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In-Depth Information
systematically toward the south and toward
Mount Tamalpais, a scenario consistent with slip
above a blind listric thrust fault (Fig. 9.13B).
During efforts to locate previously unknown
active structures, both regional maps and
swath profiles of topographic characteristics
can be very useful, particularly for identifying
anomalies or trends. In a swath profile, spatial
data on transects perpendicular to the long axis
of the swath are assessed at successive steps
(typically equivalent to the pixel size of the data)
along the swath. The data from each transect
can be examined statistically to define their
attributes: commonly maximum, mean, and
minimum of some characteristic. For a typical
swath profile of elevation in most landscapes,
the mean elevation will lie closer to the minimum
than to the maximum. If the mean elevation
approaches the maximum, this upward deflec-
tion of the mean is likely to indicate a transient
state of adjustment to an accelerated uplift rate
in which rates of hillslope lowering are not keep-
ing pace with the rate of rock uplift (Fig. 9.14).
Similarly, changes in the average hillslope angle
or topographic relief can be a signal that the
landscape has adjusted (or is adjusting) to
changes in patterns or rates of rock uplift.
Recent studies in the Nepalese Himalaya have
combined diverse topographic attributes with
bedrock cooling ages to identify what may prove
to be a major, active, and previously unrecog-
nized fault (Wobus
et al.
, 2003, 2006b). The
location of this fault was initially deduced from
maps of hillslope angles and channel steepness
indices that show an abrupt discontinuity that
lies south of the Main Central Thrust and within
the Lesser Himalaya (Plate 7). Swath profiles
across this region show concurrent increases in
both mean elevation and mean relief (Fig. 9.14A
and B) and led to the naming of this topographic
break as the “physiographic transition 2” (or
PT2). (The PT1 lies at the break between the
Tibetan Plateau and the Greater Himalaya.)
6000
New Himalayan Thrust Fault?
knickpoints
steepness
terraces
4000
2000
A
south
north
0
3000
2000
1000
Lesser
Himalaya
Greater
Himalaya
B
0
0
10
20
30
40
50
Distance (km)
10
4
south
north
10
3
10
2
10
1
n = 312
C
10
0
50
40
30
20
10
0
Distance Downstream from MCT (km)
Fig. 9.14
Topographic indices and detrital ages
related to a proposed Himalayan fault.
A. Maximum, mean, and minimum elevation along
a north-south swath focused on the Burhi Gandaki
river in central Nepal (see Plate 7). The northern and
southern traces of the Main Central Thrust (MCT)
and the north-south extent of fluvial terraces, river
knickpoints, and normalized channel steepness
indices greater than 450 are shown. Major topographic
changes occur
>
20 km south of the MCT at the
“physiographic transition 2” (PT2). Note the upward
deflection of the mean elevation toward the
maximum, indicating a transient state north of the PT2.
B. Maximum, mean, and minimum of local relief
show an abrupt increase at the PT2. Transition between
Greater and Lesser Himalaya is taken as the MCT(S).
C. Detrital muscovite cooling ages (
n
= 312) from
seven tributary catchments indicating a
to the widths of sampled catchments orothogonal to
the swath. Box top and bottom show the 25th and
75th percentiles, with horizontal lines indicating
the median age. Modified after Wobus
et al.
(2003, 2006b).
>
10-fold
offset of ages across the PT2. Box widths correspond
