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Magmatic and Tectonic Interactions Revealed by Buried
Volcanoes in Te Riu-a-Māui/Zealandia Sedimentary Basins
Journal:
New Zealand Journal of Geology and Geophysics
Manuscript ID
Draft
Manuscript Type:
Research Paper
Date Submitted by the
Author:
n/a
Complete List of Authors:
Bischoff, Alan; University of Canterbury,
Barrier, Andrea; University of Canterbury,
Beggs, Mac; University of Canterbury
Nicol, Andrew; GNS Science, Active Landscapes
Cole, Jim; University of Canterbury, Department of Geological Sciences
Sahoo, Tusar; GSN Science, Petroleum Geoscience
Keywords:
buried volcanoes, seismic reflection, Te Riu-a-Māui/Zealandia, rift
magmatism, intraplate magmatism, subduction-related magmatism,
tectonics
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1Magmatic and Tectonic Interactions Revealed by Buried Volcanoes in Te
2Riu-a-Māui/Zealandia Sedimentary Basins
3Alan Bischoffa*, Andrea Barriera, Mac Beggsa, Andrew Nicola, Jim Colea and
4Tusar Sahooab
5a School of Earth and Environment, University of Canterbury, Christchurch, New Zealand;
6b GNS Science, Wellington, New Zealand
7 alan.bischoff@canterbury.ac.nz
8 *corresponding author
9Abstract
10 Many volcanoes are buried and preserved within Te Riu-a-Māui/Zealandia sedimentary
11 basins. This paper presents the location, age, morphology and characteristic eruptive
12 styles of some representative of these volcanoes, based on interpretation of a large
13 collection of 2D and 3D seismic reflection datasets and petroleum exploration drillholes.
14 Their morphology, spatio-temporal distribution, and type of plumbing system are
15 primarily controlled by the tectonic setting and stress regime operating at each volcano.
16 Late Cretaceous volcanoes commonly formed large (>20 km3) volcanic complexes and
17 composite cones concurrently with, or immediate after, rifting and breakup of eastern
18 Gondwana. The Cenozoic volcanoes are both intraplate and subduction-related. Intraplate
19 volcanoes typically form clusters of scattered small-volume (<1 km3) submarine craters
20 and cones. Subduction-related mainly comprise large deep-submarine to subaerial
21 composite and shield volcanoes. In detail, the presence of pre-existing crustal structures
22 and types of enclosing host rocks also play important control in the location and
23 distribution of magma in the upper crust. This study demonstrates the value of detailed
24 characterization of volcanoes buried in sedimentary basins for evaluating the interplay
25 between magmatic activity and plate tectonics, which can help us understand the
26 formation and evolution of volcanoes on the entire Zealandia continent and globally.
27 Keywords: buried volcanoes; seismic reflection; Te Riu-a-Māui/Zealandia; rift
28 magmatism; intraplate magmatism; subduction-related magmatism; tectonics
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29 Introduction
30 Te Riu-a-Māui/Zealandia is a region of 4.9 Mkm2 of continental crust consisting of Cambrian-
31 Early Cretaceous basement overlaid by Late Cretaceous-Cenozoic sedimentary and igneous
32 rocks now largely (ca 95%) submerged in the Pacific Ocean (e.g. Cole et al. 1981; Laird and
33 Bradshaw 2004; Mortimer et al. 2017, 2018). Over the last 150 years, geological mapping and
34 studies of igneous rocks from onshore New Zealand, New Caledonia and several surrounding
35 small islands has provided an enormous amount of information about syn-and post Gondwana
36 breakup volcanism (e.g. Hochstetter 1867; Marshall 1932; Suggate 1978; Cole 1986; Ballance
37 1993; Mortimer and Scott 2020). More recently, onshore information has been supplemented
38 by oceanographic data from dredging the sea-floor above submarine seamounts, ridges and
39 plateaus across Zealandia (e.g. Timm et al. 2010; Bache et al. 2014a; Mortimer et al. 2018;
40 Hoernle e et al. 2020). These complementary datasets provide the basis for current geological
41 models explaining the Late Cretaceous-Cenozoic magmatic activity in Zealandia (e.g. Laird
42 and Bradshaw 2004; Hoernle et al. 2006; Timm et al. 2010; van der Meer et al. 2017; Mortimer
43 et al. 2018). However, to date, only few studies have had incorporated information on buried
44 volcanoes into these models (e.g. Herzer 1995; Uruski et al. 2019; Barrier et al. 2020a;
45 Mortimer and Scott 2020).
46 From coastal exposures, geologists have long suspected that large eroded volcanoes could
47 extend beneath the New Zealand continental shelf and possibly beyond. It was not until the
48 1960’s that igneous rocks was first documented within sedimentary strata offshore New
49 Zealand. Initially, recognition of these buried igneous systems was inferred by remote sensing
50 techniques such as seismic reflection, gravimetric and magnetometric surveys acquired during
51 petroleum exploration campaigns (e.g. Hatherton 1968; Knox 1982; Hayward 1987). Oil and
52 gas discoveries within the onshore Taranaki Basin stimulated further offshore exploration,
53 including the acquisition of new seismic surveys and drillholes in the Taranaki, Canterbury and
54 Great South basins (Figure 1). Many of these drillholes encountered volcanic and plutonic rocks
55 of various ages confirming an igneous origin of the anomalies recognised from geophysical
56 data (e.g. Milne 1975; Field et al. 1989; Bergman et al. 1992). Today, an increasing quality and
57 coverage of seismic surveys, associated to significant advances in computer processing and
58 seismic interpretation software are allowing detailed characterization of igneous rocks buried
59 in sedimentary basins (e.g. Schofield et al. 2016; Planke et al. 2017; Bischoff et al. 2017). New
60 Zealand’s freely available exploration datasets (e.g. seismic reflection lines and drillholes) has
61 enabled application of a systematic methodology for establishing the locations, geometries and
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62 timing of a variety of igneous-dominated structures belonging to diverse magmatic systems (i.e.
63 rift, intraplate, and arc Figure 2).
64 This work showcases the utility of exploration geophysics, and especially 3D seismic reflection,
65 for determining the architecture of buried volcanoes both in detail and as discrete components
66 of basin fill. Here, we review existing knowledge (Field et al. 1989; Herzer 1995; King and
67 Thrasher 1996; Giba et al. 2013; Bischoff 2019; Barrier 2019; Uruski 2019) and add new
68 information for Late Cretaceous to Pleistocene volcanoes buried and preserved within
69 Zealandia’s sedimentary basins. We focus mainly on reviewing the morphology, spatial
70 distribution and timing of the buried volcanoes, as well as their relationship with tectonic events
71 that shaped Zealandia in the last 105 million years. Due to space constraints we do not attempt
72 to systematically review the geochemistry literature for igneous rocks in Zealandia, which
73 references can be found in Johnson (1989), Timm et al. (2010) and Mortimer et al. (2018).
74 Instead, we present the best understood examples of buried volcanoes identified on 2D and 3D
75 seismic reflection imagery from a range of areas in the northern and southern Zealandia (Figure
76 1). Additional volcanoes are likely buried in other parts of Zealandia (e.g. Norfolk Ridge, Lord
77 Lowe Rise, and west of the Puysegur Trench), but because of limited seismic and drillhole
78 coverage, these areas were excluded from this study. Tables 1 and 2 show the nomenclature
79 and main references for the volcanoes presented here. Supplementary Material 1 presents a
80 glossary for the volcanic morphological terms used in this paper. Additional information is
81 available in Supplementary Materials 2 to 10.
82 Geological Background
83 Zealandia is an extensive area of mainly submerged continental crust around the New Zealand
84 and New Caledonia landmasses (Mortimer et al. 2017). For most of the Mesozoic, Zealandia
85 formed the active convergent eastern margin of the Gondwana supercontinent, separating from
86 eastern Australia and Antarctica in the Late Cretaceous (e.g. Laird 1993; Sutherland et al. 2001;
87 Mortimer 2004). An initial phase of crustal extension (ca 105-83 Ma) leading to continental
88 breakup gave rise to several rift basins throughout Zealandia. These basins were subsequently
89 filled with Late Cretaceous and younger strata up to 10 km thick. On the New Zealand landmass
90 and in near-shore areas (i.e. <200 km from the modern coast) these basins were affected by
91 Eocene to Recent plate boundary deformation associated with the Hikurangi and Puysegur
92 subduction zones, and dextral strike-slip transpression along the Alpine Fault (e.g. King et al.
93 1999; Lebrun et al. 2003; Nicol et al. 2007; Stagpoole and Nicol 2008; Figure 1 and 2).
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94 Most sedimentary basins within Zealandia have experienced recurrent and widespread igneous
95 activity (e.g. Field et al. 1989; King and Thrasher 1996; Laird and Bradshaw 2004; Sahoo et al.
96 2015). Late Cretaceous-Cenozoic magmatism formed numerous volcanic zones, complexes,
97 fields and centres, which are part of the Zealandia Megasequence and are grouped into the
98 Rūaumoko Volcanic stratigraphic unit. These volcanoes are broadly classified as having an
99 intraplate (Horomaka Supersuite) or a subduction-related (Whakaari Supersuite) origin
100 (Mortimer et al. 2014). Igneous activity related to oceanic sea-floor spreading, large igneous
101 provinces, rifting, and hotspot tracks is also reported in Zealandia and southwest Pacific region
102 (Mortimer and Scott 2020).
103 In and around New Zealand landmass, intraplate volcanoes are predominantly basaltic-alkaline
104 in composition, typically forming clusters of scattered small-volume volcanoes (<1 km3), and
105 less common large polygenetic volcanic systems not associated with plate boundaries or hotspot
106 tracks. This include those of Waiareka-Deborah and Auckland Volcanic fields (e.g. Coombs et
107 al. 1986; Cole 1986; Kereszturi and Németh 2016), as well as the large volcanic complexes that
108 shaped the Otago and Banks Peninsula (e.g. Coombs et al. 1960; Sewell 1988; Weaver and
109 Smith 1989).
110 In contrast to intraplate volcanoes, subduction-related magmatism formed linear, mainly calc-
111 alkaline volcanic arcs extending from oceanic crust north of Zealandia onto continental crust
112 towards the North Island of New Zealand (Figure 1 and Figure 2). These arc volcanoes formed
113 in association with the evolving convergent plate boundary between the westward subducting
114 Pacific Plate and the overriding Australian Plate, which may have been active from the Late
115 Eocene (e.g. Cole et al. 1981; King 2000; Mortimer et al. 2010; Seebeck et al. 2014; Sutherland
116 et al. 2017). The arcs broadly show migration of eruptive centres Pacific Plate to SE, formerly
117 extending along the Three Kings Ridge to Northland region and offshore western New Zealand
118 (ca 25-17 Ma), and from the Lau-Colville Ridge across Coromandel Peninsula to the
119 Mohakatino Belt (ca 16-4 Ma). The youngest modern arc (ca 2-0 Ma) extends southwards from
120 the Tonga-Kermadec trend into the Bay of Plenty and Taupo Volcanic Zone (e.g. Hayward
121 1987; Gamble et al. 1993; Kear 1994; Herzer 1995; Mortimer et al. 2010; Seebeck et al. 2014;
122 Illsley-Kemp et al. 2019; Figure 1 and Figure 2).
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123 Data and methods
124 Data used in this paper is primarily from six decades of oil and gas exploration of New
125 Zealand’s sedimentary basins. The data has been mainly sourced from the 2018 Petroleum and
126 Minerals Exploration Data pack, which includes a large collection of reports, maps, drillhole
127 records, 2D and 3D seismic surveys, and major interpreted chronostratigraphic surfaces loaded
128 into Kingdom© and ArcGIS© software. The spacing of the seismic lines used for the
129 interpretation of buried volcanoes is typically <2 km, with 3D datasets having line spacing of
130 no more than 12.5 m (Figure 1). Vertical resolution is usually a few tens of meters, depending
131 on the area, quality of the surveys, and seismic frequencies within the dataset. Drillhole data
132 commonly include wireline-logs together with lithological, geochemical, geochronological, and
133 biostratigraphic information from cutting samples and sometimes from drilling cores. In
134 addition to these drillhole and seismic data, we have compiled information from an extensive
135 range of publications, which are referred in each specific section of this paper.
