Anisotropy Of Low-field Magnetic Susceptibility Fabric Research Articles

Can magnetic fabric indicate the direction of a glacier movement? An example from Itararé Group and Aquidauana Formation, Paraná Basin, Brazil

Magnetic fabrics was determined on Late Paleozoic sedimentary rocks from Aquidauna Formation (50 sites) and Itararé Group (71 sites) that outcrop in Paraná Basin, applying both anisotropy of low-field magnetic susceptibility (AMS) and anisotropy of anhysteretic remanence magnetization (AARM). Rock magnetism analyses reveal that magnetite, hematite, and paramagnetic matrix minerals are responsible for the magnetic susceptibility and AMS, while the magnetite is the carrier AARM. The AARM tensor is coaxial with AMS fabric. The analysis at the individual-site scale defines two AMS fabric types (A and B). The type A shows vertical AMS foliation pole (Kmin perpendicular to the bedding plane), while Kmax and Kint are scattered within the bedding plane itself. This fabric is usually interpreted as sedimentary-compactional. Type B shows the three well-clustered AMS axes with Kmin off-vertical, this fabric is the most important since it was found in the majority of the sites. This fabric is interpreted as indicative of sediment transport that could be inferred by the imbrication of AMS foliation (Kmin). The comparison of magnetic lineation (Kmax) for both Aquidauana Formation and Itararé Group with their respective glacial striae allow inferring that Kmax can be used as an indicator of glacier movement that was from SE towards NW, even though some variation or changes in the movement orientation could happen as found in the Itararé Group (from São Paulo and Paraná states), probably related to glacier advance and retreat or different glacier lobes.

Emplacement of dike swarms from the island of Ilhabela (SE Brazil) and its relationship with the South Atlantic Ocean opening revealed by magnetic fabrics

Magnetic fabric and rock-magnetism studies were performed on 153 dikes from diabase, lamprophyre and alkaline dike swarms that outcrop on the island of Ilhabela, NE São Paulo State. The dikes crosscut Archean and Proterozoic polymetamorphic rocks. Their thicknesses range from a few centimeters to 4 or 5 m. They trend predominantly N30-60E with a very steep (~60°) to vertical dip. Magnetic fabrics were determined using anisotropy of low-field magnetic susceptibility (AMS) and anisotropy of anhysteretic remanent magnetization (AARM). Rock-magnetism measurements reveal that the magnetic mineral of the three swarms is magnetite mainly with grains in the range of 2–5 μm. For a few dikes, these grains are the carriers of bulk magnetic susceptibility but are not responsible for the AMS, which is due to Fe-bearing minerals, as shown by AARM. The main AMS fabric recognized in the swarms is due to magma flow, in which the Kmax–Kint plane is parallel to the dike's plane, and the magnetic foliation pole (Kmin) is perpendicular to it. The analysis of the Kmax inclination showed that the dikes were fed by horizontal to vertical flows. However, for the minority of the dikes, the AMS and AARM tensors are not coaxial. The AARMmax is oriented N30–60 W, approximately perpendicular to the Kmax. The AARMmax orientation is similar to the direction of a fault system located mainly in the Santos marginal basin which, was formed in the Cretaceous rifting during the South Atlantic opening. The AARM fabric is tectonic in origin, and is parallel with the extensional paleostress responsible for the Gondwana break-up. The comparison of AMS and AARM fabrics suggests that diabase and lamprophyre dikes were emplaced in three distinct events in the earliest stages of the South Atlantic opening. Later, the alkaline dikes were emplaced in the final stages of the Atlantic opening.

Emplacement of the Anta Granite from Rio de Janeiro State (SE, Brazil); determined by magnetic fabrics

