1,721,004 research outputs found
Dominant simple-shear deformation during peak metamorphism for the lower portion of the Greater Himalayan Sequence in West Nepal: New implications for hybrid channel flow-type mechanisms in the Dolpo region
Full text linkI conducted new vorticity and deformation temperatures studies to test competing models of the exhumation of the mid-crustal rocks exposed in the Dolpo region (West Nepal). My results indicate that the Main Central Thrust is located ~5 km structurally below the previous mapped locations. Deformation temperature increasing up structural section from ~450 °C to ~650 °C and overlap with peak metamorphic temperature indicating that penetrative shearing was responsible for the exhumation of the GHS occurred at "close" to peak metamorphic conditions. I interpreted the telescoping and the inversion of the paleo-isotherms at the base of the GHS as produced mainly by a sub-simple shearing (Wm = 0.88-1) pervasively distributed through the lower portion of the GHS. My results are consistent with hybrid channel flow-type models where the boundary between lower and upper portions of the GHS, broadly corresponding to the tectonometamorphic discontinuity recently documented in west Nepal, represents the limit between buried material, affected by dominant simple shearing, and exhumed material affected by a general flow dominates by pure shearing. This interpretation is consistent with the recent models suggesting the simultaneous operation of channel flow- and critical wedge-type processes at different structural depth
Shear deformation in mafic-ultramafic complex in north-western Elba island (Northern Tyrrhenian Sea)
Full text linkThe Cotoncello shear zone is developed in the mafic and ultramafic rocks cropping out in the north-western Elba island (northern Apennines). The mylonitic fabrics documented in the field, are highlighted by a pervasive mylonitic foliation and by cm- to dm-thick high-strain transpressive shear zones with a top-to-the S/SE sense of shear. At the microscopic scale, the mylonitic foliation in the highstrain shear zones is made exclusively by isoriented Mg-hornblende and tremolite. In the less deformed domains, Mg-hornblende and tremolite rich-layers enveloped small domains preserving igneous fabric and/or older mylonitic fabric. Mineral assemblage on the amphibolitic-bearing mylonitic foliation indicates that the youngest mylonitic fabric occurred under amphibolites facies condition (T > 550°C, P: 0.6-0.8 GPa). © Società Geologica Italiana, Roma 2013
Airborne dispersion of asbestos fibers induced by serpentinites mining: a simulation in the Pievescola area (Tuscany, Italy).
No full textIn Italy, the environmental hazard related to asbestos is still high, also due to airborne dispersion of fibers as a consequence to mining activities into asbestos-bearing rocks, as serpentinites or basalts. The Ministerial Decree 14/05/1996 provides a method to define the hazard deriving from the exploitation of the asbestos-bearing rocks quarrying, but the proposed procedures are unclear from a geologic point of view, and not so effective for a precise assessment. We present an integrated approach based on geological, mineralogical and petrographical standard investigations on serpentinites and we test a model of airborne dispersion of fibers in order to propose more effective procedures in order to assess the asbestos-related hazard. The adopted approach consists of 5 successive steps: 1) detailed structural mapping and collection of samples representative of the outcropping rocks, 2) petrographical study of selected thin sections of serpentinites with the aim to assess the presence of fibrous minerals, 3) X-Ray powder diffraction analyses for the identification of the species of fibrous minerals, 4) determination of the Index of Release according to the Ministerial Decree 14/05/1996 in order to determine the amount of fibrous minerals released by crushing of the asbestos-bearing serpentinites and 5) modelling of the airborne dispersion of asbestos fibers by the quarrying activity based on the data collected in the previous steps. This integrated approach is based on standard geological techniques that can be applied not only by researchers but also by geological consultants and requires standard equipment that are commonly found in public and private laboratories. This approach has been simulated for an area located west of Pievescola, south of Casole d’Elsa (Tuscany), where no quarrying activity occurs or is scheduled for the future. According to the simulation performed in the Pievescola area, this integrated approach can be regarded as effective, even if liable of improvements. The implications of the adopted approach allow to identify valuable procedures to minimize the asbestos-related hazard. Particularly, the procedures adopted can be able to provide useful suggestions to make more effective the present- day legislation
Deformation during exhumation of medium- and high-grade metamorphic rocks in the Variscan chain in northern Sardinia (Italy)
