Magnetic Fabric Development Research Articles

Plaster and magnets: Modelling magnetic fabric development in magma intrusions using scaled analogue experiments

Understanding magma behaviour during emplacement within the crust is vital for understanding the dynamic processes occurring in volcanic systems. However, linking the static record of magma flow to its dynamic origin is challenging, particularly as macroscopic indicators of magma flow are often not observed, absent and/or have been modified after emplacement. Anisotropy of magnetic susceptibility (AMS) has been used as an important tool in field studies to infer magma flow direction, using the magnetic fabric as a record of the magma intrusion dynamics and to identify magma source regions. Here, we describe a new method to explore magnetic fabric development in magma intrusions and lava flows using scaled analogue laboratory experiments. Coloured mixtures of Plaster of Paris (the magma analogue) seeded with magnetite particles were loaded concentrically into a piston and injected through a central port in the base of a box filled with compacted fine-grained wheat flour (the crust analogue). This created a series of interconnected sheet and tube-like ‘magma’ intrusions which eventually breached the surface to feed a model ‘lava flow’. Once solidified, the intrusions were excavated and sampled for AMS, with the results showing that magnetic fabrics were preserved. A new dynamic scaling analysis shows the plaster mixture represents the intrusion of dacite magma into the shallow crust. These models provide proof-of-concept that this new methodology and scaling analysis can be used to explore AMS development in viscous (dacite) magma intrusions in nature, with the potential for direct comparison with field-based indicators of magma flow dynamics.

Rock magnetism and AMS studies in Kondapalle-Pangidi layered complex, Eastern Ghats Belt, India: Remanence carriers and tectonic implications

The present study is an integrated approach towards the rock magnetism and Anisotropy of Magnetic Susceptibility (AMS) of the high-grade metamorphic rocks in the Kondapalle Pangidi Layered Complex (KPLC), Eastern Ghats Belt (EGB), India. Our study aims at determining the petrography, rock magnetic properties and the magnetic fabrics. The rock magnetic properties and the magnetic fabrics from AMS studies are used to determine their tectonic implications. Petrography reveals that the chief remanence carrier mineral is Ti-magnetite. Rock magnetic studies including Isothermal Remanent Magnetization (IRM), back-field IRM, hysteresis loop studies, and thermo-magnetic studies reveals the same. The magnetic hysteresis parameters (remanence and coercivity) are used to determine the magnetic domains from a modified Day plot. The magnetic domains lie within Stable Single Domain and Pseudo Single Domain ranges, thereby explaining their potential to record an ancient magnetic field. Besides the ferrimagnetic remanence carriers, paramagnetic silicates are also dominant which controlled the development of magnetic fabric. The magnetic fabric bears analogy with the regional structures and bear tectonic implications related to the post-rifting closure of the ocean followed by subduction of the ocean basin, accretionary orogeny and felsic magmatism in the Indo-Antarctic continental blocks.

Applicability of AMS technique as a flow fabric indicator in dykes: insight from Nandurbar–Dhule Deccan dyke swarm

Determination of the nature of magma flow (vertical or lateral) from rock fabrics in a dyke swarm holds significant importance in deciphering its emplacement mechanism, depth of magma chamber and its association with mantle plumes. Nandurbar–Dhule dyke swarm from Western India consists of ~ 210 mappable, tholeiitic, large, linear dykes intruding the country rocks of Deccan basalt. We discuss here the development of flow-controlled rock fabrics in these dykes as a function of their thicknesses. Magnetic fabric obtained from anisotropy of magnetic susceptibility (AMS) analysis of the samples collected from these dykes was used as a proxy for rock (flow) fabric. Samples from the central parts of the thicker dykes (strike perpendicular thickness > 10 m) show random and inconsistent development of magnetic fabrics than their marginal counterparts. Petrography suggests the development of relatively clear shape-preferred orientations of the elongated mineral grains only at the margin of a thick dyke. Thinner dykes (strike perpendicular thickness < 10 m) show both consistent magnetic fabrics and clear shape-preferred orientations. We propose that the center of a thick dyke fails to provide (due to high flow velocity, slower cooling rate, lack of friction-induced shearing, and draining back of magma) a favorable condition where (a) flow fabric can efficiently develop and (b) magnetic fabric can effectively mimic the flow fabric. Thinner dykes and the margins of the thicker dykes are, hence, better candidates for observing flow fabrics represented by magnetic fabrics. This work is also motivated by the reservations expressed by earlier researchers in using the AMS technique for magma flow characterization, especially for giant dyke swarms. We have critically evaluated their reservations in light of our results and commented on the applicability of the AMS technique on such studies.

