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An exposed cross-section of Late Hercynian upper and intermediate continental crust in the Sila nappe (Calabria, S. Italy)
No full textThe complete Apennines orogenic cycle preserved in a transient single outcrop near San Fele, Lucania, Southern Italy
No full textOrogenic cycles may be viewed as comprising two extensional stages that are separated by a stage of contraction. This sequence has characterized the evolution of most mountain belts resulting from continental collision, and the structural signature of individual stages is recognized on a wide variety of scales, i.e. from microscopic to regional. Whereas the history of mountain belts is generally inferred from observations carried out at different sites, the entire sequence of deformation is very rarely recorded in single exposures. The double switch in tectonic regime that led to the development of the Lucanian Apennines in southern Italy, from pre-orogenic drifting through synorogenic thrusting to post-orogenic extension, is preserved in a superb metre-scale outcrop at Serra Manarella, in the vicinity of San Fele. A Late Jurassic, synsedimentary normal fault is sealed by strata affected by a Mid-Pliocene thrust-related fold. This composite structure, in turn, is truncated by a Mid-Pleistocene normal fault. Observation of these relationships represents a unique opportunity to unequivocally establish the relative chronology of deformations in the Lucanian Apennines, and may provide useful constraints for regional cross-section restoration
COEXISTENCE OF PSEUDOTACHYLYTE VEINS AND MYLONITIC ZONES AT THE BASE OF THE DEEP CRUST: AN EXAMPLE FROM THE CALABRIA (SOUTHERN ITALY)
No full textNatural examples indicate that pseudotachylytes cohexist in the same
outcrop with mylonites and ultramylonites. Pseudotachylytes intimately
associated with mylonites and ultramylonites can develop in high strain
zone close to the brittle-ductile transition (e.g. Passchier, 1982) or entirely
within the ductile regime as result of plastic instabilities (e.g. Hobbs et al.,
1986; Handy & Brun, 2004).
This study report microstructural investigations on two pseudotachylyte
veins found within the felsic granulites at the base of the ~20-25 km thick
Variscan crustal section outcropping in the Serre Massif (southern
Calabria). Felsic granulites consist of quartz, plagioclase, K-feldspar,
biotite, garnet, sillimanite and accessory minerals. Stretched minerals and
S-C composite foliations are detected in zones crystal-plastic deformation.
In places, felsic granulites exhibit an alternance of mylonitic and
ultramylonitic bands (a few millimeters thick). Pseudotachylyte fault-veins
develop along planes, which have a parallel orientation to the mylonitic
and/or ultramylonitic foliation. Locally, pseudotachylyte fault-veins occur
along the S-C composite foliations of the felsic granulite. On the other
hand, the pseudotachylyte injection-veins cross cut the mylonitic and/or
ultramylonitic bands.
Microstructural observations indicate that the felsic granulite exhibits a
strong grain-size reduction along the S-C composite foliations and near
the contact with the pseudotachylyte veins. Back scattered electron (BSE)
images, obtained by scanning electron microscopy (SEM) and
Field-Emission Gun SEM, show that in proximity of the contact with the
pseudotachylyte the garnet of the felsic granulite is fractured and shows
rims characterized by a new cristallization of very small euhedral garnet
(3-4 m). On the other hand, the ultramylonitic bands display a very
fine-grained matrix and have a dark appearance. However, the BSE
images reveal a strong penetrative foliation, which is defined by the
alignment of biotite and by the shape preferred orientation of quartz,
plagioclase and garnet. Moreover, the ultramylonitic bands are
characterized by a new crystallization of very small crystals (a few microns
in length) of sillimanite and K-feldspar, aligned along the foliation planes.
