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Rodo Carosi

Università degli Studi di Torino, Scienze della terra, Faculty Member

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Benedetto De Vivo

Università degli Studi di Napoli "Federico II"

Kansas State University

Kelvin S. Rodolfo

University of Illinois at Chicago

Università di Pisa

Maria Rita Palombo

Università degli Studi "La Sapienza" di Roma

Università Ca' Foscari Venezia

Vanderbilt University

Alessandro Tibaldi

Università degli Studi di Milano-Bicocca

Università degli Studi "La Sapienza" di Roma

Bogdan NICULESCU

University of Bucharest

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Papers by Rodo Carosi

Schema dell’evoluzione del basamento sardo

Kinematic and geochronological constraints on shear deformation in the Ferriere-Mollières shear zone (Argentera-Mercantour Massif, Western Alps): implications for the evolution of the Southern European Variscan Belt

International Journal of Earth Sciences, 2018

In the Western Alps, a steeply dipping km-scale shear zone (the Ferriere-Mollières shear zone) cr... more In the Western Alps, a steeply dipping km-scale shear zone (the Ferriere-Mollières shear zone) cross-cuts Variscan migmatites in the Argentera-Mercantour External Crystalline Massif. Structural analysis joined with kinematic vorticity and finite strain analyses allowed to recognize a high-temperature deformation associated with dextral transpression characterized by a variation in the percentage of pure shear and simple shear along a deformation gradient. U-Th-Pb dating of syn-kinematic monazites was performed on mylonites. The oldest ~ 340 Ma ages were obtained in protomylonites, whereas ages of ~ 320 Ma were found in mylonites from the core of the shear zone. These ages indicate that the Ferriere-Mollières shear zone is a still preserved Variscan shear zone. Ages of ~ 320 Ma obtained in this work are in agreement with ages of the dextral transpressional shear zones occurring in the Maures-Tanneron Massif and Corsica-Sardinia. However, transpression in the Argentera-Mercantour Massif started earlier than in other sectors of the southern Variscan Belt. This is possibly caused by the curvature of the belt triggering the progressive migration of shear deformation. Our data allow a correlation between the Argentera-Mercantour Massif and other segments of the Southern European Variscan Belt, in particular with Maures-Tanneron Massif and Corsica-Sardinia, and contribute to fill a gap in the age of activity and in the kinematics of the flow of the system of dextral shear zones of the southern portion of the EVSZ.

Middle to late Eocene exhumation of the Greater Himalayan Sequence in the Central Himalayas: Progressive accretion from the Indian plate

Geological Society of America Bulletin, 2016

We investigated a contractional shear zone located in central Nepal, known as Kalopani shear zone... more We investigated a contractional shear zone located in central Nepal, known as Kalopani shear zone. This high-temperature shear zone triggered the early exhumation of the metamorphic core in the Himalayan belt and deeply affected the tectono-metamorphic history of the crystalline rocks soon after the collisional stage. Pseudosection modeling and inverse geothermobarometry reveal that rocks involved in the Kalopani shear zone experienced pressure-temperature conditions between 0.60 and 0.85 GPa and 600 and 660 °C. U-Th-Pb in situ laser abla tion-inductively coupled plasma-mass spectrometry and sensitive high-resolution ion microprobe dating on monazite points to retrograde metamorphism related to the Kalopani shear zone starting from ca. 41 to 30 Ma. The kine matics of the Kalopani shear zone and associated erosion and/or tectonics caused the middle-late Eocene exhumation of the Greater Himalayan Sequence in the hanging wall of the Kalopani shear zone at least 9 m.y. before the activities of the middle tectonicmetamorphic discontinuity in the Greater Himalayan Sequence (High Hima layan discontinuity), the Main Central thrust, and the South Tibetan detachment. Structural data, metamorphic conditions, and geochronology from the Kalopani shear zone, compared to those of other major tectonic discontinuities active within the Greater Himalayan Sequence in the Kali Gandaki valley, indicate that shear deformation and exhumation were not synchronous all over the Greater Himalayan Sequence but migrated downward and southward at different lower levels. These processes caused the exhumation of the hanging wall rocks of the activated shear zones. The main consequence is that exhumation has been driven since the middle-late Eocene by an in-sequence shearing mechanism progressively involving new slices of the Indian crust, starting from the metamorphic core of the orogen and later involving the outer portions of the belt. This challenges the common view of exhumation of the Greater Himalayan Sequence mainly driven by the coupled activity of Main Central thrust and South Tibetan detachment between ca. 23 and 17 Ma.

