Oligocene–Miocene Tectonic Evolution of the South Fiji Basin and Northland Plateau, SW Pacific Ocean: Evidence from Petrology and Dating of Dredged Rocks ⁎ N

Marine Geology 237 (2007) 1–24 www.elsevier.com/locate/margeo

Oligocene–Miocene tectonic evolution of the South Fiji Basin and Northland Plateau, SW Pacific Ocean: Evidence from petrology and dating of dredged rocks ⁎ N. Mortimer a, , R.H. Herzer b, P.B. Gans c, C. Laporte-Magoni d, A.T. Calvert c,1, D. Bosch e

a GNS Science, Private Bag 1930, Dunedin, New Zealand b GNS Science, PO Box 30368, Lower Hutt, New Zealand c Department of Geological Sciences, University of California, Santa Barbara, CA 93106, USA d LGPMC, EA 3325, Université de Nouvelle Caledonie, BP R4, 98851 Noumea, Nouvelle Calédonie, France e Lab. de Tectonophysique, UMR-CNRS 5568, Univ. Montpellier II, cc 56, Montpellier 34095, France Received 16 November 2005; received in revised form 5 September 2006; accepted 23 October 2006

Abstract

We present new analytical data from lavas and associated rocks dredged and/or drilled from the South Fiji Basin, Northland Plateau, Colville Ridge and Havre Trough. These results provide much-needed ground truth about the geology, age and tectonic evolution of the Cenozoic submarine ridges and basins between the active intraoceanic Tonga–Kermadec arc, and rifted continental borderlands of New Zealand, the Norfolk Ridge and New Caledonia. Key results from this study include: (1) Ar–Ar dates on Minerva Abyssal Plain oceanic crust suggest that the ages of magnetic anomalies in the South Fiji Basin have been overestimated by earlier workers; (2) subduction-related lavas are widespread across the region, are not presently organised into arc-like chains, and cluster in the age range 22–18 Ma (Early Miocene); (3) the oldest subduction-related lavas occur in the western part of the region (32–26 Ma: Norfolk and Three Kings Ridge); (4) shoshonites, interpreted as rifted arc lavas, were erupted in a narrow 20– 21 Ma interval over a wide area. Put together, these results indicate high magmatic flux and large and rapid horizontal tectonic translations and basin opening from 18–23 Ma in the region immediately north of New Zealand. We explain the Miocene tectonomagmatic development of the region by a model of rapid rollback of a single, east-facing Pacific arc–trench system that became established after Northland Allochthon emplacement. Critical testing of this, versus other, tectonic models must await drilling and dating of thus-far unsampled Kupe Abyssal Plain crust. © 2006 Elsevier B.V. All rights reserved.

Keywords: petrology; geochemistry; geochronology; subduction; back-arc basins; shoshonite; South Fiji Basin; New Zealand

1. Introduction

The South Fiji Basin area (Fig. 1) is one of the least investigated parts of the southwest Pacific Ocean. It is a ⁎ Corresponding author. Tel.: +64 3 479 9686; fax: +64 3 477 5232. E-mail address: [email protected] (N. Mortimer). region of remnant volcanic arcs, plateaus and basins that 1 Present address: U.S. Geological Survey, 345 Middlefield Road, lie between the presently active Taupo–Kermadec– Menlo Park, CA 94025, USA. Tonga arc, and the edge of Gondwanaland continental

Fig. 1. Regional setting of the study area in the SW Pacific Ocean, showing geographic names and dredge locations from Churkin and Packham (1973), Packham and Terrill (1976), Davey (1982), Adams et al. (1994), Mortimer et al. (1998), Shipboard Party (1999), Ballance et al. (1999), Bernardel et al. (2002), Mortimer et al. (2003), Herzer et al. (2004a) and GNS Science (unpublished data). Bathymetry from CANZ (1997), basins are darker grey, ridges and plateaux lighter grey. Magnetic anomaly picks from Malahoff et al. (1982) and Davey (1982). JLin = Julia Lineament, VDLL = Van der Linden Lineament. Inset shows different geographic areas of the Northland Plateau mentioned in the text and appendices. Small text labels refer to dredge numbers (new samples) and P numbers (old samples from Mortimer et al., 1998). N. Mortimer et al. / Marine Geology 237 (2007) 1–24 3 crust which extends from New Zealand along the 2001; Bosch et al., 2002; Mortimer et al., 2002; Herzer Norfolk Ridge to New Caledonia. et al., 2004b, 2005). The data comprise 81 X-ray There is general agreement that from c. 85–55 Ma, fluorescence analyses, 41 ICP-MS element analyses and the Pacific-Australian plate boundary was the Tasman 26 Ar–Ar dates of lavas from 36 different dredge sites. Sea spreading ridge, and that New Zealand, the Lord The results greatly expand our knowledge of this part of Howe Rise and Norfolk Ridge formed a rifted continen- the SW Pacific seafloor and enable a more complete tal borderland – Zealandia – on the Pacific Plate. At c. view of the Cenozoic volcanotectonic evolution of the 45 Ma, convergence began between the Pacific and large triangular area between New Zealand, New Australian plates (Sutherland, 1999), and since at least Caledonia and Fiji. In particular they challenge existing 5 Ma the convergent Pacific-Australian plate boundary tectonic models which are largely based on longstanding has been close to its present position which is the magnetic anomaly interpretations (e.g. Malahoff et al., Hikurangi–Kermadec–Tonga Trench. However, there is 1982; Davey, 1982). We present an integrated, data- major disagreement on the ages of the Norfolk and South constrained petro-tectonic model that explains the Fiji Basins and also the age, polarity and numbers of Miocene tectonic evolution of the area. volcanic arcs and subduction zones that operated in the interval 85–5 Ma (e.g. Malpas et al., 1992; Herzer and 2. Regional setting Mortimer, 1997; Mortimer et al., 1998; Ballance, 1999; Crawford et al., 2003; Sdrolias et al., 2004; Schellart 2.1. South Fiji Basin et al., 2006). In addition to these uncertainties in tectonic models, the nature and role of a newly defined tectonic The South Fiji Basin (Packham and Terrill, 1976)isa element in the southwest Pacific, the 45000 km2 generally smooth-floored, 3–4 km deep basin with up to Northland Plateau (Fig. 1, Herzer et al., 2000)is 500 m of sediment cover (Fig. 1). It is bounded to the essentially unknown. Is the Northland Plateau a piece west by the Three Kings Ridge, Loyalty Ridge and Cook of trapped Cretaceous Hikurangi Plateau (Mortimer and Fracture Zone, to the east by the Colville–Lau Ridge and Parkinson, 1996), foundered New Zealand continental to the south by the Northland Plateau. Reflection seismic crust, or a Cenozoic volcanic pile? It is important to profiles across many of these margins (work in progress) address these issues in order to understand how typical or show both rifted volcanics and volcaniclastic aprons atypical is the New Zealand–New Caledonia–Fiji area in merging into, or slightly above, the top of the oceanic terms of progressive arc and backarc basin development crust of the deep basin (e.g. Herzer et al., 2000, 2004b). and the applicability of standard trench rollback models. There is no evidence of a fossil trench or accretionary A knowledge of the timing, sequence and kinematics of prism on any side of the South Fiji Basin. The basin is SW Pacific backarc opening and volcanism is important divided into two parts, a southern Kupe Abyssal Plain for petroleum prospectivity of the Reinga, Northland and and a northern Minerva Abyssal Plain, separated by a East Coast continental shelves, as such events provide poorly surveyed and defined Central Ridge region long range driving forces for basin evolution. between the Lau and Fantail Terraces (Fig. 1). In order to help test the various tectonic models we Several groups of seamounts (Sarah, Devonport, made two cruises on the R/V Tangaroa to the Northland Margot, Marion, Coquille), some elongated, some flat Plateau in 1999 and 2002, ONSIDE (Offshore North- topped, rise from the western part of the southern South land SeIsmic and Dredging Expedition) I and II, with Fiji Basin, which also has a different satellite gravity the specific aim of recovering, analysing and dating texture from the adjacent flatter, and slightly deeper rocks in conjunction with reflection seismic profiling Kupe Abyssal Plain. Two elongated flat-topped sea- (Shipboard Party, 1999; Herzer et al., 2004a). We also mounts, Matahourua and Mascarin, lie on the eastern obtained material from contract dredging of seamounts side of Kupe Plain near the Colville Ridge (Fig. 1). in the South Fiji Basin and Colville Ridge, and The age and kinematics of the opening of the South reanalysed and dated some material from our earlier Fiji Basin are controversial. Oligocene magnetic anom- work in the Norfolk Basin (Mortimer et al., 1998) and aly (7–12) patterns outlining a triple junction have long from Deep Sea Drilling Project holes in the South Fiji been interpreted for the Minerva Plain and westward- Basin (Churkin and Packham, 1973; Stoeser, 1976) younging 7–12 anomalies have long been interpreted in (Fig. 1). the Kupe Plain (Weissel and Watts, 1975; Davey, 1982; The purpose of this paper is to present a summary of Malahoff et al., 1982). However, Sdrolias et al. (2003) our new analytical results, previously published only as reinterpreted the Kupe 7–12 anomalies as younging east, abstracts (Herzer and Mortimer, 1997; Herzer et al., not west, and connected them to a Minerva triple 4 N. Mortimer et al. / Marine Geology 237 (2007) 1–24

Table 1 Selected whole rock analyses of lavas

GNS no. Location SiO2 TiO2 Al2O3 Fe2O3T MnO MgO CaO Na2OK2OP2O5 LOI Total As Ba Minerva Abyssal Plain P63848w DSDP205 48.20 1.35 16.10 10.40 0.17 7.33 11.50 2.56 0.60 0.19 1.61 100.01 b120 P63850w DSDP285 50.10 2.02 13.80 13.10 0.21 6.54 10.40 3.13 0.12 0.19 0.36 99.97 b111 P63834w Julia 50.71 1.47 15.62 8.36 0.16 7.23 12.63 2.84 0.17 0.15 0.00 99.33 b120 P63836w Alison 46.88 2.00 17.76 10.18 0.16 4.73 11.73 3.65 0.33 0.31 1.20 98.94 10 9

