Effect of lithostatic pressure and tectonic deformation on the magnetic fabric (anisotropy of magnetic susceptibility) in low-grade metamorphic rocks

The strong morphological similitude of the block-in-matrix fabric of chaotic rock units (mélanges and broken formations) makes problematic the recognition of their primary forming-processes. We present results of the comparison between magnetic fabric and mesoscale structural investigations of non-metamorphic tectonic, sedimentary, and polygenetic mélanges in the exhumed Late Cretaceous to early Eocene Ligurian accretionary complex and overlying wedge-top basin succession in the Northern Apennines (northwest Italy). Our findings show that the magnetic fabric reveals diagnostic configurations of principal anisotropy of magnetic susceptibility (AMS) axes orientation that are well comparable with the mesoscale block-in-matrix fabric of mélanges formed by different processes. Broken formations and tectonic mélanges show prolate and neutral-to-oblate ellipsoids, respectively, with magnetic fabric elements being consistent with those of the mesoscale anisotropic “structurally ordered” block-in-matrix fabric. Sedimentary mélanges show an oblate ellipsoid with a clear sedimentary magnetic fabric related to downslope gravitational emplacement. Polygenetic mélanges show the occurrence of a cumulative depositional and tectonic magnetic fabric. The comparison of field and laboratory investigations validate the analysis of magnetic features as a diagnostic tool suitable to analytically distinguish the contribution of different mélange forming-processes and their mutual superposition, and to better understand the geodynamic evolution of subduction-accretion complexes.

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Anisotropy of magnetic susceptibility as strain indicator in a fold-and-thrust belt sandbox model above décollements with frictional contrast

10.5194/egusphere-egu2020-15507 ◽

2020 ◽

Author(s):

Thorben Schöfisch ◽

Hemin Koyi ◽

Bjarne Almqvist

Keyword(s):

Magnetic Susceptibility ◽

Complex Structure ◽

Anisotropy Of Magnetic Susceptibility ◽

Magnetic Fabric ◽

Low Friction ◽

Thrust Belts ◽

Fold And Thrust Belt ◽

Analogue Models ◽

The Individual ◽

Strain Indicator

<p>Magnetic fabric is used as strain indicator to provide further insights into different tectonic settings. Applying anisotropy of magnetic susceptibility (AMS) analysis on analogue models has shown to be a useful approach to understand details of deformation. Here we use this technique on shortened sandbox models to illustrate the relationship between rotation of grains and the influence of d&#233;collement friction in fold-and-thrust belts. Layers of sand were scraped to a thickness of 2.5 cm on top of high-friction sandpaper on one side and on low-friction fibreglass on the other side of the sandbox model. After shortening the model by 26%, samples were taken at the surface and at depth for measuring AMS. During shortening, above the high-friction d&#233;collement, a stack of imbricates was formed, which shows distinct clustering of the main principal magnetic susceptibility axes (k1 &#8805; k2 &#8805; k3) around the dip of the forethrusts. In contrast, AMS data above the low-friction d&#233;collement show a more heterogeneous AMS pattern due to complex structure development with box folds and fault bending. In general, the magnetic fabric can be differentiated between the initial model fabric in the foreland and a tectonic overprint within the hinterland. The AMS analysis show that strain increases with the development of structures towards the hinterland and additionally with depth, but differs between the two frictional d&#233;collements. At the transition zone between the two different frictional environments, a deflection zone developed where the trace of thrusts change trend causing additional rotation of sand grains within this zone perpendicular to main shortening direction, as reflected by the orientation of the k1 and k3 axes. Overall, the orientation of the AMS axes and shape of anisotropy depend on the structure geometry and movement, which are determined by the friction of the individual d&#233;collement beneath. Consequently, AMS in models indicates and describes the development of structures and reflects strain above different basal friction.</p>

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A Discussion on natural strain and geological structure - The control of ductility on the deformation of pebbles and conglomerates

Philosophical Transactions of the Royal Society of London. Series A, Mathematical and Physical Sciences ◽

