A PALAEOMAGNETIC STUDY OF JURASSIC AND CRETACEOUS SEDIMENTS

Summary. Representative samples of Cretaceous and Jurassic arenaceous rocks and some Jurassic limestone samples were collected. The arenaceous rocks were generally either too weakly magnetized for the measurement of their remanent magnetization, or were unstable; higher intensities of magnetization were recorded in the limestones. The magnetic directions of the few reversed Jurassic limestones indicate a pole position near Omsk. X-ray analysis indicates that the detrital iron ore mineral present was goethite. It is concluded that the palaeomagnetic unsuitability of these Jurassic and Cretaceous deltaic deposits results from the weathering conditions in the source area.     

Palaeomagnetism of limestones from the Gargano Peninsula (Italy), and the implication of these data

The platform limestones of Apulia are usually too weakly magnetic for precise measurement. The East Gargano basin is an autochthonous extension of Apulia and incorporates deeper water limestones which, though weakly magnetic ( J NRM ≃ 50 nG), can be reliably measured using a cryogenic magnetometer. The magnetization is attributed to the presence of detrital magnetite and the pelagic limestones yield a mean magnetic direction for the Late Cretaceous (Dec. = 335°, Inc. = 38°, α₉₅= 6.5°). The circle of confidence associated with this direction overlaps with those associated with Late Cretaceous magnetic directions from Iblei (Sicily) and from the Vicentinian (Southern) Alps. Palaeomagnetic pole positions for Iblei, Gargano/Apulia and the Southern Alps indicate that these three autochthons need not have rotated significantly relative to each other since the Late Cretaceous. An inferred Late Cretaceous pole position for Africa coincides with the pole positions obtained from these Italian data.     

Rock magnetic properties of recent soils from northeastern Bulgaria

In this paper, basic rock magnetic studies of Holocene loess-soil samples from northeastern Bulgaria are reported. The sites are related to the Danube river and located at different distances southwards, thus representing various pedogenic conditions. The study is primarily aimed at determining the main magnetic carrier(s) and their physical characteristics (grain-size distribution, magnetic enhancement, etc.). Oxyhydroxides, maghemite and titanomagnetites of various oxidation degrees are assumed to be the main ferromagnetic minerals present. Our results suggest that the uppermost part of recent soil profiles is rich in stable, near-single-domain (SD) particles, while the illuvial horizons are characterized by a gradual decrease in grain sizes, from highly viscous to a true superparamagnetic (SP) domain state. The properties of samples from carbonate-rich horizons of recent soils are basically controlled by detrital minerals, while those from sites with more intensive pedogenesis and especially grey forest soils are influenced by strongly magnetic minerals formed ' in situ '.     

The rock magnetism of the Mississippian Madison Limestone north-central Wyoming

Summary. A history of remanence acquisition in the Madison Limestone in north-central Wyoming suggests that many processes affected the primary magnetization, including (1) dolomitization, (2) solution of evaporites during uplift in the Late Mississippian and (3) partial remagnetization in the Late Cretaceous or Cenozoic. The stability and intensity of the magnetization was greatest in the dolomitic limestones, while the dolomites and the less intense fossiliferous limestones contained magnetization of intermediate directional stability. The coarse- and fine-grained limestones gave erratic directions. The preservation of the Mississippian magnetization in the dolomitic limestone from the lower portion of the Madison testifies to the delicate mechanism of the dolomitization process, and suggests that the dolomitization may even have aided in the preservation of the magnetization. The presence of solution features corresponds closely to the presence of unstable directional behaviour. It appears that the formation of these features by solution of the evaporite beds led to the destruction of the primary remanence in the upper portion of the Madison.
Thermomagnetic curves indicate the presence in the lower portion of the Madison of almost pure magnetite. Although thermomagnetic curves from the upper portion are irreversible, alternating field and thermal demagnetization experiments show intensity changes similar to those from the lower portion, suggesting a similar magnetic mineral. Isothermal remanent magnetization acquisition curves also suggest the presence of a large amount of hematite or goethite in over half the samples which apparently did not contribute significantly to the natural remanent magnetization.     

