Distinguishing the Mélange-Forming Processes in Subduction-Accretion Complexes: Constraints from the Anisotropy of Magnetic Susceptibility (AMS)

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

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.

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The emplacement mode of Upper Cretaceous plutons from the southwestern part of the Sredna Gora Zone (Bulgaria): structural and AMS study

Geologica Carpathica ◽

10.2478/v10096-009-0001-8 ◽

2009 ◽

Vol 60 (1) ◽

pp. 15-33 ◽

Cited By ~ 16

Author(s):

Neven Georgiev ◽

Bernard Henry ◽

Neli Jordanova ◽

Nikolaus Froitzheim ◽

Diana Jordanova ◽

...

Keyword(s):

Magnetic Susceptibility ◽

Shear Zone ◽

Upper Cretaceous ◽

Plate Boundary ◽

Shear Zones ◽

Anisotropy Of Magnetic Susceptibility ◽

Southwestern Part ◽

Structural Investigations ◽

Plate Convergence ◽

Microgranular Enclaves

The emplacement mode of Upper Cretaceous plutons from the southwestern part of the Sredna Gora Zone (Bulgaria): structural and AMS study Several plutons located in the southwestern part of the Sredna Gora Zone — Bulgaria are examples of the Apuseni-Banat-Timok-Sredna Gora type of granites emplaced during Late Cretaceous (86-75 Ma) times. The studied intrusive bodies are spatially related to and deformed by the dextral Iskar-Yavoritsa shear zone. The deformation along the shear zone ceased at the time of emplacement of the undeformed Upper Cretaceous Gutsal pluton, which has intruded the Iskar-Yavoritsa mylonites. A clear transition from magmatic foliation to high-, moderate- and low-temperature superimposed foliation and lineation in the vicinity of the Iskar-Yavoritsa and related shear zones gives evidence for simultaneous tectonics and plutonism. Away from the shear zones, the granitoids appear macroscopically isotropic and were investigated using measurements of anisotropy of magnetic susceptibility at 113 stations. The studied samples show magnetic lineation and foliation, in agreement with the magmatic structures observed at a few sites. Typical features of the internal structure of the plutons are several sheet-like mafic bodies accompanied by swarms of mafic microgranular enclaves. Field observations indicate spatial relationships between mafic bodies and shear zones as well as mingling processes in the magma chamber which suggest simultaneous shearing and magma emplacement. Structural investigations as well as anisotropy of magnetic susceptibility (AMS) data attest to the controlling role of the NWSE trending Iskar-Yavoritsa shear zone and to the syntectonic emplacement of the plutons with deformation in both igneous rocks and their hosts. The tectonic situation may be explained by partitioning of oblique plate convergence into plate-boundary-normal thrusting in the Rhodopes and plate-boundary-parallel transcurrent shearing in the hinterland (Sredna Gora).

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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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First sedimentary record of the pre-obduction convergence in New Caledonia: formation of an Early Eocene accretionary complex in the north of Grande Terre and emplacement of the ‘Montagnes Blanches’ nappe

BSGF - Earth Sciences Bulletin ◽

10.2113/gssgfbull.182.6.479 ◽

2011 ◽

Vol 182 (6) ◽

pp. 479-491 ◽

Cited By ~ 30

Author(s):

Pierre Maurizot

Keyword(s):

Late Cretaceous ◽

New Caledonia ◽

Sedimentary Cover ◽

Planktonic Foraminifera ◽

Foreland Basin ◽

Late Eocene ◽

Passive Margin ◽

Early Eocene ◽

Accretionary Complex ◽

The North

Abstract New Caledonia lies at the northern tip of the Norfolk ridge, a continental fragment separated from the east Gondwana margin during the Late Cretaceous. Stratigraphic data for constraining the convergence that led to ophiolitic nappes being obducted over Grande Terre during the Eocene are both few and inaccurate. To try and fill this gap and determine the onset of the convergence, we investigated the lithology, sedimentology, biostratigraphy and geodynamic context of the Late Cretaceous – Palaeogene sedimentary cover-rock succession of northern New Caledonia. We were able to establish new stratigraphic correlations between the sedimentary units, which display large southwest-verging overfolds detached along a basal argillite series, and reinterpret their interrelationships. The sediments from the Cretaceous-Paleocene interval were deposited in a post-rift pelagic environment and are mainly biogenic with minimal terrigenous input. From the base up, they comprise black organic-rich sulphide-bearing argillite, black chert (silicified equivalent of the argillite), micritic with chert, and micrite rich in planktonic foraminifera. These passive-margin deposits are found regionally on the Norfolk Ridge down to New Zealand, and on the Lord Howe Rise, and were controlled primarily by regional or global environmental factors. The overlying Eocene deposits mark a change to an active-margin regime with distal calciturbidite and proximal breccia representing the earliest Paleogene flysch-type deposits in New Caledonia. The change from an extensional to a compressive regime marks the beginning of the pre-obduction convergence and can be assigned fairly accurately in the Koumac–Gomen area to the end of the Early Eocene (Late Ypresian, Biozone E7) at c 50 Ma. From this period on, the post-Late Cretaceous cover in northern New Caledonia was caught up and recycled in a southwest-verging accretionary complex ahead of which flysch was deposited in a flexural foreland basin. The system prograded southwards until the Late Eocene collisional stage, when the continental Norfolk ridge entered the convergence zone and blocked it. At this point the autochthonous and parautochthonous sedimentary cover and overlying flysch of northern New Caledonia was thrust over the younger flysch to the south to form a newly defined allochthonous unit, the ‘Montagnes Blanches’ nappe, that is systematically intercalated between the flysch and the obducted ophiolite units throughout Grande Terre.