No full textThe Anglona and SW Gallura regions represent key places to investigate the tectonic evolution of medium- and high-grade metamorphic rocks cropping out in northern Sardinia (Italy). From south to north we distinguish two different metamorphic complexes recording similar deformation histories but different metamorphic evolution: the Medium Grade Metamorphic Complex (MGMC) and the High Grade Metamorphic Complex (HGMC). After the initial collisional stage (D1 deformation phase), both complexes were affected by three contractional deformational phases (D2, D3 and D4) followed by later extensional tectonics. The D2 deformation phase was the most significant event producing an important deformation partitioning that produced localized shearing and folding domains at the boundary between the two metamorphic complexes. We highlight the presence of two previously undocumented systems of shear belts with different kinematics but analogous orientation in the axial zone of Sardinia. They became active at the boundary between the MGMC and HGMC from the beginning of D2. They formed a transpressive regime responsible for the exhumation of the medium- and high-grade metamorphic rocks, and overall represent a change from orthogonal to orogen-parallel tectonic transport
Reply to discussion by Elter and Padovano of ‘Deformation during exhumation of medium- and high-grade metamorphic rocks in the Variscan chain in northern Sardinia (Italy)
No full textWe welcome the invaluable opportunity that the discussion
by Elter and Padovano offers us in order to corroborate
the conclusions of our paper and clarify some aspects of
the geodynamic setting of Sardinia in the context of the
Southern Variscan Belt. Our response will be focused on
aspects concerning (1) orthogonal collision, (2) High Grade
Metamorphic Complex (HGMC), (3) transcurrent tectonics
and (4) suture.
The paper by Carosi et al. (2009) reports many original
structural data from the central-northern portion of the
Variscan basement of Sardinia highlighting the occurrence
of sinistral and dextral shear zones. The final interpretation
also takes into account a huge amount of structural,
kinematic, petrological and geochronological data recorded
over the last ten years by the research group in Pisa across
several transects in northern Sardinia. It is worth noting that
none of the proposed relevant papers by Elter and Padovano
deal with the geology of the study area; they only deal with
the northeastern portion of the HGMC, or other portions of
the Southern Variscan Belt.
In their discussion of our contribution, Elter and Padovano
consider two contributions by Corsini and Rolland (2009)
and Padovano et al. (2009), both published after our paper
was accepted in September 2008. The first was published
online on 4 February 2009 and the second is an abstract
published in September 2009. Obviously, we could not
consider these two contributions when writing our own, but
with the aim of gaining a better knowledge of Variscan
geological history it is useful to discuss some of the
criticisms raised
Structural evolution of the Tuscan Nappe in the southern sector of the Apuan Alps metamorphic dome (Northern Apennines, Italy)
No full textStructural analysis carried out in the Tuscan Nappe (TN) in the southeastern sector of the Apuan Alps highlights a structural evolution much more complex than that proposed so far. The TN has been deformed by structures developed during four deformation phases. The three early phases resulted from a compressive tectonic regime linked to the construction of the Apenninic fold-and-thrust-belt. The fourth phase, instead, is connected with the extensional tectonics, probably related to the collapse of the belt and/or to the opening of the Tyrrhenian Sea. Our structural and field data suggest the following. (1) The first phase is linked to the main crustal shortening and deformation of the Tuscan Nappe in the internal sectors of the belt. (2) The second deformation phase is responsible for the prominent NWSE-trending folds recognized in the study area (Mt. Pescaglino and Pescaglia antiforms and Mt. Piglione and Mt. Prana synforms). (3) The direction of shortening related to the third phase is parallel to the main structural trend of the belt. (4) The interference between the third folding phase and the earlier two tectonic phases could be related to the development of the metamorphic domes. The two directions of horizontal shortening induced buckling and vertical growth of the metamorphic domes, enhancing the process of exhumation of the metamorphic rocks. (5) The exhumation of the Tuscan Nappe occurred mostly in a compressive tectonic setting. A new model fo r the exhumation of the metamorphic dome of the Apuan Alps is proposed. Its tectonic evolution does not fit with the previous ly suggested core complex model, but is due to compressive tectonics
EXCURSION IN THE VARISCAN BASEMENT OF NORTHERN SARDINIA (ITALY) FIELD GUIDE
No full textThe Variscan basement in Sardinia is an almost complete section across the
South Variscan belt showing the transition from the low- up to the medium-high-grade
basement ( Figure 1 ). The basement shows beautiful expositions of folded, sheared and
metamorphosed Paleozoic rocks poorly affected by Alpine tectonics. From south to north
the effect of the Variscan collision and the subsequent exhumation can be observed at
all scales.
