Constructing an Alpine Fault Paleoseismicity Record from Slumped Lacustrine Deposits in the Cascade River Valley, South Westland, New Zealand
Abstract:The Alpine Fault is a major structure in New Zealand capable of producing earthquakes of magnitude 7 or greater, which delineates the boundary between the Australian and Pacific plates. Paleoseismic records of these earthquakes indicate recurrence intervals of 300 – 400 years over the last 1,300 years. However, there are no pre-Holocene records. Documenting the late Pleistocene record of magnitude, timing, and frequency of earthquakes would significantly reduce uncertainty in hazard analyses.
The tectonically complex Cascade River Valley follows the Southern Alpine Fault, where the fault dominantly accommodates strike-slip motion. Two ~7m outcrops of proglacial lacustrine silt are exposed along the river in which, deformed rhythmites bounded by planar laminated rhythmites have been identified. These exhibit a variety of fold geometries in outcrop and x-ray computed tomography (CT) scans, all of which show some degree of asymmetry. Initial radiocarbon ages of 14,400 and 13,300 14C yr BP have been obtained from terrestrial plant material isolated from samples near the base of one outcrop. Given the age range and laminae density, these dates suggest that the rhythmites are varves, but additional radiocarbon dates and CT-scans will be used to confirm this.
The deformed horizons are interpreted to be seismites formed by slumping. Earthquake shaking triggers an increase in pore fluid pressure, which destabilises the sublacustrine slope causing failure and the release of silt into the sedimentary system. As silt is transported by downslope shear it is deformed in distinct layers. Displacement of volumes of silt also causes the formation of seiche waves that apply shear stress to lake floor sediments causing further deformation. Deviations in magnetic susceptibility and the declination of magnetic remanence observed underneath and within deformed horizons are interpreted to be a response of earthquake shaking. Data from these different proxies will be presented and compiled to generate a record of earthquake shaking from the Southern Alpine Fault.