The interplay between deformation and magma transport in southeast Iceland

Abstract

Igneous intrusions are of fundamental importance to society's ability to source critical metals, predict major natural hazards and, understand how Earth’s crust evolved. However, the factors that control how magma transits through Earth’s crust and the variables that determine if magma will arrest during the process of ascent or erupt at the surface remains a challenge. This study investigates the interface between key tectonic, host rock and magma parameters to advance our understanding of how these variables influence magma transport processes in sub-volcanic plumbing systems. Specifically, this thesis assesses the interplay of magma flow dynamics and i) deformation structures associated with the ambient extensional stress regime of SE Iceland and ii) host rock deformation processes caused by the process of laccolith intrusion. The first part of the thesis investigates how surface and overburden deformation (forced folds, faults, and fractures) correlate with the mechanism of magma emplacement. In a case study on the Sandfell Laccoith, eastern Iceland, this chapter presents new palaeomagnetic data and a modified fold test to assess the formation of a forced fold. The results show that the formation of the forced fold occurred in two distinct processes that required the temporal break between the initial injection of magma and the later inflation of the laccolith. An evolutionary model is proposed whereby the continuous forceful emplacement of magma involved initial lateral propagation of a sill-like body preferentially towards the SSW resulting in forced folding with a relatively simple geometry. Continuous addition of magma at the base of the laccolith resulted in subsequent ‘bulldozing’ of magma underlying the cooled, viscously stalled initial magma. Magma that was subsequently injected into the laccolith caused inflation facilitated by further displacement of host rock and the formation of a prominent forced fold that is today observed in outcrop. This study demonstrates the utility of a new modified fold test that provides an effective means to track the style and location of transient shallow-level magma movement associated with forced folding. Through detailed structural mapping, ²⁰⁶Pb/²³⁸U zircon dating and, anisotropy of magnetic susceptibility (AMS) analyses, the second data chapter of this thesis investigates the relationship between pre-existing fault/fracture networks and magma flow processes in the Reyðarártindur intrusion in southeast Iceland is investigated. The intrusion is interpreted as a sub-volcanic laccolith where magma propagated laterally along a NE-SW primary axis over a timeframe of 10⁴ – 10⁵ years. Pre-existing NNE-striking, faults/fractures in the host rock accommodated intrusive sheets which themselves facilitated upward and outward evacuation of magma from the laccolith. This study elucidates on how the process of magma transport is influenced by pre-existing fault/fracture networks caused by ambient extensional tectonic regimes and shows that lateral offset of eruptive edifices can be controlled by these fracture networks. The final data chapter uses fracture modelling to provide a revised regional tectonic framework for southeast Iceland. Fracture analysis of fault and joint populations identified within the structural aureole of the Reyðarártindur intrusion and across the southeast Iceland to identify different generations of Riedel shear structures that in turn define a sinistral Riedel shear system. The inferred Riedel shear system is interpreted to have formed within a Neogene WSW-ENE trending transfer zone that linked at least two rift segments. This investigation posits a new interpretation of the contemporaneous stress regime for southeast Iceland and discusses how the associated deformation influenced the transport and storage of magma. The works of this thesis highlight that investigating the interplay between magma movement and host-rock properties aids in our understanding of magma transport in sub-volcanic systems.