Year

2013

Degree Name

Doctor of Philosophy

Department

School of Earth and Environmental Sciences

Abstract

Ongoing seismicity is a characteristic feature of plate boundaries, which is not commonly observed in most continental interiors. Consistent low-level seismicity with anomalous higher magnitude events in the intraplate setting of the Mount Lofty and Flinders Ranges of South Australia, is an exception. The N-S elongated ranges are bounded by active faults, including the Eden-Burnside Fault, which produced a magnitude 5.5 Adelaide earthquake in 1954. Evidence of ongoing deformation from detailed mapping and trenching, highlights the potential seismic hazards in this intraplate setting, which requires a comprehensive characterization of faulting and a plausible driving mechanism.

The process of mountain building in the South Australian continental interior has been regarded as, E-W compressional uplift by reverse faulting on either sides of the Mount Lofty and Flinders Ranges. This is not in agreement with the local stress orientations, seismicity and fault characteristics revealed by the present study of individual fault contacts. Detailed geological mapping of stratigraphic offsets, discontinuity orientations, kinematic indicators and geomorphic features associated with the Willunga Fault at Sellicks Beach, have revealed reverse, normal and oblique fault movements. The unique orientations of these faults can be best explained through a sinistral strain-ellipse model, whereas subsidiary reverse fault splays with oblique movements radiating over a broad fault zone resembles positive flower structures. Oblique sinistral movements on the Eden-Burnside Fault and segments of the Wilkatana Fault, also display evidence of transpression, rather than purely E-W compression.

Extensive investigations on this intraplate neotectonic setting with rapid site exploration techniques, such as high resolution LiDAR digital elevation models, ground penetrating radar incorporated field trenches has revealed the presence of multiple faults in the shallow subsurface with limited surface expression. Results of the mapping and trenching indicate predominant reverse faulting towards the margins of the ranges, with comparatively steep fault contacts towards the centre. Identification of subtle, but important topographic and subsurface features, led to establish time constraints for relatively recent rupture events. Single grain OSL results on a colluvial wedge developed over a disrupted soil profile in a trench at Tarlee has constrained the timing of ~1.6 m near surface offset on a low-angle thrust of the Williamstown-Meadows Fault to be ~15 ka. Single grain OSL dates on Alma and Burra faults also yielded late Pleistocene activity from a complicated depositional environment.

As the neotectonic faulting in the Mount Lofty and Flinders Ranges largely reflect reverse and oblique slip, a revised tectonic model is proposed with left-lateral movements. Accordingly, the faulting characteristics and much of the topographic features are accommodated by transpression instead of simplistic E-W compression. The model also better explains the distribution of regional seismicity. Frequent lowmagnitude earthquakes along the middle of the ranges are largely due to the strikeslip movements, whereas intermittent higher magnitude earthquakes to the south and north are due to the terminal thrust faults that accumulate larger degrees of stress. Even though, the pre-historic structures are likely to continually evolve from a strikeslip behavioural perception, direct evidence of Delamerian structural influence on the neotectonic activities were not apparent on this localised and narrow zone of uplift.

The driving mechanism responsible for this intra-plate shallow-depth seismicity is difficult to reconcile, as the region has experienced multiple deformation episodes and the slow rates of ongoing deformation. However, it is possible to correlate major shifts in plate boundary conditions of the Indo-Australian tectonic plate, with deformations in the Australian continental interior. Renewed differential movements along the N-S striking crustal-scale southern oceanic transform faults due to the current NW-SE stress fields may have primarily impinged sinistral movements on the continental interior. The presence of these fracture zones south of the Mount Lofty and Flinders Ranges and the thermally weakened crust have been mutually facilitating the deformation, while the remainder of the plate boundary interactions are likely to enforce secondary controls. Furthermore, extensive fault branching, aseismic slip and structurally reworked, thermally weakened continental crust underneath the Mount Lofty and Flinders Ranges, may be collectively absorbing much of the seismic risk in this region. Nevertheless, neotectonic fault offsets suggest that the episodic large earthquakes are inevitable in this intraplate setting.

FoR codes (2008)

0403 GEOLOGY

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Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.