Franco Marco Elter

The East Variscan Shear Zone: new insights into its role in the Late Carboniferous collision in southern Europe

A comparison of the petro-tectonic features recorded in the Variscan Massifs scattered throughout the Alps, the Corsica-Sardinia-Maures-Tanneron Massif, the Calabria-Peloritani Arc, and the Northern Apennines, has allowed us to propose that they belonged to the same geodynamic realm until Late Carboniferous time. In the interval 330–300 Ma, the development of a regional dextral strike–slip shear zone, the East Variscan Shear Zone (EVSZ), affected all the massifs, leading to their spatial separation. The EVSZ developed, together with numerous regional shear zones, under a transpressional tectonic regime deriving from the Late Carboniferous collision between Gondwana, peri-Gondwana microcontinents (Armorica and Avalonia), and Laurussia plates. The EVSZ evidently played a key role in the evolution of the subsequent Alpine and Apenninic cycles, acting as a pre-existing tectonic barrier. Our proposed geodynamic reconstruction does not reflect the acquisition of new data, but is based on the analysis and review of the recent geological literature.

The south-western Alpine foreland: correlation between two sectors of the Variscan chain belonging to “stable Europe”: Sardinia(-)Corsica and the Maures Massif (south-eastern France).

The post-collisional Variscan evolution in Sardinia (Italy)—Corsica is characterized by Late Carboniferous—Permian extensional tectonics that affected the whole belt and was due to gravitational collapse of the previously thickened crust. Evidence of this late Hercynian extensional tectonism is widespread throughout the basement and its development is chronologically constrained by radiometric, sedimentary and paleontological data. Extensional tectonism in Sardinia may be related to a composite shear framework characterized by two events: an Early Shear Event (ESE) and a Late Shear Event (LSE). The ESE is characterized by shear zones in amphibolite facies conditions with a top-to S/SE shearing in relation to a dome structure. The LSE consists of strike slip ductile shear zones (mainly dextral shear zones) associated with retrograde metamorphism, commonly coeval with syntectonic plutons. The ESE accommodated the exhumation of the Variscan belt in Sardinia, while the LSE belongs to the Late Variscan strike-slip faulting common in SW Europe. The Late Variscan geodynamic evolution of the Maures Massif (south-eastern France) is characterized by extensional tectonism related to noncoaxial deformation in HT to LT metamorphic conditions: the close relationships between these two areas suggests that they belong to the same segment of the Variscan belt.

eo-Variscan (Devonian?) melting in the High Grade Metamorphic Complex of the NE Sardinia Belt (Italy)

New structural data pointed out the presence of an older scattered migmatization event (Devonian?, M1) overcome by the well known Variscan migmatization event (Lower-Middle Carboniferous, M2) related to the Late extensional tectonic that affected the High Grade Metamorphic Complex (HGMC) in the Variscan Belt of Sardinia (Italy). The M1 event is only recognizable in the kyanite – amphibole bearing migmatitic gneiss. Both migmatization events (M1 and M2) are characterized by a syn-tectonic non coaxial deformations (D1 and D2 deformational events). D1 shows a top to NW sense of shear while the D2 event a top to NE/SE sense of shear (the shear senses are considered at the present Sardinia – Corsica block position in the Mediterranean sea). The M2+D2 is characterized by a complicate, composite normal shear network; the M1+D1 by inverse shear zones. The M2+D2 is transposed by the late D3 strike slip shear event: the D3 is characterized by strike slip shear zones syn-kinematic to the emplacement of Late Carboniferous granitoids (320 Ma – 300 Ma). Despite the absence of geochronological data about the M1+D1 event, the field relationships suggest, for first time, an older migmatization process (Devonian?) syn-tectonic with the late stage of thickness of the Sardinia Variscan Belt. Similar evolutions are recognized in different segments of the Variscan Belt such as the Massif Central (France) or in the eastern mid-European Variscides.

Relationships between the Sakarya Zone and the Ankara–Erzincan suture (central-northern Turkey): geological and petrographic data from the Ankara–Çankiri, Çorum and Amasya areas

