Anisotropy Of Magnetic Susceptibility Analysis Research Articles

A Complex Interplay Between Pluton Emplacement, Tectonic Deformation, and Plate Kinematics in the Cretaceous Sierra Nevada Magmatic Arc, California

AbstractThe relation of plate kinematics to the structural record of arc plutons and their host rocks is complex and still not fully understood. We address this issue through a combination of field mapping, structural analysis, anisotropy of magnetic susceptibility analysis, and fabric modeling in the Late Cretaceous Tuolumne Intrusive Complex, Sierra Nevada, California. A pattern of anti‐clockwise rotation from ∼NNW–SSE to WNW–ESE steep foliations and change in fabric ellipsoid shape from oblate to prolate was revealed in successively emplaced Kuna Crest (∼95–92 Ma), Half Dome (∼92–89 Ma), and Cathedral Peak (∼89–84 Ma) granodiorites. The numerical model indicates that the Kuna Crest was emplaced in a transpressional setting with an angle of convergence α = 60–40°, whereas the Half Dome and Cathedral Peak required simultaneous vertical constriction overprinted by transpression with α = 35–15°. This transition, which occurred at ∼92 Ma, is accompanied by a shallowing of the lineation plunge observed also in other ∼88–84 Ma central Sierra Nevada plutons. Provided that the Cretaceous Sierra Nevada arc was constructed during overall dextral transpression, these transitions reflect significant changes in kinematics, where ∼107–92 Ma plutons were emplaced during pure shear‐dominated transpression, which was followed by a transition to wrench‐dominated transpression recorded in ∼92–84 Ma plutons. Such a transition in kinematics is explained as a result of progressively increasing obliquity of the relative convergence of the Farallon plate subducting beneath the North American continental margin, in agreement with most paleogeographic reconstructions.

Two phases of Cretaceous dextral shearing recorded in the plutonic rocks of NW Nevada (USA): A tectonic link between intra-arc shearing in the Sierra Nevada and Idaho batholiths

Abstract The Cretaceous intrusive units of the Sahwave and Nightingale ranges in northwestern Nevada, USA, located between the Sierra Nevada and Idaho batholiths, represent a critical segment of Cretaceous arc magmatism. U-Pb zircon age dating shows that the older, 104 Ma Power Line intrusive complex is dominantly granodioritic in composition, while the younger 94–88 Ma Sahwave Range intrusive suite (the Juniper Pass, Bob Springs, and Sahwave plutons) is similar in composition (tonalite to granodiorite) and age to the plutons of the Tuolumne intrusive suite of the east-central Sierra Nevada batholith. We present new field measurements, microstructural observations, and anisotropy of magnetic susceptibility analyses of the Power Line intrusive complex and Sahwave Range intrusive suite. The Power Line intrusive complex is characterized by a vertical, N–S-striking, solid-state foliation and down-dip lineation. Evidence of dextral shearing is observed on subhorizontal planes that are perpendicular to the lineation, which is consistent with pure shear-dominated transpression. This fabric is similar in style and timing to both the western Idaho shear zone of the Idaho batholith and mid-Cretaceous shear zones of the central Sierra Nevada. The plutons of the Sahwave Range intrusive suite are not affected by the pure shear-dominated transpressional fabric observed in the Power Line intrusive complex, which indicates that this deformation ceased by ca. 94 Ma. Rather, the Juniper Pass pluton contains an E–W-striking magmatic foliation fabric that rotates to a steep NW–SE-striking, solid-state foliation in the younger Sahwave pluton. These fabrics are strikingly similar to fabrics in the Tuolumne intrusive suite, Sierra Nevada, California, USA. Recent work in the western Idaho shear zone also indicates that late-stage deformation occurred there until ca. 85 Ma. Therefore, the intrusions of northwestern Nevada provide a tectonic link between the Sierra Nevada and Idaho batholiths, which suggests that two distinct phases of mid-Cretaceous, transpressional deformation occurred in at least three magmatic arc segments of the western U.S. margin.

