98 research outputs found
U-Pb ages of magmatic rocks of the western Austroalpine Dent-Blanche-Sesia unit
Conventional U-Pb ages on zircon and monazite demonstrate that granites
and gabbros intruded during a short time span of 5 Ma between 293 and
288 Ma in several polycyclic basement units of the Western Austroalpine
domain. This bimodal activity reflects increasing underplating of an
upwelling mantle at the base of a thinning post-Variscan continental
crust
The age of movements along the insubric line West of Locarno (Northern Italy and southern Switzerland)
Permo-Mesozoic Canavese sediments are pinched in between the pre-Alpine
high-grade metamorphic Ivrea Zone and the Alpine metamorphosed Sesia
Zone along the Insubric Line W of Locarno. According to the ``illite
crystallinity'' these sediments were deformed under anchi- and
epizonal conditions. Synkinematically formed white mica in the
mylonitized Canavese sediments yields the following K-Ar age ranges:
60-76 Ma at the southwestern end, 28-43 Ma in the central part and 19-26
Ma in the northeastern part of the Insubric Line W of Locarno. The
youngest age group dates the main uplift and dextral strike-slip
movements of the Insubric Line, comprising mylonites in the NE and
cataclasites in the SW. This activity correlates with Late Oligocene to
Early Miocene rapid cooling and uplift of the Central Alps
The Tertiary structural and thermal evolution of the central Alps - Compressional and extensional structures in an orogenic belt
The Western Alpine Are has been created during the Cretaceous and the
Tertiary orogenies. The interference patterns of the Tertiary structures
suggest their formation during continental collision of the European and
the Adriatic Plates, with an accompanying anticlockwise rotation of the
Adriatic indenter. Extensional structures are mainly related to ductile
deformation by simple shear. These structures developed at a deep
tectonic level, in granitic crustal rocks, at depths in excess of 10 km.
In the early Palaeogene period of the Tertiary Orogeny, the main
Tertiary nappe emplacement resulted from a NW-thrusting of the
Austroalpine, Penninic and Helvetic nappes. Heating of the deep zone of
the Upper Cretaceous and Tertiary nappe stack by geothermal heat flow is
responsible for the Tertiary regional metamorphism, reaching
amphibolite-facies conditions in the Lepontine Gneiss Dome (geothermal
gradient 25 degrees C/ km). The Tertiary thrusting occurred mainly
during prograde metamorphic conditions with creation of a penetrative
NW-SE-oriented stretching lineation, X(1) (finite extension), parallel
to the direction of simple shear. Earliest cooling after the culmination
of the Tertiary metamorphism, some 38 Ma ago, is recorded by the cooling
curves of the Monte Rosa and Mischabel nappes to the west and the
Suretta Nappe to the east of the Lepontine Gneiss Dome.
The onset of dextral transpression, with a strong extension parallel to
the mountain belt, and the oldest S-vergent `'backfolding'' took place
some 35 to 30 Ma ago during retrograde amphibolite-facies conditions and
before the intrusion of the Oligocene dikes north of the Periadriatic
Line. The main updoming of the Lepontine Gneiss Dome started some 32-30
Ma ago with the intrusion of the Bergell tonalites and granodiorites,
concomitant with S-vergent backfolding and backthrusting and dextral
strike-slip movements along the Tonale and Canavese Lines (Argand's
Insubric phase). Subsequently, the center of main updoming migrated
slowly to the west, reaching the Simplon region some 20 Ma ago. This was
contemporaneous with the westward migration of the Adriatic indenter.
Between 20 Ma and the present, the Western Aar Massif-Toce culmination
was the center of strong uplift.
The youngest S-vergent backfolds, the Glishorn anticline and the Berisal
syncline fold the 12 Ma Rb/Sr biotite isochron and are cut by the 11 Ma
old Rhone-Simplon Line. The discrete Rhone-Simplon Line represents a
late retrograde manifestation in the preexisting ductile Simplon Shear
Zone. This fault zone is still active today.
The Oligocene-Neogene dextral transpression and extension in the Simplon
area were concurrent with thrusting to the northwest of the Helvetic
nappes, the Prealpes (35-15 Ma) and with the Jura thin-skinned thrust
(11-3 Ma). It was also contemporaneous with thrusting to the south of
the Bergamasc (> 35-5 Ma) and Milan thrusts (16-5 Ma)
High-precision U-Pb and 40Ar/39Ar dating of an Alpine ophiolite (Gets nappe, French Alps)
Coarse-grained gabbros from two different localities in the Gets nappe
(Upper Prealps) have been dated by U-Pb and Ar-40/Ar-39 isotopic
analyses. Zircons from both gabbros gave identical concordant U-Pb ages
of 166 +/- 1 Ma (Fig. 4). Amphibole from one of them gave an Ar-40/Ar-39
plateau age of 165.9 +/- 2.2 Ma (Fig. 5). This concordance implies that
166 +/- 1 Ma is the age of magmatic crystallization of these gabbros.
The Gets wildflysch with its mafic and ultramafic lenses is an
ophiolitic melange, that we infer to come from a proximal part of the
accretionary prism at the foot of the active SE margin of the Piemont
ocean. In this position we can expect to find remnants of the oldest
parts of the Piemont oceanic crust. These are the first high-precision
dates using modern techniques from an Alpine ophiolite and are in
excellent agreement with the following:
1) The few, somewhat younger, reliable ages on ophiolites from the
probable continuation of the Piemont basin into the Apennines and
Corsica;
2) Recent data on the age of the first supra-ophiolitic sediments (Late
Bathonian to Early Callovian radiolarites);
3) The structural and stratigraphic evolution of the Brianconnais (s.s.)
domain, the future NW margin of the Piemont ocean. We note a remarkable
coincidence, in Late Bajocian time, between: (A) the end of tensile
fracturing in the Brianconnais continental crust; (B) the beginning of
its subsidence; (C) the age of the Gets ophiolites. This coincidence is
consistent with an ocean opening mechanism based on a combination of
subhorizontal extension and thermally driven vertical movements of the
lithosphere
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