Development of permeable networks by viscous-brittle deformation in a shallow rhyolite intrusion. Part 1: Field evidence

• Published in: Journal of Volcanology and Geothermal Research
• Main Authors: Witcher, Taylor, Burchardt, Steffi, Mattsson, Tobias, Heap, Michael J., McCarthy, William
• Format: Article in Journal/Newspaper
• Language: English
• Published: Uppsala universitet, Mineralogi, petrologi och tektonik 2024
• Subjects: Geology; Geologi; Iceland
• Online Access: http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-525620; https://doi.org/10.1016/j.jvolgeores.2024.108166

Efficient outgassing of shallow magma bodies reduces the risk of explosive eruption. Silica-rich magmas are too viscous for exsolved gas bubbles to escape the system through buoyant forces alone, and so volatile overpressure is often released through deformation-related processes. Here we present a case study on magma-emplacement-related deformation in a shallow (500 m depth) rhyolite intrusion (the Sandfell laccolith, Eastern Iceland) to investigate the establishment and evolution of degassing and outgassing networks in silicic sub-volcanic intrusions. We observe viscous and brittle deformation features: from vesiculated flow bands that organized into 'pore channels' in the ductile regime, to uniform bands of tensile fractures (‘fracture bands’) that grade into breccia and gouge in the brittle regime. Through field mapping, structural analysis, and anisotropy of magnetic susceptibility (AMS), we show that the deformation spectrum, observed all over the laccolith, represents stages of degassing (viscous processes) and outgassing (brittle processes) that resulted in the formation of interconnected permeable networks through the growth and linkage of fracture bands. Areas with concentrations of higher degrees of brittle deformation are proximal to abruptly changing AMS fabrics and point to laccolith-scale strain partitioning in the magma linked to different stages of laccolith growth. The establishment of intrusion-scale permeable networks through the cumulation of discrete magma fractures would have profoundly assisted the outgassing of the entire laccolith. Therefore, fracture banding captures viscous and brittle processes working in tandem as an efficient outgassing mechanism, and should be considered in sub-volcanic intrusions elsewhere.