Giovanni Musumeci

The Hercynian chain in Sardinia (Italy)

AbstractA new geodynamic model for the Sardinian segment of the Hercynian chain is presented. The improvement of knowledge regarding several geological, metamorphic, magmatic and geochronological aspects of the Sardinian Palaeozoic basement, mainly achieved in the last few years, allows us to propose a more complete picture of its evolution.The occurrence of remnants of an oceanic suture along a major tectonic lineament in northern Sardinia, as well as the products of Ordovician calc-alkaline magmatism, testifies to the presence, during the Lower Paleozoic, of an ancient (Precambrian- Cambrian) oceanic domain and its consumption along an Andean- type subduction zone. The following Carboniferous continental collision caused crustal stacking with Barrovian metamorphism and southward-migrating deformation from the suture zone toward the foreland.Early Carboniferous Culm-type facies sediments, deposited in the outermost zone of the chain, imply that continental collision took place earlier in the internal zon...

Geology of the Higher Himalayan Crystallines in Khumbu Himal (Eastern Nepal)

Abstract In this paper we present the current geological knowledge and the results of new geological and structural investigations in the Cho Oyu-Sagarmatha-Makalu region (Eastern Nepal and Southern Tibet). The tectonic setting of the middle and upper part of the Higher Himalayan Crystallines (HHC) and Tibetan Sedimentary Sequence is characterized by the presence of pervasive compressive tectonics with south-verging folds and shear zones overprinted by extensional tectonics. In the middle and upper part of the HHC two systems of folds (F2a and F2b) have been recognized, affecting the S1 high-grade schistosity causing kilometer-scale upright antiforms and synforms. The limbs of these upright folds are affected by F3 collapse folds, top-to-SE extensional shear zones and extensional crenulation cleavages linked to extensional tectonics. The uppermost portion of the HHC and the lower part of the Tibetan Sedimentary Sequence is affected by two major extensional fault zones with a top-to NE direction of movement. The lower ductile extensional shear zone brings into contact the North Col Formation with the high grade gneisses and micaschists of the HHC. It is regarded as the main feature of the South Tibetan Detachment System. The upper low-angle fault zone is characterized by ductile/brittle deformation and thin levels of cataclasites and brings the slightly metamorphosed Ordovician limestones into contact with the North Col Formation. Extensional tectonics continued with the formation of E–W trending high angle normal faults. Three metamorphic stages of Himalayan age are recognized in the HHC of the Sagarmatha-Makalu region. The first stage (M1) is eclogitic as documented by granulitized eclogites collected at the top of the Main Central Thrust Zone in the Kharta region of Southern Tibet. The second event recorded in the Kharta eclogites (M2) was granulitic, with medium P (0.55–0.65 GPa) and high T (750–770°C), and was followed by recrystallization in the amphibolite facies of low pressure and high T (M3). The first event has also been recorded in the overlying Barun Gneiss, where M1 was followed by decompression under increasing T, the M2 event, producing the dominant mineral assemblage (garnet-sillimanite-biotite), and then by strong decompression under high T, with growth of andalusite, cordierite and green spinel. Also, changes in phase compatibilities suggest an increase in metamorphic temperature (T) coupled with a decrease in metamorphic pressure (P) in some of the thrust sheets of the MCT Zone. A telescoped metamorphic zonation ranging from the sillimanite to the staurolite and biotite zones is characteristic of the ductile extensional shear zone which is the lower part of the STDS in the Sagarmatha region. Evidence for decompression under increasing temperature, anatexis and leucogranite emplacement accompanying extension in the HHC was found throughout the whole ductile shear zone, particularly in metapelites both below and above the Makalu leucogranite and in micaschists of the staurolite zone.

Late Hercynian shear zones in Sardinia

Abstract In 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.

