ABSTRACTS / RESUMES - Comitato Glaciologico Italiano

higher along the softer terrains, oriented NNW-SSE and
N -S, leading to the activation of many river captures. Almost
at the same time, together with increased uplift, extensional
tectonics reached the western side of the chain as
well as the East Tiber valley creating huge grabens of which
Gubbio, Gualdotadino, Spoleto-Foligno, Colfiorito,
Castelluccio di Norcia, Norcia, Cascia etc. constitute some
of the best examples. The activation of NE-SW faults can
also be associated to this phase.
The activation of the tectonic depressions, frequently
bounded by NNW-SSE and NE-SW faults, locally reoriented
the previous drainage systems. In the Norcia basin,
a NE-SW fault diverted N-S rivers to the west. The
Paglia, which previously crossed the Martani Mrs, was captured
by the Teverone. The upper Gubbio basin, also
oriented NS, was captured to the west by the Saonda. In
the Colfiorito area, many eastward rivers were blocked by
extentional faults and lacustrine deposition was established
at 1,1 my. In this area the Chienti River continued to drain
to the east but many parts of the basin were captured to
the west. In the Periadriatic basin, only a few superimposed
rivers continued to cross the limestone ridges, such as
the Cesano (crossing the Monti delle Cesana), the Musone
(crossing the Cingoli Ridge) and the Salinello (crossing the
Montagna dei Fiori). Many other captures are documented
and represent, to the east as well as to the west of the
chain, one of the most important mechanism in the evolution
of the drainage network in the area.
MAURO COLTORTI 1
& CLIFF OLLIER 2
Geomorphic and neotectonic evolution
of the Ecuadorian Andes
1 Dipartimento di Scienze della Terra, via delle Cerchia 3, Siena, Italy
2 Centre for Resource and Environmental Studies,
Australian National University, Canberra ACT 0200, Australia
All writers on the Ecuadorian Andes recognize an East
Cordillera, a West Cordillera and an Interandean Depression.
They usually fail to write that the East and West Cordilleras
are plateau, i.e. planation surfaces, and that there is
an accumulation plateau in the Interandean Depression.
We recognize a planation surface all along the Andean
Cordillera. There was originally a single surface, and it was
planated to base level, the Pacific sea level. It cuts folded
and faulted rocks from Precambrian to Late Miocene­
Early Pliocene age. In places it is covered by Early Pliocene
volcanic flows, ignimbrites and pyroclastites that form a
volcanic plateau. The volcanics possibly represent the first
extension associated with the beginning of uplift. The planation
surface was modelled during the Early Pliocene age
followed immediately by uplifting.
The tectonic depression that today separate the E from the
W Cordillera did not exist at that time or it would have
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been filled with volcanic products. Uplift of many hundreds
meters followed. Strato volcanoes began to erupt
about 1,8-1,5 my ago and many are still active today. The
accumulation plateau consists of conglomerates, sands, agglomerates,
lava-flows and lahar deposits. In places it is
deeply weathered by tropical soils and it is usually dissected
by river erosion up to 800 m, in others it is buried under
younger alluvial and volcanic deposits.
Folds and faults in the accumulation plateau result from
gravity tectonics. The sediments affected by huge landslides
up to 700 m thick.
The planation surface, preserved in the Cordillera at elevation
of 3,000-4,000 m, indicates an uplifting rate of about
1000 B (metres/million years). Thrust faults toward the
Costa in the West and toward the Amazonia in the east
suggest spreading of the uplifted Andean Plateau and
spreading may have created the extensional Inter-Andean
depression.
The model of vertical uplifting of a planation surface, proposed
here for the Ecuadorian Andes, may be extended to
the rest of the Andean Chain.
LYLIAN COLTRINARI
Karstic-type forms and landscape evolution
in Taubate Basin (Sao Paulo, Brazil)
Departamento de Geografia, Universidade de Sao Paulo,
C. P. 8105, 05508-900 Sao Paulo, Brazil
Morphological mapping of Sao Jose dos Campos plateau
includes in its SW extremity land forms with wide level interfluves
(700-750 m) that become lower and more dissected
(600-650 m) towards Cacapava at the NE end. Depressions
and amphitheatre-like valley-heads on hilltops and
interfluves, and flat valley bottoms with or without underfit
streams are characteristic features in this section of Taubate
basin. Gradient is low (0 - 3°) and deep oxisols cover
the area.
Interpretation of aerial photographs surveyed probably at
the beginning of the '40 before the new Rio de Janeiro ­
Sao Paulo Highway opening allowed the detection of the
quasi-original surface features in the area and comparison
with present-day forms. The choice of a morphological legend
was deliberate to avoid or at least to diminish the genetic
bias in current geomorphological maps.
Differences in shape, size and distribution of depressions
appear between the higher and the lower sectors under research.
Between J acarei and the Porangaba river depressions
appear isolated or at the valley heads and drainage
density is low. In the NE section sluggish streams move
through extensive swampy areas, depressions are bigger
and more numerous and may be isolated or arranged in
different patterns. They may be flooded or present
swampy bottoms permanently or seasonally.