10.1098/rsta.1976.0072 ◽

1976 ◽

Vol 283 (1312) ◽

pp. 109-128 ◽

Cited By ~ 10

Keyword(s):

Confining Pressure ◽

Geological Structure ◽

Metamorphic Rocks ◽

Tectonic Deformation ◽

Low Grade ◽

Rock Types ◽

Gravitational Loading ◽

The Matrix ◽

Host Rocks

Pebbles are commonly used parameters for the determination of finite strain in deformed rocks. In high grade metamorphic environments, rocks probably behave as viscous fluids and a theory exists which relates the deformation experienced by a pebble to that of the host rocks. However, some deformed conglomerates are found in low grade metamorphic rocks where the assumption of viscous behaviour is unrealistic The deformation of artificial conglomerates made of geological materials, at room temperature and varying confining pressure is described. In these experiments, pebbles deform by cataclasis at surprisingly low applied loads and large finite strains are achieved. The amount of deformation experienced by pebbles of different rock types depends mainly on their yield strengths and ductility contrasts with respect to the matrix. A theoretical analysis assuming that pebble and matrix behave as workhardening Bingham materials during deformation relates the strain experienced by a pebble to that of the host rock. The results suggest that significant pebble deformation can occur during gravitational loading of sediments. An attempt is made to verify this idea by analysing the shape of pebbles in conglomerates of the Upper Witwatersrand System. At some sites the pebbles appear to have deformed during gravitational compaction while at others a tectonic deformation has been superimposed upon the pre-tectomic strain.

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ANISOTROPY OF MAGNETIC SUSCEPTIBILITY (AMS) IN VOLCANIC FORMATIONS: THEORY AND PRELIMINARY RESULTS FROM RECENT VOLCANICS OF BROADER AEGEAN.

Bulletin of the Geological Society of Greece ◽

10.12681/bgsg.16474 ◽

2004 ◽

Vol 36 (3) ◽

pp. 1308 ◽

Cited By ~ 1

Author(s):

I. Zananiri ◽

D. Kondopoulou

Keyword(s):

Magnetic Susceptibility ◽

Flow Direction ◽

Source Region ◽

Physical Property ◽

Anisotropy Of Magnetic Susceptibility ◽

Magnetic Fabric ◽

Local Flow ◽

Magma Flow ◽

Preliminary Results ◽

Scalar Properties

The anisotropy of magnetic susceptibility (AMS) is a physical property of rocks widely used in petrofabric studies and other applications. It is based on the measurement of low-field magnetic susceptibility in different directions along a sample. From this process several scalar properties arise, defining the magnitude and symmetry of the AMS ellipsoid, along with the magnetic foliation, namely the magnetic fabric. Imaging the sense of magma flow in dykes is an important task for volcanology; the magnetic fabric provides a fast and accurate way to infer this flow direction. Moreover, the AMS technique can be used in order to distinguish sills and dykes, a task that is almost impossible by using only field observations. Finally in the case of lava flows, the method can be applied to define the local flow conditions and to indicate the position of the "paleo" source region. However, this technique is quite new in Greece. Some preliminary results from volcanic formations of continental Greece and Southern Aegean are presented (Aegina, Almopia, Elatia, Gavra, Kos, Patmos, Samos, Samothraki and Santorini).

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Magnetic properties of pseudotachylytes, Jämtland, central Sweden

10.5194/se-2019-128 ◽

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.

