Magnetic fabric from Quaternary volcanic edifices in the extensional Bransfield Basin: internal structure of Penguin and Bridgeman islands (South Shetlands archipelago, Antarctica)

Summary Studying the magnetic fabric in volcanic edifices, particularly lava flows from recent eruptions, allows us to understand the orientation distribution of the minerals related to the flow direction and properly characterize older and/or eroded flows. In this work, the magnetic fabric from recent (Quaternary) lava flows (slightly inclined in seven sites and plateau lavas in two sites), pyroclastic deposits (two sites from a scoria cone) and volcanic cones, domes and plugs (three sites) from Penguin and Bridgeman islands, located in the Bransfield back-arc basin, are presented. The volcanism in the two islands is related to rifting occurring due to the opening of the Bransfield Strait, between the South Shetlands archipelago and the Antarctic Peninsula. The direction of flow of magmatic material is unknown. Rock magnetic analyses, low temperature measurements and electron microscope observations (back-scattered electron imaging and Energy Dispersive X-ray analyses) reveal a Ti-poor magnetite (and maghemite) as the main carrier of the magnetic fabric. Hematite may be present in some samples. Samples from the center of the lavas reveal a magnetic lineation either parallel or imbricated with respect to the flow plane, whereas in the plateau lavas the magnetic lineation is contained within the subhorizontal plane except in vesicle-rich samples, where imbrication occurs. The magnetic lineation indicates a varied flow direction in Bridgeman Island with respect to the spreading Bransfield Basin axis. The flow direction in the plateau lavas on Penguin Island is deduced from the imbrication of the magnetic fabric in the more vesicular parts, suggesting a SE-NW flow. The volcanic domes are also imbricated with respect to an upward flow, and the bombs show scattered distribution.

Magnetic fabric in carbonatic rocks from thrust shear zones

<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>

Magnetic fabric in brittle faults and ductile shear-zones: Examples from cataclasites from the Iberian Peninsula

<p>Shear zones, or their counterparts in near-surface conditions, the brittle fault zones, constitute crustal-scale, narrow, planar domains where deformation is strongly localized. The variation with depth of deformation conditions (P-T), rheology and strain rates entails a wide range of fault rock types, characterized by different petrofabrics and classically grouped into mylonitic (fault rocks undergoing crystalline plasticity) and cataclasitic (fault rocks undergoing frictional deformation) series. Magnetic fabric methods (most frequently anisotropy of magnetic susceptibility, AMS) have been established as a useful tool to determine fault rock petrofabrics in shear/fault zones, being interpreted as kinematic indicators with a considerable degree of success. However, mylonites and cataclasites show remarkable differences in magnetic carriers, shape and orientation of the fabric ellipsoid. Here, we present a study of ten brittle fault zones (one of them at the plastic-brittle transition) located in various locations in the Iberian Plate, with an aim  to interpret patterns of AMS in cataclasites.</p><p>Reviewing AMS studies dealing with SC mylonites, three fundamental features can be drawn: i) the presence of composite magnetic fabrics with shape and lattice-preferred orientations, ii) the fabric is carried predominately by ferromagnetic minerals and iii) surprisingly in composite fabrics, the absolute predominance of magnetic lineations parallel to (shear) transport direction (88% of the reviewed sites), independently of fabrics being defined by paramagnetic or ferromagnetic carriers. Based on our study, magnetic fabrics in cataclasites: i) are mainly carried by paramagnetic minerals and ii) show a strong variability in magnetic lineation orientations, which in relation with SC deformational structures, are either parallel to transport direction (44% of sites) or parallel to the intersection lineation between shear (C) and foliation (S) planes (41%). Furthermore, changes between the two end-members can be frequently observed in the same fault zone. Sub-fabric determinations (LT-AMS; AIRM and AARM) also indicate that the type of magnetic lineation cannot be consistently related with a specific mineralogy (i.e. paramagnetic vs ferromagnetic minerals).</p><p>The wide range of deformation conditions and fault rocks covered in our study allowed us to analyse the factors that control these different magnetic lineation orientations, especially in brittle contexts. Plastic deformation results into a mineral stretching parallel to transport direction which can be directly correlated with the development of transport-parallel magnetic lineation. In brittle fault zones, the degree of shear deformation can be directly correlated with the type of magnetic lineation. The fault cores, where strain and slip are localized, show a predominance of transport-parallel magnetic lineations, most probably related with the development of lineated petrofabrics. Furthermore, the minor development of shear-related petrofabrics enhance the frequency of intersection-parallel magnetic lineations, also contributing the presence of inherited, host rock petrofabrics in the fault rocks.</p>

