131,121 research outputs found
Limits and biases on the three-dimensional vorticity analysis using porphyroblast system: A discussion and application to natural example
No full textIn questo contributo viene affontata una discussione sui possibili errori
sistematici associati alla misura del numero di vorticità cinematica
(Wm). Si suggerisce che, per ottenere una corretta interpretazione del
comportamento meccanico dei porfiroclasti, è necessario individuare
quali tra le curve teoriche predette dal modello di Jeffery o Mulchrone,
si adattano meglio alla distribuzione e allo Shape Preferred Orientation
(SPO) dei porfiroclasti nei casi reali.
Negli studi di vorticita’ cinematica effettuati sia in sezione sottile che
in affioramento, le principali fonti di errore possono derivare sia dalla
semplicistica assunzione meccanica del sistema matrice/porfiroclasto,
sia dalle misure dei principali parametri geometrici quali il raggio e
l’orientazione dell’ asse principale del porfiroclasto rispetto alla matrice
e/o code ricristallizzate. Questi errori sistematici limitano fortemente
l’interpretazione cinematica di Wm per valori medio bassi.
Applicando diverse tecniche di misurazione su differenti tipologie di
porfiroclasti viene proposta un’analisi dettagliata di casi reali appartenenti
a zone di taglio della zona assiale della Catena Varisica Sarda
Implications of complex eigenvalues in homogeneous flow: A three-dimensional kinematic analysis
No full text
Marche: un progetto per connettere ricerca e industria
No full textL'articolo tratta della relazione tra ricerca e industria nella regione Marche
Le pieghe asimmetriche dell'Unità di Meana Sardo, sardegna centrale (Italia): evoluzione e meccanismi di piegamento
No full textL’Unità di Meana Sardo (UMS) è caratterizzata
dalla presenza di treni di pieghe asimmetriche vergenti verso
SW, formatesi durante la fase di ispessimento crostale e litosferico
della catena e sviluppo delle falde nella catena Varisica
in Sardegna. La geometria delle pieghe chilometriche
osservate e cartografate, la cinematica della deformazione e
l’analisi della deformazione finita permettono di vincolare i
meccanismi di piegamento. Questi dati sono stati confrontati
con un modello geometrico bidimensionale di sviluppo di
deformazione omogenea non-coassiale. I risultati mostrano
che una evoluzione in tre stadii con iniziale layer parallel
shortening, seguito da amplificazione delle pieghe e successivo
taglio semplice, dovuto al sovrascorrimento del Complesso
Metamorfico di Basso Grado della Barbagia sull’UMS,
è in grado di produrre le pieghe asimmetriche osservate e le
deformazioni misurate
Fabric attractors in general triclinic flow systems and their application to high strain shear zones: A dynamical system approach
No full textHigh strain zones may deform by flow with a triclinic symmetry. This paper describes triclinic flow in a reference frame where Instantaneous Stretching Axes (ISA) are fixed. The operation of triclinic flow is described in two ways: first in terms of flow and the nature of flow eigenvectors and in the second part of the paper in terms of finite strain. In monoclinic flow, at least one of the eigenvectors of the flow coincides with one of the ISA and one or two of the eigenvectors act as attractors of foliation or lineation elements. In triclinic flow some flow eigenvectors are undefined since the two largest eigenvalues (controlling the flow) are imaginary. Imaginary eigenvalues are particularly common at high kinematic vorticity and within flow with deviation of the vorticity vector of more than 20° from one of the ISA. Strong deviation from monoclinic flow is therefore possible, but this will not produce permanent foliations or lineations. For triclinic flow that does produce permanent fabrics, the angle between ISA and the fabric is so small that it is unlikely that it can be recognised in nature. A discussion of the potential application of such results within real shear zones is presented
Asymmetric folds development in the Variscan Nappe of central Sardinia (Italy)
No full textThe Meana Sardo Unit (MSU) shows trains of asymmetric SW-verging folds, developed during the thickening stage and nappe development of the Variscan belt in Sardinia. The geometry of the folds and the finite strain analysis results in the hinge of a kilometric fold give information on the folding mechanisms. The strain and the geometry of the fold have been compared to a 2-D geometrical model of development of homogenous non-coaxial deformation. The results show that initial layer parallel shortening and fold amplification followed by simple shearing or by coeval simple and pure shearing, linked to the overthrust of the Low Grade Metamorphic Complex of Barbagia onto MSU, are able to explain the observed fold geometries and finite strain values
Active faulting controls bedform development on a deep-water fan
No full textTectonically controlled topography influences deep-water sedimentary systems. Using 3-D seismic reflection data from the Levant Basin, eastern Mediterranean Sea, we investigate the spatial and temporal evolution of bedforms on a deep-water fan cut by an active normal fault. In the footwall, the fan comprises cyclic steps and antidunes along its axial and external portions, respectively, which we interpret to result from the spatial variation in flow velocity due to the loss of confinement at the canyon mouth. Conversely, in the hanging wall, the seafloor is nearly featureless at seismic scale. Numerical modeling of turbidity currents shows that the fault triggers a hydraulic jump that suppresses the flow velocity downstream, which thus explains the lack of visible bedforms basinward. This study shows that the topography generated by active normal faulting controls the downslope evolution of turbidity currents and the associated bedforms and that seafloor geomorphology can be used to evince syn-tectonic deposition
POST-COLLISIONAL TRANSPRESSIVE TECTONICS IN NORTHERN SARDINIA (ITALY)
No full textThe aim of this work is to review and synthetize the geological and structural
analysis performed in the Variscan Basement of Northern Sardinian during the last ten
years and to add new preliminary data on the Anglona–SW Gallura area. A transpressive
crustal-scale deformation (D2), is documented in the Variscan Basement of northern Sardinia. A shear deformation parallel to the belt, overprinting previous D1 structures
related to a top-to-the S and SW nappe stacking, has been recognized. The L2 stretching
lineation points to an orogen-parallel stretching and to a general change in the tectonic
transport from D1 to D2 deformation phases. D1 phase developed during initial frontal
collision whereas D2 deformation was characterized by dextral shearing. In this sector
of the Variscan belt exhumation is due to continuing compression with an increasing component of horizontal displacement developed in a regime of decreasing pressure.
The D2 transpressional deformation enhanced telescoping of the Barrovian isogrades and the exhumation of the low- to medium-grade metamorphic rocks. The overall change of the shortening direction in a large sector of an orogenic belt with the occurrence of increasing orogen-parallel displacement, may be regarded as a general mechanism affecting the exhumation of rocks and preventing the overthickened and thermally softened collisional crust from undergoing a diffused gravitational collapse. The rotation of nearly 90° of the tectonic transport in Sardinia during collisional and post-collisional stages could be related to paleoposition of the Corsica-Sardinia block, close to southeast France and northeast Spain, and to the development of the Ibero-Armorican arc
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