Palaeowinds and depositional conditions from Holocene loess in Sweden and Finland

Nepheline syenites from the ∼1.2 Ga Ilímaussaq Complex of southern Greenland are examined to assess the utility of anisotropy of magnetic susceptibility (AMS) fabrics as proxies for silicate petrofabrics. Mineral lamination is a relatively common structural feature in cumulate rocks, including in the Ilímaussaq intrusion, but there is little consensus on the process (or processes) responsible for its formation. The Ilímaussaq AMS data are combined with rock magnetic experiments and electron backscatter diffraction (EBSD) measurements to characterize the magnetic mineralogy and compare the magnetic fabrics obtained to the silicate petrofabric. The data show that Na-amphibole (arfvedsonite) is most likely the dominant control on the AMS fabrics in the coarse-grained nepheline syenites (referred to as kakortokites), and that the AMS fabric is inverse relative to the observed silicate fabric. The EBSD data for a kakortokite sample suggests that the petrofabric is defined by arfvedsonite and is wholly planar, with evidence of only weak cross-lineation of c axes. The fine-grained nepheline syenites (lujavrites), two of which have a well-developed lamination carried by Na-pyroxene (aegirine), appear to have composite AMS fabrics that are considered to be a consequence of a mixed aegirine (normal) and arfvedsonite (inverse) response. The combined datasets shed light on the mechanisms of fabric acquisition in both lithologies. In the kakortokites, the AMS fabrics and silicate crystallographic preferred orientations, as well as the lack of observed microstructural evidence for subsolidus intra-crystal deformation, support models invoking gravitationally controlled crystal mats in the development of the macro-rhythmic layering of these rocks. In the lujavrites, the strong planar fabrics revealed by both the AMS and EBSD datasets, with some evidence of subsolidus deformation, point to fabric formation and perhaps even aegirine crystallization at the postcumulus stage. The combination of EBSD and AMS fabric datasets is a powerful means of deciphering the processes responsible for mineral alignment in igneous cumulates.

Anisotropy of Magnetic Susceptibility (AMS) fabrics within many individual granite plutons have previously been interpreted as recording the regional syn-magmatic tectonic strain field. To test this hypothesis, we compiled a regional database of AMS data from multiple granite complexes across two orogens, the French Massif Central and the British and Irish Caledonides, and critically evaluated the degree to which the magnetic fabric of the granite plutons recorded the known, regional tectonic strain. AMS fabrics from nine plutons from the French Massif Central show that all intrusions recorded the syn-magmatic late Variscan extensional collapse, with the maximum susceptibility axes (i.e., magnetic lineation) aligned with the NW-SE regional stretching direction. AMS fabrics from ten late Caledonian ‘Newer Granite’ plutons appear to reliably record the changing tectonic regimes between 430 and 390 Ma, including the switch from transpression to transtension following Iapetus closure, and then the return to transpression following the onset of the Acadian Orogeny at 400 Ma. This study indicates that comparisons between AMS fabrics and regional tectonics is best achieved qualitatively by comparing the orientation of the susceptibility axes to the known strain field, and more quantitatively through Woodcock analysis. Overall, our results indicate that pluton-scale AMS fabrics from multiple complexes spaced across an orogen can record a complex and changing regional tectonic strain field. This indicates there is significant potential to utilise pluton-scale AMS studies, alongside precise geochronological ages, to refine the timings of an orogen’s tectonic evolution.

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

Strain partitioning of deformation mechanisms in limestones: examining the relationship of strain and anisotropy of magnetic susceptibility (AMS)

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

Magnetic fabric constraints on friction melt flow regimes and ore emplacement direction within the South Range Breccia Belt, Sudbury Impact Structure

During mid-Miocene time, the Peach Springs Tuff (PST) was deposited in the pre-existing Kingman, Arizona, paleo-valley, which was bounded to the north and south by the Cerbat and Hualapai Mountains. Anisotropy of magnetic susceptibility (AMS) results from PST deposits on isolated topographic highs indicate that flow directions and magnetic rock fabrics are mainly controlled by paleotopography, such that the PST underwent different depositional and flow processes upon interacting with a gently sloping Granite Ridge and steeply sloping Scoria Cone. Stratigraphic measurements indicate the PST was density stratified prior to interacting with paleotopography, containing a lower, dense portion and upper, dilute portion. Variable flow directions, inferred from AMS, and transition from lithic- to pumice-rich ignimbrite on the Granite Ridge shows that the local roughness of the topographic high exerted a drag force on the current that encouraged deposition and blocking of the denser underflow. Deposition from a well-developed depositional system on the Ridge was replaced by turbulence near the Granite Ridge crest. AMS fabrics on the lee side point to grain-fall and/or vortex-induced flow and deposition as a result of flow separation. Strong AMS fabrics in down current deposits suggest that density stratification was reestablished after the current cleared the topographic high. AMS flow directions and fabrics at the Scoria Cone indicate that as the PST impacted against the steep topographic high, the lower underflow was laterally diverted along the lower parts of the Cone while the turbulent transport system overtook the crater rim. It is interpreted that deposition at the crater rim and down current areas resulted from a dilute, turbulent transport system, while deposition along the lower flanks of the Scoria Cone resulted from a denser, more lithic-rich current.

