Geology Reference
In-Depth Information
Drainages Amid Propagating Anticlines
Little Rough
Ridge
North Rough
Ridge
A
B
C
1003
.
820
deflected
streams
1060
.
1003
.
9
74
wind
gaps
629
.
1003
734
1024
05
km
10
South
Rough
Ridge
Rough
Ridge
South
Rough
Ridge
722
spot height (m)
wind gap
ridge crest
Fig. 10.18
Stream patterns in interfering bedrock folds.
A. Drainage network, fold crests, plunging fold noses, water gaps, wind gaps, and their elevations in the vicinity of
Rough Ridge, central Otago, New Zealand. Note that the elevation of the wind gaps decreases as the fold nose is
approached and that, within the piggyback basins, highly asymmetrical drainages extend upstream of the water gaps.
Cosmogenic radionuclide exposure ages along the nose of South Rough Ridge (lower right) indicate that it has been
propagating north at a rate of
∼
1 km/Myr (Bennett
et al
., 2000). B. Cartoon of the drainage patterns in the vicinity of
Rough Ridge. Note the wind gaps along the fold crests and the clear diversion of drainages around the nose of each
growing fold. C. Interpretation of the history of growth of the four anticlines and drainage development along their
flanks. Modified after Jackson
et al
. (1996).
asymmetrical because the defeated rivers are
usually diverted toward the nose of the fold, that
is, in the direction of fold propagation. Growth
of an asymmetric upstream catchment that
encompasses the diverted streams continues
until the discharge and stream power through a
water gap closer to the fold's nose is sufficient
to balance the uplift rate.
In central Otago, New Zealand ( Jackson
et al
.,
1996), excellent examples are displayed of wind
gaps along plunging fold crests, asymmetric
drainages in piggyback basins, diverted drainages,
and persistent water gaps that developed in
response to growth of a suite of thrust-related
folds (Fig. 10.18). The asymmetry of the
underlying folds, the relative timing of fold
growth, and the propagation directions of the
multiple folds can be readily deduced from these
folds. Whereas the full growth history of these
folds remains unclear, recent cosmogenic dating
of “sarsen” stones (residual quartzose monoliths
that sit atop the ancient erosion surface that
serves as a structural marker defining the
Drainage development from folds
to orogens
If several folds are growing simultaneously and
their noses are propagating toward or past each
other, complex river patterns and topography
can sometimes reveal the history of lateral fold
propagation and vertical growth. Envision the
propagation of a young fold adjacent and paral-
lel to an older fold. Steep, short drainages typi-
cally develop on the forelimbs of such folds
prior to any interference between them. In the
direction of fold propagation, the crest of the
younger fold is likely to decline in height, and
the width of the fold commonly narrows
(Fig. 10.18). Streams that formerly flowed unim-
peded from the older fold into the basin will be
diverted parallel to the young fold, if they have
insufficient erosive capacity to maintain their
course across the growing anticline. Such diver-
sions will leave wind gaps along the crest of the
young fold as channels are sequentially defeated.
The diverted drainage systems are typically
