Anisotropy Of Magnetic Susceptibility Fabrics Research Articles

Transport of Magma in Granitic Mush Systems; an Example From the Götemar Pluton, Sweden

AbstractGranitic magma bodies form in the ephemeral part of magma mush systems and are emplaced by a variety of mechanisms in different tectonic settings. This study investigates how granitic magma emplacement processes and tectonomagmatic interactions assert control over the architecture of mush state pluton‐scale magma transport pathways. The 1.45 Ga shallow‐crustal Götemar pluton is a 4.5 km diameter circular pluton that consists of three granite units: a coarse‐grained red granite, a medium‐grained pale to red granite, and fine‐grained pale microgranite sheets. We employed geological mapping supported by Anisotropy of Magnetic Susceptibility (AMS) to examine the magmatic and regional tectonic controls on late‐stage magma transport in the Götemar granitic magma mush system. Multiple parallel arcuate subhorizontal microgranite and medium‐grained granite sheets (from 0.1 to 10s of meters thick) were mapped within the pluton. The arcuate sheets pinch out from the northern part of the pluton toward the SE inferring magma propagation direction. A dominant set of vertical granitic sheets within the granite body strikes NW‐SE. The AMS fabrics are contact‐parallel in the main medium‐grained granite body and indicate inflation. Within the microgranite sheets, the AMS fabrics are parallel to the sheet strike and support a sheet propagation direction to the SE. The Götemar pluton displays a clear link between arcuate (concentric) magma‐transporting sheets and concentric strain‐partitioning related to the intrusion of medium‐grained granite magma. The vertical magma sheet orientations are consistent with an NE‐SW extensional stress field that is associated with the extensional back‐arc stress regime of the contemporary Hallandian Orogen.

Focal mechanism of a Late Triassic large magnitude earthquake along the Longmen Shan fault belt, eastern Tibetan Plateau

Stress states near active fault zones are key to understand their seismogenic setting, geodynamic evolution and future seismic activity. The Longmen Shan Fault Belt (LSFB) is a prominent fault zone that has had a long history of activity and major recent earthquakes in the eastern Tibetan Plateau. However, we have no records of major events prior to the 2008 Wenchuan earthquake, and this hinders progress in understanding both the past and future seismicity of the LSFB. A Late Triassic pseudotachylyte, recently discovered and dated, provides an opportunity to compare modern stress states with those acting ca. 230 Ma ago. Here we use a new paleoseismological approach, based on the magnetic fabric of the fault rocks, to determine the focal mechanism of Late Triassic seismic events, each recorded in pseudotachylyte veins at the Bajiaomiao outcrop. The anisotropy of magnetic susceptibility (AMS) of pseudotachylytes and cataclasites arises from the shape-preferred orientation of coseismic neoformed magnetite and monoclinic pyrrhotite, whereas that of protocataclasite arises mainly from the crystallographic preferred orientation of inherited clastic magnetite. The AMS of fault rocks shows well-defined magnetic foliation and lineation. The pseudotachylyte generation vein formed along the plane where seismic slip took place, while the AMS obliquity with respect to this plane indicates seismic slip with ∼248° trend and 38° dip. This result is consistent with the attitude of macroscopically visible subhorizontal striations on the pseudotachylyte plane itself. The consistent asymmetry of multiple injection veins with respect to the pseudotachylyte generation vein, along with the asymmetry of the AMS fabric, shows that seismic deformation resulted from left-lateral, strike-slip motion. This study provides the oldest example, to date, of focal mechanism determination using the AMS method. The kinematics of the Late Triassic event departs from that of the 2008 Wenchuan earthquake thrust motion. Yet, with a displacement estimated between ∼4.4 and 17.4 m, the magnitude of the Late Triassic event was likely Mw 7.5–7.9.