South Fiji Basin Seamounts Potassic suite P59786w Devonpt C 55.45 0.93 15.48 6.24 0.08 5.70 6.25 2.16 5.88 0.53 0.48 99.18 3 1067 P59788g Devonpt C 51.81 0.99 17.27 6.85 0.11 2.84 6.88 2.53 6.32 0.88 2.62 99.10 36 1144 P61717g Devonpt E 59.41 0.53 18.43 3.87 0.06 1.33 2.60 2.27 10.30 0.34 0.72 99.90 7 244 P61724g Margot 56.43 0.60 18.07 5.54 0.07 3.31 5.97 3.08 4.99 0.43 0.91 99.40 9 1382 P59769g Sarah W 53.19 0.71 15.71 8.47 0.15 5.57 9.78 2.36 2.25 0.30 1.06 99.55 5 1366 P63821w Sarah C 43.32 0.76 17.29 8.79 0.07 2.59 9.76 2.80 4.55 4.21 5.08 99.22 96 3211 P63825w Sarah N 51.95 0.61 15.94 9.03 0.14 6.69 10.72 1.91 1.83 0.28 0.50 99.60 5 1532 Ocean island basalts P63830w Marion 50.82 2.38 18.29 9.22 0.23 2.15 7.73 4.45 2.63 1.01 0.93 99.83 9 277 P63835w Coquille 47.83 2.00 18.44 9.16 0.17 5.29 7.05 4.61 2.32 1.10 1.66 99.62 2 423 P67647w Mascarin 38.88 1.52 16.88 9.73 0.16 2.01 15.00 3.36 1.61 5.77 4.92 99.83 52 125

Northland Plateau Poor Knights Seamount Chain P61712g P70L 53.08 1.10 17.32 9.21 0.26 0.93 4.49 4.04 5.34 1.25 2.48 99.50 54 556 P66794w D14B-3 50.61 1.12 20.33 8.65 0.16 2.47 5.87 4.23 1.65 0.20 4.68 99.97 9 90 P66824w D21-3 56.10 1.27 14.56 12.10 0.13 3.79 4.26 4.03 1.85 0.20 1.69 99.98 4 266 P66808w D18A-1 44.88 1.31 17.16 9.54 0.11 3.39 12.17 2.49 2.77 2.82 3.14 99.78 44 402 P66825w D21-6A 47.30 0.73 23.69 8.70 0.08 3.22 12.02 2.33 0.61 0.12 1.18 99.98 3 49

Outer Volcanic Plateau P63154m D4B-4 47.90 1.43 20.89 11.04 0.43 2.02 8.95 3.50 1.20 0.14 2.48 99.98 23 178 P63162m D5B-1,2 45.85 1.96 20.28 11.74 0.18 2.78 8.38 3.93 1.11 0.61 3.13 99.95 68 159 P63167m D6 hi K 54.60 1.16 20.13 7.15 0.09 1.34 6.13 5.30 2.07 0.48 1.24 99.69 13 187 P63165m D6 lo K 50.57 1.32 18.27 8.75 0.22 3.31 9.05 3.41 0.20 0.26 4.42 99.78 1 58 P63179m D7B-32 46.25 1.64 19.74 11.54 0.13 4.33 9.02 3.24 1.00 0.23 2.87 99.99 32 75 P66800w D20-1 44.67 1.68 18.64 13.50 0.12 3.90 10.76 3.21 0.79 0.22 2.48 99.97 40 70 P66803w D19-1 48.17 0.93 18.25 9.49 0.12 6.31 9.27 2.56 1.99 0.30 2.40 99.85 2 700 P66807w D19-9 55.33 0.69 20.34 6.07 0.12 1.69 3.87 4.44 4.24 0.26 2.49 99.58 6 1789

Havre Trough P63474m SO135-36 50.47 1.54 17.12 10.17 0.17 5.98 10.16 3.09 0.56 0.26 0.49 100.01 2 98

Colville Ridge P63206 OR99-1 57.82 0.65 22.86 10.86 bd 0.07 0.11 0.00 0.14 0.13 6.84 99.48 3 56

Continental shelf P66851w D24A-3 67.12 0.37 15.44 2.76 0.04 0.52 1.97 4.60 3.84 0.11 3.19 99.96 12 676 P66854w D24B-1ii 60.16 0.75 18.96 5.03 0.02 0.48 3.38 5.36 4.42 0.29 0.97 99.82 29 643 SeeCe text for Cr analytical Cs methods. Cu ICP-MS Dy analyses Er done Euat wWashington Ga State Gd University Hf (analyst Ho C. Knaack),LagGrenoble Lu Nb University Nd (analyst Ni C. Laporte-Magoni), mMontpellier University (analyst D. Bosch).

junction via a spreading ridge along the Julia Lineament. Kupe magnetic anomalies could be reinterpreted as In yet another model, Lawver et al. (2002) interpreted the Miocene (e.g. 5AA-5E; c. 19–13 Ma). Our dating of Julia Lineament as a major transform and incorporated Minerva Plain tholeiites (Section 6.1) further questions the suggestion of Herzer and Mortimer (1997) that the the original Davey–Malahoff interpretations. N. Mortimer et al. / Marine Geology 237 (2007) 1–24 5

Table 1 Selected whole rock analyses of lavas Ce Cr Cs Cu Dy Er Eu Ga Gd Hf Ho La Lu Nb Nd Ni

12.1 203 0.51 72 5.23 2.95 1.21 14 4.31 2.28 1.09 4.64 0.41 3.24 9.65 80 14.6 47 0.19 63 8.49 5.00 1.79 18 6.63 3.52 1.77 5.22 0.69 4.76 13.0 46 8.11 252 0.02 79 4.42 2.47 1.03 16 3.62 1.91 0.94 3.14 0.33 4.50 7.27 106 20.1 354 0.46 71 6.95 3.81 1.93 17 6.11 4.05 1.41 6.48 0.52 1.39 16.3 178

94.6 261 3.21 49 4.58 2.03 2.11 19 6.69 9.78 0.83 47.4 0.25 16.6 41.5 106 105 182 3.44 37 4.19 2.00 2.77 19 7.57 9.74 0.78 48.7 0.24 20.6 46.7 75 102.3 14 1.19 10 3.09 1.72 1.40 18 4.51 18.2 0.62 46.7 0.24 35.9 41.3 19 111 49 6.52 28 4.28 2.19 2.91 18 7.17 7.61 0.82 55.1 0.30 17.6 45.4 32 59.7 249 7.16 83 3.46 1.98 2.44 16 5.03 2.49 0.71 29.7 0.28 5.04 25.7 73 163 118 0.3 63 7.45 4.71 2.97 11 9.32 6.01 1.61 100.7 0.72 12.7 60.9 90 71.3 117 4.84 125 3.71 1.78 1.75 14 5.29 2.09 0.69 38.4 0.25 3.59 31.2 45

79.1 13 0.37 40 5.14 2.33 2.26 23 6.33 4.41 0.98 43.7 0.28 81.5 35.8 35 107 99 0.5 39 5.81 2.96 2.47 19 6.54 6.96 1.13 64.4 0.43 93.3 40.0 93 28.6 296 0.27 91 8.88 6.41 2.06 14 7.77 3.01 2.14 40.8 1.06 15.8 26.4 70

41.6 5 2.19 56 7.34 4.92 1.86 23 7.23 5.54 1.66 21.3 0.70 7.36 25.4 67 18.4 5 0.13 25 4.12 2.18 1.33 18 3.50 4.91 0.81 5.14 0.30 2.07 8.80 40 23.4 b1 2.24 111 5.85 3.42 1.37 20 5.19 3.32 1.26 10.2 0.46 4.49 14.2 16 57.1 282 0.41 64 5.83 3.48 1.77 13 5.93 5.53 1.23 35.7 0.49 14.3 29.5 58 5.78 14 2.41 92 2.49 1.45 0.62 20 2.03 0.83 0.54 2.51 0.21 0.47 4.17 11

18.7 109 0.24 170 3.02 1.62 0.98 17 3.10 3.92 0.61 9.59 0.24 6.59 11.5 62 27.9 419 0.16 118 8.02 4.99 2.02 18 7.88 4.23 1.74 30.1 0.71 15.5 28.5 106 23.2 3 1.67 76 8.13 4.85 1.95 23 7.87 5.06 1.72 15.7 0.70 3.95 25.4 20 16.8 17 0.12 98 4.38 2.70 1.19 18 4.04 2.80 0.93 6.88 0.43 2.03 12.0 25 13.2 238 0.37 83 6.23 3.35 1.64 15 6.38 3.01 1.24 21.8 0.41 1.52 21.8 98 13.1 538 0.11 118 7.19 3.72 2.10 18 7.45 2.77 1.45 22.9 0.48 2.44 23.7 90 38.7 13 1.67 104 4.1 2.08 1.52 14 4.91 2.44 0.80 18.6 0.27 3.97 20.3 49 103 0 0.74 48 3.7 1.66 2.23 15 5.12 10.7 0.66 35.5 0.21 17.5 29.8 43

16.6 161 0.21 60 4.68 2.75 1.40 18 4.47 2.57 0.98 6.14 0.40 2.85 12.9 71

20 10 – 5 –––41 –––10 – 1 – 7

64.7 b1 5.27 7 7.14 3.94 1.10 20 6.82 9.01 1.43 35.5 0.57 10.9 31.7 4 58.1 13 1.58 15 4.9 2.50 1.25 23 5.35 7.09 0.96 28.6 0.33 13.0 25.8 30 (continued on next page)

2.2. Northland Plateau bathymetric compilation of CANZ (1997).Itliesbetween the base of the clearly defined Northland continental slope The extent and shape of the Northland Plateau (Fig. 1; (c. 1500 m water depth) and the Kupe Abyssal Plain of the Davey, 1982) has become much better known since the South Fiji Basin (c. 3000 m). The Northland Plateau 6 N. Mortimer et al. / Marine Geology 237 (2007) 1–24

