Anisotropy Of Magnetic Susceptibility Studies Research Articles

New paleomagnetic data from the sedimentary cover of the Tandilia System: Further geodynamic or geomagnetic complexities in the Late Ediacaran

Anomalous paleomagnetic data have been found worldwide during the Ediacaran period, giving rise to several non-actualistic hypothesis. In order to get more information about this period, paleomagnetic, magnetic fabric and rock magnetic studies were carried out in the Avellaneda Formation (∼570–560 Ma) from two drill cores of the Alicia quarry in the Olavarría area of the Tandilia System, in the Río de la Plata craton (Argentina). Anisotropy of magnetic susceptibility studies indicate a pre-tectonic origin for the magnetic fabric of this Formation. Rock magnetic studies suggest the presence of magnetite and hematite in different proportions as the main ferromagnetic minerals carrying the remanence. After stepwise thermal demagnetization, two different characteristic remanence directions were obtained for the same unit, one corresponding to the marls (“b1”) of the lower section of the Avellaneda Formation, and the other from the claystones (“b2”) of the upper section of this unit. These results were combined with the remanence directions obtained by Franceschinis et al. (2022) for the same unit at La Cabañita quarry, located 10 km away. This procedure allows the calculation of two paleomagnetic poles for the Avellaneda Formation. The AV1 pole is located at: 2.0° S, 311.1° E, A95: 5.0°, N: 58 while the AV2 pole is at: 3.3° N, 348.9° E, A95: 11.7°, N: 7. These results confirms that the Rio de la Plata craton also presents anomalous paleomagnetic data during the Ediacaran, implying extremely fast movements in very short periods of time. This can be interpreted as evidence of two inertial interchange true polar wander events during this time, as was already proposed by other authors. An alternative possibility suggests a non-actualistic behaviour of the Earth Magnetic Field, switching from axial to equatorial positions, with an intermediate stable position between them. The likelihood and implications of these two hypotheses is discussed.

Non-coaxial deformation of foreland basement involved in a fold-and-thrust belt: a strain partitioning approach to the Eastern Variscan orogen

The general SW–NE course of the Variscan orogen in Europe is abruptly bent to the N–S course at its eastern margin, where an oblique convergence occurred. The main suture in this part of the Variscan orogenic belt is called the Moldanubian Thrust, characterized by a dominant dextral strike-slip kinematics and a minor thrust component. The deep level of erosion and the good exposure of this structure allowed us to study the mechanisms of oblique convergence and the incorporation of the foreland basement into the orogenic belt. The combination of small-scale structures with the anisotropy of magnetic susceptibility studies allowed the recognition of two deformations in the studied rocks: dextral simple shearing and drag folding. Due to oblique convergence, the deformations induced by this mechanism were non-coaxial; therefore, their contributions can be easily distinguished. Finally, an overturned, almost recumbent large-scale synformal fold structure in the footwall and an antiformal structure in the hanging wall of the Moldanubian Thrust were formed. These two folds can be interpreted as structures formed by dragging along the Moldanubian Thrust. The previously described sinistral simple shearing in the upper limb of the synform resulted from the original dextral strike-slip shearing, which was overturned during progressive deformation.

Rock magnetism and anisotropy of magnetic susceptibility studies on charnockites, southern India

Rock magnetism and anisotropy of magnetic susceptibility studies on charnockites, southern India

Anisotropy of magnetic susceptibility study and its significance in the Late Cretaceous-Cenozoic Sanmenxia Basin in the southeastern Shanxi rift, Central China