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Application of anisotropy of magnetic susceptibility (AMS) in understanding regional deformation, fabric development and granite emplacement: examples from Indian cratons

Geological Society London Special Publications ◽

10.1144/sp489-2019-292 ◽

2019 ◽

Vol 489 (1) ◽

pp. 275-292 ◽

Cited By ~ 2

Author(s):

Manish A. Mamtani ◽

Sandeep Bhatt ◽

Virendra Rana ◽

Koushik Sen ◽

Tridib K. Mondal

Keyword(s):

Magnetic Susceptibility ◽

Anisotropy Of Magnetic Susceptibility ◽

Dharwar Craton ◽

Magnetic Fabric ◽

Magnetic Data ◽

Orientation Data ◽

Granite Emplacement ◽

Regional Deformation ◽

General Importance ◽

Working Out

AbstractIn this paper the authors review various applications of analysing fabric in granites from Indian cratons using anisotropy of magnetic susceptibility (AMS). First the general importance of AMS in identifying the internal fabric in massive granitoids devoid of visible foliations/lineations is highlighted. Subsequently, three important applications of AMS in granitoids are discussed. (a) The case of Godhra Granite (southern parts of the Aravalli Mountain Belt) is presented as an example of the robustness of AMS in working out the time relationship between emplacement/fabric development and regional deformation by integrating field, microstructural and magnetic data. (b) AMS orientation data from Chakradharpur Granitoid (eastern India) are compared with field-based information from the vicinity of the Singhbhum Shear Zone to highlight the use of AMS in kinematic analysis and vorticity quantification of syntectonic granitoids. (c) Magnetic fabric orientations from the Mulgund Granite (Dharwar Craton) are presented to document the application of AMS in recognizing superposed deformation in granitoids. Moreover, AMS data from Mulgund Granite are also compared with data from another pluton of similar age (c. 2.5 Ga) from the Dharwar Craton (Koppal Granitoid; syenitic composition). This highlights the use of AMS from granitoids of similar absolute ages in constraining the age of regional superposed deformation.

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A magnetotectonic study correlating late Archean deformation in northwestern Ontario

Canadian Journal of Earth Sciences ◽

10.1139/e94-128 ◽

1994 ◽

Vol 31 (9) ◽

pp. 1449-1460 ◽

Cited By ~ 3

Author(s):

G. J. Borradaile ◽

M. M. Kehlenbeck ◽

T. W. Werner

Keyword(s):

Magnetic Susceptibility ◽

Anisotropy Of Magnetic Susceptibility ◽

The Other ◽

Metamorphic Rocks ◽

Dike Swarm ◽

Northwestern Ontario

Late Archean lamprophyre dikes crop out on both sides of the Quetico–Shebandowan subprovince boundary. They post date F1 folds in late Archean rocks (2690 Ma) but their intrusion overlapped with the waning phase of S1 development. S1 developed to varying degrees or is absent in some dikes. Nevertheless, the dikes show a cryptic tectonic fabric in the ferromagnetic minerals revealed by anisotropy of magnetic susceptibility (AMS). This is parallel to S1 in the host metamorphic rocks. Thus, the S1 fabric-forming episode may be correlated from one subprovince to the other using the dike swarm as a chronological marker.

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Palaeomagnetic and anisotropy of magnetic susceptibility data bearing on the emplacement of the Western Granite, Isle of Rum, NW Scotland

Geological Magazine ◽

10.1017/s0016756808005827 ◽

2008 ◽

Vol 146 (3) ◽

pp. 419-436 ◽

Cited By ~ 14

Author(s):

M. S. PETRONIS ◽

B. O'DRISCOLL ◽

V. R. TROLL ◽

C. H. EMELEUS ◽

J. W. GEISSMAN

Keyword(s):