Paleomagnetism of late Cretaceous dykes in the Gangdese belt: New constraints on the position and structure of the southern margin of Asia prior to the India-Asia collision

We report paleomagnetic data from late Cretaceous diorite dykes that sub-vertically intrude granodiorites in the eastern Gangdese belt near the city of Lhasa. Our research goals are to provide further constraints on pre-collisional structure of the southern margin of Asia and the onset of the India-Asia collision. Magnetite is identified as the main magnetic carrier in our study. The magnetite shows no evidence of metamorphism or alteration as determined from optical and scanning electron microscope observations. A strong mineral orientation is revealed by anisotropy of magnetic susceptibility analysis both for the intruded dykes and the country rocks. We interpret this AMS fabric to have formed during intrusion rather than deformation. Fifteen of 23 sites yield acceptable site mean characteristic remanences with dual polarities. A scatter analysis of the virtual geomagnetic poles suggests that the mean result adequately averaged paleosecular variation. The paleomagnetic pole from the Gangdese dykes yields a paleolatitude of 14.3°N ± 5.8°N for the southern margin of Asia near Lhasa. The paleolatitude corresponds to an in-between position of the Lhasa terrane during ~130-60 Ma. Furthermore, the mean declination of the characteristic remanent magnetization reveals a significant counterclockwise rotation of 18° ± 9° for the sampling location since ~83 Ma. In the light of tectonic setting of the dykes, the strike of the southern margin of Asia near Lhasa is restored to trend approximately ~310°, which is compatible with the hypothesis that the southern margin of Eurasia had a quasi-linear structure prior to its collision with India.

Emplacement dynamics of a crystal-rich, highly viscous trachytic flow of the Sancy stratovolcano, France

Emplacement dynamics of highly viscous, silicic lava flows remain poorly constrained due to a lack of consideration of crystal-rich cases. Emplacement models mostly apply to glassy or microlitic, vesiculated rhyolitic flows. However, crystalline, vesicle-free silicic lava can flow differently. We studied the Grande Cascade unit, which is a vesicle-free, phenocryst-rich, trachytic flow in the Monts Dore massif, France. Field work was carried out to define internal structures, and oriented samples were collected for chemical, petrological, and anisotropy of magnetic susceptibility analyses, allowing us to estimate emplacement temperature and viscosity. These data allow us to define a new silicic lava flow subtype that is low in temperature (800−900 °C), high in silica content (up to 66.8 wt%), high in viscosity (109−1011 Pa s), rich in phenocrysts (∼35%), and lacks vesicles. Brittle deformation of the lava occurs upon extrusion, generating a cataclasite basal layer and thin (3-m-thick) shear zone that accommodates all of the stress, allowing most of the flow’s volume to slide over its base as a 40-m-thick plug in which there is no deformation. Blocks are rare, of a single size (10 ± 1 cm), and result from localized break-up of the basal shear zone. Emplacement dynamics are different from those of glassy, pumiceous lava flows. They are closer to glacier dynamics, where most of the volume slides over a thin basal shear zone and till is generated there by abrasion and milling of the underlying layer. For the Grande Cascade lava flow, abrasion means that the flow lacks its classical blocky crust and instead the flow base is marked by a layer rich in fine-grained material. The structures and emplacement dynamics of this crystal-rich flow are consistent with ideal, gravity-driven shear flow. We thus argue for a global reassessment of silicic-rich lava emplacement based on crystal content and using a multidisciplinary approach focused on well-exposed examples in the rock record.

Comment on “Paleomagnetism of the Late Cretaceous Red Beds From the Far Western Lhasa Terrane: Inclination Discrepancy and Tectonic Implications” by Bian et al.

