Janine L. Kavanagh

Insights of dyke emplacement mechanics from detailed 3D dyke thickness datasets

Abstract: Dyke thickness datasets offer new insights into the detailed 3D geometry of dyke swarms and an exceptional opportunity to evaluate theoretical emplacement models. The Swartruggens kimberlite dyke swarm extends over 7 km along strike and intrudes a dolerite, quartzite, shale and andesitic lava succession. The Star kimberlite dykes cut shales and sandstones, intersect a large dolerite sill and extend 15 km along strike. Both dyke swarms comprise anastomosing en echelon segments, each several hundred metres long. In total 1532 Swartruggens dyke thickness measurements were taken, to 750 m below the surface over a 250 m depth range, and 3354 Star dyke thickness measurements were taken over a 520 m depth range. The Swartruggens dyke thicknesses are 0.05–1.95 m (mean 0.64 m), whereas the Star kimberlites range from <0.01 to 1.6 m thick (mean 0.40 m). Two-dimensional models of elastic cracks in a homogeneous, isotropic material subject to constant magmatic over-pressure poorly describe the dyke thickness variation, which is complex and varies along both breadth and length. The Swartruggens dyke segments thicken toward their terminations along strike, which represent regions where stresses were focused. Towards the surface, rock deformation is increasingly difficult to recover as inelastic processes such as host-rock brecciation, stoping, and magma solidification become important.

Mechanically disrupted and chemically weakened zones in segmented dike systems cause vent localization: Evidence from kimberlite volcanic systems

Deformation and alteration zones along kimberlite dikes hold clues as to how point-source volcanic vents can localize along sheet-like intrusions. Brittle deformation zones occur in host rock adjacent to kimberlite intrusions of the Swartruggens Kimberlite Dike Swarm, South Africa. Deformation includes local fracturing and brecciation and is associated with relay zones between offset dike segments. Breccia zones indicate dilation and hydraulic fracturing, and some zones along the dikes were affected by chemical corrosion, forming fresh cores surrounded by onion-skin concentric shells of altered rock. The alteration was caused by either exsolved magmatic volatiles moving in advance of the magma through the fracture, or by hydrothermal fluids. Consideration of the time scales needed for chemical corrosion of host rock requires intrusions to stall at depth prior to transport to higher crustal levels. Highly disrupted offsets could be preferred locations for explosive activity and initial vent formation as dikes approach the surface. A kimberlite pipe forms after magma breaks through to the surface; the altered zones are reamed out and fresh cores in spheroidally altered rock are incorporated into the pipe fill along with more angular country-rock material, as observed in layered volcanic breccias in kimberlite pipes at the Venetia Mine, South Africa. This model may have wider implications for the localization of conduits along dikes in other volcanic systems. Dike segmentation provides weak zones where hydrothermal fluids and magmatic volatiles can be preferentially channeled. Chemical corrosion can further weaken these zones, which may then become the locus for initial phreatic and phreatomagmatic explosions, creating shallow vents that can then channel magma to the surface during eruption.

The shapes of dikes: Evidence for the influence of cooling and inelastic deformation

We document the shape of dikes from well exposed field locations in the Isle of Rum, Scotland, 14 and Helam Mine, South Africa. The basaltic Rum dikes crop out on a smaller scale than the 15 Helam kimberlite dikes and have a smaller length to thickness ratio (~100:1 Isle of Rum, 16 ~1000:1 Helam Mine). We compare dike thickness field measurements with the geometry 17 predicted by elastic theory, finding best-fit models to estimate magma overpressure and regional 18 stress gradients at the time of dike emplacement. Most of the dike shapes fit poorly with elastic 19 theory, being too thick at the dike ends and too narrow in the middle. Our calculated 20 overpressures and stress gradients are much larger than independent estimates based on rock 21 strength. Dike shape can be explained by a combination of host rock inelastic deformation and 22 magma chilling at the dike’s tapering edges preventing its closure as magma pressure declines 23 during emplacement. The permanent wedging of the dike edges due to chilling has implications 24 for crustal magma transport and strain response in the crust due to dike emplacement

Mechanisms of Magma Transport in the Upper Crust—Dyking

Abstract Dykes are an essential component of volcanic plumbing systems and directly feed volcanic eruptions. They are the primary structure through which magmas are transported from their source to the surface. Originating from depths that span the upper mantle to the shallow crust, most dykes do not erupt and this makes dyke propagation an important contributor to the growth of the crust and volcanic edifices. Dykes have economic significance, and their ascent mechanisms affect the style, location, and frequency of volcanism and volcanic hazards. The rock record variably preserves flow fabrics within the solidified magma and deformation of the host rock, but deciphering evidence of pre-, syn- and post-intrusion events can be challenging. Laboratory experiments using analogue materials, and numerical simulations, test models of dyke ascent that are informed by field and geophysical datasets, collectively providing a theoretical framework upon which to base interpretations of dyke propagation in nature.