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>

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>

Complex kinematics in a major ductile shear zone, NW Shetland: Evidence of ductile extrusion during Caledonian transpression

2021 ◽  
Author(s):  
Timothy Armitage ◽  
Robert Holdsworth ◽  
Robin Strachan ◽  
Thomas Zach ◽  
Diana Alvarez-Ruiz ◽  
...  
Keyword(s):  
Shear Zone ◽  
Hanging Wall ◽  
Regional Scale ◽  
Shear Zones ◽  
White Mica ◽  
Strike Slip ◽  
Amphibolite Facies ◽  
Lateral Extrusion ◽  
Shear Sense ◽  
Ductile Shear

<p>Ductile shear zones are heterogeneous areas of strain localisation which often display variation in strain geometry and combinations of coaxial and non-coaxial deformation. One such heterogeneous shear zone is the c. 2 km thick Uyea Shear Zone (USZ) in northwest Mainland Shetland (UK), which separates variably deformed Neoarchaean orthogneisses in its footwall from Neoproterozoic metasediments in its hanging wall (Fig. a). The USZ is characterised by decimetre-scale layers of dip-slip thrusting and extension, strike-slip sinistral and dextral shear senses and interleaved ultramylonitic coaxially deformed horizons. Within the zones of transition between shear sense layers, mineral lineations swing from foliation down-dip to foliation-parallel in kinematically compatible, anticlockwise/clockwise-rotations on a local and regional scale (Fig. b). Rb-Sr dating of white mica grains via laser ablation indicates a c. 440-425 Ma Caledonian age for dip-slip and strike-slip layers and an 800 Ma Neoproterozoic age for coaxial layers. Quartz opening angles and microstructures suggest an upper-greenschist to lower-amphibolite facies temperature for deformation. We propose that a Neoproterozoic, coaxial event is overprinted by Caledonian sinistral transpression under upper greenschist/lower amphibolite facies conditions. Interleaved kinematics and mineral lineation swings are attributed to result from differential flow rates resulting in vertical and lateral extrusion and indicate regional-scale sinistral transpression during the Caledonian orogeny in NW Shetland. This study highlights the importance of linking geochronology to microstructures in a poly-deformed terrane and is a rare example of a highly heterogeneous shear zone in which both vertical and lateral extrusion occurred during transpression.</p><p><img src="https://contentmanager.copernicus.org/fileStorageProxy.php?f=gepj.0cf6ef44e5ff57820599061/sdaolpUECMynit/12UGE&app=m&a=0&c=d96bb6db75eed0739f2a6ee90c9ad8fd&ct=x&pn=gepj.elif&d=1" alt=""></p>

Mapping strain in the footwall of a thrust: Preliminary results from 3D bulk fabric of illite.

2020 ◽  
Author(s):  
Charles Aubourg ◽  
Gracia-Puzo Francho ◽  
Casas-Sainz Antonio ◽  
Izquierdo-Llavall Esther ◽  
Boiron Tiphaine ◽  
...  
Keyword(s):  
Cross Sections ◽  
Hanging Wall ◽  
Magnetic Fabric ◽  
Main Thrust ◽  
Blind Thrust ◽  
Highly Sensitive ◽  
The Matrix ◽  
Eigen Values ◽  
Degree Of Anisotropy ◽  
Non Destructive

<p>The Sigues fold (Aragon, Spain) presents an exceptional outcrop where 1) the footwall is largely exposed, 2) it is constituted of homogenous shales, 3) the strain varies at distance from the emergent thrust, with all steps of cleavage development. The best model to explain the strain distribution is the trishear propagation of a thrust with a P/S ratio of 1.  However, from East to West, the thrust geometry is changing progressively from blind thrust to flat ramp. The topographic surface as well as the position of the emerging part of the thrust determine the geometry of the structure. This is, therefore, a place with variable geometries, which allow us to describe the different geometric stages of the ramp-and-flat model that we are used to find in major orogenic thrusts.</p><p>To map the strain, we measured the magnetic fabric of hundreds of shale fragments (weighting a few grams) in dozens of localities. The magnetic fabric is governed primarily by illite. Hence, the magnetic fabric represents a 3D view of illite organization, i.e. the matrix of those shales (see Gracia-Puzo et al., EGU, EMRP3.8). The measurement of 3D fabric of illite takes about a minute per fragment and is non-destructive.</p><p>Magnetic fabric of shale fragments provides three useful parameters, the degree of anisotropy, the shape parameter from oblate to prolate, and the length of the confidence angle of the minimum axis of tensor. We show that all these three parameters are highly sensitive to strain. While each locality provides homogenous results from ~15 fragments (covering few square meters each), it is statistically different from one site to the other, with trends consistent with distance to the main thrust. Assuming rigid rotation of illite particles, we calculate the strain using Eigen values of magnetic fabric tensor.</p><p>Our preliminary maps shows: 1) that the strain increases considerably (from units to tens in %) when approaching the main thrust, 2) at a distance of more than 1 km, several strain gradients are detected, suggesting that blind thrusts propagate in the footwall. Serial N-S cross-sections are expected to describe the lateral variability on the structure, the deformation accumulated on the footwall and also establishing the portion of the hanging wall which is being affected and the décollement of the thrust. Our approach is thus promising to map strain in shales from deformed regions, both from natural outcrops, or from boreholes.</p>

A reinterpretation of the Snow Lake gold camp, Trans-Hudson Orogen, Canada: The use of cleavages as markers to correlate structures across deformed terranes