AbstractBian et al. (2020, https://doi.org/10.1029/2020TC006280) recently studied the paleomagnetism of the Late Cretaceous redbeds and summarized all the published paleomagnetic data obtained from the Lhasa terrane. A total of 25 poles from western and central‐eastern Lhasa terrane were collectively analyzed, of which they concluded 21 poles are reliable. These filtered directions, corrected for tilt and depositional biases following Tong et al. (2019, https://doi.org/10.1016/j.earscirev.2019.02.009), were then used to reconstruct the paleoposition and shape of the southern margin of the Asian continent. However, some of Bian et al.'s (2020, https://doi.org/10.1029/2020TC006280) interpretations are in conflict with the known geology of the Lhasa terrane. We argue that they have not applied the appropriate structural corrections to their data. The magnetic directions need to be corrected for a plunging fold axis and evaluated more thoroughly for post‐depositional strain. We recommend the data as presented in Bian et al. (2020, https://doi.org/10.1029/2020TC006280) should not be used for paleogeographic reconstructions. Moreover, cautions need to be given when applying paleomagnetic data with anisotropy of magnetic susceptibility analysis showing pencil structure to the deformational fabric to paleogeographic reconstructions.

Anisotropy of magnetic susceptibility analysis in Tunas Formation cores (Permian), Claromecó Basin, Buenos Aires, Argentina: Its relation to depositional and post-depositional conditions

Anisotropy of magnetic susceptibility (AMS) has been used as an important tool to study magnetic fabrics and as a basis of interpretation of depositional conditions and post-depositional processes in sedimentary successions. This contribution presents results of standard AMS and magnetic mineralogy analysis performed in core samples from the PANG 0003 well, located at the Claromecó Basin, Buenos Aires province, Argentina. The sedimentary sequence analyzed has been assigned to the Tunas Formation (Permian), upper Pillahuincó Group, southwestern of the Gondwana boundary. Additionally, obtained results were compared with previous Tunas Formation outcrop data at the Sierras Australes and Claromecó Basin area.Measured AMS parameters vary with depth, reflecting changes in postdepositional conditions such as mechanical compaction and the influence of tectonic stress. Analyses of the mean magnetic susceptibility, thermomagnetic properties and petrographic studies indicate that paramagnetic minerals as Fe-phyllosilicates are the main contributors for the AMS of this sequence. Obtained AMS parameters are mainly characterized by triaxial to oblate AMS ellipsoids and shortening directions (Kmin) with a tendency to cluster in vertical positions, parallel to the bedding plane poles, assigned to sedimentary or weak deformed magnetic fabrics. By contrast, toward the base of the sequence, strongly oblate AMS ellipsoid shapes and Kmin axes that migrate to horizontal positions indicate a high deformation degree product of tectonic stress. The presented results are consistent with geological, AMS and paleomagnetic data from Tunas Formation outcrops and confirm that deformation degree was gradually attenuated upwards in the sequence, to the younger strata and toward the foreland Claromecó basin. The study of AMS parameters and magnetic mineralogy have proven to be important tools to interpret the processes that acted during the burial history of Tunas Formation, increasing the knowledge about the evolution of the Claromecó Basin.

Syn‐tectonic emplacement during sinistral transpression: The Late Triassic Gaoqiao pluton in the South Qinling Belt, central China

