Anisotropy Of Magnetic Susceptibility Pattern Research Articles

Basal Décollement Splaying Induces Mid‐Crustal Tectonic Imbrication in an Intracontinental Orogen

AbstractMid‐to‐lower crustal rock exhumation is common in orogenic belts, but the deformation process exposing these rocks remains debated. Distributed deformation in low viscous crust extruding mid‐to‐lower crustal rocks as channel flow and localized deformation along shear zones imbricating rigid blocks are two end‐members that account for crustal thickening and unroofing. At the northwest of the Early Paleozoic orogenic belt in the South China Block, the Jiuling Massif includes orogenic root rocks exhumed from deep crustal level. Their structural pattern and exhumation history can improve our understanding on how continental mid‐to‐lower crust is deformed, thickened, and finally transported to the surface. Structural analysis reveals that two major mid‐crustal ductile shear zones and their splays are developed at temperatures of ∼350°C–550°C. Anisotropy of magnetic susceptibility (AMS) shows that the Southern Jiuling Batholith has a modified AMS pattern by syn‐orogenic compression, suggesting a gradually deformed rigid block. Combining surface geological evidence and deep structures by gravity modeling, we find shear zones rooted in basal décollement incrementally stacked the rigid granitic blocks. Along strike, the major shear zones evolved differently with more splays at their eastern portions. Thus, tectonic imbrication can evolve to pervasive flow‐like deformation as shear zones continue to splay and form an anastomosed shear zone system. The complexed structures by splayed shear zones segmenting and imbricating small rigid blocks may correspond to the geophysically low‐velocity zone in the crust, so shear zone splaying is a linking mechanism between tectonic imbrication and viscous flow deformation of the crust.

Deformation understanding in the Upper Paleozoic of Ventana Ranges at Southwest Gondwana Boundary

At the east of the Ventana Ranges, Buenos Aires, Argentina, outcrops the Carboniferous-Permian Pillahuincó Group (Sauce Grande, Piedra Azul, Bonete and Tunas Formation). We carried out an Anisotropy of Magnetic Susceptibility (AMS) study on Sauce Grande, Piedra Azul and Bonete Formation that displays ellipsoids with constant Kmax axes trending NW–SE, parallel to the fold axes. The Kmin axes are orientated in the NE–SW quadrants, oscillating from horizontal (base of the sequence-western) to vertical (top of the sequence-eastern) positions, showing a change from tectonic to almost sedimentary fabric. This is in concordance with the type and direction of foliation measured in petrographic thin sections which is continuous and penetrative to the base and spaced and less developed to the top. We integrated this study with previous Tunas Formation results (Permian). Similar changes in the AMS pattern (tectonic to sedimentary fabric), as well as other characteristics such as the paleo-environmental and sharp curvature in the apparent polar wander path of Gondwana, marks a new threshold in the evolution of the basin. Those changes along the Pillahuincó deposition indicate two different spasm in the tectonic deformation that according to the ages of the rocks are 300–290 Ma (Sauce Grande to Bonete Formation deposition) and 290–276 Ma (Tunas Formation deposition). This Carboniferous-Permian deformation is locally assigned to the San Rafael (Hercinian) orogenic phase, interpreted as the result of rearrangements of the microplates that collided previously with Gondwana, and latitudinal movements of Gondwana toward north and Laurentia toward south to reach the Triassic Pangea.

Anisotropy of Magnetic Susceptibility (AMS) of Sediments From Holes U1480E and U1480H, IODP Expedition 362: Sedimentary or Artificial Origin and Implications for Paleomagnetic Studies

