Evolution of the Intra-Arc Taupo-Reporoa Basin Within the Taupo Volcanic Zone of New Zealand

Home , Mamaku, Mangakino, Ohakuri, Orakei Korako, Reporoa, Taupo Volcanic Zone

Evolution of the intra-arc Taupo-Reporoa Basin within the Taupo Volcanic Zone of New Zealand

D.T. Downs1,*, J.V. Rowland1, C.J.N. Wilson2, M.D. Rosenberg3, G.S. Leonard4, and A.T. Calvert5 1School of Environment, University of Auckland, Private Bag 92019, Auckland 1142, New Zealand 2School of Geography, Environment, and Earth Sciences, Victoria University, PO Box 600, Wellington 6140, New Zealand 3GNS Science, Private Bag 2000, Taupo 3352, New Zealand 4GNS Science, PO Box 30368, Lower Hutt 5040, New Zealand 5U.S. Geological Survey, Volcano Science Center, 345 Middleﬁ eld Road, Menlo Park, California 94025, USA

ABSTRACT 58 ± 26 k.y. of Paeroa Subgroup emplace- of eruptions can provide readily datable and ment, but in two stages. The northern Paeroa identifi able time horizons that allow for high The spatial and temporal distributions block underwent uplift and associated tilting resolution (e.g., 10 to 100 k.y.) interpretation of volcaniclastic deposits in arc-related fi rst, followed by the southern Paeroa block. of a basin’s evolution (e.g., Houghton et al., basins refl ect a complex interplay between Elevations (>500 m above sea level) of lacus- 1995; Smith et al., 2008). However, these same tectonic, volcanic, and magmatic processes trine sediments within the southern Paeroa rates of volcanic production, in combination that is typically diffi cult to unravel. We take block are consistent with elevations of rhyo- with varying vent locations, positions of avail- advantage of comprehensive geothermal drill lite lavas in the Ongaroto Gorge, the outlet to able accommodation space, and extreme post- hole stratigraphic records within the Taupo- the paleolake in which these sediments were eruptive sedimentation rates, generally result in Reporoa Basin (TRB), and integrate them deposited, and indicate that the Paeroa block rapid lateral facies changes and burial of strata, with new 40Ar/39Ar age determinations, exist- has remained relatively stable since develop- greatly complicating the stratigraphic architec- ing age data, and new mapping to develop a ment. East of the Paeroa block, stratigraphic ture (Busby and Bassett, 2007). Furthermore, as four-dimensional model of basin evolution relationships show that movement along the these basins often host large-scale hydrothermal in the central Taupo Volcanic Zone (TVZ), Kaingaroa Fault zone, the eastern boundary systems, post-depositional modifi cation through New Zealand. Here, exceptional rhyolitic of the central TVZ, is associated with vol- hydrothermal alteration can add complexity productivity and high rates of extensional cano-tectonic events. Stratigraphic and age by overprinting both subsurface and exposed tectonism have resulted in the formation of data are consistent with rapid formation of deposits (Steiner, 1963, 1977; Browne, 1978; at least eight calderas and two subparallel, the paired TRB and TFB at 339 ± 5 ka, and Grindley et al., 1994). northeast-trending rift basins, each of which indicate that gradual, secular rifting is punc- Quaternary basins of the central Taupo Vol- is currently subsiding at 3 to 4 mm/yr: the tuated by volcano-tectonic episodes from canic Zone (TVZ) (Fig. 1) are no different in Taupo fault belt (TFB) to the northwest and time to time. Both processes infl uence basin their complexity than arc-related basins else- the TRB to the southeast (the main subject evolution. where. However, the tempo of their development of this paper). The basins are separated in is exceptional and affords an excellent opportu- the northeast by a high-standing, fault-con- INTRODUCTION nity to capture their evolution in high fi delity. trolled range termed the Paeroa block, which This area undergoes secular rifting, some parts is the focus of mapping for this study, and in Active convergent margins are characterized at >10 mm/yr (Wallace et al., 2004), coupled the southwest by an along strike alignment of by tectonic, volcanic, and magmatic processes with an exceptionally high rate of caldera-form- smaller scale faults and an associated region that infl uence basin development and provide ing silicic volcanism (3.8 km3/k.y. over the past of lower relief. Stratigraphic age constraints an abundance of volcaniclastic and sedimentary 1.6 m.y.), and frequent smaller scale explosive within the Paeroa block indicate that a single material that fi lls accommodation space. Under- and effusive eruptions (1 per 900 yr over the basin (~120 km long by 60 km wide) existed standing the interplay between such processes past ~61 k.y.; Wilson et al., 2009). This activity within the central TVZ until 339 ± 5 ka requires knowledge of the geochemistry of arc has resulted in the development of young, deep (Paeroa Subgroup eruption age), and it is systems, and the stratigraphic and structural (>3 km) basins with a plethora of dateable time inferred to have drained to the west through architecture of the resultant basins. On a global horizons (Houghton et al., 1995; Wilson et al., a narrow and deep constriction, the present- scale, the geochemistry of arc systems is well 2009). Although much of the older strata and day Ongaroto Gorge. Stratigraphic evidence known (Pearce and Peate, 1995). However, few structure is buried, more than 400 geothermal and fi eld relationships imply that develop- well resolved stratigraphic and structural archi- exploration and production drill holes provide ment of the Paeroa block occurred within tecture models of arc-related basins have been stratigraphic and petrographic data to depths of developed (Cas and Wright, 1987; Busby and 3.3 km (e.g., Browne et al., 1992; Rosenberg *Corresponding author e-mail: d.downs@ auckland Bassett, 2007; Manville et al., 2009; Sohn et al., et al., 2009). Synthesis of subsurface stratigra- .ac.nz 2013). In most such settings, high frequencies phy with fi eld and geophysical data provides a