Magnetic fabric and rock magnetism studies were performed on the three facies of the Anta granite (SE of Rio de Janeiro State, southern Brazil). This intrusion is roughly elliptical (∼ 14 km2), NE-SW oriented and composed of granodiorite, monzodiorite, quartz-diorite, syenogranite, and monzogranite. It shows solid-state deformation and tectonic foliation at its borders and is apparently isotropic in the central part where the internal fabrics are impossible or very difficult to measure through classical techniques. Magnetic fabrics were determined by applying both anisotropy of low-field magnetic susceptibility (AMS) and anisotropy of anhysteretic remanent magnetization (AARM). The AMS and AARM fabrics are coaxial for all facies. The parallelism between AMS and AARM tensors excludes the presence of a single domain (SD) effect in the AMS fabric. Several rock-magnetism experiments performed on one specimen from each facies show that, for all units, magnetic susceptibility and fabrics are carried by magnetite grains. Most of the magnetic foliations are steeply dipping in all facies, and are roughly parallel or sub-parallel to the foliation measured in the field and in the country rocks. In contrast, the magnetic lineations present mostly low plunges for almost the whole pluton, except for one site where it is steep (> 60°). Thin section analysis show that rocks from the borders of the Anta pluton are solid-state deformed, indicating that the borders of the pluton were affected by the regional strain during its emplacement. On the other hand, the lack of solid-state deformation at outcrop scale and in thin sections precludes deformation in the central part of the pluton to be determined, where igneous textures are preserved. This evidence allows us to interpret the magnetic fabrics observed in this part of the pluton as primary in origin (magmatic) and acquired when the rocks were solidified as a result of magma flow, in which steeply plunging magnetic lineation suggests that a feeder zone could underlain this area. Our data suggest that the regional deformation partially affected the emplacement of the Anta granite.

Magnetic fabrics of the Cretaceous dike swarms from São Paulo coastline (SE Brazil): Its relationship with South Atlantic Ocean opening

Magnetic fabric and rock magnetism studies were performed on 91 dikes from Cretaceous diabase and lamprophyre dike swarms that outcrop side by side on the beaches of NE São Paulo State coastline. The dikes crosscut Archean and Proterozoic poly-metamorphic rocks of the Costeiro Complex. Their thicknesses range from a few centimeters to 2m for lamprophyre and up to about 10m for the diabase. They trend predominantly N30°–60°E with vertical dip. Magnetic fabrics were determined using anisotropy of low-field magnetic susceptibility (AMS) and anisotropy of anhysteretic remanent magnetization (AARM). Rock-magnetism measurements reveal that magnetite grains in the range of 2–5μm are the magnetic mineral of both swarms. For most dikes, these grains are the carriers of bulk magnetic susceptibility but, surprisingly, are not responsible for the AMS which is carried by Fe-bearing minerals as shown by AARM. The main AMS fabric recognized in the swarms is due to magma flow, in which the Kmax-Kint plane is parallel to the dike's plane, and the magnetic foliation pole (Kmin) is perpendicular to it. The analysis of the Kmax inclination showed that the dikes were fed by horizontal to vertical flows. However, for the majority of the dikes the AMS and AARM tensors are not coaxial. The AARM lineation (AARMmax) is oriented N30–60W, approximately perpendicular to AMS lineation (Kmax) suggesting that magnetite grains were rotated approximately 90° anticlockwise from the dike plane. The AARMmax orientation is similar to the direction of a fault system mainly in the Santos marginal basin which was formed in the Cretaceous rifting during the South Atlantic opening. Therefore the AARM fabric is tectonic in origin, and the comparison of AMS and AARM fabrics suggests that lamprophyre and diabase dikes were emplaced in three distinct events in the earliest stages of the South Atlantic opening.

Deformation in rocks from Itajaí basin, Southern Brazil, revealed by magnetic fabrics

Magnetic measurements were performed on sedimentary and volcanic rocks from the Itajaí basin. Rock magnetism analyses reveal that magnetite and hematite and paramagnetic minerals show different contributions to magnetic susceptibility. Magnetic fabrics were determined in 19 sites by applying both anisotropy of low-field magnetic susceptibility (AMS) and anisotropy of anhysteretic remanence magnetization (AARM). The analysis at the individual-site scale defines three AMS fabric types. The first type shows Kmin perpendicular to the bedding plane, while Kmax and Kint are scattered within the bedding plane itself. This fabric is usually interpreted as primary. The second type (77% of the sites) shows the three well-clustered AMS axes with Kmin still perpendicular to the bedding plane. The third type is characterized by well-clustered Kmax in the bedding plane, while Kmin and Kint are distributed along a girdle perpendicular to Kmax. The second and third fabric types are interpreted as combinations of sedimentary-compactional and tectonic contributions at the earliest, and at a slightly later stage of deformation, respectively. AMS represents the contribution of all the rock-forming minerals, while AARM isolates the contribution of remanence-bearing minerals with coercivity <100mT. For some sites, the AMS and AARM fabrics are non-coaxial, mainly with respect to the lineation orientations. The comparison of magnetic lineations and the regional structural data indicates that the rocks, with no mesoscopic or macroscopic deformation, were affected by the two deformation phases that occurred in the region. One of them results from compressional tectonics related to the collisional orogen (D1), and the other is extensional related to post-collisional orogen (D2). After the former tectonic event the volcanic rocks were emplaced at the top of the stratigraphic sequence and, after that the Itajaí basin as a whole was deformed by the extensional tectonic (D2) that occurred after 560Ma which is the age of the intrusion.

