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Geomorphology xx (2003) xxx – xxx
www.elsevier.com/locate/geomorph
Tectonic forcing of longitudinal valleys in the Himalaya:
morphological analysis of the Ladakh Batholith, North India
S.S.R. Jamieson a,*, H.D. Sinclair b,1, L.A. Kirstein b,1, R.S. Purves c
b
a
School of GeoSciences, University of Edinburgh, Drummond Street, Edinburgh, EH8 9XP UK
School of GeoSciences, University of Edinburgh, Grant Institute, West Mains Road, Edinburgh, EH9 3JW UK
c
Department of Geography, University of Zurich, Winterthurerstrasse, 190, 8057 Zurich, Switzerland
Received 8 August 2002; received in revised form 7 April 2003; accepted 8 April 2003
Abstract
Longitudinal valleys form first order topographic features in many mountain belts. They are commonly located along faults
that separate tectonic zones with varying uplift histories. The Indus Valley of Ladakh, northern India, runs northwestwards
following the boundary between the relatively undeformed Ladakh Batholith to the north – east and the folded and thrusted
Zanskar mountains to the south – west. In this region the Shyok Valley, on the northern side of the batholith, approximately
parallels the course of the Indus. This study investigates geomorphic variations in transverse catchments that drain the Ladakh
Batholith, into the Indus and Shyok rivers. The batholith has been divided into three zones based on varying structural
characteristics of its northeastern and southwestern boundaries. Morphometric analysis of 62 catchments that drain into the
Indus and Shyok valleys was carried out using three digital datasets, and supported by field observations. Morphometric
asymmetry is evident in the central zone where the Shyok valley is considered tectonically inactive, but the Indus Valley is
bound by the northeastwardly thrusting Indus Molasse and the batholith. In this zone the catchments that drain into the Indus
Valley are more numerous, shorter, thinner and have lower hypsometric integrals than those that drain into the Shyok. By
linking these observations with the regional geology and thermochronological data it is proposed that high sediment discharge
from the deformed Indus Molasse Indus Valley has progressively raised base levels in the Indus Valley and resulted in sediment
blanketing of the opposing tectonically quiescent catchments that drain southwestwards off the batholith. The Indus Molasse
thrust front has propagated at least 36 km towards the Ladakh Batholith over the last 20 Ma. Hence it is proposed that this long
term asymmetric structural deformation and exhumation has forced the Indus longitudinal valley laterally into the Ladakh
Batholith resulting in the morphometric asymmetry of its transverse catchments.
D 2003 Elsevier B.V. All rights reserved.
Keywords: Ladakh; Himalayas; DEM; Indus; Longitudinal valley; Morphometric analysis
1. Introduction
* Corresponding author. Fax: +44-131-6502524.
E-mail addresses: [email protected] (S.S.R. Jamieson),
[email protected] (H.D. Sinclair),
[email protected] (L.A. Kirstein),
[email protected] (R.S. Purves).
1
Fax: +44-131-6683184.
Longitudinal valleys form first-order geomorphic
features in mountain belts. They develop where the
strike-parallel structural grain of the underlying geology dominates the topography. Tectonic structure and
0169-555X/03/$ - see front matter D 2003 Elsevier B.V. All rights reserved.
doi:10.1016/S0169-555X(03)00185-5
GEOMOR-01419
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S.S.R. Jamieson et al. / Geomorphology xx (2003) xxx–xxx
faults exert major controls on the course of longitudinal rivers (Koons, 1995; Hallet and Molnar, 2001).
The structural grain that determines the course of
many of these rivers is commonly enhanced by the
presence of faults. Faults can result in a zone of highly
strained, mechanically weak rocks that facilitate the
erosive capabilities of a river, and fix a river’s course
(Koons, 1995). Faults also bound zones with differing
uplift histories, hence differential rock uplift between
the two sides of major longitudinal valleys in mountain belts is to be expected.
Channelling of rivers by growing topography requires that rivers are close to the sedimentation –
erosion threshold and do not have the stream power
necessary to cut through the uplifting material (Koons,
1995). Present-day drainage patterns in the Himalaya
demonstrate a clear contrast between the High Himalaya and the foreland. In the High Himalaya longitudinal rivers such as the Tsangpo and Indus drain the
northern slopes that border Tibet (Seeber and Gornitz,
1983). Many of these rivers are low-gradient and close
to the sedimentation – erosion threshold, that is the
Fig. 1. Simplified geological map of study area with major tectonic and lithological features of interest to this study based on orthorectified
DEM (geology modified after Brookfield, 1983; Searle et al., 1990). KF—Karakorum fault; ZSZ—Zanskar Suture Zone; ITSZ—Indus –
Tsangpo Suture Zone. Inset shows the region of the Indus River in northern India analysed.
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riverbed is sufficiently draped with alluvium that the
river does not down-cut and the course is unaffected by
mechanical variability in the underlying bedrock. In
contrast high gradient transverse rivers drain the southfacing slopes in the foreland, which are fed by heavy,
monsoonal precipitation (Friend et al., 1999).
Longitudinal valleys, such as the Indus, extend for
hundreds of kilometres along structural strike and are
fed by numerous tributaries along their length. Given
the potential variability in tectonic forcing, lithology
and climate the geomorphology of a longitudinal
valley varies unsystematically downstream. Here, we
investigate a 400 km stretch of the Indus longitudinal
valley in Ladakh, northern India (Fig. 1) with the aim
of evaluating the long-term (105 to 107 year) role of
thrust-induced rock uplift and associated sediment
input on its geomorphic evolution. This is achieved
by analysing the morphology of the transverse
tributaries and linking this information with the structural geology and exhumational history of the blocks
that bound the valley.
2. Regional setting
The Indus River flows westward from its source at
Mount Kailas in southwestern Tibet towards Pakistan
through the regions of Jammu and Kashmir until it is
diverted southwards around Nanga Parbat and disgorges from the mountain front near Islamabad (Fig.