The Sanmenxia Basin, located on the southeastern margin of the Shanxi rift and filled with Cretaceous-Paleogene fluvial and lacustrine sediments, is a faulted basin bounded by a series of normal strike-slip faults. This provides a valuable opportunity to investigate the early tectonic deformation in the Fenwei graben. In this study, we report an integrated rock magnetism and AMS analysis of two sections from the Sanmenxia Basin spanning an interval from the Late Cretaceous to the Eocene. The rock magnetic results demonstrate that magnetite and hematite are the main magnetic carriers of remanence, and paramagnetic minerals and hematite are major contributors to the AMS in both sections of the Sanmenxia Basin. Together with the relatively low-corrected anisotropy values, the tightly grouped minimum principal axes are almost perpendicular to the bedding plane, and the well-defined magnetic lineation is generally parallel to the bedding trend, indicating that the primary sedimentary fabrics in the Sanmenxia Basin were overprinted by the initial deformation. The NW–SE magnetic lineation denotes the NW–SE stretching during the Late Cretaceous-Eocene. The stretching process may have been controlled by the Indian-Eurasian convergence and/or the subduction of the western Pacific plate.

Anisotropy of magnetic susceptibility study to locate the feeder zone and lava flow directions of the Rajmahal Traps (India): Implications to Kerguelen mantle plume interaction with Indian Plate

Rajmahal Traps (RT) represents the earliest eruption from the Kerguelen hotspot activity over Indian Plate. The results of anisotropy of magnetic susceptibility (AMS) fabrics depict distribution anisotropy with the maximum susceptibility axes (Kmax) predominantly clustered NE-SW with plunge in NE. It infers that the feeder zone for RT was located in the NE quadrant of the volcanic province and along the Narmada-Son-Brahmaputra Transform (NSBT) fault. Our results further suggest that the NSBT system provide the major topographic control for the lava flow channel distribution in the Rajmahal province. It implies that the fracture zone was available during the lava emplacement, which records the earliest Kerguelen plume activity. Comparison between AMS fabrics and field topography suggests the distal horizontal magma flow and fissural mode in the absence of volcanic vent in the region, when the plume centre was located in the Comei-Cona region of Tibet.

Preliminary anisotropy of magnetic susceptibility studies on 2367 Ma Bangalore-Karimnagar giant dyke swarm, southern India: Implications for magma flow

Anisotropy of magnetic susceptibility (AMS) fabrics have been studied for the first time on a well constrained Paleoproterozoic Bangalore-Karimnagar giant mafic dyke swarm at ca. 2367 Ma in the Dharwar craton, south India. A total number of 43 samples from nine sites representing five N-E trending dykes have been subjected to AMS and conventional rock magnetic studies. The magnetic fabric is principally ferromagnetic in origin as the magnetic susceptibility values are >10−3 SI, which is further supported by Isothermal Remanent magnetization (IRM) studies. Hysteresis loops, Isothermal remanent magnetisation (IRM), thermomagnetic properties and optical observations indicate that pseudo single domain (PSD) low-titanium magnetite particles dominate the low field susceptibility of samples. The magnetic fabrics results correspond well with the comprehensive field studies. However, the degree of anisotropy results ranges from 0.4% to 12.8%, with a mean total anisotropy of 4.7%. The principal AMS axes for samples from all the sites are tightly clustered on equal area plots, and two main fabrics were revealed in the Bangalore-Karimnagar giant dyke swarm. In the first, the maximum susceptibility axes is sub-horizontal and parallel to the dyke walls, minimum susceptibility axes plot near the pole of the dyke. It is interpreted as the primary magma flow direction. A second fabric with maximum susceptibility axis is sub-horizontal and perpendicular to the dyke walls, and is interpreted as a result of vertical compaction at a late stage of crystallization. Preliminary results suggest that dyke propagation in the Bangalore-Karimnagar giant dyke swarm preserve a flow-induced fabric slightly upwards to the NE and magma flow is sub-horizontal.

