Holocene motion on the Akatore Fault, south Otago coast, New Zealand

The Akatore Fault comprises the onshore leading edge of the Otago reverse fault-fold belt, part of the outboard zone of Australia-Pacific plate continental collision in the South Island. We examine stratigraphic and sedimentologic evidence of late Holocene reverse faulting along the Akatore Fault, and use radiocarbon dating to constrain fault rupture events and associated uplift of the Akatore fault block. Two fault rupture events are recorded in swamps on the downthrown, upstream side of the fault, as forest and/or peat horizons buried by silt, dated at post-3800 yr BP and 1150-1000 yr BP. The latest event coincides with the timing of uplift of a 3 m marine terrace on the coast, and the uplift of a second (6 m) is tentatively correlated to the penultimate event. The terrace height data, together with the amount of offset of alluvial gravels resting on the fluvial strath surface at the blocked swamp sites, suggests an average of 3 m of throw per uplift event. Distribution of throw along the fault suggests that the last event was not symmetrical about the centre of the scarp in the manner of the long-term throw. Offset of the third highest marine terrace, dated at 80 ka, can be wholly accounted for by the Holocene events, suggesting a period of quiescence between 80 000 and 3800 yr BP.