Syn-Emplacement Fracturing in the Sandfell Laccolith, Eastern Iceland—Implications for Rhyolite Intrusion Growth and Volcanic Hazards

Abstract

Felsic magma commonly pools within shallow mushroom-shaped magmatic intrusions, so-called laccoliths or cryptodomes, which can cause both explosive eruptions and collapse of the volcanic edifice. Deformation during laccolith emplacement is primarily considered to occur in the host rock. However, shallowly emplaced laccoliths (cryptodomes) show extensive internal deformation. While deformation of magma in volcanic conduits is an important process for regulating eruptive behavior, the effects of magma deformation on intrusion emplacement remain largely unexplored. In this study, we investigate the emplacement of the 0.57 km 3 rhyolitic Sandfell laccolith, Iceland, which formed at a depth of 500 m in a single intrusive event. By combining field measurements, 3D modeling, anisotropy of magnetic susceptibility (AMS), microstructural analysis, and FEM modeling we examine deformation in the magma to constrain its influence on intrusion emplacement. Concentric flow bands and S-C fabrics reveal contact-parallel magma flow during the initial stages of laccolith inflation. The magma flow fabric is overprinted by strain-localization bands (SLBs) and more than one third of the volume of the Sandfell laccolith displays concentric intensely fractured layers. A dominantly oblate magmatic fabric in the fractured areas and conjugate geometry of SLBs, and fractures in the fracture layers demonstrate that the magma was deformed by intrusive stresses. This implies that a large volume of magma became viscously stalled and was unable to flow during intrusion. Fine-grained groundmass and vesicle-poor rock adjacent to the fracture layers point to that the interaction between the SLBs and the flow bands at sub-solidus state caused the brittle-failure and triggered decompression degassing and crystallization, which led to rapid viscosity increase in the magma. The extent of syn-emplacement fracturing in the Sandfell laccolith further shows that strain-induced degassing limited the amount of eruptible magma by essentially solidifying the rim of the magma body. Our observations indicate that syn-emplacement changes in rheology, and the associated fracturing of intruding magma not only occur in volcanic conduits, but also play a major role in the emplacement of viscous magma intrusions in the upper kilometer of the crust.

Progressive Growth of the Cerro Bayo Cryptodome, Chachahuén Volcano, Argentina—Implications for Viscous Magma Emplacement

“…We interpret the contact‐parallel flow bands in Cerro Bayo to have formed during intrusion inflation (cf. Goto & McPhie, ; Mattsson et al, ; Stewart & McPhie, ). In the south‐western part of the massive central body of Cerro Bayo, in‐situ measured flow bands dip toward the NW and steepen toward the center of the western part of the Cerro Bayo laccolith (Figure a), which indicates subvertical outward flow of magma.…”

“…Cashman & Blundy, ; Hammer & Rutherford, ; Westrich et al, ). A strain‐ and strain rate‐dependent rheology of the magma can therefore cause the viscosity to dramatically increase, which can lead to viscous stalling of parts of the magma body (Mattsson et al, ; Pistone et al, ). Another process that may increase magma viscosity and lead to viscous stalling is porosity reduction by compaction and pore collapse or passive degassing through a fracture network (Ashwell et al, ; Heap et al, ; Kennedy et al, ; Kushnir et al, ; Rhodes et al, ).…”

“…Three lines of evidence indicate that viscous stalling (essentially solidification) of the magma as a result of strain‐induced magma degassing and not cooling controlled the transition from Stage I to II: (1) the duration of observed cryptodome emplacement events, which ranges from months to a few years (Castro et al, ; Lipman et al, ; Minakami et al, ), and (2) numerical cooling models (Mattsson et al, ). Together, both arguments highlight that cryptodome emplacement is much faster than crystallization due to conductive cooling.…”

“…Our study of the emplacement of the Cerro Bayo cryptodome highlights several processes that may be significant to the emplacement of cryptodomes (and laccoliths) in general. Exposed and solidified laccoliths have been found to either form by stacking of several sills over long time, that is, tens of thousands of years (de Saint‐Blanquat et al, ; Leuthold et al, ; Morgan et al, ; Westerman et al, ; Wilson et al, ) or by continuous inflation of a single sill in one shorter event (Mattsson et al, ). Observed cryptodome and lava dome emplacement, on the other hand, appears to be rapid and short‐lived, that is, months to some years (Castro et al, ; Lipman et al, ; Minakami et al, ).…”

“…The brittle features in cryptodomes are attributed to contact‐parallel shear during endogenous inflation and fast solidification of the intrusion (Goto & McPhie, ; Mattsson et al, ; Stewart & McPhie, ). Mattsson et al () also reported the occurrence of layers of parallel tensile and shear fractures separated by layers of coherent unfractured, ductile deformed intrusive rock in a cryptodome. These “fractured bands” have been inferred to form due to brittle deformation of more crystal‐rich layers when viscous magma is subject to high strain rates.…”