1, inset). It represents a first-order geomorphic feature
of the Himalayas, and is comparable to the east
flowing Yarlung – Tsangpo river system. In the region
of Ladakh, the Indus Valley’s northern boundary is
formed by the Ladakh Batholith which represents the
northern buttress against which the Himalaya have
been deformed (England and Searle, 1986; Fig. 1). In
this region too, the Shyok River can be found running
along the northern margin of the batholith from the
Karakoram fault in the east, to its convergence with
the Indus River system at the northwestern margin of
the Ladakh Batholith (Fig. 1).
2.1. Tectonic setting
The Ladakh Batholith forms part of the Transhimalayan Batholith system that defines the southern
boundary of the Tibetan Plateau from Ladakh in
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northern India east to Bhutan. To the southeastern
side of the batholith the high relief, deformed
Tertiary sediments of the Zanskar zone form the
southern margin of the Indus Valley. The area of
study is located between two major tectonic suture
zones, the Shyok to the north and the Indus –
Tsangpo to the south (Fig. 1). The former represents
the remnants of an ancient back-arc basin complex,
and the latter represents the main boundary zone
between the Indian and Asian plates (Searle et al.,
1990). The predominantly granodioritic Ladakh
Batholith forms part of the plutonic remnants of
the island arc that rimmed the Asian continent from
Cretaceous to Eocene times (Weinberg et al., 2000).
The structural deformation of the batholith is minimal, although evidence of localised ductile deformation is present. Crystallisation ages from the main
batholith range from 65 to 50 Ma (Weinberg and
Dunlap, 2000). Apatite fission track ages from the
batholith, reveal cooling through the approximately
110j isotherm during progressive unroofing in the
early Miocene (Sinclair and Jaffey, 2001). In the
more easterly portions of the study area, the batholith is bound to the northeast by the Karakorum
Fault (Fig. 1), a major dextral strike slip fault that
bounds the southwestern margin of the Tibetan
Plateau (Searle, 1996).
South of the batholith are the deformed sedimentary successions of the Indus Molasse. These contain
an Eocene to Miocene succession of sediments comprising limestones, mudstones and conglomerates
(Searle et al., 1990). This succession contains younger
apatite fission track ages (approximately 14 Ma) and
has been thrust northwards towards onto the Ladakh
Batholith from early Miocene times to recent (Sinclair
and Jaffey, 2001). Over 36 km of shortening has been
proposed between the Eocene and the late Miocene
(Searle et al., 1990).
2.2. Climatic setting
The climate is arid with desert conditions existing
throughout most of the area and large diurnal temperature fluctuations are accompanied by precipitation that
decreases eastwards in the valley floors from about 150
mm year 1 in the Lamayuru basin (Kotlia et al., 1997)
to 93 mm year 1 at Leh (Holmes, 1993). Precipitation
occurs during the summer months when monsoonal
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conditions over the Himalaya can spill northwards,
although most occurs as snowfall during the winter as
a result of westerly circulation. This is also the case in
the Zanskar range, to the south, although here a great
deal more precipitation falls due to its increased humidity. The harsh winters, with temperatures as low as
40 jC, and the loss of most of the water through rapid
snowmelt in the spring mean that vegetation cover is
minimal and discontinuous in Ladakh (Fort, 1983).
Whilst data concerning detailed spatial variations in
precipitation, soils and vegetation are not available,
there is no evidence to suggest that climate has caused
significant variation in drainage patterns between the
Shyok and Indus Valleys.
2.3. Geomorphological setting
The Ladakh Batholith is characterised by a long (ca.
350 km) ridge with peaks up to 6213 m (Spanpuk). This
watershed bounds numerous drainage basins of varying sizes, and is approximately equidistant from the
margins of the batholith across all but the central part
where it lies closer to the Indus Valley. The southern
side of the Indus Valley comprises a series of interfluve
ridges of the Indus Molasse that are deeply incised by
its tributaries, the largest of which is the Zanskar river.
To the southeast, 70 km from the Indus Valley, the
Zanskar Range forms a large, continuous massif contaning peaks such as Nun Kun (7156 m). Between the
Ladakh and Zanskar ridges, the Indus Valley acts as the
main sediment pathway for both of the mountain ranges
with a valley floor that varies from a 15-km wide
alluvial valley fed by large transverse fans, to a 50-m
wide mixed alluvial/bedrock channel forming steep
gorges. A longitudinal profile along 380 km of the
Indus River in this region reveals an upstream, high
gradient portion, followed by a 100-km long, relatively
flat reach, which then steepens abruptly again downstream before it cuts through the mountain belt near
Nanga Parbat (Fig. 2).
Ladakh, in common with much of the nearby
Karakoram Himalaya, displays a geomorphology that
is heavily influenced by reworking of frost shattered
rock and Quaternary glacial deposits through snowmelt, glacial runoff and mass movement processes.
Major damming of rivers has been identified as
having had a large influence on the geomorphology
that is currently visible in the region. Particularly
notable are the lake sediments, such as those found at
Lamayuru and Leh dated to around 35 –40 ka BP
(Fig. 1), where lacustrine deposits are preserved at
the distal ends of alluvial fans that discharge into the
Indus Valley (Bürgisser et al., 1982; Fort et al.,
1989). This damming is interpreted to have occurred
as the large valley glaciers retreated after the third
(Upper Pleistocene) glacial stage and the moraine
sediments were remobilised by tectonic activity (Kotlia et al., 1997). Mass movement activity such as this
has been common in the Karakoram Mountains since
the Late Pleistocene, and has resulted in a complex
system of diamicton depositional landforms (e.g.
Owen and Derbyshire, 1989; Owen, 1991; Owen
and Sharma, 1998), many of which are also present
in Ladakh.
