An Overview of the Plutons Magnetic Fabric Studies in the Hoggar Shield: Evolution of the Major Shear Zones During the Pan-African

2018 ◽  
pp. 149-166 ◽  
Author(s):  
B. Henry ◽  
M. E. M. Derder ◽  
S. Maouche ◽  
O. Nouar ◽  
M. Amenna ◽  
...  
Keyword(s):  
Shear Zones ◽  
Magnetic Fabric ◽  
Pan African

scholarly journals Decratonization by rifting enables orogenic reworking and transcurrent dispersal of old terranes in NE Brazil

2021 ◽  
Vol 11 (1) ◽  
Author(s):  
Carlos E. Ganade ◽  
Roberto F. Weinberg ◽  
Fabricio A. Caxito ◽  
Leonardo B. L. Lopes ◽  
Lucas R. Tesser ◽  
...  
Keyword(s):  
Shear Zones ◽  
Boundary Region ◽  
The North ◽  
Pan African ◽  
Juvenile Material ◽  
Oceanic Basin ◽  
Borborema Province ◽  
Isotopic Model ◽  
Late Neoproterozoic

AbstractDispersion and deformation of cratonic fragments within orogens require weakening of the craton margins in a process of decratonization. The orogenic Borborema Province, in NE Brazil, is one of several Brasiliano/Pan-African late Neoproterozoic orogens that led to the amalgamation of Gondwana. A common feature of these orogens is that a period of extension and opening of narrow oceans preceded inversion and collision. For the case of the Borborema Province, the São Francisco Craton was pulled away from its other half, the Benino-Nigerian Shield, during an intermittent extension event between 1.0–0.92 and 0.9–0.82 Ga. This was followed by inversion of an embryonic and confined oceanic basin at ca. 0.60 Ga and transpressional orogeny from ca. 0.59 Ga onwards. Here we investigate the boundary region between the north São Francisco Craton and the Borborema Province and demonstrate how cratonic blocks became physically involved in the orogeny. We combine these results with a wide compilation of U–Pb and Nd-isotopic model ages to show that the Borborema Province consists of up to 65% of strongly sheared ancient rocks affiliated with the São Francisco/Benino-Nigerian Craton, separated by major transcurrent shear zones, with only ≈ 15% addition of juvenile material during the Neoproterozoic orogeny. This evolution is repeated across a number of Brasiliano/Pan-African orogens, with significant local variations, and indicate that extension weakened cratonic regions in a process of decratonization that prepared them for involvement in the orogenies, that led to the amalgamation of Gondwana.

Magnetic fabric in carbonatic rocks from thrust shear zones

2021 ◽  
Author(s):  
Sara Satolli ◽  
Claudio Robustelli Test ◽  
Elena Zanella ◽  
Dorota Staneczek ◽  
Fernando Calamita ◽  
...  
Keyword(s):  
Cluster Analysis ◽  
Pure Shear ◽  
Shear Zones ◽  
Magnetic Fabric ◽  
Structural Deformation ◽  
Axis Orientation ◽  
Main Thrust ◽  
Magnetic Lineation ◽  
Magnetic Fabrics ◽  
And Cluster Analysis