Relative paleointensity estimates from magnetic anisotropy: Proof of concept

Relative paleointensity data from sedimentary rocks play an important role to decipher the workings of the geodynamo and to correct for atmospheric cosmogenic radionucleide production, so it is important to understand how sediments acquire remanent magnetizations and to better assess the quality of relative paleointensity data. We present experimental results from sediments deposited in controlled magnetic fields to observe the changes in magnetic anisotropy as a function of applied field strength going from near Earth-like values to almost full saturation. Relative paleointensity values followed a very well defined power law through the entire range of applied field intensities. Magnetic remanence fabrics evolved from oblate with maximum anisotropy axes in the sedimentary plane at low field strengths to prolate with maximum anisotropy axes parallel to the applied field direction at high fields. Anisotropy of magnetic susceptibility also evolved with field strength, but in a much less coherent manner than anisotropy of magnetic remanence. The experiments used well-characterized, natural sediments containing single domain magnetite, which made it possible to numerically model the data. The model matches the field dependency of both relative paleointensity and magnetic fabric development using a simple assumption that a large proportion (∼80%) of the remanence carriers in the sediments are unable to align with the magnetic field while a small fraction are free to align. Anisotropy of magnetic remanence thus holds promise to improve and assess relative paleointensity estimates and helps improve theoretical treatment of magnetic recording in sediments.

Magnetic fabric development in the Lower Seve thrust from the COSC-1 drilling, Swedish Caledonides

The COSC-1 project drilled the several hundred meters thick basal shear zone of the Lower Seve nappe with mylonites in mica schists, amphibole schists and gneisses. In zones of high magnetic susceptibility from 1910 to 2450 m we studied magnetic and petrographic fabrics, and magnetic mineralogy. Borehole imaging allowed for geographic reorientation of the samples and offered the opportunity to study anisotropy of magnetic susceptibility (AMS) in relation to tectonic evolution of the Seve thrust. We measured AMS at room temperature, added low-temperature and field-dependent AMS for a subset of samples, and compared magnetic with petrographic fabrics. Triaxial and prolate magnetic fabrics with degree of anisotropy (P′) up to 3.2 together with abundant S-C fabrics and strain partitioning around porphyroclasts indicate dominant simple shear until 2300 m. Magnetite and ilmenohematite mimic the rock fabric due to fabric parallel alignment and/or magnetic interaction and either contribute to increase or decrease of P′, depending on the dominating rock fabric elements. Field-dependency of pyrrhotite and magnetite in kmax-direction further increases P′. Homogeneous and oblate petrographic and magnetic fabrics in the greenschist-grade overprinted rocks below 2300 m with subhorizontal kmax-kint-girdle distributions indicate dominant flattening. AMS depicts shear fabrics including magnetite and ilmenohematite, and is additionally increased by retrograde magnetite-rutile intergrowth in ilmenohematites. We interpret that shape and degree of AMS are controlled by (a) tectonic deformation and strain, (b) alteration and magnetic grain interaction, and (c) field-dependency of deformed pyrrhotite and/or magnetite. We observed that all petrographic and magnetic subfabrics are coaxial, and lineations are mainly E-W to SE-NW directed confirming the transport direction of the Caledonian allochthonous. From our microstructural and AMS results we suggest that thrusting of the Lower Seve unit commenced under simple shear conditions at higher metamorphic grades and subsequently switched to more pure shear under greenschist-grade conditions.