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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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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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Anisotropy of magnetic susceptibility (AMS) of lava and volcanoclastic flows of the Stephano-Permian Cadi basin (central-eastern Pyrenees)

10.5194/egusphere-egu2020-10284 ◽

2020 ◽

Author(s):

Ana Simon-Muzas ◽

Antonio M Casas-Sainz ◽

Ruth Soto ◽

Josep Gisbert ◽

Teresa Román-Berdiel ◽

...

Keyword(s):

Magnetic Susceptibility ◽

Flow Direction ◽

Anisotropy Of Magnetic Susceptibility ◽

Lava Flows ◽

Magnetic Fabric ◽

Magnetic Mineralogy ◽

Thin Sections ◽

Andesitic Lava ◽

Magnetic Fabrics ◽

Standard Specimens

The aim of this work is to apply the anisotropy of magnetic susceptibility (AMS) to determine the primary and tectonic fabrics of lava flows and volcanoclastic materials in one of the Pyrenean Stephano-Permian basins.The Pyrenean Range is a double vergence orogen located at the northern end of the Iberian Peninsula. During Carboniferous-Early Permian times the extensional or transtensional regime dominant during the progressive dismantling of the Variscan belt resulted in the development of E-W elongated intra-mountainous basins. This process was coeval with an exceptional episode of magmatic activity, both intrusive and extrusive. The Cad&#237; basin represents a good example of these structures were Stephano-Permian rocks are aligned along an E-W continuous outcrop and reach thickness of several hundreds of meters. The stratigraphy of the study area is characterized by fluviolacustrine sediments changing laterally to volcanoclastic and pyroclastic rocks with interbedded andesitic lava flows. &#160;A total of 75 sites (733 standard specimens) were studied and analysed throughout the volcanoclastic, volcanic and intrusive materials of the Stephano-Permian outcrops in the Cad&#237; basin. Samples were drilled in the field along 5 sections with N-S or NW-SE direction in the Grey and Transition Unit. Afterwards, standard specimens were measured in a Kappabridge KLY-3 (AGICO) at the Zaragoza University to characterise the magnetic fabric. The susceptibility bridge combined with a CS-3 furnace (AGICO) was used for the temperature-dependent magnetic susceptibility curves (from 20 to 700 &#176;C) to recognize the magnetic mineralogy. In addition, textural and mineralogical recognition in thin-sections of the samples was carried out in order to recognize the relationship between magnetic and petrographic fabrics.The results shows that the bulk magnetic susceptibility of the specimens ranges between 118 and 9060&#183;10-6 SI but most of the values are bracketed between 160 to 450&#183;10-6 SI. Taking into account magnetic parameters (Km, Pj and T) there is no correlation between magnetic fabrics and magnetic mineralogy and there is a dominance of triaxial and prolate ellipsoids. Thermomagnetic curves indicate the dominance of paramagnetic behaviour in all the samples and except in one case there is a ferromagnetic contribution due to the generalised presence of magnetite.Magnetic ellipsoids can be divided into four main types depending on the orientation of the main axes and associated with the lithologic types: 1) Kmax vertical and Kint and Kmin horizontal for small intrusive bodies (no restoring); 2) Kmax horizontal or subhorizontal and Kint and Kmin included in a subvertical plane (before and after restitution) for volcanic breccias; 3) Kmin vertical (after restoring) and three directional maxima for lava flows and 4) non-defined fabric for the explosive materials (probably due to their complex depositional mechanisms). In general, a dominant E-W magnetic lineation is observed in many sites, resulting either from dominant flow direction, or to secondary processes. This is the case for some of the magnetic ellipsoids, that seems to be affected by deformation, Kmin is not normal to bedding and therefore, they do not become vertical after bedding restitution.

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