Abstract
This geological study on the Vicentinian
Alps is mainly an analysis of the
tectonic and paleomagnetic data, collected
by the author in the years 1959,
1960, and 1961.
The stratigraphy is based for the
greater part on data published in the first
decenniums of this century. In the westtern
part of the region the exposed basement
consists of quartz
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-phyllites. The
age of the strata overlying the basement
ranges from Permian, to Miocene. During
the Permian and the Mesozoic,
sedimentation was mainly the result
of a slow subsidence and a supply of
clastic material from a land in the south
(northern Adria and Po-block). Facieschanges
are due to WNW-ESE trending
uplifts, accompanied by magmatic (volcanic)
activity (the Schio-Recoaro horst).
In the late Cretaceous the southern land
area subsided and the Alpine geanticline
began to rise. The sedimentation in the
Tertiary was mainly controlled by the
presence of this source area in the north.
In this period changes in facies took
place also,which were mainly controlled
by NNW-SSE trending uplifts (e. g. EuganeiBerici-
Lessini horst).
Volcanic activity is known to have
taken place in the Vicentinian Alps in
various periods. Next to basaltic dikes
of either devonian or early permian
age, two major volcanic periods can be
distinguished; the Permo-Triassic and
the Tertiary. During the Permo-Triassic
mostly acid magmas intruded and effusive
rocks were formed in either marine or
terrestrial environment. During the Tertiary
mostly basic magmas ascended,
forming thick tuff- and lava deposits.
It is noteworthy that the centres of maximal
volcanic activity moved from southwest
to northeast in the early Tertiary
The tectonics are rather complex
and may be ass igned to three tectogenetic
periods. The first comprises the
Hercynic orogenesis, in which two phases
can be distinguished, one of subvertical
movements (primary tectonics) and
another of subhorizontal movements
(secondary tectonics). During the first
the region was elevated and heavily
eroded; the second caused WNW-dipping
overthrust and NNE-SSW striking folds
(originally, Le. prior to the permotriassic
rotation as determined by paleomagnetic
data, the overthrust planes
were dipping WSW and the fold axes
were E-W. The second tectogenetic period
is of permo-triassic age. At this
time, a horst-like doming-up was caused
in het Valli-Becoaro area by ascending
basic magmas in an ENE-WSW striking
zone of weakness. Originally, this zone
had a WNW-ESE direction (see paleo-mag-.
netism). Normal faults were formed,striking
ENE-WSW (dipping north or south)
and NNW-SSE (dipping east or west).
The thi I'd tectogenetic period coincided
with the alpine orogenesis. The Vicentinian
region was located on the intersection
of two hinge zones. One is ENEWSW
and lies between the rising Alpine
geanticline and the subsiding Adria mass;
the other is NW-SE and forms the northern
flank of the subsiding Po area. In both
hinge belts the basement and the sedimentary
cover reacted differently to
differential vertical movements. In the
basement, the older, permo - triassic
zones of weakness were reactivated;ENEWSW
striking step faults, dipping south,
were formed as reaction to the subsidence
of the Adria mass and NNW-SSE
faults dipping east originated on the
NE-side of the Po geosyncline. This
second set of faults is closely related
to the doming-up along the northeastern
margin of the subsiding Po region, which
can be interpreted as a correction of the
isostatic equilibrium. The updoming was
accompanied by volcank acti vity. The
centra of volcanic acti viiy shifted northeastward
(each time over about 30 km)
between upper Cretaceous and Oligocene.
The sedimentary cover slided off the
south flank of the alpine geanticline to
the south ("decollement "); this caused
approximately E-W (Tauern-decollement)
and NNE -SSW trending fold axes (Oetzdecollement).
Only the former have been
of importance in the Vicentinian Alps.
These southward movements of the sedimentary
cover were locally obstructed
by the permo-triassic horst ("bufferzone"),
causing the fold axes there to
be striking NE-SW.
In the younger Tertiary, NNE -SSW sinistral
wrench faults caused anti -clockwise
rotations of minor blocks.
From paleomagnetic data of permotriassic
and tertiary rocks may be concluded
that NE-Italy has been subjected
to majol' translations and rotations since
the Permian. Three relatively fixed points
are known. Firstly, the present position
of the region (450 45' N. L., 110 30'
E. L. ). Secondly, the position in the
Eocene (NE-Italy must have been located
on the +500 isocline of the Eocene, which
runs across the present position of
southern Italy). Thirdly, the position
during the Permian (NE-Italy was probably
on the intersection of the permian
-300 isocline and the Tethys zone; this
point is at the present location of the western
Himalayas). Two major movements
may be distinguished: awestdrift of the
region during the Permian and Mesozoic
(over maxima14800km) and a northdrift during
the Tertiary (over 800 km). During
the westdrift, the Vicentinian region
rotated about 600 anti-clockwise in the
late Triass ic. This rotation corresponds
with right lateral shear movements along
the north side of the Tethys zone.
The northward movement of the region
during the Tertiary may have been caused
by a NW drift of Africa.
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