Magnetic Susceptibility Fabrics Research Articles

Anisotropy of magnetic susceptibility impressed during rock magnetic procedures (AF, IRM) and information on the domain state of the magnetic carriers

We present and interpret anisotropy of magnetic susceptibility (AMS) fabrics in various rocks, focusing on the effects of Alternating Field (AF) demagnetization and Isothermal Remanent Magnetization (IRM). Our findings reveal that AMS in samples from intrusive rocks with large multidomain magnetite grains is minimally affected by IRM or static AF demagnetization. In nearly isotropic volcanic rocks with titanomagnetite pseudo single domain (PSD) carriers, AMS fabrics caused by static AF demagnetization are easily identifiable, with the most prominent effect being a well-defined AMS lineation (up to 1.04) in the direction of the applied AF demagnetization. Conversely, in samples from rapidly cooled volcanic rocks with titanomagnetite of smaller magnetic grain size, an AMS foliation (∼1.02) is observed orthogonal to the direction of the applied AF field, instead of a lineation. In such samples, an IRM produces a much larger AMS foliation up to 1.3 orthogonal to the IRM. The IRM-impressed AMS is also particularly strong in metamorphic rocks in the greenschist facies with either titano-hematite or pyrrhotite magnetic carriers. Samples with the largest IRM-impressed fabric have very high Mrs/Ms ratio (>∼0.4). Mrs/Ms ratios above 0.5 may indicate the contribution of SD magnetic grains with multiaxial anisotropy. As the apparent multiaxial anisotropy is especially observed in volcanic rocks with micron size dendrites of titanomagnetites, the complex shape of the magnetic particles and their chemical composition likely play a key role in IRM-impressed AMS. AMS fabric in volcanic rocks should not be measured after static AF demagnetization. Tumbling AF demagnetization does not alter significantly the initial magnetic fabric and could be safely used in rocks with strong magnetization related to lightning possibly recording an IRM impressed AMS.

Vertical deformation partitioning across a collapsing large and hot orogen

AbstractFeaturing 3 000‐km‐long large and hot orogen, the Mantiqueira Province provides a rare opportunity to study the process of gravitational collapse at mid to deep crustal levels. Distinct but contemporary (~500 Ma) post‐collisional intrusions show structures and anisotropy of magnetic susceptibility (AMS) fabrics related to their emplacements, recording different flow patterns. In southern deep‐seated intrusions, ellipsoidal‐shaped roots with gabbroic‐to‐hybrid cores surrounded by granitic rocks show concentric patterns of AMS fabrics that cut across the NE‐trending regional foliation. In contrast, northern intrusions, exposed as the upper sections of batholith‐size bodies of coarse‐grained granite emplaced at the shallow to mid‐crust, show general NS‐trending magnetic fabrics roughly parallel to strike of the orogen and the regional foliation of host rocks. These contrasting magnetic patterns from shallow to deeper crust suggest vertical magma migration from the overthickened orogenic core to be emplaced across its thinner stretched flanks during the gravitational collapse of the orogenic edifice.

Magma chamber-related transition from magmatic to solid-state fabrics within the Late Triassic granitic Dushan pluton (North China)

SUMMARY Penetrative petro-fabrics in plutons, particularly late-stage solid-state fabrics, are usually considered to record regional tectonism. However, these fabrics are fundamentally ambiguous to correlate with magma-chamber processes or regional tectonics. Understanding the processes of fabric formation requires a proper understanding of pluton formation and emplacement mechanism. In this paper, we provide a case study of a transition from magma chamber-related magmatic to solid-state fabrics within the Late Triassic granitic Dushan pluton (North China) based on a multidisciplinary investigation of its emplacement mechanism, including structural geology, isotope chronology, AMS data measurement, and gravity modeling. The Dushan pluton varies in a southwest direction from a megascopically isotropic monzogranite in the northeast to an arc-shaped gneissic monzogranite with locally mylonitic zones along the southwest margin. The compatible outward dipping mesoscopic and magnetic susceptibility foliations define a dome-like roof with a NE-SW long axis. Considering the absence of similar penetrative fabrics in the country rocks, a continuum of magma chamber-related fabric variations within the pluton, from magmatic to solid-state conditions, probably formed during pluton emplacement. Gravity modeling suggests that the Dushan pluton has a NE-SW trending tongue-like shape with a northeast positioned root. We propose a model in which the Dushan pluton was emplaced through a northeast feeder zone, beginning probably as a sill then evolving by inflation via several magma batches. Later magma batches inflated the country rocks to form the final pluton. Magma inflation deformed the former semi-solid magma, forming a curved gneissic to mylonitic foliation along the southwest margin of the pluton. Several multidisciplinary studies show that the North China Craton (NCC) experienced a Late Triassic regional crustal extension attending the Dushan pluton emplacement. Both mesoscopic and magnetic susceptibility fabrics of the pluton could be more related to magmatic process rather than regional tectonics, implying that such fabrics within plutons need to be used with caution for inferring regional tectonics.