<p><strong> </strong></p><p>The aim of this study is to investigate how structural deformation in shear zones is documented by the anisotropy of magnetic susceptibility (AMS). The study area is located in the Pliocene outer thrust of the Northern Apennines, which involved Cretaceous to Neogene calcareous and marly rocks. Here, brittle-ductile tectonites show different characteristics along two differently oriented thrust ramps: the NNE-SSW-trending oblique thrust ramp is characterized by the presence of S tectonites, while the NW-SE-trending frontal ramp is characterized by the presence of SC tectonites.</p><p>Samples for AMS fabric investigation were collected on shear zones from three sectors of the belt, at different distance from the main thrust to detect possible magnetic fabric variations. The three study area are characterized by different combinations of simple and pure shear, thus different degree of non-coaxiality, which has been quantified through the vorticity number W<sub>k</sub>.</p><p>Specimens were measured with an AGICO KLY-3 Kappabridge at the CIMaN-ALP Laboratory (Italy) on 15 different directions mode. Only measurements with all three F-statistics of the anisotropy tests higher than 5 were accepted as reliable. Moreover, outliers characterized by ± 2σ difference with respect to the mean value of AMS scalar parameters were excluded from further analysis. In order to distinguish groups of specimens affected by different sedimentary or tectonic processes, we identified clusters of AMS scalar parameters; when clusters were not defined by these parameters, we applied a combination of contouring and cluster analysis on each principal axis to identify different subfabrics.</p><p>The magnetic fabric revealed straightforward correlations with structural data and specific changes of AMS axis orientation depending upon the increasing of deformation (lower vorticity number) and proximity to the main thrust. Similar evolution was detected in different deformation regimes. Overall, the magnetic fabric is more sensitive to the simple shear deformation, as the magnetic lineation tends to parallelize mostly with the computed slip vector; however in pure-shear dominated regimes, the magnetic lineation becomes parallel to the transport direction when the deformation is really intense (sites at less than 15-30 cm from the thrust plane).</p><p>The applied combination of density diagrams and cluster analysis on AMS data successfully allowed discriminating subfabrics related to different events, and shows a great potential to unravel mixed sedimentary and/or tectonic features in magnetic fabrics.</p>

Unravelling heterogeneities in magnetic fabric record of the strain: a combined AMS and ApARM data analysis applied to intraplate shear zones.

2021 ◽  
Author(s):  
Claudio Robustelli Test ◽  
Elena Zanella ◽  
Andrea Festa ◽  
Francesca Remitti
Keyword(s):  
Cluster Analysis ◽  
Hanging Wall ◽  
Shear Zones ◽  
Magnetic Fabric ◽  
Plate Boundaries ◽  
Thrust Faults ◽  
Localized Deformation ◽  
Main Thrust ◽  
Shear Sense ◽  
And Cluster Analysis

<p>Deciphering the stress and strain distribution across plate boundary shear zones is critical to understanding the physical processes involved in the nucleation of megathrust faults and its behaviour. Plate boundaries at shallow depth represent complex and highly deformed zones showing structures from both distributed and localized deformation.</p><p>As magnetic minerals are sensitive to stress regime, the investigation of the magnetic fabric has proven to be an effective tool in studying faulting processes at intraplate shear zones.</p><p>Anisotropy of magnetic susceptibility (AMS) provides insights into the preferred orientation of mineral grains and the qualitative relationships between petrofabrics and deformation intensity.</p><p>We present an approach of combined Contoured Diagram and Cluster Analysis to isolate the contribution of coexisting petrofabrics to the total AMS and evaluating the significance of magnetic fabric clusters.</p><p>Our results reveal distinct subfabrics with reasonably straightforward correlations with structural data. Specific AMS pattern may be associated to the intensity of the reworking related to tectonic shearing and the structural position within the shear zone (i.e., the proximity to the main thrust faults).</p><p>Close to the main thrust the magnetic fabric is dominantly oblate with magnetic foliation consistent to the S-C fabric and/or mélange foliation and the magnetic lineation parallel to the shear sense.</p><p>Away from the thrust faults the degree of anisotropy as well as the ellipsoids oblateness gradually diminishes. Thus, the presence of subfabrics related to previous tectonic events or less intense deformation (i.e. intersection lineation fabric) became dominant. The discrimination of subfabrics also allowed to unravel the presence of minor thrust plane and qualitatively evaluate the heterogeneous registration of strain (i.e. distributed versus localized deformation).</p><p>An abrupt change in magnetic ellipsoid shape and parameters is also observed below the basal décollements showing purely sedimentary magnetic fabric or previous deformation history with minor to absent evidences of shearing in the hanging wall.</p><p>Then, the integration with anisotropy of magnetic remanence experiments in different coercivity windows (ApARM) allow to separate the contribution of different ferromagnetic subpopulation of grains, constraining the significance of the different magnetic pattern/clusters detected through the AMS analysis.</p><p>In conclusion, our results show the potential of a combination of density diagrams and cluster analysis validated by ApARM experiments in distinguishing the superposition of deformation events, unravelling strain partitioning/concentration and thus to better understand the geodynamic evolution of subduction-accretion complexes.</p>

scholarly journals Remanence stability and magnetic fabric development in synthetic shear zones deformed at 500°C