Internal structures of the pluton may provide key information on magma emplacement mechanism and the relationship with regional tectonics. We present a case study on the Late Triassic Gaoqiao pluton from South Qinling Belt, which was emplaced under a syn‐collisional setting. Multi‐approaches including zircon U–Pb geochronology, field and microstructural observations, anisotropy of magnetic susceptibility analysis was conducted to reconstruct the growth process of the pluton and to investigate the internal structures. Zircon U–Pb geochronology reveals that the pluton is a composite intrusion which was constructed by two successive units with distinct ages (~220 and ~210 Ma). Field observations show that the Gaoqiao pluton and its country rocks recorded abundant evidences of syn‐plutonic deformations, indicating a syn‐tectonic emplacement. Microstructural observations show that the internal fabrics were mainly acquired during the stage from magma flow to high‐T solid‐state deformation. The fabric distributions display ‘onion‐skin’ patterns on the pluton scale, which are characterized by zoned foliations and lineations. Combined with the syn‐collisional convergence setting, we propose a tectonic‐controlled incremental growth model for the pluton, in which the pluton was successively constructed by the Fujiahe unit and the Hetaoping unit, and the emplacement process of the pluton was controlled by the regional deformation with sinistral transpressional kinematics. Furthermore, the internal fabric patterns of the Gaoqiao pluton were probably formed by tectonic‐controlled helical magma flow, which appears to be common in fabric formation of many other plutons with similar features.

A review of anisotropy of magnetic susceptibility analysis of Indian dykes: Implications for magma emplacement

The analysis of Anisotropy of Magnetic Susceptibility (AMS) is a powerful and rapid technique to examine the preferred orientations of mineral (magnetic) fabrics and can indicate the nature of a magma transport (vertical or lateral). The relationship between magnetic fabric and geometry of a dyke swarm enables us to understand magma emplacement processes. Depending on the mutual relationship of magnetic fabric and individual dyke geometry, mode of magma transport is interpreted. The knowledge on the nature of magma transport combined with information on geometry, magmatic overpressure and geochemistry enable us to comment on dyke emplacement processes, the location of possible feeders, syn-emplacement and post-emplacement deformations and prevailing stress regime during emplacement. A number of dykes and dyke swarms have been emplaced into the Indian shield at different points in time. Their ages vary from the Mesoarchean to Tertiary. We present here a review of three case studies where AMS technique was applied to the samples collected from Indian dykes. Two case studies are on the Proterozoic dykes that intruded into the Dharwar craton and the third case study is on Mesozoic dykes that punctured the South Indian Granulite Terrain (SIGT). The dykes generally show “normal” anisotropy fabric to indicate vertical magma emplacement with few exceptions where lateral/inclined magma flow was suggested or the results were inconclusive. We present here a critical review on the interpretation of such “anomalous” fabrics and comment on further studies that can be carried out to extract more information from such results.

Interpreting Granitic Fabrics in Terms of Rhyolitic Melt Segregation, Accumulation, and Escape Via Tectonic Filter Pressing in the Huemul Pluton, Chile

AbstractThe physical process of rhyolite segregation from crystal mushes remains elusive as microstructural evidence of conventional segregation mechanisms is not available. This study provides direct fabric evidence for deformation‐assisted segregation of eruptible rhyolite in the Chilean Andean arc. The shallow (<7 km), 6.4–6.2 Ma Huemul pluton comprises domains of quartz monzonite, granite, and high‐silica granite. Compositional modeling shows that rhyolitic melt (high‐silica granite) was extracted from a granitic parent, leaving behind silicic cumulates (quartz monzonite). To understand mechanisms of rhyolite segregation, we investigate magmatic fabrics in the pluton. Anisotropy of Magnetic Susceptibility analyses reveal oblate magnetic fabrics and NNW‐striking, subvertical magnetic foliations throughout Huemul. Within the high‐silica granite, magnetic lineations are subvertical and parallel to elongate miarolitic cavities. Magnetic lineations in the quartz monzonite plunge moderately to the NNW, away from the high‐silica granite. In the quartz monzonite, the Shape‐Preferred Orientation of early feldspars is parallel to the magnetic lineation and developed while suspended in melt. Estimations of early feldspar clustering and crystallinity yield ~38% of interstitial volume loss in the quartz monzonite and no volume loss in the granite. These fabric data suggest ENE tectonic shortening coeval with rhyolite extraction. We explain these observations with a model of tectonic filter pressing in which shortening is accommodated by interstitial melt flow at slow (10−5 km3/yr) rates, segregating moderate volumes of rhyolite in Myr time scales. These interactions link plutonism, tectonic deformation, and upward mobility of eruptible rhyolite in tectonically active margins.

