Geology Reference
In-Depth Information
provide insights on a history of fold growth that
are otherwise unattainable (Burbank
et al.
, 1996c,
1999; Keller
et al.
, 1998). Several key data sets can
contribute to such a history, including: topo-
graphic data on the fold crests, limbs, and nose;
dates on geomorphic surfaces and features along
the fold; detailed analysis of the underlying struc-
tural geometry; and variations of structure along
the length of the fold.
Consider first the ways in which a fluvial
system that was previously flowing across a
relatively low-relief landscape may interact with
a growing fold or a suite of growing folds. For
any individual fold, as it emerges above the
adjacent land, a new drainage divide is defined
along the fold crest and new catchments are
formed along its flanks: it subdivides formerly
continuous drainage systems, and the new
catchment configuration is closely tied to the
fold geometry. Asymmetrical folds with steep
forelimbs will have short, steep catchments on
their forelimbs and elongate, gentler catchments
on their backlimbs. This asymmetry is readily
visible in map patterns of river courses (Talling
and Sowter, 1999; Talling
et al.
, 1997).
Streams that had formerly flowed across the
site of the growing fold either (i) are diverted
parallel to the fold axis and around the nose of
the fold, (ii) become entrenched as antecedent
streams that incise across the uplifting fold, or
(iii) bevel off the top of the emerging fold so that
it has little or no topographic expression. In
order to maintain its course through a water gap
across a rising fold, an antecedent stream must
maintain a basinward-dipping gradient across
the fold. Otherwise, the stream will be diverted
along the upstream margin of the fold. A folding
event will cause relative uplift of the channel
reach within the folding domain (Fig. 9.22A),
causing a likely instantaneous reversal of the
channel gradient immediately upstream of the
fold. Sediment will tend to be ponded in this
depression and begin to fill it. At the same time,
a knickpoint will begin to propagate upstream
from the newly steepened zone on the down-
stream end of the uplifted reach (Fig. 9.22B).
A competition, therefore, exists between the
rate of differential rock uplift in the fold and
the rate of aggradation upstream of the fold.
If the sediment load of an antecedent stream is
insufficient to aggrade as fast as the fold is
rising, the stream is likely to be diverted
(Humphrey and Konrad, 2000). Several other
factors can affect whether a channel can sustain
its course. First, if a layer of alluvium that is a
few meters thick is part of the longer-term
transport load of the channel floor, then, in
response to uplift that is less than the thickness
of the alluvium, the channel will simply incise its
bed rapidly through the alluvium and sustain its
course. Second, if rock strength is low across
the core of the fold, then a knickpoint will
more rapidly propagate from the downstream
to the upstream end of the uplift and, thereby,
restore a downstream gradient. Third, as the fold
widens during continued growth (Fig. 9.22C),
the gradient of antecedent channels across the
fold will decrease and cause a concomitant
decrease in the erosive power of the stream
(Burbank
et al.
, 1996c). Whereas the first two
conditions promote maintenance of an anteced-
ent channel, the last condition promotes its
diversion.
When analyzing the map pattern of stream val-
leys associated with growing, laterally propagating
folds, the expectation should be that a series of
wind gaps along the fold crest will record the
progressive defeat of older, antecedent streams
and that, where a stream is still antecedent, the
fold has not broadened and uplifted sufficiently
to defeat the stream. As successive antecedent
streams are defeated, the remaining streams will
capture the discharge of the defeated streams
and will augment their discharge (and probably
stream power) as they cross the anticlinal crest.
Thus, the most likely location for an antecedent
stream is near the propagating nose of a fold
(Burbank
et al.
, 1996c; Jackson
et al.
, 1996).
An illustrative example of both antecedent
and diverted channels is well displayed in
the southern Tien Shan of western China (Hubert-
Ferrari
et al.
, 2007). Here, as deformation
encroaches on the adjacent foreland, a new, elon-
gate fold that is 200-300 m high has grown in
front of an older fold that is cut by antecedent
streams. Prior to the growth of the new fold,
those antecedent streams fed alluvial fans about
10 km long whose apices lay on the downstream