AbstractRecognition of coring‐induced disturbance, which is essential for magnetic fabric and paleomagnetic studies of poorly lithified sediments, is generally not straightforward. Here, we report on anisotropy of magnetic susceptibility (AMS) and paleomagnetic data of the sediments from Holes U1480E and U1480H, IODP Expedition 362, west of the Sumatra subduction zone. AMS is characterized by steep minimum principal axes (Kmin) in undisturbed sediments. However, a considerable portion of the recovered sediments are affected by significant coring‐induced disturbance. In these cases, we observed three AMS patterns: (1) AMS principal axes are randomly distributed for sediments with mingling and distortion of beds, (2) Kmin axes of sediments with upward‐arching beds are deflected out of the splitting face of the working half, and (3) suck‐in sediments are characterized by vertical Kmax axes. These deformation‐dependent AMS patterns can be attributed to the realignment of mineral particles caused by the coring process and subsequent sampling procedures. Besides a low‐coercivity, vertical, drilling‐induced overprint, we observed a high‐coercivity component that is likely a composite of the primary magnetization with a demagnetization‐resistant portion of the drilling overprint. After accounting for the disturbed intervals, several polarity transitions can be identified in the undisturbed sediments which correlate well with the Pleistocene geomagnetic polarity timescale. These observations demonstrate that great caution is required when attributing geological significance to AMS and paleomagnetic data obtained from soft sediment cores, which are highly susceptible to coring‐induced disturbance. In addition, AMS measurements provide a potential tool for identifying core deformation for further paleomagnetic studies.

The anisotropy of magnetic susceptibility of uniaxial superparamagnetic particles: Consequences for its interpretation in magnetite and maghemite bearing rocks

AbstractA simple model that provides a quantitative description of the magnetic susceptibility of superparamagnetic to stable single‐domain uniaxial magnetic particles can be built in the framework of the theory of stochastic resonance. This model expands that of Mullins and Tile (1973) for superparamagnetic grains by considering the dependence of superparamagnetic susceptibility on the particle orientation and thus describes the anisotropy of magnetic susceptibility (AMS) of ensembles of superparamagnetic as well as single‐domain particles. The theory predicts that on the contrary of stable single domain, the maximum anisotropy of superparamagnetic particles is parallel to their easy axis and shows that the AMS of ensembles of uniaxial particle is strongly dependent on the distribution of particle grain size, coercivity, measurement temperature, and frequency. It also explains why the inverse AMS pattern expected for stable single‐domain particles is rarely observed in natural samples. We use examples of well‐characterized obsidian specimens to show that, as predicted by the theory, in the presence of significant superparamagnetic contributions, the maximum susceptibility axis of AMS is directed along the preferential direction of particles easy axis.

The Paleoproterozoic Djebel Drissa ring complex (Eglab shield, Algeria): Post-collisional intrusions in a transtentional tectonic setting

The Eburnean A-type granitic Djebel Drissa ring complex is part of the 2.08Ga post-collisional Aftout magmatic suites that intruded the 2.22–2.09Ga-old Paleoproterozoic granitoids in the Eglab shield, in West African Craton. It is composed of unfoliated granitoids arranged in near-concentric zones. The zoning is interpreted as resulting from multiple intrusions of continuous magmatic pulses, leading to in situ growth of the complex.New field observations, anisotropy of magnetic susceptibility (AMS), gravimetry and aeromagnetic data suggest that the regional deformation controlled the emplacement of this complex. The sub-circular form of the granitic body and the AMS pattern are consistent with a transtensional tectonic setting along a NW–SE major shear-zone and with an emplacement also related to an associated WNW–ESE extension fault. This well-dated regional deformation corresponds to the last main structural event of the Paleoproterozoic evolution of the Eglab shield.

Extreme effusive eruptions: Palaeoflow data on an extensive felsic lava in the Mesoproterozoic Gawler Range Volcanics

The Gawler Range Volcanics are the eroded remnants of a Mesoproterozoic dominantly felsic large igneous province. The Eucarro Rhyolite is one of three voluminous (> 675 km 3) units in the upper part of the succession. Eight stratigraphic sections through the Eucarro Rhyolite were sampled for anisotropy of magnetic susceptibility (AMS) and petrofabric analysis in order to determine the source location. The AMS foliations (Kmax/Kint plane) consistently strike northwest and most dip steeply. At sites in the western and central parts of the unit, there is a change from steep southwesterly dips near the base, to vertical or northeasterly dips in the middle and near the top. In the same area, Kmax axes plunge to the south or southwest in the lower parts, and to the north in upper parts, of the unit. At sites in the eastern part of the unit, the pattern is reversed: AMS foliations dip northerly near the base and southerly near the top, and Kmax axes plunge northerly near the base and southerly near the top. Such up-section reversals in dip or plunge directions of AMS parameters resemble the patterns shown by other lavas. A dominance of very steep dips and an up-section reversal in dip direction are also expected for flow bands in felsic lavas. Furthermore, the AMS pattern implies a northeasterly flow direction for the western and central parts of the unit, and a southerly flow direction for the eastern part. The opposing flow directions may indicate that the western and central parts of the unit versus the eastern part belong to different flow lobes, or that they came from separate sources. Lineations defined by preferred alignment of elongate phenocryst intersections on gently dipping surfaces show a less consistent pattern and, alone, could not be used to define palaeoflow directions. This study confirms the existence of felsic lavas that have dimensions comparable with the largest known mafic lavas.

