LATE HOLOCENE DEFORMATION, FOLD GROWTH AND THE SEISMIC CYCLE, KAIKOURA PENINSULA, SOUTH ISLAND, NEW ZEALAND

The northeastern portion of the South Island of New Zealand is actively converting from continent-continent collision to a subduction zone as the Hikurangi trench grows southward at plate tectonic rates of 10’s mm/yr. The Kaikoura Peninsula is located ~30 km inboard of the southern tip of the Hikurangi trench and ~ 10 km southeast of the Hope Fault, the southeasternmost splay of the Alpine Fault, which is the plate boundary between the Australian and Pacific plates. The peninsula is underlain by Late Cretaceous to middle Tertiary, shallow marine sedimentary rocks which are deformed into NE-SW trending, open folds in the hanging wall of a NW dipping thrust. Two late Holocene beach gravels are preserved on the Kaikoura Peninsula. On the hinge of the seawardmost anticline these gravels are preserved as, cobble beach berms with steep swash faces. The upper beach berm, T1, is ~2m above the High Tide Debris Line (HTDL) and is Cal AD 1640 to 1840. The lower beach berm, T2, is ~0.5 m above the HTDL and is Cal AD 1320 to 1470. However, 2 km landward near the hinge of a syncline, these beach gravels with the same age are in normal stratigraphic succession with younger over older. They occur as thin, discontinuous layers several cobbles thick and are separated by ~ 1m of colluvium derived from the adjacent sea cliff. The T1 gravel is at the HTDL and the T2 gravel is 1.2 m above the HTDL. The syncline records subsidence, followed by uplift, probably as part of the local seismic cycle for great earthquakes in the region. Subsidence created accommodation space for the deposition of ~1 m thick colluvium; followed by uplift to account for its present location above the HTDL. The high energy, steep cobble beach berms on the anticline do not preserve a subsidence facies. Differential uplift on the anticline/syncline pair for T1 is ~2 m in ~600 yrs or ~3mm/yr. NW tilting is also observed in MIS 3 and 5 marine terraces on the peninsula, although the Holocene tilting is too rapid to yield the amount of tilt in the MIS 5 terrace. The last great earthquake on the Hope Fault is estimated to be Cal AD 1780 ± 60, the mid point of the age for T1, suggesting movement on the Hope Fault is coincident with slip along the thrust under the Kaikoura peninsula and development of the folds in the hanging wall. Based on the radiometric ages for T1 and T2 the recurrence interval for great earthquakes is ~300 yrs.