New Zealand Journal of Geology and Geophysics ISSN: 0028-8306 (Print) 1175-8791 (Online) Journal homepage: http://www.tandfonline.com/loi/tnzg20 A geophysical reconnaissance survey of Great Barrier Island, North Island, New Zealand S.A. Henrys & M.P. Hochstein To cite this article: S.A. Henrys & M.P. Hochstein (1985) A geophysical reconnaissance survey of Great Barrier Island, North Island, New Zealand, New Zealand Journal of Geology and Geophysics, 28:3, 383-395, DOI: 10.1080/00288306.1985.10421193 To link to this article: http://dx.doi.org/10.1080/00288306.1985.10421193 Published online: 06 Feb 2012. Submit your article to this journal Article views: 62 View related articles Citing articles: 3 View citing articles Full Terms & Conditions of access and use can be found at http://www.tandfonline.com/action/journalInformation?journalCode=tnzg20 Download by: [203.173.181.241] Date: 14 August 2017, At: 09:41 New Zealand Journal ofGeology and Geophysics, 1985, Vol. 28: 383-395 383 0028-8306/85/2803-0383$2.50/0 © Crown copyright 1985 A geophysical reconnaissance survey of Great Barrier Island, North Island, New Zealand S. A. HENRYS described later in this study, is used, these data can M. P. HOCHSTEIN be expressed in terms of residual gravity anomalies Geology Department which reach values of about -17 mgal (-170 IlN/kg) University of Auckland near Port Fitzroy. Although these values point to Private Bag a thick sequence of volcanic rocks, no significant Auckland, New Zealand magnetic anomalies are indicated in high-level air­ borne magnetic data (Hunt & Syms 1977). Recently, marine gravity anomalies have become available (Rawson 1983) which outline the Jurassic base­ Abstract Results of a gravity and an airborne ment structure to the west and southwest of the magnetic survey of Great Barrier Island are island (Fig. 1). described. The interpretation of residual Bouguer A geophysical reconnaissance survey of Great gravity anomalies shows that a sequence of Mio­ Barrier Island was undertaken during 1980-81, to ceneandesites up to 2 km thick (Coromandel Group determine the thickness of the Miocene volcanics volcanics) has been deposited in the elongated, with respect to the underlyinggreywacke rocks, and northwest-trending Great Barrier Depression which to outline gross structural features within the extends offshore to the west and northwest of the volcanics. island. Late Miocene rhyolites (Whitianga Group volcanics) erupted from the Mt Hobson caldera, a 2 km deep depression within the Miocene volcan­ ics, and at Rakitu Island and Okupu. GEOPHYSICAL AND GEOLOGICAL Contemporaneous geothermal activity produced SETTING widespread alteration of near-surface volcanics, The geological setting of Great Barrier Island has mineral deposition, and partial demagnetisation been affected by the tectonic processes which led which is reflected in the low-amplitude pattern of to the formation of the Hauraki Rift (Hochstein & magnetic anomalies over the central part of the Nixon 1979). Geophysical studies have shown that island. this rift extends over a distance of about 300 km from Whangarei to the Taupo Volcanic Zone and Keywords gravity anomalies; magnetic anom­ that it consists of a series of NNW.trending fault­ alies; Miocene; volcanic rocks; Great Barrier angle depressions and concealed basement horsts Depression; Mt Hobson caldera; geothermal sys­ (Hochstein 1978; Ferguson et al. 1980; Tearney tems; Great Barrier Island 1980; Davidge 1982; Rawson 1983). The rift has been filled by unconsolidated Pleistocene sedi­ Downloaded by [203.173.181.241] at 09:41 14 August 2017 ments which are underlain in some parts by older INTRODUCTION volcanic rocks (Hochstein et al. in press). The basement on Great Barrier Island consists Great Barrier Island and the Coromandel Penin­ of indurated, folded sediments, slates, and lithic sula most likely are remnants of the eastern shoul­ sandstones, commonly described as greywackes, der of the Hauraki Rift (Hochstein & Nixon 1979) which have been correlated with the Late Jurassic and are separated by the Colville Channel (Fig. 1). Manaia Hill Group (Skinner 1967; 1976). These Great Barrier Island and the Coromandel Penin­ rocks crop out in the northern part and also along sula are covered extensively by volcanics of Mio­ the eastern coast at Overtons Beach (Fig. 2). cene - Early Pliocene age (Thompson 1960; A thick sequence of easterly dipping Tertiary Schofield 1967). andesitic flows and pyroclastics unconformably Published gravity data are restricted to stations overlies the basement. Similar rocks on Coroman­ along the Great Barrier Island coast (Woodward & del Peninsula have been classified as Coromandel Reilly 1972). When a regional gravity field, Group (Skinner 1976). The term Coromandel Group is here extended to include those rocks on Great Barrier Island, mapped originally as Beesons Received 12 January 1984, accepted 1 March 1985 Island Volcanics by Thompson (1960), which typ- Sig ,. 