Active Tectonics and Long-Term Deformation of the Southern Alps, New Zealand

In order to fully understand the evolution of tectonic systems it is important to consider processes that occur across all time-scales. The advent of space-based geodesy, and increasing the delivery of regular, open-access satellite imagery at a global scale allows for the measurement of ground deformation at km-scale or below at continental scales. However, these observations occur over a geologically instantaneous time-scale. Deformation at active orogenies can still take place over millions of years, and current observations, although detailed and precise, may not reflect the long-term deformational processes. The aim of this work, therefore, is to carry out a study of the Southern Alps of New Zealand - an actively forming orogeny deforming along an major plate boundary, using geodetic observations from Sentinel-1 InSAR to constrain current ground motions, and observations of micro-structures from field samples to characterise the long-term deformation mechanisms.

In Chapter 3, I produce 3-component velocity fields over the central Southern Alps, showing the along strike variations in the vertical velocity field for the first time geodetically. Peak uplift rates of ∼12 mm/yr are observed, with uplift focused within a 50 km length of strike. I run Bayesian inversions on these velocity fields in order to solve for the variation in fault geomtry and slip rates, with a 15◦ shallowing of the fault coinciding with the location of the locus of uplift. Having established that this shallowing represents a structural control on the location of uplift that is constant in time, I show that short-term measurements of surface uplift can be used as a proxy for uplift, and that a successful proxy for exhumation can be produced by combining interseismic uplift measurements with inverted co-seismic displacements.

In Chapter 4, I investigate a non-guided, automated process for the detection and correction of unwrapping errors in large interferogram datasets. The insights from this are then applied in Chapter 5, where I generate Line-of-Sight velocity maps for 10 Sentinel-1 tracks over South Island, and invert them for east and up velocities. In the process, I investigate the impact of the 2016 mw 7.8 Kaik¯ora earthquake of South Island velocity fields. I build on previous work by resolving uplift along the entire length of the Southern Alps. Finally, I will highlight the impact of fading signals as a noise source in tectonic observations.

In Chapter 6, I analyse micro-structures in rocks collected from the field. I show that although most studies have focused on dislocation creep as a dominant deformation mechanism within shear zones and fault-proximal mylonites, actually the major deformation mechanism in the bulk of the Alpine hinterland is dissolution-precipitation creep. I present a mechanism for the formation of shuffled veins, a key indicator that deformation in the Southern Alps are dominated by dissolution-precipitation. I show that this mechanism is occurring homogeneously throughout the hinterland, allowing for the accommodation of strain through distributed deformation of the Southern Alps, a result that is reflected in InSAR profiles of ground motion created in Chapters 3 and 5.