Application of anisotropy of magnetic susceptibility (AMS) fabrics to determine the kinematics of active tectonics: examples from the Betic Cordillera, Spain, and the Northern Apennines, Italy

Abstract. The anisotropy of magnetic susceptibility (AMS) technique provides an effective way to measure fabrics and, in the process, interpret the kinematics of actively deforming orogens. We collected rock fabric data of alluvial fan sediments surrounding the Sierra Nevada massif, Spain, and a broader range of Cenozoic sediments and rocks across the Northern Apennine foreland, Italy, to explore the deformation fabrics that contribute to the ongoing discussions of orogenic kinematics. The Sierra Nevada is a regional massif in the hinterland of the Betic Cordillera. We recovered nearly identical kinematics regardless of specimen magnetic mineralogy, structural position, crustal depth, or time. The principal elongation axes are NE–SW in agreement with mineral lineations, regional GPS geodesy, and seismicity results. The axes trends are consistent with the convergence history of the Africa–Eurasia plate boundary. In Italy, we measured AMS fabrics of specimens collected along a NE–SW corridor spanning the transition from crustal shortening to extension in the Northern Apennines. Samples have AMS fabrics compatible only with shortening in the Apennine wedge and have locked in penetrative contractional fabrics, even for those samples that were translated into the actively extending domain. In both regions, we found that specimens have a low degree of anisotropy and oblate susceptibility ellipsoids that are consistent with tectonic deformation superposed on compaction fabrics. Collectively, these studies demonstrate the novel ways that AMS can be combined with structural, seismic, and GPS geodetic data to resolve orogenic kinematics in space and time.

Anisotropy of magnetic susceptibility study to locate the feeder zone and lava flow directions of the Rajmahal Traps (India): Implications to Kerguelen mantle plume interaction with Indian Plate

Rajmahal Traps (RT) represents the earliest eruption from the Kerguelen hotspot activity over Indian Plate. The results of anisotropy of magnetic susceptibility (AMS) fabrics depict distribution anisotropy with the maximum susceptibility axes (Kmax) predominantly clustered NE-SW with plunge in NE. It infers that the feeder zone for RT was located in the NE quadrant of the volcanic province and along the Narmada-Son-Brahmaputra Transform (NSBT) fault. Our results further suggest that the NSBT system provide the major topographic control for the lava flow channel distribution in the Rajmahal province. It implies that the fracture zone was available during the lava emplacement, which records the earliest Kerguelen plume activity. Comparison between AMS fabrics and field topography suggests the distal horizontal magma flow and fissural mode in the absence of volcanic vent in the region, when the plume centre was located in the Comei-Cona region of Tibet.

Structural Setting, Paleomagnetism, and Magnetic Fabric of Miocene Plutons in a Transpressional Sinistral Shear Zone, Tonalá, Chiapas, Mexico: Evidence of Shortening During Magma Emplacement