Carosi et al 1998 JAES

Deformation and illite crystallinity in metapelitic rocks from the Mandas area, in the Nappe Zone of the Variscan belt of Sardinia

Structural evolution of the Southern Sulcis metamorphic complex (SW Sardinia, Italy)

Comptes Rendus de l'Académie des Sciences - Series IIA - Earth and Planetary Science, 1998

A complex deformation history from compression to extension was recognized in the metamorphic com... more A complex deformation history from compression to extension was recognized in the metamorphic complex of Southern Sulcis (SW Sardinia, Italy). The first deformation phase can be considered to be merely a relic, whereas the second phase is characterized by shear zones with top-to-the east and northeast sense of shear and upright folds, with north-south trending axes related to a general west-east shortening. The third phase is related to the liercynian extensional collapse of the metamorphic rocks in Southern Suicis. Geologicai data suggest a primary relation between the Ordovician protolith of the Mt. Filau gneiss and the I3ithia formation. The available data suggest a close relationship between Southern Sulcis and the Eastern Pyrenees and Catalonian Coastal Range. 0 AcadCmie des sciences / Elsevier, Paris.

Age and temperature constraints on metamorphism and exhumation of the syn-orogenic metamorphic complexes of Northern Apennines, Italy

Tectonophysics, 2011

The Apennine Range is a young convergent orogen that formed over a retreating subduction zone. Sy... more The Apennine Range is a young convergent orogen that formed over a retreating subduction zone. Synorogenic metamorphic rocks are exposed in the mid-Tuscan ridge, Tuscan coast and Tuscan Archipelago and provide information about processes of continental subduction and exhumation associated with accretion and retreat. Apatite and zircon fission-track analyses were applied to some of these syn-orogenic metamorphic rocks. The new low-temperature thermochronological data helped to constrain the timing of the metamorphic peak and the age and modality of exhumation of these rocks that were structurally buried at 20 to 40 km depth during the main tectonogenic phase of the Northern Appenine belt. In the Giglio and Elba islands and in the Monti dell'Uccellina, along the coast, minimum ages of partially reset zircon samples yielded a first time constraint on the Late-Oligocene age of the metamorphic peak. The ca. 27 Ma obtained at the Monti dell'Uccellina is coeval to that found in the Alpi Apuane (located at the northern termination of the mid-Tuscan ridge). The data from the Tuscan Archipelago point to a slightly older age. In the Monti Pisani, south of the Alpi Apuane, and in the Argentario Promontary along the coast, south of the Monti dell'Uccellina, completely reset zircon samples, but composed by different populations, were behaving as multiple thermochronometers and confirm the already proposed isothermal nature of the exhumation occurred after the metamorphic peak. Zircon P1 peak indicated that at 13-10 Ma rocks were already at shallow crustal levels (b 9 km). Zircon minimum ages and apatite ages point to a sharp increase in the exhumation rate at~6-4 Ma. Data indicate a common thermal history for the whole mid-Tuscan ridge and coastal outcrops. The 6-4 Ma increase in exhumation rate can be attributed either to unroofing due to the extensional tectonics or to a contractional "out-of-sequence" or "re-activation" event occurring in the internal sector of Tuscany already affected by a regional extensional regime.

A comparative U?Th?Pb (zircon?monazite) and40Ar?39Ar (muscovite?biotite) study of shear zones in northern Victoria Land (Antarctica): implications for geochronology and localized reworking of the Ross Orogen

