Making space for magma fingers and sheet intrusions: the importance of intrusion tip velocities

Magma transport through the Earth’s crust is commonly described to occur through interconnected planar sheet intrusions such as dykes and sills, which form so called magma plumbing systems. Elongate intrusion geometries (i.e., magma fingers and segments), hereafter referred to as elements, may form during magma transport due to viscous and/or elastic instabilities at the propagating intrusion tip, and they are often observed at the outer margin of solidified sheet intrusions. Field observations, geophysical datasets, and analogue models further show that when elements grow in width, they can coalesce, indicating that planar sheet intrusions can form and grow by the amalgamation of individual elements. Previous studies suggest that the emplacement and growth of elements is accommodated by one dominating emplacement end-member process, namely: i) tensile-elastic fracturing, ii) shear failure, or iii) viscous deformation (e.g., host rock fluidisation). However, the interplay between individual end-member processes remains poorly understood. Here we present field observations of elongate magma fingers located at the SE margin of the Paleogene Shonkin Sag laccolith (Montana, USA) to assess how host rocks (Cretaceous Eagle Sandstone) deform to make space for the magma. We combine drone photogrammetry surveys with field mapping and microstructural analyses to describe and quantify host rock deformation in the vicinity of 37 magma fingers, and we conduct thermal modelling to further evaluate the conditions at which viscous deformation due to host rock fluidisation is feasible.

Our field observations show that all three proposed end-member processes accommodated the emplacement of magma fingers at the SE margin of the Shonkin Sag laccolith. Brittle deformation, shear failure, and folding of host rock mainly occurs in the compressional regime between two adjacent magma fingers, whereas host rock fluidisation and mobilisation is predominantly observed at the cross-sectional, lateral finger tips. Our photogrammetric analyses show that up to 40 % of the finger thickness is accommodated by elastic host rock uplift. Critically, this range of host rock deformation mechanisms is observed in one outcrop at metre scale, and in some cases associated with an individual magma finger. Thermal modelling of temperatures ahead of a propagating intrusion tip indicates that intrusion induced host rock fluidisation is only possible at low tip velocities of ≤ 10^-5 m/s, which can vary depending on the emplacement depth, magma temperature, and the thermal diffusivity of the host rock.

Overall, we conclude that the emplacement of magma fingers at the outer margin of the Shonkin Sag laccolith was accommodated by a combination of elastic host rock uplift and both brittle and ductile host rock deformation. Based on our field observations and thermal modelling results, we suggest that intrusion tip velocities and the resulting strain rate are key parameters that control the dominating space-making mechanisms during magma emplacement. Due to the elongate geometry of elements and the resulting different strain rates at their lateral and frontal tips, we further propose that deformation mechanisms observed at lateral tips in cross sectional outcrops are likely decoupled from those at frontal tips such that they may not be equivalent.