Early–Middle Miocene subtle compressional deformation in the Ebro foreland basin (northern Spain); insights from magnetic fabrics

The results of anisotropy of magnetic susceptibility analyses of 19 sites carried out on magnetostrigraphically-dated, Lower to Middle Miocene (20.4 to 13.7Ma) “non-deformed” mudstones from the central part of the Ebro basin reveal the presence of a subtle tectonic overprint lasting at least until the Langhian (Middle Miocene) in the southern Pyrenean foreland. Magnetic ellipsoids show a sedimentary fabric in 42% of sites and a weak and well-defined magnetic lineation in 47% and 11% of sites. The magnetic lineation is roughly oriented around the east–west direction, compatible with a very weak deformation occurring there and related to the north–south compression linked to the convergence between Europe, Iberia and Africa during the Early–Middle Miocene. A slight variation of the magnetic parameters Km, Pj, and T exists through time, probably due to changes in the sedimentary conditions in the basin.

Paleomagnetic and petrologic study of the age, origin, and significance of early and late Paleozoic events in the Long Mountain Granite, Wichita Mountains, Oklahoma

The Cambrian Long Mountain Granite, exposed in the western Wichita Mountains, Oklahoma, is red, granophyric, alkali feldspar granite. The red granite abruptly transitions into a green granite in the subsurface; both red and green granites have similar geochemical signatures. Anisotropy of magnetic susceptibility analysis shows that the green granite retains a primary magnetic fabric that is consistent with the sill-like emplacement of the Wichita Granite Group. Demagnetization of green granite specimens yields a characteristic remanent magnetization (ChRM) residing in magnetite with east declinations and moderate down inclinations. The paleopole (9.0°N, 314.6°E) is interpreted as a primary Cambrian thermal remanent magnetization or an early magnetization related to deuteric/hydrothermal alteration. The paleopole position is consistent with some other poles for Cambrian igneous rocks in southern Oklahoma but is to the southwest of the Cambrian part of the apparent polar wander path. The red granite contains abundant secondary hematite that occurs in fractures, as grain boundary coatings, and along cleavage and exsolution planes in alkali feldspars. The iron in the hematite appears to be sourced from the oxidation of primary magnetite and ilmenite and the breakdown of silicate ferromagnesian minerals. The red granite has about two orders of magnitude lower magnetic susceptibility and natural remanent intensity than the green granite. The magnetic fabric is interpreted as an alteration fabric. The ChRM of red granite has southeast declinations and shallow inclinations and is interpreted as a chemical remanent magnetization (CRM) residing in hematite. The paleopole (44.9°S, 304.9°E) falls near the 290–300 Ma segment of the North American apparent polar wander path, which is consistent with inferred timing of exposure of the Long Mountain Granite. The CRM is interpreted to have been acquired during alteration by low-temperature weathering fluids near the surface during the late Paleozoic. The results from the red granite are not consistent with alteration caused by widespread paleoclimatic conditions in the Late Permian, and they are interpreted as related to local tectonic and/or weathering events.

Paleomagnetism and magnetic fabric of the Eastern Cordillera of Colombia: Evidence for oblique convergence and nonrotational reactivation of a Mesozoic intracontinental rift