384 New Zealand Journal of Geology and Geophysics, 1985, Vol. 28 I · · · ~· oo f ·"S··-;' an. ~ ' ;O ' 'lS .~ !;i ' o 80 ·0 t u tt e Barrier [ . '" 0-, 65 0 75 ·0 a wa u €J [ .D " ,", J 700 ~ ' ~'·E " I ,.- < ....,.; )),~ 0 '$ Downloaded by [203.173.181.241] at 09:41 14 August 2017 ~ 'V' . s' ~ Juras si c Grf ywa ckf o Gravit y Sta t ion. I"~ \.1 '"60'0, Rfg lonal.. Gravity con lours(mgals Ok m FI RT H OF o/ Frf e air a noma lies (seal I I TH A MES Bouguer anomalies (land l ,.,/"" - u rea l Barrier Depression Fig. 1 Map of the Hauraki Gulf showing smoothed gravity anomalies over and around Great Barrier Island together with the regional gravity field. The dashed line around Great Barrier Island outlines the approximate extent of the Great Barrier Depression which has been filled with Miocene volcanic rocks. Henrys & Hochstein-Geophysical survey, Gt Barrier I. 385 ically consist of poorly bedded volcanic breccias GRAVITY AND MAGNETIC SURVEYS and lava flows of marginally subaqueous mode of deposition (Ramsay 1971; Hayward 1973). These During the summer of 1980-81, 242 gravity stations rocks have not been dated yet, but Pliocene lignite (Fig. 2) were established in addition to the existing lenses interbedded with Coromandel Group 70 DSIR stations. The measurements were made breccias near Medlands Stream contain leaf with Worden Pioneer meters (Nos 240 and 697). impressions of late Mangapanian - Hautawan age Stations were located along roads and tracks, form­ (Henrys 1982). Radiometric ages for the Coroman­ ing closed nets. Grid co-ordinates were obtained del Group volcanics on the Coromandel Peninsula within an error of ± 100 m from airphotos and range from 16.2 to 2.54 Ma (Robertson 1983), but 1:50000 topographic maps (NZMS 259) and draft the northernmost andesites conformably overlie copies of recently compiled 1:25 000 maps (?)Otaian marine beds of the Colville Formation. (NZMS 270). Co-ordinates and elevations of 26 The Coromandel Group volcanics are uncon­ stations were determined by tacheometry. Heights formably overlain by rhyolitic volcanics in the cen­ of all the remaining stations were determined by tral part of Great Barrier Island (i.e., around Mt barometry using two Paulin altimeters according to Hobson), which are very similar to rocks of the the single altimeter method of Hodgson (1970). Whitianga Group on Coromandel Peninsula Gravity measurements and altimeter readings were (Thompson 1960, 1961). Similar rocks can also be repeated at 4 h intervals at intermediate base found further north on the Mokohinau Islands, stations. Drift-corrected data indicate mean closure Fanal Islands, and the Poor Knights Islands (Wod­ errors of about ±0.2 mgal for gravity and about zicki & Bowen 1979). Lithologies ofthe Whitianga ±3 m for altimeter data except for the Mt Hobson Group rocks on Great Barrier include crystal-free, net (±5 m). The gravity and elevation data were flow-banded rhyolites, breccias of perlitic and vitric tied into the DSIR base station at Port Fitzroy. All rhyolites, ignimbrites and pyroclastic flows, as well gravity data have been deposited with DSIR, Geo­ as rhyolitic domes. A remnant of a presumably physics Division (station numbers 02/782-1013). more extensive pyroclastic flow sheet forms a 230 In computing Bouguer anomalies, the terrain m high plateau at Mt Te Ahumata (Fig. 2) which, effects (density of 2.67 X 103 kg/m ') of Hammer according to Erceg (1981), originated from a vent Zones E-M were obtained by using a digitised ter­ in the Mt Hobson area or from the Okupu Rhyolite rain model of Great Barrier Island (Henrys 1982). Dome. Fission track dating of Whitianga Group The validity of the digitised terrain model was rhyolites from Great Barrier Island and the Coro­ checked by determining separately the terrain effect mandel Peninsula gave ages between 8.9 ± 1.3 Ma of 20 widely spaced stations using Hammer Zone and 2.3 ± 0.2 Ma (Rutherford 1978; Leach et al. graticules. The rms difference in the terrain effect 1981).Obsidian fragments near Awana, Mt Young, using the two procedures was found to be about and Mt Te Ahumata gave dates of 8.9 ± 1.3, 8.3 ±0.4 mgal for most of the island, although in the ± 0.6, and 4.6 ± 0.9 Ma respectively (Rutherford steeply dissected terrain around Mt Hobson, errors 1978; Leach et al. 1981). up to ± 1.5 mgal might occur. The resulting mean Faults in the basement rocks trend NW-NNW error ofthe Bouguer anomaly was estimated to be (Bartrum 1921; Hayter 1954); intrusive dikes, dated ±1.4 mgal for all other stations. at 18.1 Ma, in the northern part of the island, follow The effect of deeper-seated crustal masses was this trend but also strike NNE, parallel to a minor reduced by constructing a regional field for the Downloaded by [203.173.181.241] at 09:41 14 August 2017 fold direction (Thompson 1960).