136 Buried volcanoes have been identified using a combination of seismic reflection and drillhole
137 data interpretation. The age of the volcanic rocks in the subsurface is rarely determined by
138 radiometric dating, and was primarily derived by mapping chronostratigraphic surfaces that
139 correlate seismic anomalies possibly of igneous origin with biostratigraphic markers identified
140 in the drillholes. Chronostratigraphic mapping follows seismic and sequence stratigraphic
141 principles such as stratal reflection relationships and depositional trends within seismic facies
142 (e.g. Mitchum et al. 1977; Hunt and Tucker 1992; Catuneanu 2010), giving time resolution in
143 the order of ca 1 Myr in many New Zealand sedimentary basins. In addition, we have
144 undertaken seismic volcano-stratigraphic analysis on the buried volcanoes by mapping their
145 pre-eruptive and post-eruptive surfaces, following principles described in e.g. Herzer (1995),
146 Planke et al. (1999), Giba et al. (2013), Reynolds et al. (2016), Bischoff (2019) and Barrier
147 (2019). Interpretation of the environment in which the eruptions occurred was determined by
148 plotting the location of volcanoes on paleogeographic maps of similar age (e.g. Arnot and Bland
149 2016), and/or by calibration with paleoenvironmental data obtained from microfossils from a
150 variety of drillholes across the studied areas.
151 Detailed characterization of the buried volcanoes is based on criteria such as the geometry,
152 internal and external configuration of seismic reflections, deformation of enclosing strata, and
153 stratal relationship within the seismic images (Mitchum et al. 1977), followed by analysis of
154 the volcanic and sub-volcanic architecture (Bischoff et al. 2017, 2019 a, b, and c). Best results
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155 were obtained by applying a range of techniques such as seismic attribute analysis, opacity
156 rendering of the seismic signal, decomposition and merging of an array of frequencies of the
157 seismic wavelength spectrum, and 3D mapping of geobodies with similar seismic response (e.g.
158 Chopra and Marfurt 2005; Marfurt 2018; Bischoff and Nicol 2019).
159 With the exception of the Maahunui Volcanic Field, in which the morphological analysis
160 estimates the effects of compaction and erosion (Bischoff et al. 2019b), the height of the buried
161 volcanoes presented here are inferred from their present day morphology (i.e. after erosion and
162 compaction during burial). As demonstrated in Bischoff et al. 2019b, these estimations are only
163 approximations, but reasonable variations do not affect the final interpretation. Thus, the
164 present day height of the volcanoes now buried in the subsurface (i.e. the relief) was estimated
165 by recording the distance between the pre- and post-eruptive surfaces in two-way-travel (TWT)
166 in seconds, multiplied by the estimated acoustic velocity of the volcanic rocks within the
167 seismic anomaly. Massive and unaltered basalts typically have acoustic velocities between
168 5000-7000 meters per second (m/s), while non-welded pyroclastic rocks range around 3000 m/s
169 (e.g. Planke et al. 2000; Holford et al. 2012; Klarner and Klarner 2012). These values were
170 calibrated with data available in the literature (e.g. Schutter 2003; Nara et al. 2011; Millett et
171 al. 2015; Heap and Kennedy 2016; Reynolds et al. 2016; Heap et al. 2017; Cant et al. 2018;
172 Bischoff 2019), and when possible, with velocities of volcanic rocks recorded in wireline-logs
173 from drillholes penetrating the buried volcanoes (e.g. Milne 1975; Field et al. 1989; Rad 2015.
174 According their morphology, each volcanic edifice was approximated as a cone, a trapezoidal
175 prism, or as a spherical cap to roughly estimate the minimal volume of erupted material.
176 Northern Zealandia
177 Vulcan Volcanic Group – Late Cretaceous
178 Vulcan Volcanic Group comprises seven large Late Cretaceous volcanoes, as well as several
179 small vents and numerous intrusive bodies presently buried beneath ca 4000 m of sedimentary
180 strata in the Deepwater Taranaki Basin (Figure 1-3). This group of volcanoes has been referred
181 to in petroleum reports and literature after gods and goddesses of Greek mythology, as well as
182 after common New Zealand sheep breeds. In this paper, we follow this nomenclature where
183 possible, and extend it to describe volcanoes not previously named (Table 1).
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184 Late Cretaceous volcanic activity in this part of the Deepwater Taranaki Basin was initially
185 inferred by interpretation of seismic reflection lines (McAlpine 1999; Uruski and Warburton
186 2010). In 2014, the Romney-1 drillhole penetrated a sequence of volcanic rocks interbedded
187 with coal measures of Haumurian age (83.6 to 66 Ma), as well as sandstones and mudstones of
188 the Rakopi Formation, confirming a volcanic origin of the anomalies observed in seismic
189 imagery. These volcanic rocks are overlain by shallow marine sandstones and siltstones of the
190 latest Cretaceous North Cape Formation, indicating that eruptions occurred near paleo-
191 shoreline (Schiøler et al. 2014). Drill cuttings were initially identified as moderately- to highly-
192 altered tuffs and epiclastic rocks of basaltic composition (Rad 2015), but our petrographic, SEM
193 and EDS analysis indicates that both mafic and felsic (possibly alkaline) rocks are present in
194 the Romney-1 drillhole (Supplementary Material 2).
195 Vulcan Volcanic Group erupted in two phases (Zhu et al. 2017). Eruptions in Chronos mark the
196 onset of volcanic activity in the area at ca 85 Ma. The present day morphology of this volcano
197 is semi-conical, 4.5 km wide and ca 900 m high. Rhea erupted shortly after Chronos, also forms
198 a semi-conical edifice at least 7 km across and ca 1400 m high (Figure 3 andFigure 4). Magma
199 composition of Chronos and Rhea volcanoes are not known as neither has been drilled. The
200 large volume of each edifice (>20 km3) suggests that they were constructed by multiple eruptive
201 cycles, rather than a single volcanic episode. This inference together to the width vs. height
202 ratio of the volcanoes (certainly >10⁰ inclination) indicates that they have had an architecture
203 similar to stratocones, possibly comprising interbedded lavas and pyroclastic flows deposits,
204 and volcanogenic sedimentary rocks (e.g. Cas and Wright 1992; Manville et al. 2009; de Silva
205 and Lindsay 2015). Both volcanoes are aligned in a NE orientation parallel with the
206 contemporaneous trend of the Zealandia Rift (Strogen et al. 2014).
207 The onset of the second eruptive phase is marked by eruptions in the Coopworth Volcanoes at
208 ca 74 Ma, shortly followed by activity in the Romney Volcanic Field (Figure 3 andFigure 4).
209 Coopworth comprises two main small-volume cones and minor scattered vents. The northern
210 volcano is smaller (1000 m across and 160 m high) than the southern (1800 m across and 200
211 m high). Next to the vents, a distinctive, sub-horizontal, continuous and high-amplitude
212 reflection form a semi-circular apron that extends as far as 8 km from the eruptive vents. A
213 decreasing seismic amplitude from the vent towards the basin indicates changes in the density
214 of material (e.g. Herzer 1995; Reynolds et al. 2017; Bischoff et al. 2017), in the case of
215 Coopworth Volcanoes, probably corresponds to the transition of low viscosity lava-flow
216 deposits and basin sedimentary strata (Figure 4). The small relief edifices are associated with
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217 large aprons of high amplitude (i.e. dense) material, and are possibly the product of Hawaiian
218 and/or Strombolian eruptions that formed scattered spatter cones and extensive lava fields, a
219 characteristic observed in basaltic monogenetic fields (Németh 2010). The amalgamation of the
220 pre- and post-eruptive surfaces of these volcanoes with increasing distance from the eruptive
221 centres indicates that Coopworth volcanoes had short-lived activity (Figure 4).
222 Romney Volcanic Field comprises a large shield volcano, as well as numerous (possibly
223 hundreds) of scattered small-volume vents, and several shallow intrusions typically of saucer-
224 shaped morphology (Figure 3c and Figure 5). The large shield volcano is 15 km across and ca
225 400 m high as preserved in the basin. This shield volcano has two main eruptive centres, one at
226 its summit, and the other offset towards the NW, as well as diverse small parasitic vents erupted
227 on its flanks. Many of these parasitic vents show a preferential orientation parallel with normal
228 faults (Figure 5b), which is commonly observed in volcanoes associated with rifting and
229 extension (e.g. Galland et al. 2014; Vries and Vries 2018; Burchardt et al. 2018). The
230 extremities of the shield volcano show high-amplitude reflections with spreading toe-like
231 terminations (Figure 5c), a morphology commonly associated with both Pahoehoe and a′ā lava
232 flows sourced in Hawaiian eruptions (Gregg 2017).
233 In contrast to the shield volcano, the scattered small-volume vents of the Romney Volcanic
234 Field typically show cone-type morphology with extensive flat-lying aprons of high-amplitude
235 reflections that possible correspond to spatter cones and related lava fields (Figure 3) . However,
236 a small number of crater-type volcanoes also occur, suggesting that at least some may
237 correspond to tuff-rings or maar-diatremes formed by high-energy pyroclastic eruptions. The
238 location of both cones and craters, as well as observed in the shield volcano, shows preferential
239 structural control, evident by cone alignments and crater rows of NW-SE to NE-SW orientation
240 and parallel with regional normal faults of the West Coast-Taranaki rift (Strogen et al 2015;
241 Figure 3c). In the western part of the field, there is a narrow NE trending structure that resemble
242 a fissure vent (Figure 5a). This structure is also located along a normal fault, reinforcing the
243 interpretation of volcanic activity simultaneously with extension, as commonly described in rift
244 volcanoes (Sigmundsson et al. 2018; Neal et al. 2019). Dome-like structures are also seen in
245 seismic imagery. The drillhole Romney-1 penetrates the NW flank of one of these domes that
246 probably contain at least some felsic and alkali rocks (Supplementary Material 2), which may
247 suggest eruptions of lava-domes sourced by relatively viscous magmas.
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248 Volcanic activity in Hades Caldera occurred simultaneously or immediately after eruptions in
249 Romney Volcanic Field, at around 73 Ma. In plain view, Hades has a semi-circular structure 10
250 km across and ca 800 m high with a central depression 3.5 km wide and 1 km deep bounded by
251 ring faults (Figure 3d). In cross-section, localized small displacement (<0.02 TWT) normal
252 faults mark the centre of the structure, while both flanks dip away from the centre and display
253 sets of parallel, continuous and mod-amplitude reflections (Figure 4). This morphology
254 indicates two volcanic processes: (i) the bedded circular flanks suggest that material was spread
255 relatively uniformly and deposited in tabular layers around the vent, as common of high energy
256 pyroclastic eruptions (Kereszturi and Németh 2013), while (ii) the central depression and ring
257 faults are frequently observed in explosive and non-explosive calderas volcanoes (Cole et al.
258 2005; Martí et al. 2008). The moderate amplitude and continuous reflections on Hades flanks
259 suggests fragmented material rather than dense lava-flow deposits, which are generally
260 characterised by high-amplitude reflections (e.g. Planke et al. 2000; Holford et al. 2012;
261 Bischoff et al. 2019b). These observations collectively suggest that Hades was a Late
262 Cretaceous explosive caldera volcano.
263 Vulcan and Hestia are two large long-lived composite volcanoes that interfinger with basin
264 strata deposited over at least 8 Myr (Figure 3a). This temporal relationship is evident by
265 interbbeding of high-amplitude reflections of the Hestia edifice, with mod-amplitude and
266 continuous reflections that correspond to sedimentary strata of the North Cape Formation
267 (Figure 4). Vulcan is 25 km across and ca 1500 high, while Hestia is 22 km across and ca 1000
268 m high. Volcanic activity started in Vulcan at ca 74 Ma and ended while Hestia was still active,
269 which is confirmed by the western flank of Hestia overlapping the eastern flank of Vulcan
270 (Figure 4). The first stage of eruptions in Hestia likely produced low viscosity lavas that extend
271 up to 30 km from the eruptive centre (Figure 4). These eruptions formed amalgamated lava-
272 channels complexes such as those observed in Kilauea Volcano in the 2018 eruptions (Bischoff
273 2019). Volcanic activity ceased in Hestia at around 68 Ma, marking the end of volcanism in the
274 Vulcan Volcanic Group. The magma composition and eruptive mechanisms that constructed
275 Vulcan and Hestia remain poorly calibrated in absence of drillholes, but possibly include
276 pyroclastic and lava-flow deposits interbedded with each other and with volcanogenic
277 sedimentary rocks, as common of large long-lived composite volcanoes (Manville et al. 2009;
278 de Silva and Lindsay 2015).