AbstractWe studied the structure, paleomagnetism, and magnetic fabric of Miocene granitoids intruded along the left‐lateral, transpressional, Tonalá shear zone in Chiapas, southern Mexico. Magmatic fabrics delineate patterns of regional folding striking parallel to the shear zone, suggesting syn‐magmatic shortening. Magnetic susceptibility fabrics are well developed, and mean magnetic susceptibility averages 18.6 × 10−3 SI. The magnetic fabric of the plutons is controlled primarily by MD and finer PSD magnetite, which also carries the magnetic remanence. The orientations of macroscopic mineral fabrics and magnetic susceptibility fabrics are consistently parallel in protomylonites, but their coincidence is good to poor in granitoids that preserve magmatic fabrics. Most site mean directions are NW‐NE directed of moderate positive inclination (or antipodal), but on a local scale site, mean directions are very discordant. The overall mean direction, omitting highly discordant sites, is of Decl. = 359.6° and Incl. = +41.9° (k = 14.4, α95 = 8.1°, N = 24 sites); it is nearly concordant with the North America reference direction, and the slight difference may indicate a gentle, north‐side down regional tilt. High directional dispersion is interpreted to reflect relative rotation about local vertical to sub‐vertical axes of plutons, or parts of plutons, along the shear zone, with some rotations interpreted to be caused by progressive deformation within the shear zone. Paleomagnetic and AMS data from the protomylonite rocks are interpreted to have been affected by the distortional strain, which accommodated shape and volume change in the rock. We suggest that the introduction of granitic magma into middle crustal levels critically enhanced the crustal response to shortening within the shear zone, possibly due to melt compressibility.

Preliminary anisotropy of magnetic susceptibility studies on 2367 Ma Bangalore-Karimnagar giant dyke swarm, southern India: Implications for magma flow

Anisotropy of magnetic susceptibility (AMS) fabrics have been studied for the first time on a well constrained Paleoproterozoic Bangalore-Karimnagar giant mafic dyke swarm at ca. 2367 Ma in the Dharwar craton, south India. A total number of 43 samples from nine sites representing five N-E trending dykes have been subjected to AMS and conventional rock magnetic studies. The magnetic fabric is principally ferromagnetic in origin as the magnetic susceptibility values are >10−3 SI, which is further supported by Isothermal Remanent magnetization (IRM) studies. Hysteresis loops, Isothermal remanent magnetisation (IRM), thermomagnetic properties and optical observations indicate that pseudo single domain (PSD) low-titanium magnetite particles dominate the low field susceptibility of samples. The magnetic fabrics results correspond well with the comprehensive field studies. However, the degree of anisotropy results ranges from 0.4% to 12.8%, with a mean total anisotropy of 4.7%. The principal AMS axes for samples from all the sites are tightly clustered on equal area plots, and two main fabrics were revealed in the Bangalore-Karimnagar giant dyke swarm. In the first, the maximum susceptibility axes is sub-horizontal and parallel to the dyke walls, minimum susceptibility axes plot near the pole of the dyke. It is interpreted as the primary magma flow direction. A second fabric with maximum susceptibility axis is sub-horizontal and perpendicular to the dyke walls, and is interpreted as a result of vertical compaction at a late stage of crystallization. Preliminary results suggest that dyke propagation in the Bangalore-Karimnagar giant dyke swarm preserve a flow-induced fabric slightly upwards to the NE and magma flow is sub-horizontal.

Burial Diagenesis and Tectonism Inferred From Paleomagnetism and Magnetic Fabrics in the Wolfcamp Shale, Midland Basin, Texas, USA

AbstractAn integrated rock magnetic, paleomagnetic, geochemical, and electron microscopy study was performed on three cores penetrating the Wolfcamp Shale interval in the Midland Basin, West Texas. This work presents a temporally constrained diagenetic history of the Wolfcamp Shale, refines our understanding of tectonic influences in the Midland Basin, and postulates an interplay of late diagenetic processes that may contribute to the formation of authigenic magnetite on the surfaces of chlorite/illite. Paleomagnetic results indicate a well constrained, lithofacies independent, Jurassic‐aged chemical remanent magnetization carried by SD‐PSD magnetite. 1D basin modeling suggests that the Jurassic chemical remanent magnetization occurred within the oil window (100–120 °C) during a tectonically quiescent phase. Burial temperatures, electron microscopy, and inorganic geochemistry provide lines of evidence for maturation/migration of hydrocarbons, dissolution of carbonates, and chloritization/illitization. We interpret that the interaction of these processes may have created conditions suitable for the neoformation of magnetite on the surfaces of chlorite/illite. Anisotropy of magnetic susceptibility (AMS) fabrics are carried by paramagnetic clays, and ferroan dolomite is observed in all cores. Geographically corrected data show NE‐SW lineations among normal AMS fabrics and streaked inverse AMS fabrics trending NW‐SE. Textural observations indicate that authigenic ferroan dolomite formed in undercompacted conditions and likely behaved as passive structural markers. The orientation of AMS fabrics and the relative timing of ferroan dolomite authigenesis suggest that layer‐parallel shorting occurred shortly after the deposition of the Wolfcamp Shale and likely corresponds with the timing of Marathon‐Ouachita suturing.

