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

Abstract

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.

Strain localisation within ductile shear zones beneath active faults: The Alpine Fault contrasted with the adjacent Otago fault system, New Zealand

“…Limited paleoseismic data on the Dunstan Fault (Beanland et al, 1986) is compatible with this whereas recent movement on the Akatore Fault (Litchfield and Norris, 2000) is faster. What is remarkable is that all four faults investigated paleoseismically (Pisa Fault, Beanland and Berryman, 1989;Dunstan Fault, Beanland et al, 1986;Titri Fault, Litchfield, 2001;Akatore Fault, Litchfield and Norris, 2000) show periods of quiescence of more than 100 ka, i.e.…”

Marine submersion of an archaic moa-hunter occupational site, Shag River estuary, North Otago

“…The 65-km-long Akatore Fault has a maximum of c. 120 m of Quaternary uplift (Litchfield & Norris 2000). Detailed studies of the onshore section have identified two rupture events within the Holocene (Litchfield & Norris 2000;Litchfield & Lian 2004). The youngest rupture is well constrained by 14 radiocarbon ages at AD 800Á950 (Litchfield & Norris 2000), although McFadgen (2007) has reinterpreted the constraining dates and extended the possible age to lie between AD 900 and 1450.…”

“…Detailed studies of the onshore section have identified two rupture events within the Holocene (Litchfield & Norris 2000;Litchfield & Lian 2004). The youngest rupture is well constrained by 14 radiocarbon ages at AD 800Á950 (Litchfield & Norris 2000), although McFadgen (2007) has reinterpreted the constraining dates and extended the possible age to lie between AD 900 and 1450. In an independent study Hayward et al (2007) recorded a sudden subsidence event on the downthrown side of the Akatore Fault in Akatore estuary with the drowned saltmarsh peat having a date of AD 630Á900, supporting the earlier age for the last movement on this fault.…”

“…1). The 65-km-long Akatore Fault has a maximum of c. 120 m of Quaternary uplift (Litchfield & Norris 2000). Detailed studies of the onshore section have identified two rupture events within the Holocene (Litchfield & Norris 2000;Litchfield & Lian 2004).…”

“…Fault is debatable (e.g. Litchfield & Norris 2000;Hayward et al 2007;McFadgen 2007) and that coseismic compaction of estuarine sediments along a 300 km length of coastline from Waitaki to Bluff (as suggested by McFadgen 2007) associated with one major upper-plate fault-related earthquake (Murihiku Event) would be highly unusual.…”