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279 Kaiwero Volcanic Field – Eocene
280 The Kaiwero Volcanic Field is a cluster of >50 scattered Eocene volcanoes and several shallow
281 (<1 km) intrusions currently buried by 300 to 1500 m of sedimentary strata offshore western
282 North Island, New Zealand (Figure 6). The name comes from the Māori for the large submarine
283 plateau (Challenger Plateau) adjacent to the area were the volcanoes are located (Figure 1 and
284 Table 1). Volcanism in the Kaiwero Volcanic Field occurred over an area of ca 17,000 km2,
285 extending from the NE boundary of the Challenger Plateau towards the SW Deepwater
286 Taranaki Basin. At the location of the field, there are no drillholes providing physical evidence
287 of a volcanic origin of the seismic anomalies. Volcanic rocks of Late Eocene age are reported
288 in western Challenger Plateau (Nelson et al. 1986; Carey et al. 1991, Wood 1991; apud Uruski
289 2019), and from several drillholes in the western Taranaki Basin (e.g. Amokura-1, Patele-1 and
290 Kiwi-1), where they are marked by bentonite clays of the Turi Formation, referred to in
291 petroleum exploration reports as “the ash unit”.
292 The seismic expression of these volcanoes indicates mound and cone morphologies, typically
293 of high-amplitude reflections immediately overlying the early Eocene chronostratigraphic
294 horizon (Figure 6b and c). Above these mound- and cone-like anomalies, reflections usually
295 show a dome configuration, a feature commonly interpreted to be formed by differential
296 compaction between volcanic and sedimentary rocks (e.g. Planke et al. 2005; Holford et al.
297 2017; Bischoff et al. 2017). Internal configuration includes chaotic, rubble and bedded seismic
298 facies dipping away from the centre of the structure, which likely represent material deposited
299 proximal to a vent zone (e.g. Holford et al. 2012; Reynolds et al. 2016; 2017). These mounds
300 and cones are frequently associated with a single peripheral sub-horizontal and high-amplitude
301 reflection up to 8 km across, possible material deposited distal to the vent. Below the pre-
302 eruptive surface, numerous high-amplitude seismic anomalies display a cross-cutting
303 relationship with enclosing strata, frequently showing saucer-shaped or inclined sheet
304 geometries, such as observed where intrusive bodies have been emplaced in sedimentary strata
305 (e.g. Hansen et al. 2006; Senger et al. 2017; Galland et al. 2018). Reflections above these
306 intrusions are often jacked-up and onlapped by younger sedimentary strata at the Eocene level,
307 reinforcing the interpretation of volcanic activity of this age (Figure 6b and c).
308 Eruptions in the Kaiwero Volcanic Field were short-lived, as indicated by the amalgamation of
309 the pre- and post-eruptive surfaces with increasing distance from the eruptive centres (Figure
310 6). The volcanoes erupted onto a low-gradient submarine sea-floor dipping N to NE, in paleo-
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311 water-depths ranging from 50 to 1500 m (Figure 6). Most volcanoes are cone-type, less than 1
312 km across and <200 m high. Some edifices appear to be formed by overlapped cones up to 5
313 km across and ca 300 high. A single small (<1 km across) crater-type volcano was observed in
314 the central part of the field (line DTB01-31). The volcanoes are often located above the tips of
315 saucer-shaped intrusions or associated with high-amplitude reflections that cross-cut the
316 Cretaceous to Paleocene sedimentary sequence.
317 Little is known about the eruptive styles and edifice growth mechanisms of these volcanoes.
318 The association of vents with large aprons of high-amplitude reflections suggest eruptions of
319 fluidal lavas, which together with the internal mound and rubble morphology of the vents, may
320 indicate that some of them correspond to submarine pillow-mounds. The volcanoes that show
321 bedded flanks may have been formed by submarine pyroclastic eruptions (e.g. submarine
322 Strombolian and eruption-fed density currents), which is reinforced by the widespread
323 occurrence of the “ash unit”. The scattered distribution of small-volume volcanoes gives the
324 Kaiwero Volcanic Field a characteristic observed in monogenetic volcanic fields (Cas et al.
325 1989; Németh 2010; Kereszturi and Németh 2013). The location of the volcanoes do not show
326 relationship with plate boundaries or pre-existing crustal structures and it is likely that these
327 volcanoes have an intraplate origin such as those of the Horomaka Supersuite (Mortimer et al.
328 2014).
329 Northland-Mohakatino Volcanic Belt – Early Miocene to Recent
330 Due to extensive petroleum prospectivity of the region and excellent correlative outcrops, the
331 Northland-Mohakatino Volcanic Belt is the best studied group of volcanoes buried in Zealandia
332 sedimentary basins. The name is a combination of Northland and Mohakatino volcanic belts
333 (Table 1). These volcanoes has been identified by geophysical tomography since the 1960’s
334 (Hatherton 1968). Several drillholes (beginning with e.g. Mangaa-1 in 1968, and including
335 Kora-1 to 4, Albacore-1, and Tangaroa-1) drilled into volcanic rocks of Miocene age,
336 confirming a volcanic origin of the geophysical anomalies.
337 The Northland-Mohakatino Volcanic Belt form a geographically continuous chain of more than
338 70 large basaltic-andesitic volcanic edifices, hundreds of parasitic and satellite vents, as well as
339 numerous intrusive bodies (Figure 1 and Figure 7). The age of the volcanoes range from Early
340 Miocene (ca 25 Ma) in the north, to the active Mount Taranaki in the south, forming a roughly
341 north-south trending belt oblique to the modern strike of the Taupo Rift and to the strike of the
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342 subducted Pacific Plate. Most volcanoes are deeply buried (>1000 m) offshore of the west coast
343 of the North Island of New Zealand and correlated in age and lithologies with volcanic rocks
344 outcropping in the Northland and Coromandel Peninsulas, and with modern volcanoes of the
345 Taranaki Peninsula and Taupo Volcanic Zone, conferring them a subduction-related origin
346 (Whakaari Supersuite; Bergman 1992; Herzer 1995; Giba et al. 2013; Seebeck et al. 2014;
347 Mortimer et al. 2018).
348 Their morphology include large stratocones (e.g. Kora), shields (e.g. Waipoua), and volcanic
349 complexes with several overlapping edifices (e.g. within the Parihaka 3D and Nimitz seismic
350 volume). Many of these volcanoes were long-lived systems active for >8 Myr (e.g. Kora,
351 Manukau, Te Kumi; Bergman et al. 1992; Herzer 1995; Giba et al. 2013). Typically, in the
352 north, the volcanoes are wider and show low-angle flanks (e.g. Manukau, 75 km across and ca
353 1600 m high), while smaller and steeper volcanoes predominantly occur in the south (e.g. Kora,
354 15 km across and ca 1200 m high). Comparing the location and age of the volcanoes to the
355 paleogeographic maps of Arnot and Bland (2016), suggests that eruptions were predominately
356 in deep submarine settings (>1000 m), and mainly subaerial at the ends of the belt. The
357 submarine volcanic edifices include a complex variety of lithofacies comprising tuffs to tuff-
358 breccias, and pillow-lavas commonly interbedded with volcanogenic sandstones and
359 conglomerates (e.g. Bergman et al. 1992; Bear and Cas 2013; Bischoff et al. 2017; Shumaker
360 et al. 2018; Figure 8c and d).
361 The plumbing system of the Northland-Mohakatino Volcanic Belt emplaced hundreds of km3
362 of magma into pre-existing Cretaceous grabens and associated normal-fault zones, which form
363 a large mass of igneous rocks N-S trending and extending semi-continuously for >500 km
364 (Figure 8). Observations in the Kora and Nimitz 3D datasets indicate that most of these
365 intrusions occur in association with large polygenetic volcanoes. The intrusions show
366 seismically detectable sheet-like shapes such as dikes and sills, and relatively small (<7 km
367 across) swarms of hybrid bodies with a variety of geometries such as saucer- and stock-like
368 shapes that cross-cut each other and the sedimentary host rocks (e.g. Bischoff et al. 2017;
369 Morley 2018; Kumar et al. 2018). In detail, dikes and sills of polygenetic volcanoes usually
370 form along, or branched out from, pre-existing Cretaceous faults, commonly converging
371 towards a relatively stationary central vent (Figure 8a and b). In contrast, small
372 parasitic/satellite volcanoes are typically, but not exclusively, located above the tips of shallow
373 (<2 km) saucer-shaped intrusions (Bischoff 2019). Refer to Herzer (1990), Bergman et al.
374 (1992), Herzer (1995), King and Thrasher 1996; Stagpoole and Funnell (2001), Hayward et al.
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375 (2001), Nicol et al. (2007), Giba et al. (2010 and 2013), Seebeck et al. (2014), Bischoff et al.
376 (2017), Morley (2018), Kumar et al. (2018), Shumaker et al. (2018), Bischoff (2019) and
377 Kutovaya et al. (2019) for further details of these volcanoes.
378 West Ngatutura Volcanic Field – Pliocene
379 The West Ngatutura Volcanic Field is a Pliocene cluster of at least 40 scattered small-volume
380 cone-type volcanoes varying in size from <1 km across and <400 m high, distributed over an
381 area of ca 2,100 km2, and currently buried by 400 to 800 m of sedimentary strata offshore of
382 the Waikato and Auckland coasts (Figure 1 and Figure 9). We refer to this cluster of volcanoes
383 as the western equivalent of the onshore Ngatutura Volcanic Field (Table 1). The volcanoes
384 erupted within the lower part of the Pliocene sequence of the Taranaki Basin (ca 3.5 Ma), onto
385 a low-gradient westerly dipping paleo-sea-floor, in water depths around 2000 m. There is no
386 evidence of volcanic rocks of this age from nearest drillholes. However, mound- and cone
387 seismic anomalies are underlain by pull-up of seismic velocities artefacts, indicating that rocks
388 within the anomalies have a higher acoustic impedance than surrounding sedimentary strata,
389 supporting the interpretation of a volcanic origin (Uruski 2019; Figure 9b). This interpretation
390 is reinforced by numerous high-amplitude reflections cross-cutting host strata and below the
391 pre-eruptive surface of these volcanoes, as is typical of igneous intrusions. In addition, doming
392 of reflections overlying cone-type volcanoes are often observed, commonly when volcanic
393 rocks compact less then surrounding sedimentary strata (e.g. Planke et al. 1999; Holford et al.
394 2012; Bischoff et al. 2017).