Table 1 (continued ) GNS no. Pb Pr Rb Sc Sm Sr Ta Tb Th Tm U V Y Yb Zn Zr Minerva Abyssal Plain P63848w 1.54 1.86 7.3 44 3.42 165 0.39 0.78 0.30 0.42 0.13 256 28.2 2.64 79 82 P63850w 1.50 2.34 1.4 46 5.02 112 0.80 1.26 0.32 0.71 0.12 326 47.2 4.45 87 120 P63834w 1.03 1.24 1.9 50 2.72 148 0.42 0.69 0.29 0.35 0.74 262 32.6 2.13 96 76 P63836w 1.81 3.17 6.6 38 5.26 357 0.44 1.09 0.09 0.54 0.20 241 36.6 3.36 85 175

South Fiji Basin Seamounts Potassic suite P59786w 22.3 10.6 138 24 8.67 715 1.58 0.89 16.1 0.27 3.71 160 22.6 1.59 115 366 P59788g 20.5 12.5 126 9 8.59 756 1.78 0.95 15.6 na 3.08 193 24.2 1.72 91 429 P61717g 19.0 11.9 283 7 5.66 449 2.50 0.67 8.15 na 2.11 49 18.5 1.70 46 734 P61724g 46.7 12.6 110 15 8.41 918 1.23 0.95 29.3 na 6.76 119 25.3 2.00 53 326 P59769g 25.9 6.80 70.9 12 5.2 798 0.70 0.67 14.9 na 3.47 247 20.5 1.84 87 100 P63821w 75.7 16.6 33.6 28 11.8 1453 1.33 1.30 71.7 0.68 13.4 184 74.9 4.21 120 185 P63825w 29.2 7.85 52.0 39 6.85 967 0.61 0.72 16.1 0.26 3.17 228 18.5 1.58 71 74 Ocean island basalts P63830w 1.93 8.66 40.3 20 7.14 720 2.63 0.97 3.86 0.32 0.99 176 48.4 1.80 94 367 P63835w 4.63 10.9 38.2 16 7.73 1535 6.14 1.00 9.17 0.43 2.52 126 30.7 2.71 75 335 P67647w 2.68 5.80 19.3 35 6.24 570 1.09 1.29 1.64 0.96 2.52 245 93.5 6.17 179 121

Northland Plateau Poor Knights Seamount Chain P61712g 17.9 5.94 105 19 6.27 193 0.98 1.17 7.72 na 4.04 188 56.7 4.59 109 221 P66794w 17.8 1.78 16.5 31 3.17 197 0.16 0.65 3.42 0.32 0.29 101 17.6 2.00 155 137 P66824w 6.60 2.96 46.5 35 4.53 189 0.32 0.92 3.58 0.49 1.21 263 31.9 2.97 122 100 P66808w 6.44 7.27 47.4 34 6.06 493 1.00 0.91 6.48 0.50 2.38 259 43.6 3.05 128 190 P66825w 7.09 0.78 18.9 44 1.48 182 0.03 0.37 0.61 0.21 0.29 389 14.7 1.30 187 23

Outer Volcanic Plateau P63154m 14.0 2.47 13.0 25 2.68 279 0.65 0.47 2.13 0.24 0.62 246 15.4 1.54 194 168 P63162m 11.4 6.15 13.0 35 6.08 271 0.90 1.19 1.46 0.70 1.73 291 65.1 4.40 155 191 P63167m 2.97 5.10 34.7 22 6.19 263 0.41 1.21 1.16 0.69 0.27 134 58.5 4.37 111 201 P63165m 3.99 2.43 1.7 30 3.17 250 0.39 0.65 1.09 0.40 0.42 304 27.6 2.59 561 108 P63179m 2.16 4.54 15.6 40 4.92 350 0.39 0.95 0.17 0.45 0.67 197 36.2 2.72 175 118 P66800w 7.65 5.02 7.3 42 6.51 325 0.15 1.18 0.44 0.52 0.65 315 38.7 3.11 183 122 P66803w 47.3 4.63 51.9 31 5.18 456 0.27 0.72 6.19 0.29 1.41 226 20.8 1.77 144 85 P66807w 44.7 7.72 60.5 11 6.35 531 1.66 0.69 51.2 0.23 0.72 73 17.0 1.41 85 276

Havre Trough P63474m 1.91 2.49 8.3 35 3.51 256 0.84 0.70 0.62 0.39 0.15 295 26.2 2.55 72 92

Colville Ridge P63206 10 – 645– 38 ––b5 – 4 207 20 – 22 87

Continental shelf P66851w 17.8 8.00 98.4 11 7.47 171 0.95 1.16 15.6 0.58 2.50 17 38.4 3.71 64 334 P66854w 16.7 6.43 116 8 5.84 265 0.97 0.84 12.7 0.35 2.30 81 25.0 2.18 56 259

varies in width from 100–200 km and was divided by herein referred to as the Van der Linden Lineament or Herzer et al. (2000) into the Inner Sedimentary Basins VDLL) marks a fundamental change in the crust between (Whangaroa and Knights Basins) the Central Seamount the Inner Sedimentary Basins and the combined Poor Chain (Poor Knights Seamount Chain) and Outer Knights Seamount Chain–Outer Volcanic Plateau. The Volcanic Plateau. A prominent magnetic and gravity VDLL is parallel to, but not continuous with, the better lineament (Van der Linden “Fault” of Sutherland, 1999, known Vening Meinesz Fracture Zone (VMFZ), a left- N. Mortimer et al. / Marine Geology 237 (2007) 1–24 7 stepping dextral strike–slip fault system that separates the therein). To the north, the Colville Ridge becomes the Norfolk Basin from the Reinga Ridge (Herzer and Lau Ridge which, in turn, connects bathymetrically with Mascle, 1996). Fiji. The only dated lava from the entire Colville Ridge Sillimanite schists, rapidly exhumed in the Early is a 5.5 Ma (K–Ar whole rock age) arc basalt (Adams Miocene, have been dredged from Cavalli Seamount et al., 1994). At c. 32°S many features of the Colville- which lies near a jog in the continental shelf edge Lau Ridge change: to the north it becomes wider and between the Whangaroa and Knights Basins (Mortimer shallower and has a more northerly trend, and the Lau et al., 2003). Either side of Cavalli, the borders of the Terrace abuts the Minerva Plain (Fig. 1). Morphological Inner Sedimentary Basins against the continental shelf changes in the Kermadec Ridge and intervening Havre– and Poor Knights Seamount Chain (VDLL; Fig. 1 inset) Lau basins also occur at c. 32°S. are roughly linear and NW-trending; in contrast the Ballance et al. (1999) report a 7.8 Ma K–Ar age for a border of the Outer Volcanic Plateau with the South Fiji lava from the Kermadec Trench as well as Oligocene to Basin is very embayed. The western boundary of the Miocene microfossils from volcaniclastic sediments on Plateau (as drawn in Fig. 1) with the Three Kings Ridge the Kermadec Ridge. They interpret that, prior to inferred is somewhat arbitrary (magnetic and gravity trends of cessation of Colville volcanism and opening of the Havre the two features appear continuous with each other). The Trough at about 5 Ma, the formerly conjoined Kermadec– eastern part of the Northland Plateau is more obviously Colville ridge was volcanically active. transected by the NNE structural and volcanic trends of the Colville Ridge and Havre Trough (Fig. 1). 3. Methods