Magma flow pattern of the 1.78 Ga dyke swarm of the North China Craton during the initial assembly of the Supercontinent Nuna/Columbia: Constraints from rock magnetic and anisotropy of magnetic susceptibility studies

The geodynamic origin of the 1.78 Ga dyke swarm in the North China Craton (NCC) is long debated. It is considered either a post-collision process or paleoplume-related. Constraining its origin is essential for the reconstruction of the NCC position within the supercontinent Nuna (also referred to as Columbia). Here, we report on the rock-magnetism and anisotropy of magnetic susceptibility (AMS) of thirty-three 1.78 Ga dykes, to constrain the magma center of the igneous event. The samples were collected from the entire swarm across the NCC. Magnetite appears to be the dominant magnetic mineral in all the dykes, occasionally with minor maghemite, while the magnetite grain size within the dykes is generally increasing from south to north. AMS results of the dykes portray a magma flow-related fabric. Over half of the studied dykes show a predominance of horizontal to subhorizontal magmatic flow, while a few have been formed by vertical intrusion. These observations imply that the 1.78 Ga dyke swarm may have been generated by sustained magma flow from a magmatic source centered at the south margin of the NCC. During dyke propagation, some localized magma subchambers may have been formed; they could be responsible for the less common vertically intruded dykes and the EW-trending dykes. Therefore, a plume-related tectonic setting for the 1.78 Ga dyke swarm is more compelling. With the eruption center along the margin of the NCC, it may serve as an essential criterion to search for possible counterparts in other tectonic units within Nuna/Columbia to evaluate the proximity of the NCC with them.

Nature of Flow Patterns of Rajahmundry Lava, Gowripatnam Area, West Godavari, India:Insights from AMS Studies

Anisotropy of magnetic susceptibility (AMS) data for a single basaltic lava flow, herein named the Gowripatnam lava flow from the Rajahmundry Traps, are evaluated for determining precisely the mechanism of lava flow. At Rajahmundry, lava flows are found on both banks of the Godavari River and sandwich intertrappean sedimentary layers in between. The ones on the west bank of the river are studied here. This study has an implication as the mechanism of lava flow or nature of lava flow patterns of Rajamundry Trap basalt is still a debatable issue. AMS directions is a powerful tool for investigating the source and direction of lava. Its application to single lava flow from Rajahmundry indicates maximum direction of susceptibility axes in almost all possible directions, indicating radial flowage from radial vent source(s). Two AMS directions (towards north-west – 14%, and north – 7%), however, have the maximum number of petals. This indicates that at least there were palaeo-flowage patterns towards these directions. Incidentally the Godavari lineament strikes NW–SE and the flow dip is due south. However, the two dominant magnetic lineation directions are due north. Hence, the possibility of the earlier suggested river piracy model for lava flow can be ruled out. The fissure eruption near Rajahmundry is pointed out as the then prevailing lava flow mechanism. This is further supported by an overall random distribution of maximum susceptibility axes apart from two dominant flow directions.

Anisotropy of Magnetic Susceptibility of Earthquake-Affected Soft Sediments: Example from Ther Village, Latur, Maharashtra, India

Anisotropy of magnetic susceptibility (AMS) of deformed and undeformed unconsolidated clay samples of Deccan Trap terrain from the ~2000-year-old palaeoearthquake site of Ther village, Maharashtra, India, was studied. Such deposits are rare in this region and give an exceptional opportunity to test the efficacy of AMS and other magneto petrofabric studies, which can have a bearing on magnetic granularity ellipsoids, inclination and declination directions impacting palaeomagnetic studies. The undeformed clay samples exhibit typical sedimentary fabric with an oblate AMS ellipsoid, whereas the deformed samples are tightly grouped in the inferred compression direction, probably effected by an earthquake, exhibiting prolate as well as oblate AMS ellipsoids. The temperaturedependent magnetic susceptibility revealed the occurrence of titanomagnetite and magnetite in both the deformed and undeformed samples signifying similar sediment material. The site mean ChRM direction for undeformed clay samples is D = 3 and I = 44.7 ( k = 92.7, α 95 = 4.5), whereas for deformed samples D = 336 and I = 39 ( k = 39, α 95 = 8). The present study reveals that sediments can be severely deformed without deflecting minimum susceptibility directions. AMS methodology can be effective in the DT region and can throw up new results to build the chronology of past earthquakes.