Contractional deformation between extensional dome exhumation in Central Pamir at 17–15 Ma constrained by metamorphic and paleomagnetic data from the Bartang volcanic rocks, Tajikistan

The debated mechanism and timing of formation of the Pamir orogenic salient provides an ideal case to combine paleomagnetic and metamorphic analyses. Opposing views argue for either oroclinal bending of the Pamir since the India-Asia collision or for an originally arcuate shape, which can be tested using paleomagnetism to estimate vertical-axis rotations. Furthermore, Pamir deformation can be dated and characterized by the analysis of a well-expressed regional metamorphism. However, paleomagnetism is seldom applied to metamorphic rocks such that the significance of their rock magnetic signal with respect to deformation remains poorly understood. We studied a > 2 km thick sequence of slightly metamorphosed Cretaceous-Paleogene volcanic and volcaniclastic rocks from the western Central Pamir Mountains using metamorphic petrology, geochronology, and paleomagnetic analyses. These rocks present a medium-grade metamorphism and have undergone fine grained recrystallisation of biotite due to NW-SE compression. 40Ar/39Ar and Rb/Sr ages (∼17 to 15 Ma) on whole rock and biotite, confirm that the greenschist facies metamorphism is related to the coeval exhumation of Pamir gneiss domes during the Middle Miocene. The rocks generally show homogenous magnetic properties. High field magnetic properties and SEM/EDS data show that the main magnetic remanent carrier is titano-hematite. Thermal demagnetizations yield complex characteristic remanent magnetization with site-mean directions obtained for only 14 of the 44 studied sites but scattered mainly within the foliation plane defined by the anisotropy of magnetic susceptibility (AMS). These rocks have also unusual magnetic characteristics where strong-field isothermal remanent magnetizations induce an AMS fabric, likely related to the specific composition and structure of the metamorphic titano-hematite but independent of the original anisotropy fabrics. Combined paleomagnetic and geochronological data from the Bartang volcaniclastics rocks enable us to identify a ∼ 17–15 Ma compressive deformation event contemporaneous with the emplacement of the gneiss domes and suggest no oroclinal bending since that event.

Reconstruction of Subaqueous Eruption‐Fed Current Characteristics and Source Location Using Anisotropy of Magnetic Susceptibility (AMS) at Otapahi, Dunedin Volcano, New Zealand

AbstractInformation about birth and early eruptions of composite volcanoes is scarce, but erosion has revealed very early deposits of the Dunedin Volcano, and we apply anisotropy of magnetic susceptibility (AMS) techniques to retrieve information about the earliest known, submarine, eruptive centre. Although AMS has been used to recover information from subaerial eruptions, and to examine turbidites, this is its first use in investigating depositional processes and the source location for submarine volcaniclastic deposits. AMS fabrics of the deposits, emplaced by submarine eruption‐fed density currents, show basal portions of the currents were decoupled from more‐buoyant upper parts. Triangulation of AMS vectors and orientations yields a vent site location, with some imprecision reflecting the chaotic nature of deposits rapidly aggraded within 500 m of the source vent. This method elucidates depositional processes of subaqueous volcanic currents as well as subaerial ones, while still yielding sufficient consistency for triangulation of flow source sites.

Preferred Orientations of Magnetic Minerals Inferred from Magnetic Fabrics of Hantangang Quaternary Basalts

This paper presents a comprehensive analysis of the anisotropy of magnetic susceptibility (AMS) and paleomagnetic data from Quaternary basalt outcrops along the Hantangang River, Korea. A total of 554 samples were collected from 20 sites, representing three distinct units, Unit I, Unit II, and Unit III. Paleomagnetic data reveal a difference in the timing of eruptions between Units I and II, suggesting distinct periods by volcanic episodes. The mineral magnetic analysis identified titanomagnetite as the dominant magnetic carrier in the samples. AMS results indicated weak anisotropy and scattered AMS directions, indicating a low degree of preferred orientation of grains within the basalt rocks. The inverse AMS fabrics observed at specific sites are attributed to single-domain (SD) grains. Comparing the AMS data with the anisotropy of anhysteretic remanent magnetization (AARM) data, three distinct types of magnetic fabrics (normal, intermediate, and inverse) were discerned. The magnetic fabric was utilized to ascertain the flow direction based on the findings obtained from the AMS results. The findings suggest that the Quaternary basalts in this study’s area were primarily confined to the Hantangang River channel and its immediate vicinity during lava flow. However, distinct flow patterns are observed in the southwestern region, implying the presence of unknown volcanic sources.

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.

