scholarly journals Polyphase evolution of a crustal-scale shear zone during progressive exhumation from ductile to brittle behaviour: a case study from Calabria, Italy

2015 ◽  
Vol 7 (1) ◽  
pp. 909-955
Author(s):  
E. Fazio ◽  
G. Ortolano ◽  
R. Cirrincione ◽  
A. Pezzino ◽  
R. Visalli
Keyword(s):  
Shear Zone ◽  
Regional Scale ◽  
Structural Evolution ◽  
Structural Features ◽  
Normal Faults ◽  
Field Variation ◽  
Axis Orientation ◽  
Progressive Development ◽  
Deformational History ◽  
Scale Shear

Abstract. Mylonitic rocks involved within a polyphase crustal-scale shear zone, cropping out in the Aspromonte Massif (Calabria, Italy), has been investigated to reveal the meso- and micro-structural evolution (from ductile- to brittle-type deformation) occurred during exhumation trajectory. A relatively small area (about 4 km2) has been selected in the central-eastern part of the massif to constrain the sequence of the structural features from the earliest ones (Hercynian in age), almost totally obliterated by a pervasive mylonitic foliation (plastic regime), up to recent ones, consisting of various sets of veins typical of semibrittle to brittle regime. The former ductile evolution was followed by a compressive thin-skinned thrusting stage developed during the Apennine phase of the Alpine Orogeny, interested by a second brittle stage, consistent with the switching from compressive to extensional tectonics. This last stage accompanied the final exhumation process causing the activation of regional scale normal faults, which partly disarticulated previous mylonitic microstructures. A suite of oriented specimens were collected and analyzed to complete the deformational history already recognized in the field. Quartz c axis orientation patterns confirm the greenschist facies conditions of the former ductile exhumation stage with a dominant top-to-NE sense of shear. Microstructural investigations highlighted the progressive development from plastic- to brittle-type structures, allowing to constrain each step of the multistage exhumation history, and to establish the relative timing of the stress field variation causing thrusting and subsequent normal faulting. Obtained results support a continue compressional exhumation of this sector since the opening of Tyrrhenian basin (10 Ma).

scholarly journals Structural features in the Miocene sediments of the Pécs-Danitzpuszta sand pit (SW Hungary)

2021 ◽  
Vol 151 (4) ◽  
pp. 411-422
Author(s):  
Krisztina Sebe
Keyword(s):  
Late Miocene ◽  
Structural Evolution ◽  
Bedding Plane ◽  
Structural Features ◽  
Normal Faults ◽  
Time Interval ◽  
Block Rotation ◽  
Lake Pannon ◽  
Basin Inversion ◽  
Upper Miocene

The Pécs-Danitzpuszta sand pit in southern Hungary exposes middle and upper Miocene (Badenian to Pannonian/Langhian to Tortonian) sediments along the mountain front fault zone of the Mecsek Mts and preserves an essential record of tectonic events during and after the early late Miocene, which are not exposed elsewhere in the region. In this paper we present structural observations recorded over 20 years of work, date the deformation events with mollusk biostratigraphy and make inferences on the structural evolution of the area. At the beginning of the time interval between 10.2–10.0 Ma, NNW–SSE (to NW–SE) extension created normal faults and negative flower structures. These show that extension-related fault activity lasted here up to the late Miocene. Shortly thereafter, still in the early part of the time interval between 10.2–10.0 Ma, N–S to NNW–SSE compression ensued and dominated the area ever since. Deformations under this stress field included reverse faulting in the Pannonian marls and sands, folding of the whole succession, with bedding-plane slip and shearingelated block rotation in the already deposited middle and upper Miocene marl layers and continuously changing bedding dips and southward thickening layers in the Pannonian sands. Lake level changes of Lake Pannon must have played a role in the formation of an angular unconformity within the sands besides compression. The compressional event can be explained by the Africa (Adria) – Europe convergence, but cannot be correlated regionally; it pre-dates basin inversion-related events reported from the region so far.

