ABSTRACTS / RESUMES - Comitato Glaciologico Italiano

logical province. Gypsum bedrock outcrops total at least
80,000 km', although most are mantled by glacial deposits.
Interstratal salt deposits are abundant in the interior lowlands
at depths greater than 100 m and there are outcrops
in the high arctic islands. Types of karst that occur are determined
chiefly by relations between (i) formation thickness
and purity, (ii) regional topography and hydraulic gradient,
(iii) effects of receding Wisconsinan and earlier glaciers,
and (iv) extent of modern permafrost.
Exposures of bare karst on thick, pure sulfate formations
are comparatively rare. Two principal landform types
found on them are (1) high density polygonal karst (microdoline
densities up to 40,000 km") where hydraulic gradients
are high and tills are thin; morphometric analysis
has determined that doline distribution approaches uniformity,
with Nearest Neighbour values generally greater than
1.40; (2) hills and ridges of blocks uplifted and fractured
by hydration (anhydrite) tectonics at paleo-icefront positions
where hydraulic gradients are low. Bare salt karst is
largely limited to seasonally active (thaw) zone features in
the extreme northern permafrost. Covered karst is abundant
on sulfates conformably overlain by carbonate strata;
collapse dolines are the principal landform..Very large
breccia pipes (up to 25 x 15 km) are associated with deep
subrosion of salt during glacier recessions in Saskatchewan
and Alberta. Polycyclic breccia karst is a third, distinct category
created in some formations of thin, interbedded dolostones
and sulphates in the Northwest Territories. Where
these are exposed to high groundwater hydraulic gradients,
deep calcite-cemented breccias form by incongruent
dissolution and common ion effects in a first cycle,
upon which doline and pinnacle karsts and remarkable
dissolution draped topographies develop rapidly in subsequent
cycles.
MONIQUE FORT
Large scale geomorphic events in the Nepal Himalaya
and their role in the evolution of the landscape
Laboratoire de Geographie Physique,
Universite Denis-Diderot, Case 7001,
2 Place Jussieu, 75251 Paris Cedex 05, France
The Himalaya, a still growing mountain created by the
collision between Indian and Asian plates, is eroding
down at a very fast rate, as attested by the volume of sediments
annually trapped in the large sedimentary reservoirs
of the Indus and Ganga-Brahmaputra fluvial, deltaic and
submarine systems. In fact, the overall characteristics of
the mountain are such that all favorable factors for intense
denudation, acting at all scales and frequencies, are repre-
sented: bedrock uplift, tecto-seismic activity, steep and
long slopes, monsoon climate and/or glacially dominated
slopes.
This contribution is aimed at documenting and assessing
the significance of large scale, low-frequency events recorded
in the Nepal Himalaya, which is very representative of
the entire Himalayan Range. These events are related to three
types of major processes: tectonic/unloading collapse,
slope weathering and landsliding, and river downcutting
and flooding. Large magnitude events typically associate at
least two, if not the three types of processes, as illustrated
by selected study cases.
1. Gigantic landslides, mostly found in the Greater Himalaya
(cf in Mustang, Manang, Langtang, Khumbu), typically
affect entire valley slopes. Their morphology and associated
displaced materials (109-lOm3) are generally complex,
and currently induce specific low-magnitude, high
frequency mass wasting processes which would otherwise
be probably absent. Their frequency is very low (> 10 4 - 5
years recurrence interval) and they durably impact the overall
morphology of the valleys.
2. Catastrophic debris-flow equally characterize the upper
and middle himalayan valleys, as attested by very thick,
very coarse, mud-supported deposits (>10 8 - 9 m 3 sediments
accumulated for one event) (cf Pokhara and Marsyangdi
valleys). Their development is related to an exceptional
combination of favorable conditions (steep topographic
gradient, glaciated areas, deep fluvial incision, adequate
lithologies, high seismicity). The recurrence interval (10 2 - 4
years) is not well constrained, because of later reworking
of the catastrophically carried material 'by smaller, more
frequent events. .. . '"i; ';:
3. Exceptionnal floods affect both the Greater and Lower
Himalaya. Whereas in the upper valleys these are mostly
glacial lake outburst floods {Khumbu, 'Mustang); in the
lower valleys, they are basically climaticallyinduced floods
(rainfalls of exceptionnal intensities and/or duration). The
volume of sediments transported is considerable (10 4 - 5
m 3/km2/flood; Bagmati 1993 event). Their large extent may
affect on a large scale the himalayan foothills, the most threatened
and densely populated areas.
The origin of these events is undoubtedly natural, compared
to small-to-medium scale events, induced both by natural
and/or human factors. It is shown that these high magnitude
events playa major role in the shapingand denudation
history of the entire range. The more high .magnitude-low
frequency, the longer rate of landscape recovery, In
turn, the time of recovery may be indirectly used for esti .;
mating the magnitude of a given event.
High magnitude events not only contribute to a large transfer
of materials across and out of the mountain; their geomorphic
and sedimentary remains also indirectly control
the present zoning of geomorphic instabilities and. the nature
and intensity of current processes, thus eventually the
distribution of potential hazards. Further studies are
however necessary to better predict sediments yields and
geomorphic remodelling of the landscape by these extreme
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