Magnetic fabrics and their relationship with the emplacement of the Piracaia pluton, SE Brazil

Magnetic fabric and rock-magnetism studies were performed on the four units of the 578 ± 3-Ma-old Piracaia pluton (NW of Sao Paulo State, southern Brazil). This intrusion is roughly elliptical (~32 km2), composed of (i) coarse-grained monzodiorite (MZD-c), (ii) fine-grained monzodiorite (MZD-f), which is predominant in the pluton, (iii) monzonite heterogeneous (MZN-het), and (iv) quartz syenite (Qz-Sy). Magnetic fabrics were determined by applying both anisotropy of low-field magnetic susceptibility (AMS) and anisotropy of anhysteretic remanent magnetization (AARM). The two fabrics are coaxial. The parallelism between AMS and AARM tensors excludes the presence of a single domain (SD) effect on the AMS fabric of the units. Several rock-magnetism experiments performed in one specimen from each sampled units show that for all of them, the magnetic susceptibility and magnetic fabrics are carried by magnetite grains, which was also observed in the thin sections. Foliations and lineations in the units were successfully determined by applying magnetic methods. Most of the magnetic foliations are steeply dipping or vertical in all units and are roughly parallel to the foliation measured in the field and in the country rocks. In contrast, the magnetic lineations present mostly low plunges for the whole pluton. However, for eight sites, they are steep up to vertical. Thin-section analyses show that rocks from the Piracaia pluton were affected by the regional strain during and after emplacement since magmatic foliation evolves to solid-state fabric in the north of the pluton, indicating that magnetic fabrics in this area of the pluton are related to this strain. Otherwise, the lack of solid-state deformation at outcrop scale and in thin sections precludes deformation in the SW of the pluton. This evidence allows us to interpret the observed magnetic fabrics as primary in origin (magmatic) acquired when the rocks were solidified as a result of magma flow, in which steeply plunging magnetic lineation suggests that a feeder zone could underlie this area.

Emplacement mechanism of the main granite pluton of the Lavras do Sul intrusive complex, South Brazil, determined by magnetic anisotropies

Magnetic fabric and rock magnetism studies were performed on apparently isotropic granite facies from the main intrusion of the Lavras do Sul Intrusive Complex pluton (LSIC, Rio Grande do Sul, South Brazil). This intrusion is roughly circular (~ 12 × 13.5 km), composed of alkali-calcic and alkaline granitoids, with the latter occupying the margin of the pluton. Magnetic fabrics were determined by applying both anisotropy of low-field magnetic susceptibility (AMS) and anisotropy of anhysteretic remanent magnetization (AARM). The two fabrics are coaxial. The parallelism between AMS and AARM tensors excludes the presence of a single domain (SD) effect on the AMS fabric of the granites. Several rock-magnetism experiments performed in one specimen from each sampled site show that for all sites the magnetic susceptibility is dominantly carried by ferromagnetic minerals, while mainly magnetite carries the magnetic fabrics. Lineations and foliations in the granite facies were successful determined by applying magnetic methods. Magnetic lineations are gently plunging and roughly parallel to the boundaries of the pluton facies, except at the few sites in the central facies which have a radial orientation pattern. In contrast, the magnetic foliations tend to follow the contacts between the different granite facies. They are gently outerward-dipping inside the pluton , and become either steeply southwesterly dipping or vertical towards its margin. The lack of solid-state and subsolidus deformations at outcrop scale and in thin sections precludes deformation after full crystallization of the pluton. This evidence allows us to interpret the observed magnetic fabrics as primary in origin (magmatic) acquired when the rocks were solidified as a result of processes reflecting magma flow. The foliation pattern displays a dome-shaped form for the main LSIC-pluton. However, the alkaline granites which outcrop in the southern part of the studied area have an inward-dipping foliation, and the steeply plunging magnetic lineation suggests that this area could be part of a feeder zone. The magma ascent probably occurred due to ring-diking.