2.3. Three Kings Ridge Each rock dredge was sorted on board into broadly different rock types. Further examination, sawing and The Three Kings Ridge has generally been treated as sorting of rocks were done at GNS Science. About 130 a simple, narrow bathymetric feature extending north representative samples were selected for thin sectioning, from Northland, which separates the Norfolk and South micropaleontology and geochemical work. Prior to Fiji Basins (e.g. Davey, 1982; Ballance, 1999). Recent chemical analysis, manganese and weathering rinds gravity, seismic and bathymetric surveys have shown were removed to leave as fresh a sample as possible, and the ridge to be a much wider and more complex thin rock slabs were soaked in deionised water for a week composite feature (Herzer and Mascle, 1996; Herzer to remove seawater salt. No acid leaching was done. The et al., 2000; Bernardel et al., 2002; Sdrolias et al., 2004). rocks were crushed in a tungsten carbide ring mill; Strong north–south gravity lineaments are present that, analyses of quartz blanks crushed at GNS Science have based on the identification of the fault-bounded Cagou revealed no issues with Nb or Ta contamination for Trough and Weta Terrace, are of probable tectonic, normal sized samples (Appendix A, see also Roser et al., rather than volcanic, origin. The main ridge is flanked 2003). However, given the variable Nb/Ta ratios for the by the Fantail Terrace and Sarah Seamount group to the often very small samples in this study, we have not used east, and the Bates Plateau and Weta and Tuatara Ta in our geochemical interpretations. Major elements, Terraces (new names for the informal upper and lower loss on ignition, and As, Ba, Ce, Cr, Cu, Ga, La, Nb, Ni, Three Kings terraces of Herzer and Mascle, 1996) to the Pb, Rb, Sc, Sr, Th, U, V, Y, Zn and Zr were determined west. These latter terraces are basically fault-bounded for all samples by XRF (X-ray fluorescence) analysis at platforms between the crest of the Three Kings Ridge Spectrachem Analytical, Wellington. For the subset of and the abyssal Norfolk Basin (see Mortimer et al., 1998 samples shown in Table 1, Cs, Hf, Pb, Ta, Th, U, Zr and Fig. 3). Rock samples have been dredged from seven rare earth elements (REEs) were determined by ICP-MS sites on the Three Kings Ridge (Fig. 1) and consist of (inductively coupled plasma mass spectrometry) meth- serpentinite, Late Eocene boninite, Early Miocene ods at Washington State University, Pullman, and at andesite and Early Miocene shoshonite (Mortimer Grenoble and Montpellier Universities. Trace element et al., 1998; Bernardel et al., 2002; Sdrolias et al., 2004). ICP-MS analyses follow methods described in Knaack et al. (1994), Barrat et al. (1996) and Lapierre et al. (1997). 2.4. Colville Ridge Samples were dated by conventional argon step heating at the University of Santa Barbara using techniques similar The Colville Ridge is regarded as a classic remnant to Faulds et al. (1996). Samples were irradiated in the arc, separated from the active Kermadec arc by the cadmium-lined tube at Oregon State University in three Havre Trough (Ballance et al., 1999 and references separate packages for 3–8 MW-h. The irradiations were 8 N. Mortimer et al. / Marine Geology 237 (2007) 1–24 monitored with Taylor Creek Sanidine (Dalrymple and K-feldspar basalts, andesites and trachytes. Orange- Duffield, 1988) using an assigned age of 27.92 Ma. brown smectite replaces groundmass and olivine, and a Geochemical, electron microprobe and geochrono- K–Na zeolite is common in amygdules but despite the logical data are stored on the PETLAB database (http:// alteration, original phenocrysts are commonly preserved; data.gns.cri.nz/pet) and listed in Appendix A. Analyses (2) less porphyritic, commonly amygdaloidal, olivine of rock standards, silica blanks and detection limits are +plagioclase or aphyric basalts with chlorite+ calcite also presented in Appendix A. alteration assemblages. The distinction between these two petrographic suites is developed in Section 5.2 below. 4. Sample descriptions 4.3. Northland Plateau Samples and their locations are described in the ONSIDE I and II cruise reports (Shipboard Party, 1999; 4.3.1. Lavas and volcanic breccias Herzer et al., 2004a) and in Appendix A. In general, the The majority of lavas from the Northland Plateau are antiquity of the – mainly Miocene – rocks meant that plagioclase+pyroxene±olivine±biotite±hornblende almost all dredged samples were covered by Mn crusts, porphyritic basalts and/or andesites (Appendix A). The with Mn oxides also having penetrated joint planes and largest quantity of material was obtained from D21 grain boundaries. Zeolites, too, were almost always (Purerua Seamount). Small (b1 kg) pieces of Mn present (particularly in amygdules), as was orange crusted and altered lavas were obtained from un- smectite clay as an alteration product of glass and olivine. named volcanic peaks at D5, D6, D7 and D18 and apparent fault scarps at D19 and D20 (Fig. 1). 4.1. Minerva Abyssal Plain At sites D2 (Whangaroa Seamount), D4, D6, D14 and D25 analytical work was done on individual cm-sized The basaltic rocks penetrated at the bottom of Deep clasts in volcanic breccias. The polymict nature of these Sea Drilling Project (DSDP) 205 and 285A have been breccias, on slopes far from canyon systems, suggests to described by Churkin and Packham (1973) and Stoeser us they are fault scarp talus sampling a section through a (1976). Plagioclase and olivine in the dolerites are fresh volcanic pile, and are therefore representative of the local but glassy groundmass is somewhat altered to smectite in situ geology. D2 material consists of palagonite and celadonite. The Julia dredge site (Fig. 1) is an east- breccia with clasts of plagioclase+clinopyroxene basalt. facing fault scarp from which a suite of altered basaltic Zeolite is present in amygdules. Petrographic examina- lavas and basaltic breccias was recovered. A range of tion of seven lava samples from dredge D6, at the edge of textural types was present, from dolerite to chilled basalt. the Northland Plateau indicated the presence of at least Secondary alteration to chlorite+actinolite+epidote two different suites of lavas. The dominant set (P63164, assemblages is common. The Alison dredge site 66, 67) was like most of the other Northland Plateau (Fig. 1; new name) is on a volcanic rise in the seafloor lavas in that they contain the phenocryst assemblage east of the Loyalty Ridge. Samples from Alison are plagioclase+pyroxene+biotite and have minor smectite slightly altered olivine+plagioclase+pyroxene porphy- and zeolite alteration. However P63165, 68, 72, from the ritic pillow basalts. Some pillows have glassy rinds. same dredge, contain just plagioclase phenocrysts and have epidote+sericite+carbonate alteration. These lavas 4.2. South Fiji Basin seamounts were also slightly harder to crush. Carbonate alteration in a D6 rhyolite clast, P63169, suggests a correlation with Altered lavas, volcaniclastic sedimentary rocks, and the latter basalt–andesite suite. some limestones were recovered from three seamounts in the Sarah and Devonport seamount groups, and from 4.3.2. Polymict volcano-sedimentary breccias Margot, Marion and Coquille seamounts. All these At sites P70, D9A and B, D11, D12, D16 and D17 seamounts are in the western part of the South Fiji Basin significant proportions of breccias containing clasts other and appear to be constructional volcanoes. Small than basaltic–andesitic lavas were recovered in the dredges amounts of Mn-crusted lavas were also dredged from (Appendix A). In the case of D9, D11 and D12 extremely Mascarin and Matahourua, two large seamounts that rise soft and porous altered rhyolitic pumice breccias dominate. from the eastern Kupe Abyssal Plain. Breccia from D9B contains three sorts of clast: (1) cpx Petrographically, the lavas can be divided into two +plag+hbl lavas of the sort common on the Northland groups: (1) highly porphyritic, variably amygdaloidal cli- Plateau volcanic peaks (described above); (2) rarer nopyroxene+plagioclase+FeTi oxide±olivine±biotite± epidotised lava and dolerite; (3) a single clast of quartz– N. Mortimer et al. / Marine Geology 237 (2007) 1–24 9 plagioclase granofels. Breccia from D9A additionally chemical interpretations. In interpreting the dredged contained greywacke, actinolitised gabbro and individual lavas, we have placed most emphasis on elements clastic grains of reddish garnet and biotite. In all cases, generally considered to be less mobile during secondary secondary quartz and epidote alteration preceeded (re-) alteration e.g. Ti, Y, Zr, Nb, V, Th, REE. deposition. Greywacke clasts and/or isolated pebbles have also been found at dredge sites D16 (The Slab) and at 5.1. Minerva Abyssal Plain P70. Large, well-rounded basalt and dolerite pebbles at D17 resemble those found in the D9 breccias. The All analysed samples from Minerva Abyssal Plain polymict breccias are not considered further in this paper. are subalkaline basalts with moderate TiO2 contents All dredges thus far made on Cavalli Seamount have yielded exclusively plutonic and high grade metamorphic rocks. Rocks from the ONSIDE I cruise have been described in detail by Mortimer et al. (2003).

4.4. Other sites

A basalt from the central volcanic ridge of the Havre Trough (Sonne 135–36 dredge site; Haase et al., 2002)is a relatively fresh and unaltered black vesicular, plagioclase–porphyritic basalt. The only new material obtained from the Colville Ridge for this study (OR99 dredge site) is hydrothermally altered sparsely porphyritic andesite. Two dredges (D24A, B) on a spur of the continental shelf near Cavalli Seamount yielded a variety of porphyritic basalts, andesites and rhyolites.

5. Geochemistry

Following petrographic examination, representative lavas were chosen for geochemical analysis (see Table 1 and Appendix A). The emphasis was on analysing petrographically different assemblages to characterise different igneous suites at each location. In some cases, lack of sample amount meant that only one lava from each dredge site could be analysed. As mentioned above, most rocks contain some secondary zeolite, smectite clay, phosphate, manganese oxide and/or calcite. Smectite alteration imparts red and yellow hues to the rocks. We selected the least altered samples for geochemical analysis. Relative to typical Northland Plateau lavas, a manganese crust from site D7 is enriched in Fe, Mn, P, LOI, As, REE, Pb, Nb, Y, Zn and Zr and depleted in Si, Cr, Rb, Sc, U and Th (P63180, Appendix A). Our previous experience with pre- Quaternary submarine lavas is that P, Ca, Rb, U and K contents can also be particularly unreliable in terms of primary values (Mortimer and Parkinson, 1996). Even after careful sample preparation, some analysed samples were found to have significant MnO, P O and Fig. 2. Binary diagrams showing broad compositional ranges of 2 5 dredged rocks. A. Nb/Y versus Zr/Ti (Pearce, 1994); B. SiO versus loss on ignition contents (N0.2, N1 and N2 wt.% 2 K2O; C. SiO2 versus TiO2. All elements normalised to anhydrous respectively). Where other material was available from values. Havre Trough and Northland Arc reference fields from data in the same dredge, such samples were not used for geo- Middleton (1983), Ruddock (1990) and Gamble et al. (1993). 10 N. Mortimer et al. / Marine Geology 237 (2007) 1–24

(Fig. 2). Normalised multi-element patterns generally the abyssal samples from this study, as is P57025 from show flat shapes with downward slopes at the large ion the east edge of the Norfolk Basin (Figs. 1 and 3A; lithophile element (LILE) end (Fig. 3A). None of the Mortimer et al., 1998). These have been interpreted as samples show convincing Nb anomalies, as might be backarc basin basalts (BABBs). expected if there had been significant subducted slab involvement in their petrogenesis. Likewise, low Ba/Nb 5.2. South Fiji Basin seamounts and moderate Ti/V are consistent with mid-ocean ridge basalt (MORB)-like compositions (Fig. 4). 5.2.1. Less porphyritic (ocean island) basalts The glass and holocrystalline rock analyses from the Lavas from Matahourua, Mascarin, Marion, Coquille FAUST-2 dredge site in the Cook Fracture Zone and dredge SF9801-D3 on Margot have the highest reported by Bernardel et al. (2002) are very similar to TiO2 and Nb contents of the entire sample set and are

Fig. 3. Multielement diagrams for selected representative samples, and some P number samples from Mortimer et al. (1998), normalised to primitive mantle (Sun and McDonough, 1989) values. Cs, Rb, U, Ta, K, Pb and P have been omitted in order to give a clearer visual representation of primary igneous patterns. N. Mortimer et al. / Marine Geology 237 (2007) 1–24 11

incompatible element abundance patterns have straight- ish negative slopes, steeper than that of the OIBs except for Nb and Ti troughs, and LILE peaks (Fig. 3C). These latter features are indeed similar to shoshonites and further suggest a partly subduction-related origin. The shoshonitic nature is confirmed on the Ba vs Nb plot (Fig. 4). Electron microprobe analyses of clinopyrox- enes (Appendix A) also reveal the typically high Ca+Na and low Ti of shoshonites (Mortimer et al., 1998). The most primitive lava in the shoshonitic suite is a Sarah West lava which contains Cr spinel, lacks biotite and has 5.6 wt.% MgO and 249 ppm Cr. The most evolved lava is a biotite+sanidine porphyritic trachyte from Devonport East seamount. The inferred primary potassium content of these lavas (estimated by extrap- olation of normalised REE trends on Fig. 3C) far exceeds that of Clark Volcano in the modern Kermadec arc (Gamble et al., 1997).