We report the paleomagnetic and magnetic fabric results of 58 sites from Cretaceous-Miocene marine and continental strata from the Eastern Cordillera (EC) and the Cucuta zone, at the junction between the Santander Massif and the Merida Andes of Colombia. The EC is an intracontinental doubly vergent range inverting a Triassic to Early Cretaceous rift zone. Twenty-three sites reveal nonsystematic tectonic rotations, including unrotated areas of the EC range with respect to stable South America. Our data show that the EC inverted a NNE oriented rift zone and that the orientation of the Mesozoic rift and the mountain chain roughly correspond. Interestingly, magnetic lineations from anisotropy of magnetic susceptibility analysis do not trend parallel to the chain but rather are oblique to the main orogenic trend. By also considering GPS evidence of a ~1 cm/yr ENE displacement of central western Colombia accommodated by the EC, we suggest that the Miocene-Recent deformation event of this belt arises from ENE oblique convergence reactivating a NNE oriented rift zone. Oblique shortening was likely partitioned into pure dip-slip shear characterizing thick-skinned frontal thrust sheets (well known along both chain fronts) and by range-parallel right-lateral strike-slip faults, which have not been identified yet, but likely exist in the axial part of the EC. Finally, the 35° ± 9° clockwise rotation observed in four post-Miocene magnetically overprinted sites from the Cucuta zone reflects late Cenozoic and ongoing right-lateral strike-slip displacement occurring along faults parallel to the Boconó fault system, possibly connected with the right-lateral faults inferred to exist along the axial part of the EC.

Depositional processes, paleoflow patterns, and evolution of a Miocene gravelly fan-delta system in SE Korea constrained by anisotropy of magnetic susceptibility analysis of interbedded mudrocks

Anisotropy of magnetic susceptibility (AMS) has been used as a proxy of grain fabrics and as a basis of interpretation of depositional processes and paleocurrent directions in sedimentary successions. In this paper, we present the results of the AMS analysis of over 900 core specimens from the sandy to muddy deposits intercalated between the gravelly deposits of a Miocene fan-delta system in SE Korea. Analyses of the mean magnetic susceptibility, thermomagnetic property, and hysteresis show that paramagnetic minerals are the main contributors to the AMS of the deposits. The AMS analysis shows that the deposits have four types of magnetic fabrics, including 1) horizontal and type-A imbricated fabrics, characterized by girdle-distributed to weakly clustered k1 and k2 axes on the bedding plane and subvertical to variably tilted k3 axes, 2) type-B imbricated fabric, characterized by well-clustered k1 and k2 axes and subvertical to tilted k3 axes, and 3) rolling fabric, characterized by streak-distributed k2 and k3 axes along a plane perpendicular to the k1 axes. These fabric types are interpreted to have resulted from suspension settling (horizontal and type-A imbricated fabrics), turbidity currents (type-B imbricated fabric), and debris flows (rolling fabric). Modification of the primary fabrics during sediment compaction on a slope resulted in a wide range of imbrication angles in all fabric types. Paleocurrent directions inferred from the AMS data are generally consistent with those obtained by previous field studies of the gravelly sequences and provide additional information of the temporal change of the paleoflow pattern, which is related to the growth, exhumation, and dissection of the fan-delta system. This study thus suggests that much information can be drawn from the AMS analysis of mudrocks regarding not only the depositional processes and paleoflow pattern but also the evolution of a depositional system.

Incremental emplacement of the sheeted Glas Bheinn Porphyritic Dolerite, Ardnamurchan, NW Scotland

Synopsis The c . 58 Ma Ardnamurchan Central Complex represents the deeply eroded roots of an ancient volcanic edifice that purportedly formed during three discrete phases of igneous activity. Although only a relatively small component of the first phase of magmatism (Centre 1), the Glas Bheinn Porphyritic Dolerite provides important insights into igneous emplacement mechanisms and age relationships on Ardnamurchan. New field observations combined with anisotropy of magnetic susceptibility analyses, which allow primary magma flow patterns to be reconstructed, suggest that the Glas Bheinn Porphyritic Dolerite consists of numerous inclined sheet intrusions that, overall, have a flower intrusive structure. Vertically ascending sheets that do not appear to be related to the Centre 1 focus are proposed to have been deflected into the inclined sheets and sills by the load of the overlying volcanic edifice. Cross-cutting relationships further suggest that some sheets of the Glas Bheinn Porphyritic Dolerite are later than several Outer Centre 2 cone sheets. These observations suggest that the Glas Bheinn Porphyritic Dolerite cannot be attributed to Centre 1. Supplementary material: AMS block sample locations and magnetic data are available at www.geolsoc.org.uk/SUP18562