The curved Magallanes fold and thrust belt: Tectonic insights from a paleomagnetic and anisotropy of magnetic susceptibility study

The Magallanes fold and thrust belt (FTB) presents a large-scale curvature from N-S oriented structures north of 52°S to nearly E-W in Tierra del Fuego Island. We present a paleomagnetic and anisotropy of magnetic susceptibility (AMS) study from 85 sites sampled in Cretaceous to Miocene marine sediments. Magnetic susceptibility is lower than 0.0005 SI for 76 sites and mainly controlled by paramagnetic minerals. AMS results indicate that the sedimentary fabric is preserved in the undeformed areas of Tierra del Fuego and the more external thrust sheets units, where an incipient lineation due to layer parallel shortening is recorded. Prolate AMS ellipsoids, indicating a significant tectonic imprint in the AMS fabric, are observed in the internal units of the belt. AMS results show a good correlation between the orientation of the magnetic lineation and the fold axes. However, in Península Brunswick, the AMS lineations are at ~20° counterclockwise to the strike of the fold axes. Pretectonic stable characteristic remanent magnetizations (ChRM) were determined in seven sites. A counterclockwise rotation (21.2° ± 9.2°) is documented by ChRM data from four sites near the hinge of the belt in Península Brunswick and near Canal Whiteside while there is no evidence of rotation near the nearly E-W oriented Vicuña thrust within Tierra del Fuego. The curved shape of the Cenozoic Magallanes FTB is not related to vertical axis rotation, and thus, the Magallanes FTB can be considered as a primary arc.

Anisotropy of magnetic susceptibility study in two classical localities of the Gastre Fault System, central Patagonia

Abstract The Central Patagonian Batholith is a suite of acid and meso-silicic rocks cropping out in central Patagonia. The emplacement of these rocks has been proposed to be related to the activity of a system of dextral transcurrent faults, the NW-SE Gastre Fault System. This fault system has been ascribed a transcontinental magnitude and a ∼500 km dextral displacement during Gondwana dismembering in Jurassic times. However, the timing, kinematics and amount of displacement of the Gastre Fault System are still controversial. In this work we have visited two localities which were subject of controversial observations, in order to perform petrographical, microstructural and anisotropy of the magnetic susceptibility studies to contribute to the ongoing discussion. The results mostly agree with the findings von Gosen and Loske (2004) in that rocks spatially and temporally associated to the Gastre Fault System do not show evidence supporting the existence of a major dextral fault system active during Jurassic times.

Superposed deformation fabrics in the Precambrian metabasic rocks of the Iron Ore Group, Singhbhum craton, Eastern India: Evidences from anisotropy of magnetic susceptibility studies

Anisotropy of magnetic susceptibility (AMS) data have been utilized to build up the multiple deformation sequences and strain variation from an apparently undeformed metabasic lava sequence of the basal part of the Paleo-Mesoarchean greenstone succession of the Iron Ore Group rocks in the western Iron Ore basin, Singhbhum craton, eastern India. The basal lava crops out bordering the regional horseshoe shaped outcrop (the horseshoe synclinorium structure) of the overlying BIF rocks. AMS data of 892 cylindrical specimens drilled from 83 samples collected from different structural domains within the synclinorium structure are presented in the analysis. AMS orientation data have been analyzed in the light of superposed fold (Type-1 interference) fabrics developed within BIF as a result of successive deformation episodes (D 1 and D 2). It is found that the lava samples ubiquitously present two consistent magnetic fabrics homogeneous over smaller sub-areas, which have been successfully correlated with the D 1 and D 2 deformation fabrics from the BIF rocks. Moreover, a regional scale variation in intensity and nature of strain is noticed from the AMS data, which have been utilized to decipher folding mechanism as well as the changing deformation pattern during successive folding events.