The aim of the field trip is to show the effects of progressive deformation and metamorphism along three transects in the Variscan belt in Northern Sardinia. The proposed
transects offer very good exposures of Palaeozoic rocks along the western coast of the Nurra peninsula, in the Regional Park of the Asinara Island, in the Anglona and South- Western Gallura regions and in the Baronie area ( Figure 2 )
The intra-pontide suture zone, northern turkey: an updated view
No full textThe tectonic setting of Turkey can be described as a puzzle where several continental terranes are boubded by ophiolite-bearing suture zones (Sengör and Yılmaz, 1981). Among these suture zones, the Intra-Pontide Suture Zone (IPSZ) is probably the less known one. The IPSZ is an east-west trending, suture zone that stretches for about 600 km dividing the Istanbul-Zonguldak terrane, in the north, from the Sakarya terrane, in the south. This suture zone is well exposed along the Daday-Arac-Kursunlu and Tosya – Emirköy geotraverses in northern-central Turkey where its tectonic features can be fully reconstructed.
The study of the two geotraverses indicates that the imbricate stack of the IPSZ consists of several distinct tectonic units, all characterized by the occurrence of mafic rocks. These units include the Aylı Dağ ophiolite Unit, the Arkot Dağ Mèlange and three metamorphic units (the Daday, Domuz Dağ and Devrekani Units). This imbricate stack is probably the result of several episodes of out-of-sequence thrusts that affected the whole IPSZ.
The Aylı Dağ Unit (Göncüoglu et al., 2012) includes an about 5 km-thick, not metamorphic ophiolite sequence topped by the middle Bathonian to early Callovian radiolarian cherts. In addition, a metamorphic sole is present at the base of the serpentinized peridotites. The geochemical evaluation of pillow-basalts and dykes highlights subduction-related characteristics, similar to IAT- and BABB-type lavas generated in a back-arc oceanic basin.
The Arkot Dağ Mèlange (Göncüoglu et al., 2014) consists of an assemblage of slide-blocks, with different size and lithology, enclosed in a Late Santonian sedimentary matrix. The slide-blocks also include ophiolitic lithologies, mainly represented by basalts, but gabbros and peridotites are also found. The slide-blocks of basalts display affinites to IAT- and BABB-type magmas, signifying the involvement of subduction component, whereas no MORB-like basalt have been found. The age of the radiolarian chert blocks ranges between Middle-Late Triassic (Tekin et al., 2012) to Late Cretaceous.
The Daday unit (cf. Martin Unit by Okay et al., 2013) is characterized by metasedimentary and metabasic slices deformed under low-grade blueschist metamorphic facies conditions (in the Early Cretaceous (ca. 110 Ma; Rb–Sr method on mica; Okay et al., 2012). The metasedimentary rocks include mica-bearing schists, fine-grained marbles and quartzites, which inclue detritial zircons mainly derived from an Early Carboniferous felsic magmatic source. The metabasic lithologies, on the other hand, comprise actinolite-bearing schists and Na-amphibole-bearing varieties possibly derived from basaltic and gabbroic protoliths. The metabasic rocks have a wide range of chemical compositions, displaying N-MORB-, E-MORB-, OIB- BABB- and IAT-type signatures.
