Hydrothermal footprint related to regional-scale shear zone-controlled scheelite mineralization, Seridó W-skarn system, northeastern Brazil

2020 ◽  
Vol 103 ◽  
pp. 102755 ◽  
Author(s):  
R.S. Corrêa ◽  
C.G. Oliveira ◽  
E.L. Dantas ◽  
N.F. Botelho
Keyword(s):  
Shear Zone ◽  
Regional Scale ◽  
Northeastern Brazil ◽  
Scale Shear

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>

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 Monazite U–Th–Pb geochronology of the Central Metasedimentary Belt Boundary Zone (CMBbz), Grenville Province, Ontario Canada

2018 ◽  
Vol 55 (9) ◽  
pp. 1063-1078 ◽  
Author(s):  
Michelle J. Markley ◽  
Steven R. Dunn ◽  
Michael J. Jercinovic ◽  
William H. Peck ◽  
Michael L. Williams
Keyword(s):  
Shear Zone ◽  
Crustal Thickening ◽  
Boundary Zone ◽  
Grenville Province ◽  
Metamorphic History ◽  
Crustal Thinning ◽  
Tectonic Significance ◽  
History Of ◽  
Central Gneiss Belt ◽  
Scale Shear

The Central Metasedimentary Belt boundary zone (CMBbz) is a crustal-scale shear zone that juxtaposes the Central Gneiss Belt and the Central Metasedimentary Belt of the Grenville Province. Geochronological work on the timing of deformation and metamorphism in the CMBbz is ambiguous, and the questions that motivate our study are: how many episodes of shear zone activity did the CMBbz experience, and what is the tectonic significance of each episode? We present electron microprobe data from monazite (the U–Th–Pb chemical method) to directly date deformation and metamorphism recorded in five garnet–biotite gneiss samples collected from three localities of the CMBbz of Ontario (West Guilford, Fishtail Lake, and Killaloe). All three localities yield youngest monazite dates ca. 1045 Ma; most of the monazite domains that yield these dates are high-Y rims. In comparison with this common late Ottawan history, the earlier history of the three CMBbz localities is less clearly shared. The West Guilford samples have monazite grain cores that show older high-Y domains and younger low-Y domains; these cores yield a prograde early Ottawan (1100–1075 Ma) history. The Killaloe samples yield a well-defined prograde, pre- to early Shawinigan history (i.e., 1220–1160 Ma) in addition to some evidence for a second early Ottawan event. In other words, the answers to our research questions are: three events; a Shawinigan event possibly associated with crustal thickening, an Ottawan event possibly associated with another round of crustal thickening, and a late Ottawan event that resists simple interpretation in terms of metamorphic history but that coincides chronologically with crustal thinning at the base of an orogenic lid.

scholarly journals Eburnean deformation pattern of Burkina Faso and the tectonic significance of shear zones in the West African craton

2020 ◽  
Vol 191 ◽  
pp. 2 ◽  
Author(s):  
Dominique Chardon ◽  
Ousmane Bamba ◽  
Kalidou Traoré
Keyword(s):  
Burkina Faso ◽  
Shear Zone ◽  
Regional Scale ◽  
Kinematic Model ◽  
Shear Zones ◽  
West African ◽  
Deformation Pattern ◽  
The West ◽  
West African Craton ◽  
Regional Flow

Shear zones of the Paleoproterozoic Eburnean accretionary Orogen (West African craton) are investigated by means of large-scale structural mapping. Regional scale (10-100 km) mapping was based on the aeromagnetic survey of Burkina Faso and craton-scale (1000 km) mapping on a compilation of fabric data. At both scales, shear zones are arranged as an anastomosed transpressional network that accommodated distributed shortening and lateral flow of the orogenic lithosphere between the converging Kénéma-Man and Congo Archean provinces. Structural interference patterns at both scales were due to three-dimensional partitioning of progressive transpressional deformation and interactions among shear zones that absorbed heterogeneities in the regional flow patterns while maintaining the connectivity of the shear zone network. Such orogen-scale kinematic patterns call for caution in using the deformation phase approach without considering the “bigger structural picture” and interpreting displacement history of individual shear zones in terms of plate kinematics. The West African shear zone pattern is linked to that of the Guiana shield through a new transatlantic correlation to produce an integrated kinematic model of the Eburnean-Transamazonian orogen.

Sedimentary and metamorphic lithofacies of the Lower Negaunee Iron Formation, Marquette District, Michigan, USA

2013 ◽  
Vol 50 (12) ◽  
pp. 1165-1177
Author(s):  
Natalie J. Pietrzak-Renaud
Keyword(s):  
Shear Zone ◽  
Mineral Assemblage ◽  
Regional Metamorphism ◽  
Regional Scale ◽  
Open Pit ◽  
Iron Formation ◽  
Banded Iron Formation ◽  
Brittle Fractures ◽  
Granular Iron ◽  
The Republic

The base of the Proterozoic Negaunee Iron Formation is exposed in the open pit at Tilden Mine, Marquette, Michigan. Juxtaposed against the Archean-aged Palmer Gneiss, it is bounded by the regional-scale Southern Shear Zone and cut by two sets of dykes: an older chloritic and schistose set and a younger 1.1 Ga Keweenawan set. Tilden Mine is dominated by a 100 m scale plunging northwest-anticline and is cut by a growth fault locally termed the Tower Hill Fault that intersects the Southern Shear Zone. The base of the exposed iron formation is composed of three lithofacies, including lower clastics that grade into the overlying banded iron formation that in turn grades upward into granular iron formation. This succession is capped by chloritic metadiabases locally termed the Summit Hill Sill and Pillar Intrusive. Petrographic and mineral chemical investigations document primary or early diagenetic hematite, siderite and possibly ferri-hydrite, metamorphic and related hydrothermal magnetite, chlorite, late martite overgrowing earlier magnetite and growth of specularite. All three lithofacies are cut by brittle fractures and late quartz veins. Brittle fractures are coated with chlorite, carbonate minerals, fluor-apatite, and sparse Cu-sulphides. These lithofacies document initial clastic sedimentation of strained detrital quartz into a subsiding fault trough. Over time, as subsidence slowed or sea level fluctuated, clastic deposition competed with quiescent chemical sedimentation, leading to deposition of the banded iron formation facies. As a stable shelf platform emerged, the granular iron formation facies was deposited via wave reworking of hardgrounds. Subsequent diagenesis initiated dissolution of carbonate and chert and promoted diagenetic replacement of primary iron minerals and chert. Regional metamorphism during Penokean orogeny at 1875–1835 Ma produced a suite of secondary metamorphic and related hydrothermal minerals. Metamorphism and hydrothermal flux related to the 1750 Ma development of the Republic Metamorphic Node overprinted the iron formation at Tilden to greenschist facies and infilled brittle fractures with a unique mineral assemblage. This unique mineral assemblage exhibits some striking similarities to Mn, Au, and Cu-sulphides documented at Champion Mine, west of Tilden, and proximal to the core of the Republic Node.

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