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Mamtani M.A.,Indian Institute of Technology Kharagpur | Piazolo S.,University of Stockholm | Piazolo S.,Macquarie University | Greiling R.O.,Karlsruhe Institute of Technology | And 3 more authors.
Earth and Planetary Science Letters | Year: 2011

This study evaluates the fabric defined by magnetite grains in a syntectonically deformed granite and deciphers the processes that led to magnetite fabric development. Anisotropy of anhysteretic remanence magnetization (AARM) analysis is performed in samples taken from different parts of the granite to establish that the magnetite grains define a fabric. Along with microstructural studies, the AARM data help conclude that this fabric is on account of shape preferred orientation (SPO) of the magnetite grains. The intensity of magnetite fabric (degree of anisotropy of the AARM ellipsoid) is higher in the southern parts as compared to the north, which is inferred to indicate a strain gradient. Electron back scattered diffraction (EBSD) analyses of magnetite grains were performed to determine if there are intracrystalline deformation features that could have influenced magnetite shape and SPO, and thus AARM data. Detailed crystallographic orientation data coupled with orientation contrast imaging did not reveal any subgrains and/or significant variations in crystallographic orientations within magnetite grains. Instead, grains exhibit fractures and are in places associated with quartz pressure fringes. Hence, neither the SPO nor the variation in the magnetite fabric intensity in the granite can be attributed to intracrystalline deformation of magnetite by dislocation creep. It is concluded that the magnetite grains were rheologically rigid and there was relative movement between the magnetite and the matrix minerals (quartz, feldspar and biotite). These matrix minerals actually define the fabric attractor and the magnetite grains passively rotated to align with it. Thus it is demonstrated that the magnetite fabric in the granite stems from rigid body movement rather than dislocation creep. © 2011 Elsevier B.V.

Hrouda F.,AGICO Inc. | Hrouda F.,Charles University | Faryad S.W.,Charles University | Chlupacova M.,AGICO Inc. | Jerabek P.,Charles University
Tectonophysics | Year: 2014

The largest body of metabasic rocks in the Bohemian Massif, the Mariánské Lázně Complex (MLC), represents an Early Paleozoic ophiolite complex, which experienced poly-metamorphic evolution with an early high-pressure amphibolite, followed by eclogite and subsequent amphibolite facies overprint. This study aims to investigate the fabric relationships among the rock bodies and to analyze the deformation-recrystallization history of the MLC using the anisotropy of magnetic susceptibility (AMS).Magnetic susceptibility of the MLC rocks varies according to the presence of para- and ferromagnetic minerals as well as according to the primary lithologies. In weakly magnetic rocks (Km<1×10-3 [SI]), the main carrier of susceptibility is paramagnetic mafic silicates, amphibole and biotite in amphibolites, clinopyroxene and garnet in eclogites, olivine and serpentine minerals in serpentinized ultramafite, and amphibole in gabbro. In highly magnetic rocks (Km≫1×10-3), ferromagnetic minerals, such as magnetite, titanomagnetite, general Fe-Ti oxides, and subordinately also pyrrhotite can play an important role.The results of AMS indicate that the orientation of magnetic minerals in most rocks is related to the last exhumation and retrogression event that occurred in amphibolite facies conditions. However, there are some well preserved eclogites possessing the magnetic fabrics that are at least partially related to prograde or peak PT conditions during metamorphism. In addition, massive metagabbros that suffered only weak deformation partially preserve the intrusive magnetic fabrics. © 2014 Elsevier B.V.

Hrouda F.,AGICO Inc. | Hrouda F.,Charles University | Jezek J.,Charles University
Tectonophysics | Year: 2014

In some geological processes, new very fine-grained magnetic minerals may originate. The variation in content of these minerals is routinely investigated by frequency-dependent magnetic susceptibility, which is traditionally interpreted in terms of the presence of viscous superparamagnetic (SP) particles in addition to stable single domain (SSD) and multidomain (MD) magnetic particles. In addition, the fabric of these grains can be investigated through the frequency-dependent AMS. Through standard AMS measurement at different frequencies, one can evaluate the contribution of SP particles to the whole-rock AMS; appropriate methods were developed. Various rocks, soils and ceramics, showing frequency-dependent magnetic susceptibility, were investigated. Measurable changes of AMS with operating frequency were revealed and attempts are made of their fabric interpretation. © 2014 Elsevier B.V.