Tectonic fabric revealed by AARM of the proterozoic mafic dike swarm in the Salvador city (Bahia State): São Francisco Craton, NE Brazil

Magnetic fabric and rock magnetism studies were performed on 25 unmetamorphosed mafic dikes of the Meso-Late Proterozoic (∼1.02 Ga) dike swarm from Salvador (Bahia State, NE Brazil). This area lies in the north-eastern part of the São Francisco Craton, which was dominantly formed/reworked during the Transamazonian orogeny (2.14–1.94 Ga). The dikes crop out along the beaches and in quarries around Salvador city, and cut across both amphibolite dikes and granulites. Their widths range from a few centimeters up to 30 m with an average of ∼4 m, and show two main trends N140–190 and N100–120 with vertical dips. Magnetic fabrics were determined using both anisotropy of low-field magnetic susceptibility (AMS) and anisotropy of anhysteretic remanent magnetization (AARM). The magnetic mineralogy was investigated by many experiments including remanent magnetization measurements at variable low temperatures (10–300 K), Mössbauer spectroscopy, high temperature magnetization curves (25–700 °C) and scanning electron microscopy (SEM). The rock magnetism study suggests pseudo-single-domain magnetite grains carrying the bulk magnetic susceptibility and AARM fabrics. The magnetite grains found in these dikes are large and we discard the presence of single-domain grains. Its composition is close to stoichiometric with low Ti substitution, and its Verwey transition occurs around 120 K. The main AMS fabric recognized in the swarm is so-called normal, in which the K max– K int plane is parallel to the dike plane and the magnetic foliation pole ( K min) is perpendicular to it. This fabric is interpreted as due to magma flow, and analysis of the K max inclination permitted to infer that approximately 80% of the dikes were fed by horizontal or sub-horizontal flows ( K max < 30°). This interpretation is supported by structural field evidence found in five dikes. In addition, based on the plunge of K max, two mantle sources could be inferred; one of them which fed about 80% of the swarm would be located in the southern part of the region, and the other underlied the Valéria quarry. However, for all dikes the AARM tensors are not coaxial with AMS fabrics and show a magnetic lineation (AARM max) oriented to N30–60E, suggesting that magnetite grains were rotated clockwise from dike plane. The orientation of AARM lineation is similar to the orientation of a system of faults in which the Salvador normal fault is the most important. These faults were formed during Cretaceous rifting in the Recôncavo-Tucano-Jatobá assemblage that corresponds to an aborted intra-continental rift formed during the opening of the South Atlantic. Therefore, the AARM fabric found for the Salvador dikes is probably tectonic in origin and suggests that the dike swarm was affected by the important tectonic event responsible for the break-up of the Gondwanaland.

Magnetic fabrics and rock magnetism of Archaean and Proterozoic dike swarms in the southern São Francisco Craton, Brazil

Magnetic fabric and rock magnetism studies were performed on three mafic dike swarms (total of 38 dikes) from the southernmost part of the São Francisco Craton (SFC) (Minas Gerais State, SE Brazil). They cut Archaean granite–gneiss–migmatite and paleoprototerozoic terranes. These swarms are classified as basic–noritic (Sm–Nd age ∼ 2.65 Ga), basic (Rb–Sr age ∼ 1.87 Ga) and metamorphic (Rb–Sr age ∼ 1.87 Ga) suites, in which the second is the most important. Magnetic fabrics were determined by applying both anisotropy of low-field magnetic susceptibility (AMS) and anisotropy of anhysteretic remanent magnetization (AARM). In most sites magnetic susceptibility is dominantly carried by ferromagnetic minerals, however, in some sites the paramagnetic contribution exceeds 70% of bulk susceptibility. Mainly coarse to fine-grained Ti-poor titanomagnetite up to pure magnetite carry the magnetic fabrics. Three primary AMS fabrics are recognized which are all coaxial with the AARM fabric. Normal AMS fabric is dominant in the basic suite (16 of 20 analyzed dikes) and occurs in 4 and 3 dikes from the basic–noritic and metamorphic suites, respectively. This fabric is interpreted as a result of magma flow in which the analysis of K max inclination permitted to infer that the majority of dikes were fed by inclined flows (30° < K max < 60°), although 44% of dikes from the basic suite were fed by horizontal or sub-horizontal flows ( K max < 30°). Intermediate AMS fabric was found in 50% of dikes from the basic–noritic and metamorphic suites, but in only 2 dikes from the basic suite. It is interpreted as due to vertical compaction of a static magma column with the minimum stress along the dike strike. Inverse AMS fabric is a minority (2 dikes from each suite). The parallelism between AMS and AARM tensors for dikes with abnormal fabrics suggests a primary origin for them. Gyroremanent magnetization (GRM) effect was negligible for the majority of dikes, but it was found in two dikes from the basic suite with normal AMS fabric. Magnetic fabrics recognized for the three studied swarms do not depend on magnetic mineralogy, geochemical composition, dike strikes, nor the age of the swarms since the same magnetic minerals and magnetic fabric types are found in dikes from all suites. Inclined and horizontal flows allow us to infer the relative position of at least three magma sources (or magma chambers) from which the dikes were fed.