The role of regional-scale shear zones in paleogeographic reconstructions: the case study of the Variscan belt in the Mediterranean area

2021 ◽  
Author(s):  
Matteo Simonetti ◽  
Rodolfo Carosi ◽  
Chiara Montomoli ◽  
Salvatore Iaccarino
Keyword(s):  
Earth Sciences ◽  
Shear Zone ◽  
Regional Scale ◽  
Shear Zones ◽  
Mediterranean Area ◽  
Variscan Belt ◽  
Metamorphic Evolution ◽  
The Mediterranean ◽  
Scale Shear

<p>Paleogeographic reconstruction and recognition of the tectono-metamorphic evolution of ancient orogenic belt is often complex. The combination of an adequate amount of paleomagnetic, metamorphic, structural and geochronological data is necessary. Fundamental data derive from the study of regional-scale shear zones, that can be directly observed, by combining detailed field work with structural analysis, microstructural analysis and petrochronology. The Southern European Variscan Belt in the Mediterranean area was partially overprinted by the Alpine cycle (Stampfli and Kozur, 2006) and correlations are mainly based on lithological similarities. Little attention has been paid to the compatibility of structures in the dispersed fragments. A main debate is the connection among the Corsica-Sardinia Block (CSB), the Maures-Tanneron Massif (MTM) and the future Alpine External Crystalline Massifs (ECM) (Stampfli et al., 2002; Advokaat et al., 2014) and if these sectors were connected by a network of shear zones of regional extent, known as the East Variscan Shear Zone (EVSZ).</p><p>We present a multidisciplinary study of shear zones cropping out in the CSB (the Posada-Asinara shear zone; Carosi et al., 2020), in the MTM (the Cavalaire Fault; Simonetti et al., 2020a) and in the ECM (the Ferriere-Mollières and the Emosson-Berard shear zones; Simonetti et al., 2018; 2020b).</p><p>Kinematic and finite strain analysis allowed to recognize a transpressional deformation, with a major component of pure shear and a variable component of simple shear, coupled with general flattening deformation. Syn-kinematic paragenesis, microstructures and quartz c-axis fabrics revealed that shear deformation, in all the studied sectors, occurred under decreasing temperature starting from amphibolite-facies up to greenschist-facies. A systematic petrochronological study (U-Th-Pb on monazite collected in the sheared rocks) was conducted in order to constrain the timing of deformation. We obtained ages ranging between ~340 Ma and ~320 Ma. Ages of ~340-330 Ma can be interpreted as the beginning of the activity of the EVSZ along its older branches while ages of ~320 Ma, obtained in all the shear zones, demonstrate that they were all active in the same time span.</p><p>The multidisciplinary approach revealed a similar kinematics and tectono-metamorphic evolution of the studied shear zones contributing to better constrain the extension and timing the EVSZ and to strength the paleogeographic reconstructions of the Southern Variscan belt during Late Carboniferous time, with important implications on the evolution of the Mediterranean area after the Late Paleozoic. This case study demonstrates how paleogeographic reconstructions could benefit from datasets obtained from large-scale structures (i.e., shear zones) that can be directly investigated.</p><p> </p><p>Advokaat et al. (2014). Earth and Planetary Science Letters 401, 183–195</p><p> </p><p>Carosi et al. (2012). Terra Nova 24, 42–51</p><p> </p><p>Carosi and Palmeri (2002). Geological Magazine 139.</p><p> </p><p>Carosi et al. (2020). Geosciences 10, 288.</p><p> </p><p>Simonetti et al (2020a). International Journal of Earth Sciences 109, 2261–2285</p><p> </p><p>Simonetti et al. (2020b). Tectonics 39</p><p> </p><p>Simonetti et al. (2018). International Journal of Earth Sciences. 107, 2163–2189</p><p> </p><p>Stampfli and Kozur (2006). Geological Society, London, Memoirs 32, 57–82</p><p> </p><p>Stampfli et al. (2002). Journal of the Virtual Explorer 8, 77</p>

Geometrical similarity in successively developed folds and sheath folds in the basement rocks of the northwestern Indian Shield