Journal of Metamorphic Geology, 2007

Mylonitic granites from two shear zones in northern Victoria Land (Antarctica) were investigated ... more Mylonitic granites from two shear zones in northern Victoria Land (Antarctica) were investigated in order to examine the behaviour of the U-Th-Pb system in zircon and monazite and of the 40 Ar-39 Ar system in micas during ductile deformation. Meso-and micro-structural data indicate that shear zones gently dip to the NE and SW, have an opposite sense of shear (top-to-the-SW and-NE, respectively) and developed under upper greenschist facies conditions. In situ U-Pb dating by laser-ablation inductively coupled plasma-mass spectrometry of zircon areas with well-preserved igneous zoning patterns (c. 490 Ma) confirm that granites were emplaced during the Early Cambrian to Early Ordovician Ross-Delamerian Orogeny. Monazite from the Bier Point Shear Zone (BPSZ) mainly yielded U-Th-Pb ages of c. 440 Ma, in agreement with in-situ Ar laserprobe ages of syn-shear muscovite and with most Ar ages of coexisting biotite. The agreement of ages derived from different decay schemes and from minerals with different crystal-chemical features suggests that isotope transport in the studied sample was mainly controlled by (re)crystallization processes and that the main episode of ductile deformation in the BPSZ occurred at c. 440 Ma. Cathodoluminscence imaging showed that zircon from the BPSZ contains decomposed areas with faint relics of oscillatory zoning. These areas yielded a U-Pb age pattern which mimics that of monazite but is slightly shifted towards older ages, supporting previous studies which suggest that ÔghostÕ structures may be affected by inheritance. In contrast, secondary structures in zircon from the Mt. Emison Shear Zone (MESZ) predominantly consist of overgrowths or totally recrystallized areas and gave U-Pb ages of c. 450 and 410 Ma. The c. 450-Ma date matches within errors most monazite U-Th-Pb ages and in-situ Ar ages on biotite aligned along the mylonitic foliation. This again suggests that isotope ages from the different minerals are (re)crystallization ages and constrains the time of shearing in the MESZ to the Late Ordovician. Regionally, results indicate that shear zones were active in the Late Ordovician-Early Silurian and that their development was partially synchronous at c. 440 Ma, suggesting that they belong to a shear-zone system formed in response to NE-SW-directed shortening. Taking into account the former juxtaposition of northern Victoria Land and SE Australia, we propose that shear zones represent reactivated zones formed in response to stress applied along the new plate margin as a consequence of contractional tectonics associated with the early stages (Benambran Orogeny) of the development of the Late Ordovician-Late Devonian Lachlan Fold Belt.

The structural evolution of the southern Apuan Alps: new constraints on the tectonic evolution of the Northern Apennines (Italy)

Comptes Rendus Geoscience, 2002

Structural analysis performed in the southern sector of the Apuan Alps Metamorphic Complex (AAMC)... more Structural analysis performed in the southern sector of the Apuan Alps Metamorphic Complex (AAMC) and on the overlaying Tuscan Nappe (TN) pointed out a structural evolution much more complex than that outlined till now. The comparison between the structural evolutions of the two tectonic units could provide new important constraints on the tectonic evolution of the whole belt. The two tectonic units recorded different tectonic evolution during the first stages of compression, while they shared the same deformation history later after the Tuscan Nappe overlapped the AAMC. The coupled tectonic units have been then deformed by two systems of folds, in a compressive tectonic regime. Extensional tectonics affected the units later, at upper crustal levels. To cite this article: R.

Strain Analysis in the Millen Range of Northern Victoria Land, Antarctica

Deformation history of the Verrucano of Pisani Mts.(Northern Apennines, Italy)

Pressure–temperature and deformational evolution of high-pressure metapelites from Variscan NE Sardinia, Italy

Lithos, 2013

Chloritoid schists crop out north of the village of Lula in the Inner Zone of the Variscan chain ... more Chloritoid schists crop out north of the village of Lula in the Inner Zone of the Variscan chain of Sardinia consisting of a variety of metamorphic rocks. The S 1 and S 2 foliations in these schists are defined by the orientation of muscovite, paragonite, and chloritoid. Chlorite is an additional mineral oriented along S 2. Late margarite grew at the expense of chloritoid included in garnet. Garnet porphyroblasts, enclosing quartz, chloritoid, rutile, Fe-oxide, apatite and paragonite, show a progressive decrease of spessartine component from 17 to 7 mol% and an increase of pyrope component from 4 to 6 mol% from core to rim. The grossular content firstly increases from the inner (Grs~2 1) to the outer core (Grs~2 7) and then decreases towards the outermost rim (Grs~1 5). Compositional mapping of white mica also revealed zoning and a wide range in Si content (from 6.0 to 6.6 pfu). The highest Si content is related to the highest Fe and Mg contents and the lowest Na content. P-T pseudosections were calculated in the system Na 2 O-K 2 O-CaO-FeO-MnO-MgO-Al 2 O 3-TiO 2-SiO 2-H 2 O for compositions of chloritoid schists. The highest Si contents of K-white mica and the garnet core composition suggest pressures close to 1.8 GPa and temperatures of 460-500°C. The garnet rim composition and low Si contents in K-white mica are compatible with re-equilibration at 540-570°C and 0.7-1.0 GPa. These results suggest an HP-metamorphic imprint during the D 1 deformation phase which occurred before the Barrovian amphibolite-facies metamorphism of NE Sardinia. D 2 folding and shearing occurred at decreasing P-T conditions during the exhumation of the metamorphic complex.