A conceptual magnetic fabric development model for the Paks loess in Hungary

Abstract We describe magnetic fabric and depositional environments of aeolian (loess) deposits from Paks, Hungary, and develop a novel, complex conceptual sedimentation model based on grain size and low-field magnetic susceptibility anisotropy data. A plot of shape factor (magnetic fabric parameter) and dry deposition velocity estimated from grain-size reveals primary and secondary depositional processes during the sedimentation of loess. Primary ones are driven by gravity, with poorly oriented MF for fine grain materials, and by tangential stress, with flow-aligned or flow-transverse fabric for coarser grain sediments. The fabric developed by a primary process is called depositional magnetic fabric. Secondary processes develop in unconsolidated sediments, beginning right after deposition and terminating before the start of diagenesis. Under slow sedimentation conditions, deposited materials are likely to be exposed near the surface for longer periods. Therefore, relatively strong winds with a stable direction can alter the fabric of non-buried surficial sediments. As a result, grain orientations may change from scattered, non-flow oriented fabric to flow-oriented fabric. This type of fabric, developed by a secondary process, is called transformed magnetic fabric, and is characterized by relatively well-defined grain orientation, which allows us to estimate a dominant wind direction.

Laboratory study of fabric development in shearing till: The importance of effective pressure and shearing rate

Herein we present data on the shearing rate (glacier velocity) and effective pressure (difference between the ice-overburden pressure and pore-water pressure) in the development of magnetic fabric (anisotropy of magnetic susceptibility) using a rotary ring-shear device. A Wisconsin-age basal till was used in the experiments and deformed to its critical state at shear strains as high as 93. We also present data from hysteresis and high temperature susceptibility experiments to identify the magnetic carrier in the basal till. Results showed little change in fabric strength when varying the shearing rate in the speed range of 110–860myear−1. Moreover, the effective pressure tests also showed an inconsistency in fabric between 30 and 150kPa; however, a slight strengthening effect was documented. Thus, the k1 magnetic fabric strength is independent of the shearing rate and effective pressure. This suggests that the fabric strength upon these variables cannot be used as a benchmark for estimating shear deformation to the geological record. The k1 fabric strength in this study; however, remained consistent with respect to other till particle fabric methods (e.g., sand and pebble) in which the same conclusion was drawn; all particles align parallel to the direction of shear and plunge mildly up glacier.

Development of magnetic fabrics with tunable hydrophobicity

ABSTRACTPolystyrene (PS) fiber mats incorporating iron (Fe) particles were fabricated by electrospinning and the hydrophobicity of the resulting magnetic fabrics was investigated with and without an applied magnetic field. The results show that the hydrophobicity increases in the presence of a magnetic field and the hysteresis in the advancing/receding contact angle decreases in the presence of a magnetic field. It is also shown that the contact angle and hysteresis increase with decreasing fiber diameter. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 2352–2358, 2013

Development of magnetic fabric in sedimentary rocks: insights from early compactional structures

The authors acknowledge the La Rioja Government for the permission to develop investigations within the Rioja palaeontological heritage. We also thank Dario Bilardello and an anonymous reviewer for all their constructive suggestions that improved substantially this paper. Besides, we acknowledge the use of Servicio de Preparacion de Rocas y Materiales Duros (Servicio General de Apoyo a la Investigacion-SAI, Universidad de Zaragoza) and Amelia Blasco for the calcimetries performing. This study has been financed by the research project CGL2009-08969 of the MICINN (Spanish Ministry of Science and Innovation). CGL also acknowledges the MICINN for the research grant BES-2010-037509.

