Lithospheric deformation in the Southern Lakes, New Zealand
Authors:   , Warren-Smith Emily 
Published: July 2022

<p><b>Geological and geophysical data from the Southern Lakes region, South Island, New Zealand, are utilised in this thesis to examine active and Cenozoic deformation between the Australian and Pacific plates. Using published seismic and gravity data, the volume of the crustal root beneath the Southern Alps is estimated and combined with constraints on Cenozoic exhumation to form a simple 2D model of crustal shortening. This approach shows that the volume of crust (plus eroded material) requires up to three times the Cenozoic plate boundary orthogonal motion as constrained by plate reconstructions. Crustal thickening in the Southern Alps must therefore be contributed to by a significant amount of along-strike motion. It is proposed that this motion is accommodated by a low-angle mid-crustal detachment between an underthrust Australian plate hyper-extended rift margin and the overlying Pacific plate.</b></p>
<p>Quantification of Cenozoic deformation is approached using apatite and zircon fission-track thermochronology. Previous fission-track studies in the Southern Lakes have been hindered by extremely low uranium concentrations (<1 ppm) in apatite, leading to underestimates of cooling ages and overestimates of exhumation. To overcome this problem, a new maximum likelihood fission-track age calculation method is developed for low uranium samples. These new, more reliable age estimates indicate that a zone of fully reset apatite ages (<5 Ma) is restricted to within 30 km southeast of the southern Alpine Fault, a value consistent with geological estimates of exhumation and observations along the plate boundary farther north. Fission-track ages are used to develop the first thermo-kinematic models for the Southern Lakes to examine the sub-surface geometry of the plate boundary zone. Modelling results indicate that the observed cooling age pattern can be reproduced by a listric reverse fault geometry beneath the Southern Lakes, which shallows to a low-angle detachment in the midcrust. This detachment may surface offshore to the south of Jackson’s Bay. This detachment corresponds with a mid-crustal low attenuation zone and P-wave velocity inversion, and provides support for the proposed along-strike tectonic model. The model also provides an explanation for the reversal in throw (upthrown to the northwest) across the southern Alpine Fault, by considering the fault as a weakened secondary structure, approximating an antithetic pro-shear zone above the overlying detachment.</p>
<p>Modes of active deformation are examined through the deployment of the Central Otago Seismic Array (COSA) in June 2012, a network of eight broadband seismographs. The longest (15 month) focused microseismic catalogue in the region to date is presented and interpreted. Precise hypocenter locations reveal microseismicity is not associated with known active fault traces and is diffuse throughout the crust. A deepening of the seismogenic zone is observed to the southeast of the Alpine Fault, increasing from c. 10 km within 20 km of the southern Alpine Fault, to c. 20 km >40 km from the fault. This deepening of seismicity approximates the mid-crustal detachment constrained by thermo-kinematic modelling.</p>
<p>Focal mechanisms for 154 events (0.1
</p><p>By considering the 4th May 2015 ML6.0 Wanaka earthquake, it is shown that the same kinematic process also controls large magnitude events. The mainshock and 99 aftershock waveforms are used as template events to perform matched-filter cross-correlation detection of further aftershocks. This method detects 2544 aftershocks over 26 days, 27 times more than recorded by the national network. By generating lagged single channel cross-correlation derived phase picks, precise double-difference locations of these detected aftershocks are computed. Hypocentres highlight a steeply northwest dipping (c. 70°) fault striking at c. 250°, which aligns well with the dextral plane of the mainshock focal mechanism and the plate convergence direction. Analysis of aftershock slip vectors reveals secondary slip occurred on synthetic Riedel shears oriented c. 20° from the principal slip plane.</p>
<p>This thesis contributes a new fission-track age calculation method, 41 new fission-track ages, thermo-kinematic models and two detailed microseismic catalogues for an important transitional section of the Australian-Pacific plate boundary zone. These data are interpreted in the context of a new tectonic model for the region, involving a large component of dextral unthrusting to explain the crustal and lithospheric roots.</p>