Magnetic Susceptibility ◽

Country Rock ◽

Anisotropy Of Magnetic Susceptibility ◽

Magnetic Fabric ◽

Contact Aureole ◽

Data Set ◽

The North ◽

Early Paleocene ◽

Magma Emplacement

AbstractThe Western Granite is the largest of several granitic bodies around the margin of the Rum Central Igneous Complex. We report palaeomagnetic and anisotropy of magnetic susceptibility (AMS) data that bear on the emplacement and deformation of the Western Granite. The collection includes samples from 27 sites throughout the Western Granite, five sites in adjacent feldspathic peridotite, and two sites in intermediate to mafic hybrid contact aureole rocks. Palaeomagnetic data from 22 of the 27 sites in the granite provide an in situ group mean D = 213.2°, I = −69.5°, α95 = 5.5° that is discordant to an early Paleocene reverse polarity expected field (about 184°, −66°, α95 = 4.3°). The discrepancy is eliminated by removing an inferred 15° of northwest-side-down tilting about a best fit horizontal tilt axis trending 040°. Data from the younger peridotite and hybrid rocks of the Rum Layered Suite provide an in situ group mean of D = 182.6°, I = −64.8°, α95 = 4.0°, which is statistically indistinguishable from an early Paleocene expected field, and imply no post-emplacement tilting of these rocks since remanence acquisition. The inferred tilt recorded in the Western Granite, which did not affect the younger Layered Suite, suggests that emplacement of the ultrabasic rocks resulted in roof uplift and associated tilt of the Western Granite to make space for mafic magma emplacement. Magnetic fabric magnitude and susceptibility parameters yield two subtle groupings in the Western Granite AMS data set. Group 1 data, defined by rocks from exposures to the east and south, have comparatively high bulk susceptibilities (Kmean, 29.51 × 10−3 in SI system), stronger anisotropies (Pj, 1.031) and oblate susceptibility ellipsoids. Group 2 data, from rocks in the west part of the pluton, have lower values of Kmean (15.89 × 10−3 SI) and Pj (1.014), and triaxial susceptibility ellipsoids. Magnetic lineations argue for emplacement of the granite as a tabular sheet from the south–southeast toward the north and west. Moderate to steeply outward-dipping magnetic foliations, together with deflection of the country rock bedding in the north, are consistent with doming accompanying magma emplacement.

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Deformation understanding in the Upper Paleozoic of Ventana Ranges at Southwest Gondwana Boundary

Scientific Reports ◽

10.1038/s41598-021-99087-1 ◽

2021 ◽

Vol 11 (1) ◽

Author(s):

Guadalupe Arzadún ◽

Renata Nela Tomezzoli ◽

Natalia Fortunatti ◽

Nora Noemi Cesaretti ◽

María Belén Febbo ◽

...

Keyword(s):

Magnetic Susceptibility ◽

Buenos Aires ◽

Anisotropy Of Magnetic Susceptibility ◽

Tectonic Deformation ◽

Thin Sections ◽

Polar Wander ◽

Apparent Polar Wander Path ◽

Upper Paleozoic ◽

San Rafael

AbstractAt the east of the Ventana Ranges, Buenos Aires, Argentina, outcrops the Carboniferous-Permian Pillahuincó Group (Sauce Grande, Piedra Azul, Bonete and Tunas Formation). We carried out an Anisotropy of Magnetic Susceptibility (AMS) study on Sauce Grande, Piedra Azul and Bonete Formation that displays ellipsoids with constant Kmax axes trending NW–SE, parallel to the fold axes. The Kmin axes are orientated in the NE–SW quadrants, oscillating from horizontal (base of the sequence-western) to vertical (top of the sequence-eastern) positions, showing a change from tectonic to almost sedimentary fabric. This is in concordance with the type and direction of foliation measured in petrographic thin sections which is continuous and penetrative to the base and spaced and less developed to the top. We integrated this study with previous Tunas Formation results (Permian). Similar changes in the AMS pattern (tectonic to sedimentary fabric), as well as other characteristics such as the paleo-environmental and sharp curvature in the apparent polar wander path of Gondwana, marks a new threshold in the evolution of the basin. Those changes along the Pillahuincó deposition indicate two different spasm in the tectonic deformation that according to the ages of the rocks are 300–290 Ma (Sauce Grande to Bonete Formation deposition) and 290–276 Ma (Tunas Formation deposition). This Carboniferous-Permian deformation is locally assigned to the San Rafael (Hercinian) orogenic phase, interpreted as the result of rearrangements of the microplates that collided previously with Gondwana, and latitudinal movements of Gondwana toward north and Laurentia toward south to reach the Triassic Pangea.

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