Seismic Tomography at Volcanic Rift Zones and Transpressional Plate Boundaries Askja Caldera and Macquarie Island

Abstract

This thesis consists of two main parts: imaging the seismic structure beneath Askja caldera, Iceland, located in a divergent tectonic setting, and Macquarie Island, south-west Pacific, that sits within an intra-oceanic transpressive regime. For decades, the presence or absence of a shallow magma body beneath Askja caldera has been a source of speculation in numerous studies. However, conclusive evidence from seismic tomography, a powerful tool for imaging subsurface seismic structure, has been lacking. In this study, I utilised P- and S-wave arrival times obtained from local earthquakes to image the seismic velocity structure beneath the caldera. To complement this work, seismic attenuation analysis was also performed to further explore the magmatic plumbing system beneath Askja. The results of the tomographic study reveal the presence of a cylindrical low-velocity and high Vp/Vs (P-wave velocity divided by S-wave velocity) zone beneath Askja caldera that is ∼3 km wide and extends to ∼8 km below sea level (bsl). Within this zone, a shallow low-velocity and high Vp/Vs anomaly is located ∼0.5 km bsl beneath the caldera. This body is interpreted as a shallow magma reservoir and sits above the other previously identified and interpreted magma reservoir at ∼6 km bsl that has also been imaged. Significantly, the geographical location of the shallow anomaly aligns closely with the centre of uplift caused by a recent inflation episode, first detected in August 2021 by Interferometric Synthetic Aperture Radar (InSAR) and Global Navigation Satellite System (GNSS), which is consistent with shallow melt accumulation. Seismic attenuation, a sensitive indicator of partial melt, was derived from a shallow local earthquake dataset to corroborate the presence of the shallow magma reservoir. A new 3-D model of absorption reveals a shallow high-absorption anomaly beneath the caldera, in a similar location to the low velocity and high Vp/Vs zone found through the inversion of arrival times. One significant difference is that the anomaly is characterised by a lens shape, which aligns more closely with recent geodetic modelling results and may be attributed to its sensitivity to partial melt as well as improved path coverage at shallow depths. The two sets of findings strongly suggest that the recent inflation of the Askja caldera is closely linked to increased pressure within the shallow magma reservoir, likely driven by the influx of melt from deeper sources. This new study not only advances our understanding of Askja’s volcanic plumbing system but also underscores the utility of integrated geophysical techniques in unravelling complex geological phenomena. Macquarie Island stands as the exposed part of the Macquarie Ridge Complex, situated along the tectonic boundary between the Australian and Pacific plates. While incipient subduction has been proposed for the northern and southern segments of the ridge, the plate configuration underlying Macquarie Island, located within the central segment, remains unclear. In this part of the study, teleseismic tomography is utilised to construct a 3-D model of relative P-wave velocity variations in the uppermost mantle in the neighbourhood of Macquarie Island, thus providing fresh insight into its subsurface seismic structure and immediate surroundings. The teleseismic tomography results reveal distinct features beneath the Macquarie Island region, including a notable lower-velocity zone identified beneath the southern part of the island, which could be attributed to partial melt and high-temperature material generated by plate spreading before its transition to a transpressional plate boundary. Furthermore, a contrast to significantly higher velocity is observed to the east, under the adjacent trough area. This higher-velocity zone raises the possibility of underthrusting of the Pacific plate beneath Macquarie Island, something that has been previously postulated based on evidence from gravity, focal mechanisms, and several geological features. However, the tomography model is unable to provide any insight into whether subduction initiation is taking place or not, owing to its limited resolution. The findings from this study contribute to a clearer understanding of the geological structure and dynamics of Macquarie Island within the broader context of the Macquarie Ridge Complex. The identification of distinct velocity zones provides evidence for past and ongoing geological processes, although further work, such as deploying a much denser Ocean Bottom Seismometer array, is needed to address the question of subduction initiation.