How does orogenic crust deform? Evidence of crustal-scale competent behaviour within the partially molten middle crust during orogenic compression

Abstract

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Strain Partitioning Within Bending Orogens, New Insights From the Variscan Belt (Chiroulet‐Lesponne Domes, Pyrenees)

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The understanding of how convergence is accommodated by the lithosphere remains an important issue in tectonics, for both recent and old orogenic belts (e.g., Conand et al, 2020;Torvela & Kurhila, 2020). Mass transfer within a crust subjected to convergence can be variably accommodated, depending on the rheological behavior of the colliding lithospheres (e.g., Brun, 2002;Davy, 1986).

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“…This phase (c. 1850−1800 Ma) refolded the F1 flat-lying crustal fabrics and recumbent folds into upright folds with E-W striking fold axes (F2; Fig. 2c; Torvela & Kurhila 2020). Particularly from c. 1825 Ma onwards, the deformation was synchronous with wide-spread low-P, high-T anatectic granite magmatism and migmatisation, metamorphism and formation of steep, local-to crustal-scale, strike-slip shear zones in the Svecofennian domain (e.g.…”

“…4a; Nironen & Kurhila 2008); and local extensional structures such as boudinaged pegmatites (Skyttä & Mänttäri 2008). However, Kurhila and Torvela (2020) show that despite voluminous partial melts in the crust, the convergence and crustal thickening was very much active during this age bracket, based e.g. on the fact that the 1.83 Ga Perniö granite is folded showing ~20% shortening after the full or partial solidification of the granite sheet.…”

“…The overall evolution of the Svecofennian domain of Southern Finland is fairly well constrained, but the role of the syn-orogenic extensional/transtensional features is still debated (e.g. Skyttä & Mänttäri 2008;Torvela & Kurhila 2020). This is partly because of the paucity of both field observations and age data from such features, limiting the further understanding of the time-space development of this complex orogenic belt during the 1.85−1.80 Ga 'Fennian' convergence.…”

“…1; Skyttä & Mänttäri 2008). This interpretation of large-scale flattening is slightly problematic as the convergence and significant crustal shortening, with large-scale folding and crustal thickening, evidently continued around this time: Torvela and Kurhila (2020) show that the migmatisation and crustal shortening were coeval, and that the thickening/folding of the partially molten crust was enabled by the rapidly crystallising, competent granitic sheets. However, extensional structures do facilitate some deformation within the shortening crust, particularly towards the later stages of an orogeny, and some extensional/ transtensional shear zones have been interpreted in the field and from seismic reflection data near and north of Vihti (Fig.…”