Anisotropy of magnetic susceptibility and petrofabric studies in the Garhwal synform, Outer Lesser Himalaya: Evidence of pop‐up klippen

Abstract Geological field, petrographic, and anisotropy of magnetic susceptibility studies help in understanding the evolutionary history of the Garhwal synform that lies in the western Outer Lesser Himalaya. Orientations of the magnetic susceptibility axes reveal large variations at short distances as a result of superimposed deformation, and predominant stress conditions favorable for normal faulting. Rocks forming the outer limbs of the Garhwal Synform are metamorphosed up to the lower greenschist facies. The metamorphic grade increases to chlorite zone in the inner limb and the core is characterized by chlorite–biotite to garnet zones. The different grades of metamorphism are separated by thrusts and the structure is described as a pop‐up klippen.

Anisotropy of magnetic susceptibility study of kaolinite matrix subjected to biaxial tests

AbstractThe potential for structural failure of consolidated clay materials, which is of great importance in many applications, typically are assessed by measuring the localized strain bands that develop under anisotropic load stress. Most methods are precluded from providing a full understanding of the strain anisotropy because they only give two-dimensional information about the stressed clay blocks. The purpose of the present study was to investigate three-dimensional strain localization in a kaolinite matrix, caused by strain anisotropy due to a biaxial plane-strain test, using a relatively new method known as Anisotropy of Magnetic Susceptibility (AMS). This method involves induction of magnetism in an oriented sample in different directions and measurement of the induced magnetization in each direction. The AMS analyses were performed on core samples from different parts of the deformed kaolinite matrix. The degree of magnetic anisotropy (P′), which is a measure of the intensity of magnetic fabric and a gauge of strain intensity, was shown to be greater in cores containing shear bands than in those containing none. A threshold value for P′ for the deformed kaolinite matrix was identified, above which shear bands may develop. The comparison of the shape parameter (T), obtained from undeformed and deformed samples, illustrated a superimposition of prolate strain over the original oblate fabric of the kaolinite matrix. The orientation of the principal strain axis revealed that reorientation or rotation of the principal axis occurred along the shear bands.

Anisotropy of magnetic susceptibility studies in Tertiary ridge-parallel dykes (Iceland), Tertiary margin-normal Aishihik dykes (Yukon), and Proterozoic Kenora–Kabetogama composite dykes (Minnesota and Ontario)

Mafic dykes of different ages were collected from three different tectonic settings and analyzed using anisotropy of magnetic susceptibility (AMS) as a proxy for magmatic flow during intrusion. In Iceland, ridge-parallel basaltic dykes were sampled on each side of the active tectonic boundary. The dykes are < 10 m wide along a 1–2 km strike, and are the result of a single intrusion from 1–2 km deep magma chambers in oceanic crust. Thirteen samples were collected (7 N. American plate; 6 European) and 153 cores were analyzed by AMS and preserve a vertical K max orientation indicating vertical emplacement. The Eocene Aishihik dyke swarm intrudes the Yukon–Tanana terrane in the Yukon province, Canada over an area ~ 200 by 60 km. These dykes were intruded normal to the accretionary margin, are porphyritic andesites, and have an intermediate geochemical signature based on major and trace element analyses. Ten dykes were sampled and 111 cores analyzed using AMS, and the dykes preserve a vertical K max orientation, indicating intrusion was vertical through ~ 30 km of continental crust. The 2.06 Ga Kenora–Kabetogama dykes in northern Minnesota and western Ontario crosscut a variety of Archean terranes (thickness ~ 50 km) in a radiating pattern. The unmetamorphosed basaltic dykes are 1–120 m wide, 10–110 km in length, are vertical in orientation and can be grouped as either being single intrusion or multiple intrusion (composite) dykes. AMS data preserve a vertical K max orientation for the southerly locations (2 dykes, n = 53) and horizontal K max for the remainder to the northwest (15 dykes, n = 194). Maximum magnetic susceptibility axes (4 dykes, n = 92) for composite dykes are scattered and yield inconsistent flow directions with regard to the dyke margin. Almost all of our results are “normal” in that, the magnetic foliation (the plane containing K max and K int, normal to K min) is parallel to the dyke planes, which gives us confidence that the magnetic lineations (i.e., K max orientations) are parallel to magmatic flow.

