1,721,060 research outputs found
Ductile wrench tectonics and exhumation of hercynian metamorphic basement in Sardinia: Monte Grighini Complex
No full textSillimanite-bearing shear zones in syntectonic leucogranite: fluid-assisted brittle-ductile deformation under amphibolite facies conditions
No full textIn the Imja Kola valley (eastern Nepal), Miocene peraluminous leucogranite dykes were emplaced within transtensional shear zones synchronously with regional amphibolite–facies metamorphism in the metamorphic host rocks. Within the dykes the magmatic foliations, parallel to the metamorphic schistosity, are cross-cut by thin (millimetre to centimetre size) shear zones, which are made up of prismatic and/or fibrolitic sillimanite with a minor amount of white mica. These structures have a heterogeneous distribution and form networks of strongly deformed and foliated layers wrapping around poorly deformed granitic pods. The textural and strain features are consistent with development of sillimanite shear zones as a result of heterogeneous deformation and fluid action, which allowed the replacement of strained magmatic minerals such as feldspar and biotite by means of incongruent pressure solution. The described example illustrates that nucleation and development of heterogeneous shear zones under amphibolite facies conditions, is also strongly favoured by brittle fracturing and fluid action at the grain-scale along pre-existing planes of weakness. The growth of cracks and fractures can be regarded as a sequence of short lived brittle events, due to work hardening, which punctuate a history of plastic deformation
Magmatic belts in accretionary margins, a key for tectonic evolution: the Tonalite Belt of North Victoria Land (East Antarctica)
No full textOccurring in the Early Palaeozoic palaeomargin of the East Antarctica Craton, the Tonalite Belt of North Victoria Land is a narrow linear magmatic belt up to 200 km long, made up of strongly foliated tonalitic and granodioritic intrusions. They were synkinematically emplaced into the Lanterman-Murchison Shear Zone, a major tectonic structure across which different tectono-metamorphic terranes were accreted during the Late Cambrian Ross Orogeny. The deformational and kinematic features of the intrusions indicate that strain and displacement during terrane accretion were partitioned into oblique thrust, high-angle thrust and strike-slip shear zones. From this it is deduced that a transpressional regime prevailed during Early Palaeozoic deformation and terrane accretion. This and other magmatic belts which exploited major tectonic structures in collisional margins, such as the Coast Plutonic Complex of NW America, are basic keys for understanding deformational processes and kinematic regimes during terrane accretion. Moreover, the occurrence of such types of magmatic belts might be regarded as a reliable indication of large scale strike-slip motion between accreting terranes in an oblique convergent setting
Structure of the Deep Freeze Range - Eisenhower Range of the Wilson Terrane (North Victoria Land Antarctica): emplacement of magmatic intrusions in the Early Palaeozoic deformed margin of the East Antarctic Craton
No full textIn North Victoria Land (Antarctica), the Wilson Terrane is a portion of the palaeomargin of the East
Antarctic Craton, deformed during the Late Cambrian-Early Ordovician Ross Orogeny. Crustal deformation,
from westward subduction of the palaeo Pacific plate and terrane accretion on this palaeomargin, gave rise to
the development of a transpressive fold belt and a wide magmatic arc. In the inner portion ofthe Wilson Terrane,
(Deep Freeze Range-Eisenhower Range) a large portion of this magmatic arc is made up of intrusions and dyke
systems. Intrusive rocks range from large unfoliated plutons to well foliated sheet intrusions emplaced in low
and medium-high grade metamorphic rocks respectively. Field and structural data on intrusive rocks and
metamorphic host rocks, coupled with parameters relative to deformation mechanism and magmatic processes
(crystallization and cooling) rates, make it possible to outline an episode of diffuse synkinematic magmatism
in the Wilson Terrane. The emplacement of intrusions in both the middle and upper crust was coeval and related
to the development of transpressional and transtensional structures along dextral strike-slip shear zones.