High‐Velocity Slip and Thermal Decomposition of Carbonates: Example From the Heart Mountain Slide Ultracataclasites, Wyoming

AbstractThe Heart Mountain Slide in Wyoming is one of the largest known terrestrial gravity slides (3,500 km2) formed ∼49 Ma ago by the nearly horizontal detachment of Paleozoic‐Eocene cover sliding on top of autochthonous formations. At the White Mountain locality, exposures offer an exceptional opportunity to investigate high strain rate/high velocity processes in carbonates. Here we use the anisotropy of magnetic susceptibility (AMS) of 274 samples to shed light on ultracataclastic deformation along this detachment. Contrary to predictions, the carbonate ultracataclasite displays a consistent AMS fabric, particularly in the upper ultracataclasite. The AMS in this unit is controlled primarily by magnetite formed through the breakdown of iron sulfides caused by frictional heating. Additional thermomagnetic experiments reveal that the new magnetic fabric began forming ∼250ºC and continued up to ∼400ºC when calcination of carbonate minerals caused a major drop in friction. The main cataclastic slip direction inferred from AMS is ∼N033°, at odds with the previously accepted NNW‐SSE direction. We validate these AMS fabrics through 3D shape preferred orientation analysis and micro X‐ray scanning of the same specimens. These results, however, may only represent cataclastic flow directions at the local scale as a result of synkinematic rotation of the White Mountain block. Alternatively, these results may call for a re‐evaluation of the large scale movement of the slide. Finally, this study demonstrates the usefulness of a magnetic approach in deciphering deformation processes in carbonates, particularly in high strain rate cases such as seismic faults.

Magnetic fabrics reveal three-dimensional flow processes within elongate magma fingers at the margin of the Shonkin Sag laccolith (MT, USA)

Unravelling magma flow in ancient sheet intrusions is critical to understanding how magma pathways develop and feed volcanic eruptions. Analyzing the shape preferred orientation of minerals in intrusive rocks can provide information on magma flow, because crystals may align parallel to the primary flow direction. Anisotropy of magnetic susceptibility (AMS) is an established method to quantify such shape preferred orientations in igneous sheet intrusions with weak or cryptic fabrics. However, use of AMS data to characterize how magma flows within the individual building blocks of sheet intrusions (i.e., magma fingers and segments), hereafter referred to as elements, has received much less attention. Here we use a high spatial resolution sampling strategy to quantify the AMS fabric of the Eocene Shonkin Sag laccolith (Montana, USA) and associated elongate magma fingers. Our results suggest that magnetic fabrics across the main laccolith reflect sub-horizontal magma flow, and inferred flow directions are consistent with an underlying NE-SW striking feeder dyke. Within the magma fingers, we interpret systematic changes in magnetic fabric shape and orientation to reflect the interaction between competing forces occurring during finger-parallel magma flow (i.e., simple shear) and horizontal and vertical inflation (i.e., pure shear flattening). For example, we highlight how local crossflow of magma between coalesced fingers increases the complexity of magma flow kinematics and related fabrics. Despite these complexities, the AMS data in coalesced magma fingers maintain their internal flow- and inflation-related fabrics, which suggests that magma flow within the fingers remains channelized after coalescence. Given that many sheet intrusions consist of amalgamated elements, our findings highlight the need to carefully consider element distribution and sample locations when interpreting magma flow based on AMS measurements.

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.

Magnetic fabric of the early-Ediacaran Itapetim monzogranitic pluton: magma flow during oblique extension along strike-slip shear zones (Eastern Brazil)

The Itapetim pluton is a sigmoid-shaped intrusion bounded by the NE-SW, sinistral Itapetim and Taperoá shear zones. The Anisotropy of Magnetic Susceptibility (AMS) and Anhysteretic Remanent Magnetization (AARM) techniques and microstructural analysis were employed in order to characterize the pluton's emplacement mechanism and its relationships with regional tectonics. Microstructures are mainly sub-magmatic/incipient solid-state. Magnetic susceptibilities are typically paramagnetic (k ≤ 0.5 mSI). Thermomagnetic curves and hysteresis loops further indicate that negligible ferrimagnetic susceptibilities may be the result of fine-grained, oxidized magnetite inclusions. AARM fabrics do not show direct correlations with AMS fabrics. A WNW to E-W trending, shallowly-plunging magnetic lineation is associated with an intermediate-to steep-dipping magnetic foliation. The magnetic lineation locally displays intermediate plunges that may represent possible feeder zones for magma ascent during the initial stages of emplacement. The proposed emplacement model for the Itapetim pluton combines bulk NNW-SSE shortening with NE-SW trending simple shear along the Itapetim and Taperoá shear zones, and a WNW-ESE/E-W dextral stretching in the splay between both shear zones. The crystallizing magma mush flowed laterally along the splay during bulk NNW-SSE transpression, which is consistent with continental collision and subsequent extrusion of the Central Domain of the Borborema Province during the Neoproterozoic.