395 Most vents have an internal bedded seismic facies dipping away from their central regions,
396 suggesting that the edifice was constructed from a single eruptive vent. However, the NE-SW
397 portion of the field also contain amalgamated mounds and cones, indicating that some edifices
398 were formed by overlapping vents (Figure 9a). Below the volcanoes, interruptions and
399 deformation of basin-strata reflections (i.e. host sedimentary rocks) indicate the location of
400 narrow (<50 m) sub-vertical magma feeders, suggesting that some volcanoes were connected
401 with a deep source at least 5 km below resolvable reflectivity in the seismic lines. In contrast,
402 a number of vents are located above the tips of saucer-shaped intrusions (Figure 9b), indicating
403 that these igneous bodies likely act as shallow (<1 km) stationary magma chambers priory
404 eruptions. The distribution of the volcanoes is typical of the intraplate Horomaka Supersuite
405 (Mortimer et al. 2014). However, a NE alignment of vents is observed in the east part of the
406 field, which is above a cretaceous fault of same direction, and parallel with the concurrent
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407 opening of the Taranaki Northern Graben, located 20 km east of the field (King and Thrasher
408 1996). Such alignment of vents are common for the Neogene volcanoes erupted in the North
409 Island (Le Corvec et al. 2013).
410 Southern Zealandia
411 East Waiareka Volcanic Field – Paleocene to Oligocene
412 The East Waiareka Volcanic Field is an extension of the Waiareka-Deborah Volcanic Field,
413 evidenced by a number of mound- and cone seismic anomalies within Paleocene-Oligocene
414 strata of the Canterbury Basin (Table 1; Figure 10). This group of volcanoes is presently buried
415 by up to 3 km of sedimentary strata offshore of the Waitaki district, in the South Island of New
416 Zealand. Some of these volcanoes where intersected by the Endeavour-1, Cutter-1, Galleon-1,
417 Clipper-1, Oamaru-1 and Oamaru-2 drillholes (Field et al. 1989). Volcanic rocks comprise
418 basaltic tuffs up to 124 m thick interbedded with marine mudstones, sandstones and limestones
419 (e.g. Katiki and Hampden formations, and Amuri Limestone), indicating that eruptions
420 occurred in a shallow (<500 m) marine environment during passive tectonic subsidence of the
421 Canterbury Basin (Schiøler et al. 2011). The spatial distribution of the volcanoes form a cluster
422 of >50 randomly distributed small-volume cone-type vents, typically varying in size from <1
423 km across and <400 m high, over an area of ca 3,500 km2 and locate onto a paleo-sea-floor
424 gently dipping towards the east.
425 As is the case for their onshore correlatives, the offshore volcanoes form both overlapping and
426 isolated cones such as those of the Kakanui Head and Cape Wanbrow (Figure 10a and c).
427 Paleocene edifices are generally larger, in some cases up to 5 km across and probably represent
428 overlapping vents (Figure 10). In most cases, the volcanic edifices appear to be partially eroded,
429 with erosion scars and wavy tops morphologies in seismic imagery, perhaps due their location
430 in a continental shelf. The flanks of well-preserved cones usually show bedded seismic facies
431 that dip outward from a central zone of chaotic and/or inward dipping reflections of moderate
432 amplitude (Figure 10), suggesting accumulation of pyroclastic material (Bischoff et al. 2019b),
433 which is reinforced by the tuffaceous facies described in the drillholes (Field et al. 1989). Some
434 minor high-amplitude reflections located next to the vents may indicate the presence of pillow-
435 lava deposits, as observed in volcanoes onshore (e.g. Cas et al. 1989; Corcoran and Moore
436 2008; Moorhouse et al. 2015). Disrupted seismic facies commonly occur below the vents and
437 likely indicate the presence of magma conduits. Large (>3 km) intrusive bodies are rarely
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438 observed below this volcanic field. This contrasts with other monogenetic volcanic fields
439 presented in this study, suggesting that the East Waiareka volcanoes are mainly feed directly
440 from a magma source at least deeper than the seismic resolution of our dataset (i.e. >5 km),
441 which is in agreement with petrological models (Coombs et al. 1986). Based on its distance
442 from plate boundaries and in correlation with the onshore counterparts, the East Waiareka
443 Volcanic Field can be classified as having an intraplate origin (Horomaka Supersuite).
444 Papatowai Volcanic Field – Oligocene or Early Miocene
445 The Papatowai Volcanic Field comprises at least 30 scattered small-volume cone-type
446 volcanoes located offshore of the Clutha district, South Island, New Zealand (Figure 11). We
447 refer to this cluster of volcanoes using the name of the town and river adjacent to the area were
448 they are located (Table 1). The presence of pull-up velocity artefacts underlying the volcanoes,
449 as well as high-amplitude reflections with saucer-shaped geometry cross-cutting host pre-
450 eruptive strata indicate a volcanic origin (Figure 11). In addition, based on the interpretation of
451 aeromagnetic data, Tulloch et al. (2019) suggest the presence of Cenozoic volcanic rocks in the
452 region that correspond to the Papatowai Volcanic Field. To date, no drillholes or dredge samples
453 provide physical evidence of the origin of the seismic anomalies here interpreted to be buried
454 volcanoes.
455 Volcanism in this field occurred over an area of ca 1,600 km2. Eruptions were short-lived, as
456 indicated by the amalgamation of the pre- and post-eruptive surfaces with increasing distance
457 from the eruptive centres (Figure 11). The volcanoes erupted onto a low-gradient submarine
458 sea-floor dipping E, in paleo-water-depths <500 m. It is not possible to precisely determine the
459 age of these volcanoes based exclusively on seismic lines because they are buried within a thin
460 condensed section of sedimentary rocks of Oligocene to Early Miocene age, with epoch
461 boundaries are below the seismic resolution of the dataset at that location.
462 Most edifices are presently buried in <500 m of sedimentary strata of the Great South Basin,
463 however some are outcropping in the modern sea-floor (Figure 11b), which could be a target
464 for future dredge sampling to confirm the origin and precise age of these seismic anomalies. As
465 is the case in the East Waiareka and Kaiwero volcanic fields, the flanks of the Papatowai cones
466 usually show bedded seismic facies outward dipping from a central zone of chaotic and/or
467 inward dipping reflections of mod-amplitude (Figure 11b and c), suggesting some pyroclastic
468 eruptions. In addition, most cones have peripheral high-amplitude reflections, which could
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469 correspond to pillow-lavas or other forms of dense submarine lava-flow deposits. The
470 Papatowai volcanoes likely have an intraplate origin such as those of the Horomaka Supersuite.
471 Maahunui Volcanic Field – Middle Miocene
472 The Maahunui Volcanic Field is a cluster of Middle Miocene volcanoes and shallow (<1 km)
473 intrusions currently buried by ca 1000 m of sedimentary strata in the offshore Canterbury Basin,
474 southeast of Banks Peninsula (Figure 1 and 12). The name comes from the legendary canoe that
475 the Māori demigod Maui sail the Pacific Ocean (Maahunui), aka the local name of the coast
476 south of Banks Peninsula and adjacent to the area where the volcanoes are now buried (Table
477 1).
478 In this area, volcanic activity of Middle Miocene age has been inferred from 2D seismic
479 reflection surveys and was confirmed by igneous rocks collected from the Resolution-1
480 drillhole (Milne 1975; Field et al. 1989; Bischoff et al. 2019a and b). Volcanism in the
481 Maahunui Volcanic Field covered an area of ca 1,520 km2, comprising at least 31 scattered
482 small-volume (<1 km3) cone- and crater- type volcanoes. Eruptions were entirely submarine
483 (500 to 1500 m), producing deep-water morphologies equivalent to maar-diatreme and tuff
484 cones (Figure 12d). A number of intrusive bodies are observed in seismic lines, commonly
485 emplaced within organic-rich Cretaceous-Paleocene sedimentary strata. Saucer-shaped
486 intrusions are the typical morphology and are interpreted to have fed magma to many of the
487 volcanoes (Bischoff et al. 2019a and b).
488 Representative volcanic rock in the Resolution-1 drillhole suggest that the magmatic products
489 are primarily alkali basalts in composition. The extrusive rocks are interbedded with lower
490 bathyal siltstones of the Tokama Formation, supporting the interpretation of a deep-water
491 setting at the time of the eruptions. These volcanic rocks contain abundant shards of glass,
492 spheroidal fragments enveloped in a palagonite film, broken phenocrysts, and limestone and
493 coal lithics, typically showing a moderate- to high degree of alteration (Figure 12b).
494 Volcaniclastic rocks correlate with the onshore Wairiri Volcaniclastite, which was erupted in
495 shallow-waters and outcrops near Coalgate, northern Canterbury Basin (Carlson et al. 1980;
496 Bischoff et al. 2019a). The intrusive rocks comprise a monzogabbro that contains miarolitic
497 cavities and ophitic texture of plagioclase and augite (Figure 12c), suggesting that the intrusions
498 were emplaced at a relatively shallow level. Decompaction of sedimentary strata overlying the
499 monzogabbro intrusion indicate an emplacement depth around 900 m below the paleo-seafloor
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500 (Bischoff et al. 2019a; Magee et al. 2019). Both volcaniclastic and intrusive rocks show similar
501 mineral paragenesis and chemical signature, indicating a co-genetic process of formation
502 (Bischoff et al. 2019a).
503 The volcanoes show random distributions of cone- and crater-types relative to the water depths
504 in which they have erupted. The primary volcanic morphology is interpreted to be mainly
505 controlled by high-energy pyroclastic eruptions (Bischoff et al. 2019a and b), in which coeval
506 thermogenic gases and CO2 incorporated into the magmatic system could have had an important
507 role in the fragmentation and dispersion of erupted material (e.g. Svensen et al. 2004; Aarnes
508 et al. 2015; Agirrezabala et al. 2017). Eruptions typically produced small-volume volcanoes
509 (<1 km3) controlled by a plumbing system that fed magma to dispersed eruptive centres, a
510 characteristic of monogenetic volcanic fields (Németh and Kereszturi 2015). After volcanism
511 ceased around 11.5 Ma, the Maahunui Volcanic Field was progressively buried by an increase
512 in sediment influx from the NW, which is interpreted to be derived from the early uplift along
513 the Southern Alps (Field et al. 1989). This volcanic field has no clear association with a plate
514 boundary or subducting plates and can based in the morphology and distribution of the vents,
515 can be classified as having an intraplate origin (Horomaka Supersuite).
516 Other Volcanoes Buried Offshore New Zealand
517 Other Late Cretaceous (and possible older) volcanoes identified offshore the North Island of
518 New Zealand are: (a) Tikati Volcanic Complex (Uruski 2019; Supplementary Material 4), and
519 (b) intrusions and possible vents in the area of the Matuku-3D dataset (Supplementary Material
520 5).
521 Numerous Late Cretaceous (and possible Early Cretaceous) volcanic centres are mapped in
522 Canterbury and Great South basins (Field et al. 1989; Bischoff et al. 2016; Barrier et al. 2017;
523 Barrier 2019 and Barrier et al. 2020a; Figure 14a and b; Table 1 to 4). Many of these volcanoes
524 occur immediately above the basement of the host sedimentary basin, which, due to similarity
525 in the seismic signal of volcanic and basement rocks, makes their characterization difficult
526 (Barrier et al. 2020a). Similar seismic facies are identified SE of Banks Peninsula (e.g. line
527 IP256-98-001), where volcanic rocks of the Mount Somers Volcanic Group were intersected
528 by the Leeston-1 and J.D. George-1 drillholes, which at least in some locations, confirm a Late
529 Cretaceous volcanic origin for these seismic facies. Other large polygenetic volcanoes are: (a)
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530 Sloop (Supplementary Material 7), (b) Barque (Supplementary Material 8), and (c) Tapuku East
531 (Supplementary Material 9) volcanic complexes.
532 The northwester limit of the Deepwater Taranaki Basin contains a large (ca 100 km) W-E
533 elongated structure oblique to the NW orientation of the New Caledonia Trough, which
534 bathymetric expression is referred to as Aotea Seamount (Brodie 1965). This structure
535 comprises a main volcanic body with multiple parasitic and satellite vents. Based on its
536 morphology in seismic data, we refer it to as the Aotea Volcanic Complex (Supplementary
537 Material 6; Tables 1 and 3). The volcanoes of Aotea erupted immediately above the base of the
538 Miocene chronostratigraphic surface indicating an age of ca 23 Ma, which is supported by Ar-
539 Ar dating of 22.5 Ma, while a single dredge sample have an ultra-alkaline signature, suggesting
540 an intraplate composition (Mortimer et al. 2018).
541 The Waka-3D seismic survey located offshore the Canterbury Basin contains several large (ca
542 5 km across) saucer-shaped intrusions, mainly aligned with pre-existing Cretaceous faults
543 (Blanke 2012; Figure 13). Above these intrusions, a number of volcanic vents are located onto
544 the Miocene paleo-sea-floor, here referred to as the East Waipiata Volcanic Field (Tables 2 and
545 4), which have been erupted at a water depth of at least >500 m. The shape of the volcanoes
546 include small-volume cone-type with extensive high-amplitude aprons that likely corresponds
547 to submarine spatter cones and related lava fields. The lateral extent of this volcanic field is
548 unclear due to the limited resolution of the seismic data outside the area of the 3D survey, but
549 they correlate in age and morphology with those from the Waipiata Volcanic Field located
550 onshore (Figure 13). Another field of scattered small-volume volcanoes and shallow intrusions
551 probably of Pleistocene age are clustered ca of 100 km SE of the Stewart Island, near the
552 location of the Toroa-1 drillhole (Supplementary Material 10; Tables 2 and 4). Both fields are
553 likely intraplate in origin.