Model of successive granite sheet emplacement in transtensional setting: Integrated microstructural and anisotropy of magnetic susceptibility study

This study presents a model of successive emplacement of three granite plutons in transtensional deformation regime controlled by the preextensional synconvergent history of the lower and middle crust in the central Vosges Mountains (France). The complex compressional orogenic structure recorded a vertical exhumation‐related fabric in midcrustal levels which was overprinted by the regional extension. Three successively emplaced granite sheets exploited the inherited vertical anisotropy obliquely oriented with respect to the applied tensile stress. This progressive opening of original steep fabric leads to the sequential generation of free spaces toward the south, and emplacement and deformation of granite sheets. Repeated granite intrusions are responsible for reheating of southern margins of the already partially solidified granites, which became reactivated under lower ambient thermal conditions related to overall ∼6 Ma cooling of rapidly exhumed crust. The results of anisotropy of magnetic susceptibility (AMS) fabric modeling suggest a highly partitioned oblique extension divided into pure shear‐dominated deformation close to the central and northern margins of intrusions and wrench‐dominated shear along the southern margins. The AMS modeling suggests the importance of overprinting of the intrusive fabrics by the transtensional deformation.

Structural and magnetic fabric study of the Marimanha granite (Axial Zone of the Pyrenees)

The structural and magnetic fabric study of the Marimanha granite, Axial Zone of the Pyrenees, provides new data to characterize the zonation and the internal structure of the pluton. The Marimanha granite intrudes Cambro–Ordovician clastic rocks and Silurian–Devonian limestones and slates. The zonation of the low field magnetic susceptibility, consistent with the petrological zonation of the igneous body, indicates a concentric arrangement of rock types, with more basic compositions at the external areas. This pluton is characterized by a low susceptibility, and rock–magnetic studies indicate a majority of “paramagnetic” samples. Magnetic foliations strike parallel to petrographic contacts and to contours of zonation of magnetic susceptibility, and show a dominant NE–SW strike, steeply dipping towards the North. Locally, in the northern border of the pluton foliation directions become perpendicular to petrographic contacts and depict sigmoidal trajectories. Magnetic lineations are characterized by the predominance of NE–SW trends with shallow plunges to the NE and SW. These foliations and lineations are parallel to the slight elongation of internal petrographic zonation. Magnetic fabric within the granitoid body and internal elliptical shape of petrographic zonation, suggest an intrusion contemporary with a transpressional regime and NNW–SSE shortening, syntectonic with the late stages of the Variscan orogeny. These results are in accordance with anisotropy of magnetic susceptibility studies of others plutons in the Pyrenees. To explain the origin of the Marimanha granite we propose magma ascent through faults at depth and emplacement by ballooning in situ at the rheological boundary between Cambro–Ordovician and Siluro–Devonian metasediments.

Fabric evolution across a discontinuity between lower and upper crustal domains from field, microscopic, and anisotropy of magnetic susceptibility studies in central eastern Eritrea, NE Africa