Record of Late Pleistocene Glaciation and Deglaciation in the Southern Cascade Range. I. Petrological Evidence from Lacustrine Sediment in Upper Klamath Lake, Southern Oregon

Petrological and textural properties of lacustrine sediments from Upper Klamath Lake, Oregon, reflect changing input volumes of glacial flour and thus reveal a detailed glacial history for the southern Cascade Range between about 37 and 15 ka. Magnetic properties vary as a result of mixing different amounts of the highly magnetic, glacially generated detritus with less magnetic, more weathered detritus derived from unglaciated parts of the large catchment. Evidence that the magnetic properties record glacial flour input is based mainly on the strong correlation between bulk sediment particle size and parameters that measure the magnetite content and magnetic mineral freshness. High magnetization corresponds to relatively fine particle size and lower magnetization to coarser particle size. This relation is not found in the Buck Lake core in a nearby, unglaciated catchment. Angular silt-sized volcanic rock fragments containing unaltered magnetite dominate the magnetic fraction in the late Pleistocene sediments but are absent in younger, low magnetization sediments. The finer grained, highly magnetic sediments contain high proportions of planktic diatoms indicative of cold, oligotrophic limnic conditions. Sediment with lower magnetite content contains populations of diatoms indicative of warmer, eutrophic limnic conditions. During the latter part of oxygen isotope stage 3 (about 37–25 ka), the magnetic properties record millennial-scale variations in glacial-flour content. The input of glacial flour was uniformly high during the Last Glacial Maximum, between about 21 and 16 ka. At about 16 ka, magnetite input, both absolute and relative to hematite, decreased abruptly, reflecting a rapid decline in glacially derived detritus. The decrease in magnetite transport into the lake preceded declines in pollen from both grass and sagebrush. A more gradual decrease in heavy mineral content over this interval records sediment starvation with the growth of marshes at the margins of the lake and dilution of detrital material by biogenic silica and other organic matter.     

Nature and origin of magnetic minerals within the Middle Jurassic shallow-water carbonate rocks of the Paris Basin, France: implications for magnetostratigraphic dating

The Middle and Upper Jurassic Bathonian-Oxfordian shallow-water carbonate rocks from the Paris Basin, France, consist mainly of oolitic and bioclastic limestones that are hydrocarbon reservoirs in the subsurface. Despite a preliminary positive study, these deposits have been considered to be largely remagnetized (Rochette, private communication), and hence not amenable to palaeomagnetic dating. To establish their magnetic mineralogy and test this remagnetization hypothesis, we have used an integrated investigation combining petrographic, geochemical, rock-magnetic and palaeomagnetic measurements on samples extracted from five cores from the Paris Basin and from outcrops in Burgundy. Magnetic minerals in the Bathonian-Oxfordian carbonates include: (1) primary biogenic single-domain magnetite and detrital multi-domain Ti-magnetite and their oxidized form, maghemite; (2) authigenic spheres of magnetite probably related to hydrocarbons; and (3) goethite, either restricted to ferruginous ooid layers or resulting from surficial alteration, notably replacement of pyrite framboids. Rock-magnetic experiments carried out on 68 samples reveal H _cr/ H _c and M _rs/ M _s ratios ranging from 1.88 to 5.58 and 0.017 to 0.314, respectively. These values are clearly distinct from diagnostic values for a chemical remagnetization. Pyrrhotite was not identified within these sediments. Moreover, the average H _cr/ H _c ratio of 3.14 is significantly different from the value of 1.333 for natural pyrrhotite (Dekkers 1988). These results have a direct implication for the preservation of the primary magnetization; consequently, these deposits are selectively amenable for magnetostratigraphic dating and possible regional correlations.     

The acquisition of post-depositional detrital remanent magnetization in a variety of natural sediments

Summary. Post-depositional detrital remanent magnetization (pDRM) is the primary means whereby many sediments acquire their palaeomagnetic signal. We have studied the acquisition of this magnetization in a variety of natural sediments. Our technique involves determining the magnetic direction recorded by a sediment as a function of the water content present in the sediment when the sediment experiences a change in the direction of the applied magnetic field. Most of the sediments used in this study were collected wet from natural environments and were preserved in their original state until they were used in the experiments. Grain sizes were measured by the settling tube method which led to the determination of the clay, silt and sand fractions in each sediment. Isothermal remanent magnetization acquisition studies indicated that the predominant magnetic carriers were magnetite. In the pDRM acquisition studies two distinct modes of behaviour were found. For sediments with a sand content less than 60 per cent, the original direction of magnetization was preserved regardless of the water content. Such behaviour is not consistent with a theoretical model which assumes that at high water contents the magnetic carriers remain mobile within fluid-filled voids and hence are able to realign along a new magnetic field direction. For sediments with a sand content in excess of 60%, remagnetization along a new magnetic field direction occurred as expected, provided the sediments were sufficiently wet. Studies of natural sediments and corresponding samples of dried and reconstituted sediments have demonstrated that the magnetic characterization of a sediment can be reliably determined even for older, desiccated sediments.     