2010 ◽  
Vol 148 (1) ◽  
pp. 171-182 ◽  
Author(s):  
DEEPAK C. SRIVASTAVA
Keyword(s):  
Shear Zone ◽  
Regional Scale ◽  
Structural Evolution ◽  
Advanced Stage ◽  
Indian Shield ◽  
Hinge Line ◽  
Bulk Strain ◽  
Geometrical Similarity ◽  
Ductile Shearing ◽  
Deformed Lineation

AbstractAn intensely deformed gneiss–migmatite terrane and a relatively undeformed granulite–granitoid terrane constitute the bulk of Precambrian basement in the northwestern Indian Shield. This article traces the structural evolution in the gneiss–migmatite terrane, where traditional methods of structural analysis are difficult to apply, and shows how successively developed folds can assume identical geometry and orientation at an advanced stage of progressive ductile shearing. The gneiss–migmatite terrane exemplifies a regional-scale ductile shear zone that preserves the history of polyphase folding and sheath folding. Geometrical similarity between individual/domain-scale sheath folds and mesoscopic/regional-scale folds implies that sheath folding is common at all scales in the gneiss–migmatite terrane. As the mylonite foliation that traces successive folds is curviplanar, the successively initiated hinge lines were curvilinear from their inception in the shear zone. At the advanced stage of ductile shearing, the hinge line curvatures were accentuated due to their rotation towards subvertically directed maximum stretching (X), and variably oriented fold axial planes were brought into approximate parallelism with the upright principal plane (XY) of the bulk strain ellipsoid. Eventually all the folds, irrespective of their relative order of development, became strongly non-cylindrical, extremely tight, isoclinal and approximately co-planar with respect to each other. It is due to the above geometrical modifications during ductile shearing that folds, irrespective of their order of development, now appear identical with respect to isoclinal geometry, axial plane orientation and hinge line curvilinearity. Evidence from the fold orientations, the deformed lineation patterns and the sheath fold geometry suggest that the shearing occurred in a general shear type of bulk strain, and NNW–SSE-directed subhorizontal compression resulted in subvertically directed stretching in the gneiss–migmatite terrane.

scholarly journals Structural Geology of the Basement Complex Rocks in Iwaraja Area, Southwestern Nigeria

2016 ◽  
Vol 58 ◽  
pp. 16-28 ◽  
Author(s):  
Blessing Adeoti ◽  
Chukwuemeka Tony Okonkwo
Keyword(s):  
Shear Zone ◽  
Structural Evolution ◽  
Structural Features ◽  
Granitic Rocks ◽  
Basement Complex ◽  
Southwestern Nigeria ◽  
Basement Rocks ◽  
Deformation Phase ◽  
Two Phases ◽  
Orogenic Belts

The geological and structural evolution assessment of the Basement Complex rocks in Iwaraja area, southwestern Nigeria showed that the basement rocks comprise quartzites and quartz-schists of the Effon Psammite Formation, quartz-mica schists, biotite schists, migmatitic gneisses, granitic gneisses, late and post-tectonic granitic rocks including pegmatites, which have been subjected to two phases of deformation. The first phase is mainly a fabric forming deformation that also led to the development of moderately plunging mineral lineation. No minor fold of this phase was recognized. The second deformation phase gave rise to the development of Iwaraja (ductile) shear zone. Mylonitic foliation is sub-vertical and steeply dipping and while the mineral lineation is sub-horizontal and shallowly plunging. Within granitic gneiss mylonites, the shear zone is characterised by early and late open to tight folds of pegmatite dykes. Adjacent the shear zone, ptygmatic folds of quartzo-feldspathic veins in gneisses and crenulated cleavages that overprinted earlier schistosity surfaces in the quartz-mica schist also characterised this deformation phase. Extensional fractures in the various lithologies generally show bimodal orientations mainly in the WNW-ESE and NW-SE directions, which are nearly perpendicular to the regional metamorphic foliations. These deformation conditions depict structural features that are associated with internal zones of orogenic belts. Within the study area, the structures document late-Precambrian deformation during the closing stages of Pan-African orogenesis.