Furthermore the development of foliated or unfoliated fabrics is related to competition between rates of
deformation and magmatic processes, which is a function of the thermal state of the host rock
Late Visean mylonitic granitoids in the Argentera Massif (Western Alps): age and kinematic constrains on the Ferrière-Mollières shear zone
No full textThe Argentera Massif in the western Alps is made of up two medium–high-grade metamorphic complexes of
Variscan age. They are separated by a regional-scale northwest-trending shear zone, the Ferrière–Mollières Line, which
consists of low-grade mylonitic rocks. Muscovite-bearingmylonitic leucogranites crop out as subvertical northwest-trending
intrusions, in the northern portion of the shear zone. A Rb/Sr muscovite–wr age of 327±3 Ma on foliated leucogranite gives
a lower limit for the age of mylonitic deformation for which kinematic indicators indicate a dextral sense of shear. This strike-slip
tectonics is compatible with the extensional regime that occurred during Carboniferous in the whole western Europ
Alpine tectonothermal evolution of the Tuscan Metamorphic Complex in the Larderello geothermal field (northern Apennines, Italy)
No full textOn the internal (Tyrrhenian) side of the northern Apennines, high heat flow and geothermal gradient
in the Larderello geothermal field are evidence of a large-scale thermal anomaly, related to the emplacement
of a large batholith of Pliocene age extending from the upper to middle crust. These intrusions relate to the
Miocene to Recent northern Apennines magmatism, which developed as a result of crustal extension and
upwelling of the asthenosphere at the base of the thinned crust. In the subsurface of the Larderello geothermal
field, medium- and high-grade metamorphic rocks (Tuscan Metamorphic Complex) cored in several
geothermal wells, have previously been considered to be Variscan basement. New structural and petrographic
data on these metamorphic rocks indicate an Alpine tectonometamorphic history and provide evidence that
medium- to high-grade mineral assemblages are related to Pliocene thermal metamorphism centred on
intrusions at shallow depth. These data allow a new interpretation of the Tuscan Metamorphic Complex as a
tectonic wedge of the Late Palaeozoic–Triassic units, and in particular highlight the considerable extent of
Pliocene thermal metamorphism and hydrothermal circulation as a result of convective cooling of intrusions
Hydrofracturing-related sill and dyke emplacementat shallow crustal levels: The eastern Elba Dyke Complex, Italy
No full textAn example of sheet-like intrusion emplacement at very shallow crustal levels
on Elba Island, Italy, is described. The Eastern Elba Dyke Complex (EEDC) consists of
decimetre- to metre-thick sheeted aplites emplaced within intensely folded low-grade metamorphic
rocks. Field data indicate that sill and dyke emplacement was controlled by mechanical
discontinuities, represented by fractures in the host rocks, and was strongly favoured by
magma overpressure. The occurrence of angular fragments of host rocks in the dyke border
zones and the branching of sills testify to hydraulic fracturing. Analysis of the spatial distribution
and geometry of EEDC sills and dykes provides clues on fluid pressure conditions
and the stress state at the time of magma emplacement, as well as on the depth of
emplacement. The calculated stress ratio and driving pressure ratio were used to estimate a
magma overpressure of 6–54 MPa at the time of emplacement of the EEDC at a depth of
about 2 km
Relationships between geomorphic proprieties and tectonic uplift in the Argentera Massif (Western Alps, Italy): geodynamics implication.
No full textInteractions between low-angle normal faults and plutonism in the upper crust: Insights from the island of Elba, Italy: Comment
No full textThe interplay between magmatism and tectonics is pivotal for deciphering the tectonic and geodynamic evolution of mountain belts. In the northern Apennines - northern Tyrrhenian Sea orogenic system large amounts of magma were emplaced at shallow crustal levels since the middle-late Miocene to recent (e.g. Serri et al., 1993; Rosenbaum et al., 2008). In this sector of the Apennine chain the mutual relationships between magmatism and tectonics are still a matter of debate (e.g., Rossetti et al., 1999; Acocella and Rossetti, 2002; Musumeci et al., 2005; 2008; Rosenbaum et al., 2008). Elba Island, located in the Tyrrhenian Sea about 30 km west of the inner portion of the chain, is a key area where tectonics - magmatism relationships can be studied
Late Hercynian shear zones in Sardinia
No full textIn the past few years two main late Hercynian shear zones have been identified in Sardinia: the Mount Grighini shear zone in the centre of the island and the Posada Valley shear zone in the north. Both are dextral wrench shear zones, affecting the Hercynian basement during the Late Carboniferous, and characterized by a simple shear deformation with an evolution from cataclastic to ultramylonitic rocks. The meso-and microstructural deformation features are discussed here. On the basis of these, two maps of the different deformative zones within the shear zones have been prepared and both displacements have been calculated. An attempt has also been made to correlate with other late Hercynian shear zones in continental Europe on the basis of common features and geological constraints
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