5.3. Northland Plateau

Unlike for the South Fiji Basin there is no simple match between geographic site, petrography and chemistry for Northland Plateau lavas. Basalts and andesites from the Poor Knights Seamount Chain (Purerua, Whangaroa and P70) and the Outer Plateau (D4 lava, three clasts from D6, D18-20) all plot in and/or parallel to the field of Northland Fig. 4. Binary diagrams showing variation in chemistry between different Arc lavas on an SiO2 vs TiO2 diagram (Fig. 2). On a multi- areas. A. Ba vs Nb (Perfit et al., 1980); B. Ti vs V (Shervais, 1982). element normalised diagram many basalts and andesites show distinctive Nb and Ti troughs and relatively high concentrations of LILEs typical of subduction-related interpreted as ocean island basalts (OIBs) (Figs. 2 lavas (Fig. 3D). Compositions range from a very depleted and 4). The lavas also show convex-up mantle-normal- plagioclase-porphyritic (accumulative?) basalt from Pure- ised patterns, with peaks at normalised Nb (Fig. 3B; rua (D21) to very LILE-enriched hornblende trachyande- Matahourua and Mascarin lavas are strongly contami- site at D19. Some trachyandesites (P70 and D19) are nated by phosphate which increases heavy REE). Binary probably high-K to shoshonitic but, because of LILE diagrams show the typical low Ba/Nb and high Ti/V mobility we cannot be sure they are shoshonites. ratios of OIB suites (Fig. 4). Possibly the most LILE-depleted lavas on the Northland Plateau are present at the eastern, outer 5.2.2. More porphyritic (potassic) lavas region at sites D6 and D7 (Fig. 3E). However the LILE Lavas dredged from Devonport East and Central, concentrations of these D6 clasts and D7 lava are Sarah North, Sarah West, Sarah Central and dredge comparable to those reported for the active northern SF9801-D2 on Margot, are trachybasalts and trachyan- Kermadec–Tonga arc (Haase et al., 2002), Nb and Ti desites and distinctly different in composition to the anomalies are still evident, and the light REE contents OIBs. As shown below they belong to shoshonitic rock are atypical of MORB and backarc basin basalts. Like series (Morrison, 1980). These lavas have high to most of the rest of the Northland Plateau, we interpret extreme total alkali contents, largely due to high the D6 and D7 lavas as subduction related. potassium, coupled with low TiO2 contents (Fig. 2). A single, highly altered lava from the edge of the Given the almost ubiquitous smectite and zeolite Plateau at D5 is a subalkaline basalt (Fig. 2). It has LILE alteration, all of the high K contents cannot be assumed enrichment, and high Zr and Hf concentrations but no Nb to be primary igneous values, but the presence of biotite depletion. This is the only non-arc type lava dredged phenocrysts confirms a potassic suite. The normalised from the Northland Plateau and is plotted with the 12 N. Mortimer et al. / Marine Geology 237 (2007) 1–24 petrologically similar OIBs of Kupe Abyssal Plain in positional range of the Northland Arc and Three Kings Fig. 3B. Ridge (Fig. 3F).

5.4. Other sites 6. 40Ar/39Ar geochronology

The Havre Trough basalt is an important backarc Where present, fresh biotite and hornblende were basin reference with which to compare our other lavas. dated from porphyritic lavas, fresh plagioclase was A normalised multi-element plot of the SO135-36 basalt dated from other porphyritic lavas, and groundmass shows a negative Nb anomaly together with Ti to Yb concentrates were dated from unaltered aphyric basalts. normalised values slightly lower than MORB (Fig. 3A). Plagioclase separates typically had very low K/Ca ratios Overall, the lava is similar (except in age, see Section 6) and particular attention was paid to monitoring the to some of the less depleted abyssal plain basalts. tailing correction in order to give accurate ages. Results The porphyritic basalts, andesites and rhyolite from are summarised in Table 2 and gas release spectra from D24 on the Northland continental shelf edge show a selected samples are shown in Fig. 5. In the text and range of high-K arc-like compositions (Figs. 2, 3F Table 2 2σ errors are reported (note only 1σ error bars and 4). In this respect they bracket the entire com- are plotted in Fig. 5). For almost all plagioclase ages, our

Table 2 Ar–Ar ages (Ma) of lavas analysed at University of California Santa Barbara (analysts P.B. Gans and A.T. Calvert) GNS # Location Rock Mat. UCSB # K/Ca TF age Plateau age Isochron age Preferred age Minerva Abyssal Plain P63847 DSDP205 Basalt Plag SB50-07 0.004–0.006 26.4 25.9±0.6 26.2±2.9 26.0±1.0 P63849 DSDP285A Dolerite Plag SB50-05 0.011–0.013 23.1 22.7±0.3 22.7±1.6 22.8±0.4 P63850 DSDP285A Dolerite Plag SB36-45 0.004–0.005 22.0 21.9±0.8 21.5±2 21.9±0.7 P68211 Julia Basalt clast Plag SB52-25 0.005–0.007 21.8 22.1±1.8 22.2±2.4 22.1±1.8 P68215 Alison Basalt Plag SB50-04 0.002–0.003 19.2 19.3±1.3 19.4±3.3 19.3±1.5

South Fiji Basin seamounts Potassic suite P59772 Devonport Central Trachyandesite Biot SB18-72 40–209 20.7 20.7±0.1 20.8±0.2 20.7±0.05 P59788 Devonport Central Trachyandesite Biot SB18-73 53–148 20.8 20.7±0.1 20.6±0.1 20.7±0.05 P61717 Devonport East Trachyte Biot SB24-15 56–195 20.0 19.9±0.1 19.9±0.1 19.9±0.1 P61724 Margot Trachyandesite Biot SB24-17 34–78 20.5 20.5±0.1 20.5±0.1 20.5±0.1 P59769 Sarah West Basaltic andesite wr SB18-74 0.46–0.48 21.0 21±2 15.5±1.2 21±2 P63825 Sarah North Basaltic andesite Plag SB36-53 0.025–0.040 20.2 20.4±0.22 21.8±0.6 20.6±0.5

Ocean island basalts P63830 Marion Trachybasalt Plag SB36-49 0.017–0.023 16.1 16.2±0.2 16.3±0.3 16.2±0.2 P67646 Matahourua Basalt wr SB50-12 0.014–0.075 14.7 14.1±0.2 15.1±0.4 15±2

Northland Plateau P61712 Poor Knights Chain P70 Trachyandesite Plag SB24-16 0.023–0.024 19.5 19.7±0.2 19.5±0.3 19.7±0.2 P66825 Poor Knights Chain D21 Basalt Plag SB50-16 0.001 19.2 19.2±2.1 18.6±6 18.5±4 P66794 Poor Knights Chain D14 Basaltic andesite Plag SB50-22 0.005 14.9 14.7±0.6 14.6±1.5 14.7±1.0 P63153 Outer Plateau D4 Basalt clast Plag SB31-04 0.005–0.007 18.5 17.9±0.8 18.0±1.4 17.9±0.8 P63158 Outer Plateau D4 Andesite clast hbl SB31-05 0.028–0.030 21.1 21.9±0.3 22.6±0.3 21.9±0.3 P63165 Outer Plateau D6 Bas andesite clast Plag SB31-06 0.004 19.9 20.1±1.9 19.5±3.6 20.1±1.9 P63166 Outer Plateau D6 bas trachyand clast Plag SB31-07 0.007 16.1 17.6±0.9 – 17.6±0.9 P63179 Outer Plateau D7 Basalt Plag SB31-08 0.008 36.6 22±9.3 – 22±9 P66802 Outer Plateau D20 Andesite hbl SB50-84 0.015–0.030 31.6 31.7±0.2 31.5±0.8 31.7±0.2 P66803 Outer Plateau D19 Basalt Plag SB52-23 0.003–0.006 20.0 20.4±0.8 21.1±1.8 21.1±1.8 P66807 Outer Plateau D19 Trachyandesite Biot SB50-20 100–400 20.4 20.4±0.04 20.4±0.07 20.4±0.05 P66798 Outer Plateau D25 Andesite Plag SB50-09 0.008–0.012 2.5 1.04±0.8 1.4±1.4 1.2±0.8

Havre Trough P63474 Havre Trough SO135-36 Basalt wr SB31-09 0.021–0.068 1.3 1.12±0.38 0.76±0.64 1.1±0.4 Errors are 2 sigma. Mat = material; bas = basalt; plag = plagioclase; biot = biotite; wr = whole rock; hbl = hornblende; TF = total fusion. N. Mortimer et al. / Marine Geology 237 (2007) 1–24 13

Fig. 5. Selected Ar–Ar spectra. Error bars are ±1 sigma without error in the J parameter. cited preferred age error is greater than analytical un- irregular only in the high temperatures of gas release certainty, a conservative approach that we consider (Fig. 5). Different samples from DSDP285A are within appropriate given the ultra low K/Ca of the samples. error of each other and also within error of the (less precise) Julia age (Table 2). Plagioclase from Marion 6.1. Minerva Abyssal Plain and South Fiji Basin seamount also gives a good, flat, spectrum that is seamounts distinctly younger than the abyssal DSDP samples. The higher K/Ca from the Marion OIB gives a more precise Plagioclase separates from the DSDP drillcores and age than from the abyssal tholeiites (Fig. 5). There is no Julia dredge site give good flattish, spectra that become compelling evidence to suggest that the crystallisation 14 N. Mortimer et al. / Marine Geology 237 (2007) 1–24 age of the plagioclases is anything older than the ages which means it could be younger, and less precise, than cited in Table 2. the plateau age reported by Mortimer et al. (1998).We Biotite from the Devonport East trachyte and Margot reinterpret the crystallisation age for the basalt, as the trachyandesite (Fig. 5) both yielded plateaus in slightly isochron age of 15.8±3.4 Ma. U-shaped argon release spectra. For both these shoshonitic samples, highly precise inverse isochron ages of 7. Discussion 19.9±0.1 and 20.5±0.1 Ma respectively were obtained using 100% of the gas. 7.1. Age of South Fiji Basin ocean crust