Continental collision from two perspectives: A review of Variscan metamorphism and deformation in northern Sardinia

Periodico Di Mineralogia, 2015

Structural, petrological and geochronological data from the Variscan chain of northern Sardinia a... more Structural, petrological and geochronological data from the Variscan chain of northern Sardinia are discussed, with the aim to improve our knowledge on the tectono-metamorphic evolution of the northern Sardinian basement and its role in the tectonic puzzle of the southern Variscan massifs. In northern Sardinia, which corresponds to the Inner zone of the chain, the metamorphic grade rapidly increases towards the NE from biotite to sillimanite+K-feldspar zone. The highest grade rocks cropping out in the Migmatite zone reached amphibolite facies conditions in the sedimentary and igneous-derived migmatite and eclogite/granulite facies in the enclosed metabasite lenses. Evidence of HP metamorphism has also recently been found more to the south in micaschist from the garnet zone. The D 1 deformation can be observed in the southern part of the Inner zone, whereas, moving progressively northwards, the D 2 deformation progressively obliterates the previous foliation. The other, less pervasiv...

Crustal strength control on structures and metamorphism in collisional orogens

Tectonophysics, 2018

Although the upper crust is the most accessible among other crustal units, so far little attentio... more Although the upper crust is the most accessible among other crustal units, so far little attention was paid about its role on the continental collision style and on exhumation processes of metamorphic rocks. With this respect, several 2D thermo-mechanical numerical experiments of post-subduction continental collision were performed, which allowed investigating the interplay between surface processes, thermal history and rheology of the upper crust units. The viscous strength of upper and lower plate upper crust (U.U.C. and L.U.C.) was systematically changed, obtaining a reference set of experiments to which subsequently we applied enhanced erosion patterns and variable radiogenic heat productivity of the sediments. We obtained three classes of collisional style: 1) decoupled asymmetric; 2) coupled asymmetric; 3) coupled symmetric. In general, for high-rates of radiogenic heating, a strong U.U.C. leads to onesided, asymmetric and coupled hot orogen, which shifts to two-sided symmetric for weaker U.U.C.. Lower radiogenic heat productivities yield colder orogens, promoting more asymmetric orogens and eventually the decoupling between upper and lower plates. The enhanced erosion leads to narrower and colder orogens, affecting the collisional style only for a relatively strong L.U.C. rheology. Exhumation of high-grade metamorphic rocks mainly depends on the L.U.C. strength: a weak viscous rheology promotes pure shear thickening of the incoming crust in the orogenic front, which inhibits both burial and exhumation processes. Higher viscous strengths promote deeper subduction of the L.U.C., with subsequent heating, delamination, partial melting and exhumation of high-grade rocks via channel flow. Enhanced erosion, although not a necessary condition, favours the extrusion of the partially melted mid-crustal channel.

The Higher Himalayan Crystallines, Alaknanda – Dhauli Gan-ga Valleys, Garhwal Himalaya, India

Journal of the Virtual Explorer

Geological field trips in the Himalaya, Karakoram and Tibet

Pressure–temperature–time–deformation path of kyanite-bearing migmatitic paragneiss in the Kali Gandaki valley (Central Nepal): Investigation of Late Eocene–Early Oligocene melting processes

Lithos, 2015

Pressure-temperature-time-deformation path of kyanite-bearing migmatitic 20 paragneiss in the Kal... more

A geological journey through the deepest gorge on Earth: the Kali Gandaki valley section, west-central Nepal

The Kali Gandaki valley in the Central Himalaya is flanked by the eight thousand meter high Annap... more The Kali Gandaki valley in the Central Himalaya is flanked by the eight thousand meter high Annapurna and Dhaulagiri massifs forming the deepest gorge in the world. It offers an invaluable opportunity to directly observe a cross section through the continental crust involved in the Himalayan orogen. The north-south trend of the valley runs almost perpendicular to the dominant regional strike and cross cuts the major tectonic units that comprise the Himalaya including the Lesser Himalayan Sequence, the Greater Himalayan Sequence and the Tethyan Sedimentary Sequence. The valley is easily accessible compared to many other sections across the belt and its history as a popular trekking route ensures services and lodging that are often not available elsewhere. We propose a one-week itinerary across the low metamorphic grade Proterozoic quartzites of the Lesser Himalayan Sequence in the south, through the high-grade, migmatitic metamorphic core of the Greater Himalayan Sequence to the unme...

