EXPLORING THE GEOMETRY AND GEOCHEMISTRY OF THE VOLCANIC INTRUSION UNDERLYING URUMAU RIDGE: LYTTELTON, NEW ZEALAND

The morphology of intrusive volcanic features can provide insight into their emplacement conditions. Typical intrusives of the Lyttelton Volcanic Complex, on Banks Peninsula, New Zealand, are basaltic to trachytic dykes or domes, which form resistant highs in the now eroded topography. This study investigates a prominent ridge to the east of Lyttelton, within the Urumau Reserve, known as the Urumau Ridge. At the base of the ridge, exposed in a roadcut, is the previously mapped “Windy Point Sill”. This study uses ArcCollector assisted field mapping and Google Earth visualizations to examine the ridge morphology and volcanic processes. The ridge is comprised of lava flows and scoria deposits which host intrusions. The ridge forming intrusion extends from the lower ridge along Sumner Road, connecting up slope to the ridge crest, with exposures demarked by inclined columnar joints and inclined intrusive contacts. ArcCollector aided field mapping correlated the lower “Windy Point Sill” intrusive to the ridge forming intrusive body, connected due to the presence of cooled margins and cooling joints, with the cross-cutting path of the body. This is further supported by geochemical analysis of the intrusive body, with pXRF analysis confirming linear related trachytic chemistry, as they are both products of the same magma source.

To visualize the morphology of the intrusion, the intrusive contacts were expressed in Google Earth by connecting contact exposures from the ridge crest to the exposures near the lower ridge. This visualization revealed the intrusion dips perpendicular to the ridge crest, which is uncharacteristic of the vertical to subvertical radial dykes found in the Lyttelton Volcanic Complex. This suggests that underlying tectonic controls influenced the stress regime within the volcanic edifice at the time of intrusion. The resultant inclined, resistant, cohesive intrusion now forms Urumau Ridge, with the geomorphology controlled by the intrusion forming the backbone of the ridge, and the southeastern side as a major erosional discontinuity forming the inner harbour slope. This study highlights the use of digital data collection methodologies and the use of simple 3d visualizations in Google Earth.