High altitude cirque glaciers are present above ca.
5100 m, with broad ‘U’-shaped valleys above approximately 4800 m (Holmes, 1993). Surrounding these
glacially dominated regions, periglacial environments
persist (Fort, 1983), and this, combined with the frost
succeptibility of the rock, results in large volumes of
frost shattered debris that blanket the slopes. Below
this, the valleys are ‘V’-shaped with mean slopes of
25 –30j. These slopes are dissected by gulleys that
feed steep, debris-flow and snowmelt-runoff dominated alluvial fans that are typical of many parts of
Ladakh and Karakoram Himalayas; between these
fans, the bases of the slopes are locally draped by
colluvium. In the transverse catchments that drain into
the Indus and Shyok valleys from the batholith, the
river channels range from steep bedrock reaches, to
lower angle, braided alluvial forms as they near the
Fig. 2. Digital elevation model (USGS GTOPO30 data) covering the study area illustrating drainage catchments analysed, coverage of datasets,
the subdivision of the area into zones A, B and C based on the tectonic characteristics of the Indus and Shyok Valleys, and two topographic
profiles across the study area. Note: dashed line indicates division between zones, angular brackets indicate dataset coverage. A, B, C and D mark
the end points of the topographic profiles: A – B is a cross-section from the Zanskar Range in the southwest to the Karakorum Batholith in the
northeast, C – D is a longitudinal profile of the Indus River as it runs adjacent to the Ladakh Batholith. KF—Karakorum Fault; ZBT—Zanskar
backthrusting.
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longitudinal valley floors. The imprint of various
stages of glaciation can still be seen on the landscape,
particularly in southern Ladakh near the Indus, where
numerous terminal moraines exist in the transverse
valley floors, e.g. at Leh and at nearby Basgo, Phyang
and Tharu (Fort, 1983).
3. Methods
3.1. Morphometric analysis
Morphometric characterisation is not a new concept, and techniques such as hypsometric (area-elevation) analysis (Strahler, 1952) have become firmly
rooted in our understanding of geomorphology and its
links with tectonic uplift or relative base-level change
(Summerfield, 1991). Hence, the shape of the earth’s
surface is often used to interpret the nature of the subsurface processes that contributed to its formation
(Koons, 1995; Hutchinson and Gallant, 2000; Snyder
et al., 2000). In recent years, data have become widely
available that record the shape of the landscape in a
simple, regularly spaced altitude matrix (a Digital
Elevation Model or DEM) allowing the statistical
analysis and comparison of different terrain types at
multiple scales (Burrough and McDonnell, 1998;
Wood, 1996a,b). The application of such analyses
can be achieved through the use of spatial analysis
‘toolboxes’ provided by Geographic Information Systems (GIS). Such systems are increasingly being used
in morphometric analyses due to their ability to
provide repeatable, quantifiable measures of shape
parameters for landscape units—for instance the mean
slope or curvature of catchments. However, caution
should be used in the interpretation and discussion of
results which may be strongly dependent on the scale
of the DEM (Wolock and Price, 1994; Wood, 1996a;
Schneider, 2001). Furthermore, the selection of the
parameters to be measured must in itself take into
account the scale of the DEM and consider it in
conjunction with the likely scales at which the processes shaping the forms being measured operate.
Nonetheless, in remote areas where limited tools are
available, morphological analysis provides a useful
tool in comparing the form of landscape units.
The advent of DEMs as structures for storing
landscape shape, and of GIS as a tool for analysis of
such data has led to the rejuvenation and further
development of morphometric analysis as a technique
for investigating landscape evolution. Derivatives of
landscape form such as slope, aspect and curvature as
discussed by Evans (1980) allow us to enumerate the
form of a landscape represented by a DEM in a way
that is meaningful for surface process studies. Furthermore, drainage basins and networks, as we will
show, can also be easily extracted from a DEM to
provide further ways of numerically describing the
landscape and investigating its past tectonic and
geomorphic history. In the Mendocino triple junction
area of northern California, for example, Snyder et al.
(2000) used DEMs to quantify variations in landscape
morphometry, and more specifically, river long-profiles, associated with along-strike changes in crustal
and climatic setting.
3.2. Strategy
This study aims to provide insights into the effects
of tectonic forcing on the geomorphic character of the
Indus Valley. Specifically we investigate how asymmetric structural evolution, exhumation and sedimentation may have determined the long-term (106 years)
development of the valley form. In order to characterise a tectonic input into the system, we have to
characterise the non-tectonic, background character
of the basins that discharge into both the Indus and
the Shyok valleys. We have chosen to subdivide the
Ladakh region into three zones (Fig. 2), each identified by distinct tectonic characteristics. In the northwest of the area, the Indus River crosses through and
is bounded on both sides by the Ladakh Batholith,
whilst the Shyok River follows the boundary between
the Ladakh Batholith and the Shyok Suture Zone; this
area is termed zone A (Fig. 2). In zone A it is believed
that, where their courses are directly adjacent to the
batholith, neither of the rivers follow lines of Tertiary
structural activity. Zone B stretches southeastwards
from zone A to the zone of intersection of the
Karakorum fault and the Shyok Valley (Fig. 2).
Hence, in zone B, the Shyok Valley still flows along
the inactive Shyok Suture Zone. In contrast, the Indus
Valley of zone B is traced along the boundary between
the northeastward vergent fold and thrust belt comprising the deformed Indus Molasse to the southwest,
and the Ladakh Batholith to the northeast. Zone C is
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defined by the remaining section of the batholith
within the eastern-most portion of the area of interest.
Here, the Shyok Valley follows the route of the active
Karakorum fault as it bounds the batholith to the north
(Fig. 1). To the south, the Indus Valley continues to
follow the boundary between the deformed Indus
Molasse and the Ladakh Batholith.