6.2. Northland Plateau 7.1.1. Minerva Abyssal Plain DSDP 205 was drilled in crust interpreted to be of Lavas from the Poor Knights Seamount Chain gave anomaly 12 (c. 31 Ma) age. Our Ar–Ar age of 26±1 Ma Ar–Ar plagioclase ages of 19.7±0.2 Ma (P70; possibly for the DSDP205 basalt suggests that this magnetic shoshonitic) and 18.5±4 Ma (D21, Purerua; Table 2). interpretation requires modification. Our basalt age is, These are essentially coeval with the time of rapid however, in agreement with Churkin and Packham's cooling and exhumation of the nearby Cavalli Seamount (1973) interpretation of pillow basalt eruption contem- schists (Mortimer et al., 2003). poraneous with, and/or injection of basalt into, unlithi- Hornblende from an andesite dredged from Outer fied late mid Oligocene nanno ooze (c. anomaly 10 age). Plateau site D20 gave the oldest age from the Northland DSDP 285 was drilled in crust of interpreted anomaly Plateau (31.7±0.2 Ma; Fig. 5). Ages of plagioclase 8 (c. 26 Ma) age (Davey, 1982; Fig. 1). As with DSDP from nearby D19 basalt give 21.1±1.8 Ma and of 205, our Ar–Ar ages of 22.8±0.4 and 21.9±0.7 Ma for biotite from a D19 trachyandesite (possibly shoshoni- samples from DSDP 285 are significantly younger than tic), 20.4± 0.05 Ma. Two separate breccia clasts from the inferred magnetic anomaly picks (Figs. 1 and 6). In Outer Plateau volcanic peak D4, gave ages of 17.9± DSDP 285A, the actual basalt-sediment contact was not 0.8 Ma (plagioclase) and 21.9±0.3 Ma (hornblende). observed but was inferred to possibly be intrusive Two clasts from the D6 breccia were dated: plagioclase (Stoeser, 1976). We re-interpret the contact relations as from an epidote-altered basaltic andesite gave 20.1± early Middle Miocene sediments deposited on a thick 1.9 Ma and plagioclase from a zeolite-altered por- Early Miocene basalt with a chilled flow top. The basalt phyritic trachyandesite 17.6±0.9 Ma. These ages would be proper oceanic crust basement not a later, off- overlap but the more altered rock has a permissibly axis sill. Thus although we have not re-examined the older age. The arc basalt from Outer Plateau D7 gave core, our new Ar dates are consistent with the reported a very imprecise plagioclase age of 22±9 Ma. The fossil ages from both DSDP sites. youngest Miocene age was given by plagioclase from Further evidence that the Minerva spreading centres D14 from the eastern Northland Plateau: although the persisted into the Miocene and did not cease at spectrum is quite irregular, the age of 14.7±1.0 Ma is anomaly 7 (c. 25 Ma) comes from the 23 Ma age of a distinctly younger than the other Northland Plateau basalt from the Cook Fracture Zone (Fig. 6; Meffre lavas (Fig. 5). et al., 2002; Sdrolias et al., 2004), 22.1±1.8 Ma age from the Julia Lineament basalt and 19.3±1.5 Ma age 6.3. Other sites from the Alison dredge (this paper). An MORB from the east edge of the Norfolk Basin abyssal plain has an Havre Trough SO135-36 basalt is essentially unal- age of 19.8±0.8 Ma (Mortimer et al., 1998). Thus five tered. A whole rock (groundmass) separate gave a low out of the six sites at which MORB and/or BABB precision but distinctly young age of 1.12±0.38 Ma age. lavas have been sampled in the South Fiji and Norfolk A low seamount on the easternmost Northland Plateau, Basins have given Early Miocene ages, and DSDP 205 D25, gave a plagioclase age of 1.2±0.8 Ma. Although it has given a Late Oligocene, not an Early Oligocene, is physically part of the Northland Plateau, the young age (Fig. 6). age must be regarded as an expression of westernmost Colville volcanism. 7.1.2. Kupe Abyssal Plain We have reinterpreted an age reported in Mortimer There are no dredge or drill samples of abyssal ocean et al. (1998). This is a whole rock age from P57142, an crust from Kupe Abyssal Plain (Fig. 6). Local minimum OIB from a seamount in the South Norfolk Basin. We ages for various parts of the Kupe Plain are provided by now judge the sample to possibly have excess argon the OIB and shoshonite ages from Devonport Central, N. Mortimer et al. / Marine Geology 237 (2007) 1–24 15

Fig. 6. Summary of the age and composition of lavas in the Norfolk Ridge–New Zealand–Colville Ridge area. New ages are in bold type; other data from Mortimer et al. (1998), Ballance et al., 1999, Hayward et al. (2001), Meffre et al. (2002) and Sdrolias et al. (2004). See Fig. 1 for geographic names and abbreviations.

Margot, Sarah West and Sarah North (21 Ma), Devon- tion of the Northland Plateau by CANZ (1997), Herzer port East (20 Ma), Marion (16 Ma), and Matahourua et al. (2000) and Stagpoole (2002), the linear magnetic (15 Ma) seamounts. Devonport East sits astride, and anomalies from Kupe Abyssal Plain appear to penetrate amplifies the westernmost interpreted linear magnetic up to 100 km into the Northland Plateau (Fig. 1), anomaly in Kupe Plain (Fig. 1) which has been although their sizes increase and their shapes change interpreted as either anomaly 7 (c. 25 Ma; Davey, radically; Sdrolias et al. (2003), perhaps justifiably, stop 1982; Malahoff et al., 1982) or anomaly 12 (c. 31 Ma, them at the foot of the Northland Plateau. The acoustic Sdrolias et al., 2003). With the more accurate delinea- basement and volcaniclastic aprons of the 18–22 Ma 16 N. Mortimer et al. / Marine Geology 237 (2007) 1–24 volcanoes at Outer Plateau sites D4 and D7 merge with possible high-K to shoshonitic lavas are present at D19, the southernmost Kupe oceanic crust (Herzer et al., D24 and P70. Prior to the discovery of the South Fiji 2000). Collectively, these points lead us to believe that Basin seamount shoshonites reported here, Mortimer the matching of the Kupe anomalies to simple abyssal et al. (1998) interpreted the two shoshonites from the spreading patterns of Oligocene age is highly suspect. Three Kings and Norfolk Ridge to have been emplaced A critique of existing Kupe magnetic anomaly above the backmost part of a single, west-facing, Late interpretations is beyond the scope of this paper but Oligocene–Early Miocene arc that subsequently mi- our new radiometric ages and seismic stratigraphic grated rapidly east in response to Pacific trench retreat. interpretations suggest that at least some of the Kupe Our discovery of shoshonites east of the Three Kings Plain opened as the Northland Plateau grew, i.e. in the Ridge potentially complicates this interpretation. Early Miocene. Shoshonitic volcanism has been reported from a If Sdrolias et al. (2003) are correct that the Julia number of tectonic settings (Morrison, 1980). In the Lineament is a fossil spreading centre then our 22.1± case of the intraoceanic South Fiji Basin seamounts we 1.8 Ma basalt from there could approximate the age of can probably discount origins involving melting of, or cessation of a Kupe spreading centre arm. However in significant contamination by, continental crust. The the sparse seismic and bathymetric data (Davey, 1982), HFSE-depleted nature of the lavas points to a partly updated by several recent multibeam echo sounding and subduction-related origin either: (1) in the backmost part seismic crossings of the feature (Herzer et al., 2005), the of a mature arc system (Morrison, 1980; Kepezhinskas, Julia Lineament more closely resembles a fault scarp 1995); (2) associated with arc deformation in the (see also Fig. 7). The true origin of the lineament will termination of a subduction phase, including subduction only be revealed with further surveys. flip or slab breakoff (Morrison, 1980; Davies and von Blanckenburg, 1995); (3) rifting of a volcanic arc (Stern 7.2. Significance of shoshonitic lavas et al., 1988; Gill and Whelan, 1989; Kepezhinskas, 1995). Shoshonitic lavas have now been dredged from eight The presence of high-K to shoshonitic rocks both of the sites in Fig. 6 (inverted triangle symbols) and west and east of the Three Kings Ridge means that, until tectonic features are better defined by seismic work, it is inadvisable to use any of the shoshonites as evidence for local subduction polarity as was done by Mortimer et al. (1998). Their apparent short 19.9–20.7 Ma time range of eruption, and association with both OIBs and low- medium K suites (Fig. 6), leads us to invoke arc-rifting models for their petrogenesis. In the Marianas and Fiji, like in the South Fiji Basin-Northland Plateau region, shoshonites erupt shortly after the start of arc rifting and/ or basin opening and extend across the entire arc and into bordering basins.