554 Discussion
555 Complementary studies of both onshore and offshore New Zealand, New Caledonia and smaller
556 islands are helping to reveal the patterns of volcanism in Zealandia and southwest Pacific
557 regions (Mortimer and Scott 2020). Insights from volcanoes buried in sedimentary basins can
558 help to fill important gaps in understanding these volcanic patterns across the entire Zealandia
559 (e.g. Herzer 1995; King and Thrasher 1996; Giba et al. 2013; Bischoff and Nicol 2019).
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560 However, identification and detailed characterization of these buried volcanic systems is in its
561 infancy and will benefit from future studies. The variable distribution, quality and resolution of
562 the seismic surveys, largely motivated by resource exploration, places limitations on the
563 synthesis of the volcanic processes within the basins. Vast areas are still poorly imaged (e.g.
564 West Coast, Aotea, Bellona, Emerald, Northeast Slope basins, and towards east of the
565 Canterbury and Great South basins (Figure 1 and Figure 14), or only covered by sparse 2D
566 seismic grids that cannot give insights in the same resolution as 3D datasets (e.g. Reinga-
567 Northland Basin).
568 In addition, regions of high erosion or those affected by tectonic processes associated with the
569 Hikurangi and Puysegur subduction zones, and with uplift of the Southern Alps (e.g. Ballance
570 1993; Lebrun et al. 2003; Nicol et al. 2007) generally have poor scope for burial and
571 preservation of volcanoes. The following sections discuss the first-order regional features
572 observed for offshore volcanoes buried in New Zealand sedimentary basins, where preservation
573 was enhanced by low deformation, little or no erosion, and seismic reflection data are present
574 in great quantity.
575 Late Cretaceous syn- and immediate post-rift volcanism
576 Late Cretaceous volcanic activity primarily occurred under the influence of a tensional
577 lithospheric stress field (i.e. σ1 vertical, σ2 and σ3 horizontal) associated with the break-up and
578 separation of Zealandia from eastern Gondwana (Figure 2, Figure 14a and b). The product of
579 this volcanism typically formed large (20-1400 km3) composite cones and volcanic complexes
580 that were active for ca 1 to 15 Myr, and less commonly fields of scattered small-volume (<1
581 km3) volcanoes erupted into subaerial to shallow marine environments.
582 In the Taranaki and Deepwater Taranaki basins, two eruptive phases of volcanism are observed
583 for the Late Cretaceous and were at least in part concurrent with two rifting events (i.e.
584 Zealandia and West Coast-Taranaki rifts; Strogen et al. 2017). During the first eruptive phase,
585 ca 85-84 Ma and possibly older, Chronos, Rhea and at least two additional unnamed volcanoes
586 (Uruski 2019) erupted along the NW-SE rift system of the New Caledonia Trough (Figure 3
587 andFigure 14a). The precise timing of cessation of this eruptive phase is difficult to determine
588 based on the available data, but it was contemporaneous with, or followed by, deposition of the
589 lowermost Taranaki Delta around 83 Ma (Schiøler et al. 2014). In northern Zealandia, the first
590 rift phase shut down at ca 83 Ma, perhaps due to the onset of seafloor spreading in the Tasman
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591 Sea and the associated cessation of normal faulting in the interior of Zealandia (Gaina et al.
592 1998; Strogen et al. 2017). Relaxation or reduction of the tensional stresses across Zealandia
593 with the advent of sea floor spreading is a possible explanation for the termination of the first
594 eruptive phase.
595 The second eruptive phase (ca 74-68 Ma) formed a broad N-S alignment of volcanoes along
596 the extension of the West Coast-Taranaki rift (Figure 2 and Figure 14b). These include
597 Coopworth, Romney, Hades, Vulcan, Hestia (Figure 3 andFigure 4), Tikati (Supplementary
598 Material 4), and volcanoes within the Matuku-3D dataset (Supplementary Material 5). The
599 West Coast-Taranaki rift is described to have formed a narrow and confined N-S trending
600 aulacogen in these basins, active from ca 80-55 Ma (Strogen et al. 2017). Results from this
601 paper indicates that the second rifting event extended further west, across the Deepwater
602 Taranaki Basin (Supplementary Material 3). We observe that many, if not most, volcanoes of
603 the second eruptive phase are typically located along concurrent N-S trending normal faults
604 (Figure 3). This is also documented in Zhu et al. (2017) and Uruski (2019) and suggests that
605 the second eruptive phase is linked with extension concurrent with the West Coast-Taranaki
606 rifting phase.
607 Uruski (2019) argues that crustal extension does not control the location of volcanoes in
608 Deepwater Taranaki Basin, since many volcanoes erupted above local basement highs with
609 magma migrating through thicker crust. However, the locus of eruptions along continental rifts,
610 such as those of Late Cretaceous age in Zealandia, is thought to be controlled by the interplay
611 between the rate of extension of the crust and the presence of pre-existing structures (e.g. Corti
612 2009; Le Corvec et al. 2013; Vries and Vries 2018). For example, rifts associated with high
613 rates of extension tend to form volcanoes at the centres of the rift axis, where intense crustal
614 thinning occurs (e.g. Afar Rift; Illsley-Kemp et al. 2017), while rifts with low extension rates
615 typically have volcanoes located at the rift margins, onto local basement highs (e.g. Limagne
616 Rift; Vries and Vries 2018). Both volcanoes erupted at the rift axis and margins are buried in
617 offshore New Zealand. We infer that the locations of these volcanoes probably reflects the
618 interplay of tectonic stress during volcanic activity, and the presence of pre-existing crustal
619 structures (Figure 5a).
620 A relationship between extension and the location of eruptive centres is also observed in
621 southern Zealandia. In the Canterbury Basin, 70% of the syn-rift volcanoes are within 5 km of
622 a rift fault (Barrier et al. 2020a). These authors also suggest that elongated features formed
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623 along rift faults may correspond to volcanoes erupted in fissures, typical of volcanism related
624 with extension elsewhere (e.g. Corti 2009; Sigmundsson et al. 2018; Neal et al. 2019). Large
625 (>500 km3) polygenetic volcanoes erupted post Zealandia breakup (e.g. Barque, Sloop and
626 Tapuku) are also aligned with both NE-SW and NW-SE trending rift faults (Figure 14b; Barrier
627 et al. 2020a). Similarly, volcanoes of Late Cretaceous age erupted in the Great South Basin
628 follow the NE-SW trend of concurrent rifting (Figures 14a and b). In addition, Mortimer and
629 Scott (2020) speculatively suggest that a sub-category of Zealandia intraplate volcanoes are
630 related with Late Cretaceous rifting of the eastern Gondwana, which is indicated by areas of
631 high-positive magnetic anomaly interpreted to be caused by mafic rift-related igneous rocks
632 (Tulloch et al. 2019).
633 Collectively, evidence presented in this paper indicate that during separation of Zealandia from
634 eastern Gondwana, magmatism and continental rifting were closely linked in time and space.
635 In a tectonic sense, we propose that these rift volcanoes should be distinguished from the
636 intraplate Horomaka Supersuite (Mortimer et al. 2014). We propose here a new stratigraphic
637 unit, Te Kiekie Supersuite, to describe Late Cretaceous volcanoes that have a rift affinity. Te
638 Kiekie is the Māori name for the Mount Somers, inland Canterbury Basin, which represents
639 volcanic activity contemporaneous with Zealandia rifting (e.g. Laird and Bradshaw 2004;
640 Mortimer et al. 2014; Van der Meer et al. 2017).
641 Cenozoic diffuse intraplate volcanism
642 The term ‘intraplate’ is generally applied to describe volcanic activity distal from plate
643 boundaries with magma generation not directly related to plate interaction (LaFemina 2015). In
644 New Zealand and Eastern Australia, ‘intraplate’ has been used more in a petrological and
645 geochemical sense, rather than adhering strictly to the tectonic definition (Johnson 1989; Timm
646 et al. 2010; Hoernle et al. 2020), with intraplate volcanics classified as a group of rocks that
647 have distinct composition from those found in mid-ocean ridges and in arc-trench systems
648 (Horomaka Supersuite; Mortimer et al. 2014).
649 Disregarding the Late Cretaceous rift-related volcanoes (here named Te Kiekie Supersuite),
650 intraplate volcanos in Zealandia and SE Australia are well represented by numerous Cenozoic
651 basaltic-alkaline monogenetic fields, such as the Newer (Cas et al. 2017) and Waiareka-
652 Deborah (Cas et al. 1989) volcanic fields. Similarly, buried volcanoes erupted throughout the
653 Cenozoic and located away from the Australian-Pacific plate boundary (>200 km) typically
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654 form clusters of scattered small-volume (<1 km3) craters and cones. The distribution of these
655 volcanoes in time and space appears to be unrelated to plate boundary processes, and the locus
656 of eruptions, at least at a kilometre scale, predominately show randomly distributed (Figure 6
657 and Figure 9-Figure 14).
658 In detail, alignments of vents and intrusions does occur, but at a much more local scale than
659 volcanoes of Late Cretaceous age (Figure 9a and Figure 13). These alignments are observed in
660 two Neogene volcanic fields (West Ngatutura and East Waipiata) that erupted <300 km from
661 an active plate boundary (Figure 1Figure 2 and Figure 14c and d). In both cases, the vents and
662 intrusions are located parallel to the trend of pre-existing faults, which is commonly also
663 observed for Neogene monogenetic volcanic fields in the North Island of New Zealand (Le
664 Corvec et al. 2013). In contrast, during a period of tectonic quiescence across most New Zealand
665 in the Paleogene, neither onshore nor offshore monogenetic fields show any clear relationship
666 with pre-existing crustal structures.
667 The influence of tectonic activity on fault displacement is reported to extend far from plate
668 boundaries. For example, in the intra-cratonic Paraná Basin, southern Brazil, extension and
669 reactivation of normal faults along the boundaries of crustal blocks are linked with the timing
670 of terrain accretion on the western Gondwana plate boundary, which was located at least 1500
671 km from the basin (Milani and Ramos 1998; Holz et al. 2006). Magma erupted distal from plate
672 boundaries (intraplate setting) rises through thicker (>10 km) crust, which is facilitated mostly
673 by the existence of a extensional stress field in the lithosphere, and in interaction with the
674 presence of pre-existing crustal structures (e.g. Shaw, 1980; Le Corvec et al. 2013; Vries and
675 Vries 2018).
676 For instance, after ca 40 Myr of predominantly monogenetic intraplate volcanic activity in
677 Zealandia, the large polygenetic volcanoes of Banks, Otago, and Aotea erupted concurrently
678 with Neogene tectonic activity, in which the primary plate boundary structures were located
679 <250 km from the locus of the eruptions (e.g. Coombs et al. 1960; Sewell 1988; Mortimer et
680 al. 2018). Banks Peninsula erupted at the intersection of WNW-ESE and NE-SW trending
681 basement structures of Cretaceous age (i.e. Chatham Rise and Endeavour High; Field et al.
682 1989; Barrier et al. 2020b). Eruptive centres formed along normal faults of similar orientation,
683 presumably due to fault reactivation concurrently with volcanic activity (Hampton and Cole
684 2009). Otago Peninsula erupted in the core of the Otago Schist Belt, near the limit of the Rakaia
685 and Caples terranes, and is also aligned with the Titri fault and Taieri graben (Coombs et al.
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686 1960; Deckert et al. 2002). Aotea Volcanic Complex erupted onto the West Norfolk Ridge,
687 with normal faults along its boundary with the New Caledonia Trough to the SW
688 (Supplementary Material 3). These spatial associations between major structures and volcanoes
689 may suggest that the stress field imposed at plate boundaries could be at least in part responsible
690 for fault reactivation, extension and eruptions in an intraplate setting some distance from the
691 primary plate boundary structures.