The Devrekani Unit is represented by an assemblage of amphibolites, marbles and micaschists showing a metamorphic climax developed under upper amphibolite facies in the Late Jurassic time (ca. 163 Ma; mica Ar-Ar geochronology; Marroni et al., 2014). Amphibolites recorded pressures of ~0.80–0.99 GPa and temperatures of ~600°C. Comparable P-T conditions were obtained for the micaschist (P ~0.86 GPa and T ~ 640°C)(Marroni et al., 2014). The amphibolites display E-MORB-, OIB- BABB- and IAT-type signatures.
The Domuz Dağ Unit is in turn mainly represented by eclogites and amphibolites with minor micaschist, gneisses and marbles deformed under eclogite facies metamorphism (490°C and 1,7 Gpa; Okay et al., 2006) of Early Cretaceous age (ca. 105 Ma; mica Ar-Ar geochronology; Okay et al., 2006).
The three metamorphic units, referred by Okay et al (2006; 2013) as the Central Pontide Supercomplex, represent subduction-related mèlanges developed at different depths in a subduction zone during the Middle Jurassic-Early Cretaceous time span. This subduction zone probably developed since Early Jurassic in the Intra-Pontide oceanic basin (IPOB) as result of the convergence between Istanbul-Zonguldak and Sakarya continental margins. The occurrence of calc-alkaline volcanic rocks of Late Cretaceous in the sedimentary cover of Istanbul-Zonguldak terrane strongly suggests that the subduction was north-dipping. In this framework, the ophiolites from the Aylı Dağ Unit can be interpreted as the remnants of a Late Jurassic back-arc basin opened above this north-dipping subduction. As result of the continuous convergence, the back-arc basin started to close in the Late Cretaceous time leading to the obduction of the Aylı Dağ ophiolites onto the Sakarya continental margin. During the obduction, the Arkot Dağ Mèlange originated. In the Late Paleocene, the final closure of the IPOB led to development of the imbricate stack of tectonic units that characterizes the IPSZ, whose structures are sealed by the Early Eocene shallow-water deposits
Geological setting and geochemical signatures of the mafic rocks from the Intra-Pontide suture zone: implications for the geodynamic reconstruction of the Mesozoic Neotethys.
Full text linkA number of suture zones exist in Turkey, which is believed to represent the closure of Paleo and NeoTethyan oceanic basins. Regarding the development of the latter oceanic entity, namely Neotethys, the geodynamic evolution of the Intra-Pontide branch, the northernmost one of a number of oceanic basins remains enigmatic. The Intra-Pontide Suture Zone (IPSZ) in Northwest Turkey includes several tectonic units most of which are characterized by the occurrence of mafic rocks with distinct geochemical signatures. In this paper, the mafic rocks collected from four of these units (the Domuz Dağ Unit, the Saka Unit, the Daday Unit and the Arkot Dağ Mèlange) have been studied in detail along two selected transects. The Domuz Dağ Unit is characterized by amphibolites, micaschists and marbles, which have been overprinted by low-grade metamorphism.The Saka Unit is in turn represented by an assemblage of slices of amphibolites, marbles and micaschists metamorphosed under upper amphibolite facies metamorphic conditions in the Late Jurassic time. In these units, the amphibolites and their retrograded counterparts display E-MORB-, OIB- BABB- and IAT-type signatures. The Daday Unit is characterized by metasedimentary and metamafic rocks metamorphosed under blueschist to sub-greenschist facies conditions. The metamafic rocks comprise actinolite-bearing schists and Na-amphibole-bearing varieties possibly derived from basaltic and gabbroic protoliths. They have a wide range of chemical compositions, displaying N-MORB-, E-MORB-, OIB- BABB- and IAT-type signatures. The Arkot Dağ Mèlange consists of a Late Santonian assemblage of slide-blocks mainly represented by basaltic lithologies showing affinities ranging from N-MORB- and IAT- to BABB-type magmas. The geochemical signature of the studied mafic rocks indicates that the tectonic units documented along the two studied transects of the Intra-Pontide Suture Zone have been derived from a supra-subduction zone. This hypothesis corroborates the available data collected from the Aylı Dağ Ophiolite Unit cropping out in the westernmost studied transect. This finding can provide new insights for the reconstruction of the geodynamic history of the Intra-Pontide domain
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