On the use of geochronology of detrital grains in determining the time of deposition of clastic sedimentary strata

Placing geologic events in a temporal framework is essential to telling the story of Earth history. However, clastic sedimentary rocks can be difficult to date in an absolute reference because they are made up of grains that are older than the rock in which they are now found, and some clastic rocks do not contain fossils that allow precise reference to the Geologic Timescale. For such rocks, the isotopic dating of detrital minerals can be used to estimate the time of deposition; the clastic rock must be younger than the youngest grain analysed. However, many researchers eschew this simple and straightforward approach in favour of schemes that estimate the maximum allowable depositional age as the weighted mean of the age of several grains, chosen by a variety of selection criteria. This is a mistake; in the absence of a geochemical resemblance apart from the similarity of their age, detrital grains should not be assumed to have originated in the same system and therefore any averaging or other manipulation of such data is statistically invalid and produces results without geologic significance. In the absence of interbedded volcanic rocks or index fossils, dating of detrital minerals can be an important aid in understanding the time of deposition of clastic rocks, but the best estimate will come from taking note of the youngest single grain and not by inappropriately averaging data.     

Laboratory studies of depositional DRM

Summary. Acquisition of magnetic remanence in slurries of fme grained organic muds settling in long tubes is investigated using a cryogenic magnetometer. The average settling behaviour of remanence carrying grains relative to the whole sediment gives information about the relative magnetic grain size spectrum, whereas the response of settled deposits to vibration gives an indication of the degree of alignment of particles and their average shape.
Two classes of behaviour are apparent in both the time and field dependence of detrital remanent magnetization (DRM) acquisition. Dilute slurries settling in the Earth's field (analogous to detrital sedimentation) acquire a remanence which reaches a maximum after about 2 day, whereas for concentrated slurries (analogous to slumped or bioturbated sediments) this takes only a matter of minutes. The field dependence of DRM in dilute slurries is in plausible quantitative agreement with Stacey's extension of the classical Langevin expression for the susceptibility of a paramagnetic gas, whereas concentrated slurries show a quasi-linear dependence of DRM on the applied field. Inclination errors are generally absent, but do appear in fields less than about 0.5 Oe, and when the magnetic fraction settles out preferentially. A weak negative dependence of DRM on temperature is found, but the results are too crude to provide a further test of Stacey's theory.
Remanence acquisition in slurries settled in zero field indicates that short term post-depositional magnetization processes are relatively unimportant in slurries that have ceased to compact rapidly. A large increase in remanence from naturally occurring sediments to slowly redeposited long cores, to rapidly settled slurries is best explained as a feature of compaction.     

The Palaeomagnetism of the Great Dyke of Southern Rhodesia

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Oriented cores have been secured from fourteen sites in the Great Rhodesian Dyke, by means of a portable sampling drill. The natural remanent magnetizations showed high dispersion at all sites except one. After demagnetization in alternating magnetic fields, nine sites gave well grouped directions of primary magnetization. These sites include five rock types distributed among three Complexes of the Great Dyke and two satellite dykes, over 200 miles of the length of the Dyke and through several thousand feet in depth as the rocks were originally intruded. The nine site mean directions of primary magnetization are closely grouped and are believed to represent directions of thermo-remanent magnetization at the date of intrusion of the Great Dyke. It is suggested that the dates of magnetization at the sites must cover a sufficient time interval to give a mean pole position close to the axial geocentric dipole freed from secular variation. On the assumption of a geocentric dipole field, the position of the mean South magnetic pole is 211/2 °N, 611/2 °E, with radius of 95 per cent confidence 9°. This pole position is close to positions of North magnetic poles given by studies of the palaeo-magnetism of the Pilansberg Dykes and Bushveld gabbro.     