High-temperature magnetic fabric development from plastically deformed magnetite in experimental shear zones

Magnetic fabric development has been studied in synthetic magnetite-silicate aggregates in a series of high-temperature simple shear deformation experiments. Samples composed of magnetite grains with a nominal size of 20-40 µm dispersed at 3 wt % in a matrix of plagioclase were deformed between 1000 and 1200 °C with a confining pressure of 300 MPa, shear stresses in the range 10-130 MPa, and shear strains up to γ = 3. We determined that both magnetite and plagioclase were deformed plastically at these conditions. An oblate magnetic fabric ellipsoid develops subparallel to the shear plane and the degree of AMS rapidly increases with strain up to a value of 2.5. Theoretical strain response models of magnetic fabric development were applied to the data to obtain estimated magnetite strains. The shape-preferred orientation of magnetite grains after deformation, determined from reflected light image analysis, was used to calculate independent magnetite strain estimates. These results were then compared with strains estimated from theoretical magnetic anisotropy development. Both strain estimation methods indicate strain partitioning between magnetite and the plagioclase matrix and the results are used to determine approximate viscosity ratios between the two phases at the experimental conditions.

Magnetic fabric development in a highly anisotropic magnetite-bearing ductile shear zone (Seve Nappe Complex, Scandinavian Caledonides)

Magnetite-bearing mylonitic garnet–micaschists close to the major suture between the Baltica and Iapetus terranes (Seve Nappe Complex, Scandinavian Caledonides) show very high anisotropy of magnetic susceptibility (AMS) with corrected degree of anisotropy (P′) up to 4.8. Three different magnetic fabric types can be distinguished. They correspond to protomylonite (type I, P′ 3), respectively. The orientation of the ellipsoid axes from all applied magnetic fabric methods in this study is similar with shallow dips of the metamorphic foliation toward WSW and subhorizontal, mostly NW–SE trending mineral lineation. Differences between subfabrics were minimized under high shear strain as all markers tend to align parallel with the shear plane. The very high anisotropies and mostly oblate ellipsoid shapes of type III correlate with high magnetic susceptibility (kmean up to 55 × 10−3 SI units) and are related to the concentration of magnetite aggregates with shape-preferred orientation. They show a distinct field dependence of magnetic susceptibility of up to 10% in the kmax-direction. We attribute this field dependence to a “memory” of high strains in the domain walls of the crystals acquired during synkinematic magnetite growth during shear zone fabric development at temperatures of 550–570°C.

Remanence stability and magnetic fabric development in synthetic shear zones deformed at 500°C

Shear experiments were performed on magnetite‐bearing calcite aggregates to examine magnetic fabric development and remanence stability in a deforming system using elevated temperature and pressure to encourage deformation by crystal‐plastic processes. Samples composed of 1 wt % pseudo‐single‐domain magnetite (1–2 μm) in a calcite matrix were created with either strong or weak initial fabrics and deformed in coaxial simple shear to strains up to γ = 1.5 at constant strain rates between 6 × 10−5 and 1 × 10−4 s−1 at 500°C and confining pressure of 300 MPa. Samples were given weak field thermal remanent magnetizations prior to deformation. Demagnetization of postdeformation remanence reveals that a primary remanent magnetization can withstand deformation at pressures and temperatures approximately equivalent to greenschist facies metamorphic conditions on laboratory time scales, but this stability is found to depend on the character of the predeformation fabric. The origin of secondary remanence components acquired during deformation is uncertain but is likely to partially result from thermal viscous remagnetization. Complete postdeformation remagnetization in initially anisotropic samples appears to involve a stress‐softening or piezoremanent magnetization mechanism. Postdeformation anisotropy measurements show progressive changes in magnetic fabric strength with strain. In the absence of a strong initial magnetic anisotropy, magnetic fabric intensity increases linearly as a function of strain; however, deformation that overprinted an existing fabric results in an apparent decrease of the initial anisotropy at low strains followed by rapid increases in magnetic fabric strength with increasing strain. Our results underscore the important role that initial fabric can play in determining the character of deformation fabrics.