2010 ◽  
Vol 11 (12) ◽  
pp. n/a-n/a ◽  
Author(s):  
J. L. Till ◽  
M. J. Jackson ◽  
B. M. Moskowitz
Keyword(s):  
Shear Zones ◽  
Magnetic Fabric

scholarly journals New insights into the Gondwana breakup at the Southern South America by apatite fission-track analyses

2019 ◽  
Vol 47 ◽  
pp. 1-15 ◽  
Author(s):  
Cristiane H. Gomes ◽  
Delia Almeida
Keyword(s):  
Fission Track ◽  
Shear Zones ◽  
Temperature History ◽  
Early Paleozoic ◽  
Apatite Fission Track ◽  
Pan African ◽  
History Of ◽  
Dom Feliciano Belt ◽  
Eastern Domain ◽  
Exhumation History

Abstract. Apatite fission-track (AFT) analyses, applied to Southern Brazil and Uruguay samples, was employed aiming to understand the low temperature history of the Dom Feliciano Belt Segment. The Dom Feliciano Belt formed during the Neoproterozoic to Early Paleozoic, linked to the Brasiliano/Pan-African Orogeny. Twenty-four samples were dated, and confined track lengths of twenty samples were measured. The spatial distribution of ages shows three domains with different evolution cut by shear zones and, or suture zones in the Dom Feliciano Belt. The Western Domain exhibits AFT ages > 250 Ma (Permian to Devonian) while the Eastern Domain shows AFT ages < 230 Ma (Paleogene to Triassic). In the Central Domain, the AFT ages range from ∼196 to 130 Ma (Jurassic to Early Cretaceous). The thermal modeling in the domains revealed a complex evolution, with cooling and reheating phases, and a denudation of ∼2600 m. The AFT ages clearly postdate the Gondwanide, Paraná-Etendeka and Rio Grande Cone exhumation history of the Dom Feliciano Belt.

scholarly journals Magnetic properties of pseudotachylytes, Jämtland, central Sweden

2019 ◽  
Author(s):  
Hagen Bender ◽  
Bjarne S. G. Almqvist ◽  
Amanda Bergman ◽  
Uwe Ring
Keyword(s):  
Magnetic Susceptibility ◽  
Shear Zones ◽  
Anisotropy Of Magnetic Susceptibility ◽  
Fault Zones ◽  
Small Sample ◽  
Magnetic Fabric ◽  
Normal Faults ◽  
Magnetic Fabrics ◽  
Nappe Complex ◽  
Fault Kinematics

Abstract. Nappe assembly in the Köli Nappe Complex, Jämtland, Sweden, has been associated with in- and out-of-sequence thrusting. Kinematic data from shear zones bounding the Köli Nappe Complex are compatible with this model, but direct evidence from fault zones internally subdividing the nappe complex does not exist. We studied a series of pseudotachylyte exposures in these fault zones for deciphering the role seismic faulting played in the assembly of the Caledonian nappe pile. To constrain the fault kinematics, microstructural and magnetic fabrics of pseudotachylyte in foliation-parallel fault veins have been investigated. Because the pseudotachylyte veins are thin, we focused on small (c. 0.2 cm3) samples for measuring the anisotropy of magnetic susceptibility. The results show inverse proportionality between specimen size and anisotropy of magnetic susceptibility degree, which is most likely an analytical artifact related to instrument sensitivity and small sample dimensions. This finding implies magnetic anisotropy results acquired from small specimens demand cautious interpretation. However, analysis of structural and magnetic fabric data indicates that seismic faulting occurred during exhumation into the upper crust but yield no kinematic in-formation. Structural field data suggest that seismic faulting was postdated by brittle E–W extensional deformation along steep normal faults. Therefore, it is likely that the pseudotachylytes formed late during out-of-sequence thrusting of the Köli Nappe Complex over the Seve Nappe Complex.

