Physical and structural controls on monogenetic basaltic volcanism, and implications for the evolution of the Auckland Volcanic Field

Abstract

Monogenetic volcanic fields (MVFs) comprise numerous volcanic centers, distributed across large areas. Understanding the relative importance of various tectonic and magmatic controls on field evolution is pivotal to hazard analyses, and is of particular importance in New Zealand, where the highest population density resides on the active Auckland Volcanic Field (AVF). This thesis provides insights into this problem through studies at different scales (from multiple volcanic fields, to a single volcanic field, to propagation of a single dike) and different dimensions (from 2D surficial spatial analysis to 4D spatio-temporal analysis of the chemical composition of erupted products). Tectonic control or the influence of pre-existing faults is often invoked to explain alignments and statistical distributions of volcanic centers in MVFs; however, a global comparison has been lacking. I present a new global analysis of the spatial distribution of volcanic centers within 37 MVFs, and show: 1) a common clustered distribution of volcanic centers, 2) independency of clustering on tectonic environment, 3) dependency of volcanic field shape on tectonic environment, and 4) influence of tectonic environment on the number of preferred orientations of volcanic alignments. Controls on volcanic alignments are explored using analogue models to evaluate the impact of pre-existing faults on dike propagation. These reveal that lateral distance between fault(s) and dikes, and angle of approach, have the greatest control on the tendency of a dike to modify its trajectory to intercept pre-existing faults, thus modifying the distribution of volcanic centers. Pre-existing faults also affect dike geometry and velocity, as did dike volume. The AVF is an outlier in the global comparison, having an apparently random distribution. Taking advantage of newly available geochemical data and geochronological models, I present a new methodology that combines time, location and chemistry of each eruption, to evaluate the evolution of this MVF. The method looks for the spatio-temporal evolution of volcanic centers using the nearest neighbour analysis and statistical correlations between chemical composition of erupted products and time, distance and volume between successive eruptions. In spite of data limitations, the nearest neighbour analysis reveals that the magma source shows a constant spatial behaviour through time and the statistical correlations show that the spatial distribution of volcanic centers, i.e. the release of magma, is controlled by the behaviour of the source. As well as offering insight to the deep workings of the AVF, this method offers considerable potential to understand the behaviour of monogenetic basaltic volcanism elsewhere.