Magnetic fabric of sheared till: A strain indicator for evaluating the bed deformation model of glacier flow

Wet‐based portions of ice sheets may move primarily by shearing their till beds, resulting in high sediment fluxes and the development of subglacial landforms. This model of glacier movement, which requires high bed shear strains, can be tested using till microstructural characteristics that evolve during till deformation. Here we examine the development of magnetic fabric using a ring shear device to deform two Wisconsin‐age basal tills to shear strains as high as 70. Hysteresis experiments and the dependence of magnetic susceptibility of these tills on temperature demonstrate that anisotropy of magnetic susceptibility (AMS) develops during shear due to the rotation of primarily magnetite particles that are silt sized or smaller. At moderate shear strains (∼6–25), principal axes of maximum magnetic susceptibility develop a strong fabric (S1 eignevalues of 0.83–0.96), without further strengthening at higher strains. During deformation, directions of maximum susceptibility cluster strongly in the direction of shear and plunge “up‐glacier,” consistent with the behavior of pebbles and sand particles studied in earlier experiments. In contrast, the magnitude of AMS does not vary systematically with strain and is small relative to its variability among samples; this is because most magnetite grains are contained as inclusions in larger particles and hence do not align during shear. Although processes other than pervasive bed deformation may result in strong flow parallel fabrics, AMS fabrics provide a rapid and objective means of identifying basal tills that have not been sheared sufficiently to be compatible with the bed deformation model.

Development of magnetic fabrics during hydrothermal alteration in the Soultz-sous-Forêts granite from the EPS-1 borehole, Upper Rhine Graben

Abstract The Variscan, magnetite-bearing Soultz-sous-Forêts granite is found between 1420 and 2230 m of the EPS-1 borehole situated in the Upper Rhine Graben (France). Our study focuses on the changes of magnetic properties that occur during the progressive hydrothermal alteration and fracturing of the Soultz granite after emplacement. The magnetic susceptibility (κ) of the granite is between 10 and 80 × 10 −3 SI, and suggests that ferrimagnetic minerals are the primary carrier. During cooling and later tectonic and hydrothermal overprints, including the formation of the Rhine Graben, the granite was deformed under brittle conditions and partially altered by hydrothermal fluids. Along with this fluid activity, oxidation of magnetite to hematite occurred and reduced κ (&lt; 1 × 10 −3 SI). AMS analysis on oriented samples documents the history of progressive transformation from primary magmatic fabric to tectonic fabric during hydrothermal alteration and faulting. The fresh granite with multidomain magnetite grains shows sub-horizontal magnetic foliations and randomly oriented magnetic lineations within the foliation plane. This fabric is similar to the magmatic fabric reflected by biotite. Transformation of the magnetic fabric started with localized magnetite oxidation along NW-SE oriented micro-cracks, which are probably associated with a late-magmatic alteration (stage I). Elongated and co-aligned magnetite relics within the newly formed hematite caused a well-defined NW-SE trending magnetic lineation and steeper magnetic foliation. Later alteration associated with intense brittle deformation (stage II) initially adopted this magnetic fabric, but intense cataclasis destroyed it. The geometry and orientation of magnetic fabric clearly indicate a hydrothermal alteration, which relates to the acting tectonic stresses in the post-emplacement history of the Soultz granite.

How deformed are weakly deformed mudrocks? Insights from magnetic anisotropy

Abstract For over thirty years the anisotropy of magnetic susceptibility (AMS) has been exploited in rock fabric studies. Our knowledge of the mechanisms leading to magnetic fabric development in deformed rocks has grown substantially, even though some details are still under debate. This paper reviews recent developments in AMS studies applied to the characterization of early deformation stages in mudrocks. From the current data set of AMS in rocks deformed at low temperature and low pressure it is possible to revisit the concept of weakly deformed rocks. We contend that weak deformation in mudrocks and the appearance of magnetic cryptofabric are concatenated. Furthermore, magnetic anisotropy studies in weakly deformed rocks suggest that cleavage fabric, the most common planar fabric in deformed mudrocks, builds up gradually from the earliest stages of deformation rather than suddenly at some strain threshold.