7.3. Nature of the Northland Plateau

Samples recovered from the Northland Plateau in this study consist mainly of inferred flows and breccias of subduction-related basalts, andesites and rhyolites that range in age from 32–14 Ma (with most in the 22– 19 Ma age range). The 32 Ma lava is from a rifted terrace at the junction of the Outer Northland Plateau and eastern Three Kings Ridge (Herzer et al., 2004b). At Fig. 7. Vector calculation of Vening Meinesz Fracture Zone (VMFZ) no site were exclusively fine-medium grained sedimen- slip rates at c. 20 and 16 Ma based on possible hotspot trails in the tary rocks dredged, as might be expected for an South Fiji Basin. CFZ = Cook Fracture Zone. Same area as Fig. 1 with shaded predicted bathymetry background (Stagpoole, 2002). 67± accretionary prism. Greywacke and dolerite clasts are 5 mm/yr Early–Middle Miocene Australian-hotspot plate motion from present in breccias at the D9 and P70 dredge sites on the McDougall and Duncan (1988). Poor Knights Seamount Chain, and high grade schists N. Mortimer et al. / Marine Geology 237 (2007) 1–24 17 have been obtained from Cavalli Seamount (Mortimer or tectonic juxtaposition of formerly separate arcs (see et al., 2003). Section 8 below). Thus the acoustic basement of the Outer Northland Plateau and much of the Poor Knights Seamount Chain 7.5. Hotspot migration trails and basin opening rates appears to be fundamentally a constructional volcanic feature with Late Oligocene and Early Miocene The present day half spreading rate for the northern subduction-related basalts, andesites and rarer rhyolites Lau Basin is c. 50 mm/yr (Taylor et al., 1996). This is exposed in several places. Magnetic and gravity comparable to the calculated half spreading rate for anomalies, along with morphostructural features, indi- Minerva Plain between DSDP 205 and 285 of c. 50 mm/ cate that the Plateau northeast of the Van der Linden yr, measured perpendicular to magnetic anomalies using Lineament (Fig. 6) is a probable along-strike continua- our new argon dates (see Section 7.1.1 above). It is tion of the Three Kings Ridge, and that Colville Ridge instructive to compare these values with bracketed structural and magmatic features have been super- spreading rates for the Norfolk Basin and Kupe Plain, imposed on its eastern part. The continent–ocean crust based on various assumptions. Excluding the rifted boundary could lie close to the Van Der Linden Line- terraces east and west of the Three Kings Ridge, the ament, but its precise position has been obscured by widths of both the eastern Kupe Plain and South Norfolk Oligocene–Miocene volcanism. Basin are c. 300 km. The opening of each could thus be There is thus no compelling evidence, at present, to achieved with a (not unreasonable) half spreading rate of suggest that the Poor Knights Seamount Chain and 50 mm/yr in only 3 m.y. Outer Plateau comprise (a) thickened or overthrust Kupe The E to ENE elongation of Devonport–Margot, Abyssal Plain backarc crust without arc-related volca- Mascarin, Matahourua and Marion seamounts is nics; (b) a sedimentary accretionary prism; (c) an reminiscent of backarc basin migration trails of the offshore continuation of the Late Cretaceous–Oligocene kind described for the Havre Trough by Wright et al. Northland Allochthon (see also Herzer et al., 2000)or (1996). OIB and potassic lavas with possible astheno- (d) a trapped piece of Cretaceous Hikurangi Plateau spheric sources have been sampled from these Kupe (Mortimer and Parkinson, 1996). Plain seamounts which raises the possibility that they can be treated as very short-lived, scattered hotspot 7.4. Widespread Early Miocene volcanism trails. Fig. 7 is a simple analysis of these putative short hotspot tracks assuming that a rigid Kupe microplate Prior to the present study and that of Mortimer et al. moves solely in response to northward motion of the (1998), known Early Miocene subduction-related vol- Australian Plate and to ESE motion along the VMFZ, canism in the area of Fig. 6 was restricted to onland both relative to a fixed mantle hotspot frame. Decreas- northern New Zealand (the “Northland Arc”; Ballance ing VMFZ slip rates of c. 100–67 mm/yr are suggested et al., 1982; Brothers, 1986; Spörli, 1989). Mortimer et in the interval 21–15 Ma (Fig. 7). If the Kupe Plain was al. (1998) showed that at least parts of the Three Kings fully open during this time (Malahoff et al., 1982; Ridge and Norfolk Basin consisted of Early Miocene Sdrolias et al., 2004) then the 15–16 Ma figures need andesites, shoshonites, abyssal tholeiites and alkali only match the opening rate, and age, of the Norfolk basalts. The present study and that of Bernardel et al. Basin. However if Kupe Plain was also opening in the (2002) has still further expanded the known area of Miocene then a more complicated analysis is required. Early Miocene lavas to include Cook Fracture Zone, Some Norfolk Basin spreading took place at 20– Minerva Abyssal Plain, Julia Lineament, all seamounts 23 Ma (the ages of backarc tholeiites reported by thus far sampled rising from Kupe Plain, and much of Mortimer et al., 1998 and Sdrolias et al., 2004), though the Northland Plateau. Ballance et al. (1999) from fossil there was probably extension and asymmetric rifting of evidence show that the Kermadec and Colville Ridges the Three Kings and southern Loyalty Ridges to form were sites of volcaniclastic deposition in the Early their western flanking terraces and plateaus, prior to this. Miocene. Fiji also has an Early Miocene arc volcanic There are few direct constraints on when Norfolk Basin record (Wharton et al., 1995). spreading ceased but Herzer et al. (1997) note that The presence of Early Miocene subduction-related synsedimentary deformation along the VMFZ continued lavas in a vast, non-linear 400×300 km region enclosing through into the Middle Miocene (16–11 Ma). Available the Northland Plateau and onland Northland (Fig. 6) evidence is thus consistent with our kinematic analysis demands an explanation. This distribution has been connecting the ages and shapes of the Kupe Basin achieved either by unusual plate tectonic processes and/ seamounts with hotspot tracks related to VMFZ 18 N. Mortimer et al. / Marine Geology 237 (2007) 1–24 movement and Norfolk (and possibly Kupe) Basin were erupted onto the Northland Allochthon (Spörli, opening. 1989; Hayward et al., 2001); (4) throughout the Cenozoic, New Caledonia was much further from the 8. Tectonic models Pacific-Australia pole of rotation than New Zealand meaning that the total amount of trench rollback and There exists a variety of tectonic models of the width of backarc opening would have been significantly Cenozoic evolution of the New Zealand–New Caledo- less near the New Zealand pivot point (Sutherland, nia–Fiji area. Many of these (e.g. Ballance et al., 1982; 1999; Ballance, 1999). Brothers, 1986; Spörli, 1989; Malpas et al., 1992)predate any significant offshore work. Those that do use offshore 8.2. Northeast-dipping subduction? data are largely based on swath bathymetry and potential field datasets, and also use the New Caledonia obduction New Caledonia models use an inferred NE-dipping model of Aitchison et al. (1995) as a key part of offshore subduction zone to explain the overthrust geometry of interpretations near New Zealand (e.g. Crawford et al., the ultramafic nappe (Cluzel et al., 1994, 2001; 2003; Sdrolias et al., 2003; Schellart et al., 2006). Aitchison et al., 1995; Mauffret et al., 2002; Lagabrielle et al., 2005 and references therein). The arc generated by 8.1. New Caledonia vs New Zealand this east-dipping subduction is believed to be the Loyalty Ridge which lies c. 150 km NE of the Both New Caledonia and the Northland area of New allochthon front (Fig. 8A). Crawford et al. (2003) and Zealand can be regarded as emergent parts of the Schellart et al. (2006) have continued this model south Norfolk and Reinga Ridges. From the Late Cretaceous into the New Zealand area and regard the Northland to Eocene, much of this strip of Gondwanaland accreted Allochthon emplacement and Three Kings Ridge– terranes was a submerged, continental borderland. Northland Plateau volcanism as being due to the final Along and outboard of this borderland, the Poya and shutting down of this NE-dipping subduction system Tangihua basalts were erupted in submarine basins and near New Zealand at the end of the Oligocene. were subsequently thrust over continental crust in both The alternative model for Northland Allochthon onland areas. It could be argued, given the still- emplacement involves crustal delamination of a west- submerged nature of much of the Zealandia continental dipping Pacific slab (e.g. Spörli, 1989; Rait et al., 1991; crust, that the thrusting/obduction events were confined Malpas et al., 1992) possibly involving a triple junction to these two geographic areas, i.e. because it is still (Bradshaw, 2004). A west-dipping slab model readily submerged, none of the intervening Norfolk Ridge has explains the presence of the 26 Ma shoshonitic breccias been overthrust. on the Norfolk Ridge (Mortimer et al., 1998) and the Significant differences in the geological history of linear 23–15 Ma Northland arc chain that erupted on the New Zealand and New Caledonia include: (1) the continent and allochthon. The absence of thick ultramafic obduction event in New Caledonia took place in the sheets in the Northland Allochthon does not necessarily Late Eocene (37–34 Ma) (Cluzel et al., 1994, 2001; demand a lithosphere-penetrating megathrust. Aitchison et al., 1995; Lagabrielle et al., 2005) but in Our present dating and petrological study provides Northland the allochthon was emplaced during the no new information on the origin or tectonic mode of the Waitakian stage (25–22 Ma) (Spörli, 1989; Isaac et al., 25–22 Ma emplacement of the Northland Allochthon 1994); (2) the uppermost New Caledonia nappe (see Bradshaw, 2004 for a recent discussion), nor on the comprises a thick section of mantle peridotites whereas age of the Northland Allochthon basalts (Whattam et al., the Northland Allochthon comprises mainly basalts and 2005). We accept that both NE-subduction and flake sedimentary rocks (Spörli, 1989; Malpas et al., 1992; tectonics models are possible and the first two panels of Isaac et al., 1994). The only significant peridotite mass Fig. 8 are a summary of the two end-member options. is at North Cape (Fig. 6); (3) the post-Eocene history of Note that in both models the Norfolk Basin is closed, or New Caledonia is one of very minor magmatism and nearly closed, at 27 Ma. great distance from a plate boundary (Cluzel et al., 2001, 2005; Lagabrielle et al., 2005) whereas the Northland 8.3. Preferred model Allochthon emplacement coincided with inception of the Alpine Fault as the major Australia-Pacific plate The main themes in our 26–15 Ma tectonic model boundary (Sutherland, 1999) and, during but largely (Fig. 8C–F) are: (1) driving force of a single, west- following thrusting, abundant Miocene arc volcanics dipping, retreating Pacific slab for post-Northland N. Mortimer et al. / Marine Geology 237 (2007) 1–24 19

Fig. 8. Schematic model showing possible stages in development of the South Fiji Basin and surrounding ridges and plateaux. 20 N. Mortimer et al. / Marine Geology 237 (2007) 1–24

