Geology Reference
In-Depth Information
Kyrgyzstan (Burbank
et al
., 1999; Sobel
et al
.,
2006), where folded and faulted bedrock ridges
have been lifted more than 2 km above local base
level. Prior to uplift, the bedrock was overlain by
about 5 km of Cenozoic sediment. Today, these
sediments have been stripped from nearly all fold
flanks rising more than 300-500 m above local
base level. These exhumed bedrock ridges display
generally bow-shaped topographic profiles,
suggestive of more recent and lesser displacement
near the fold tips, whereas, along the central part
of the forelimbs of many of the larger folds, faults
have cut the surface.
With the common exception of detachment
folds, most folds are asymmetric, with steeper
forelimbs than backlimbs (Fig. 4.36). If rainfall
is uniform or at least symmetric across the fold
crest, then the channels on the forelimb would
be expected to erode faster than backlimb
channels. As a consequence, the drainage divide
should migrate toward the backlimb until equi-
librium slopes are developed whereby rock
uplift is balanced by erosion on each flank of
the fold (Fig. 10.16). Now consider the impact
Range-Scale Folds & Drainage Divides
fold hinge
drainage divide
low local
relief, but
convex channels
pattern of
tectonic uplift
(iii)
migrating divide
high
local relief
(iii)
migrating divide
beheaded
valleys
folded
terraces
(iii)
migrating divide
base-level
drop via
axial drainage
increased
local relief
beheaded valleys
oversteepened
channels
A
(iv)
outpaces erosion, whereas in more equilibrium phases
(iii) and (iv), erosion and rock uplift are in balance.
(i) Initially, incipient erosion is much slower than rock
uplift, such that the structural crest of the fold and the
drainage divide coincide. Erosion may be greater on the
backlimb because it has bigger catchment areas. (ii) As
fold growth continues, the divide migrates toward the
backlimb owing to both gentler slopes and its higher
base level. (iii) Over time, relief diminishes with respect
to rock uplift, and the divide occupies an equilibrium
position between drainages with equivalent concavity.
(iv) Base-level fall near the backlimb steepens the local
channel gradients and drives migration of the divide to a
new equilibrium position closer to the forelimb. Such
migration could behead channels that formerly drained
to the forelimb. Modified after Ellis and Densmore (2006).
B. Example of divide migration with respect to structural
crest of a range-scale fold, eastern Kyrgyz Range,
Kyrgyzstan. The range has propagated eastward over
time, such that a space-for-time substitution can be made.
More retreat of the drainage divide toward the backlimb
has occurred in the western, older part of the range,
whereas the divide and the structural crest coincide in
the eastern, younger part of the range. In this range,
divide migration has been enhanced by headwall erosion
by glaciers. Modified after Oskin and Burbank (2005).
B
~1550 m
Divide Migration
g
Drainage Divide
Thrust Fault
Anticlinal Axis
Divide Migration
Local Base Level
0
5
km
10
~2050 m
Fig. 10.16
Range-scale folds and migration of
drainage divides.
A. Conceptual model for divide migration in response to
the differential dip of fold limbs and to different local
base levels on opposite flanks of the fold. Pattern of
tectonic rock uplift remains constant in each panel. In the
topographic building stages (i) and (ii), tectonics