Magnetic and structural fabrics of the melange in the Shimanto accretionary complex, Okinawa Island: Implication for strain history during decollement‐related deformation

The magnetic fabrics of the melange in the Shimanto accretionary complex of Okinawa Island were obtained from the anisotropy of magnetic susceptibility (AMS) data. The magnetic fabrics show a marked contrast between the structurally lower and upper parts of the melange; the lower part records deformation in the footwall of the decollement (D1), whereas the upper part documents that in the hanging wall of the decollement (D2). In the lower part of the melange the magnetic fabrics show good correlations with S1 layer‐parallel foliation, reflecting strain geometries under layer‐perpendicular flattening. As S‐C fabrics were locally developed by heterogeneous shear during D1, some of the previous magnetic fabrics were rotated with S1. In this case, the resultant magnetic fabrics reflect “S” surfaces, and maximum AMS axes are reoriented parallel to shear directions without clear change in shape and anisotropy degree of the AMS ellipsoids. Thus “S” surfaces did not act as flattening planes but may represent the deformation through independent particulate flow and mechanical rotation. The lower part of the melange was pervasively sheared during D2, reflecting that it laid immediately above the decollement. However, D1 magnetic fabrics were not modified because shear strain was accomplished by brittle microfaults without development of penetrative strain. In contrast, penetrative strain was significant in the upper part of the melange, resulting in the magnetic fabrics recording incremental strain associated with D2. Therefore deformation mechanism and proximity to a decollement play an important role on development of magnetic fabrics in a melange.

Correction to “Magnetic fabric development in the tertiary accretionary complex in the Boso and Miura Peninsulas of central Japan”

Correction to “Magnetic fabric development in the tertiary accretionary complex in the Boso and Miura Peninsulas of central Japan”

Magnetic fabric development in the Tertiary Accretionary Complex in the Boso and Miura Peninsulas of central Japan

It has been demonstrated that the anisotropy of magnetic susceptibility (AMS) is a sensitive indicator for distinguishing various processes that had acted during and after the deposition of sediments. Recently, much attention has been focused on proving its usefulness in distinguishing early deformation signatures in sediments which show no visible grain‐scale deformation. Here, we present the AMS results of the Neogene Miura Group of Central Japan, which was formed by arc‐arc collision tectonics with subdivisions of two contrasting tectonic provinces: the southern province, a highly deformed accretionary prism and slope basin, and the northern province, a gently deformed forearc basin. Samples were collected from 21 sites in both tectonic settings. We obtained from axes orientations three typical fabric domains considered to represent sedimentary, tectonically deformed, and their intermediate types. The shape parameters of AMS also revealed distinctions clearly. Sedimentary fabrics are more foliated and anisotropic. Deformed fabrics show a more prolate shape and are closer to being isotropic. The sedimentary fabrics predominate in the northern forearc basin sequence whereas the intermediate and deformed fabrics occur predominantly in the southern province. The general trend of the shape of the magnetic fabric in studied sediments reveals a clear evolution in the pattern of the isotropic tectonic fabric constructed from foliated sedimentary fabric.

The magnetic fabrics of experimentally deformed artificial clay-water dispersions

The development of magnetic fabrics in artificial clay-water dispersions and natural, hematite-bearing mudstones is investigated in plane-strain pure shear laboratory experiments under strain rates of 1.6 × 10 −5 and 2 × 10 −4 s −1. The mixtures contain 0,15, 30 and 45% chlorite in an illite matrix, and 0, 1, 3, 6 and 8% magnetite in a kaolin matrix. Shortening up to 40% is imposed. The resulting fabrics show the following characteristics: (1) In the clay mixtures, the principal susceptibility axes ( k max ≥ k int ≥ k min) rotate away from the well defined initial fabric orientations into the princip strain directions ( e 1 ≥ e 2 ≥ e 3) at strains > 30%. (2) Both mineralogical composition and initial magnetic fabric, but not the applied strain, influence the magnitudes of the principal susceptibility axes. (3) The illite-chlorite mixture series show an almost linear correlation between mineral concentration and susceptibility magnitudes. (4) Magnetite dominates the fabric of the magnetite-kaolin mixtures; the fabric is independent of the magnetite concentration.

