slip faults depart from verticality
and when the fault trace bends into the path of
the fault-slip vector, the resultant fault-normal
stresses cause contraction and uplift, thereby
forming a pressure ridge (Fig. 4.19). If the
structural anomaly is persistently attached to a
block on one side of the fault, the anomaly
acts as a point source that drives uplift of the
opposing block as it slides by. Once a given
segment of the opposing block moves past the
anomaly, uplift ceases. Thus, a very discrete
spatial and temporal window exists in which
the opposing block transitions, first, to experi-
encing the accelerated rock uplift and, second,
to exiting the zone of uplift. The spatial extent
of the window depends on the length of the
structural anomaly parallel to the fault trace,
whereas the pattern of uplift depends on the