Magnetic properties of siliceous marine sediments in Northern Hokkaido,Japan: a quantitative tectono‐sedimentological study of basins along an active margin

The formation processes of the late Neogene sedimentary basins in Northern Hokkaido have been investigated on the basis of rock magnetism, structural geology and numerical modelling. Untilted site‐mean directions of remanent magnetization of the Wakkanai Formation, obtained from oriented core samples in Horonobe, suggest remarkable counterclockwise block rotation (ca. 70°) since the late Neogene. Uniform microscopic fabric of the siliceous sediments was inferred from the alignment of the principal axes of the anisotropy of magnetic susceptibility (AMS). After correction for tectonic rotation, the maximum axis of AMS, which reflects the sedimentary fabric of the dominant paramagnetic minerals, is in an E‐W direction, which is concordant with the influx direction of diatomaceous particles into the N‐S elongate sedimentary basins. The difference in the bulk initial magnetic susceptibility of the siliceous sediments of the Wakkanai Formation between the depocenter of the basin and its peripheral part implies that terrigenous non‐magnetic fraction has been sorted out during transportation of the detrital grains as gravity flows. As for the development mechanism of the N‐S elongate late Neogene basins in Northern Hokkaido, their depocenter arrangement and subsidence pattern indicates dextral motions upon a longitudinal fault zone along the Eurasian convergent margin. Dislocation modelling was adopted to explain vertical displacement and rotational motion around the study area and successfully restored the deformation pattern based on the assumption of dextral slip at a left‐stepping part of a strand of the transcurrent fault.     

Curie Temperature Analyses of Upper Jurassic and Lower Cretaceous Pelagic Limestones

Summary. Magnetic extracts were prepared from samples of Upper Jurassic and Lower Cretaceous pelagic limestones from France and Spain. Thermomagnetic analysis of the magnetic extracts using a microbalance required careful monitoring of base weight changes during heating. Heating in argon gas atmosphere induced production of magnetite during heating while slight oxidation occurred during heating in air. the dominant Curie temperature detected by the thermomagnetic analyses was the 585°C Curie temperature of magnetite. the 680°C Curie temperature of haematite was only detected when isothermal remanent magnetism (IRM) data indicated large concentrations of haematite. Even when IRM data indicated its presence, the thermomagnetic analyses did not detect the Neel temperature of goethite. Although thermomagnetic analyses of magnetic extracts provide more direct identification of the dominant, strongly ferromagnetic minerals, IRM acquisition and subsequent thermal demagnetization is a superior technique in detecting high coercivity, weakly ferromagnetic minerals such as goethite and haematite.     

Angular Dispersion Due to Random Magnetization

Summary. The angular probability distribution generated by adding a set of randomly oriented vectors to a set of constant parallel vectors has been studied. If one random vector of constant length is added to each constant vector, the resultant angular probability distribution departs widely from the distribution used by Fisher as the basis for palaeo-magnetic analysis. However, if n random vectors of equal length are added to each constant vector and if n > 6, the resulting angular probability distribution is nearly identical to that proposed by Fisher over most of the range of angular dispersions encountered in palaeo-magnetic investigations. Equations are developed from which the mean intensity of random magnetization can be calculated, given the mean total intensity and the angular dispersion. These parameters, in turn, are related to the ratio of random to non-random magnetization for each ferromagnetic mineral grain in a rock specimen.     

Greigite (Fe3S4) as an indicator of drought – The 1912–1994 sediment magnetic record from White Rock Lake, Dallas, Texas, USA

Combined magnetic and geochemical studies were conducted on sediments from White Rock Lake, a reservoir in suburban Dallas (USA), to investigate how land use has affected sediment and water quality since the reservoir was filled in 1912. The chronology of a 167-cm-long core is constrained by the recognition of the pre-reservoir surface and by 137Cs results. In the reservoir sediments, magnetic susceptibility (MS) and isothermal remanent magnetization (IRM) are largely carried by detrital titanomagnetite that originally formed in igneous rocks. Titanomagnetite and associated hematite are the dominant iron oxides in a sample from the surficial deposit in the watershed but are absent in the underlying Austin Chalk. Therefore, these minerals were transported by wind into the watershed. After about 1960, systematic decreases in Ti, Fe, and Al suggest diminished input of detrital Fe-Ti oxides from the surficial deposits. MS and IRM remain constant over this interval, however, implying compensation by an increase in strongly magnetic material derived from human activity. Anthropogenic magnetite in rust and ferrite spherules (from fly ash?) are more common in sediment deposited after about 1970 than before and may account for the constant magnetization despite the implied decrease in detrital Fe-Ti oxides. An unexpected finding is the presence of authigenic greigite (Fe3S4), the abundance of which is at least partly controlled by climate. Greigite is common in sediments that predate about 1975, with zones of concentration indicated by relatively high IRM/MS. High greigite contents in sediment deposited during the early to mid-1950s and during the mid-1930s correspond to several-year periods of below-average precipitation and drought from historical records. Relatively long water-residence times in the reservoir during these periods may have led to elevated levels of sulfate available for bacterial sulfate reduction. The sulfate was probably derived via the oxidation of pyrite that is common in the underlying Austin Chalk. These results provide a basis for the paleoenvironmental interpretation of greigite occurrence in older lake sediments. The results also indicate that greigite formed rapidly and imply that it can be preserved in the amounts produced over a short time span (in this lake, only a few years). This finding thus suggests that, in some lacustrine settings, greigite is capable of recording paleomagnetic secular variation.     

