Geology Reference
In-Depth Information
age, some had been uplifted more than 100 m!
Moreover, a complex pattern of folding in the
youngest terrace was progressively amplified in
successively older terraces. Each fold appears
to reflect subtle changes in dip in the under-
lying fault that cause spatial variations in rock
uplift rates (Fig. 7.25B). Given the dip of the
fault and rates of terrace uplift, Lavé and Avouac
(2000) were able to argue that, during at least
the Holocene, the entire plate convergence that
occurs between India and southern Tibet
(
∼
20 mm/yr) has been accommodated on the
Main Frontal Thrust. This argument is consist-
ent with the megathrust earthquakes that have
occurred on the plate-boundary fault descend-
ing northward from the Main Frontal Thrust
(Fig. 6.31), and it emphasizes again that the cur-
rently slow geodetic shortening across this
thrust is most likely a response to interseismic
elastic strain being stored within the Greater
Himalaya (Fig. 5.7) until the next megathrust
earthquake (Avouac, 2003).
Usually, as rivers incise into their valleys,
terraces along their sides are gradually eroded
away by hillslope processes. Thus reconstruc-
tion of older river profiles can be nearly impos-
sible in many mountainous regions. Sometimes,
however, fortuitous circumstances provide a
different means of preserving former valley
bottoms. In the Sierra Nevada of California, for
example, lava flowed down some alpine val-
leys during Miocene times and overran the for-
mer river bed. Because these lavas were more
resistant to erosion than were the surrounding
rocks, they have been better preserved and
actually stand above the modern valley bot-
toms, where they provide fine examples of
inverted topography. These lavas help define
the former profile of a major river flowing
west from the Sierra crest. When combined
with the modern river gradient in the same
valley and the age of the flows, the difference
between the profiles constrains the amount
and rate of differential bedrock uplift since the
lava flow engulfed the valley (Huber, 1981): in
essence, these flows are huge tilt-meters.
Although the flows seem like robust markers,
they are preserved much closer to the edge of
the range than to its crest. Hence, their tilt
150
A
Deformed Himalayan
Terraces
South
North
9.1 ka
9.2 ka
100
T
1
6.1 ka
T
2
50
terrace cover
T
3
strath surface
radiocarbon age
9.6
ka
0
12
B
Uplift Rates
9
T
2
T
3
6
T
1
3
0
C
Main Frontal Thrust
Himalayan
Structure
0
4
8
12
16
20
Distance (km)
Fig. 7.25
River terraces deformed above the Main
Frontal Thrust of the Himalaya.
A. Surveyed Holocene terraces along the Bagmati River,
Nepal, in the hanging wall of the Main Frontal Thrust
(MFT) within the Himalayan foreland. Note the
undulating upper terrace surfaces. B. Relative rock-uplift
rates derived from the age of each terrace and its height
above the modern river. The spatial coincidence of rates
for each terrace results from their kinematic tie to subtle
changes in dip of the MFT. The persistence of uplift
rates through time argues for steady Holocene slip on
the MFT. C. Geometry of the MFT reconstructed from
surface dips and slip rate from deformed terraces (B).
Modified after Lavé and Avouac (2000).
that is substracted from the observed profiles,
such that differential uplift of any portion of the
terrace becomes apparent. In a stunning study
of deformed terraces within the Himalayan
foreland (Fig. 7.25A), Lavé and Avouac (2000)
made detailed surveys across a 20 km wide
deformed zone that lies above the Main Frontal
Thrust, the active fault defining the southern
edge of the Himalayan deformation (Fig. 7.25C).
Although all of the terraces were of Holocene