These three zones that bound all (zones B and C),
or part (zone A) of the Ladakh Batholith are spatially
analysed and examined for variations in shape characteristics that may be related to the contrasting
tectonic settings between them. Specifically, the morphometry of the transverse catchments that drain from
the batholith into the Shyok and Indus valleys are
examined in detail. The testable prediction is that in
the zone where there is maximum contrast in the
degree of tectonic activity along the Shyok versus
the Indus Valley, one would expect maximum contrast
in the morphometry of the transverse catchments that
drain into the valleys from the batholith. Hence in
zone B, the catchments that drain the batholith into the
Indus Valley should show a signal that is different
from those that drain into the Shyok if a pervasive
tectonic regime is the principal control on form. This
asymmetry should be unique to this region and should
not be present in zones A and C as they are defined by
different surrounding tectonic characteristics. These
analyses of modelled surface shape are supported by
localised geomorphic field observations.
3.3. Data
In order to identify whether any significant morphological difference exists within the Ladakh region,
DEMs were analysed using a commercial GIS: ArcGIS (ESRI, 2002). Three datasets of varying resolutions, derivations and spatial extents (Table 1) were
subjected to similar analysis in order to gain a detailed
insight into the geomorphic form of the Ladakh
Batholith and because of limitations in the spatial
extent of the highest available resolution data. The
GTOPO30 data is a global dataset derived from a
number of different types of data (USGS, 2002),
although in Ladakh, it is simply a down-sampled
version of NIMAs DTED Level 1 data (NIMA,
2002). The 100 m GRID dataset was created on a
local scale from 1:250,000 cartographic sources (US
Army, 1954) and approximates the resolution quality
7
Table 1
Digital elevation data
DEM
Cellsize Coverage
(m)
GTOPO30
1000
100 m DEM 100
SPOT DEM
50
Entire batholith
NW portion
North/central
portion
Vertical Source
accuracy
(m)
F 30
F 30
F 100
USGS
Cartographic
SPOT stereo
image
Vertical accuracy is absolute linear error at 90% probability with the
exception of the SPOT DEM which is tested against the 100 m data
at an absolute linear error of 60% probability.
of a DTED Level 1 DEM, and the SPOT DEM was
derived from a stereo pair of SPOT satellite images.
The application of DEMs to morphometric analysis
is constrained by the resolution of the DEM and its
precision and accuracy in the vertical and horizontal,
all of which contribute to uncertainty in ascribing
signals to real differences in form or artefacts and
errors in the DEM. This is particularly true where
derivatives of the elevation (such as slope, aspect and
curvature) are involved (Bolstad and Stowe, 1994) so
it is important to note that none of the data employed
here is without error. Hutchinson and Gallant (2000)
indicate that because most applications of DEMs
depend on their ability to depict landsurface shape
and drainage structure, absolute elevation errors alone
cannot provide a full assessment of DEM quality. A
number of visual and statistical tests were therefore
applied to the three datasets in order that their accuracy could be tested. Slope plots were analysed for
evidence of an overly ‘stepped’ terrain that might
indicate poor quality interpolation results, and a
shaded relief plot was used to identify any random
or systematic error evidenced by local anomalies in
the DEMs. Frequency histograms of elevation were
also produced to test for any obvious contour bias in
the DEMs that might have been present. These histograms would show increased frequencies at the regularly spaced contour elevations if the original
interpolation method were poor (Hutchinson and
Gallant, 2000; Wood, 1996a). In the case of the data
covering the study area, no such patterns were visible,
suggesting that the methods used to create the DEMs
were sufficiently accurate. However, it was very
difficult to get accurately positioned control points
for the derivation of the SPOT DEM due to the remote
nature of the area and the existence of snow cover in
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high altitude areas of the SPOT images. Visual comparison of a river network derived from this DEM to
the original orthorectified SPOT image however
showed that planform drainage calculated by ArcGIS
compared well, and did not deviate significantly from
the path of the associated stream network in the SPOT
image. However, a notable deterioration in accuracy
occurs over the snow-covered areas in the upper
reaches of the transverse river where DEM construction relied heavily on interpolation across a spectrally
homogenous region.
Walker and Willgoose (1999) found that DEMderived drainage catchments can often be significantly
different from reality, particularly if the catchments
are small. To test the inter-comparability of the three
datasets, simple morphometric data (including measures of basin size) was extracted and statistically
compared over a number of overlapping basins in
zones A and B in order to establish the degree of
correlation between all three data types. Where the
three DEMs overlapped, calculations showed up to
96% correspondence between basin areas across the
datasets inferring that a wider study of the entire area
using the lower resolution, but more spatially extensive GTOPO30 data would be of adequate accuracy.
More focussed studies in the higher resolution data
areas could then be used to identify more closely the
issues relating to any spatial variations in the relationship between geomorphology and tectonism.
3.4. Morphometric extraction
In order to allow comparison between the different
DEMs it was necessary to derive the morphometric
parameters that were robust in their ability to describe
landsurface shape as well as being sensitive to the
processes of formation. Specifically, the parameters
outlined in Table 2 were derived from each DEM on a
catchment by catchment basis; the drainage basin
being an easily definable morphological unit allowing
direct comparisons both across and along strike in
Ladakh. The extraction of drainage basins and networks upon which the other morphological parameters are then derived, is based upon pitless versions of
the DEMs where topographic hollows are filled and
can consequently create flat areas of topography
(Jenson and Domingue, 1988). This makes the assumption that all ‘pits’ are artefacts caused by eleva-
Table 2
Summary of morphometric parameters and their calculation
Parameter
Basin-wide
Area
Perimeter
Hypsometry
Hyps. integral
Basin length
Elevation max.
Elevation min.