Matrix of the pseudotachylytes is microcrystalline and contains abundant
clasts (>50%) made up of quartz, plagioclase, K-feldspar, garnet and rare
biotite. Clasts in the matrix are aligned with a parallel orientation to
oblique foliation of the mylonitic granulite. At the margin of the vein,
garnet of the pseudotachylyte may occur in two habits: 1) garnet
microlites with very small sizes (3-4 m) and an idiomorph habit, which
formed by direct crystallization from the frictional melt, and 2) garnet
clasts (a few ten micrometres in size), with rims characterized by a new
crystallization of very small garnets (<2 m) and with a similar aspect to
the garnet rims of the host rock. In the vein centre, the matrix is mainly
composed of skeletal plagioclase and biotite (a few microns in lenght).
Plagioclase and biotite microlites often nucleated on rounded clasts of
quartz or plagioclase. Garnet microlites are absent in the vein centre.
These data, combined with the indications for the formation depth of the
pseudotachylytes (21-23 km) obtained by Altenberger et al. (2010),
indicate that during propagation of the seismic rupture the shear
deformation was highly heterogeneous and took place through the
development of alternating pseudotachylyte and ultramylonite, as result of
plastic instabilities.
References
Altenberger, U., Prosser, G. & Grande, A. (2010): Workshop
Physico-chemical processes in seismic faults, 11
Handy, M.R. & Brun, J.P. (2004): Earth Planet. Sc. Lett., 223, 427-441
Hobbs, B.E., Ord, A. & Teyssier, C. (1986): Pure Appl. Geophys., 124,
309-336
Passchier, C.W. (1982): J. Struct. Geol., 4, 69-7
The abundance of 55 elements and petrovolumetric models of the crust in the Sila nappe, Calabria (site 7). In SASSI F.P. The abundance of 55 elements and petrovolumetric models of the crust in 9 type areas from the crystalline basements of Italy, with some Geophysical and petrophysical data
No full textBrittle deformation of Triassic dolostones
No full textPreliminary results of an on-going project aimed at deciphering the micromechanics of brittle deformation of Triassic dolostones are here presented. Samples collected from large, trastensional high-angle faults (several 100’s m-throw) crosscutting the Mesozoic carbonates of the Neo Tethys (Campanian-Lucanian Platform) are investigated by mean of a detailed field structural survey, optical microscopy and SEM methodologies, and image analysis. The goal is to characterize in detail both composition and, mainly, texture of the cataclastic dolomitic rocks in order to assess the 3D structural architecture of the fault cores. Moreover, the present study addresses the relative timing of different mechanisms such as intergranular extensional crushing, chipping and shear fracturing, which all caused grain comminution during grain rolling and crushing. Finally, based upon the absolute thickness of key texture types recognized in the study cataclastic fault rocks, a simple model is assumed to compute the amount of overburden acting at time of deformation on the exhumed fault cores now days cropping out nearby the Basilicata-Campania border of southern Italy.
Study hand samples pertain to from well-exposed fault cores pertaining to high-angle faults oriented either NW-SE, parallel to the Apennine belt, or NE-SW, almost orthogonal to it. The faults crosscut the entire Meso-Cenozoic carbonate succession, and offset the low-angle thrust faults juxtaposing the aforementioned carbonates (fault hanging wall) against the Meso-Cenozoic, mixed carbonate-silicate Lagonegro succession (fault footwall). The amount of throw of individual high-angle faults was actually measured taking into account the displaced thrust faults as markers. Based upon large scale field mapping (1:10,000 scale) and 3D modelling of an about 100 km2 area (Move software), the NW-SE high-angle faults postdate the NE-SW ones, suggesting a different time of activity.
In the field, hand samples were selected based upon their distribution within the fault cores and, mainly, distance from main slip surfaces. Microscale analysis of 66 oriented fault rock samples shows that, mostly, the study cataclastic rocks are made up of dolomite. In a few cases, some small portions of silicate grains deriving from the Lagonegro succession (Scisti Silicei Fm.) are found. In order to characterize the main textural types, a great attention is been paid to the following parameters: 1) grain-matrix ratio; 2) grain sphericity; 3) grain roundness; 4) grain sorting. Moreover, by suing an automatic box-counting technique (Image-J software), the fractal dimension of selected samples was computed.