Magnetic fabric and sedimentary characterization of near-slope to basinal deposits from the Chicontepec Formation, central and southern region of the Tampico-Misantla basin

We report results of a study of the Chicontepec Formation magnetic fabric in the central and southern region of the Tampico-Misantla basin in the state border region between Veracruz and Hidalgo. Samples were collected at 16 sites corresponding to two main facies associations: channel-fill facies and channel overbank facies. The channel facies dominate the relief developing prominent geoforms adjacent to low hills developed in facies dominated by shales. Measurements were made on channel and overbank facies. Laboratory analyses include anisotropy of magnetic susceptibility (AMS) and remanence anisotropy (AARM), as well as isothermal remanence acquisition (IRM) and thermomagnetic curves to characterize magnetic mineralogy. The magnetic susceptibility values of the entire collection are of the order of 40 to 70 x10-6 SI, so the susceptibility fabric is controlled by the paramagnetic fraction. The IRM acquisition curves are near saturation with inductions < 0.3 T and can be modeled with a low coercivity component contributing ~ 90% and a high coercivity component contributing < 10%. The thermomagnetic curves are dominated by paramagnetic phases. In the AMS fabric, three types of behavior are observed, corresponding to a lesser extent to sedimentary fabrics (k3 ~ vertical), and mostly to incipient and well-developed tectonic fabrics. The sedimentary fabrics are characterized by the imbrication of the magnetic foliation perpendicular to the paleocurrent data obtained in the field, which are generally consistent from NW to SE. At sites with tectonic fabric, which generally correspond to sites closer to the deformation front, the magnetic lineaments are well clustered in the NW quadrant with low plunges. AMS and AARM fabrics may combine in complex patterns. AARM appears to record either maximum axis directions that correspond to alignment of elongated particles perpendicular to flow or the direction of thrust motion.

Deformation of the colliding Luzon volcanic arc: Strain analysis using magnetic fabrics of syn-orogenic mudstones in the Coastal Range, eastern Taiwan

Exposed in the Coastal Range of eastern Taiwan is a portion of the Luzon volcanic arc that has collided with the South China continent since 6 Ma, as well as syn-orogenic sedimentary rocks deposited on the arc and subsequently intensely folded and thrusted. This study sampled syn-orogenic mudstones for the measurement of anisotropy of magnetic susceptibility (AMS) at four sites on the sides of the range. The AMS fabrics reveal a contribution from burial compaction and, less significantly, tectonic shortening which varies slightly across the range. This lateral variation is considered as, for simplicity, a result of two-phase deformation because these rocks were involved in an eastward propagating back thrust system on the colliding arc. A new strain analysis method is developed to estimate the overall tectonic strain by unstraining the pseudo-strains with respect to the AMS measurements at each site with the knowledge about the distribution of the susceptibility ellipsoids in the unstrained state. Strain results reveal that shortening phases occurred before and after the folding, with a shortening ratio of 0.177–0.195 and 0.181–0.248, respectively. The post-folding shortening on the western side is sub-parallel to the range, and the pre-folding shortening on both sides is layer-parallel and nearly perpendicular to the range. Therefore, the mudstones underwent layer-parallel shortening, fold-related tilting and eventually orogen-parallel shortening after deposition. This sheds light on the complexity in deformation of the colliding Luzon arc.

Magnetic fabric and geomorphic characteristic of Neotectonic activity along strike direction of North Almora Thrust, Kumaun Lesser Himalaya, India