2020 ◽  
Author(s):  
Kate Elizabeth Rubingh ◽  
Bruno Lafrance ◽  
Harold L. Gibson
Keyword(s):  
Boundary Conditions ◽  
Time Lag ◽  
Volcanic Rocks ◽  
Shear Zones ◽  
Thrust Faults ◽  
Volcanic Stratigraphy ◽  
Shear Sense ◽  
Flin Flon ◽  
Shear Sense Indicators ◽  
Superior Craton

The Snow Lake gold camp is located within amphibolite facies volcanic rocks of the ca. 1.88 – 1.87 Ga Flin Flon-Glennie Complex (FFGC) in the Trans-Hudson Orogen, Manitoba. During thrusting and collision with the Archean Sask craton, volcanic rocks were interleaved with turbidites of the ca. 1.855 - 1.84 Ga Burntwood Group and sandstone and conglomerate of the ca. 1.845 - 1.835 Ga Missi Group. The main cleavage in the turbidites was previously attributed to thrusting and used as a marker for correlating structures across the camp. A re-examination of this cleavage suggests that it overprints the thrust faults and formed during later collision between the FFGC and the Archean Superior craton. This has important implications as it further suggests that (1) previously unrecognized, early brittle thrust faults repeat volcanic stratigraphy and may have created the boundary conditions that enabled the formation of ductile thrust faults, fold nappes, and mega sheath folds; (2) shear sense indicators along ductile thrust faults formed during their reactivation as sinistral shear zones rather than during thrusting; and (3) peak metamorphic conditions were caused by thrusting and stacking during collision with the Sask craton but were attained later during collision with the Superior craton due to the time lag between orogenesis and the re-equilibration of regional isotherms. Results from this study may be applicable to other complexly deformed terranes where the dominant regional cleavage differs in expression in mixed volcanic and sedimentary successions and has been used as a marker for correlating structures.

The limit of the contemporary financial system in Russia

2020 ◽  
Vol 16 (7) ◽  
pp. 1223-1245
Author(s):  
V.V. Smirnov
Keyword(s):  
Cluster Analysis ◽  
Foreign Currency ◽  
Financial System ◽  
Descriptive Statistics ◽  
High Tech ◽  
Monetary Aggregate ◽  
Internal System ◽  
Monetary Relations ◽  
And Cluster Analysis ◽  
Structural Preservation

Subject. The article focuses on the modern financial system of Russia. Objectives. I determine the limit of the contemporary financial system in Russia. Methods. The study is based on methods of descriptive statistics, statistical and cluster analysis. Results. The article shows the possibility of determining the scope of the contemporary financial system in Russia by establishing monetary relations as the order of the internal system and concerted operation of subsystems, preserving the structure of the financial system, maintaining the operational regime, implementing the program and achieving the goal. I found that the Russian financial system correlated with the Angolan one, and the real scope of the contemporary financial system in Russia. Conclusions and Relevance. As an attempt to effectively establish monetary relations and manage them, the limit of the contemporary financial system is related to the possibility of using Monetary Aggregate M0 to maintain the balance of the Central Bank of Russia. To overcome the scope of Russia’s financial system, the economy should have changed its specialization, refocusing it on high-tech export and increasing the foreign currency reserves. This can be done if amendments to Russia’s Constitution are adopted. The findings expand the scope of knowledge and create new competence in the establishment of monetary relations, order of the internal system and concerted interaction of subsystems, structural preservation of the financial system and maintenance of its operational regime.

Zircon geochronology and paleomagnetism of an Archean harzburgite intrusion, eastern Bighorn Mountains, Wyoming

2020 ◽  
Vol 57 (1) ◽  
pp. 21-40
Author(s):  
Alexandra Wallenberg ◽  
Michelle Dafov ◽  
David Malone ◽  
John Craddock
Keyword(s):  
Hanging Wall ◽  
Vertical Axis ◽  
Shear Zones ◽  
Strike Slip ◽  
Zircon Geochronology ◽  
Weighted Mean ◽  
Mafic Complex ◽  
Laramide Orogeny ◽  
Axis Rotation ◽  
Bighorn Mountains

A harzburgite intrusion, which is part of the trailside mafic complex) intrudes ~2900-2950 Ma gneisses in the hanging wall of the Laramide Bighorn uplift west of Buffalo, Wyoming. The harzburgite is composed of pristine orthopyroxene (bronzite), clinopyroxene, serpentine after olivine and accessory magnetite-serpentinite seams, and strike-slip striated shear zones. The harzburgite is crosscut by a hydrothermally altered wehrlite dike (N20°E, 90°, 1 meter wide) with no zircons recovered. Zircons from the harzburgite reveal two ages: 1) a younger set that has a concordia upper intercept age of 2908±6 Ma and a weighted mean age of 2909.5±6.1 Ma; and 2) an older set that has a concordia upper intercept age of 2934.1±8.9 Ma and a weighted mean age 2940.5±5.8 Ma. Anisotropy of magnetic susceptibility (AMS) was used as a proxy for magmatic intrusion and the harzburgite preserves a sub-horizontal Kmax fabric (n=18) suggesting lateral intrusion. Alternating Field (AF) demagnetization for the harzburgite yielded a paleopole of 177.7 longitude, -14.4 latitude. The AF paleopole for the wehrlite dike has a vertical (90°) inclination suggesting intrusion at high latitude. The wehrlite dike preserves a Kmax fabric (n=19) that plots along the great circle of the dike and is difficult to interpret. The harzburgite has a two-component magnetization preserved that indicates a younger Cretaceous chemical overprint that may indicate a 90° clockwise vertical axis rotation of the Clear Creek thrust hanging wall, a range-bounding east-directed thrust fault that accommodated uplift of Bighorn Mountains during the Eocene Laramide Orogeny.

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