F. Caratori Tontini

Gravity anomaly grids for the New Zealand region

ABSTRACT We present 1 arc-minute Bouguer, Faye, free air and topography corrected gravity anomaly grids for the New Zealand region, 25°S to 60°S and 160°E to 170°W. The grids were compiled from existing terrestrial, marine and satellite altimetry-derived gravity data enhanced with new airborne gravimetry data that were acquired for improvement of the New Zealand vertical datum. The airborne data seamlessly cover onshore and offshore areas over New Zealand’s North, South and Stewart islands with a uniform flight line spacing of 10 km. All data were corrected for the gravitational effect of the Geodetic Reference System 1980 (GRS80) reference ellipsoid and tied to the International Gravity Standardization Net 1971 (I.G.S.N.71) gravity datum. The gravity anomaly data from all sources were combined using the method of least squares collocation with a three dimensional logarithmic covariance function. Terrain corrections for gravity anomaly grids were calculated using an 8 m digital elevation model for topography above sea level and a 250 m seafloor topography model.

Colville II '15 Cruise report. Wellington to Auckland September 5 to 25 2015

This research cruise was successful in swath mapping the mid-part of the Colville Ridge and collecting underway-geophysical measurements as part of an ongoing project to survey the Colville Ridge within New Zealand’s Exclusive Economic Zone (EEZ). The main outcome of this expedition is providing fundamental base maps for future mineral exploration. The Colville Ridge is a prominent tectonic feature that marks the western boundary of the Kermadec Arc-Havre Trough backarc system. Very little is known about this large feature on the seafloor, which extends over 1000 km within NZ’s EEZ. It is thought to be at least 2 Myr old (and likely much older) and is possibly composed of various volcanic centres. EM 300 standard multi-beam data was largely missing for the Colville Ridge, with only single ship’s tracks crossing the ridge in a few areas. In 2013 a comprehensive survey of the southern portion of the Colville Ridge was completed during the 15 day OS2020 Colville ’13 cruise that included 12 days on station, with 11 days used for swath mapping and 1 day for dredging. This cruise was highly successful, having produced the first complete map of detailed bathymetry, backscatter, magnetics and gravity over a large part of the southern Colville Ridge. Rocks were also collected from 13 separate stations. The 21 day Colville II ’15 cruise was successful in acquiring gravity and magnetic measurements, along with bathymetry and backscatter data, during 17 days on station, over an area of ~21,320 km2. The datasets have 100% coverage of these parameters, enabling us to ‘stack’ the gravity and magnetic maps on top of the bathymetry and backscatter, enabling a multi-disciplinary approach to interpretation. Rock samples were collected from 20 dredge sites that covered the length of the ridge mapped during the survey, and which included a variety of bathymetric features. Together, the information collected are crucial in our ability to identify prospective sites to carry out more focused, follow-up surveys by AUVs, ROVs, and possibly manned submersibles along the Colville Ridge. The results from this survey will be published as maps and papers in international journals and will be publicly accessible online at GNS. (auth)

Inversion of magnetic and gravity data reveals subsurface igneous bodies in Northland, New Zealand

ABSTRACT Magnetic anomalies identified in the 2011 aeromagnetic survey data of Northland generally correlate with regional geology, but several anomalies have neither surface geological expression nor identifiable associations with the known geology of Northland. Inversion of magnetic data suggests that three circular positive anomalies located east of Kawakawa, east of Kaitaia and southwest of Houhora Heads are associated with buried volcanoes and/or igneous intrusions. Models suggest the magnetic anomalies are caused by subsurface bodies with magnetisation values up to 0.4 A m–1 that are c. 5 km in diameter and extend from depths of 1–3 km to within a few hundred metres of the ground surface. The shape and magnetisation of the bodies suggest that they may be buried rhyolite or andesite volcanoes and/or large granite or diorite igneous intrusions. These previously unknown igneous bodies contribute to our understanding of the volcanic history of Northland and, by analogy to known mineral deposits in Northland, may also host mineralisation that could be of interest to mineral explorers.