692 Cenozoic subduction-related volcanism
693 Arc-related volcanoes onshore and offshore New Zealand mainly comprise large deep-
694 submarine to subaerial composite, shield and caldera volcanoes formed in association with
695 thousands of cubic kilometres of intrusive rocks emplaced into sub-volcanic sedimentary strata
696 (Ballance et al. 1985; Cole 1986; Bergman et al. 1992; Herzer 1995; King and Thrasher 1996;
697 Figure 7 and 8). The relative locations of the arcs are complex, and has been interpreted to
698 reflect progressive steepening of the slab and southward migration of crustal extension of the
699 upper plate during the Miocene to Recent (Kamp 1984; Mortimer et al. 2010; Giba et al. 2010;
700 Seebeck et al. 2014). Until about 12 Ma, significant volcanism did not extend south of about
701 38o S latitude, migrating ca 60 km further south during 12-8 Ma, with voluminous eruptions
702 then in the area of the Parihaka-3D survey up to about 4 Ma (Hayward et al. 2001; Giba et al.
703 2013; Figure 7). After 4 Ma, volcanism migrated further SE to its present location at the Taupo-
704 Kermadec-Tonga trend (Cole 1986; Figure 7).
705 The active volcanoes of the Central North Island are now situated about 80-100 km above the
706 Pacific Plate subducting slab, with magma interpreted to be produced by slab dehydration and
707 partial melting of the asthenosphere (Seebeck et al. 2014; Illsley-Kemp et al. 2019; Figure 7).
708 Southwest of the Central North Island and south of the Mt Taranaki, seismicity indicates that
709 the subducting slab reaches depths of >100 km, however, there is no evidence of volcanic
710 activity in seismic reflection surveys and drillholes on this area (Figure 1). A similar southward
711 termination also occurred for the Miocene Mohakatino volcanoes, although this termination
712 was west and north of the contemporary termination of the Central North Island volcanoes.
713 During both time intervals volcanism was absent from regions primarily accommodating crustal
714 contraction, where near-horizontal σ1 stress may generally reduce fracture apertures and retard
715 the rise of magma. In areas where crustal extension is sufficiently low that it is not manifest as
716 normal faults, the rise of magma through the crust may be facilitated by pre-existing Cretaceous
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717 faults (Giba et al. 2013). Conversely, volcanism is also absent in some areas where normal
718 faulting and crustal extension has occurred. At the present time, for example, the transition from
719 crustal tension to crustal compression in the southern Taranaki Basin occurs midway between
720 North and South Islands (Nicol et al. 2007; Townend et al. 2012; Rajabi et al. 2016; Massiot et
721 al. 2019), and yet there is no volcanism south of Mt Taranaki. The absence of volcanism in this
722 case may reflect the presence of thickened continental crust due to late Cenozoic thrusting and
723 shortening, and this thickening may inhibit the upward flow of magma to the ground surface.
724 Conclusions
725 Mapping and characterization of volcanoes buried in Zealandia sedimentary basins is a work in
726 progress. In this paper, we propose a systematic classification of the best understood examples
727 of volcanoes buried offshore New Zealand, according their location, morphology, age and
728 tectonic setting of eruption. Detail characterization indicates that magmatic activity syn-and
729 post-breakup of the eastern Gondwana was primarily controlled by the tectonic setting and
730 stress regime operating at each volcano. Late Cretaceous volcanoes mainly comprise large (>20
731 km3) long-lived systems, including composite cones and volcanic complexes that erupted
732 concurrently in time, space and orientation of continental rifts and spreading centres formed by
733 the separation of Zealandia from the eastern Gondwana. Cenozoic intraplate volcanism is
734 mainly randomly distributed across Zealandia, and is characterized by scattered small-volume
735 (<1 km3) cone- and crater-type volcanoes associated with numerous shallow (<2 km) magmatic
736 bodies, typically with saucer-shaped morphology. In detail, only Cenozoic intraplate volcanoes
737 erupted concurrently with tectonic activity show a spatial relationship with pre-existing
738 structures, suggesting that fault reactivation, extension and volcanism in an intraplate setting
739 could be influenced by the stress field imposed near (<300 km) plate boundaries. Subduction-
740 related volcanism dominantly occurs in the Northern Zealandia, and has formed a
741 geographically continuous chain of over 70 large strato- and shield volcanoes from Early
742 Miocene in the north to the active Mt Taranaki in the south. These volcanoes comprise multiple
743 arc systems associated with the initiation and evolution of the Kermadec subduction zone from
744 the Eocene. Whatever the volcanic setting (i.e. rift, intraplate or subduction-related), the
745 volcanic plumbing systems emplaced intrusions preferentially into mudstones and carbon-rich
746 Cretaceous-Paleocene sedimentary rocks. Studying the volcanoes buried in sedimentary basins
747 can help advance understanding of syn- and post-Gondwana breakup magmatism across the
748 entire Zealandia continent and globally.
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749 Acknowledgments
750 We would like to thank IHS Markit and Schlumberger for providing academic licence to use
751 the Kingdom and Petrel software. We are grateful to the New Zealand Petroleum and Minerals
752 and the Ministry of Business, Innovation and Employment of New Zealand for providing the
753 dataset and funding for this study. Thanks for the constructive reviews of xxxx and xxxx, and
754 for Marcos Rossetti for the petrographic discussions.
755
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756 List of Figures
757
758 Figure 1: Map showing the dataset, main regional physiography, and location of buried
759 volcanoes discussed in this study. Numbers and abbreviations are: (1) Vulcan Volcanic Group;
760 (2) Tikati Volcanic Complex; (3) Matuku Volcanics; (4) Kaiwero Volcanic Field; (5)
761 Northland-Mohakatino Volcanic Belt; (6) Aotea Volcanic Complex; (7) West Ngatutura
762 Volcanic Field; (8) Sloop Volcanic Complex; (9) Barque Volcanic Complex; (10) Tapuku East
763 Volcanic Complex; (11) East Waiareka Volcanic Field; (12) Papatowai Volcanic Field; (13)
764 Maahunui Volcanic Field; (14) East Waipiata Volcanic Field; (15) Toroa Volcanic Field; (TVZ)
765 Taupo Volcanic Zone; (TKR) Three Kings Ridge. Volcanoes numbered in grey circles are only
766 shown in the supplementary material of this paper.
767
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768
769 Figure 2: Simplified Late Cretaceous-Cenozoic volcano-stratigraphic framework and main
770 tectonic events offshore wester North Island and offshore eastern South Island of New Zealand.
771 Number in red are the estimated volume of erupted magma in km3. Symbols in black indicate
772 the volcanic type and are: Composite Cone (CC), Volcanic Complex (VC), Volcanic Field
773 (VF), Caldera (Ca) and Shield (Sh). Colours in the bars correspond to the active phase of the
774 volcanoes and their tectonic setting of eruptions and are: syn-rift (dark green), immediate post-
775 rift (green), scattered intraplate (blue), subduction-related (red).
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776
777 Figure 3: (a) Regional map of the post-eruptive surface of the Vulcan Volcanic Group (ca 68
778 Ma). Numbers are: (1) Chronos Composite Volcano; (2) Rhea Composite Volcano; (3)
779 Coopworth Volcanoes; (4) Romney Volcanic Field; (5) Hades Caldera; (6) Vulcan Composite
780 Volcano; and (7) Hestia Composite Volcano. (b) 2D cross-section across the Romney Volcanic
781 Field. Black arrows indicate the location of eruptive vents. Blue lines correspond to fault plains
782 and red lines to the location of possible conduits. (c) Plan view map of Coopworth Volcanoes
783 and Romney Volcanic Field. Dots are located at the position of eruptive vents. Dashed lines
784 mark the terminal front of lava-flow deposits and the boundaries of the volcanic fields. (d)
785 Detail chronostratigraphic map showing the proxy post-eruptive surface of the Hades Caldera
786 at ca 74 Ma.
787
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788
789 Figure 4: Uninterpreted (a) and interpreted (b) 2D seismic section showing the morphology and
790 stratigraphic relationship of some of the large volcanoes of the Vulcan Volcanic Group. Po is
791 the post-eruptive surface of each volcano, while Pr is the pre-eruptive surface, these surfaces
792 are the upper and lower bounds of each coloured polygon. Numbers indicate the stratigraphic
793 order relative to the onset of volcanic activity in each volcano. Note that some volcanoes
794 interfinger (e.g. Vulcan and Hestia) indicating that volcanic activity was contemporaneous,
795 while in others (i.e. Coopworth and Hades) the Pr and Po amalgamate with increasing distance
796 from the eruptive centre, indicating that eruptions were relatively short-lived compared to the
797 rate of sedimentary and volcanic processes that control the local basin infill. Blue lines
798 correspond to fault planes and red lines locate possible magma conduits. The location of this
799 line is shown in Figure 3.
800
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801
802 Figure 5: (a) Plan view map of the Romney Volcanic Field revealed by spectral decomposition
803 at the 4100 millisecond time-slice. (b) Opacity rendering of the Romney-3D showing the
804 morphology of a double-summit shield-like volcano composed by several eruptive vents. (c)
805 Time-slice section at 4200 millisecond coupled with a 2D cross-section across the shield
806 volcano shown in (b). Note the “bird-feet” morphology evidencing the termination of lava
807 flows, while the concentric structures mark the location of conduits. S1 and S2 are mapped
808 summits of the shield volcanoes. WCTRT is the West Coast-Taranaki rift trend. Note the
809 structural control of faults in the location of eruptive vents.
810
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811
812 Figure 6: (a) Mmap showing the distribution of volcanoes in the Kaiwero Volcanic Field and
813 available 2D seismic grid. The coloured bars show the location and present burial depth of the
814 volcanoes in TWT. The area highlighted in red corresponds to the limit of occurrence of
815 intrusive and extrusive bodies of Eocene age. (b) 2D seismic section (line MER10-004)
816 showing a small cone-type volcano located at the tip of a saucer-shaped intrusion. Both the pre-
817 eruptive surface and forced folding above the intrusions indicate an Eocene age. Red lines
818 represent the location of possible magma conduits. (c) 2D seismic section (line DTB01-20)
819 shows a small cone-type volcano and an intrusion, both of Eocene age. Numbers are: (1)
820 intrusions; (2) jacked-up dome; (3) conduits; (4) seismic artefact (loss of reflectivity); (5) cone-
821 type volcano.
822
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823
824 Figure 7: (a) Simplified map showing the distribution and age of subduction-related volcanoes
825 onshore and offshore the North Island of New Zealand, in which the black dashed lines bound
826 the volcanoes in the Northland-Mohakatino Belt. Coloured polygons show the location and age
827 of volcanoes mapped in seismic data, while dots correspond to dredged samples of volcanic
828 origin. White dashed lines bond the trends of subduction-related volcanoes by age groups. Note
829 a progressive younging in the age of the volcanoes from NW to SE. Aotea Volcanic Complex
830 have a single representative dredge sample comprising a basalt Ar-Ar dated at 22.5 Ma
831 (Mortimer et al. 2018), indication a intraplate origin. See discussion and Supplementary
832 Material 6 for more details. VMFZ is the Vening Meinesz Fault Zone. Mapping of the offshore
833 volcanoes was combined with information from Herzer (1995), Hayward et al. (2001),
834 Mortimer et al. (2010), Giba et al. (2013), Seebeck et al. (2014), Global Volcanism Programme,
835 and GNS Geological Map (Heron 2014). (b) Schematic section across the Hikurangi subduction
836 zone. AP is the Australian Plate and PP is the Pacific Plate. Dashed black lines show
837 approximate location of the subducted Pacific Plate. Red arrow indicates eastward directed slab
838 roll-back. From Giba et al. (2013).