Distinguishing coeval patterns of contraction and collapse around flow lobes in mass transport deposits

Gravity-driven mass transport deposits (MTDs) form by the downslope-directed movement of sediment associated with slope failure. Simple models suggest that extension forms at the upslope (head) area, contraction is focussed in the downslope toe of the slump, while differential shear associated with strike-slip is restricted to the lateral margins of the slump. Although the head and toe are considered to be dominated by layer-parallel shear (LPS), differential layer-normal shear (LNS) may be generated around the lateral margins of slumps and potentially also within MTDs where flow has been separated into different ‘lobes’. Despite this realisation that LNS must form, there has been little work into the geometries and spatial relationships of resulting structures. Using the late Pleistocene Lisan Formation exposed around the Dead Sea Basin as our case study, we examine detailed (<10 m) relationships of folds and thrusts created during LNS and LPS, as well as investigating the role of broadly coeval extension that may reactivate these structures. We also undertake analysis of anisotropy of magnetic susceptibility (AMS) fabrics to determine flow and shear relationships around folds and detachments created during LNS and LPS. Our study shows that LPS results in gently-curvilinear fold hinges that arc around the transport direction while LNS results in cylindrical fold hinges developed oblique or sub-parallel to transport. Such folds may be recumbent or upright, and associated with lateral ramps marking areas of differential LNS within the MTD. These structures are interpreted to accommodate variations in the amount and direction of downslope-directed movement resulting in LNS around the margins of individual flow ‘lobes’ that are developed over tens of metres. These ‘lobes’ display broadly down-slope transport with locally radial flow that results in along-strike shortening between lobes. Our analysis of AMS fabrics shows that they are controlled by slump folds, but magnetic fabrics do not differentiate how these folds were created in zones of LPS or differential LNS. AMS taken from gouge formed along detachments marked by differential LNS provide a first-order indicator for the transport direction. In addition, AMS fabrics in gouge or fluidised layers directly beneath thrust ramps, reveals prolate fabrics marking a component of strike-parallel flow along the branching intersections of thrust ramps and flats. Extensional faults directly reactivate existing thrusts, or create new extensional faults that are sub-parallel to thrusts or cut across them at steeper angles. Extension is part of the same MTD event as a sedimentary cap that is deposited out of suspension following slope failure, overlies and locally thickens into the hangingwall of extensional faults to create ‘growth’ sequences. Extensional reactivation and ‘collapse’ of original thrusts may help explain why contraction is apparently ‘missing’ from many seismic sections across MTDs.

Inverse Magnetic Susceptibility Fabrics in Pelagic Sediment: Implications for Magnetofossil Abundance and Alignment

AbstractSingle‐domain magnetite particles exhibit minimum susceptibility along their elongation, resulting in so‐called inverse fabric of the anisotropy of magnetic susceptibility (AMS). We report the discovery of inverse AMS fabrics from pelagic clay recovered by a ∼12 m long piston core from the western North Pacific. A previous study identified fossil single‐domain magnetite produced by magnetotactic bacteria (magnetofossils) as the dominant ferrimagnetic mineral in the sediment. The inverse AMS fabrics were found in a ∼2 m zone. The ∼6 and ∼4 m of sediment above and below this zone showed normal, horizontal AMS fabrics. Rock magnetic data and ferromagnetic resonance spectroscopy indicated that magnetofossils account for most of the mean susceptibility regardless of normal or inverse AMS. This was explained by the mixing models where the inverse fabric from magnetofossils is nearly balanced by the normal fabrics of terrigenous minerals. The corrected degree of AMS carried by magnetofossils in the sediment was estimated to be ∼1.01, which is comparable to that of typical pelagic sediment at shallow depth. On the other hand, terrigenous minerals in the sediment were estimated to have higher degree of anisotropy, possibly reflecting burial and subsequent erosion of &gt;80 m of sediment, which was also suggested by a subbottom acoustic stratigraphy. This suggests that inverse AMS fabrics due to magnetofossils may be widespread in pelagic clay without strong compaction.