Overprinting of magnetic fabrics in granites by small strains: numerical modelling

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

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

Studying extinct volcanoes where erosion has exposed dykes and sills provides direct access to the fossil remnants of magma movement, however, linking crystallized magma to emplacement dynamics is challenging. This study investigates how magma flow varies across the thickness of a thin (6 m thick) mafic sill. We use a high-resolution sampling regime to measure micro-scale variations in magnetic anisotropy, which is associated with the orientation of the magnetic particles present within the crystalline rock. Fieldwork was conducted on exposed sills of the British and Irish Palaeogene Igneous Province, Isle of Skye, Scotland. Here Jurassic sedimentary rocks have been intruded by a series of sills, of picrite to crinanite composition, from the Little Minch Sill Complex (c.60 Ma). Anisotropy of magnetic susceptibility (AMS) and anisotropy of anhysteretic remanent magnetization (AARM) signals have been used to separate a crinanite sill into distinct magnetic groupings. We identified two AMS groups (the upper and lower sill margins, and the central region) and four AARM groups (the lower margin, the middle region, a region just below the upper margin, and the upper margin). Both AMS and AARM signals originate from titanomagnetite of multi-domain or vortex-state to single-domain sized grains, respectively. The AMS and AARM fabrics are aligned with each other in the margin regions preserving a history of magma flow toward the North during initial emplacement. However, in the sill interior the magnetic fabrics are oblique to each other, thus reflecting multiple origins. We suggest the AMS fabrics have recorded magma flow during sill growth, and AARM fabrics have recorded melt percolation flow as the interstitial melt migrated upward through a solidifying crystal mush. We demonstrate that when AMS and AARM are used in combination they enable a detailed understanding of magma flow and solidification dynamics to be obtained, from initial emplacement to solidification. Overall, our detailed sampling and analysis indicates that magnetic fabrics can be highly variable over small distances, supporting the suggestion of horizontal flow restriction and propagation path migration within growing sills, and that previous reports of magma flow and solidification dynamics based on under-sampled bodies may require reconsideration.

The relation between magnetite and silicate fabric in granitoids of the Adamello Batholith

SUMMARY Interpretation of palaeomagnetic data requires the detection of magnetofossils in sedimentary rocks and an understanding of their influence on magnetic properties. Subsamples collected from IODP site M0061 lost up to 90 per cent of their initial bulk magnetic susceptibility (MS) during cold-room storage of 5 months, which was attributed to the alteration of single-domain magnetosomal greigite (Fe3S4). To test if the magnetic susceptibility loss affected the anisotropy of MS (AMS) we resampled site M0061 with a Kullenberg piston corer (3 cores), took palaeomagnetic subsamples and undertook time-dependent AMS measurements over 1 yr in a controlled cool, humidified environment and exposed to air. Most subsamples possessed an initial normal oblate AMS fabric predicted for laminated sediments (horizontal with respect to the bedding plane) but we also detected a negative trend between the degree of anisotropy (Pj) and MS. In accordance with previous observations, MS decreased over 1 yr, which we accredit to oxidation of magnetosomal greigite and conversion into a less magnetic phase (probably FeO(OH)) that does not make a detectable contribution to AMS. These results allowed us to isolate, through application of an AMS tensor subtraction routine, the fabric of the magnetosomal greigite component that had decayed. In subsamples with the largest MS loss over one year, the decayed component had a prolate, inverse AMS fabric (defined as the principal susceptibility axis perpendicular to the bedding place) but relatively low Pj. We conclude that the initial (in-situ) AMS ellipsoid consisted of a mixture of a typical normal, oblate sedimentary fabric and the prolate, inverse magnetosomal fabric. The mostly inverse nature of the separated fabric indicates that the long axis of the magnetosomal greigite (as individual single-domain magnetofossils or chains) must be oriented parallel to the bedding plane, which implies that the magnetosomal greigite was deposited from the water column and contributes to a depositional remanent magnetization (DRM). Our results indicate that greigite magnetofossils can (i) explain the inverse AMS fabrics that have been reported in similar sedimentary environments and (ii) carry DRM with a median destructive field (MDF) of approximately 20 mT, although this remanence is transient under ambient laboratory conditions and is prone to oxidation.

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

<p>Field impressed AMS fabric, although it has been recognized for a very long time, has been the subject of very few publications in the paleomagnetic literature. This effect has been mainly described in samples with magnetite as a main magnetic carrier. This fabric is usually of low magnitude and observed mainly in nearly isotropic rock after application of static AF demagnetization or after acquisition of an isothermal remanent magnetization (IRM). Forty four paleomagnetic sites have been sampled in a >2 km thick sequence of Cretaceous volcano-clastic rocks from the western Central Pamir mountain (Tadjikistan). These rocks present a medium grade level of metamorphism characterized by fine grained recrystallisation of biotite. The magnetic properties are very homogeneous across the sequence. Bulk magnetic susceptibilities vary between 150-250 μ SI. The AMS magnetic fabrics correspond to triaxial tensors with a well defined foliation plane and a steeply dipping magnetic lineation. The degree of anisotropy varies between 1.03 and 1.2. This fabric was likely acquired during the deformation associated with the emplacement of Middle Miocene gneiss domes. SEM/EDS data indicate that the main iron oxide mineral is hematite with up to 15% of ilmenite in solid solutions. This is in agreement with unblocking temperatures of SIRM around 630 °C, lower than the one of pure hematite. One of the most surprising magnetic characteristics of these rocks is the effect of strong-field remanent magnetizations upon the AMS. During the acquisition of an Isothermal Remanent Magnetization (IRM), the initial AMS is progressively obliterated by a new AMS fabric. The field-impressed AMS is characterized by a decrease of the magnetic susceptibility along the direction of the IRM and an increase in magnetic susceptibility in the orthogonal plane. The field-impressed AMS is thus mainly oblate with a degree of anisotropy usually between 1.2 and 1.4. As far as we know, such a strong effect has never been reported. In sandstone with detrital hematite as the main carrier, the degree of the induced AMS fabric is less than 1.02 suggesting that the ilmenite content in the metamorphic hematite is the main cause of the large observed field induced fabric in these rocks.</p><p> </p>