Geology Reference
In-Depth Information
time). In theory, the loss of contributions from
the Punjab catchments should be detectable in
records from the Bay of Bengal where the
Ganges debouches its sediment load. But, the
addition of new, isotopically distinctive source
areas (as happens when the Punjab rivers are
captured by the Indus) is much easier to detect
than the incremental decrease in an existing and
more isotopically homogeneous source area, as
would be the case for the Ganges catchment.
As is clear from the Yangtze example (Fig. 10.19),
river-network anomalies that represent departures
from expected geometries may point to sites
where former catchments have been significantly
modified. In addition to features that may connote
capture events, such departures include anomalous
spacing of drainages or downstream narrowing of
catchments. Whereas no specific tectonic events
are known to be associated with the capture
sequence on the Yangtze or in the Punjab, clear
tectonic inferences can be drawn from some river
patterns. Consider, for example, the southeastern
Tibetan Plateau, where three of the region's great
rivers (the Mekong, Salween, and Yangtze) rotate
from easterly to southerly courses as they swing
around the eastern tip of the Himalaya and exit
the plateau (Fig. 10.20A). The most curious aspect
of their courses through this bend is that they con-
verge, such that they flow parallel to each other
and are spaced only a few tens of kilometers apart
(Hallet and Molnar, 2001). This region of closely
spaced, parallel channels represents a major width
anomaly in which the catchments are about
10-fold narrower than would be expected at these
downstream positions (Fig. 10.20B, C, and D). So,
what causes such striking narrowing of these
catchments? Hallet and Molnar (2001) propose a
tectonic model (Fig. 10.20E) in which crustal-scale
shear occurs around the northeast corner of the
Indian craton as Eurasia (and the Tibetan Plateau)
converges with it. North of the rigid indentor
(India), crustal shortening in Tibet would be
expected to cause narrowing of catchments ori-
ented east-west. East of the indentor, large-scale
dextral shearing would be expected. Simple geo-
metric assessments suggest that the shear strain
that is required to cause both the proximity and
the parallelism of the rivers is about 6
±
1. Shear
strain is defined here as the ratio of shear displace-
ment to the width of the shear zone, and, in this
setting, such strain is equivalent to about 900km
of dextral shear across a 150-km-wide shear
zone where the anomalous drainages occur
(Fig. 10.20E). Such strain is consistent with
modern geodesy (see Fig. 5.15A) which depicts
a striking turning and flow of crustal material
around the eastern syntaxis where the three
rivers described here are in close proximity.
Steady state and pre-steady state
Because convergence between two plates can be
sustained for millions of years, rock uplift in
contractional mountain belts can persist for
similarly long time spans. But the height of
mountains cannot grow forever. At some point,
rates of erosion or tectonic extension ought to
balance the rates of rock uplift, such that the
range comes into a topographic steady state.
Prior to that time, the mean surface elevation can
increase through time, reflecting rates of rock
uplift that outpace erosion rates, whereas in post-
steady-state conditions, erosion outstrips rock
uplift. Even in steady-state conditions, rock uplift
will not be balanced by erosion at every point in
the landscape at all time scales. For example, we
expect that changes in erosion rates are coupled
to changes in climate at time scales of decades to
many millennia, but, in the general absence of
evidence indicating similarly paced changes in
tectonic rates, we deduce that tectonic rates are
steadier and that short-term imbalances between
erosion and uplift are likely. On average, however,
under steady-state conditions, a balance should
exist at time scales that span several climate
cycles. Thus, we examine here how a balance
between rock uplift and erosion is attained in
such circumstances and how we may recognize
whether or not this balance has been achieved.
Categories of steady state
With respect to mountain ranges or orogens, at
least four categories of steady state can be
defined: flux steady state, thermal steady state,
exhumational steady state, and topographic
steady state (Willett and Brandon, 2002).