The effect of magnetic anisotropy on paleomagnetic directions in high-grade metamorphic rocks from the Juiz de Fora Complex, SE Brazil

The Juiz de Fora Complex is mainly composed of granulites and granodioritic–migmatite gneisses and is a cratonic basement of the Ribeira fold belt, which was formed during the last stage of Brasiliano orogeny. Samples widely distributed over the studied region (SE Brazil, Rio de Janeiro State) were collected for paleomagnetic and magnetic anisotropy determinations. After measurement of anisotropy of low-field magnetic susceptibility (AMS) the same specimens were submitted to both alternating field (for anisotropy of remanent magnetization, ARM, determinations) and thermal demagnetization for paleomagnetic analyses. Demagnetization processes allowed isolating an eastern, steep, positive-inclination characteristic remanent magnetization (ChRM) direction from the rocks. The studied specimens are strongly heterogeneous at sample scale regarding the amount of magnetic minerals even for specimens from the same core. Rock magnetism indicates that there is a complex mixture of magnetic minerals with both low and high coercivity which are associated with coarse-grained (titano)magnetite and either fine-grained (titano)magnetite or ‘titanohematite’ grains, respectively. These magnetic minerals are responsible for both remanent direction and magnetic anisotropies, even though for some specimens paramagnetic minerals are the carriers of AMS fabric. The ChRM direction found for the rocks should represent the cooling phase of the Brasiliano event in the area, and it was acquired after the magnetic fabric was formed. The AMS and ARM fabrics are coaxial and tectonic in origin, and compare favorably with the mesoscopic-scale fabrics in the adjacent areas. However, ARM measurements have reached only low-coercivity grains and it could be determined in few specimens. Then only an estimate of ARM effect on ChRMs correction is shown. The rocks are strongly magnetically anisotropic (>50%) and foliated. The paleomagnetic directions from each specimen within a site tend to approach its AMS or ARM foliation ( K max– K int or ARM max–ARM min plane). These directions were corrected, specimen-by-specimen from the sites, taking into account the pole of AMS or ARM foliation ( K min or ARM min) and the value of the degree of anisotropy ( P). Results show that even for specimens with high P, the angular difference between corrected and uncorrected magnetization direction may be insignificant if the geomagnetic field at the time of acquired magnetization was approximately parallel to the magnetic foliation of the specimens. Our data set suggests that AMS is a powerful tool to correct the ChRM deviation from paleofields in rocks with complex magnetic mineralogy such as those from the Juiz de Fora Complex.

Magnetic fabrics in the Holum granite (Vest-Agder, southernmost Norway): implications for the late evolution of the Sveconorwegian (Grenvillian) orogen of SW Scandinavia

The Holum granite (Vest-Agder, southernmost Norway) has been considered to be the oldest (980 Ma) undeformed (“post-tectonic”) pluton of a late-Sveconorwegian granitoid series that form two belts in southern Norway and SW Sweden. An integrated structural study of this granite pluton, based on a survey of its anisotropy of low-field magnetic susceptibility (AMS), complemented by microstructural observations, optical microscopy and rock magnetism investigations (including measurements of partial anhysteretic remanent magnetization and its anisotropy, pAARM), is presented here. The magnetic susceptibility and the AMS of the Holum granite are carried by large magnetite grains. Coaxiality between the magnetite-controlled AMS fabric and the rock fabric is verified, which allows the use of the former as a proxy for the latter. Microstructural evidence of a deformation continuum in the granite from magmatic to solid state, and homogeneity and continuity of the AMS fabrics across the pluton demonstrate that the Holum granite is not post-tectonic, but that it was emplaced in a tectonic strain field. A model of emplacement and deformation during the last Sveconorwegian regional folding phase evidenced in the country rocks of the pluton is proposed.