839
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840
841 Figure 8: 3D seismic visualization and volcanogenic rocks of the Kora Volcano, a long-lived
842 Miocene stratocone of the Northland-Mohakatino Belt. (a and b) Main chronostratigraphic
843 surfaces and structures coupled with geobody mapping of Kora Plumbing System. Note that
844 intrusions preferentially were emplaced into pre-existing Cretaceous-Paleocene grabens and
845 along normal faults. (c) Photograph of a core sample from the drillhole Kora-1a at the depth of
846 1783 m showing a moderately sorted lapilli-tuff. (d) A polymictic volcanic conglomerate from
847 1908 m in Kora-1a. The location of the Kora Volcano is shown in Figure 7. Modified from
848 Bischoff et al. 2017 and Bischoff and Nicol 2019.
849
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850
851 Figure 9: (a) Map showing the location of the West Ngatutura Volcanic Field, regional fault
852 structures and possible onshore correlated volcanic fields. The coloured bar show the isochron
853 horizon between the pre- and post-eruptive surfaces, revealing the location and morphology of
854 the volcanoes. Onshore volcanic rocks are from GNS Geological Map (Heron 2014) and faults
855 are from Giba et al. 2013. (b) Interpreted 2D seismic section (line OA07-025) showing the
856 morphology of some volcanoes and intrusions in the West Ngatutura Volcanic Field. Number
857 are: (1) feeder dikes; (2) saucer-shaped intrusion; (3) jacked-up dome; (4) seismic artifact
858 (multiple); (5) seismic artifact (hyperbole); (6) seismic artifact (pull-up velocity); (7) small-
859 volume cone-type volcano; (8) lava-flows?; (9) vents.
860
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861
862 Figure 10: (a) Map showing the location and age of volcanoes onshore and offshore eastern
863 South Island of New Zealand (Horomaka Supersuite). (b and c) 2D sections showing the age
864 and morphology of small-volume cone-type volcanoes buried in the Canterbury Basin.
865 Numbers are: (1) intrusions; (2) conduits. (d) An outcrop within the Kakanui Head, a volcano
866 of the Waiareka-Deborah Volcanic Field showing an angular contact between amalgamated and
867 poorly-sorted pyroclastic beds inward-dipping towards the central crater of the edifice, while
868 well-sorted and tabular beds dip outward. Onshore volcanic rocks are from GNS Geological
869 Map (Heron 2014).
870
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871
872 Figure 11: (a) TWT map on the proxy post-eruptive surface of small-volume cone-type
873 volcanoes of the Papatowai Volcanic Field. (b and c) 2D sections (lines OMV08-042 and
874 Husky-H-104) showing the morphology of some of the volcanoes and part of their plumbing
875 system. Number are: (1) feeder dikes, (2) intrusion; (3) jacked-up dome; (4) seismic artifact
876 (pull-up velocity); (5) pillow mounds? (6) small-volume cone-type volcano; (7) pre-eruptive
877 surface; (8) post-eruptive surface.
878
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879
880 Figure 12: (a) Composite paleogeographic and paleoenvironmental montage of the area of the
881 Maahunui Volcanic Field. Red dashed line shows the approximate paleo-bathymetry at the
882 onset of eruptions in the field. Pink dashed line show the location of the terminations of lava-
883 flow deposits sourced from Banks Peninsula Volcanoes. The inland part of the map show
884 lithologies from the GNS Geological Map (Heron 2014). (b and c) Thin section of cutting and
885 core samples from the drillhole Resolution-1. (b) Shows volcaniclastic fragments with
886 microporphyritic and vitrophyric textures from 1130 to 1140 m. Minerals are pyroxene (py),
887 plagioclase (pl) and palagonite (pal). (c) Shows a monzogabbro intrusive rock with ophitic
888 texture of plagioclase and augite (aug) collected from a core sampled at 1962 m. (d) Interpreted
889 2D strike-oblique section (line CB82-15) showing the morphology of some volcanoes and their
890 underlying plumbing system. Number are: (1) disrupted blocks and shallow intrusions, (2)
891 feeder dikes; (3) saucer-shaped intrusion; (4) small-volume crater-type volcano; (5) small-
892 volume cone-type volcano. BP abbreviation refers to Banks Peninsula volcanoes. Modified
893 from Bischoff et al. 2019a, b and c.
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894
895 Figure 13: Examples of intrusions and vents of the East Waipiata Volcanic Field. (a) Amplitude
896 opacity rendered seismic volume (Waka-3D) revealing a number of intrusion emplaced into
897 Paleocene sedimentary strata and vents erupted onto the Miocene paleo-sea-surface. Note the
898 spatial relationship between the intrusive bodies and vents. (b) Geobody extraction of a saucer-
899 shaped intrusion showing its geometry, relationship with enclosing strata, conduits, and co-
900 genetic eruptive vents. Modified from Bischoff and Nicol, 2019. Include cretaceous fault.
901
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902
903 Figure 14: Simplified geological evolution of the Zealandia region showing four main tectonic-
904 magmatic phases. (a) The first phase (ca 100-85 Ma) is characterised by widespread
905 intracontinental break-up (Zealandia Rift) and simultaneous volcanism broadly associated with
906 lithosphere thinning and extension. (b) The second phase (ca 85-66 Ma) corresponds to the
907 opening of the Tasman Sea and separation of Zealandia from eastern Australia and Antarctica.
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908 This process was followed by extension (West Coast-Taranaki Rifting) and simultaneous
909 volcanism in western New Zealand, and volcanic activity immediately after cessation of rifting
910 in the southern Zealandia. Volcanic activity in both first and second phases mainly formed large
911 (>50 km3) volcanoes. Diffuse intraplate alkaline-type volcanism is characteristic of the third
912 phase (ca 66 to 30 Ma), which is followed by drifting of Zealandia from Australia and
913 Antarctica. In New Zealand and immediately offshore, volcanic activity mainly formed fields
914 of small-volume volcanoes, with exception of the large volcanic complexes of Banks and Otago
915 peninsulas, and Aotea Seamount. In addition, initiation of the subduction between the Pacific
916 and Australian plates forms part of the Norfolk Ridge in the Eocene-Oligocene. The fourth
917 phase (ca 30-0 Ma) is characterized by both subduction related volcanism associated with the
918 evolution of Tonga-Kermadec system, and by semi-continuous diffuse intraplate magmatic
919 activity. Maps are compiled and updated from Herzer (1995), Hayward et al. (2001), Mortimer
920 et al. (2010), Giba et al. (2013), Seebeck et al. (2014), Bache et al. (2014b), Mortimer et al.
921 (2018), Bischoff et al. (2019b), Tulloch et al. (2019), Uruski (2019), Barrier et al. (2020),
922 Global Volcanism Programme, and GNS Geological Map (Heron 2014). Triangles correspond
923 to the location of buried volcanoes mapped in seismic reflection datasets and petroleum
924 exploration drillholes. Circles are the location of volcanoes recognized onshore and by dredge
925 sampling of the sea-floor. Yellow polygons correspond to the area of the volcanoes mapped in
926 seismic lines and are only visible in the online version of this paper.
927
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928 List of Tables
929 Table 1: Nomenclature and main references of the volcanoes buried in the Taranaki, Deepwater
930 Taranaki and Reinga-Northland basins. Some of these volcanoes have been previously refer to
931 as petroleum exploration targets, which is not discussed in detail in this paper.
This study
Uruski 2019
Zhu et al. 2017
King and
Thrasher
1996
Herzer
1995
Brodie
1965
Chronos Composite
Volcano
-
Coopworth
Volcano
-
-
-
Rhea Composite
Volcano
Vulcan-
Hestia
volcanoes
Hestia-Vesta
volcanoes
-
-
-
Coopworth
Volcanoes
-
Coopworth
Volcano
-
-
-
Romney Volcanic
Field
Romney
Volcano
Romney Volcanic
Complex
-
-
-
Hades Caldera
-
-
-
Vulcan Composite
Volcano
-
-
-
Vulcan-
Volcanic
Group
Hestia Composite
Volcano
Vulcan-
Hestia
volcanoes
Hestia-Vesta
volcanoes
-
-
-
Tikati Volcanic Complex
Tikati
Volcanic
Field
Corriedale-East
V.F. and
Corriedale V.C.
-
-
-
Matuku Volcanics
obs: Cretaceous volcanic rocks
are reported in Arnberger
(2015)
-
-
-
-
-
Kaiwero Volcanic Field
Paleogene
mounds
-
-
-
-
Northland-Mohakatino
Volcanic Belt
-
-
Mohakatino
Volcanic
Centre
Northland
Volcanic
Belt
-
Aotea Volcanic Complex
Aotea
Seamount
-
-
-
Aotea
Seamount
West Ngatutura Volcanic Field
Pliocene
Volcanic
Field
-
-
-
-
932
933
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934 Table 2: Nomenclature and references of the volcanoes buried in the Canterbury and Great
935 South basins. At the region of the Papatowai Volcanic Field, Tulloch et al. (2019) mention
936 possible Cenozoic volcanic rocks based in interpretation of aeromagnetic data.
This study
Barrier 2019
Bischoff et
al. 2019
Blanke 2012
Beckman
2012
Field et al.
1989
Sloop Volcanic
Complex
Sloop Volcanic
Complex
-
-
Sloop
Volcano
Galleon
volcanics
Barque Volcanic
Complex
Barque Volcanic
Complex
-
-
-
Galleon
volcanics
Tapuku East Volcanic
Complex
Tapuku East
Volcanic Complex
-
-
Tapuku
East
volcanics
-
Papatowai Volcanic
Field
-
-
-
-
-
East Waiareka
Volcanic Field
Paleocene to
Oligocene
volcanoes
-
-
-
Endeavour
volcanics
Maahunui Volcanic
Field
Maahunui Volcanic
Field
Maahunui
Volcanic
Field
-
-
Volcanics of
Banks Peninsula
East Waipiata Volcanic
Field
Igneous intrusions
-
Saucer-
shaped sills
-
-
Toroa Volcanic Field
-
-
-
-
-
937
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938 Table 3: Main characteristics of the volcanoes buried in the Taranaki, Deepwater Taranaki and
939 Reinga-Northland basins. Symbol (*) refers to composition estimated from seismic
940 geomorphological analysis and (#) from integration of seismic and drillhole data. Abbreviations
941 are: late Cretaceous (lC), Eocene (E), early Miocene-Quaternary (eM-Q), middle Miocene
942 (mM), Pliocene (P).
Volcano
Age
Tectonic
setting
Type
Environ.
eruption
Dominant
comp.
Chronos
Composite
Volcano
lC
~85 Ma
Rhea Composite
Volcano
lC
~84 Ma
Zealandia
rift
Composite Cone
Unknown
Coopworth
Volcanoes
lC
~74 Ma
Cluster of small-volume
cone-type volcanoes
Basaltic *
Romney
Volcanic Field
lC
~74 to 73 Ma
Cluster of small-volume
cone-type volcanoes
fissures and crater-rows,
and a large shield
volcano
Basaltic to
intermediate.
Rhyolitic? #
Hades Caldera
lC
~73 Ma
Caldera
Vulcan
Composite
Volcano
lC
~73 to 70 Ma
Hestia
Composite
Volcano
lC
~72 to 68 Ma
Composite Cone
Paralic
Tikati Volcanic
Complex
lC
~70 Ma
Cluster of small-volume
cone-type volcanoes and
one large multi-vent
complex
Shallow-
submarine
Unknown
Matuku
Volcanics
lC
~68 Ma
West
Coast-
Taranaki
rift
Cluster of intrusions and
relative small-volume
vents
Paralic
Basaltic #
Kaiwero
Volcanic Field
E
~55 Ma
Intraplate
Cluster of small-volume
cone-type volcanoes
Shallow to
deep-
submarine
Basaltic *
Northland-
Mohakatino
Volcanic Belt
eM-Q
~23 to 0 Ma
Intra-arc
Belt of large composite
and shield volcanoes
Deep-
submarine to
subaerial
Basaltic-
Andesitic #
Aotea Volcanic
Complex
eM
~23 Ma
Intraplate
Composite multi-vent
massif
Basaltic #
West Ngatutura
Volcanic Field
Pli
~3.5 Ma
Intraplate
Cluster of small-volume
cone-type volcanoes
Deep-
submarine
Basaltic *
943
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944 Table 4: Main characteristics of the volcanoes buried in the buried in the Canterbury and Great
945 South basins. Symbol (*) refers to composition estimated from seismic geomorphological
946 analysis and (#) from integration of seismic and drillhole data. Abbreviations are: late
947 Cretaceous (lC), Paleocene-Oligocene (P-O), Oligocene (O), early Miocene (eM), middle
948 Miocene (mM), Miocene (M), Pleistocene (Ple).
Volcano
Age
Tectonic
setting
Type
Environ.
eruption
Dominant
composition
Sloop Volcanic
Complex
lC
~83 to 70 Ma
Rift and
Post-rift
Composite multi-vent massif
Unknown
Barque
Volcanic
Complex
lC
~ 79 to 67 Ma
Post-rift
SW edifice: Strato-shield,
central caldera and several
overlapping small vents. NE
edifice: stratocone
Probably
Basaltic *
Tapuku East
Volcanic
Complex
lC
~83 to 70 Ma
Rift and
Post-rift
Composite multi-vent massif
Paralic
Unknown
East Waiareka
Volcanic Field
P-O
Cluster of small-volume
cone- and crater-type
volcanoes
Basaltic
alkaline #
Papatowai
Volcanic Field
O or eM
Cluster of small-volume
cone-type volcanoes and
pillow mounds?