Leucogranite intruding the south tibetan detachment in western nepal: implications for exhumation models in the himalayas

Kinematics of the crust in southern Tibet and Higher Himalayan Crystalline –a paleomagnetic approach

Schema dell’evoluzione del basamento sardo

Kinematic and geochronological constraints on shear deformation in the Ferriere-Mollières shear zone (Argentera-Mercantour Massif, Western Alps): implications for the evolution of the Southern European Variscan Belt

International Journal of Earth Sciences, 2018

In the Western Alps, a steeply dipping km-scale shear zone (the Ferriere-Mollières shear zone) cr... more In the Western Alps, a steeply dipping km-scale shear zone (the Ferriere-Mollières shear zone) cross-cuts Variscan migmatites in the Argentera-Mercantour External Crystalline Massif. Structural analysis joined with kinematic vorticity and finite strain analyses allowed to recognize a high-temperature deformation associated with dextral transpression characterized by a variation in the percentage of pure shear and simple shear along a deformation gradient. U-Th-Pb dating of syn-kinematic monazites was performed on mylonites. The oldest ~ 340 Ma ages were obtained in protomylonites, whereas ages of ~ 320 Ma were found in mylonites from the core of the shear zone. These ages indicate that the Ferriere-Mollières shear zone is a still preserved Variscan shear zone. Ages of ~ 320 Ma obtained in this work are in agreement with ages of the dextral transpressional shear zones occurring in the Maures-Tanneron Massif and Corsica-Sardinia. However, transpression in the Argentera-Mercantour Massif started earlier than in other sectors of the southern Variscan Belt. This is possibly caused by the curvature of the belt triggering the progressive migration of shear deformation. Our data allow a correlation between the Argentera-Mercantour Massif and other segments of the Southern European Variscan Belt, in particular with Maures-Tanneron Massif and Corsica-Sardinia, and contribute to fill a gap in the age of activity and in the kinematics of the flow of the system of dextral shear zones of the southern portion of the EVSZ.

Middle to late Eocene exhumation of the Greater Himalayan Sequence in the Central Himalayas: Progressive accretion from the Indian plate

Geological Society of America Bulletin, 2016

We investigated a contractional shear zone located in central Nepal, known as Kalopani shear zone... more We investigated a contractional shear zone located in central Nepal, known as Kalopani shear zone. This high-temperature shear zone triggered the early exhumation of the metamorphic core in the Himalayan belt and deeply affected the tectono-metamorphic history of the crystalline rocks soon after the collisional stage. Pseudosection modeling and inverse geothermobarometry reveal that rocks involved in the Kalopani shear zone experienced pressure-temperature conditions between 0.60 and 0.85 GPa and 600 and 660 °C. U-Th-Pb in situ laser abla tion-inductively coupled plasma-mass spectrometry and sensitive high-resolution ion microprobe dating on monazite points to retrograde metamorphism related to the Kalopani shear zone starting from ca. 41 to 30 Ma. The kine matics of the Kalopani shear zone and associated erosion and/or tectonics caused the middle-late Eocene exhumation of the Greater Himalayan Sequence in the hanging wall of the Kalopani shear zone at least 9 m.y. before the activities of the middle tectonicmetamorphic discontinuity in the Greater Himalayan Sequence (High Hima layan discontinuity), the Main Central thrust, and the South Tibetan detachment. Structural data, metamorphic conditions, and geochronology from the Kalopani shear zone, compared to those of other major tectonic discontinuities active within the Greater Himalayan Sequence in the Kali Gandaki valley, indicate that shear deformation and exhumation were not synchronous all over the Greater Himalayan Sequence but migrated downward and southward at different lower levels. These processes caused the exhumation of the hanging wall rocks of the activated shear zones. The main consequence is that exhumation has been driven since the middle-late Eocene by an in-sequence shearing mechanism progressively involving new slices of the Indian crust, starting from the metamorphic core of the orogen and later involving the outer portions of the belt. This challenges the common view of exhumation of the Greater Himalayan Sequence mainly driven by the coupled activity of Main Central thrust and South Tibetan detachment between ca. 23 and 17 Ma.