The study was performed in central-northern Anatolia (from Ankara to Amasya) to investigate the relationships of the Sakarya Zone units and the Izmir–Ankara–Erzincan suture (IAES) melange. It reveals that all the Sakarya Zone units are metamorphic and three main tectonostratigraphic units have been distinguished for the first time: the BAA (metasiliciclastic rocks capped by metacarbonates and varicoloured phyllite), the BKC (poly-metamorphic garnet-bearing micaschist and metabasite with a well-preserved relict HP–LT amphibole in a low-amphibolitic to greenschistfacies framework) and the AMC (meta-arkose passing vertically to carbonate–phyllitic alternations and, then, to a thick succession of prevailing acidic to intermediate–basic metavolcanites and volcanicrich metasediments). The BAA and AMC, whose metamorphic frameworks are of Cimmerian age, underlie the Mesozoic carbonate cover sequences (e.g. t2-3, j3–k1) that often show tectonic detachments and slicing. The piling up of the BAA above the HP–LT BKC can be correlated to the tectonic superposition of two similar units (i.e. the Cimmerian Çangaldağ Complex and the Alpine Middle– Upper Cretaceous Domuzdağ Complex, respectively) defined by previous authors in other sectors of the Central Pontides front. The ophiolitic melange generally underlies the Sakarya Zone, but locally (e.g. SE of Amasya) tectonically rests above the latter, probably owing to back-thrusting that occurred during the Tertiary syn-collisional shortenings and the later strike-slip tectonics. We hypothesize that, also in these areas, the Sakarya Zone–IAES consists of a complex tectonic stack of different units, belonging to different palaeogeographic domains and orogenic events (Cimmerian versus Alpine orogenies), but originated within a single long-lived (since Late Triassic to Paleocene/Eocene times), prograding subduction–accretion system in front of the Laurasian continent.

The high-grade metamorphics from Pittulongu to Golfo Aranci (NE Sardinia): an attempt of lithological reconstruction

Lucchi, Renata G. ... et. al.-- 87° Congresso della Societa Geologica Italiana e 90° Congresso della Societa Italiana di Mineralogia e Petrologia, The Future of the Italian Geosciences - The Italian Geosciences of the Future, 10-12 September 2014, Milan, Italy.-- 1 pageThe Montellina Spring (370 m a.s.l.) represents an example of groundwater resource in mountain region. It is a significant source of drinking water located in the right side of the Dora Baltea Valley (Northwestern Italy), SW of Quincinetto town. This spring shows a morphological location along a ridge, 400 m from the Renanchio Torrent in the lower sector of the slope. The spring was investigated using various methodologies as geological survey, supported by photo interpretation, structural reconstruction, NaCl and fluorescent tracer tests, discharge measurements. This multidisciplinary approach, necessary due to the complex geological setting, is required for the importance of the Montellina Spring. It is interesting in the hydrogeological context of Western Alps for its high discharge, relatively constant over time (average 150 l/s), and for its location outside a fluvial incision and suspended about 40 m above the Dora Baltea valley floor (Lasagna et al. 2013). According to the geological setting, the hydrogeological reconstruction of the area suggests that the large amount of groundwater in the basin is essentially favoured by a highly fractured bedrock, covered by wide and thick bodies of glacial and gravitational sediments. The emergence of the water along the slope, in the Montellina Spring, is essentially due to a change of permeability between the deep bedrock and the shallow bedrock and/or surficial sediments. The deep bedrock, showing closed fractures and/or fractures filled by glacial deposits, is slightly permeable. The shallow bedrock, strongly loosened as result of gravitational phenomena, and the local gravitational sediments are, on the contrary, highly permeable. The concentration of water at the spring is due to several reasons. a) The spring is immediately downward a detachment niche, dipping towards the spring, that essentially drains the water connected to the change of permeability in the bedrock. b) It is along an important fracture, that carries a part of the losses of the Renanchio Torrent. c) Finally, it is favored by the visible and buried morphology. Although it is located along a ridge, the spring occurs in a small depression between a moraine and a landslide body. It also can be favored by the likely concave trend of buried base of the landslide. At last, tracer tests of the Renanchio Torrent water with fluorescent tracer are performed, with a continuous monitoring in the Montellina Spring. The surveys permit to verify and quantify the spring and torrent hydrogeological relationship, suggesting that only a small fraction of stream losses feeds the spring.

COMMENT TO: “VOLCANIC ACTIVITY FROM THE NEOGENE TO THE PRESENT EVOLUTION OF THE WESTERN MEDITERRANEAN AREA. A REVIEW” BY M. LUSTRINO

In the Discussion and Conclusions of the paper “Volcanic activity from Neogene to the present evolution of the Western Mediterranean sea. A review” (Ofioliti, 25 (2): 87- 101), by Michele Lustrino, mainly deals with the Neogene to present volcanic activity and it presents a “continuous” model from Hercynian time to present: the paper, particularly because it wants to be a review, must take in better and more complete consideration the present-day knowledge on bibliography, data and interpretations, about the Hercynian, Late Hercynian and post-Hercynian geodynamic evolution in Western and Southern Europe. The Discussion and Conclusions chapter reports, in particular, some considerations about the Hercynian and post- Hercynian geodynamic evolution that do not take in consideration present interpretations developed by recent geological, structural and petrological-geochemical investigations carried out by several authors. The presentation of some main topics of this evolution is particularly inadequate for an up-to-date review published on an international bulletin.