Refined model of incremental emplacement based on structural evidence from the granodioritic Newry igneous complex, Northern Ireland

Although many intrusions are now known to have been incrementally emplaced, the mechanisms through which this takes place are generally poorly understood. The Newry igneous complex was incrementally emplaced within the Southern Uplands-Down-Longford terrane of Northern Ireland during late Caledonian sinistral transtension. This study uses a variety of new and existing data and techniques to provide a fuller and firmer understanding of incremental emplacement than has previously been available, addressing both deep-crustal processes and those operating within the emplacement site. Host-rock orientations suggest that some of the accommodation space for the Newry igneous complex was generated due to pull-apart tectonics operating within the Southern Uplands-Down-Longford terrane. Local host-rock deflections, concentric igneous foliations, and concentric linear anisotropy of magnetic susceptibility (AMS) fabrics show that inflation due to magma overpressure also generated significant space. Strong AMS fabrics close to the boundaries of some magma pulses in turn suggest that inflation was accomplished by injection of individual magma pulses and was thus incremental. The dome-like orientations of mineral foliations within plutons and the truncation of steep local host-rock tracts by the Newry igneous complex imply that the complex consists of four laccolithic bodies. On a larger scale, it is suggested that the deep-seated Argyll and Newry lineaments represent faults that allowed magma generated at depth to ascend to the crustal level of the Southern Uplands-Down-Longford tract boundaries. It is also inferred that sinistral movement along the Argyll and Newry lineaments may have produced the releasing bend within the Southern Uplands-Down-Longford terrane. Higher in the crust, reduced confining pressure resulted in tectonic opening along this releasing bend. This local stress field induced horizontal magma flow and emplacement of the Newry igneous complex as laccolithic bodies. This study suggests that simplistic emplacement models should largely be abandoned in favor of holistic models incorporating the multiple interdependent processes operating during magma ascent and emplacement.

A multi-proxy lacustrine record of last deglacial–early Holocene environmental variability in the lower Yangtze region from the Chaohu Lake Basin, eastern China

AbstractThe Rb, Sr, and Ti content, Rb/Sr ratio, grain size, magnetic susceptibility, and magnetic fabric in sediments of the BZK1 core were utilized to reconstruct the evolution of the climatic environment in the Chaohu Lake Basin between the last deglacial and the early Holocene. Multi-proxy analyses indicate that lacustrine sediments in Chaohu Lake clearly record the Bølling-Allerød interstadial, the Younger Dryas event and dry-cold climate events occurring between 10.7 cal ka BP and 10.5 cal ka BP. At approximately 15.6–14.8 cal ka BP, the waters became deeper and the climate turned cool. The climate subsequently shifted to a relatively humid period and the lake was largest from 14.8 to 12.8 cal ka BP. From 12.8 to 11.7 cal ka BP, the climate abruptly turned dry and cold and the lake shrank to its lowest level. During 11.7–10.7 cal ka BP, the climate became relatively humid but, from approximately 10.7 to 10.5 cal ka BP, suddenly reverted to a dry and cold state. These climatic change records suggest that lacustrine sediments from the Chaohu Lake Basin in the lower Yangtze region responded actively to global climate changes, comparable with the environmental records from stalagmites and other lacustrine sediments in the region.

Mechanical anisotropies and mechanisms of mafic magma ascent in the middle continental crust: The Sondalo magmatic system (N Italy)