Hrouda F.,AGICO Ltd. | Hrouda F.,Charles University
Geophysical Journal International | Year: 2011

Mathematical models of the frequency-dependent susceptibility in rocks, soils and environmental materials have been adapted to measurements performed with multiple operating frequencies (465, 976, 3904, 4650, 15 616, 100 000 and 250 000 Hz) on the basis of log-normal volume distribution of magnetic particles. The XFD parameter depends, in addition to the amount of SP particles, also on the operating frequencies, whose values should be therefore also presented. The model curves of the XFD parameter versus arithmetical mean (μ) of the logarithms of grain volume are roughly bell-like shaped. The width and peak position of these curves is controlled by mean and standard deviation of the logarithmic volume distribution. Magnetic susceptibility contributions from paramagnetic minerals, and from ferrimagnetic particles not belonging to a unimodal SP/SD volume distribution, tend to decrease the XFD parameter. Therefore, low XFD values do not therefore necessarily indicate low amount of SP particles, but can also be indicative of the presence of the paramagnetic fraction. A new parameter XR is introduced based on susceptibility measurements at three operating frequencies; it is insensitive to dia- and paramagnetic fractions and helps us to differentiate between wide and narrow size distributions of ferromagnetic particles. A new XFB parameter is introduced that originates through normalizing the XFD parameter by the difference of natural logarithms of operating frequencies and related to the decade difference between the frequencies. It is convenient for comparison of the Bartington MS-2 Susceptibility Meter data with the MFK1-FA Kappabridge data. © 2011 The Author Geophysical Journal International © 2011 RAS.

Hajna J.,Charles University | Zak J.,Charles University | Zak J.,Czech Geological Survey | Kachlik V.,Charles University | And 2 more authors.
Precambrian Research | Year: 2010

The Teplá-Barrandian unit (TBU) of Central Europe's Bohemian Massif exposes perhaps the best preserved fragment of an accretionary wedge in the Avalonian-Cadomian belt, which developed along the northern active margin of Gondwana during Late Neoproterozoic. In the central TBU, three NE-SW-trending lithotectonic units (Domains 1-3) separated by antithetic brittle faults differ in lithology, style and intensity of deformation, magnetic fabric (AMS), and degree of Cadomian regional metamorphism. The flysch-like Domain 1 to the NW is the most outboard (trenchward) unit which has never been significantly buried and experienced only weak deformation and folding. The central, mélange-like Domain 2 is characterized by heterogenous intense deformation developed under lower greenschist facies conditions, and was thrust NW over Domain 1 along a SE-dipping fault. To the SE, the most inboard (arcward) Domain 3 is lithologically monotonous (dominated by graywackes and slates), was buried to depths corresponding up to the lower greenschist facies conditions, where it was overprinted by a pervasive SE-dipping cleavage and then was exhumed along a major NW-dipping normal fault. We interpret these domains to represent allochtonous tectonic slices that were differentially buried and then exhumed from various depths within the accretionary wedge during Cadomian subduction. The NW-directed thrusting of Domain 2 over Domain 1 may have been caused by accretion at the wedge front, whereas the SE-dipping cleavage and SE-side-up exhumation of Domain 3 may record inclined pervasive shortening during tectonic underplating and subsequent horizontal extension of the rear of the wedge. The boundary faults were later reactivated during Cambro-Ordovician extension and Variscan compression. Compared to related terranes of the Cadomian belt, the TBU lacks exposed continental basement, evidence for regional strike-slip shearing, and extensive backarc magmatism and LP-HT metamorphism, which could be interpreted to reflect flat-slab Cadomian subduction. This, in turn, suggests that Cadomian accretionary wedges developed in a manner identical to those of modern settings, elevating the TBU to a key position for understanding the style, kinematics, and timing of accretionary processes along the Avalonian-Cadomian belt. © 2009 Elsevier B.V. All rights reserved.

Measurement of Anisotropy of Magnetic Susceptibility (AMS) has become an important tool for Structural Geological analysis in the past few decades. In the past, AMS data have been used for petrofabric analysis of deformed rocks as well as for gauging strain. However, the AMS of some rocks can be carried by both ferromagnetic and paramagnetic minerals. Separating effects of these mineral groups on the rock's AMS is difficult because of expensive and commercially less available instrumentation. On the other hand, instrumentation is available in most rock magnetic and palaeomagnetic laboratories for resolving bulk susceptibility into ferromagnetic and paramagnetic components. Mathematical modelling was made of the relationship between bulk susceptibility and AMS. If the contribution of the ferromagnetic or the paramagnetic fraction, to the rock susceptibility is dominant (let us say higher than 80%), the resultant AMS is relatively near to the AMS of the dominating fraction in all aspects, the degree of AMS, shape parameter and orientation of principal susceptibilities. In the interpretation of the AMS of rocks with dominating one fraction, the resolution of the AMS into paramagnetic and ferromagnetic components is not necessary, the resolution of bulk susceptibility into components is sufficient that can be made using the instrumentation available in most rock magnetic and palaeomagnetic laboratories. © GEOL. SOC. INDIA.

