Complex mid‐crustal flow within a growing granite–migmatite dome: An example from the Variscan belt illustrated by the anisotropy of magnetic susceptibility and fabric modelling

Abstract

This study describes anisotropy of magnetic susceptibility (AMS) of migmatites associated with crustally derived granites of the Pelhřimov core complex, Bohemian Massif. In combination with numerical modelling, we use this case example to discuss some of the general complexities related to interpreting flow patterns of anatectic lower and middle crust from magnetic fabric during growth of metamorphic domes. Magnetic lineations, reflecting tiling or zone axis of magnetically oblate grains, commonly tend to develop along the principal stretching direction. Our fabric modelling, however, shows that a lineation may also develop at a high angle to the overall flow (tectonic transport) direction as a relict of a pre‐existing fabric or due to a component of core complex axis‐parallel stretching and may be maintained for a significant period of time. Furthermore, we demonstrate that the finite magnetic fabric orientation is largely independent on the initial migmatite fabric intensity (formed before dome growth) but is rather a function of (a) shear strain, causing magnetic foliation and lineation to rotate towards the shear plane, and (b) magnitude of dome growth, causing their external rotation. Similarly, the degree of anisotropy P is also a function of accumulated shear strain and increases over time. These findings suggest that low fabric intensities may point to rapidly cooling, less sheared domains that are more likely to preserve early formed fabrics. In dome limbs undergoing protracted simple shear, low and high degrees of oblateness (as expressed by the shape parameter T) may discriminate between domains with weak and strong initial fabrics and may thus be used to map out domains with different strain histories and perhaps also with different melt proportions and rheological behaviour within metamorphic domes.

Multiscale Characterisation of Fracture Patterns of a Crystalline Reservoir Analogue

“…In the granitic pluton, the pluton magmatic fabric can control the structural architecture [72][73][74] and by fault systems and their associated clustering. The background fractures show strikes that follow the regional trend and a relatively random spacing.…”

Geochemistry of Granitic Rocks of the Moldanubian Batholith (Central European Variscides)

“…The ~NNE-SSW trending segment as the eastern part of the batholith is formed by large continuous exposures of granitic rock pluton, whereas the ~NW-SE as the western part of the batholith is formed by a number of isolated smaller plutons and bodies, some of which seem to be roughly parallel to the ~NW-SE trending regional shear zones (the Pfahl and Danube shear zones [13][14][15]). Both batholith branches are closely associated with the host migmatites [10,16,17]. Estimates of the pluton emplacement depths range from a number of separate smaller plutons, some of which seem to be from 18 to 20 km in the NW-SE segment and/or 7 to 9 km in the NNE-SSW segment of the Moldanubian batholith [10,12].…”

Decoding the Link Between Magmatic Cyclicity and Episodic Variation of Tectonics and Crustal Thickness in the Overriding Plate

“…The AMS method is used to refine the structural elements of granite to understand its geometry. This method has been widely applied in isotropic granite to interpret the complex tectonic evolution, which was proved to be efficient (Archanjo et al., 2002; Bouchez, 1997; Feng et al., 2022; Ji, Faure, et al., 2018; Lehmann et al., 2013; Rabillard et al., 2015; Turrillot et al., 2011; Wei et al., 2014a, 2016; Žák et al., 2015, 2019). Sampling and analytical methods of AMS are described in Text S3 in Supporting Information S1, and AMS data of all specimens measured and site‐mean values are presented in Tables S2 and S3.…”