Pliocene shortening direction in Nankai Trough off Kumano, southwest Japan, Sites IODP C0001 and C0002, Expedition 315: Anisotropy of magnetic susceptibility analysis for paleostress

Integrated Ocean Drilling Program Expedition 315 recovered cores from accretionary units, which are overlain by cover sequences, at a site on the hanging wall of the mega‐splay fault (C0001), and at a seaward margin of a forearc basin (C0002) in Nankai Trough off Kumano, Japan. In order to investigate the amount and the style of deformation and shortening direction of the accretionary prism units, anisotropy of magnetic susceptibility (AMS) of cored samples were measured. AMS of the late Pliocene to late Miocene accretionary prism at C0001 reveals a deformed magnetic ellipsoid (prolate type), and a restored direction of the AMS orientation indicates northwest–southeast shortening which is the same as the present‐day stress field measured by borehole breakout. The older Miocene accretionary prism cored beneath the forearc basin at C0002 does not show a typical prolate type but more oblate feature in parallel to plunged bedding planes. This type can be interpreted as an intermediate type between prolate and oblate types, by indicatives of AMS parameters, which was formed under a bedding vertical loading and a relatively weakly lateral compaction. The restored AMS orientation of the accretionary prism beneath the forearc basin also indicates the northwest‐southeast shortening which is different from the present‐day principal horizontal stress orientations at C0002. It is supposed that the shortening direction had been recorded in Pliocene time probably when the sequence was sited in the outer wedge. The northwest–southeast directions of AMS in the Pliocene time agree with the past subducting direction of the Philippine Sea Plate since the Pliocene.

How are saucer‐shaped sills emplaced? Constraints from the Golden Valley Sill, South Africa

Magmatic sill intrusions develop saucer‐like geometries in layered sedimentary basins intruded by large volumes of magma. Saucer‐shaped sill emplacement models are based on the analysis of the intrusion geometry and their spatial relationships with potential feeders, not on magma flow pattern. The Karoo Basin of South Africa hosts hundreds of saucer‐shaped sills. Among these, the Golden Valley Sill is well exposed and displays connections with adjacent and nested saucers. A combination of detailed field observations and anisotropy of magnetic susceptibility measurements (AMS) were used to identify strain markers that can be interpreted in terms of magma flow directions. A total of 113 localities (six specimens per site), mostly including pairs of opposite sill margins, were sampled for anisotropy of magnetic susceptibility analyses. The magnetic properties were determined by measuring hysteresis loops and K‐T curves. The majority of the localities display well‐defined magnetic foliations that consistently dip outward from the center of the Golden Valley Sill. This orientation of the magnetic foliation most likely represents inflation/deflation cycles of the intruding sill that interacts with nonstatic enclosing walls. In addition, magma channels and ropy flow structures were identified and display an imbrication of the magnetic foliation indicating an outward magma flow direction. The observed magma flow geometries derived from macroscopic flow indicators and the AMS data correlate well and constrain an emplacement model for the Golden Valley Sill Complex. Our emplacement model integrates our observations with dynamic emplacement processes characterized by inflation and deflation cycles.

Anisotropy of magnetic susceptibility analysis of deformed kaolinite: implications for evaluating landslides

Plane strain tests were performed on seven kaolinite blocks, each of which developed shear bands. Anisotropy of magnetic susceptibility (AMS) analysis of the kaolinite reveals a threshold degree of magnetic anisotropy (P′) exceeding which shear bands develop. Since P′ is a strain-intensity gauge and soils are known to develop shear bands prior to landsliding, it is concluded that soil in every landslide-prone region must have its unique threshold P′ exceeding which it develops shear bands before failing. Therefore, AMS monitoring of soil in landslide prone regions is proposed as a potential tool in the management of natural hazard zones.

