GEOMORPHOLOGY OF THE HIMALAYA: A CLIMATO-

TECTONIC FRAMEWORK

Landscapes are shaped by tectonic movements and sculptured by

denudation processes

In mountain areas, regional variation in geomorphology,

topography and erosional processes reflect the spatial variation in

geology and the rate of uplift, and denudation

In part, geology may mirror the variation in uplift and denudation

Variation in uplift is controlled by the plate tectonic setting and

denudation is usually affected by precipitation-driven

processes, is controlled by global atmospheric circulation pattern

with the mountains

The regional geomorphology of the Himalaya reflects the

interaction of mountain building and precipitation variation

The N-S and E-W variation of Himalayan geomorphology is a

function of the climate, tectonics, geology, topography, erosional

processes, sediment yield etc.

Topography: Parallel to

strike, four physiographic

division

1. Low/Fore/Outer or

Sub-Himalaya. Average

height: 900-1500m and

10-13 km wide

2. Middle: North of the Outer Himalaya, average height: 700-2500

m, average width: 60-130 km. In central Nepal, it can be divided

into highly dissected Mahabharat Range (north of MBF)

3. The Higher/Greater: Average height: 50-100 km, average

elevation: 6000m and numerous peaks rising to 8000 m. This unit

asymmetrically rises sharply from the south and gradually merges

with the Tebetian plateau to the north.

4. To the north of the highest mountain. Within this unit, the

Tibetan marginal range forms a 6000-7000m high southern

rampart to the Tibetan plateau to the north

PLATE TECTONIC SETTING

The Himalaya is the product of the Indian-Eurasia collision that

started t 50My ago.

But the present mountain building is much younger as the

Himalaya did not significantly block moisture from the south until

Mid Pleistocene

The Himalaya shows longitudinal and transverse seismic

inhomogenity which is well reflected from the southward- different

rate of thrust sheet movement and distribution of earthquake

epicentres (Bilham, 2004)

The longitudinal and transverse dissection of the Himalaya shows

distinct geomorphological characteristics

CENTRAL SEISMIC GAP

The central seismic gap (Bilham et al., 2001, Seeber and

Armbruster, 1981) lies in between the 1905 Kangra Eq. in the

west and the 1934 Bihar-Nepal Eq. in the east within which

earthquake >8M has not been reported within last 300 years

Three prominent seismic

gaps have been identified in

the Himalayan terrain based

on the historical great

earthquake (M>8) record

The conditional probability of occurring great Eqs in western

Himalaya in time window 100 years is 27% in Kashmir gap, 6% in

Kangra region, 52% in Central seismic gap and 3% in Bihar-

Nepal region (Khatri, 1999)

PRECIPITATION

The interaction of the southwestern monsoon with the Himalaya is

the major influence on the climate of the areaThin black arrows indicate present-day, weak monsoonal wind directions. Bold orange arrows show prevailing wind directions of strong monsoon inferred to represent intensified monsoon phases (IMP) during late Pleistocene and Holocene. Dashed lines depict temporal evolution of Indian summer monsoon and its northwestward propagation

From June to September, the Himalaya force the prevailing winds

to rise, and most precipitation occurs on the southern flank of the

range, in the Low and Middle Himalaya.

In general the Siwalik range does not obstruct the monsoon and

maximum precipitation occurs on the southern flanks of Pir Panjal,

Middle Himalaya and Mahabharat ranges

Rainfall decreases northward of the Middle Himalaya

But again it increases on coming within about 40 km of the crest

of the Greater Himalaya, with annual rainfall of 2500-3500mm .

About 20-30 km south of the crest of the Himalaya rainfall drops

sharply to < 1000mm-~600 mm.

On the northern slope of the Great Himalaya, rain shadow is

pronounced

Over most of the Himalaya (including the rain shadow area) ~90%

rainfall occurs during the summer monsoon.

At high elevations in the Greater

Himalaya, most snow falls

during the monsoon or late

winter.

The uplift rates

across the different

segments of the

Himalaya is poorlydefined

UPLIFT RATES

The Himalaya shows along

and across variation of uplift

rated due to the segmented

nature

A proportional measure

of uplift rates across the

Himalaya can be inferred

from the river terraces

The profile of the highest

terrace suggests that

uplift rates are highest in

the low and high

Himalayas.

EROSION PROCESS

Mass wasting is the dominant process in the Himalayas: Due to

tectonism, freezing and thawing, gravity, pore fluid pressure

Geomorphic mapping in the Middle

Himalayas indicate over 75% of landscape

is undergoing moderate to rapid or

extremely rapid mass wasting

Shallow landslides are by far the most

numerous and most failure occurs in the

monsoon

Geological factors such as faulting, shearing, weathering and

weak lithology appear the most important determinants of slopestability

Ramsay (1978) studied the erosion processes and mass wasting

in parts of the Middle Himalaya (Phewa valley in 122 km2) and

found that there is average 2-3 mm surface lowering annually in

the area

Another recent study by Caine and Mool (1982) suggest

~12mm/yr lowering of the area though landslide presently

occupying only 1% of the area.

Many Himalayan landscapes

appear to undergo cyclic

behavior with periods of

relative stability separated by

spells of instability during

which large number of

landslides occur.

Most mass flow materials are transported out of the Himalaya by

streams and form alluvial fans at the foot hill

Observation by Starkel (1972) of landslide following a heavy rain

in the Darjeeling area suggest that such catastrophic events

should be considered the most significant for slope evolution in

parts of the Himalaya.

Such single events may

result in annual erosion of ten

times more than average

Fluvial erosion rates in the Himalaya is poorly documented

although Ciene and Mool (1981) indicate a maximum probable

rate of bank erosion of few rivers and tributaries are 12mm/yr.

With increasing altitude, glacial and periglacial processes increase

in importance. Glacial processes are common on the highest

ranges.

SEDIMENT YIELDS

Different authors estimated sediment yield for same drainage

basin, but all the data are inaccurate, imprecise and misses the

peak yield

Though the data quality is poor, still it suggests some regional

variation as it appears that, the sediment yield is the highest in the

Low and Middle Himalaya in contrast with the high Himalaya or

Tibetian plateau

Sediment yield appear greatest

from areas with highest rainfall

Most sediments likely to be

produced from only a small part

of the basin (The zones where

mass movement debouches

into the fluvial system).

CONCLUSION

Regionally, the Geomorphology of the Himalaya reflects variations

in uplift and erosion rates, and their differences

Uplift is rapid and presently appears to be greatest in the Low

and Higher Himalaya in the vicinity of MBT and MCT.

This uplift has forced a topographic barrier into the monsoon

circulation, so controlling the precipitation pattern, and in turn , as

precipitation is a primarily the driving force for erosion, controlling

the variation of the denudation rates.

Precipitation is high in the Lesser Himalaya and so as the erosion

rate. But the rate of uplift compensate the denudation

Glacial landforms occur at higher elevations but at lower

elevations V-shaped valleys occur with alluvial, fluvio-glacial,

glacial and landslide deposits on the valley floors

Depositional landforms are more common than in the Lesser

Himalaya.

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