1,721,019 research outputs found
Metamorphic evolution of the Saka Unit (Central Pontides, Northern Turkey): new implications for the Mesozoic convergence-related processes in the Intra-Pontide suture zone
Full text linkIn Northern Turkey, the Intra-Pontide suture (IPS) zone represents an east-west trending belt of deformed and/or metamorphic rocks bounded by the Istanbul-Zonguldak Terrane (IZ) to the north and the Sakarya Composite Terrane (SK) to the south (e.g. Göncüoğlu et al., 1997). Despite its importance for the geodynamics reconstructions of the Black Sea and Eastern Mediterranean areas during the Mesozoic, the IPS zone has been poorly investigated; only recently the role of its metamorphic units during the syn-collisional evolution was explored (Marroni et al., 2014; Frassi et al., 2016; 2017)
In order to provide new insights on the Mesozoic-Tertiary geodynamic reconstruction of the southern margin of the Laurasia, we present new data from the Saka Unit from the eastern portion of the IPS. Using a multidisciplinary approach that includes lithological, structural, metamorphic and petrographic investigations, we constrained the P-T-t path during the Mesozoic subduction and consequent exhumation and accretion to Laurasia. Following the determination of mesostructures, we analysed a series of samples of metabasite and metapelite. Mineral chemistry of phases at equilibrium in the pervasive S2 foliation was determined, and PT determinations were performed through pseudosection calculation of three samples. PT conditions are within the epidote-amphibolite facies, with pressures of 1.2 ± 0.15 GPa and Temperatures 600 ± 50 °C, thus higher than those determined previously, suggesting that S2 could be a composite foliation where a progressive evolution acquired during onset of exhumation was recorded. A comparison with the PT evolutions of the nearby Daday and Domuz-Dag units, shows that they record lower T at D2 at different ages, and that the pre-D2 conditions, when they are preserved, are different in the three units.
References
Frassi C., Göncüoğlu M.C., Marroni M., Pandolfi L., Ruffini L. Ellero A., Ottria G. & Sayit K. 2016. The Intra-Pontide Suture Zone in the Tosya-Kastamonu area, Northern Turkey. J. of Maps. 12, 211-219
Frassi, C, Marroni, M., Pandolfi, L., Göncüoğlu, M. C., Ellero, A., Ottria, G., Sayit, K., McDonald, C.S., Balestrieri, M.L. & Malasoma, A. 2018. Burial and exhumation history of the Daday Unit (Central Pontides, Turkey): implications for the closure of the Intra-Pontide oceanic basin. Geol. Mag., 155, 356-376.
Göncüoğlu, M. C., Marroni, M., Sayit, K., Tekin, U. K., Ottria, G., Pandolfi, L. & Ellero, A., 2012. The Ayli Dag ophiolite sequence (central-northern Turkey): A fragment of middle Jurassic oceanic lithosphere within the Intra-Pontide suture zone. Ofioliti, 37, 77–91.
Marroni, M., Frassi, C., Göncüoğlu, C. M., Di Vincenzo, G., Pandolfi, L., Rebay, G., Ellero, A. & Ottria, G. 2014. Late Jurassic amphibolite-facies metamorphism in the Intra-Pontide Suture Zone (Turkey): an eastward extension of the Vardar Ocean from the Balkans into Anatolia? J. Geol. Soc., 171, 605–608
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
Le metamorfiti dell'Anglona e della Gallura sud-occidentale (Sardegna settentrionale, Italia): Nuove implicazioni sull'evoluzione post-collisionale della catena sarda
No full textThe Rigid Grain Net (RGN): An alternative method for estimating mean kinematic vorticity number (Wm)
No full textThe use of porphyroclasts rotating in a flowing matrix to estimate mean kinematic vorticity number (Wm) is important for quantifying the relative contributions of pure and simple shear in penetratively deformed rocks. The most common methods, broadly grouped into those that use tailed and tailless porphyroclasts, have been applied to many different tectonic settings; however, attempts have not been made to unify the various methods. Here, we propose the Rigid Grain Net (RGN) as an alternative graphical method for estimating Wm. The RGN contains hyperbolas that are the mathematical equivalents to the hyperbolic net used for the porphyroclast hyperbolic distribution (PHD) method. We use the RGN to unify the most commonly used Wmplots by comparing the distribution of theoretical and natural tailless porphyroclasts within a flowing matrix. Test samples from the South Tibetan detachment, Tibet yield indistinguishable results when the RGN is compared with existing methods. Because of its ease of use, ability for comparing natural data sets to theoretical curves, potential to standardize future investigations and ability to limit ambiguity in estimating Wm, the RGN makes an important new contribution that advances the current methods for quantifying flow in shear zones. © 2006 Elsevier Ltd. All rights reserved
The Variscan basamentof Northern Sardinia (Italy): field guide to the Escursion in the Baronie region
No full textAfter a short and updated geologic overview of
the Variscan belt in Sardinia a field trip in the Baronie area
(in the northeastern portion of the island) is described. We
choose six stops that can be easily reached and visited in one
day. They are among the best outcrops where the tectonometamorphic
history of this sector of the Variscan belt was
unravelled
Proiezioni stereografiche in geologia strutturale
No full textDedicato all'applicazione delle proiezioni stereografiche nel campo della geologia latu sensu e rivolto agli studenti dei corsi legati alle scienze della terra che hanno la necessità di trattare oggetti lineari e planari nello spazio e , quindi, di acquisire le basi delle tecniche delle proiezioni stereografiche
Shear belts in the Variscan axial zone in northern Sardinia (Italy)
No full textLa zona assiale della catena Varisica sarda è attraversata da una fascia
milonitica di spessore chilometrico e con cinematica destra, conosciuta
come Linea Posada-Asinara (PAL), che separa un complesso
metamorfico di medio e basso grado da un complesso metamorfico
di alto grado.
