Geology Reference
In-Depth Information
the San Gabriel Mountains of California, many
river valleys debouch on to densely populated
areas of the Los Angeles basin. Because much
of the historical damage from floods has been
caused by large debris flows, Los Angeles County
engineers have built debris basins at the mouth of
each of the canyons. These basins are efficient
sediment traps and have been reasonably effec-
tive in preventing widespread damage by debris
flows to the expensive homes built on the alluvial
fans below the mouths of the canyons. When the
debris basins approach capacity during flooding,
the deposits are excavated and removed by trucks.
The amount of debris removed provides an effec-
tive measure of the denudation in the catchment
(Scott and Williams, 1978) and is recorded in
county registers in units of truckloads! Such
records provide a rich data set with which to
examine the effects of fire, storms, human inter-
ference, and revegetation on sediment fluxes and
erosion rates (Lavé and Burbank, 2004).
Even when sediment volumes, their ages, and
the catchment source area are very well known,
however, calculations of meaningful erosion rates
can be difficult (Church and Slaymaker, 1989).
The data from the tide-water glaciers illustrate
some of the pitfalls and challenges of recon-
structing erosion rates. Almost all components
for an erosion-rate calculation were well con-
strained: sediment volumes from reliable isopachs
in the fiord at the glacier's toe, good time control
on a 40-year-long record, and a well-defined
source area. Yet, the resulting average erosion
rates (Fig. 7.7) appear to greatly overestimate the
likely long-term rates (Koppes and Hallet, 2006).
Because the glaciers have been strongly out of
equilibrium in the past century, their sediment
fluxes are probably atypical. It is important to
recognize that these caveats do not invalidate the
calculated erosion rates: those rates are extremely
informative for retreating tide-water glaciers, but
they probably should not be interpreted as char-
acteristic of equilibrium glacial erosion.
was removed during the growth of a fold or slip
on a fault, long-term erosion rates can be
calculated. In order to do this, several conditions
must be met. Both the pre- and the post-
deformational geometry of the displaced mass
must be known, such that the volume of eroded
material can be calculated. The ages of key over-
lapping and cross-cutting relationships between
structures and strata must be known, so that the
initiation and termination of deformation and
erosion can be determined.
Whereas such volumetric calculations are
simple in concept, they can be reliably applied
only in unusually well-documented situations,
and, even then, they may require a set of
commonly untested assumptions. One poten-
tially straightforward geometry to consider is the
erosion of the hanging wall of a thrust sheet. The
south-vergent Salt Range Thrust in the Himalayan
foreland fold-and-thrust belt of Pakistan pro-
vides a clear example for application of this
methodology (Burbank and Beck, 1991). The
hanging wall of this thrust comprises Eocambrian
evaporites of variable thickness, a Paleozoic-to-
Eocene carbonate and clastic succession about
1 km thick, and a 2.5-3 km thickness of Miocene
and younger fluvial strata of the Himalayan fore-
land basin, the Siwalik molasse (Fig. 7.8). Prior
to deformation, the molasse strata formed a pre-
dictable wedge-shaped geometry that gradually
thinned toward the south. The regional taper
and thickness of the foreland strata can be deter-
mined from the minimally eroded foreland strata
abutting the hinterland edge of the uplifted
hanging wall. The known initial thickness and
taper are keys to determining how much erosion
occurred subsequently. During thrusting, the
hanging wall was raised up a 1-km-high footwall
ramp and was translated about 20 km to the
south (Baker
et al.
, 1988). Stratigraphic evidence
indicates that translation took place in two
stages: between 6.3 and 5.8 Ma, about 5 km of
shortening occurred (Fig. 7.8); and the remain-
der occurred since 1.5 Ma. During the first stage
of thrusting, erosion of the fluvial strata from the
leading edge of the hanging wall was great
enough to expose Permian rocks at the thrust
tip, as evidenced by the presence of distinctive
lithologies within the 6 Myr old foreland strata.
Rates of erosion based on structural
and stratigraphic controls
When it is possible to delineate both the duration
of erosion, as well as the volume of rock that