Magnetic fabrics of the Bloodgood Canyon and Shelley Peak Tuffs, southwestern New Mexico: implications for emplacement and alteration processes

Anisotropy of magnetic susceptibility (AMS) of the middle Tertiary Bloodgood Canyon and Shelley Peak Tuffs of the Mogollon-Datil volcanic field has been used to (1) evaluate the ability of AMS to constrain flow lineations in low-susceptibility ash-flow tuffs; (2) establish a correlation between magnetic fabric, magnetic mineralogy, tuff facies, and characteristics of the depositional setting; and (3) constrain source locations of the tuffs. The tuffs are associated with the overlapping Bursum caldera and Gila Cliff Dwellings basin. The high-silica Bloodgood Canyon Tuff fills the Gila Cliff Dwellings basin and occurs as thin outcrops outside of the basin. The older Shelley Peak Tuff occurs as thin outcrops both along the boundary between the two structures, and outside of the complex. AMS data were collected from 16 sites of Bloodgood Canyon Tuff basin fill, 19 sites of Bloodgood Canyon Tuff outflow, and 11 sites of Shelley Peak Tuff. Sites were classified on the basis of within-site clustering of orientations of principal susceptibility axes, based on the categories of Knight et al. (1986). Most microscopically visible oxide minerals in the Bloodgood Canyon Tuff outflow and basin fill, and in the Shelley Peak Tuff are members of the hematite-ilmenite solid solution series. However, IRM acquisition data indicate that Bloodgood Canyon Tuff basin fill and Shelley Peak Tuff have magnetic mineralogy dominated by single- or pseudo-single-domain magnetite, and that the magnetic mineralogy of the Bloodgood Canyon Tuff outflow is dominated by hematite. Hematite in Bloodgood Canyon Tuff outflow is likely to be the result of deuteric and/or low-temperature alteration of magnetite and iron silicate minerals. Bulk magnetic susceptibility is higher in magnetite-dominated ash-flow tuff (Bloodgood Canyon Tuff basin fill and Shelley Peak Tuff) than it is in hematite-dominated ash-flow tuff (Bloodgood Canyon Tuff outflow). Bloodgood Canyon Tuff outflow has the highest total anisotropy (H) of the three units, followed by Shelley Peak Tuff and Bloodgood Canyon Tuff basin fill. All three ash-flow tuffs are genearlly characterized by oblate susceptibility ellipsoids, with those of the Bloodgood Canyon Tuff basin fill nearest to spherical. At high values of total anisotropy, Shelley Peak Tuff susceptibility ellipsoids attain a prolate shape; those of Bloodgood Canyon Tuff outflow attain an increasingly oblate shape. Three factors may influence differences in total anisotropy and susceptibility ellipsoid shape: (1) ash which travelled the greatest distance before deposition may show the best development of magnetic fabric, particularly of magnetic lineation; (2) deposition of ash in a closed basin may inhibit laminar flow throughout the sheet and the resulting development of flow textures; and (3) replacement of magnetite and iron silicates preferentially oriented within the foliation plane by hematite with strong crystalline anisotropy may enhance the magnetic susceptibility within that plane. Scatter in AMS axis orientation within sites may result from: (1) greater orientation inaccuracy in block-sampled than in fielddrilled samples; (2) rheomorphism; and (3) low accuracy of AMS measurement in low-susceptibility ashflow tuffs. Evaluation of flow lineation based on AMS of sites with well-clustered K1 axes indicates that (1) Bloodgood Canyon Tuff basin fill flowed along a generally northwest-southeast azimuth; (2) Shelley Peak Tuff located on the boundary of the Bursum caldera and the Gila Cliff Dwellings basin flowed along a nearly east-west azimuth; and (3) Bloodgood Canyon Tuff outflow sites have K1 susceptibility axes generally radial to the Bursum-Gila Cliff Dwellings complex, but within-site scatter of K1 orientations is generally too large to draw conclusions about flow lineation orientation. Limited petrographic work on pilot thin sections adds flow direction information to AMS-derived flow lineation information.