Magnetic fabrics of dike swarms from SE Bahia State, Brazil: their significance and implications for Mesoproterozoic basic magmatism in the São Francisco Craton

Magnetic fabrics were determined by applying two techniques: anisotropy of low-field magnetic susceptibility (AMS); and anisotropy of anhysteretic remanence (AAR). AMS was performed in 81 dikes from Ilhéus, Olivença and Itaju do Colônia swarms located in SE Bahia. The dikes intrude high-grade metamorphic terrains as old as 3.2 Ga. Dikes are vertical or subvertical in dip and trend mainly E–W. They are tholeiitic in composition and are classified into two groups, characterized by high TiO 2 (>2%) and incompatible elements contents, and low TiO 2 and incompatible elements contents, respectively. Thicknesses vary from a few centimeters to ca. 20 m (average of 3 m). 40Ar/ 39Ar data indicate ages of 1.012 (Ilhéus dikes) and 1.078 Ga (Olivença dikes). Rock magnetism shows that Ti-poor titanomagnetites up to pure magnetites pseudo-single-domain/multidomain grain sizes carry the magnetic fabrics. Three types of AMS fabric are recognized: (i) ‘Normal’ fabric (64 dikes) defined by the clustering of K 1– K 2 axes on the dike plane whereas K 3 axes are nearly perpendicular to it. This fabric is interpreted as due to magma flow. (ii) ‘Intermediate’ fabric (seven dikes) characterized by K 1– K 3 axes clustering close to dike plane and K 2 axes perpendicular to the dike wall. It was interpreted as due to vertical compaction of a static magma column with the minimum stress along the dike direction; and (iii) ‘Inverse’ fabric (nine dikes) defined by K 2– K 3 plane parallel to the dike plane and K 1 perpendicular to dike wall. AAR measurements were performed in three dikes; two of them with ‘inverse’ and one with ‘normal’ AMS fabrics. AAR was coaxial for ‘normal’ AMS fabric and it resulted in ‘intermediate’ and ‘normal’ for ‘inverse’ AMS fabrics. A combination of AMS and AAR fabrics suggests that the magmatic fabric for these dikes that should be flow, was overprinted by some local event, probably related to Brasiliano Orogeny processes, after dike emplacement. ‘Inverse’ AMS fabric would record this event.‘Normal’ fabric is the most important one found for the studied swarms. The analysis of K 1 inclination permitted one to infer that the dikes from Ilhéus were fed dominantly by horizontal fluxes ( K 1<30°) whereas those from Olivença and Itaju do Colônia were mainly fed by inclined up to vertical fluxes ( K 1>60°). Dikes from Itaju do Colônia region have the steepest K 1 suggesting that this region could be closer to a magma source. On the other hand, a magma source near to Olivença area could be also inferred since the majority of the dikes were fed by inclined flow. The existence of more than one source is supported by geochemical, geochronological and paleomagnetic data which have shown that contemporaneous dikes were fed by compositionally distinct magmas. The AMS data also suggest that the majority of dikes were fed from upward flow coming from west to east and only a few of them were fed from upward flow from east to west.

Paleoremanence dispersal across a transpressed Archean terrain: Deflection by anisotropy or by late compression?

The Quetico Belt is an Archean metasedimentary terrain sandwiched between greenstone subprovinces. A single regional schistosity is subparallel to magnetic foliation defined by anisotropy of low‐field magnetic susceptibility (AMS) and anisotropy of anhysteretic remanence (AARM). Aligned micas are the primary cause of AMS. Aligned pyrrhotite is the sole cause of AARM. Each magnetic subfabric is due to minerals that grew in overlapping intervals in the strain history. Their slight obliquity may be due to crystallization during noncoaxial strain. Magnetic fabric ellipsoids suggest varying severity of flattening: very strong flattening on the northern boundary, less severe on the southern boundary, and least severe in the interior. The interior and the southern boundary of the belt stretched horizontally and the shortening direction varies gradually, from NS in the north to NNW in the south. The paleomagnetic record carried by pyrrhotite shows characteristic remanences dispersed around a great circle subparallel to the regional schistosity and magnetic foliations. Late stress effects rotated paleoremanences into this pattern; we discount deflection of the paleofield by anisotropy. The pattern of dispersal indicates uniaxial oblate flattening of the region with NNW‐SSE shortening, late in the metamorphic cooling history (&lt;300°C). This postdates schistosity, AMS and AARM fabrics but it is controlled by the same kinematic pattern.