Shallow to
deep-
submarine
Unknown
Maahunui
Volcanic Field
mM
12.7 to 11.5 Ma
Cluster of small-volume
cone- and crater-type
volcanoes
Basaltic
alkaline #
East Waipiata
Volcanic Field
M
~ 18 Ma
Cluster of intrusions and
relative small-volume vents
Deep-
submarine
Probably
Basaltic *
Toroa Volcanic
Field
Ple
?
Intraplate
Cluster of small-volume
cone- type volcanoes
Shallow to
deep-
submarine
Unknown
949
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950 Supplementary Material
951 Supplementary Material 1: Glossary
952 Volcano: A vent in the Earth’s surface through lava, rock fragments, hot vapour, and gas are
953 or have been erupted from the subsurface.
954 Buried volcano: The remains of one of multiple ancient volcanoes now buried and preserved
955 in sedimentary basins. Also informally called fossil volcano.
956 Volcanic group: An area characterized by an assemblage of related volcanoes and associated
957 rock strata largely of volcanic origin.
958 Volcanic complex: A large volcanic structure or massive comprising diverse overlapping
959 volcanoes erupted by multiple episodes.
960 Volcanic field: An area with a large number of scattered volcanic edifices and craters that share
961 a genetic association and were erupted in the same interval of time.
962 Polygenetic volcano: Large volcanic structure formed by multiple eruptive episodes bounded
963 by unconformities. Here the term unconformity refers to a surface separating two rock masses
964 or strata of different ages.
965 Monogenetic volcano: Typically small-volume volcanic structure where the eruptive activity
966 ceases after a continuous or semi-continuous episode.
967 Composite or stratovolcano: A cone-shaped volcanic morphology constructed of
968 accumulations of lava and pyroclastic deposits mainly erupted from a central stationary vent
969 located at the summit of the structure, and from parasitic vents located at its flanks.
970 Shield volcano: A shield-shaped volcanic morphology that shows a large width if compared
971 with its height, typically formed by overlapping highly fluidal lava-flows erupted from a
972 relatively stationary central vent located at the summit of the structure, and from parasitic vents
973 located at its flanks.
974 Caldera: A volcanic structure that show a low-angle outward-dipping apron with a central
975 topographic depression bounded by ring faults, typically resulting from collapse of the roof of
976 a magma chamber.
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977 Lava field: An area of nearly flat-lying and overlapping deposits of lava-flows typically
978 originated from highly fluidal magmas.
979 Parasitic vents: Small-volume volcanoes that erupted from the flanks of a large volcanic
980 structure of same age.
981 Satellite vents: Small-volume volcanoes that erupted in the surroundings of a large volcanic
982 structure of same age.
983 Cone-type volcano: A small-volume volcano (<5 km3, typically <1 km3) dominantly showing
984 a cone- or mound-shaped morphology constructed above its pre-eruptive surface. This class
985 includes pillow mounds, and cinder, tuff, scoria, and spatter cones.
986 Crater-type volcano: A small-volume volcano (<5 km3, typically <1 km3) dominantly
987 showing a crater- or funnel-shaped morphology excavated into its pre-eruptive surface. This
988 class includes tuff rings and maar-diatreme volcanoes.
989
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990
991 Supplementary Material 2: Photographs of cutting samples and thin-sections of volcanic rocks
992 perforated by the drillhole Romney-1 at the depth interval of 3680 to 3785 m. (a) Felsic volcanic
993 rock comprising plagioclase, quartz and rare mafic minerals from the depth of 3746 to 3752 m.
994 (b) SEM image of a volcanic rock comprising k-feldspar (k-felds), quartz (qz) and an
995 moderately altered matrix. The number in the blue boxes are from EDS spectrometry analysis
996 and show the composition of two minerals in the image. (c) Thin section in cross-polarized light
997 and (d) SEM image of fragments of volcanic rocks comprising feldspars (felds), quartz and a
998 matrix altered to palagonite (pal) from the depth of 3773 to 3779 m. (Fe) is an iron oxide. The
999 typical high degree of alteration of the volcanic interval sampled in Romney-1 make mineral
1000 characterization and geochemistry analysis difficult in many cases. However, it was possible to
1001 observe that the mafic rocks present high content of plagioclase and are commonly altered to
1002 palagonite, which suggests an original basaltic composition. The felsic rocks were likely
1003 erupted from more evolved silica-rich magmas, evident by a high content of k-feldspar and
1004 quartz, and general lack of ferromagnesian minerals such as olivine and pyroxene.
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1005
1006 Supplementary Material 3: Uninterpreted (a) and interpreted (b) 2D seismic section across the
1007 shield volcano of Romney Volcanic Field, and transversal to the main N-S fault that cross the
1008 extension of the volcanic field. Note that the fault offsets the eastern flank of the volcano and
1009 is associated with growth strata in the hanging wall of the fault, which indicates that the fault
1010 was active during magmatic activity. The age of the fault is contemporaneous with the opening
1011 of the West Coast-Taranaki rifting. Rift phases names from Strogen et al. 2014.
1012
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1013
1014 Supplementary Material 4: Uninterpreted (a) and interpreted (b) 2D seismic section across some
1015 of the volcanoes in the Tikati Volcanic Complex. Note that the pre- and post-eruptive surfaces
1016 of the volcanoes are confined between the basement and the base Paleocene reflection (~66
1017 Ma), which indicate a Cretaceous age for this volcanic field.
1018
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1019
1020 Supplementary Material 5: Evidence of magmatic activity of late Cretaceous age in the Matuku
1021 area, south Taranaki Basin. (a) 2D section showing an intrusive body with saucer-shaped
1022 geometry and potential correlated vents. Note that the strata above the intrusion is forced-
1023 folded, which is onlapped by strata of early Paleocene age. (b) 2D section across some of the
1024 vents in Matuku area. Note that the vents form mound-like structures that are onlapped by strata
1025 of Cretaceous age. (c and d) Maps showing the distribution of intrusions and vents in the
1026 Matuku area. Red dashed lines mark the location of vents, while white dashed lines show the
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1027 location of intrusions. Note a correlation of vents typically locate at the tip of the saucer-shaped
1028 intrusions. (e and f) show fragments of volcanic rocks sampled by the drillhole Matuku-1 from
1029 a depth of 4785 to 4795 m. Most volcanic fragments are clasts in sedimentary rocks that could
1030 be eroded from the basement, however, isolate volcanic fragments are also present. One sample
1031 from 4325-4326 m contain three detrital zircon dated in 68.5 ± 4.7 Ma, much younger than any
1032 other granitic rock in the area, but consistent with the age of some eruptive centres in Westland
1033 and Deepwater Taranaki Basin (GNS consultancy reports 2014/176 and 2014/273). The zircon
1034 age is coherent with the stratigraphic ages of intrusions and eruptive vents mapped in seismic
1035 reflection data, thus, we interpret that at a local magmatic event have happened in the Matuku
1036 area at ca 68 Ma. Note that, regionally, these vents are roughly aligned with the N-S trend of
1037 late Cretaceous volcanoes in the Taranaki and Deepwater Taranaki basins (Figure 1and Figure
1038 14).
1039
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1040
1041 Supplementary Material 6: (a and b) show 2D seismic sections across Aotea Volcanic Complex.
1042 The maps in the bottom right hand corners correspond to the top of the volcanic edifice (a),
1043 while (b) shown the structural map of the basement. Aotea Volcanic Complex was constructed
1044 immediately above the base of the Miocene chronostratigraphic surface which indicate an early
1045 Miocene age (ca 23 Ma). This age is supported by Ar-Ar dating of 22.5 Ma (Mortimer et al.
1046 2018), suggesting that the volcanic structure was probably constructed in less than 1 Ma, which
1047 is reinforced by the broad amalgamation of the pre-and post-eruptive surfaces with increasing
1048 distance from the eruptive centre. Note that the structure comprises a large main body with
1049 multiple parasitic and satellite vents. The volcanic complex seems to have erupted parallel with
1050 a WSW-ENE basement structure, which may explain its elongated form. However, this minor
1051 trend may be a seismic artefact caused by pull-up of velocity below the edifice. More notable
1052 is the summit of the structure that is located in the boundary between a high of the basement of
1053 NW orientation and the SE part of the New Caledonia Trough, suggesting some sort of pre-
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1054 existing structures control the location of the eruptive centres. If the small scale WSW-ENE
1055 trend shown in (b) is not a seismic artefact, it is likely that magma has used pre-existing
1056 structures to arise to the surface.
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1057
1058 Supplementary Material 7: Uninterpreted (a) and interpreted (b) 2D seismic section across the
1059 Sloop Volcanic Complex. The mound-like structure overlap reflections of Cretaceous age and
1060 its onlapped by reflections older than 66 Ma, indicating a late Cretaceous age for the formation
1061 of the volcanoes. Note that the structure is composed by multiple overlapping volcanic edifices,
1062 a characteristic of volcanic complexes.
1063
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1064
1065 Supplementary Material 8: (a) 2D seismic section across the Barque Volcanic Complex. (b)
1066 shown the chronostratigraphic surface that correspond to the top of the SW volcanic edifice. (c)
1067 is a magnetic map (1st derivative) over the SE edifice. Barque Volcanic Complex comprise a
1068 large SW strato-shield volcano (17 km across and ca 1000 m high) with a central negative
1069 magnet anomaly at the centre of the structure (caldera?), and a large NE stratocone, as well as
1070 several overlapping volcanoes and numerous intrusive bodies. This volcanic complex was
1071 initially inferred from 2D seismic lines acquired during the 1960’s to 1980’s. In 2013 a high-
1072 resolution 3D seismic survey imaged the volcanic edifice in detail, revelling a substantial
1073 volcanic edifice emplaced from ca 79 Ma and capped by a marine erosional surface dated as
1074 latest Cretaceous (Beggs 2016). To date, there are no drillholes confirming a volcanic origin of
1075 the seismic anomalies. (b and c) are modified from Beggs 2016.
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1076
1077 Supplementary Material 9: Uninterpreted (a) and interpreted (b) 2D seismic section across the
1078 Tapuku East Volcanic Complex. The mound-like structure overlap reflections of Cretaceous
1079 age and its onlapped by reflections older than 66 Ma, indicating a late Cretaceous age for the
1080 formation of the volcanoes. Note that the structure comprise multiple summits likely to
1081 represent overlapping volcanic edifices, a characteristic of volcanic complexes. Note a younger
1082 intrusion with saucer-shaped morphology, likely of Miocene age and possible correlated with
1083 the saucer-intrusions imaged in the Waka-3D seismic survey (Figure 10 and Figure 13).
1084
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1085
1086 Supplementary Material 10: Uninterpreted (a) and interpreted (b) 2D seismic section across the
1087 Toroa Volcanic Field. The high-amplitude of the mound-like anomalies, associated with pull-
1088 up velocity seismic artefacts indicate that these anomalies are composed of hard material, likely
1089 to be of volcanic origin. The Toroa-1 drillhole did not recover samples from the volcanic
1090 interval.
1091
1092
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