How to make space for magma fingers?

Abstract:

Magma transport in large volcanic plumbing systems is portrayed to occur via channel-like sheet intrusions (i.e., dykes and sills). Finger-like geometries often emerge from the outer margins of these planar sheets during their migration through the crust and can form connectors in volcanic plumbing systems. Previous studies suggest that magma finger emplacement happens in response to either 1) mode I failure, 2) mode II failure, or 3) host rock fluidization. In this study, we present field-based data collected at the margin of the Paleogene Shonkin Sag laccolith (SSL), Montana, USA, to assess how host rocks (Cretaceous Eagle Sandstone) deform to make space for magma fingers and whether or not deformation mechanisms influence finger geometries. We use a combination of unmanned aerial vehicle (UAV or drone) photogrammetry surveys, field mapping, and microstructure analyses.

Our field observations show that the emplacement of magma fingers at the margin of the SSL resulted in both brittle and ductile deformation, as well as host rock fluidization. The latter is mainly observed at the cross-sectional tips and, less commonly, at the top and bottom of magma fingers, while shear failure, thrusting, and folding occur between adjacent fingers. Host rock uplift only makes a minor contribution to making space for individual fingers. It is important to note that three mechanisms are observed in the same outcrop at meter scale and in some cases even associated with a single finger. Preliminary results of photogrammetric analyses suggest that fingers associated with host rock fluidization have rounded or parabolic tip geometries in vertical cross sections perpendicular to the long dimension of the finger. This contrasts with fingers associated with brittle and or ductile deformation, which have blunt or rectangular cross-sectional tip geometries.

The meter-scale spatial variation in host rock deformation indicates that magma finger emplacement is likely facilitated by more than one deformation mechanism. We conclude that a combination of host rock fluidization, and brittle and ductile deformation were the dominant mechanisms that accommodated magma finger emplacement at the margin of the SSL.