Elevation mean
Slope mean
Relief ratio
Elongation ratio
Drainage network
Stream order
Mean lengths
Drainage density
Stream frequency
Bifurcation ratios
Stream profiles
Description
Area of the basin
Perimeter of the basin
Area vs. elevation plot
Area under the hypsometric curve
Length of a straight line between
the basin pour point and the furthest
point in Euclidean space from
the pour point
Maximum elevation in the basin
Minimum elevation in the basin
Mean elevation across the basin
Mean slope across the basin
Relief/basin length (relates
elavations to basin length)
A measure of basin shape
(length to width ratio)
Strahler stream ordering
Stream lengths for each
stream section
Mean length of stream
channels/basin area
Number of streams of
particular order/basin area
No. of streams of order0/
No. of streams of order+ 1
(tells us about network structure)
Long-profiles of full stream systems
(1st to Nth order) within the
transverse catchments
tion underestimation rather than overestimation
(Martz and Garbrecht, 1998). Pit filling is necessary
in many approaches to drainage extraction because
they rely on having a hydrologically consistent surface that allows flow to continue uninterrupted
throughout the entire basin (Wood, 1996a).
The derivation of drainage systems (both basins
and networks) has attracted much attention in the
modelling community (e.g. Band, 1986; PalaciosVélez and Cuevas-Renaud, 1986; Fairfield and Leymarie, 1991; Tribe, 1992; Tarboton, 1997). Specifically, the different algorithms used to delineate
drainage can produce more or less realistic results
depending upon which is employed. Here, the deterministic eight flow direction matrix (D8) was
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employed, this being the simplest algorithm that
derives flow direction from a gridded dataset (Tarboton, 1997) and being known to be robust in its ability
to delineate basins quickly and adequately (Gallant
and Wilson, 2000). The derived stream networks for
the 100 m DEM are thought to be of reasonable
quality because areas of erroneously flat surface data
are minimal, thus avoiding problems of assigning
flow over such areas (Garbrecht and Martz, 1997;
Martz and Garbrecht, 1998; Turcotte et al., 2001).
Perhaps a greater issue relating to precision of the
drainage networks in this case relates to the fact that
when they are derived, a threshold number of cells are
used to infer their initiation of flow (Montgomery and
Foufoula-Georgiou, 1993). Helmlinger et al. (1993)
indicate that attempts to predict appropriate values of
flow initiation have been largely inconclusive, and
therefore the largely subjective choice of an initiation
value is more likely to skew the results than slight
imprecision in a particular stream. As an assessment,
drainage networks for a number of basins from the
100 m DEM were overlayed and compared with
Digital Chart of the World river data (DCW, 2002),
and the overlapping orthorectified SPOT image. The
river networks appeared consistent with each other
despite the differing resolutions of data, with streams
generally being within 1 pixel distance of each other
in the GTOPO30 data.
Many of the parameters outlined in Table 2 were
extracted basin by basin using simple analysis techniques available in ArcGIS. However, these were supported by GIS algorithms written to extract hypsometry (modified after Holmes, 2000), and for the
extraction of river long-profiles and associated finite
difference measurements. The latter was based on the
principle of least cost path analysis—a standard spatial analytical method employed in GIS to calculate a
path of least resistance through a number of points.
Furthermore, due to the manner in which ArcGIS
handles stream ordering and stream network topology
after conversion from raster to vector data, algorithms
were written to create a vector stream network that
was accurate in its depiction of Strahler stream ordering (Strahler, 1952). This is important because Strahler ordering is a requirement for accurate and
consistent calculation of bifurcations ratios (the number of streams in a particular order divided by the
number in the next highest order), one of the metrics
9
applied to identify any differences in river network
form.
4. Results
As indicated previously, a fundamental way of
measuring the shape of the landscape is through the
extraction of hypsometric data. These describe the
relative altitudinal distribution of land area within the
study area. A number of key results (mean elevation,
basin area, the hypsometric integral, and elongation
ratio) are illustrated in Fig. 3 and point to variability in
morphometry both along strike, and across the central
drainage divide.
Statistical analysis of the morphometric data
extracted from each of the DEMs was carried out to
test the null hypothesis that no significant difference
existed between the landscape either side of the
Ladakh drainage divide. Strong parametric tests could
not be employed as it was not possible to normalise the
data, so the non-parametric Mann – Whitney U-test
was employed to compare the two independent groups
of data (north versus south). Non-parametric tests
make no assumptions about the distribution of the
data, and are therefore weaker than, for example, the
parametric t-test, because they have less information to
rely on in testing for significance and hence stand less
chance of finding statistical significance (Shaw and
Wheeler, 1994). Hypsometry and stream long-profiles
were plotted in a graphical form to allow visual
analysis, with the hypsometric integral providing the
means to scrutinize the hypsometry numerically.
4.1. Zone A results
This area is covered fully by the 100 m DEM and
the GTOPO30 data. Covering 15 basins, the morphometric data was analysed and the results subjected to
statistical analysis which indicated that very little
significant difference could be identified between
the morphometry of the northeastward draining catchments versus those draining to the south –west. Indeed, as identified in Table 3, both the datasets
consistently show that the morphometry on both sides
of the drainage divide is very similar. Two of the
strongest correlations occur with the basin area and
the elongation ratio (the diameter of a circle of the
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S.S.R. Jamieson et al. / Geomorphology xx (2003) xxx–xxx
Fig. 3. Plots illustrating the variations in geomorphic parameters from across the area. (a) Basin area, (b) Mean elevation, (c) Hyposometric
integral, (d) Elongation ratio. Note: class breaks determined statistically by finding adjacent feature pairs that have relatively large gaps between
them (natural breaks method). Dashed line indicates divisions between zones.
same area as the basin, divided by the length of the
basin) measurements. These are represented visually
in Fig. 3 and show that basins on both sides of the
central drainage divide have areas ranging from ca. 30
to ca. 340 km2 and elongation ratios ranging from ca.
