Geology Reference
In-Depth Information
the lower crust, rather than the upper mantle, is
the zone where isostatic compensation occurs.
+1000
model rigidities
5 x 10
21
N
m
2 x 10 Nm
20
Shaping landscapes during orogenic growth
1 x 10
Nm
19
graben
0
Outward flow of the lower crust is one of the
many potential modes of orogenic growth
(Fig. 10.3). Wherever erosion is both localized
and intense, some crustal response is expected
to the focused removal of mass. Typically, the
climate-erosion link is conceptualized in the
context of heavy rainfall, such as the monsoon
rains of the Himalaya. But, the key is truly
erosion, irrespective of the presence of a specific
climatic driver. Large rivers that run through arid
landscapes can potentially erode just as fast as if
they were flowing through a rainforest. The key
role played by the river is in its efficient removal
of any combination of bedrock from beneath it
and of detritus along its flanks.
The two largest river catchments in the
Himalaya are those of the Tsangpo and the Indus
(Fig. 10.52A). These rivers each flow parallel to
the range for more than 1000 km in southern
Tibet before turning south and exiting to the
foreland near the eastern or western edge of the
indentor formed by the Indian craton. As these
rivers leave the plateau and head toward the
lowlands, they enter deep, steep gorges that flow
along the flank of perhaps the most rapidly
uplifting massifs in the Himalaya (Zeitler
et al
.,
2001b). The foreland-veering rivers and uplifting
massifs occur at what have been termed “indentor
corners,” and the uplifting massifs of Nanga
Parbat in the west and Namche Barwa in the east
have been referred to as “tectonic aneurysms” - a
term meant to invoke accelerated deformation
due to weakening of the confining stresses
(Zeitler
et al
., 2001a). In each case, the weakening
is accomplished by the rapid incision of large
rivers in the topographic “gap” adjacent to the
massif (Fig. 10.52B). This incision increases the
near-surface thermal gradient, weakens the crust,
and creates a positive feedback that enhances
the development of a crustal-scale shear zone
(Koons and Kirby, 2007). Whereas the climate
is moderately wet where Tsangpo flows past
Namche Barwa, it is considerably drier than
Flexure, Wavelength, and Rigidity
-1000
0
10
20
3
0
4
0
50
Kilometers
Fig. 10.51
Tibetan topography and flexure across
a rift-flank uplift.
Predicted deformation is depicted for three different
rigidities. Only the general trend of the topography,
not the small-scale wiggles, should be compared
to the curves. The mismatches between the curves and
the actual topographic trends provide an estimate of
the precision attainable with this approach. For these
data, rigidities in the range (2-50) × 10
20
N m provide
the best match. Modified after Masek
et al
. (1994a).
flow in southeast Tibet is broadly consistent with
both the regional geodetic patterns (Fig. 5.15A)
and the history of regional shear inferred for the
Three Rivers region (Fig. 10.20).
Although the presence across much of Tibet
of a widespread, ductile lower crust remains
debated, Masek
et al
. (1994a) showed 15 years
ago that the deflections associated with many
Tibetan grabens was most consistent with
compensation associated with a weak lower
crust (Fig. 10.51). The deflections associated
with normal faults are largely isostatic in nature
and occur because the air and sediment that fill
the grabens act as a negative load (in compari-
son to the original, unfaulted crust). Owing to
the long length scale associated with the flex-
ural rigidity of the lithosphere, the isostatic
response to this negative load is more regional
than the load itself and induces uplift of the
flanks of the graben (Masek
et al
., 1994a; Small
and Anderson, 1995; Weissel and Karner, 1989).
Even though the Tibetan crust is 60-70 km thick
in the vicinity of several of these grabens, the
flexural wavelength is consistent with an effec-
tive elastic thickness of <10 km and implies that