Hrouda F.,AGICO Inc. | Hrouda F.,Charles University | Faryad S.W.,Charles University | Franek J.,Czech Geological Survey | Chlupacova M.,AGICO Inc.
Gondwana Research | Year: 2013

In the high-grade Moldanubian Zone of the European Variscides, numerous bodies of ultramafic rocks occur embedded in granulite. The anisotropy of magnetic susceptibility and its low-field variation as well as the anisotropy of magnetic remanence were used to investigate magnetic fabrics of some ultramafic bodies and host granulite. In granulite, the magnetic foliation is roughly parallel to the metamorphic foliation and the magnetic lineation is near the mineral alignment lineation. In ultramafite, the magnetic foliation is relatively scattered spatially, but mostly oriented in a different way than that in granulite. The magnetic lineation is also scattered, but still relatively well defined spatially. Again, its orientation is mostly different than that of granulite. The magnetic fabric in ultramafic rocks is therefore different from that in the host granulite even though both rock types underwent at least partially common structural history. The componental movements forming the granulite fabric, mostly during amphibolite facies retrograde metamorphism, were evidently not intensive enough to strongly overprint the magnetic fabric of ultramafite. The ultramafite is therefore strong enough to maintain its pre-metamorphism fabric even at such high temperatures and pressures that are characteristic of high amphibolite facies retrograde metamorphism. © 2012 International Association for Gondwana Research.

Hrouda F.,AGICO Ltd. | Hrouda F.,Charles University | Pokorny J.,AGICO Ltd.
Studia Geophysica et Geodaetica | Year: 2011

Frequency-dependent magnetic susceptibility, which is an important tool for environmental and palaeoclimatologic research, is usually relatively low, mostly less than 15% of the low frequency value and there is a demand for high precision of its measurement. The accuracy required for the susceptibility measurement at individual operating frequencies for precise determination of the parameter characterizing the frequency-dependent susceptibility was investigated theoretically and also experimentally through repeated measurement of artificial specimens and sediments of a loess/palaeosol sequence using the MFK1 Kappabridge. It was found that the variation in the frequency-dependence in the order of 1% is well reproducible and the measurements can be interpreted in terms of magnetic granulometry even in weakly magnetic materials. © 2011 Institute of Geophysics of the ASCR, v.v.i.

Hajna J.,Charles University | Zak J.,Charles University | Kachlik V.,Charles University | Chadima M.,AGICO Inc. | Chadima M.,Academy of Sciences of the Czech Republic
International Journal of Earth Sciences | Year: 2012

The Teplá-Barrandian unit (TBU) has long been considered as a simply bivergent supracrustal 'median massif' above the Saxothuringian subduction zone in the Variscan orogenic belt. This contribution reveals a much more complex style of the Variscan tectonometamorphic overprint and resulting architecture of the Neoproterozoic basement of the TBU. For the first time, we describe the crustal-scale NE-SW-trending dextral transpressional Krakovec shear zone (KSZ) that intersects the TBU and thrusts its higher grade northwestern portion severely reworked by Variscan deformation over a southeastern very low grade portion with well-preserved Cadomian structures and only brittle Variscan deformation. The age of movements along the KSZ is inferred as Late Devonian (~380-370 Ma). On the basis of structural, microstructural, and anisotropy of magnetic susceptibility data from the KSZ, we propose a new synthetic model for the deformation partitioning in the Teplá-Barrandian upper crust in response to the Late Devonian to early Carboniferous subduction and underthrusting of the Saxothuringan lithosphere. We conclude that the Saxothuringian/Teplá-Barrandian convergence was nearly frontal during ~380-346 Ma and was partitioned into pure shear dominated domains that accommodated orogen-perpendicular shortening alternating with orogen-parallel high-strain domains that accommodated dextral transpression or bilateral extrusion. The synconvergent shortening of the TBU was terminated by a rapid gravity-driven collapse of the thickened lithosphere at ~346-337 Ma followed by, or partly simultaneous with, dextral strike-slip along the Baltica margin-parallel zones, driven by the westward movement of Gondwana from approximately 345 Ma onwards. © 2012 Springer-Verlag.

Studynka J.,AGICO Inc. | Chadima M.,AGICO Inc. | Chadima M.,Academy of Sciences of the Czech Republic | Suza P.,AGICO Inc.
Tectonophysics | Year: 2014

A 3D rotator (for MFK1-FA or MFK1-A Kappabridges) was developed to increase the speed and comfort of anisotropy of magnetic susceptibility (AMS) measurements. The 3D rotator rotates the specimen simultaneously about two axes with different velocities. The two-axis rotation enables the determination of 320 directional susceptibilities during a single anisotropy measurement. These directions are very well distributed on a sphere which makes the measuring design almost rotatable. The actual measurement is fully automated in such a way that, once the specimen is mounted into the rotator, it requires no additional manipulation to measure the full AMS tensor. The approximate duration of one anisotropy measurement is 1.5. min. Fundamental ideas of data acquisition and processing, AMS tensor fitting and respective error analysis are described and discussed. Calculation of the anisotropy tensor, respective error analysis and immediate data visualization is provided by Safyr5 software. Examples comparing the 3D rotator data with previously developed systems of AMS determination are presented. © 2014 Elsevier B.V.

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