ABSTRACTS / RESUMES - Comitato Glaciologico Italiano

Analytical results reveal that long-term cooling rates for
the region could only be confidently resolved for the Mesozoic
era. Due to the limitations of fission-track predictive
models for critical temperatures lower than >- 60°C, Cenozoic
trends were only inferred by extrapolation. Assuming
a common geothermal gradient for the region, cooling rates
suggest a slow, steady style of regional denudation with,
however, different values according to sample elevation
and lithology: on charnockites, which form South Asias relict
highland surfaces, rates are one half to two thirds of
the rates measured on upland tonalite-trondhjemite gneiss,
and one third of the rates obtained for the Palghat Gap,
locus of a major breach in the Western Ghats escarpment.
This preliminary evidence confirms the idea, long-held by
geomorphologists but insufficiently addressed in recent
numerical models of plateau uplift, that lithology and geological
structure exert an important control over denudation
patterns. The findings also challenge the prevailing
idea that the high-level charnockite massifs are horsts and
that the South Indian and Sri Lankan palaeosurface staircases
are related to Cenozoic fault reactivation.
The elevational consistency of planation levels throughout
the entire region, highlighted by flat-lying topography indifferent
to stratification, schistosity or foliation and often
by characteristic palaeosol cappings, excludes interpreting
the relative rates of surface lowering between different
rock units as evidence for a purely «etchplanational» view
of landscape evolution. Discrepancies between rates may
have increased in the Cenozoic due to the growing isolation
of charnockite palaoesurfaces from active drainage sytems.
The timing and magnitude of tectonic events documented
for Indias eastern and western passive margins vindicate
the periodicity of peaks of continental erosion as a
major factor of landscape development. A comprehensive
and composite polycyclical denudation chronology, driven
by regional uplift, is therefore proposed for southern India
and Sri Lanka. The quasi-insular status of South India, as
indeed Sri Lanka or Madagascar, may explain that the
landscape exhibits a clearer expression of cyclical response
to endogenic activity than other, larger continents with vast
and remote hinterlands. This scale factor implies that the
boundary conditions usually imposed on geomorphological
modelling exercises for Atlantic, Antarctic or Australian
passive margins cannot be straightforwardly adopted
for southern India, where two competing base levels (Bay
of Bengal since 100 Ma, Arabian Sea since 65 Ma), and
two Mesozoic palaeosurfaces on charnockite have imbricated
Cenozoic drainage and landscape evolution beyond the
simplifying rift-shoulder uplift rationale of existing models.
Structural heterogeneity in the Indian shield has enhanced,
rather than obscured, the staircase morphology of episodic
rejuvenation. In addition to its cyclical components, landscape
change over time was also directional: while the periodicity
between base level events became increasingly
shorter than the relaxation time necessary to attain cycle
completion, lithological contrast played an increasingly selective
role in denudation patterns through geological time,
possibly due to Cenozoic climatic regimes becoming
less equable and promoting opportunities for poorly
194
weathered debris to be transported and for bedrock channel
scour and incision to increase. This proviso appears ne- ,
cessary to account for the lithology-dependent denudation
rates. Ridge-and-valley relief in the Cuddapah Proterozoic
basin also points to the growing control of rocks on erosion
in South India.
Against the background of «slow variables» borne out by
the cooling patterns, the «faster» variables of sea level
fluctuation, climate change, domal uplift and medium wavelength
lithospheric buckling, all documented with
varying degrees of accuracy, are expected to have had an
effect on headward erosion rates and surface lowering patterns
unresolved by fission-tracks. In the crystalline shield
of South India, the structural grain of the basement is generally
transverse to the major river courses; matters are
thus complicated by the likelihood that a single continental
base level change will give rise to a flight of partial planation
levels located at different elevations and controlled
by local, structural base levels such as greenstone ridges.
These prevail in the greenschist facies region of the
Dharwar Craton but are absent in the granulite terrain of
Tamil Nadu. Contemporary predictive modelling exercises
in geomorphology need to address such issues of structural
predesign.
A VUlT GUPTA 1 & RAFl AHMAD 2
Geomorphology and the urban tropics
1Department of Geography, National University of Singapore,
Singapore 119260
2 Department of Geology, University of the West Indies, Mona,
Kingston 7, Jamaica
The developing countries, located almost entirely within
the tropics, are currently undergoing urbanisation at a rapid
pace. Their urban population, measured as the percentage
of the total population, rose from 16.9 in 1950 to
approximately 34 in 1990; and is projected to climb to 57
by 2025. Although this growth occurs across a wide range
of urban settlement sizes, it is the large cities with several
million inhabitants which are growing at a particularly rapid
rate.
In general, many of these cities are not older than a few
centuries, being established to function as regional trading
posts or administrative centres by either the colonial or the
regional powers. It is doubtful that the site conditions were
taken into consideration, and many of these cities were
established in hazardous or difficult areas. As these cities
developed over time, they spread across a wide range of
terrain conditions much of which are unsuitable, being
floodplains, coastal swamps, steep slopes or sand dunes.
The original location of levees (Calcutta, Bangkok) or a
Pleistocene terrace above the floodplain of an active river
(Allahabad) are no longer spacious enough. For a number