Structurally, the North Almora Thrust (NAT), is placed in the northern proximity of the Almora Nappe (AN) in the Kumaun Lesser Himalaya. The NAT is characterized by the presence of a zone of mylonitic rocks of basal Saryu Formation (1800 ± 100 Ma) in the hanging wall and the metasedimentaries of Inner Lesser Himalayan rocks as the footwall block. The NAT has been dextrally offset by two estabilished major faults i.e. Saryu River Fault (SRF) in Saryu Valley and Dwarahat-Chaukhutia Fault (DCF) in Ramganga valley. In present study, we identified four new faults, based on field study, geomorphic landform and magnetic fabric analysis. These faults are N–S trending Pancheshwar Fault, NE–SW Rameshwer Fault, NNE-SSW trending Kosi Fault, and NNE-SSW trending Gagas Fault (GF). Our results show that the zone is bound by cross cutting relation with NAT and these fault zones are comparatively more active than other regions. Furthermore, we suggest that the steep and NW–SE orientation of magnetic foliation within the NAT zone is a result of NE–SW oriented progressive regional compression. The magnetic foliations represent the unseen internal foliations in the rocks developed due to preferred alignment of magnetic minerals and can be found through the Anisotropy of magnetic susceptibility (AMS) study. Variation in the alignment of the field and magnetic foliations are developed due to superimposed brittle deformation along the faults over the pre-existing field foliations. Magnetic foliations represent the impact of last stage deformation and significant to find brittle deformation and finite strain in the rocks of the study area. The lowering of anisotropy (Pj) away from the fault zone represents distribution of strain across the NAT zone. AMS fabric confirms the presence of faults developed across the NAT zone and also explains the deformation pattern along these faults. The geomorphic anomalies and steepness changes across the NAT zone are correlated with active deformation along the NAT and associated transverse faults.

Deformation in SW Anatolia (Turkey) Documented by Anisotropy of Magnetic Susceptibility Data

AbstractConvergence between the Eurasian and the African plates in the West Anatolian‐Aegean region results in a trench retreat due to slab roll‐back and tearing of the subducted African lithosphere. The upper plate response of this process gave way to back‐arc extension in the region. We have conducted a very detailed anisotropy of magnetic susceptibility (AMS) study on the Neogene rocks in SW Anatolia to unravel the style and magnitudes of deformation. For this purpose, from 83 sites in 11 structurally homogeneous domains, 1,680 paleomagnetic samples were analyzed. The results show that AMS fabrics are related to the tectonic deformation and that the magnetic lineation (maximum susceptibility axis, k1) is parallel to inferred maximum extension, while minimum susceptibility (k3) is typically normal to the bedding plane, corresponding to a preserved compaction associated with deposition fabric. The intermediate axis (k2) is parallel to a second extension direction and indicates that the region has been under the control of multi‐directional extension during the Neogene. Two main magnetic lineation directions are identified and represent Oligocene to middle Miocene E‐W, and late Miocene to Pliocene NW‐SE oriented extension. The magnetic lineation directions are dominantly parallel or perpendicular to the general strikes of the normal faults. The results show that the deformation in the region resembles two differentially stretched rubber sheets under the influence of SW oriented extension, exerted by the southward retreating Eastern Mediterranean subduction system.

Emplacement and Segment Geometry of Large, High-Viscosity Magmatic Sheets

Understanding magma transport in sheet intrusions is crucial to interpreting volcanic unrest. Studies of dyke emplacement and geometry focus predominantly on low-viscosity, mafic dykes. Here, we present an in-depth study of two high-viscosity dykes (106 Pa·s) in the Chachahuén volcano, Argentina, the Great Dyke and the Sosa Dyke. To quantify dyke geometries, magma flow indicators, and magma viscosity, we combine photogrammetry, microstructural analysis, igneous petrology, Fourier-Transform-Infrared-Spectroscopy, and Anisotropy of Magnetic Susceptibility (AMS). Our results show that the dykes consist of 3 to 8 mappable segments up to 2 km long. Segments often end in a bifurcation, and segment tips are predominantly oval, but elliptical tips occur in the outermost segments of the Great Dyke. Furthermore, variations in host rocks have no observable impact on dyke geometry. AMS fabrics and other flow indicators in the Sosa Dyke show lateral magma flow in contrast to the vertical flow suggested by the segment geometries. A comparison with segment geometries of low-viscosity dykes shows that our high-viscosity dykes follow the same geometrical trend. In fact, the data compilation supports that dyke segment and tip geometries reflect different stages in dyke emplacement, questioning the current usage for final sheet geometries as proxies for emplacement mechanism.

Cenozoic deformation in the Tethyan Himalaya, SE Tibet: Insights from magnetic fabrics and structural analysis of Upper Triassic flysch

