Abstract
The southeast Otago region forms the present leading edge of the outboard zone of continental collision between the Pacific and Australian plates. This study focuses on Quaternary motion of two northeast-striking, steeply southeast-dipping, reverse faults, the Akatore and Titri Faults, and subsidence of the Taieri Basin to the west.
The Akatore Fault (~65km long) alternates between onshore and offshore segments; maximum throw along the central onshore segment is 130m. Holocene fault scarps are well preserved along the central onshore segment. Buried peat and wood horizons in blocked swamps along the fault trace, and two marine terraces preserved along the seaward edge of the block, together record two uplift events post-loess deposition. Radiocarbon dating constrains these events to ~1.15 and ~3.8ka. Uplift per event averages 3m, but increases to a maximum of 4m near the south end of the central onshore segment. Evidence for Pleistocene motion is predominantly from marine terraces; two terraces (35 and 65m) restricted to the Akatore block are interpreted to be 105 and 125ka in age, and indicate a period of Akatore Fault uplift between 80-125ka. From 80-3.8ka, however, the fault appears to have undergone a period of quiescence.
The Titri Fault System is also ~65km long, but is probably linked at depth to the reverse Castle Hill Fault to the southwest. Structure contours drawn on basement of the coastal range indicates the "Titri Fault" is segmented; segment lengths range from 13-25km. Maximum total throw is ~650m. The fault system consists of a master fault and several frontal strands; the latter locally deform loess-covered alluvial fans. There is no evidence for Holocene
motion. Alluvial fans can be divided into four sets; the oldest two are everywhere deformed, whereas the second-youngest is locally deformed near Moneymore. Optically stimulated luminescence (OSL) dating provides some control on fan ages, as well as loess stratigraphy. These ages indicate that the last widespread deformation along the Titri Fault System was ~150-70ka, with localised deformation (Moneymore) occurring between 60 and 25ka. Uplift
of a marine terrace interpreted to be 80ka in age further constrains the last period of widespread motion to ~80-70ka. Uplift of higher marine terraces indicates earlier fault motion between 125 and 400ka, with evidence from deformed alluvial fans for a major period of deformation and erosion during 01 stage 7 (245-186ka).
The Taieri Basin is a tectonic depression on the downthrown side of the northern Titri Fault System. It is also faulted on its north-western margins by the west-dipping, reverse, Maungatua and North Taieri Faults. The latter faults have deformed alluvial fans of interpreted penultimate glacial and antepenultimate age, but not last glacial age, indicating
middle and late Quaternary activity. Water bore logs, drillhole logs, a high resolution seismic survey and gravity surveys indicate the basin is asymmetric, with maximum depths of ~200- 300m occurring adjacent to the Titri Fault System (southeast side), suggesting the Titri Fault System is controlling subsidence.
Synthesising the above evidence for timing of fault movement leads to the interpretation that the Akatore Fault and Titri Fault System are moving episodically, on the time scale of tens of thousands of years. Furthermore, there is some evidence for switching between the two. Episodic behaviour is also recognised in central Otago, and supports the interpretation that the Otago reverse faults are linked by a sub-horizontal, mid-crustal ductile shear zone. Episodic behaviour has significant implications for seismic hazard analysis, both in Otago and worldwide.