The preliminary results are consistent with four main textural types present within the study fault cores. The distinction is made on the basis of the grain-matrix ratio, which increases generally in the vicinity of the main slip surfaces. In particular, protocataclasites characterized by intense intergranular extensional fracturing are present adjacent to the fractured and fragmented dolomitic damage zones, whereas cataclasites and ultracataclasites in the inner portions of the fault cores. A negative correlation between grain-matrix and grain sphericity, roundness and sorting is documented for the latter rocks, which means that the more deformed ultracataclasites show more spherical and rounded smaller grains relative to cataclasites. The fourth type of texture, which is discontinuously present along main slip surfaces of the fault cores, consists on cataclasites and ultracataclasites cemented with calcite minerals.
On the basis of the aforementioned box-counting analysis, the three uncemented textural types show dissimilar D-values (fractal dimensions): D protoclasites = -1.82 (+ 1 sigma standard deviation 0.02); D cataclasites = - 1.72 (+ 0,05); D ultracataclasites = -1,56 (+ 0.04). This result shows therefore a positive correlation between D-value and fault maturity, and it might be helpful to quickly assess the degree of cataclasis in subsurface faults from analysis of cores recovered from exploration wells. Moreover, by taking into account only the cumulative thickness of the ultracataclastic bands present within the study faults, the result of the present study is consistent with the cores of the NW-SE faults being subjected to a lithostatic load about 6 times higher than those pertaining to the NE-SW faults. Ongoing 3D modelling of the study area will shed more lights on the slip distribution along individual faults to assess the relationship between ultracataclastic band thickness and fault intersection geometries
Stratigraphy and structural evolution of Lagonegro units from the Southern Apennines (Basilicata, Italy)
No full textStrain localisation driven by marble layers: the Palmi shear zone (Calabria-Peloritani terrane, Southern Italy)
No full text
Cooling and exhumation history of deep-seated and shallow level, late Hercynian granitoids from Calabria
No full textThe thermal and exhumation history of late Hercynian granitoids from
Calabria (Sila and Serre massifs) has been studied using thermobarometry
and radiometric age determinations. The uplift and erosion which
followed contractional tectonics of Tertiary age exposed in Calabria a
nearly complete section of the Hercynian crust. Field data, constrained
by igneous thermobarometrical data, have enabled us to draw simplified
crustal profiles. In both the Sila and Serre massifs, granitoids make up
the intermediate portions of the crustal sections and are stacked as
tabular intrusions for up to 13 km cumulative thickness. Shallow
granitoids are characterized by a weak fabric, mostly developed in the
magmatic stage, whereas deep-seated granitoids display a strong fabric
developed in the magmatic state and, with decreasing temperatures, in
the subsolidus state.
The intrusive bodies were emplaced at 300-290 Ma, at a time when the
Calabrian crust was undergoing extensional tectonics and crustal
thinning. The subsequent post-Hercynian evolution is recorded by Rb-Sr
dates of micas and fission track ages of zircon and apatite obtained
from granitoids emplaced at different depths. A decrease in Rb-Sr and
fission track ages is observed as depth of emplacement increases. Data
on the post-Hercynian geological evolution of Calabria were used to
model in three stages the cooling and exhumation history of deep-seated
and shallow granitoids. The first stage, in Permian to Triassic times,
was characterized by slow erosion. It was followed by a second stage of
extensional tectonics in Jurassic times. The third stage was exhumation
during the Apenninic Orogeny. The model has generated two P-T-t arrays,
one for deep-seated and the other for shallow granitoids of the Serre
massif. The T-t paths. suggest that the dates of micas, zircon and
apatite are cooling ages. They also show that deep-seated granitoids
remained at temperatures above the brittle-plastic transition for a long
time, whereas shallow granitoids cooled rapidly. Distinct P-T-t paths
explain why deep-seated and shallow granitoids display different fabric
and microstructural features. Copyright (C) 2000 John Wiley & Sons, Ltd
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