Magma transfer between lower and upper continental crust is a fundamental process linking the dominantly mafic composition of the lower crust with the more felsic composition of the upper crust. We explore the mechanisms of mafic magma ascent and emplacement in the middle crust by studying a mid-crustal gabbroic to dioritic magmatic system (Sondalo complex, Eastern Central Alps, N Italy). We characterize the structure and Anisotropy of Magnetic Susceptibility (AMS) fabric of concentric gabbroic to dioritic intrusions. The significance of AMS fabrics is discussed using Anisotropy of Anhysteretic Remanent Magnetization (AARM) and Crystallographic Preferred Orientation (CPO) data acquired on different test sites. The magmatic and magnetic fabrics of the pluton were acquired during its emplacement in the Permian and were not subsequently tilted: the fabrics are essentially vertical, indicating vertical magma transfer through the crust with a two-phase intrusion history. (1) The concordant orientation between the magmatic foliation and the host-rock xenoliths in the center of the pluton suggest that the first magma ascent phase occurred along pathways subparallel to the vertical fabric of the host metasedimentary rocks. (2) The second magma ascent phase was controlled by a change in the rheology of the host-rock and the mafic magma. Heat dissipation to the contact aureole induced partial melting, thereby lowering the mechanical strength of the host-rocks, whereas the viscosity of the mafic magma increased due to cooling and associated fractional crystallization. This caused an en-masse rise of the pluton resulting in the formation of a structural aureole, i.e. a vertical foliation in the contact aureole and a weaker but concordant magmatic foliation at the rim of the pluton. This ascent phase accounts for the P−T evolution recorded by metasedimentary rocks in the contact aureole of the pluton.

Fault-related fold kinematics recorded by terrestrial growth strata, Sant Llorenç de Morunys, Pyrenees Mountains, NE Spain

Foreland basin growth strata are ideal recorders of deformation rates and kinematics in tectonically active regions. This study develops a high-resolution chronostratigraphic age model to determine folding rates in the Eocene-Oligocene terrestrial growth strata of the Berga Conglomerate Group, NE Spain. The Berga Conglomerate Group was sampled for rock magnetic, magnetostratigraphic, and magnetic susceptibility (χ) cyclostratigraphy analyses. Analysis of rock magnetic measurements indicate a mixed mineral assemblage with both paramagnetic and ferromagnetic minerals. A new magnetic reversal stratigraphy constrains the time frame of folding and is in agreement with previous interpretations. Time series analysis of χ variations show statistically significant power at expected orbital frequencies and provides precession-scale (20 kyr) temporal resolution. Strain measurements including anisotropy of magnetic susceptibility (AMS) fabrics and bedding plane strain worm burrow distortion are consistent with fixed hinge, flexural folding kinematics. Fault-related folding was modeled using χ cyclostratigraphy timing and strain measurement kinematic constraints. The onset of folding was at 33.85 Ma and the end of deformation is less constrained but is younger than 31.06 Ma. Deformation and sediment accumulation rates are unsteady at 20 kyr time scales but appear artificially steady at polarity chron time scales.

Diverse magma flow directions during construction of sheeted dike complexes at fast- to superfast-spreading centers

Dike intrusion is a fundamental process during upper oceanic crustal accretion at fast- to superfast-spreading ridges. Based on the distribution of magma along fast-spreading centers inferred from marine geophysical data, models predict systematic steep flow at magmatically robust segment centers and shallow magma flow toward distal segment ends. Anisotropy of magnetic susceptibility (AMS) fabrics from 48 fully-oriented block samples of dikes from upper oceanic crust exposed at Hess Deep Rift and Pito Deep Rift reveal a wide range of magma flow directions that are not consistent with such simple magma supply models. The AMS is interpreted to arise from distribution anisotropy of titanomagnetite crystals based on weak shape-preferred orientation of opaque oxide and plagioclase crystals generally parallel to AMS maximum eigenvectors. Most dike samples show normal AMS fabrics with maximum eigenvector directions ranging from subvertical to subhorizontal. The distributions of inferred magma flow lineations from maximum eigenvectors show no preferred flow pattern, even after structural correction. We use a Kolmogorov–Smirnov test (KS-test) to show that the distribution of bootstrapped flow lineation rakes from Pito Deep are not statistically distinct from Hess Deep, and neither are distinguishable from Oman and Troodos Ophiolite AMS data. Magma flow directions in sheeted dikes from these two seafloor escarpments also do not correlate with available geochemistry in any systematic way as previously predicted. These results indicate distinct compositional sources feed melt that is injected into dikes at fast- to superfast-spreading ridges with no preference for subhorizontal or subvertical magma flow. Collectively, results imply ephemeral melt lenses at different along-axis locations within the continuous axial magma chamber and either direct injection or intermingling of melt from other deeper ridge-centered or off-axis sources.

