ABSTRACTS / RESUMES - Comitato Glaciologico Italiano

planation affecting calcareous successions (which, conversely,
in the past where much less exposed).
Relief generation and/or increase by downcutting prevailed,
in the inner belt, starting from the Late Pliocene and
during the whole Pleistocene. The increase in rivers downcutting
rates (i.e, increase in river gradients) was due to: (i)
shortening of river valleys due to migration of the Tyrrhenian
rifting towards the NE which had drawn the coastline
close to the present position by the end of Pliocene time;
(ii) increase in regional elevation due to uplift affecting wide
sectors of the belt; (iii) increase in local relief due to
vertical displacements occurring in perityrrhenian and intramontane
structural depressions.
In the outer belt, prevailing relief reduction occurred from
Late Pliocene to the beginning of Early Pleistocene; it was
due to both sedimentation occurring in wide perched basins
and erosion affecting the intervening smooth highs. A
major phase of relief generation and/or increase by downcutting
started during the Early Pleistocene, even though
its debut was diachronous being younger towards the foredeep
that emerged and was uplifted only at the Early­
Middle Pleistocene boundary.
ASFAWOSSEN ASRAT 1 , OGBAIGHEBRIEL BERAKHI 1
,
LUDOVICO BRANCACCIO 2, FRANCESCO DRAMIS 3
& MUHAMMAD UMER 1
Gravitational slope phenomena along the eastern
escarpment of Welo (Ethiopia)
1Department of Geography, University of Addis Ababa,
p.o. box 1176, Addis Ababa, Ethiopia
2 Dipartimento di Scienze della Terra, Universita di Napoli Federico II,
largo S. Marcellino 10, 80134 Napoli, Italy
3 Dipartimento di Scienze Geologiche, Universita di Roma Tre,
via Ostiense 169, 00154 Roma, Italy
The eastern escarpment of Welo is characterized by a
number of north-northwest - south-southeast and north ­
south trending graben-like depressions, connected to normal
faults which have affected Late Tertiary basalts («Termaber
basalts» of Miocene-Pliocene age) and interbedded
volcanic tuffs. The depression of Dessie is filled by a thick
(up to some dozens meters) sequence of alluvial, colluvial
and lacustrine deposits, mostly made of fine gravels, coarse
(sometimes pyroclastic) yellow sands and white diatomites,
also including two buried vertisols ic.a 1 m thick). The upper
part of the sequence is made of blackish soil sediments
and debris, which indicate a recent period of widespread
slope denudation.
Due to regressive erosion, the Dessie depression was recently
captured by the Borkena river, which cut a 300 m
deep gorge (locally named «Doro Mezleya», literally
«Chicken Jump») into the outer threshold. As a consequence
of the valley cutting, deep seated gravitational slope
deformations and large-scale landslides, including lateral
spreadings, rotational slides, flows and rock falls (these
latter affecting the inner fault scarp) were triggered. Particularly
important in this context is a system of regressive
rotational slides, migrating upslope towards Dessie. These
landslides, which already affect the eastern outskirts of the
town, could produce catastrophic consequences in the
next future.
FRANCK A. AUDEMARD
Morpho-structural expression of active thrust systems
in humid tropical foothills of Colombia and Venezuela
Funvisis, Apartado postal 76.880, Caracas l070-A, Venezuela
Morpho-structural expression of thrust faults in fold-andthrust
belts is largely dependent on several factors, that
may act independently or interact jointly, such as: dip of
thrust fault planes (gentle or steep dip), kinematics of thrust
faults (pure dip slip or oblique slip), lateral continuity of
structures and depth of fault plane (outcropping or blind
thrust). This paper will present geomorphological and/or
geological evidences of either blind or outcropping, gentlydipping,
tectonically-active, pure dip-slip thrust faults of
fold-and-thrust belts associated to the Llanos (eastern)
foothills of the Andes Cordillera of Venezuela and the Andean
Eastern Cordillera of Colombia.
In any' case, thebest geomorphic evidences of active thrust
faults in these foothills areas, where much thin-skinned
tectonics, is involved, are: A) flexural scarps that correspond
either to the warping of the sedimentary sequence
at the shallowest tip of blind thrusts (fault-propagation
folds) or to the steeper forelimbs of foreland-vergent faultbend
folds; B) drainage patterns and anomalies which reflect
very subtle topography modifications. Among these
anomalous drainage behaviours we might mention: (1) radial
drainage, indicative of periclinal clousure of folds, (2)
densely dissected (morphological) scarps, (3) tilt of Quaternary
alluvial units on the anticline backlimb, reflected
by river pattern inversion (flow from the basin towards the
range), (4) diversion (diffluence) of either large rivers or
small river channels, suggesting either dyachronic and/or
differential fold growth or faster tectonic uplift rates than
linear erosion rates (erosion by river flow), (5) beheaded
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