Carosi et al 1998 JAES

Deformation and illite crystallinity in metapelitic rocks from the Mandas area, in the Nappe Zone of the Variscan belt of Sardinia

Structural evolution of the Southern Sulcis metamorphic complex (SW Sardinia, Italy)

Comptes Rendus de l'Académie des Sciences - Series IIA - Earth and Planetary Science, 1998

A complex deformation history from compression to extension was recognized in the metamorphic com... more A complex deformation history from compression to extension was recognized in the metamorphic complex of Southern Sulcis (SW Sardinia, Italy). The first deformation phase can be considered to be merely a relic, whereas the second phase is characterized by shear zones with top-to-the east and northeast sense of shear and upright folds, with north-south trending axes related to a general west-east shortening. The third phase is related to the liercynian extensional collapse of the metamorphic rocks in Southern Suicis. Geologicai data suggest a primary relation between the Ordovician protolith of the Mt. Filau gneiss and the I3ithia formation. The available data suggest a close relationship between Southern Sulcis and the Eastern Pyrenees and Catalonian Coastal Range. 0 AcadCmie des sciences / Elsevier, Paris.

Age and temperature constraints on metamorphism and exhumation of the syn-orogenic metamorphic complexes of Northern Apennines, Italy

Tectonophysics, 2011

The Apennine Range is a young convergent orogen that formed over a retreating subduction zone. Sy... more The Apennine Range is a young convergent orogen that formed over a retreating subduction zone. Synorogenic metamorphic rocks are exposed in the mid-Tuscan ridge, Tuscan coast and Tuscan Archipelago and provide information about processes of continental subduction and exhumation associated with accretion and retreat. Apatite and zircon fission-track analyses were applied to some of these syn-orogenic metamorphic rocks. The new low-temperature thermochronological data helped to constrain the timing of the metamorphic peak and the age and modality of exhumation of these rocks that were structurally buried at 20 to 40 km depth during the main tectonogenic phase of the Northern Appenine belt. In the Giglio and Elba islands and in the Monti dell'Uccellina, along the coast, minimum ages of partially reset zircon samples yielded a first time constraint on the Late-Oligocene age of the metamorphic peak. The ca. 27 Ma obtained at the Monti dell'Uccellina is coeval to that found in the Alpi Apuane (located at the northern termination of the mid-Tuscan ridge). The data from the Tuscan Archipelago point to a slightly older age. In the Monti Pisani, south of the Alpi Apuane, and in the Argentario Promontary along the coast, south of the Monti dell'Uccellina, completely reset zircon samples, but composed by different populations, were behaving as multiple thermochronometers and confirm the already proposed isothermal nature of the exhumation occurred after the metamorphic peak. Zircon P1 peak indicated that at 13-10 Ma rocks were already at shallow crustal levels (b 9 km). Zircon minimum ages and apatite ages point to a sharp increase in the exhumation rate at~6-4 Ma. Data indicate a common thermal history for the whole mid-Tuscan ridge and coastal outcrops. The 6-4 Ma increase in exhumation rate can be attributed either to unroofing due to the extensional tectonics or to a contractional "out-of-sequence" or "re-activation" event occurring in the internal sector of Tuscany already affected by a regional extensional regime.

A comparative U?Th?Pb (zircon?monazite) and40Ar?39Ar (muscovite?biotite) study of shear zones in northern Victoria Land (Antarctica): implications for geochronology and localized reworking of the Ross Orogen

Journal of Metamorphic Geology, 2007

Mylonitic granites from two shear zones in northern Victoria Land (Antarctica) were investigated ... more Mylonitic granites from two shear zones in northern Victoria Land (Antarctica) were investigated in order to examine the behaviour of the U-Th-Pb system in zircon and monazite and of the 40 Ar-39 Ar system in micas during ductile deformation. Meso-and micro-structural data indicate that shear zones gently dip to the NE and SW, have an opposite sense of shear (top-to-the-SW and-NE, respectively) and developed under upper greenschist facies conditions. In situ U-Pb dating by laser-ablation inductively coupled plasma-mass spectrometry of zircon areas with well-preserved igneous zoning patterns (c. 490 Ma) confirm that granites were emplaced during the Early Cambrian to Early Ordovician Ross-Delamerian Orogeny. Monazite from the Bier Point Shear Zone (BPSZ) mainly yielded U-Th-Pb ages of c. 440 Ma, in agreement with in-situ Ar laserprobe ages of syn-shear muscovite and with most Ar ages of coexisting biotite. The agreement of ages derived from different decay schemes and from minerals with different crystal-chemical features suggests that isotope transport in the studied sample was mainly controlled by (re)crystallization processes and that the main episode of ductile deformation in the BPSZ occurred at c. 440 Ma. Cathodoluminscence imaging showed that zircon from the BPSZ contains decomposed areas with faint relics of oscillatory zoning. These areas yielded a U-Pb age pattern which mimics that of monazite but is slightly shifted towards older ages, supporting previous studies which suggest that ÔghostÕ structures may be affected by inheritance. In contrast, secondary structures in zircon from the Mt. Emison Shear Zone (MESZ) predominantly consist of overgrowths or totally recrystallized areas and gave U-Pb ages of c. 450 and 410 Ma. The c. 450-Ma date matches within errors most monazite U-Th-Pb ages and in-situ Ar ages on biotite aligned along the mylonitic foliation. This again suggests that isotope ages from the different minerals are (re)crystallization ages and constrains the time of shearing in the MESZ to the Late Ordovician. Regionally, results indicate that shear zones were active in the Late Ordovician-Early Silurian and that their development was partially synchronous at c. 440 Ma, suggesting that they belong to a shear-zone system formed in response to NE-SW-directed shortening. Taking into account the former juxtaposition of northern Victoria Land and SE Australia, we propose that shear zones represent reactivated zones formed in response to stress applied along the new plate margin as a consequence of contractional tectonics associated with the early stages (Benambran Orogeny) of the development of the Late Ordovician-Late Devonian Lachlan Fold Belt.