Significant rotations related to cover–substratum decoupling: Example of the Dôme de Barrôt (Southwestern Alps, France)

The Barrôt area presents an important contrast of deformation between a foliated, faulted and non-rotated Permian substratum and a folded and strongly rotated Meso-Cenozoic sedimentary cover. The Permian substratum and the sedimentary cover are separated by a décollement level of Triassic gypsum. While numerous studies have been done on the Dôme de Barrôt (i.e., the Permian substratum), we focused our work on the deformed Meso-Cenozoic sedimentary cover around the Dôme de Barrôt where we measured and analyzed both the magnetic susceptibility fabrics and the paleomagnetic directions. Our results highlight an arcuate pattern of the directions of shortening trajectories revealed by type II–III fabrics of anisotropy of magnetic susceptibility (which predates the folding) and a later syn- or post-folding anticlockwise rotation of large amplitude (64±14°) for the sedimentary cover. The arcuate pattern of the directions of shortening and the different rotations between the Permian substratum and the sedimentary cover can be partly explained after restoring the paleogeographic location of the sedimentary cover before the Oligocene Alpine compression event. We tend to interpret the remaining 30° anticlockwise rotation as a result of the high efficiency of the Triassic gypsum décollement surface, a nearby regional left-lateral shear zone (RDFZ), and a possible near-field gravitational sliding near the later exhumed Dôme de Barrôt.

Internal structure of basalt flows: insights from magnetic and crystallographic fabrics of the La Palisse volcanics, French Massif Central

We present a new interpretation of anisotropy of magnetic susceptibility (AMS) fabrics in basaltic lava flows based on the detailed study of magnetic mineralogy and silicate crystallographic fabric of a Quaternary lava flow from the French Massif Central (La Palisse). We consider the model of AMS fabric imbrication between magnetic foliation and flow surface, as initially proposed for dykes. At the two sampling sites, the concordance between the flow direction deduced from the AMS foliation and that deduced from field observations indicates that the imbrication model could apply to the lava flows. However, the flow senses inferred fromAMSare systematically opposed between the two sampling sites suggesting permutations between K1 and K3 AMS axes, a configuration referred to as inverse fabric. Electron backscatter diffraction (EBSD) measurements show strong lattice-preferred orientations (LPO) for plagioclase, especially the (010) plagioclase plane, which tends to be parallel to the flow. Clinopyroxene LPO remains less marked than plagioclase LPO, whereas titanomagnetite does not display a significant LPO. Comparison between magnetic and crystallographic fabrics suggests that the AMS fabric of the lava flow results from the distribution of titanomagnetite grains, which is in turn controlled by the fabric of the silicate framework. Magnetic hysteresis parameters and anisotropy of remanent magnetization (ARM) measurements exclude a significant contribution from single-domain grains, often called upon to explain inverse magnetic fabrics. The origin of the observed inverse magnetic fabric may relate to the dip of the palaeosurface, which is the only remarkable difference between the two sampling sites. AMS appears as a good tool to determine the direction of basaltic lava flows and coupling with local crystallographic fabric data provides a valuable control of relationships between magnetic fabrics and flow and thus contributes to better constrain the AMS signature of lava flows.

The flow dynamics of an extremely large volume pyroclastic flow, the 2.08-Ma Cerro Galán Ignimbrite, NW Argentina, and comparison with other flow types