Deformation and magnetic fabric of the Capinha granite (Fundão, Central Portugal): ascent and emplacement mechanisms during the late-Variscan crustal thinning

<p>The Capinha area is located in the Central Iberian zone and is characterized by several Variscan granites intruded in the Neoproterozoic–Cambrian metasedimentary rocks. The main goal of the study is to identify the deformation patterns and provide crucial information to investigate the evolution of the magnetic fabrics in a post-Variscan granite emplaced during the crustal thinning, at the end of the Variscan orogeny. In order to achieve these purposes, fieldwork, petrography, microstructures and anisotropy of magnetic susceptibility (AMS) analysis were undertaken. The AMS was measured in 160 oriented cores, collected from 20 sampling sites homogeneously distributed, allowing the quantification of scalar (magnetic susceptibility, K; paramagnetic anisotropy, P<sub>para</sub>; magnetic ellipsoid shape, T) and directional data (magnetic lineation, //K<sub>1</sub>; magnetic foliation, perpendicular to K<sub>3</sub>). The Capinha granite (CG), exposed over an area of about 7 km<sup>2</sup>, is a small circular circumscribed outcrop in the NE-SW contact between the regional Belmonte–Caria granite (301.1±2.2 Ma) and the metasedimentary sequences. The CG is cut by two main fracturing systems: N30º-40ºE and N110º-120ºE, both subvertical. The contact is sharp, intrusive and discordant with the general trending of the D<sub>1</sub> and D<sub>3</sub> Variscan structures registered in the metasedimentary rocks. The CG is homogeneous in the whole area and consists of a fine- to medium-grained, muscovite-biotite leucogranite. The CG exhibit a paramagnetic behaviour with a K mean of 73 µSI, belonging to the ilmenite-type granites. At several scales, the CG does not show any magmatic flow or ductile deformation patterns displaying P<sub>para</sub> of about 1.6%, which corresponds to dominant magmatic to submagmatic microstructures. The P<sub>para</sub> highest values are concentrated in the NE border associated to prolate ellipsoids (linear fabric). Based on the interpretation of the magnetic fabric, is possible to observe that the orientation of the magnetic foliation is variable ranging from NNW-SSE to NNE-SSW. Generally, the magnetic foliations are sub-horizontal, being the vertical dips observed in the NE border, near the intersection of the N100º-120ºE and the N30º-40ºE fractures. The arrangement of the magnetic foliations follow concentric trajectories, with the symmetry axe parallel to the major axis of the outcrop (roughly NNE-SSW). The magnetic lineations are mainly sub-horizontal NNE-SSW parallel to the granite major axis; although, in the SW border the lineations tend to be parallelized to the contact. The magnetic lineation arrangement develops linear trajectories converging to the NE zone, where the dip is strong. The common gently magnetic fabric suggests the roof of the CG intrusion. During the late stages of the Variscan orogeny (D<sub>3</sub>, 321-300 Ma), ductile extensional detachments promoted the thinning of a previously thickened crust, providing the opening of pre-existing structures and the production of new ones. These structures act as conduits for a passive magma ascending and emplacement at shallow levels. Therefore, it is suggested that the CG magma ascent and emplaced in the intersection of pre-existing fractures, located in the NE zone, and flowed to the SW, developing a small asymmetric laccolith, poorly eroded, with a tongue-shaped body.</p>

ANIZOTROPIE MAGNETICKÉ SUSCEPTIBILITY HORNIN NA KONTAKTU METABAZITOVÉ A DIORITOVÉ ZÓNY BRNĚNSKÉHO MASIVU V OKOLÍ VELKÉ BABY U JINAČOVIC

An anisotropy of magnetic susceptibility and temperature dependence on magnetic susceptibility were used to reveal an evolution history along the contact of the Metabasite and Diorite zones of Brno massif north of Brno-Řečkovice. The analysis of temperature dependence of magnetic susceptibility indicated that magnetic properties of all rocks in this area are essentially controlled by magnetite with a very small contribution of pyrhotite and hematite. These minerals were formed later than the primary magmatic minerals. Therefore we assume that magnetic fabrics in studied rocks reflect deformational processes which affected these rocks. There are three patterns in anisotropy of magnetic susceptibility (AMS) in studied rocks. In the first pattern detected in diorites, the magnetic foliation is striking NE–SW, dipping to the NW and there is subvertical magnetic lineation. The second planar magnetic indicates a rotational movement of microgranite rocks along the contact of the Metabasite and Diorite zones. The last pattern found in rocks of the Metabasite zone is magnetic foliation striking NNE–SSW dipping on the NWW and magnetic lineation trending to the SW with plunge of 42° and it shows normal faulting of studied area.