Deformational history of an area with well-developed tectonic slides: Dalradian rocks of Horn Head, NW Irish Caledonides

1983 ◽  
Vol 73 (3) ◽  
pp. 151-171 ◽  
Author(s):  
Donald H. W. Hutton
Keyword(s):  
Shear Zone ◽  
Marginal Zone ◽  
Minor Importance ◽  
Progressive Development ◽  
Deformational History ◽  
Close Proximity ◽  
Thrust Zone ◽  
History Of ◽  
Moine Thrust Zone

ABSTRACTA structural reinterpretation of Appin Group metasediments of the Dalradian Supergroup in part of the northwestern marginal zone of the Irish Caledonides is presented. No major F1 folds have been recorded although bedding occasionally faces upwards NW in S1. The structure is dominated by a series of NW-facing F2 recumbent folds and associated D2 tectonic slides. The slides, which have thrust the overlying rocks to the NW, are generally oblique to the fold axial planes and commonly cut out major F2 fold hinges. Rotation of the hinges towards the NW-SE (Caledonian) stretching direction in areas adjacent to the slides may have contributed to fold excision: an example of a major dislocated sheath fold is described. DM3 produces major SE-verging inclined folds. Although D4, D5 and D6 are of minor importance in the area D6 intensifies towards the SE to become the principal fabric in a major NE–SW-trending sinistrai shear zone that contains the c. 400 Ma Main Donegal granite. The early major structures of this area are compared with those in Dalradian Supergroup of adjacent parts of Scotland. It is suggested that the absence of a major F1 fold (analagous to the Islay anticline) and the occurrence of D2 recumbent folds and low angle thrusts in NW Ireland reflect the close proximity of the foreland, composed of the Lewisian complex, to the NW; such a situation is analagous to the Moine thrust zone in Scotland. The progressive development of low angle structures and thrusts to the W of the Scottish mainland is attributed to a westward shallowing of the Lewisian basement between the SW Highlands of Scotland and NW Ireland.

scholarly journals Polyphase evolution of Pelagonia (northern Greece) revealed by geological and fission-track data

Solid Earth ◽  
2015 ◽  
Vol 6 (1) ◽  
pp. 285-302 ◽  
Author(s):  
F. L. Schenker ◽  
M. G. Fellin ◽  
J.-P. Burg
Keyword(s):  
Early Cretaceous ◽  
Late Cretaceous ◽  
Fission Track ◽  
Regional Scale ◽  
Normal Faults ◽  
Recent Sediment ◽  
Northern Greece ◽  
Rapid Cooling ◽  
Fission Track Ages ◽  
Pelagonian Zone

Abstract. The Pelagonian zone, situated between the External Hellenides/Cyclades to the west and the Axios/Vardar/Almopias zone (AVAZ) and the Rhodope to the east, was involved in late Early Cretaceous and in Late Cretaceous–Eocene orogenic events whose duration and extent are still controversial. This paper constrains their late thermal imprints. New and previously published zircon (ZFT) and apatite (AFT) fission-track ages show cooling below 240 °C of the metamorphic western AVAZ imbricates between 102 and 93–90 Ma, of northern Pelagonia between 86 and 68 Ma, of the eastern AVAZ at 80 Ma and of the western Rhodope at 72 Ma. At the regional scale, this heterogeneous cooling is coeval with subsidence of Late Cretaceous marine basin(s) that unconformably covered the Early Cretaceous (130–110 Ma) thrust system from 100 Ma. Thrusting resumed at 70 Ma in the AVAZ and migrated across Pelagonia to reach the External Hellenides at 40–38 Ma. Renewed thrusting in Pelagonia is attested at 68 Ma by abrupt and rapid cooling below 240 °C and erosion of the gneissic rocks. ZFT and AFT in western and eastern Pelagonia, respectively, testify at ~40 Ma to the latest thermal imprint related to thrusting. Central-eastern Pelagonia cooled rapidly and uniformly from 240 to 80 °C between 24 and 16 Ma in the footwall of a major extensional fault. Extension started even earlier, at ~33 Ma in the western AVAZ. Post-7 Ma rapid cooling is inferred from inverse modeling of AFT lengths. It occurred while E–W normal faults were cutting Pliocene-to-recent sediment.

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