Geosphere; February 2014; v. 10; no. 1; p. 185–206; doi:10.1130/GES00965.1; 13 ﬁ gures; 2 tables; 1 supplemental ﬁ le. Received 21 July 2013 ♦ Accepted 3 December 2013 ♦ Published online 14 January 2014

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Figure 1. Summary map of central North 176°E Island showing the extents of rhyolitic Coromandel Volcanic Zone TVZ calderas TA Taupo (active) Taupo ignimbrites from the central Taupo Vol- WH Whakamaru Volcanic canic Zone (TVZ), and volcanic rocks from MA Mangakino Australian Zone OH Ohakuri plate the earlier Coromandel Volcanic Zone. The PLVZ Paeroa North RE Reporoa three main volcanic segments, major cal- Hauraki Rift Island KA Kapenga dera centers, and high temperature geother- RO Rotorua mal ﬁ elds of the TVZ are displayed. Lines OK Okataina (active) labeled old TVZ and young TVZ (after Wil- son et al., 1995) represent envelopes around HSM known or inferred rhyolitic vents from Bay of Plenty 1.6 Ma to 349 ± 4 ka and 349 ± 4 ka to pres- Pacific plate ent, respectively. Active faults (last rupture Alpine Fault 20 ka or younger) are courtesy of the GNS South Island Science Active Faults Database (2011, http:// Kilometers data .gns .cri .nz /af/). KFZ—Kaingaroa Fault 300 zone. The inset shows the TVZ location relative to the Hikurangi subduction margin A (HSM). Map is in the World Geodetic Sys- R tem 84 reference grid. 2222

Waikato River 1919 2121 2323 RORO 2200 2244 1188 OKOK rich framework with which to interpret the four- KAKA 1177 dimensional (4-D) evolution of selected vol- 1166 cani clastic ﬁ lled basins within the TVZ. 6 OHOH 1155 8 PLVZPLVZ Here we present new ﬁ eld mapping and geo- MAMA 7 9 1414 1313 RREE chronology from an active intra-arc basin within 1100 th Island 5 central TVZ, the Taupo-Reporoa Basin (TRB). 1111 1122