U–Pb zircon dating and nature of metavolcanics and metarkoses from the Monte Grighini Unit: new insights on Late Ordovician magmatism in the Variscan belt in Sardinia, Italy

In the external units of the Sardinian Variscides Nappe Zone, volcanic and volcanoclastic successions of Middle Ordovician age follow Lower Paleozoic calc-alkaline magmatism developed at the northern Gondwana margin. We present geochemical and zircon U–Pb isotopic data for the Truzzulla Formation, a low-to-medium-grade metamorphic volcanic–volcanoclastic succession belonging to the Monte Grighini Unit, the deepest unit in the Nappe Zone. Geochemical and radiometric data allow us to define a Late Ordovician (Katian) magmatic (volcanic) event of calc-alkaline affinity. These new data, in conjunction with previously published data, indicate that in the Sardinian Variscides, the age of Lower Paleozoic Andean-type calc-alkaline magmatism spans from Middle to Late Ordovician. Moreover, the age distribution of calc-alkaline volcanics and volcanoclastic rocks in the Nappe Zone is consistent with a diachronous development of Middle–Late Ordovician Andean-type magmatic arc through the portion of the northern Gondwanian margin now represented by the Sardinian Variscides. This reconstruction of the Sardinian Variscides reflects the complex magmatic and tectonic evolution of the northern margin of Gondwana in the Lower Paleozoic.

Magmatic belts in accretionary margins, a key for tectonic evolution: the Tonalite Belt of North Victoria Land (East Antarctica)

Occurring 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.

Neogene deformation and granite emplacement in the metamorphic units of northern Apennines (Italy): Insights from mylonitic marbles in the Porto Azzurro pluton contact aureole (Elba Island)

In the northern Tyrrhenian Sea, late Miocene intrusions (Monte Capanne pluton and Porto Azzurro pluton) were emplaced at upper crustal levels (

Pliocene crustal shortening on the Tyrrhenian side of the northern Apennines: evidence from the Gavorrano antiform (southern Tuscany, Italy)

The northern Tyrrhenian Sea and the inner northern Apennines are classically regarded as a late Miocene–Pleistocene back-arc system developed as a consequence of slab rollback along active subduction zones. We present new geological and structural data on the Gavorrano antiform, a key sector of the inner northern Apennines. Lying close to the northern Tyrrhenian Sea, it provides clear evidence of Pliocene shortening deformation and magma emplacement. The orientation of σ1 (N50°E–N80°E) derived by fault slip data inversion is consistent with a general ENE–WSW shortening direction. Furthermore, this ENE–WSW-trending orientation of σ1 is compatible with the compressive deformation recorded in coeval sedimentary basins. On this basis we suggest that the inner northern Apennines were affected by crustal shortening during the Pliocene. This scenario matches well geophysical data suggesting that since the Late Messinian (6–5 Ma) subduction rollback and back-arc extension strongly decreased in the northern Tyrrhenian Sea, whereas they continued as active processes in the southern Tyrrhenian Sea.

Sillimanite-bearing shear zones in syntectonic leucogranite: fluid-assisted brittle–ductile deformation under amphibolite facies conditions

Abstract In 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.

Tectonic control on laccolith emplacement in the northern Apennines fold-thrust belt: the Gavorrano intrusion (southern Tuscany, Italy)

Abstract Mechanical discontinuities within the crust, represented by tectonic structures (faults) or lithological heterogeneities, strongly control the emplacement of magmas as tabular intrusions within the middle-upper crust. The occurrence of mechanical layering is a common feature in fold and thrust belts. In the northern Apenniness, a Cenozoic fold-thrust belt affected in its inner part by Neogene magmatism, the Gavorrano laccolith (southern Tuscany) is a particularly suitable example for studying the relationships between magmatism and tectonic structures. New geological mapping, together with a large amount of subsurface data available from historical mining activity in the area, have allowed the reconstruction of: 1 the original relationships of the intrusion within the nappe pile, and 2 the laccolithic shape of the intrusion. Using the Gavorrano laccolith as an example, we propose that the emplacement of Neogene intrusions in southern Tuscany was strongly controlled by the occurrence of mechanical discontinuities represented by thrust zones in the nappe pile.

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

In 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 of the 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 rocks.