Magnetic fabrics in Portuguese Variscan granites: structural markers of the Variscan orogeny

2020 ◽  
Author(s):  
Helena Sant Ovaia ◽  
Ana Gonçalves ◽  
Claudia Cruz ◽  
Fernando Noronha
Keyword(s):  
Stress Field ◽  
Shear Zones ◽  
Ductile Deformation ◽  
Magnetic Fabric ◽  
Variscan Orogeny ◽  
Maximum Compression ◽  
Compression Direction ◽  
Magnetic Fabrics ◽  
Central Portugal ◽  
Magnetic Lineations

&lt;p&gt;This work focuses on the magnetic fabric of 20 variscan granitic massifs from northern and central Portugal and considers the Anisotropy of Magnetic Susceptibility (AMS) results obtained in about 750 sampling sites. In the northern and central Portugal, three main ductile deformation phases were recognized and described: D&lt;sub&gt;1&lt;/sub&gt;, D&lt;sub&gt;2&lt;/sub&gt; and D&lt;sub&gt;3&lt;/sub&gt;, being the variscan magmatism events mainly related to D&lt;sub&gt;3&lt;/sub&gt; phase. D&lt;sub&gt;3&lt;/sub&gt; produced wide amplitude folds with NW-SE subhorizontal axial plane and subvertical dextral and sinistral ductile shear zones, forming obtuse angles with the maximum compression direction, &amp;#963;1, NE-SW oriented. The post-D&lt;sub&gt;3&lt;/sub&gt; brittle phase was responsible for the development of conjugate faults (NNW-SSE, NNE-SSW and ENE-WSW), related to a N-S maximum compression. The studied granites were subdivided according to U-Pb dating, field observations and considering the chronology of their emplacement relative to the D&lt;sub&gt;3&lt;/sub&gt; phase of Variscan orogeny. Therefore, the studied granites are subdivided into: (1) syn-D&lt;sub&gt;3&lt;/sub&gt; two-mica granites, ca. 311 Ma; (2) late-D&lt;sub&gt;3&lt;/sub&gt; monzogranites, biotite-rich and two-mica granites, ca. 300 Ma; (3) post-D&lt;sub&gt;3&lt;/sub&gt; monzogranites and biotite-rich granites, ca. 299 &amp;#8211; 297 Ma. Magnetic fabric gives two types of directional data, magnetic foliations and magnetic lineations, which provide important information regarding the orientation of the magmatic flow, feeder zone location, relationship between the magma emplacement and tectonics and, also, the stress field. The data obtained for the magnetic fabric, based on AMS technique, allowed concluding: (i) syn-D&lt;sub&gt;3&lt;/sub&gt; granites show magnetic foliations and lineations consistent with the syn-D&lt;sub&gt;3&lt;/sub&gt; variscan structures ca. N110&amp;#176;-120&amp;#176;E, related to a NE-SW maximum stress field . The foliations are, mainly, subvertical (&gt; 60&amp;#186;), which may indicate a high thickness of the granitic body and deep rooting; on the other hand, the magnetic lineations exhibit variables plunges. (ii) Late-D&lt;sub&gt;3&lt;/sub&gt; granites are characterized by foliations and lineations, dominantly NNW-SSE to NNE-SSW oriented. The foliations are subvertical dips (&gt; 60&amp;#186;) and the lineations have, generally, soft plunges. (iii) Post-D&lt;sub&gt;3&lt;/sub&gt; granites have, in general, magnetic foliations and lineations associated with important regional post-D&lt;sub&gt;3&lt;/sub&gt; brittle structures, which display NNE-SSW and ENE-WSW trending. The subhorizontal fabric may suggest a small thickness of the granitic bodies. In all granite sets under study there is a dominance of weakly dipping lineations (slope &lt;60&amp;#186;), indicating that the feeding zones are deep, which supports the idea of an emplacement at high structural levels.&lt;/p&gt;&lt;p&gt;Acknowledgments: The authors thank Department of Geosciences, Environment and Spatial Planning at Faculty of Sciences of the University of Porto and the Earth Sciences Institute (Porto Pole, Project COMPETE 2020 (UID/GEO/04683/2013), reference POCI-01-0145-FEDER-007690).&lt;/p&gt;