Magnetostratigraphy of Early Cretaceous Maiolica sections of the Southern Alps (Cismon and Pra da Stua, Italy)

The bottom part of the Cretaceous Cismon section in the Southern Alps was sampled for high-resolution magnetostratigraphy. Although the almost pure pelagic nannofossil limestones ( c. 90 per cent CaCO₃) of the Maiolica/Biancone Formation are extremely weakly magnetized, stepwise thermal and alternating-field demagnetization removed overprints and isolated a characteristic remanent magnetization which is interpreted as a primary magnetization. The dominant magnetic carrier mineral is magnetite; a small fraction of haematite may be present. A clear reversal pattern can be correlated unambiguously with Mesozoic polarity chrons CM10N to CM8. A less well-constrained magnetostratigraphy from the Pra da Stua section could not be directly correlated with the global polarity scale, but biostratigraphic information allows its assignment to the interval CM10-CM5. A counterclockwise rotation of 56 and a northward translation of 28 latitude for the Cismon locality since the Early Cretaceous are derived from the palaeomagnetic data, consistent with previous results from the Southern Alps. The high-resolution magnetostratigraphy of the Cismon section is used in an effort to refine the Cretaceous timescale by the combination of magnetostratigraphic and cyclostratigraphic results from the same section. The cyclostratigraphic duration estimates of chrons CM10N to CM8 are compared to their equivalents in a number of traditional timescales and found to be shorter by a factor of 1.26-2.58.     

A theoretical and experimental comparison of the anisotropies of magnetic susceptibility and remanence in rocks and minerals

Summary. Susceptibility, thermo-remanent magnetization (TRM) and isothermal remanent magnetization (IRM) anisotropy ellipsoids have been determined for several rock samples. The results indicate that the ellipsoid of initial susceptibility is less anisotropic than the TRM and low field IRM ellipsoids which are found experimentally to be of identical shape. This suggests that palaeomagnetic data for anisotropic rocks may be corrected by using the anisotropy ellipsoid determined from magnetically non-destructive low field IRM measurements. Such IRM measurements can also be used to obtain anisotropy axes of samples which are inherently anisotropic but which have a susceptibility which is too weak to be accurately measured. The results for a series of artificial anisotropic samples containing magnetite particles of different sizes (in the range 0.2–90 μm) were very similar to those for the rocks. In contrast, a comparison of the susceptibility and IRM ellipsoids for anisotropic samples containing particles from a magnetic tape gave very different results in accordance with theory. Such results imply that susceptibility and IRM ellipsoids could be used to determine whether anisotropic rocks contain uniaxial single-domain particles (magnetization confined to the easy axis) or whether the particles are essentially multidomain.     

Efficiency of heavy liquid separation to concentrate magnetic particles

Low-temperature rock magnetic measurements have distinct diagnostic value. However, in most bulk marine sediments the concentration of ferrimagnetic and antiferromagnetic minerals is extremely low, so even sensitive instrumentation often responds to the paramagnetic contribution of the silicate matrix in the residual field of the magnetometer. Analysis of magnetic extracts is usually performed to solve the problems raised by low magnetic concentrations. Additionally magnetic extracts can be used for several other analyses, for example electron microscopy or X-ray diffraction. The magnetic extraction technique is generally sufficient for sediments dominated by magnetite. In this study however, we show that high-coercivity components are rather underrepresented in magnetic extracts of sediments with a more complex magnetic mineralogy. We test heavy liquid separation, using hydrophilic sodium polytungstenate solution Na₆[H₂W₁₂O₄₀], to demonstrate the efficiencies of both concentration techniques. Low-temperature cycling of zero-field-cooled, field-cooled and saturation isothermal remanent magnetization acquired at room temperature was performed on dry bulk sediments, magnetic extracts, and heavy liquid separates of clay-rich pelagic sediments originating from the Equatorial Atlantic. The results of the thermomagnetic measurements clarify that magnetic extraction favours components with high spontaneous magnetization, such as magnetite and titanomagnetite. The heavy liquid separation is unbiased with respect to high- and low-coercive minerals, thus it represents the entire magnetic assemblage.     