Magnetic fabric studies of the Nipissing sill province and Senneterre dykes, Canadian Shield, and implications for emplacement

The 2.21 Ga Nipissing diabase sills intrude the Huronian Supergroup of the Southern Province. Coeval Senneterre dykes in the adjacent portion of the Superior Province are potential feeders for the sills. Anisotropy of magnetic susceptibility (AMS) data were obtained from both of these units to study flow patterns and test emplacement models. The sills exhibit three distinct paleomagnetic directions that are likely primary and represent a time-correlation tool within the magmatic event. Two of the three directions are also found in Senneterre dykes. Sites in the northern Cobalt Embayment carry an N1 (reverse) paleomagnetic signature. They possess maximum AMS axes that trend north-northwest–south-southeast, interpreted as the direction of magma emplacement. In one area, the data rule out a cone sheet emplacement mechanism. The N3 paleomagnetic signature is restricted to a sill and (or) dyke-like bodies near Lake Timiskaming, where the AMS axes are dominantly north-northwest–south-southeast, similar to that of the nearby N1 (reverse) sites. Sites in a geographically restricted area farther south carry an N1 (normal) paleomagnetic direction. The AMS patterns in these sites exhibit very poor within- and between-site consistency, compatible with the magma having crystallized statically in this volumetrically minor magmatic pulse. In the Sudbury – Elliot Lake area, the Nipissing diabase is deformed, and outcrops exhibit east–west elongation parallel to the fold axes of the enclosing Huronian sedimentary rocks. AMS fabrics are interpreted as reflecting this deformation rather than primary magmatic flow. Three of eight Senneterre dyke sites show horizontal flow patterns, whereas the remainder have more complicated AMS patterns.

New constraints on source location and emplacement mechanisms of extensive felsic units in the Gawler Range Volcanics, South Australia

Eucarro Rhyolite is one of three voluminous (>675 km3 ) units in the upper part of the Gawler Range Volcanics. Eight stratigraphic sections through the Eucarro Rhyolite were sampled for anisotropy of magnetic susceptibility (AMS). The unit has AMS characteristics similar to those of lavas. AMS foliations (Kmax/Kint plane) consistently strike northwest and are very steeply dipping. Seven sections show a change from steep southwesterly dips near the base to vertical or northeasterly dips in the middle and near the top. Furthermore, the AMS pattern implies outflow from a source located to the southwest of present outcrops. In the remaining section, the AMS pattern implies outflow from the northeast. The opposing flow directions may be delineating the boundary between adjacent lava flow lobes. This study suggests that felsic lavas can have dimensions comparable with those of mafic lavas.

Injection mechanism of clay‐rich sediments into dikes during earthquakes

Clastic dikes may form by simultaneous fracture propagation in rocks and injection of clastic material into the fractures resulting from strong seismic shaking. We studied the mechanisms of clastic‐dike formation within the seismically active Dead Sea basin, where hundreds of clastic dikes cross‐cut the soft rock of the late Pleistocene lacustrine Lisan Formation. We analyzed the anisotropy of magnetic susceptibility (AMS) of dikes with known formation mechanisms and defined the characteristic AMS signatures, mainly of dikes developed by injection process. Most of the dikes were emplaced due to fluidization of clay‐rich sediment and are characterized by triaxial AMS ellipsoids. The dominant triaxial AMS ellipsoids along the dike widths suggest that the fluidization mechanism of clay‐rich sediment is different from the known liquefaction process of sand. The AMS analysis supported by field evidence indicates that the injection of clay‐rich sediment is characterized by two main regimes: (1) Vertical flow characterized by subvertical V2 axes and subhorizontal V1 and V3 axes. The V2 axes may indicate the flow directions during fast flow. (2) Horizontal slow flow characterized by subvertical V3 axes and subhorizontal V1 and V2 axes. A streaked AMS pattern mainly composed of V2 and V3 axes represents a turbulent flow that generated local eddies simultaneously with the clastic transport. The AMS parameters along the dikes and possible grain imbrications along dike walls support organization of grains under high strain rates. This application of the AMS method provides a petrofabric tool for identifying seismites and inferring their flow kinematics in complex geologic areas.