Analizzando la sezione centrale della PAL è emersa una complessa
geometria e cinematica non documentata negli altri settori di affioramento.
Le differenze in orientazione e in cinematica documentate nelle tre
sezioni, evidenziano una evoluzione molto complessa della PAL,
probabilmente indotta o da una forte irregolarità dei margini o da un
cambiamento nella direzione di convergenza continentale
Kinematics and vorticity of the flow in the shear belt of the Variscan axial zone in Sardinia (Italy)
No full textLa zona assiale della catena Varisica sarda è attraversata da una fascia
milonitica di spessore chilometrico e con cinematica destra, conosciuta
come Linea Posada-Asinara (PAL), che separa un complesso
metamorfico di medio e basso grado da un complesso metamorfico
di alto grado.
Analizzando la sezione centrale della PAL è emersa una complessa
geometria e cinematica non documentata negli altri settori di affioramento.
Le differenze in orientazione e in cinematica documentate nelle tre
sezioni, evidenziano una evoluzione molto complessa della PAL,
probabilmente indotta o da una forte irregolarità dei margini o da un
cambiamento nella direzione di convergenza continentale
Fault tectonics of the Tuscan Nappe in the eastern sector of the Apuan Alps (Italy)
Full text linkWe present the geological-structural map of the Tuscan Nappe exposed on the eastern border of the Apuan Alps metamorphic dome (Tuscany, Italy). The 1:6,500 scaled Main Map covers an area of about 10 km(2). It contains the first detailed overview of the fault tectonics affecting the Tuscan Nappe during the exhumation and uplift of the Tuscan Metamorphic Units. We documented a polyphase fault tectonics that initially produced low-angle extensional faults and later high-angle faults. The latter started within a transtensional tectonic regime that produced left-lateral strike-slip faults. Lately a pure extensional tensor, indicating a switch of the maximum compression sigma 1 axis from sub-horizontal to sub-vertical, produced faults with a dominant dip-slip component. In our reconstruction the lateral thickness variations documented in several formations of the Tuscan Nappe is mainly controlled by tectonics and not by stratigraphy, as previously suggested
The metamorphic complexes of Anglona and SW Gallura (Northern Sardinia, Italy): New constraints on the post-collisional evolution of the Variscan belt in Sardinia
No full textThe Variscan belt in Sardinia can be divided in three tectonometamorphic units that show an increasing both in the metamorphic grade and in the intensity of deformation moving from South toward North. A wide dextral shear zone (Posada Asinara line: PAL, CAPPELLI et alii, 1992) with a WNW-ESE trend divided the high(HGMC) and the medium-grade metamorphic (MGMC) complexes. This study is focused on a transect of the inner zone of the belt in Northern Sardinia through the Anglona and South-West Gallura regions where, structural and kinematic analyses at mesoscopic and microscopic scale highlight a similar tectonic evolution for all the studied complexes. After the continental collisional (D1 deformational phase), the metamorphic complexes were affected by a polyphase post-collisional tectonics characterized by three deformation phases (D2, D3 and D4) followed by a later extensional tectonic phase associated with collapse folds. The D2 phase, developed after the Barrovian metamorphism related to the collisional phase, is the main deformation phase in all the complexes. In SW Gallura, the S2 foliation shows an heterogeneous shear deformation, with a welldeveloped mylonitic fabric especially near the phyllonitic high strain zone. CPO and vorticity analyses, performed along a transect perpendicular to the main trend of high strain zone, testify a dominant non-coaxial flow with a major simple shear component. Two systems of shear zones has been recognized in this area. The stretching lineation shows a sub-horizontal plunge and its trend is generally parallel to the main trend of the belt and to F2 and F3 fold axes
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