Nor WWHH fault system We focus on outcrops within the upstanding Paeroa block along the basin’s northwest mar- 4 aroa Key Geothermal gin (Figs. 2 and 3). These new data sets are 3 KFZ TVZ boundaries fields KaingPlateau 1 Tokaanu integrated with a reconsidered stratigraphic Inferred caldera 2 Lake Taupo 3 Wairakei-Tauhara R TTAA Figure 2 boundaries framework based on geothermal drill cores and 2 4 Rotokawa Lake Geothermal fields 5 Mokai cuttings (e.g., Steiner, 1963, 1977; Rae, 2007; A Taupo 6 Mangakino Central TVZ rhyolitic 7 Ongaroto Rosenberg et al., 2009, 2010), using existing 8 Atiamuri (Wilson et al., 2009, 2010) and new age deter- 1 lavas & ignimbrites 9 Ohakuri (extinct) Coromandel Volcanic 10 Orakei Korako minations to develop a 4-D evolutionary model 39°S 38°S11 Ngatamariki 37°S Zone rocks 12 Ohaaki for the entire TRB. We demonstrate that deposit Tongariro Active andesite 13 Reporoa volcanoes 14 Te Kopia geometries are controlled by both tectonic and 15 Waiotapu-Waikite Ngauruhoe TVZ faults 16 Waimangu volcanic processes, and that the time scales of 17 Horohoro these processes vary considerably over the his- Ruapehu North Island fault 18 Rotorua Old TVZ boundary system & Hauraki 19 West Rotorua tory of the basin. Our 4-D reconstruction of the Rift faults 20 East Rotorua Young TVZ boundary 21 Rotoiti TRB provides a context for understanding the A TVZ andesite regions 22 Taheke 23 Rotoma evolution of relict and active arc systems, and R TVZ rhyolite region 24 Kawerau contributes to our knowledge of punctuated, and interconnected, tectonic, volcanic, and magmatic events. Hikurangi Plateau at the Hikurangi subduction ners, 2013). This migration is indicated by the margin ~10 Ma induced extension within the southeastward younging of volcanism (Black TECTONIC AND GEOLOGIC SETTING overriding plate (Reyners, 2013). Initially, this et al., 1992; Adams et al., 1994; Houghton et al., extension was focused along the Hauraki Rift, 1995), geothermal activity (Rowland et al., The northeast-trending TVZ is the most which is a north-northwest–trending feature that 2010; Mauk et al., 2011), and fault-controlled recent (past ~2 m.y.) manifestation in a >17 has been active since at least ~7 Ma, parallels volcaniclastic basins formed in Mesozoic m.y. record of similarly oriented arc vol canism basement terrane boundaries, and presumably metasedimentary basement rocks (Villamor and associated with subduction of the Paciﬁ c plate intersects the TVZ near the Whakamaru cal- Berryman, 2006). beneath the North Island of New Zealand dera (Fig. 1) (Wilson et al., 1986; Hochstein The TVZ is segmented both structurally and (Fig. 1) (Mortimer et al., 2010). From the Late and Ballance, 1993). Since ~6 Ma, extension volcanically. Arc-related composite cone-build- Miocene to ~6 Ma, the locus of volcanism has been localized along the axis of the arc as it ing andesitic volcanism aligned along the axis was located farther northwest along the line of rapidly migrated to the southeast, concomitant of the rift occurs to the northeast and southwest the Colville arc. However, subduction of the with rollback of the subduction hinge (Rey- of a central 120 km long, 60 km wide segment

186 Geosphere, February 2014

Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/10/1/185/3333253/185.pdf by guest on 25 September 2021 Taupo Volcanic Zone Basin Evolution E ′ Most recent Kaingaroa Fault zone movement Geothermal fields Ongaroto Gorge MO Maungaongaonga Rolles Peak RP 176°30 ka- trata. WF Whakaheke Fault Plateau Age date locations Kaingaroa