The structural evolution of the southern Apuan Alps: new constraints on the tectonic evolution of the Northern Apennines (Italy)

Comptes Rendus Geoscience, 2002

Structural analysis performed in the southern sector of the Apuan Alps Metamorphic Complex (AAMC)... more Structural analysis performed in the southern sector of the Apuan Alps Metamorphic Complex (AAMC) and on the overlaying Tuscan Nappe (TN) pointed out a structural evolution much more complex than that outlined till now. The comparison between the structural evolutions of the two tectonic units could provide new important constraints on the tectonic evolution of the whole belt. The two tectonic units recorded different tectonic evolution during the first stages of compression, while they shared the same deformation history later after the Tuscan Nappe overlapped the AAMC. The coupled tectonic units have been then deformed by two systems of folds, in a compressive tectonic regime. Extensional tectonics affected the units later, at upper crustal levels. To cite this article: R.

Strain Analysis in the Millen Range of Northern Victoria Land, Antarctica

Deformation history of the Verrucano of Pisani Mts.(Northern Apennines, Italy)

Pressure–temperature and deformational evolution of high-pressure metapelites from Variscan NE Sardinia, Italy

Lithos, 2013

Chloritoid schists crop out north of the village of Lula in the Inner Zone of the Variscan chain ... more Chloritoid schists crop out north of the village of Lula in the Inner Zone of the Variscan chain of Sardinia consisting of a variety of metamorphic rocks. The S 1 and S 2 foliations in these schists are defined by the orientation of muscovite, paragonite, and chloritoid. Chlorite is an additional mineral oriented along S 2. Late margarite grew at the expense of chloritoid included in garnet. Garnet porphyroblasts, enclosing quartz, chloritoid, rutile, Fe-oxide, apatite and paragonite, show a progressive decrease of spessartine component from 17 to 7 mol% and an increase of pyrope component from 4 to 6 mol% from core to rim. The grossular content firstly increases from the inner (Grs~2 1) to the outer core (Grs~2 7) and then decreases towards the outermost rim (Grs~1 5). Compositional mapping of white mica also revealed zoning and a wide range in Si content (from 6.0 to 6.6 pfu). The highest Si content is related to the highest Fe and Mg contents and the lowest Na content. P-T pseudosections were calculated in the system Na 2 O-K 2 O-CaO-FeO-MnO-MgO-Al 2 O 3-TiO 2-SiO 2-H 2 O for compositions of chloritoid schists. The highest Si contents of K-white mica and the garnet core composition suggest pressures close to 1.8 GPa and temperatures of 460-500°C. The garnet rim composition and low Si contents in K-white mica are compatible with re-equilibration at 540-570°C and 0.7-1.0 GPa. These results suggest an HP-metamorphic imprint during the D 1 deformation phase which occurred before the Barrovian amphibolite-facies metamorphism of NE Sardinia. D 2 folding and shearing occurred at decreasing P-T conditions during the exhumation of the metamorphic complex.

