1,883 research outputs found
Structure of the Variscan metamorphic complexes in the central transect of the Posada-Asinara Line (SW Gallura region, Northern Sardinia, Italy)
No full text<p>This study represents the first detailed tectono-metamorphic map of the metamorphic complexes cropping out in the inner portion of the Variscan belt in north-central Sardinia Island (Italy). The Main Map is at a 1:30.000 scale and covers an area of <i>c.</i> 148 km<sup>2</sup>. It is based on 1: 10.000 scale classic field mapping and represents an overview of the lithological and structural complexities documented in the metasedimentary and migmatite domes cropping out along the central transect of the Posada-Asinara Line (PAL). The PAL is a crustal scale discontinuity that divides migmatites from the metasedimentary sequences affected by greenschist- to amphibolite-facies metamorphism. The map shows the orientations of the superimposed foliations, fold axes and mineral lineations on the basis of geometric crosscutting relationships and, for the first time, the location of ductile-brittle and brittle shear zones developed during the long-lived activity of the Posada-Asinara Line.</p
Structural map of the metamorphic complexes in SW Gallura region (Northern Sardinia, Italy)
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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
No full textI 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
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
Le metamorfiti dell'Anglona e della Gallura sud-occidentale (Sardegna settentrionale, Italia): Nuove implicazioni sull'evoluzione post-collisionale della catena sarda
No full textStructural 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 )
POST-COLLISIONAL TRANSPRESSIVE TECTONICS IN NORTHERN SARDINIA (ITALY)
No full textThe aim of this work is to review and synthetize the geological and structural
analysis performed in the Variscan Basement of Northern Sardinian during the last ten
years and to add new preliminary data on the Anglona–SW Gallura area. A transpressive
crustal-scale deformation (D2), is documented in the Variscan Basement of northern Sardinia. A shear deformation parallel to the belt, overprinting previous D1 structures
related to a top-to-the S and SW nappe stacking, has been recognized. The L2 stretching
lineation points to an orogen-parallel stretching and to a general change in the tectonic
transport from D1 to D2 deformation phases. D1 phase developed during initial frontal
collision whereas D2 deformation was characterized by dextral shearing. In this sector
of the Variscan belt exhumation is due to continuing compression with an increasing component of horizontal displacement developed in a regime of decreasing pressure.
The D2 transpressional deformation enhanced telescoping of the Barrovian isogrades and the exhumation of the low- to medium-grade metamorphic rocks. The overall change of the shortening direction in a large sector of an orogenic belt with the occurrence of increasing orogen-parallel displacement, may be regarded as a general mechanism affecting the exhumation of rocks and preventing the overthickened and thermally softened collisional crust from undergoing a diffused gravitational collapse. The rotation of nearly 90° of the tectonic transport in Sardinia during collisional and post-collisional stages could be related to paleoposition of the Corsica-Sardinia block, close to southeast France and northeast Spain, and to the development of the Ibero-Armorican arc
Biases in the three-dimensional vorticity analysis using porphyroclast system: limits and application to natural examples
No full textA description of the systematic errors associated with the measurement of the vorticity number from poryhroclasts in natural systems is presented and discussed. We show that strong biases and systematic errors could derive both from some erroneous physical (i.e. no slip across clast/matrix boundary, homogeneity within the matrix) as well as geometrical assumptions (i.e. the radius ratio and angular measurements carried out in two dimensions on outcrop surfaces and thin section). By comparing natural datasets of porphyroclast shape preferred orientation (SPO) with different theoretical curves plots, we suggest that at least one of the Jeffery physical assumptions can be tested when applying vorticity techniques. The comparison of different possible sources of systematic errors indicates that, for medium-to-low vorticity numbers (W m< 0.8), vorticity data are strongly biased and that a minimum systematic error of 0.2 should be taken into account. Finally, we use data from natural shear zones from the Southern Variscan Belt in Sardinia to test and discuss the starting assumptions of the Jeffery model
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