Anisotropy of magnetic susceptibility (AMS) analyses and structural observations of Triassic flysch in the Tsetang area, SE Tibet, were carried out to assess the Cenozoic tectonic evolution and kinematics of deformation of the eastern Tethyan Himalaya Thrust Belt. We analysed a total of 202 standard-sized specimens from four sample sites across the Renbu–Tsetang Thrust from north to south. Rock magnetism and the detailed mineralogy demonstrated that ferromagnetic and paramagnetic minerals both contribute to the AMS. However, the paramagnetic contribution is dominant in the study area. The AMS results, together with field-based structural investigations and analyses of microstructures, revealed that layer-parallel shortening (LPS) was accommodated mainly by isoclinal folds, cleavage, and thrust faults. Shear fabrics (K1 axes oriented parallel to the direction of local tectonic transport) were found mainly in the north thrust fault zone and were linked to fault movement. Microstructural analyses of quartz and metamorphic minerals such as muscovite (sericite) and chlorite in slates and sandstones imply that the peak temperature during the deformation did not exceed 300 °C. The shear AMS fabric in north Qonggyai, near the thrust zone, shows that the strain variation is related to the main activity (11–19 Ma) of the Renbu–Tsetang Thrust. The magnetic lineation that plunges towards the east in the southern Qonggyai area indicates extension that was linked to an E–W extensional event during the early Miocene. Based on the available geochronological data, two stages of tectonic deformation can be defined: (a) syncollisional layer-parallel shortening at ~40 Ma ago, and (b) E–W extension and continuing compaction resulting in shearing at 11–19 Ma.

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.

Deformation mechanisms and kinematics of a soft sedimentary bed beneath the Scandinavian Ice Sheet, north-central Poland, revealed by magnetic fabrics

Characterising deformable substrates of modern and past ice sheets is a key factor in understanding ice mass behaviour and dynamics. This is challenging when field-scale signatures of deformation in soft sedimentary beds are weakly developed or absent. To overcome these difficulties, in this study, we combine micro- and macroscale studies and quantitative analyses to decipher the nature of subglacial sediment deformation under a Pleistocene fast-flowing (streaming) ice sheet. We investigated a complex sedimentary succession at Nieszawa in north-central Poland. A multi-proxy approach involving till micromorphology in thin sections, ripple-cross lamination orientation, till fabric, clast microfabric and the anisotropy of magnetic susceptibility (AMS) data in a closely sampled vertical profile throughout the fluvial sand and till was used to reveal subglacial bed deformation mechanisms and kinematics. This records subglacial deformation of the sedimentary succession at Nieszawa under low basal shear stresses, where simple shear was the dominant bed deformation mechanism. Intergranular advection, decoupling at the ice-bed interface related to high subglacial water pressure, clast ploughing, and basal sliding also took place. The main finding of this study is that AMS reveals the shear strain development and ice flow direction in the structurally and texturally complex deposits under weak coupling conditions at the ice-bed interface beneath a fast flowing/streaming ice sheet, where strain and kinematic indicators other than AMS fabrics are vague or absent.

Post-Depositional Fluid Flow in Jurassic Sandstones of the Uncompahgre Uplift: Insights From Magnetic Fabrics

The anisotropy of magnetic susceptibility (AMS) in sedimentary rocks results from depositional, diagenetic, syn- and post-sedimentary processes that affect magnetic grains. Some studies have also shown the potential role played by post-depositional fluid flow in detrital and carbonate formations. Here we present a new case study of Middle-Upper Jurassic sandstones where secondary iron oxides, precipitated from fluids that migrated through pores, give rise to the AMS. These sandstones are well exposed in the Uncompahgre Uplift region of the Central Colorado Trough, Colorado. The magnetic foliation of these undeformed, subhorizontal strata consistently strike NE-SW over a large distance with an average 45° dip to the SE. This steep AMS fabric is oblique with respect to the regional subhorizontal bedding and therefore does not reflect the primary sedimentary fabric. Also, outcrop-scale and microscopic observations show a lack of post-depositional plastic (undulose extinction) or pressure-solution (stylolites) deformation microstructures in these sandstones, hence precluding a tectonic origin. The combination of magnetic hysteresis, isothermal remanent magnetization, and thermal demagnetization of the natural remanent magnetization indicate that these rocks carry a chemical remanent magnetization born primarily by hematite and goethite. High-field magnetic hysteresis and electron microscopy indicate that detrital magnetite and authigenic hematite are the main contributors to the AMS. These results show that post-depositional iron remobilization through these porous sandstones took place due to the action of percolating fluids which may have started as early as Late Cretaceous along with the Uncompahgre Uplift. The AMS fabric of porous sandstones does not systematically represent depositional or deformation processes, and caution is urged in the interpretation of magnetic fabrics in these types of reservoir rock. Conversely, understanding these fabrics may advance our knowledge of fluid flow in porous sandstones and may have applications in hydrocarbon exploration.