0.9 to ca. 1.3 (where a basin is longer the elongation
ratio tends towards zero). Moreover, mean elevation
remains relatively constant throughout zone A at
between 3900 and 4700 m—the northern side showing only a marginally higher (not statistically significant) elevation overall. The hypsometric integral does
vary across the central divide, but not significantly.
The values range between 0.5 and 0.7 although again,
the northern side displays slightly higher integrals
indicating a relatively large proportion of land at high
elevation. This pattern is supported by the raw hypsometry which indicates that in general, the distribution
of landsurface area does not vary significantly between the two sets of drainage catchments. In zone A,
the results show that approximately 60 –75% of the
landsurface area can be found in the top 40% of
elevation.
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N = no significant difference at 90% confidence, Y = significant difference at 95% confidence, B = borderline-significant difference at 90% confidence. Blank cells indicate no calculation of the parameter for that particular dataset. Note: zones
A, B and C are as outlined in Fig. 2.
Y
Y
Y
B
N
N
N
N
N
N
N
B
N
N
Y
B
N
N
N
N
Y
B
B
All
A
A
B
B
C
GTOPO30
GTOPO30
100 m DEM
GTOPO30
SPOT DEM
GTOPO30
N
N
N
Y
B
N
N
N
N
Y
B
N
N
N
N
Y
Y
N
Y
N
N
Y
B
N
N
N
N
Y
Y
N
B
N
N
Y
Y
N
N
N
N
N
N
N
Y
N
N
Y
Y
B
N
N
N
N
Y
N
Elongation
ratio
Hypsometric
integral
Hypsometry
Basin
perimeter
Basin
area
Parameter
Extent
(zone/s)
DEM
Table 3
Morphometric differences identified north and south of the drainage divide
Basin
length
Elevation
maximum
Elevation
minimum
Elevation
mean
Slope
mean
Relief
ratio
Number of
1st order
1st mean
length
1st drainage
density
1st stream
frequency
Bifurcation
ratios
Stream
longprofiles
S.S.R. Jamieson et al. / Geomorphology xx (2003) xxx–xxx
11
Data extracted for river networks from the 100 m
DEM in this zone did not differ significantly across
the central divide. However, in this zone, river longprofiles were additionally extracted from the 100 m
DEM. These are shown in Fig. 4 and indicate that in
the north, river bed profiles are either convex –concave or convex, whereas in the south a number of
concave profiles are also found. The mean gradients
of the streams flowing into the Indus range between
5j and 22j—the steeper gradients being associated
with the shorter catchments. On the north, the mean
gradients are only marginally steeper overall, ranging
between 6j for the longer streams, to 23j in the
shorter streams, mirroring the pattern found in the
south. On the south side, the rivers are generally
around the same length (5 – 20 km) as the north
although two of the rivers are approximately 5 km
longer (one of these is from the basin at the right
angled kink in the Indus in the northwest corner of the
study area—see Fig. 2).
4.2. Zone B results
The data for zone B come largely from the
GTOPO30 dataset, but this area is also covered
partially by the SPOT DEM, and to a lesser extent
by the 100 m DEM. The zone supports 22 basins
overall, and is noticeable for their distribution—16 are
located on the southern side of the central divide
compared with only six basins to the north. In this
case, as shown in Table 3, all the parameters measured
in the north show a significant difference when
compared to their southern counterparts, apart from
the minimum elevation and the mean slope. The
clearest indicators of this difference are basin area
and basin shape, as measured by the elongation ratio
(Fig. 3). The basins on the south are all smaller than
210 km2 whereas none of those to the north are below
230 km2, with some as large as 500 km2. The pattern
of elongation ratios corresponds closely to this pattern, with generally smaller ratios occurring in the
south (between 0.89 and 1.17) than in the north (0.94
to 1.58), confirming that the basins flowing into the
Indus are quantitatively smaller and narrower than
those that meet with the Shyok. Mean absolute
elevations in zone B vary from 4038 to 4810 m in
the south and from 4742 to 4968 m in the north. This
pattern is supported by the hypsometric integral which
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S.S.R. Jamieson et al. / Geomorphology xx (2003) xxx–xxx
Fig. 4. River long-profiles extracted from the 100 m DEM allowing comparison between the vertical components of the drainage systems. Left:
Rivers flowing south into the Indus. Right: Rivers flowing north into the Shyok. Solid line indicates profile is taken within zone A, dashed line
indicates zone B. Note: in this area, the Indus River lies between 2380 and 3000 m a.s.l. and the Shyok River lies between 2520 and 3260 m
a.s.l.
is generally lower in the south (0.36 – 0.66) than in the
north (0.58 –0.72). The hypsometry indicates that in
the north, the proportion of land at higher elevations is
greater than in the south. In the northern part of zone
B, 50 –70% of the land surface is constantly within
the top 40% of the elevation. The southern catchments, however, have a lower altitude distribution of
land, with 20 – 60% of the area being distributed
within the upper 40% of the altitude range.
A number of the extracted network parameters
(such as drainage density and first order stream
lengths) from the SPOT DEM suggest a north – south
difference, although bifurcation ratios did not show up
this pattern, despite their well-documented relationship with basin elongation (Summerfield, 1991). River profiles derived from the 100 m DEM in zone B
(Fig. 4) are difficult to compare given that only a
single profile could be extracted for the Shyok side.
However, the difference in lengths of the drainage
networks on either side is clearly illustrated, with the
northern stream being ca. 7 km longer than the longest
southern stream.
4.2.1. Field observations of zone B
In conjunction with the analysis of the DEMs, field
observations were carried out. These observations
were limited primarily to zone B due to access
restrictions in other parts of this region. The approach
taken in the field was to evaluate the nature of the
river channels; whether broad alluvial channels, or
incised bedrock-type channels. It was also important
to assess the influence of the glacial record.