Allochthon Miocene volcanism and basin opening near 8.4. Comparison with other models New Zealand; (2) progressive southward propagation of backarc basin opening in the Oligocene–Miocene from Mortimer et al. (1998) ignored the South Fiji Basin in (in order) the North Loyalty, Minerva, Kupe and finally their simple model of Miocene Norfolk Basin and Three Norfolk Basins; (3) rapid, tandem Early Miocene Kings Ridge development. The present work, based on opening of the Norfolk Basin and Kupe Plain, and new data from the South Fiji Basin and environs, significant co-eval opening of the Minerva Plain; (4) remedies that. We agree with Ballance et al. (1982), local doubling up of the width of the Early Miocene arc Brothers (1986), Spörli (1989), Malpas et al. (1992) and as the Three Kings–Northland Plateau segment was Ballance (1999) that a west-dipping Pacific slab model is tectonically juxtaposed outboard of the Northland feasible for the Miocene volcanism, albeit accompanied segment along the VMFZ and VDLL. by rapid tectonic and magmatic change offshore in the In the 25–22 Ma interval the Northland Allochthon 23–19 Ma interval. Crawford et al. (2003) and Schellart was emplaced onto Northland continental crust at et al. (2006) have proposed NE-dipping subduction as a bathyal depths. Spreading from the North Loyalty mechanism, not just for Northland Allochthon emplace- Basin propagated south into Minerva Plain (DSDP ment, but also for Three Kings–Northland Plateau sites) and into western Kupe Plain (beneath the slightly volcanism. In this scenario, the Kupe Plain shoshonites younger Sarah seamounts). 23 Ma tholeiites at the would probably be interpreted as backmost-arc, collision eastern end of the Cook Fracture Zone and exhumation and/or slab-breakoff lavas (Morrison, 1980; Davies and of serpentinites in the Cagou Trough (Bernardel et al., von Blanckenburg, 1995). However, unlike at New 2002; Sdrolias et al., 2004) may indicate incipient Caledonia latitudes, the Poya–Tangihua backarc basin Norfolk Basin spreading and/or Three Kings Ridge formed near New Zealand may not have been wide rifting. Features such as the Cagou Trough, Tuatara and enough to make enough subducted slab to generate 12 m. Weta Terraces and related prominent north–south y. of volcanism on the Northland Plateau; what Eocene– lineaments (Fig. 1) are plausibly extensional rift features Oligocene subduction there was near New Zealand was rather than aligned volcanoes (Mauffret et al., 2002). likely “amagmatic” (Ballance, 1999). If a NE-dipping slab 21–20 Ma was a period of rapid tectonic change existed (e.g. to emplace the Northland Allochthon), it accompanied by voluminous magmatism. The Minerva could possibly have been undercut and replaced by a spreading centres propagated into the Norfolk Basin and dominant Pacific slab from 23 Ma. The volume and eastern Kupe Plain. Rhomboidal opening of the Norfolk continuum of onland Northland volcanism from 23– Basin took place along the Cook and Vening Meinesz 15 Ma seems more consistent with a gradual SE migration Fracture Zones: movement on the latter, cutting the of one west-dipping, east-retreating subduction system magmatic arc at a shallow angle, started to shuffle the (Fig. 8B–F) than a subduction flip during this time. Three Kings–Northland Plateau segment of the Three With west-dipping slab models, there has been Kings–Northland Plateau–Northland Arc outboard of disagreement as to the orientation of the Pacific trench the Northland segment. Rift related shoshonites in that generated the onland Miocene Northland arc. western Kupe Basin and Weta Terrace erupted in this Ballance et al. (1982) proposed a NW-striking trench short time period, concomitant with fast slip on the with a switch to a NNE trend in the Late Miocene. In VMFZ. Arc volcanism waned on the Three Kings Ridge contrats, Brothers (1986) advocated a retreating trench and it became a remnant arc. In what is now onland essentially striking subparallel to the present NNE Northland the active arc magmatism started to shift east Kermadec trend. Our model of Norfolk and Kupe Basin with the retreating trench. At c. 20 Ma, rapid exhumation opening in Fig. 8 provides a possible compromise from 10 km depth of the schists of the probable explanation in that inception of onland Northland arc metamorphic core complex of Cavalli Seamount (Fig. volcanism at 23 Ma was formed by Pacific Plate 1; Mortimer et al., 2003) was a response to rapid Kupe subduction along a NW-striking trench whereas by Basin opening and/or VMFZ/VDLL movement. 15 Ma, Pacific Plate subduction was along a NNE- The 19–15 Ma interval saw final spreading in the striking trench because of eastward movement of the Norfolk Basin and seamount-free parts of eastern Kupe Colville arc and the Northland Plateau past onland Plain. By this time, active arc volcanism had migrated Northland. In a more complex model, Ballance (1999) south and east of Auckland. From about 15 Ma, rapid postulated three separate Miocene trenches to generate trench retreat ceased, the Australia-Pacific plate margin the Three Kings, Colville and Northland arcs. As shown stabilised and the NNE–SSW Colville arc trend was in Fig. 8, we regard these as possibly rapidly rifted and established through the North Island (Herzer, 1995). dispersed segments of the same, single, east-facing arc. N. Mortimer et al. / Marine Geology 237 (2007) 1–24 21

The Norfolk Basin appears to be one of the youngest Swath bathymetric and detailed seismic surveys of of the Oligocene–Miocene backarc basins and it is the region are desirable in order to fully assess the rift, unusual that the Norfolk Basin opened so far behind the transform or trench significance of the structures Pacific trench. Sdrolias et al. (2004) have invoked a indicated by the Fantail, Weta and Tuatara Terraces, mantle plume mechanism as a possible driving force for the Cagou Trough, the Julia and Van der Linden Early Miocene Norfolk Basin opening, additional to Lineaments and Cavalli Seamount. lithospheric tensional stresses due to trench rollback. Our study lends some support to this idea as we have 9. Conclusions shown that, at Norfolk Basin latitudes, normal subduction-related volcanism was occurring at least at 32 Ma, New analytical data from altered volcanic rocks shoshonitic volcanism was largely confined to the 20– rocks dredged or drilled from 38 sites between the Havre 21 Ma period, and OIBs were widespread at 15–16 Ma. Trough, Three Kings Ridge and Northland Plateau Thus there may be a trend to tap deeper mantle sources provide useful information on the tectonic evolution of with time. We go one step further than Sdrolias et al. Cenozoic arcs and basins in the SW Pacific Ocean. (2004) and suggest that many of the elevated features of Early Miocene (18–22 Ma) volcanism is widespread the Norfolk Basin (e.g. Kingston Plateau, Nepean and includes a variety of different petrological suites. Saddle) could be thick piles of Miocene OIB eruptives, New Ar–Ar dates on abyssal tholeiites suggest that the as seems to be indicated by existing dredges (Fig. 6; crust of much of the Minerva Abyssal Plain of the Mortimer et al., 1998). northern South Fiji Basin may be Early Miocene in age, not Oligocene as previously interpreted using magnetic 8.5. Further tests anomalies. Subduction-related lavas, identified by their LILE enrichment and HFSE depletion are present on the The biggest data gap in the region is knowledge of Northland Plateau. Shoshonitic lavas occur on both the age of crust and spreading directions in Kupe sides of the Three Kings Ridge and are interpreted to Abyssal Plain. Longstanding and recent magnetic have formed during ultra-rapid arc rifting at 20–21 Ma. anomaly interpretations suggest an Oligocene (anomaly A range of models can be used to explain our results, 7–12) age but with controversy about younging including microplate arrangements with NE-dipping directions and geometry (Malahoff et al., 1982, Davey, slabs. However, we believe that from the Early Miocene 1982; Sdrolias et al., 2003). Our radiometric dating of (c. 23 Ma), the simplest model consist with the onland- lavas (this paper) and seismic stratigraphic constraints offshore data is one of eastward rollback of a single arc– (Herzer et al., 2000) instead suggest an earliest Miocene trench system controlled principally by a west-dipping age but are based on features not directly associated with Pacific slab. abyssal ocean crust. The geometric–kinematic relationship between the non-parallel Cook and Julia linea- Acknowledgements ments, and the seamount elongation (Fig. 7) remains enigmatic. If an Oligocene age for Kupe Plain can be We thank the Captain and crew of the R/V Tangaroa, independently confirmed, then our single Early Miocene and our colleagues Jean Mascle, Bryan Davy, Etienne west-dipping Pacific slab model (Fig. 8)mustbe Ruellan, Dan Barker, Kim Rose, Anya Duxfield and abandoned as the Three Kings arc to Pacific trench Steve Wilcox for assistance with dredging on the gap of N500 km would be unreasonably large. We ONSIDE I and II cruises. Cornel de Ronde, the scientific regard establishing the age of different parts of Kupe staff and crews from the Sonne 135 and 167 cruises, Plain as a critical test for discriminating between our Orange Roughy Management Ltd., the National Institute model and that of Crawford et al. (2003). of Water and Atmospheric Research and the staff of the Knowing whether there is, or is not, Cretaceous ODP archive in San Diego supplied additional samples oceanic crust in the Norfolk Basin is also important for from the Havre Trough, Colville Ridge, Northland the subduction polarity issue. The presence of a Poya– Plateau and Minerva Plain. Valuable technical support Tangihua Cretaceous basin is a key test of the Crawford was provided by Neville Orr, John Simes, John Hunt, et al. (2003) model (Fig. 8A), whereas no Cretaceous Charles Knaack, Diane Johnson, Martin Wong, Belinda crust is predicted by the combined Kermadec–Norfolk Smith Lyttle and Lorraine Paterson. We have had useful (“Nordec”) Arc model (Fig. 8B). Dredging and direct discussions with Rupert Sutherland, Peter Ballance, dating of lavas or sedimentary rocks in the deepest parts Philippa Black, Sebastian Meffre, Bryan Davy, Dan of the Norfolk Basin is sorely needed. Barker, Mike Isaac, Tony Crawford, Bernhard Spörli and 22 N. Mortimer et al. / Marine Geology 237 (2007) 1–24

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