Geology Reference
In-Depth Information
Table 9.1
Effects of slope steepening and lengthening on geomorphology and surface processes.
Feature/process
Effect of steepening and slope lengthening
Reason behind effect
head of first-order streams
channels begin higher upslope
channel initiation is function of
(slope × area)
ruggedness (mean
relief × drainage density)
increases with steepening and lengthening
more first-order channels lower on
slope and deeper channels to match
relative base level
creep and landslides
increases with steepening and lengthening
shear stress is a function of slope:
s
=
r
gh
sin
a
exported material
increases with steepening and lengthening
flux proportional to slope (d
y
/d
x
)
balance of uplift versus incision
uplift tends to outpace incision for
antecedent streams
width of structure increases with time;
discharge stays constant, but river slope
and mean stream power decrease
of folding. Following uplift, however, surface
processes begin to modify the surface in several
ways. Channel heads appear on slopes exceeding
about 5
°
(Keller
et al.
, 1998; Talling and Sowter,
1999). Antecedent streams incise into the
growing fold and, in the particular situation at
Wheeler Ridge, are commonly localized by
transverse tear faults (Mueller and Talling, 1997).
As gullies deepen, the pristine, but uplifted,
alluvial surface becomes more dissected, and
landsliding begins to occur on the sides of the
larger gullies. As the steep forelimb lengthens,
creep and shallow landsliding become increas-
ingly important, and gullies extend their heads
toward the crest of the fold. If fold widening and
vertical uplift are sufficiently rapid, antecedent
streams are defeated and diverted, leaving
behind wind gaps (Fig. 4.39A).
Modification of the fold's geomorphic surface
can be analyzed from at least two perspectives:
changes along the length of the anticline; and
contrasts between the forelimb and the backlimb.
The western parts of the anticline are older,
have greater topographic relief, and commonly
are steeper than the younger, eastern parts of
the structure. In any transverse cross-section of
this fold, the forelimb is consistently steeper than
the backlimb (Fig. 9.24). All slope-dependent
processes would, therefore, be expected to
attack the forelimb more vigorously than the
backlimb. Similarly, catchment areas, discharge,
and relief generally increase toward the west.
The net results are that the fold becomes
increasingly dissected toward the west and that
the forelimb is more dissected than the back-
limb along any transverse section. Qualitative
predictions of changes along the fold (Table 9.1)
suggest how various surface processes will be
influenced by the growing relief and both
lengthening and steepening of the fold limbs.
Analysis of a 30-m digital elevation model
(DEM) of Wheeler Ridge permits further
quantification of several aspects of the geomor-
phology. Slopes and relief were calculated
within a sliding 150 × 150 m window along the
fold limbs, excluding the wind and water gaps
(Brozovic
et al.
, 1995) (Fig. 9.26). The resulting
distributions show that both slope and relief
(Fig. 9.26) are consistently higher (i) on the
forelimb than on the backlimb and (ii) in
the older parts of the fold. These data support
the hypothesis that the enhancement of slope-
dependent processes promotes greater dissec-
tion of the fold limbs (Table 9.1).
As shown earlier (Fig. 7.6), digital topography
can also be used to calculate minimum volumes
of eroded material along an anticline. At Wheeler
Ridge, the pre-dissection topographic surface
can be reconstructed by connecting undissected
remnants of the uplifted surface of the fold.
Subtraction of the current topography from that
pre-dissection surface defines the magnitude of
erosion throughout the fold (Plate 8). These cal-
culations indicate that the most extensive erosion
has occurred in the older, more strongly uplifted
segments of the fold, whereas very little erosion