The 2.08-Ma Cerro Galan Ignimbrite (CGI) represents a >630-km3 dense rock equivalent (VEI 8) eruption from the long-lived Cerro Galan magma system (∼6 Ma). It is a crystal-rich (35–60%), pumice (<10% generally) and lithic-poor (<5% generally) rhyodacitic ignimbrite, lacking a preceding plinian fallout deposit. The CGI is preserved up to 80 km from the structural margins of the caldera, but almost certainly was deposited up to 100 km from the caldera in some places. Only one emplacement unit is preserved in proximal to medial settings and in most distal settings, suggesting constant flow conditions, but where the pyroclastic flow moved into a palaeotopography of substantial valleys and ridges, it interacted with valley walls, resulting in flow instabilities that generated multiple depositional units, often separated by pyroclastic surge deposits. The CGI preserves a widespread sub-horizontal fabric, defined by aligned elongate pumice and lithic clasts, and minerals (e.g. biotite). A sub-horizontal anisotropy of magnetic susceptibility fabric is defined by minute magnetic minerals in all localities where it has been analysed. The CGI is poor in both vent-derived (‘accessory’) lithics and locally derived lithics from the ground surface (‘accidental’) lithics. Locally derived lithics are small (<20 cm) and were not transported far from source points. All data suggest that the pyroclastic flow system producing the CGI was characterised throughout by high sedimentation rates, resulting from high particle concentration and suppressed turbulence at the depositional boundary layer, despite being a low aspect ratio ignimbrite. Based on these features, we question whether high velocity and momentum are necessary to account for extensive flow mobility. It is proposed that the CGI was deposited by a pyroclastic flow system that developed a substantial, high particle concentration granular under-flow, which flowed with suppressed turbulence. High particle concentration and fine-ash content hindered gas loss and maintained flow mobility. In order to explain the contemporaneous maintenance of high particle concentration, high sedimentation rate at the depositional boundary layer and a high level of mobility, it is also proposed that the flow(s) was continuously supplied at a high mass feeding rate. It is also proposed that internal gas pressure within the flow, directed downwards onto the substrate over which the flow was passing, reduced the friction between the flow and the substrate and also enhanced its mobility. The pervasive sub-horizontal fabric of aligned pumice, lithic and even biotite crystals indicates a consistent horizontal shear force existed during transport and deposition in the basal granular flow, consistent with the existence of a laminar, shearing, granular flow regime during the final stages of transport and deposition.

When an ‘inverse’ fabric is not inverse: an integrated AMS‐SPO study in MORB‐like dykes

Terra Nova, 23, 49–55, 2011AbstractMagma flow direction is recorded in the macroscopic mineral petrofabric and magnetic fabric of the MORB‐like Rooi Rand dyke swarm. The bulk magnetic fabric is sub‐parallel to the dyke plane, resulting from preferred orientation of grains during magma flow. This is referred to as type‐A fabric. A study of mineral shape preferred orientation reveals an additional fabric, which is orthogonal to the average dyke orientation in 30% of the data. This fabric is carried by both plagioclase and opaque grains and would typically be interpreted as ‘inverse’. It is referred to here as type‐B fabric. However, because it is carried by the shape preferred orientation of macroscopic grains and is in most cases not coaxial to the anisotropy of magnetic susceptibility fabric, it is interpreted as having been acquired during increased grain interaction during late‐stage magma flow associated with decreasing magma pressure.

Anisotropy of magnetic susceptibility in weakly deformed red beds from the Wyoming salient, Sevier thrust belt: Relations to layer-parallel shortening and orogenic curvature

Anisotropy of magnetic susceptibility (AMS) and structural studies of red beds in the Wyoming salient were completed to evaluate relations of magnetic fabrics to layer-parallel shortening and vertical-axis rotation in curved fold-thrust systems. The red beds display cleavage, fractures, veins, minor folds, and minor faults that accommodated widespread early layer-parallel shortening and minor strike-parallel extension. Magnetic susceptibility is carried mostly by paramagnetic phyllosilicates and ferromagnetic hematite that have composite fabrics related to sedimentary deposition, diagenesis, and tectonic processes. Anisotropy of magnetic susceptibility fabrics range from distinctly oblate ellipsoids parallel to bedding that reflect dominant sedimentary fabrics (type 1), to moderately oblate ellipsoids with weak magnetic lineations roughly parallel to the intersection of weak layer-parallel shortening fabrics and bedding (type 2), to triaxial and prolate ellipsoids with distinct magnetic lineations parallel to the intersection of moderate layer-parallel shortening fabrics and bedding (type 3). Type 1 sites occur mostly in the central, frontal part of the salient where layer-parallel shortening is <5%, whereas type 3 sites are found mostly in more interior thrust systems and toward the salient ends where layer-parallel shortening is >15%. Magnetic lineations are subparallel to structural trend and exhibit a tangential pattern around curved fold-thrust systems. Regional patterns of anisotropy of magnetic susceptibility are broadly similar to patterns of finite strain estimated from reduction spots. Combined with paleomagnetic data, anisotropy of magnetic susceptibility data indicate that early layer-parallel shortening fabrics started with minor primary curvature and then underwent significant vertical-axis rotation during large-scale thrusting. Correlations with finite strain, structural, and paleomagnetic data sets indicate that analysis of anisotropy of magnetic susceptibility in weakly deformed red beds is useful for evaluating kinematic evolution of thrust systems.