Overlap of Karoo and Ferrar Magma Types in KwaZulu-Natal, South Africa

A suite of mafic dykes from the Underberg region of southern KwaZulu-Natal (South Africa) were intruded at 178 Ma, coincident in age with the major Okavango Dyke Swarm of Botswana, and also coincident with minor Karoo-related intrusions of the northern and central Lebombo. The dykes are all low-Ti-Zr tholeiites, they trend NW-SE and are presumed to continue into the Karoo central area of the Lesotho Highlands. In many respects, the Underberg dykes are similar to the majority of the low-Ti-Zr volcanic and subvolcanic intrusions of the Karoo; however, their 87Sr/86Sr and Nd isotope ratios are either ‘Ferrar-like’ (87Sr/86Sr 0·710; Nd < -3) or transitional between Karoo low-Ti-Zr and Ferrar low-Ti magmas. A potential Ferrar source for at least some of the Underberg dykes is supported by anisotropy of magnetic susceptibility analyses of the dyke suite, which demonstrate absolute flow direction from the SE to the NW, consistent with Gondwana reconstructions. The role of crustal contamination and combined fractional crystallization is also demonstrated to have played a key role in the petrogenesis of the Underberg dykes, involving a local upper crust contaminant. However, the composition of the ‘Ferrar-like’ dykes cannot be easily explained by AFC processes, but they do demonstrate that melting of a lithospheric mantle source enriched to a small degree by subduction-derived fluid was also important.

A short, reverse polarity interval within the Jaramillo subchron: Evidence from the Jingbian section, northern Chinese Loess Plateau

A high‐resolution paleomagnetic and rock magnetic investigation supported by scanning electron microscopy observations, X‐ray diffraction determinations, and anisotropy of magnetic susceptibility analyses has been carried out on a loess‐paleosol sequence from Jingbian (37.4°N, 108.8°E), at the northern extremity of the Chinese Loess Plateau. The results indicate that the magnetic assemblage is dominated by pseudo‐single‐domain magnetite with associated maghemite and traces of a higher‐coercivity phase, probably hematite. The Brunhes‐Matuyama and upper Jaramillo transitions have been carefully documented and a record of a short reverse polarity interval (SRI), entirely within the Jaramillo normal polarity subchron, has been obtained. The duration of the SRI, recorded over a stratigraphic interval of 48 cm, is 10.1 kyr (29 kyr if the polarity transitions are included). These results seem to exclude that rock magnetic variations are adversely affecting the recording process and compel us to reconsider the Jaramillo subchron as characterized by a pair of normal polarity intervals interrupted by a short reverse polarity interval.

Emplacement of the Santa Rita Flat pluton as a pluton-scale saddle reef

Regional mapping indicates that the Middle Jurassic Santa Rita Flat pluton, exposed in the Inyo Range of eastern California, is situated within the core of a south-plunging synform defined by bedding in the surrounding metasedimentary wall rocks, which dip beneath the pluton. However, bedding in rocks preserved above the pluton defines a south-plunging antiform. Anisotropy of magnetic susceptibility analysis revealed that magnetic foliation within the pluton also defines a south-plunging antiform, and the accompanying magnetic lineation plunges subparallel to the axis of this antiform. Our data indicate that the pluton was initially intruded as a sill in the hinge zone of the enveloping synform. Subsequent vertical inflation of the sill resulted in upward doming of the overlying roof and formation of the antiform now observed at the current erosion level in the pluton and preserved roof rocks. Emplacement of the pluton at 164 ± 1.5 Ma (U-Pb zircon age) overlaps in time with regional thrust faulting and folding ca. 185–148 Ma recognized in the southern Inyo Range. We speculate that space for initial emplacement of the pluton was produced during folding by layer-parallel slip and hinge-zone dilation, producing a saddle reef-like structure.