A gravity analysis of the Alpine Fault and the DFDP-2 drill site, Whataroa valley, South Westland, South Island, New Zealand
The second phase of drilling into the Alpine Fault (DFDP-2), was completed in the Whataroa River valley, a former glacial valley located in central Westland, South Island, New Zealand. The site is located next to a steep hillside on the hanging-wall, ~1 km southeast of the mapped surface trace of the Alpine Fault. Projection of the hillside suggests a sediment thickness of 100 ± 40 m at the drill site; however, the sediment thickness was approximately double pre-drill estimates. Additionally, the surface expression and shallow geometry of the Alpine Fault in the Whataroa River valley, is not well-defined due to post-glacial burial of the fault zone. This thesis describes a gravity study designed to better constrain sub-surface structure beneath the DFDP-2 drill site and across the Alpine Fault. During this study, 466 new high-precision gravity observations were collected (standard error = 0.015 mGal) and amalgamated with 134 existing gravity stations, yielding comprehensive coverage of gravity data across the study area. A high density of observations was achieved within pre-determined zones, in addition to regional measurements so that residual gravity anomaly maps could be produced. The maps reveal: a negative residual gravity anomaly interpreted as a dextrally-offset glacial channel at least 350-450 m deep; steep localised gravity gradients near the Alpine Fault and DFDP-2 drill site that are interpreted as faulted and/or eroded boundaries; and a negative gravity anomaly adjacent to the DFDP-2 drill site that is interpreted as the deepest point of an over-deepened glacial lake. Gravity models were used to estimate the bedrock-sediment interface geometry near the DFDP-2 drill site and Alpine Fault. Structural inversion of the density boundary next to the drill site suggests either a moderately-dipping reverse fault or sub-vertical erosional wall exists beneath the hillside. Additional constraints on physical properties from direct density measurements or seismic velocity determinations and direct constraints on sediment thickness and layer geometry from seismic surveys will in future allow this new high-precision gravity dataset to be modelled more effectively.