In the Neoproterozoic East African Orogen (EAO) of Eritrea, lower to middle crustal high‐grade metamorphic rocks are juxtaposed against low‐grade upper crustal rocks along diffuse tectonic contact zones or discontinuities. In the central eastern part of Eritrea, such a tectonic zone is exposed as a low‐angle shear zone separating two distinct high‐ and low‐grade domains, the Ghedem and Bizen, respectively. Integrated field, microfabric, and anisotropy of magnetic susceptibility (AMS) studies show that this low‐angle shear zone formed during late deformation, D2, with top‐to‐the‐E/SE sense of motion. The hanging wall upper crustal volcanosedimentary schists are mainly paramagnetic and the footwall middle crustal mylonitized orthogneisses are mainly ferrimagnetic. Magnetic fabric studies revealed a good agreement between metamorphic/mylonitic and magnetic foliations (Kmin) and helped to explain fabric development in the shear zone. The magnetic lineations (Kmax) reflect stretching lineations where stretched mineral aggregates dominate fine‐grained mylonitic matrices and intersection lineations where microstructural studies revealed two fabric elements. AMS directional plots indicate that the orientations of the magnetic lineation and of the pole to the magnetic foliation vary systematically across the shear zone. While Kmax axes form two broad maxima oriented approximately N–S and E–W, the Kmin axes change from subhorizontal, generally westward inclination in the west to moderate to steep inclination in the direction of tectonic movement to the east. Because there is a systematic change in inclination of Kmin for individual samples, all samples together form a fairly well defined cluster distribution. The distribution of Kmin in combination with the E–W scattered plot of the Kmax is in accordance with the E/SE flow of mylonites over exhumed Damas core complex in the late Neoproterozoic. During the Cenozoic, the Red Sea rift‐related detachments exploited the late orogenic shear zone, indicating that the discontinuities between ductile middle and brittle upper crustal layers in the region are reactivated low‐angle shear zones and possible sites of core complexes.

Crystallization in flow - II. Modelling crystal growth kinetics controlled by boundary layer thickness

SUMMARY As in several other anisotropy of magnetic susceptibility studies, the main direction of the magnetic lineation analysed in part I of this work, as well as crystal elongation, have been found to be roughly aligned with the direction of the surrounding flow. In order to explain the mechanisms responsible for such crystal shape anisotropy in a hydrodynamic context, we derive a mathematical model based on Falkner‐Skan self-similar boundary layers at high Reynolds numbers. The model allows calculating local growth rates out of diffusion processes in the concentration boundary layer for crystal faces orientated arbitrarily in the range 90 ◦ to −18 ◦ with respect to the flow direction, and for any flow velocity. Hence, our work generalizes rationally previous attempts already done in the case of a flow parallel to the crystal face. This crystal growth model is applied to a natural case of calcite growth rate in 2-D section

Emplacement and deformation of the Cachoeirinha pluton (Borborema province, NE Brazil) inferred through petrostructural studies: Constraints on regional strain fields

The East Pernambuco shear zone system (EPSZ) conventionally has been taken as the limit between the central and southern domains of the oriental sector of the Neoproterozoic Borborema province (NE Brazil). To constrain the tectonic evolution of the southern domain during the Brasiliano orogeny, the authors undertake a structural and anisotropy of magnetic susceptibility study of the 580 Ma Cachoeirinha pluton, located 10 km south of the EPSZ. The results suggest a model for its emplacement and deformation in which (1) successive magma batches accumulated at the gently dipping interface between orthogneisses and overlying schists and paragneisses and (2) homogeneous NNE–SSW stretching of the pluton was followed by strain localization along conjugate subvertical corridors of noncoaxial shear during the last stages of crystallization. Outcrop-scale, magmatic to solid-state dextral and sinistral shear zones that affect the pluton have similar orientations to the large transcurrent mylonitic belts that characterize the central domain of the Borborema province. Thus, at the time of intrusion of the Cachoeirinha pluton, the central and southern domains of the Borborema province were undergoing deformation related to the same far-field stresses, and the EPSZ is not a major suture zone between domains with distinct geological histories. Instead, the present-day separation of the eastern Borborema province into central and southern domains resulted from progressive deformation that led to the development of the EPSZ. The absence of large shear zones in the southern domain can be explained by the faster regional cooling than in the central domain, where the longer magmatism duration resulted in persistent elevated crustal temperatures, which in turn allowed a greater amount of finite strain to accumulate.