Secondary ferrimagnetic minerals in Welsh soils: a comparison of mineral magnetic detection methods and implications for mineral formation

A study of mineral magnetic parameters was carried out on a Late Pleistocene and Holocene sedimentary sequence (of nearly 18 m) from Lake Bledowo (central Poland). Sediments of Lake Bledowo have already been analysed for bulk sediment mineralogy and biogenic materials. The mineral magnetic stratigraphy confirms the major changes in palaeo-environmental conditions that have been deduced from other methods. The most important mineral magnetic change results from the authigenetic formation of ferrimagnetic greigite, Fe₃S₄, during the beginning of lacustrine conditions (± 12 000 yr BP). Our data also indicate a detrital origin of overlying ferrimagnetic iron oxides. It is suggested that they originate from brown soils developed on the boulder clay constituting the west side of the lake shore. Variations of ferrimagnetic iron oxide size are related to the early diagenetic processes in the sediment. Larger particles are present in periods with early diagenesis of organic matter in anoxic conditions. This indicates the dissolution of fine magnetic particles by iron-oxide-reducing bacteria and results in homogeneous magnetic grain sizes, despite their origin from soils, characterized by a multimodal grain-size distribution.     

Current-scattered (?) detrital remanence directions in the Zn-rich Pennsylvanian Stark black shale, USA

Palaeomagnetic results are reported from the metalliferous Stark black shale in the Upper Pennsylvanian (Missourian/Kasimovian) Kansas City Group. Palaeomagnetic analysis of 400 specimens from 28 sites gives a characteristic remanent magnetization in 17 sites of the shale that yields a Late Mississippian to Middle Pennsylvanian palaeopole at 32.2°N 128.5°E (dp = 4.7° and dm = 8.8°). The observed palaeomagnetic age is slightly older than the host rock, indicating that the mineralization of the Stark Shale has, excluding recent alteration, a primary sedimentary or syngenetic origin. The reason for the slightly older age is likely due to trace modern hematite that slightly steepens the remanence inclination. The large oval of 95 per cent confidence is interpreted to be caused by clay–magnetite aggregates that formed during sediment transport and the biasing effect of the gentle palaeocurrent at each site acting on the large aggregates. Therefore, the scattered distribution of the site mean remanence declinations found for the Stark Shale is evidence of a detrital remanent magnetization that is formed by primary sedimentary processes with an enriched metallic content and not remagnetization with mineralization by secondary hydrothermal processes.     

AMS study of the Pont-de-Montvert–Borne porphyritic granite pluton (French Massif Central) and its tectonic implications

Thermomagnetic experiments, isothermal remanent magnetization (IRM), hysteresis loops, X-ray reflection analyses, optic microscopic observations, bulk magnetic susceptibility and natural remanent magnetization (NRM) show that the susceptibility signal of 56 sites in the Pont-de-Montvert–Borne pluton is derived mostly from biotite with a very small proportion of ferromagnetic material. Low anisotropy degree ( P parameter) and consistent AMS orientations among monzonite, enclaves and aplitic dykes indicate that the AMS was acquired during pluton emplacement in the subsolidus phase. Magnetic fabrics demonstrate that linear deformation is prominent in the area where the granite is in direct contact with micaschists, and that planar deformation becomes more important in the area where the pluton is surrounded by non-porphyritic peraluminous granites. AMS measurements also show that shallow plunging E–W-stretching lineations are the dominant structure over most of the studied area. Foliation orientations follow the pluton contour in the western part, indicating that the Pont-de-Montvert–Borne pluton is probably rooted in the west and extruded towards the east. These AMS results agree to some extent with fabric inferred from mineral preferred orientation. The AMS data support an E–W extensional tectonic setting during the Pont-de-Montvert–Borne pluton emplacement, which resulted from the Late Carboniferous crustal thinning of the Hercynian orogeny.