Initial observations of the smaller catchments to
the south of the drainage divide were made in the
region of Leh (Fig. 2). At the intersection with the
main Indus Valley at about 3500 m, up to an elevation
of 4300 m, the valley floors are blanketed with
alluvium. In the lowermost parts of the valleys, both
alluvial and colluvial deposits coalesce between the
interfluve ridges to leave isolated granite hills protruding through extensive accumulations of sediment
at the margins of the Indus Valley (Fig. 5a). Above
these alluvial valleys, the stream networks sub-divide
and form steeper gradient, bedrock streams with short
intervals of waterfalls and rapids (Fig. 5b).
North of the drainage divide, the larger catchments
terminate abruptly northwards into the Shyok Valley
as steep scarp features. There is no evidence of broad
alluvial valleys opening up into the Shyok, instead,
steep-sided valleys, with narrow valley floors that
comprise a more limited extent of alluvial cover typify
these systems, e.g. the Hunda Valley. The interfluve
ridges in the north retain their high elevations northward to the Shyok Valley where they drop steeply
down into the main valley.
4.3. Zone C results
Representing the most southeasterly part of the
study area, zone C covers 25 basins (12 in the north,
13 in the south) that were studied here using solely the
GTOPO30 1 km DEM data. Visually, the shape and
size of the drainage catchments appear very similar
throughout this portion of the study area. Statistical
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13
elevation showing slight signs of difference across the
central Ladakh divide. The size of the basins vary
between ca. 30 and 355 km2 on the north side of the
Ladakh drainage divide, and on the south, a slightly
smaller range of between 60 and 240 km2 is evident.
However, as the statistics show, the majority of basins
are comparable in size. Zone C, as shown in Fig. 3, is
the part of Ladakh that has the most consistently high
mean elevation at approximately 4580– 5480 m in the
south, and 5100 –5600 m in the north, and the results
indicate that only a minor difference is discernable
between the two sets of elevation data. The elongation
ratios of the basins in this area are similarly variable
along-strike on both sides of the central divide (between ca. 1 and 1.35) and no particular pattern of the
distribution of basin shape can be recognised within
zone C. Hypsometric measurements are extremely
variable across this part of the study area, with basins
displaying 40 – 75% of their area within the top 40%
of the cumulative altitude. This lack of obvious
north –south difference is supported by hypsometric
integral values ranging from 0.4 to 0.7 in the south
and from 0.5 to 0.7 in the north.
5. Interpretation of results
Fig. 5. Photographs of portions of catchments in zone B. (a) Lower
portions of southwestwardly directed catchments showing extensive
sediment accumulation and draping of interfluve ridges. (b) Upper
reaches of southwest flowing stream beds illustrating steep valley
sides and heavily incised bedrock.
analysis of the morphometry upholds this assertion, as
indicated in Table 3, with only the relief ratio (basin
relief divided by maximum basin length) and mean
The three zones A, B and C show significant
variation in their degree of north to south morphometric symmetry. Zones A and C reveal no highly
significant differences in the morphometric nature of
the catchments that drain south-westwards into the
Indus versus those that drain north-eastwards into the
Shyok Valley. However, zone B shows significant
asymmetry in a number of parameters.
In zone A, there is no obvious asymmetry in the
catchments and a high proportion of high elevated
landsurface and convex and straight long profiles are
observed. Mountain rivers may display straight or
convex profiles due to their inability to incise rapidly
enough to keep pace with tectonic uplift, base-level
fall or climate change (Wohl, 2000). Similarly, glacial
erosion of the upper reaches of a catchment may also
reduce the concavity of a profile. Another possibility
is that the Indus and Shyok valleys that form the local
base-levels for the transverse catchments are developed along pre-existing geologic weaknesses. Therefore, the longitudinal rivers are likely to have been
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S.S.R. Jamieson et al. / Geomorphology xx (2003) xxx–xxx
able to downcut faster than the tributaries that are
incising into homogenous granodiorite. Again, this
would encourage the tendency towards straight or
convex longitudinal profiles.
As with zone A, zone C lacks any variability that
would distinguish between the catchments north and
south of the drainage divide. The area is characterised
by high mean elevations as would be expected of an
area whose local base-levels, defined by the longitudinal valleys, are upstream of the other zones. The
hypsometry again indicates a high proportion of
highly elevated land with a range of hypsometric
integrals similar to zone A.
The greatest morphometric contrasts are observed
in zone B; basins draining south – east into the Indus
Valley are smaller, shorter, proportionally thinner,
have lower mean elevations, and a higher percentage
of lower elevation terrain than those that drain northwest into the Shyok. The hypsometric integrals are
lower on the Indus side with values of 0.36 –0.66,
than the Shyok side with values of 0.58 – 0.72. This
asymmetry is supported by field observations of
extensive accumulations of alluvium and colluvium
on the lower reaches of the tributaries that discharge
into the Indus River. Equivalent deposits are not seen
on the Shyok side of the batholith, and it is interpreted
that this is the reason for the lower hypsometric
integrals on the Indus side.
The reason for the extensive sediment accumulations in the lower reaches of the tributaries of the
Indus side of the batholith is linked to the broad
alluvial valley floor of the Indus in zone B. The
longitudinal profile of the Indus River (Fig. 2) shows
a low gradient reach bounded upstream and downstream by a high gradient portion; this low gradient
reach equates to the broad alluvial valley floor of the
Indus River in zone B. Hence, it is interpreted that
aggradation of the valley floor in this region has
resulted in extensive accumulations of sediment in
the lower portions of the tributaries that flow into the
Indus from the batholith. Whilst the Shyok Valley is
also an alluvial channel in zone B, there is no
evidence of rising local fluvial base-levels in the
tributaries of this side of the batholith.