A magnetic fabric study of the Aigoual–Saint Guiral–Liron granite pluton (French Massif Central) and relationships with its associated dikes

In the southeastern French Massif Central, the Aigoual–Saint Guiral–Liron pluton consists of porphyritic and microgranitic types. The latter is encountered within dikes forming the northern end of the pluton. Both types show prefull crystallization microstructures indicating weak subsolidus deformations. An anisotropy of magnetic susceptibility (AMS) study has been carried out to determine the granite fabric. Biotite, local hornblende, and small grains of magnetite are the main carriers of AMS in both types. Porphyritic granite and dikes display different AMS patterns related to magma dynamics and regional deformation recorded during crystallization. In the porphyritic type, the AMS lineation is mainly consistent with the regional, NW‐SE to E‐W trending, extensional event coeval with emplacement and crystallization of the pluton indicating an influence of regional tectonics on the linear fabric development. The dome shaped foliation pattern of the Saint Guiral massif likely corresponds to internal deformation‐related processes within the pluton. In the microgranite dike swarm, the NNE‐SSW trending lineations with shallow plunges record magmatic flow processes within dikes, i.e., magma injection and filling of dikes from a probable source located southwestward. Regional tectonics played a significant role in the pluton geometry and fabric development. For example, the NE‐SW trend of the dikes suggests that extensional fractures took place in the same extensional strain field as elsewhere in the pluton. Different fabric development modes were therefore responsible for the contrasted fabric patterns between the microgranite and the porphyritic granite.

Anisotropy of magnetic susceptibility (AMS) analysis of basalt dikes at Cathedral Cliffs, WY: implications for Heart Mountain faulting

Mafic dikes pervade the upper plate of the Heart Mountain Detachment (HMD), yet the dike concentration in the lower plate is sparse. Previous workers interpreted that these dikes were emplaced either coeval with or subsequent to the emplacement of the upper plate. The magnetic fabrics of 32 mafic dikes at Cathedral Cliffs were analyzed using low-field anisotropy of magnetic susceptibility (AMS) as a proxy for flow. These dikes intrude Ordovician–Mississippian carbonate and overlying Eocene volcanic rocks and are truncated along the nearly horizontal HMD. The dikes trend between N10°W and N20°E, are all steeply dipping, and range in width between 0.5 and 3 m. Flow directions for the dikes were determined by the bearing and plunge of the K max (maximum principal susceptibility) axes relative to the dike orientation. About 66% of the dikes sampled show typical dike AMS patterns with K max and K int in the plane of the dike and K min normal to the dike plane. About 66% of the dikes sampled have K max inclinations >45° and thus were emplaced upward; 16% of the dikes have K max inclinations of <10° and thus were emplaced laterally. The remaining dikes have intermediate K max inclinations. With numerous dikes showing vertical to sub-vertical emplacement directions and with no magmatic source immediately below the detachment the dikes must predate emplacement of the upper plate. Therefore, upper plate dilation by dike intrusion could not be a driving force for protracted extension. Our date is consistent with a single catastrophic emplacement event, and inconsistent with an extensional allochthon model of incremental emplacement over long intervals of time.

Deformation regime variations in an arcuate transpressional orogen (Ribeira belt, SE Brazil) imaged by anisotropy of magnetic susceptibility in granulites