Magnetotelluric investigation of the Precambrian crust and intraplate Cenozoic volcanism in the Gour Oumelalen area, Central Hoggar, South Algeria

2020 ◽  
Vol 223 (3) ◽  
pp. 1973-1986
Author(s):  
Zakaria Boukhalfa ◽  
Abderrezak Bouzid ◽  
Yixian Xu ◽  
Abderrahmane Bendaoud ◽  
Bo Yang ◽  
...  
Keyword(s):  
Cross Sections ◽  
Lower Crust ◽  
Broad Band ◽  
Sedimentary Basins ◽  
Shear Zones ◽  
Mobile Belt ◽  
Impedance Tensor ◽  
Pan African ◽  
Eastern Boundary ◽  
Cenozoic Volcanism

SUMMARY The Tuareg Shield was assembled by oceanic closures and horizontal movements along mega-shear zones between approximately 20 terranes during the Pan-African Orogeny (750–550 Ma). Although there is an ongoing debate about its origin, the exhumation of the Tuareg Shield is assumed to be related to Cenozoic intraplate volcanism. The Gour Oumelalen is a key region of the Tuareg Shield and is located in the northeastern part of the Egéré-Aleksod terrane, corresponding to the eastern boundary of the Archean–Palaeoproterozoic microcontinent LATEA (Central Hoggar). The eastern boundary of the study area corresponds to a Neoproterozoic suture zone separating two old microcontinents, LATEA and the Orosirian Stripe. We deployed two magnetotelluric (MT) profiles consisting of 33 broad-band MT stations and combined these with aeromagnetic data, aiming to define the crustal structure in detail. The resistivity cross-sections obtained from the 3-D inversion of full impedance tensor and tipper data from stations along the profiles, confirm the main Precambrian faults, some of which are covered by Quaternary sediments and hence, have not yet been deciphered. The cross-sections also highlight the Cretaceous–Quaternary sedimentary basins represented by low resistivities. The upper crust is typically cratonic with a high electrical resistivity. On the contrary, the lower crust shows a drastic drop in resistivity (&lt;10 Ωm). The most plausible hypothesis is that the study area corresponds to a Cretaceous rifting zone. The Cretaceous magmatic event and its related fluids and mineralization as well as the recent fluids associated with Cenozoic volcanism, are plausible causes of a very conductive lower crust. However, we cannot exclude other reasons such as: (i) a high-temperature and strongly sheared mobile belt or (ii) a contribution of inheritance involving Pan-African events that affected this former suture area.

scholarly journals Magnetic fabric evolution in ductile shear zones: examples in metagranites of the Aar Massif (Swiss Central Alps)

Terra Nova ◽  
2015 ◽  
Vol 27 (3) ◽  
pp. 184-194 ◽  
Author(s):  
Jessica L. Till ◽  
Jean-Pascal Cogné ◽  
Didier Marquer ◽  
Jean-Charles Poilvet
Keyword(s):  
Shear Zones ◽  
Magnetic Fabric ◽  
Central Alps ◽  
Ductile Shear Zones ◽  
Fabric Evolution ◽  
Aar Massif ◽  
Ductile Shear
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