Continental collision from two perspectives: A review of Variscan metamorphism and deformation in northern Sardinia

Periodico Di Mineralogia, 2015

Structural, petrological and geochronological data from the Variscan chain of northern Sardinia a... more Structural, petrological and geochronological data from the Variscan chain of northern Sardinia are discussed, with the aim to improve our knowledge on the tectono-metamorphic evolution of the northern Sardinian basement and its role in the tectonic puzzle of the southern Variscan massifs. In northern Sardinia, which corresponds to the Inner zone of the chain, the metamorphic grade rapidly increases towards the NE from biotite to sillimanite+K-feldspar zone. The highest grade rocks cropping out in the Migmatite zone reached amphibolite facies conditions in the sedimentary and igneous-derived migmatite and eclogite/granulite facies in the enclosed metabasite lenses. Evidence of HP metamorphism has also recently been found more to the south in micaschist from the garnet zone. The D 1 deformation can be observed in the southern part of the Inner zone, whereas, moving progressively northwards, the D 2 deformation progressively obliterates the previous foliation. The other, less pervasiv...

Crustal strength control on structures and metamorphism in collisional orogens

Tectonophysics, 2018

Although the upper crust is the most accessible among other crustal units, so far little attentio... more Although the upper crust is the most accessible among other crustal units, so far little attention was paid about its role on the continental collision style and on exhumation processes of metamorphic rocks. With this respect, several 2D thermo-mechanical numerical experiments of post-subduction continental collision were performed, which allowed investigating the interplay between surface processes, thermal history and rheology of the upper crust units. The viscous strength of upper and lower plate upper crust (U.U.C. and L.U.C.) was systematically changed, obtaining a reference set of experiments to which subsequently we applied enhanced erosion patterns and variable radiogenic heat productivity of the sediments. We obtained three classes of collisional style: 1) decoupled asymmetric; 2) coupled asymmetric; 3) coupled symmetric. In general, for high-rates of radiogenic heating, a strong U.U.C. leads to onesided, asymmetric and coupled hot orogen, which shifts to two-sided symmetric for weaker U.U.C.. Lower radiogenic heat productivities yield colder orogens, promoting more asymmetric orogens and eventually the decoupling between upper and lower plates. The enhanced erosion leads to narrower and colder orogens, affecting the collisional style only for a relatively strong L.U.C. rheology. Exhumation of high-grade metamorphic rocks mainly depends on the L.U.C. strength: a weak viscous rheology promotes pure shear thickening of the incoming crust in the orogenic front, which inhibits both burial and exhumation processes. Higher viscous strengths promote deeper subduction of the L.U.C., with subsequent heating, delamination, partial melting and exhumation of high-grade rocks via channel flow. Enhanced erosion, although not a necessary condition, favours the extrusion of the partially melted mid-crustal channel.

The Higher Himalayan Crystallines, Alaknanda – Dhauli Gan-ga Valleys, Garhwal Himalaya, India

Journal of the Virtual Explorer

Geological field trips in the Himalaya, Karakoram and Tibet

Pressure–temperature–time–deformation path of kyanite-bearing migmatitic paragneiss in the Kali Gandaki valley (Central Nepal): Investigation of Late Eocene–Early Oligocene melting processes

Lithos, 2015

Pressure-temperature-time-deformation path of kyanite-bearing migmatitic 20 paragneiss in the Kal... more

A geological journey through the deepest gorge on Earth: the Kali Gandaki valley section, west-central Nepal

The Kali Gandaki valley in the Central Himalaya is flanked by the eight thousand meter high Annap... more The Kali Gandaki valley in the Central Himalaya is flanked by the eight thousand meter high Annapurna and Dhaulagiri massifs forming the deepest gorge in the world. It offers an invaluable opportunity to directly observe a cross section through the continental crust involved in the Himalayan orogen. The north-south trend of the valley runs almost perpendicular to the dominant regional strike and cross cuts the major tectonic units that comprise the Himalaya including the Lesser Himalayan Sequence, the Greater Himalayan Sequence and the Tethyan Sedimentary Sequence. The valley is easily accessible compared to many other sections across the belt and its history as a popular trekking route ensures services and lodging that are often not available elsewhere. We propose a one-week itinerary across the low metamorphic grade Proterozoic quartzites of the Lesser Himalayan Sequence in the south, through the high-grade, migmatitic metamorphic core of the Greater Himalayan Sequence to the unme...

Leucogranite intruding the south tibetan detachment in western nepal: implications for exhumation models in the himalayas

Kinematics of the crust in southern Tibet and Higher Himalayan Crystalline –a paleomagnetic approach

Geological field trips in the Himalaya, Karakoram and Tibet

by Sandeep Singh and Rodo Carosi