The Ribeira belt of southeastern Brazil displays an arcuate shape, with a structural trend that varies from ∼NS in the northern domain to ENE–WSW in the southern domain. This curvature is accompanied by a transition from contraction-dominated to transcurrent-dominated tectonics. The transition in deformation regime is accommodated in the central domain of the belt where granulitic rocks dominate and mineral-stretching lineations are commonly concealed by metamorphic recrystallization. We present anisotropy of magnetic susceptibility (AMS) data from 664 samples from 62 sites in high-temperature gneisses, granulites and migmatites of the transitional, central domain of the belt, with the aim of investigating: (1) how well AMS allows one to map the mineral-stretching lineation and foliations in domains displaying a complex kinematic framework and (2) to investigate the kinematic pattern at the transition between the thrusting dominated and a wrench-faulting dominated orogenic segments. The mean magnetic susceptibility is 7.54×10 −3 SI. The degree of anisotropy varies from 1.32 up to 4.31, with an average value of 1.53. The shape parameter T is generally >0 meaning that the AMS ellipsoid is dominantly oblate. Magnetic lineations and foliations form a consistent pattern correlated with the modification of the structural characteristics observed along the Ribeira belt. In the southern wrench-fault-dominated domain, the magnetic lineation is subhorizontal, parallel to the trend of the steeply dipping magnetic foliation. This correlation with the fabric observed in mylonites suggests that the magnetic fabric is a valid proxy of the tectonic fabric in granulites. Results from the northern domain show that it comprises two sub-domains both displaying a ∼NS-trending magnetic foliation. Eastward, over a broad area, the magnetic foliation is consistently steeply dipping and bears a shallowly to moderately plunging magnetic lineation. Westward, the dip of the foliation decreases progressively and the lineation rotates from nearly parallel to nearly orthogonal to the trend of the foliation, suggesting a progressive transition from transcurrent to contractional tectonics. At the junction between the northern and southern domains, the magnetic fabric is composite: the magnetic foliation displays a constant NNE–SSW orientation, but the magnetic lineation spreads within the foliation plane from parallel to normal to the foliation direction. The AMS pattern mapped in the Central Ribeira belt supports a scenario in which the transcurrent and contractional deformation regimes are coeval, with a transition between the two regimes accommodated progressively over a large transpressional area where both strain regimes were active.

Magnetic study of Late Pleistocene loess/palaeosol sections from Siberia: palaeoenvironmental implications

Summary Rock-magnetic properties, including anisotropy of magnetic susceptibility, were investigated in detail for two loess/palaeosol sections in south-western Siberia (Bachat and Kurtak). The results obtained agree with the ‘Alaskan’ type of susceptibility variations for the Kurtak section, showing maximum susceptibility values in loess horizons and minima in palaeosols. For the Bachat section the palaeoenvironmental record, expressed through susceptibility, is not clear, in spite of the geographical proximity of the two sections. Instead, frequency-dependent magnetic susceptibility (FD%) discriminates well between loess and palaeosol units, showing maxima in pedocomplexes and minima in loess units. This suggests certain pedogenic formation of fine ferrimagnetic grains in soils. Studies of anisotropy of magnetic susceptibility (AMS) reveal a well-defined sedimentary magnetic fabric for both sections. Different origins of loess deposition, pure aeolian at Bachat and aeolian affected by secondary processes at Kurtak are reflected in the AMS patterns. The two sections also show different high-temperature behaviour of magnetic susceptibility. While for the Bachat section only one type of thermomagnetic curve was found, the Kurtak section shows different behaviour for loess and palaeosol units. Magnetic properties of samples from the two sections are discussed in terms of palaeoclimatic and depositional conditions.

Anisotropie de susceptibilité magnétique (ASM) et chimie des traces : une nouvelle méthodologie pour démêler processus hydrothermaux et supergènes. Application au gisement à Ba–Fe–F de Chaillac (Indre, France)

Abstract Anisotropy of magnetic susceptibility (AMS) determinations and trace-element analyses have been firstly carried out, at Chaillac, on two ferruginous sandstone sections of hydrothermal and lateritic origins, respectively. The goethite has been identified as the main AMS carrier. Distinct AMS patterns and characteristic Ti–As anomalies have been observed, which allows to quantify the importance of hydrothermal and lateritic processes when both are superimposed. These preliminary results show that the ‘Fe–Ba complex’ of hydrothermal origin at Chaillac has been lateritized for a few meters from the present erosion surface.

