Gravity studies of the Whakatane graben in the Bay of Plenty, North Island, New Zealand

Abstract

Whakatane graben, in the Bay of Plenty, is situated at the northeast end of the Taupo Volcanic Zone. This zone is within a larger region of widespread volcanic and hydrothermal activity - the Central Volcanic Region, classified as a volcano-tectonic depression within a continental environment. The present study deals with the gravity investigation of Whakatane graben. Gravity measurements of 210 new stations were done using La Coste & Romberg gravimeter G-179, and readings were tied to the New Zealand Primary Gravity Network. Station elevations were determined by barometric levelling. Previous measurements stored in the Geophysics Division Gravity Catalogue were incorporated in the analysis. Gravity data reduction are in accordance with the procedure used by the Geophysics Division of DSIR, to facilitate comparison of results. The regional anomaly was defined by the coefficients of a low order, two-way polynomial, fitted by least squares to anomalies on thick greywacke rocks flanking the Central Volcanic Region. A secondary residual anomaly was derived by subtracting a secondary regional anomaly from the primary residual anomaly along profile lines, using the graphical approach. 21/2-Dimensional interpretation of the gravity anomalies, supplemented by density data, seismic refraction result, and surface as well as sub-surface geology defined the gravity structure of the graben. The graben is widest at the coast to the northeast, and narrows in steps to the southwest. It shares a common eastern boundary with the Taupo Volcanic Zone and the larger Central Volcanic Region. This boundary appears as a stepwise distribution of greywacke fault blocks in plan and section, associated with broad, positive residual anomalies. On the other hand, the western boundary of the graben is defined by a series of northeast trending normal faults such as Matata and Otakiri faults, characterised by steep negative gravity gradient in the south, and by Manawahe-White Island ridge immediately west of the faults, associated with negative gravity gradients in the north. The western and eastern boundaries dip to the east and west respectively, varying from 11° to 70°. The western boundary within the volcanic region dips generally more steeply than the eastern boundary. The floor of the graben dips 4° to the northeast i.e. from about 1 km beneath Kawerau to about 2 km near the coast between Tarawera and Rangitaiki rivers. Thickness of the low-density, low-velocity graben-infill vary from a few hundred meters near the boundaries, to about 1 to 2 km in the centre of the graben. Underlying these materials is a 1 to 3 km thick body having a density contrast of -0.15 Mg/m3, consistent with a 4.62 km/s seismic velocity basement recorded in the graben. The gravity method is an effective means for obtaining a model for the gross structural feature of the graben. It is also effective in mapping buried or concealed structures. Steep negative gravity gradients from Whakatane to Matata coincide with the northern extension of Otakiri fault near the western boundary, and of Waiohau fault near the eastern boundary of the graben. A broad and elongated negative anomaly trending northwest from Mt. Edgecumbe to Manawahe may be related to the western boundary of the North Island Shear Belt.