Dyke emplacement mechanisms across the brittle-ductile transition

Dyke emplacement mechanisms across the brittle-ductile transition Hans Jørgen Kjøll1, Olivier Galland2, Loic Labrousse3 & Torgeir Andersen1 1 Center for Earth Evolution and Dynamics (CEED), University of Oslo 2 Physics of Geological Processes, the Njord Center, Department of Geosciences, University of Oslo 3 Institut des Sciences de la Terre Paris, ISTeP, CNRS-INSU, Sorbonne Université Dyking is the main process of magma transport through the Earth’s lithosphere. Dykes are thin sheets exhibiting shapes similar to fractures, so that the main models of dyke emplacement assume that they form by mode I hydraulic fracturing following the σ1-σ2 plane. Because of the rapid strain rates accommodating dyking, it is assumed that dyke propagation and emplacement are only governed by brittle processes, even in the ductile crust. However, the contribution of ductile deformation in dyke emplacement has not been assessed. Here we report detailed and spectacular field observations from northern Sweden and Norway of a ~605 Ma old dyke complex emplaced near the brittle-ductile transition. The dyke complex formed during continental rifting and opening of the Iapetus Ocean, and is now exposed in the Scandinavian Caledonides. In northern Sweden, observations are made along a 1.5 km long continuously exposed cliff providing unique and exceptional overview images of the dyke complex. The detailed structural analysis of the dykes and of the structures related to their emplacement allows us to identify distinct dyke emplacement mechanisms, sub-divided into: 1) Brittle dykes that exhibit straight contacts with the host rock, sharp tips, en-echelon segments with either broken bridges or intact bridges between the segments. The dyke thicknesses follow a Weibull distribution, commonly applied to fracture mechanics; 2) Brittle-ductile dykes that exhibit ductile bridges with complex patterns. Both brittle-ductile and ductile-brittle features are observed, i.e. where ductile flow induced by inflating dykes overprint brittle structures associated with dyke emplacement and vice versa; 3) Ductile “dykes” that show mingling textures between the soft ductile host rock and the intruding mafic magma as well as irregular magmatic boudinage. The dykes exhibit two distinct orientations, and are mutually cross-cutting, suggesting that the dykes did not form as vertical sheets perpendicular to regional extension. Thanks to the well-exposed layering of the dykes’ host rock, we performed a kinematic restoration to quantify the strain induced by the dyke complex. As expected, the dyke complex accommodated >100% extension in agreement with the rifting. However, counter-intuitively it also accommodated 12% of crustal thickening, in agreement with local shortening structures near the dyke walls, showing the forceful mechanism of magma emplacement. Our observations underline the complexity of magma emplacement mechanisms near the brittle-ductile transition and show that dyke emplacement cannot be described as simple mode-I brittle fractures that are being passively filled and inflated by magma.