Sedimentary fabric on deep-sea sediments from KODOS area in the eastern Pacific

An anisotropy of magnetic susceptibility (AMS) study on three sediment cores taken from the Korea Deep Ocean Study (KODOS) area in eastern Pacific was carried out to understand the relationship between magnetic fabric patterns and paleo-depositional conditions of deep-sea sediments. Most AMS measurements reveal normal sedimentary fabric patterns indicating a stable sedimentation mode. However, some intervals show deformed AMS patterns (magnetic lineation), and this anomalous fabric is mainly observed at the boundary where sediment color changes. The abrupt change of AMS parameters at this boundary provides evidence that the linear fabric was induced by input of intensified bottom currents. Based on the trend of microfossil occurrences, it is interpreted that the discontinuous color boundary was formed by hiatus between Tertiary and Quaternary age. The change of magnetic fabric at the topmost color boundary coincides with the time of prevailed antarctic bottom water (AABW) activity. In this study, the AMS properties are shown to be useful tools for making inferences concerning the paleoenvironment of abyssal basins.

Rock magnetic properties and structural development in the core of the Hazara‐Kashmir Syntaxis, NE Pakistan

Paleomagnetic and structural analyses have been conducted on three well‐exposed sections through a 6–8 km thick pile of molassic red beds (Murree formation) in the lowermost tectonic unit of the Hazara‐Kashmir Syntaxis. Micropaleontological age determinations of the lowermost Murree sediments indicate Late Paleocene deposition (55 m.y.). From south to north, the sections are situated in the Jhelum, Neelum (both in Azad Kashmir), and Kaghan (northeastern Pakistan) valleys. Thermal demagnetization experiments suggest that haematite with high unblocking temperatures carries stable characteristic remanence directions. The relationship between finite strain and magnetic fabric was established by mapping deformed reduction spot strain markers and by measuring the anisotropy of magnetic susceptibility (AMS). For the Jhelum valley the weakly tectonized Murree beds are characterized by flattened AMS ellipsoids resulting from diagenetic compaction. Inclination values suggest that the Murree foreland basin started to develop at about 8°N during the early suturing of India and the development of island arcs to the north. India has moved northward by at least 2600 km since collision with Eurasia in the Paleocene. Declination values suggest 45° of clockwise rotation of the block supporting the Murree formation relative to the Indian craton. For the Neelum and Kaghan valleys, quantitative strain mapping shows a progressive increase of deformation northward. NRM directions rotate passively toward the cleavage plane which parallels the foliation of the AMS ellipsoids. NRM directions and AMS ellipsoids are deformed because of superposition of tectonic strain on a primary compactional strain. The AMS pattern is interpreted in light of this superposition, and a regional deformation path from south to north is suggested. A tectonic rotation model is proposed which is consistent with the transport directions around the Hazara‐Kashmir Syntaxis and the rotation of thrust sheets indicated by the NRM data. This model relates the convergent transport directions with the mean indentation direction of India into Asia.

Strain and magnetic fabric in the Santa Catalina and Pinaleno Mountains Metamorphic Core Complex Mylonite Zones, Arizona

Anisotropy of magnetic susceptibility (AMS) is capable of recording finite strain in weakly magnetized rocks. AMS was measured for 228 samples from 20 sites in two mylonite zones with the same deformational history. AMS measurements were compared with finite strains determined from dike rotations and from foliation orientations. In one zone (the Santa Catalina Mountains) the orientations of susceptibility and finite strain ellipsoids are in excellent agreement, and there is a logarithmic relationship between susceptibility difference and finite strain magnitude. In the second zone (the Pinaleno Mountains) minimum susceptibility is perpendicular to the finite flattening plane, but the maximum susceptibility does not parallel the maximum extension direction, and there is no systematic relationship between susceptibility magnitude and strain magnitude. Oriented polished thin sections indicate that magnetite in the protolith of the Santa Catalina mylonite occurs as randomly oriented, elongate grains. With subsequent deformation, the long axes are rotated into the maximum extension direction. In the Pinaleno mylonites, both equant and elongate magnetite grains are present. With deformation, the elongate magnetite grains are rotated into the maximum flattening plane but show no preferred orientation within this plane. AMS in the two mylonite zones appears to be predominantly controlled by the orientation of elongate magnetite grains with respect to the megascopic fabric. The final orientation of the elongate grains is a function of their initial orientation as well as the finite strain. Therefore, despite similar deformational histories, the two zones display different AMS patterns due to the differences in occurrence, initial orientation, and shape of ferromagnetic grains.