Rapid viscous flow of crustal rocks controls dyke emplacement in the ductile crust

In the upper crust, dykes are commonly assumed to propagate as elastic fractures, exhibiting thin, tapered geometries, and propagating through mode I, tensile opening at the fracture tip. In the lower ductile crust, which is often assumed to have a Maxwell-type rheology, the fast strain rates associated with dyke emplacement are thought to embrittle the host rock. This reasoning has led to the assumption that ductile deformation of the host rock is negligible during dyke emplacement in the deep crust. In Sarek National Park, northern Sweden, a dyke complex was emplaced during a ca 608 Ma continental rifting event at depths of 10–15 km and temperatures reaching ca 650 °C. The dyke swarm was emplaced into carbonate and sandstone host rocks. Detailed field observations from glacially polished outcrops demonstrate that significant ductile deformation of the host rock accommodated dyke emplacement. We quantify that approximately 25% of the dyke thickness is accommodated by ductile folding of the host rock. Thermal modelling is used to estimate magma crystallization times, which in turn allow estimating ductile strain rates on the order of 10^-3 to 10^-6 s^-1. These strain rates are 6 to 10 orders of magnitude higher than typical tectonic ductile strain rates in the middle crust (10^-12 - 10^-15 s^-1). Furthermore, we document how the weak rheology of the host rock influenced the shape and geometry of the mafic dykes. These results have implications for our understanding of dyke emplacement as well as general deformation and rock strength in the ductile crust, which constitute a significant part of the pathway for magma to reach the surface.