Regional Gravity and Magnetic Interpretation of the East Coast and Pegasus Basins, Offshore New Zealand

Abstract

The East Coast and Pegasus basins are situated between the North Island of New Zealand and the Hikurangi Trench. DHI's and structural traps are observed on 2D seismic data over the East Coast and Pegasus basins. These observations promote interest in exploration of the basins. However, seismic imaging is complicated by presence of gas clouds and widespread volcanic deposits and intrusions as well as the folding and thrusting in the west. To better understand the geologic framework of the basement and distribution of the volcanics, Schlumberger WesternGeco acquired marine gravity and magnetic data over the basin for a qualitative structural and geologic interpretation. Interpretation of standalone gravity and magnetic data results in non-unique solutions. When combined, the solution becomes less non-unique. In order to correct for bathymetry and Moho effects, the Free Air gravity data was first complete Bouguer corrected at 2.2g/cc, then regionally and residually corrected. The Total Magnetic Intensity data was corrected by Reduction To Pole (RTP). The CRUST2.0 Moho model has been compared with the New Zealand Extended Continental Shelf SEEBASE Moho model. The SEEBASE model has more information in the SE East Coast and Pegasus basins although the overall anomaly pattern of the two residual maps shows good similarity. Radial Average Power Spectrum Analysis of Residual corrected gravity and RTP magnetic data yielded a selection of filter maps and their wavelengths compared to the seismic data. Kinematic analysis for the basement structural interpretation of lineaments from filter maps was conducted taking into account the regional NW-SE compressive stress regime orientation (Beavan etal, 2002) and Scotese (2014) plate motion model. Some major findings are: A transpressive pattern of strike-slip fault zones that terminate on the Hikurangi Trench on both sides. The subduction is oblique given the orientation of the trench and the stress regime. The trench itself displays a stepping geometry where the transfers terminate and near the trench igneous intrusions are observed that intruded into the sedimentary sequence are visible on the seismic data, hence may have had an effect on the thermal history of the sediments. These intrusions manifest as high frequency chaotic magnetic anomaly patterns and gravity highs. Buried seamounts on the subducting plate are also observed in the seismic data and correspond exactly to the gravity and magnetic highs.

AAPG Datapages/Search and Discovery Article #90217 © 2015 International Conference & Exhibition, Melbourne, Australia, September 13-16, 2015