Whilst the presence of an aggrading longitudinal
valley can explain the present accumulation of sediment in the bases of the transverse catchments, and
hence the lower mean elevations and lower hypso-
metric integrals, it does not explain the foreshortened
character of the catchments. In order to achieve this,
the long-term sediment aggradation in the main Indus
Valley would have to have completely blanketed the
lower reaches of the transverse catchments. At the
same time, the Indus River channel would have to
have translated laterally and upwards over the lower
reaches of the transverse catchments (Fig. 6).
In order to provide a mechanism for lateral translation of the valley, it is important to consider the
erosional tectonic history of the two margins of the
valley. Younger apatite fission track cooling ages to
the southwest of the Indus Valley in the Indus Molasse
relative to those on the northeastern side of the Indus
Valley (i.e. the margin of the batholith) attest to higher
rates of long-term erosional denudation. The relatively
high erosional denudation has been caused by northeastward thrusting of the Indus Molasse over the
Indus Valley since at least 14 Ma (Sinclair and Jaffey,
2001; Clift et al., 2003). Sedimentological and structural data from the Indus Molasse suggest it has been
thrusting northeastward for the last 20 Ma, and that
this has involved at least 36 km of shortening of the
Indus Molasse basin (Searle et al., 1990; Sinclair and
Jaffey, 2001). As a result of the high erosional
denudation of the Indus Molasse, large alluvial fans
now extend out from the Indus Molasse thrust front
over the Indus Valley; this has forced the course of the
river to the northeastern side of the valley as is seen
around the town of Leh (Fig. 1). In contrast to the
southwestern side of the Indus Valley, the northeastern
side formed by the batholith has been tectonically
quiescent, with slower long-term erosional denudation.
Hence, the interpretation provided here for the
morphometric characteristics of the catchments of
zone B that drain into the Indus Valley is that they
have been progressively draped by sediment during
long-term aggradation and lateral translation of the
Indus Valley (Fig. 6). The controlling mechanism for
this aggradation of the main valley floor is long-term
(107 years) northeastward thrusting of the Indus
Molasse over the valley. This has generated large
alluvial fans that have provided excess sediment,
raising base levels, and pushing the river’s course
towards the opposite side of the valley. This provides
a mechanism whereby major longitudinal valleys in
mountain belts are able to translate laterally in re-
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15
and they become thinner. It is evident that the outlet
spacing of transverse catchments scales to the length
of the catchments (Hovius, 1996); this implies that
there is a mechanism by which the number of transverse catchments can evolve during changes in the
half-width of a massif, or a mountain belt. Although
we do not understand this process, it is a basis for
current research.
6. Conclusions
Fig. 6. Cartoon illustrating the interpreted influence of asymmetric
erosional denudation and structural deformation across longitudinal
valleys, and its impact on transverse feeder catchments. (A)
Symmetric massif with dendritic transverse catchments draining
into two longitudinal valleys that follow the regional geological
strike of the mountain belt. (B) Thrust deformation from the
southwest leads to increased erosional denudation of bedrock and
sediment yield from this region into the longitudinal valley. The
increased sediment yield from one side of the valley pushes the
main river course onto the opposing side of the valley, and leads to
sediment aggradation in the valley floor. The transverse catchments
of the opposing slopes are affected by a rise in base-level and
blanketing of their lower reaches in alluvium. This is interpreted as
the control on the asymmetry of the transverse catchments across
the Ladakh Batholith in zone B, with the Indus Molasse having been
thrust northeastwards onto the Indus Valley. This mechanism of
asymmetric exhumation across longitudinal valleys has the potential
to translate valleys laterally.
sponse to differential exhumation on either side of the
main valley. An apparent consequence of this interpretation is that during the foreshortening of the
transverse catchments that drain the Ladakh Batholith,
there is also an increase in the number of catchments,
(1) Analysis of digital elevation models from the
central portion (Zone B) of the Ladakh Batholith
reveal a high degree of variance in the morphometric character of the transverse catchments that
drain southwestwards into the Indus Valley versus
those that drain north-eastwards into the Shyok
Valley. The 16 catchments that drain into the
Indus Valley are significantly shorter, thinner,
have a lower mean elevation, and a lower
proportion of their area at high elevations than
the six equivalent catchments that drain into the
Shyok Valley.
(2) Field observations indicate that the lower mean
elevations and lower hypsometric integrals are
explained by the presence of thick accumulations
of alluvium and colluvium in the lower reaches of
the catchments that drain into the Indus Valley.
These deposits are not found to any great extent
in the opposing catchments that drain into the
Shyok Valley.
(3) It is proposed that the character of the smaller
catchments on the Indus side of the Ladakh
Batholith have been influenced by increased
sedimentation associated with northward thrust
propagation and erosional unroofing of the Indus
Molasse. High sediment discharge into the Indus
Valley from the deformed Indus Molasse has
elevated base-levels and pushed the course of the
Indus River northeastward. This has resulted in
rising base-levels in the lower parts of the
transverse catchments that drain the Ladakh
Batholith, and the accumulation of extensive
alluvium and colluvium. It is interpreted that
long-term (107 years) asymmetric erosional
denudation and thrusting across the Indus Valley
has resulted in the lateral translation of the valley
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S.S.R. Jamieson et al. / Geomorphology xx (2003) xxx–xxx
northeastward into the Ladakh Batholith, and the
foreshortening of opposing transverse catchments
by sediment blanketing.
Acknowledgements
The authors would like to thank Dr. Bill Phillips
for his helpful comments throughout the course of this
project, and on an early draft of this paper. Additionally, SSRJ thanks Chris Place and Jez Everest for their
work with the SPOT data, and Keith Morrison for his
technical assistance. LAK gratefully acknowledges
support from a European Union Marie Curie Fellowship HPMF-CT-2000-00515. HDS is grateful to
financial support from the Royal Society. Two
anonymous reviewers are thanked for their insightful
comments.
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