TVZ boundaries Inferred caldera boundaries Faults 281 ka 281 16 K Key Andesite/Dacite Volcanoes TRB Bounding Faults MK Maungakakaramea TH Tauhara KF Kaiapo FaultPF Paeroa Fault OKF Orakei Korako Fault Leonard et al. ed from M MK MK 15 O 13 Reporoa MO M

k ka 349

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O O OKF OKF 103 ka Whakamaru Ar age determinations and sample locations are shown for reference. See Figure 3 for the 3 for See Figure reference. shown for age determinations and sample locations are Ar 3 39 9 Ar/ 40

centre

8 WF Maroa volcanic

Ohakuri KF 7

Taupo 5 Western Dome Belt Dome Western stratigraphic key, legend, and descriptions of all units. Map is in the World Geodetic System 84 reference grid. Geodetic System 84 reference World legend, and descriptions of all units. Map is in the stratigraphic key, Figure 2. Geologic map of the Taupo-Reporoa Basin (TRB) showing the distribution of lavas, and volcaniclastic sedimentary s Taupo-Reporoa 2. Geologic map of the Figure Wha Orakei Korako, Fault zone and Paeroa, delineated by the Kaingaroa TRB are The eastern and western boundaries of the (2010). New heke, and Kaiapo faults, respectively. New mapping within the Paeroa block is outlined; deposits outside the Paeroa block and Reporoa caldera are modiﬁ caldera are block and Reporoa block is outlined; deposits outside the Paeroa New mapping within the Paeroa

E ′ Mangakino Lake Taupo 175°50

S 38°20 38°30 S 38°40 S ′ ′ ′

Geosphere, February 2014 187

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Stratigraphic Architecture Stratigraphic Key Age Source Rock Type & Description

Surficial deposits ≤25.4±0.2 ka Taupo Primarily consists of the unwelded Oruanui and Taupo formation ignimbrites, with minor fall deposits & lacustrine sediments (see Manville, 2001; Wilson, 2001; Manville and Wilson, 2004; Wilson et al., 2009). Hydrothermal breccia Unknown Te Kopia(?) † Hydrothermal eruption breccia - Multicolored (alteration induced), clast-supported, sparse sinter & ignimbrite clasts, mostly laminated fine-grained clasts up to 2 m-size, angular clasts, poorly-sorted, silicified, pumice-absent, crystal-poor, & sparse matrix.

† Rhyolitic ignimbrites interbedded with lacustrine sediments - Unwelded (but case hardened), * 281±9 to Locally Mihi Breccia diffusely bedded (cm- to m-scale), multicolored, crystal-poor to -moderate (quartz, plagioclase), 239±3 ka sourced pumice-rich (breadcrusted textures), lithic-rich (up to 1.5 m-size with rhyolite lavas & fine-grained sediments), moderately sorted, displays overall normal grading, vitric fine-ash to coarse sand- size matrix, typically devitrified, zeolitized, & silicified. Small-scale crossbedding in the basal 4 cm. Soft sediment deformation & fine-grained clastic dikes identified throughout (see Grindley, 1959, 1961; Healy et al., 1964). Kaingaroa Formation * 281±21 ka Reporoa † Rhyolitic ignimbrite - 1) gray lower unit, 2) black middle unit, 3) pink upper unit. Variably welded (locally silicified), massive, vapor-phase altered, poorly-sorted, pumice-rich, crystal-poor (plagioclase, quartz), lithic-rich to -poor (up to 2 m-size with rhyolite lavas, crystal-rich ignimbrite, recycled Kaingaroa ignimbrite, fine-grained sediments), some vapor-phase alteration, vitric fine- ash matrix (areas of devitrification), & locally accretionary lapilli-bearing (see Nairn et al., 1994; Beresford, 1997; Beresford and Cole, 2000; Beresford et al., 2000). Ohakuri Formation * ~280 to Ohakuri † Rhyolitic ignimbrite - Massive to parallel bedded to large-scale low-angle trough crossbedded 290 ka (cm- to m-scale), unwelded (locally silicified), gray to tan colored, crystal-poor to -moderate (quartz, plagioclase), pumice-poor to -rich (2 types identified), lithic-poor (localized lithic-rich areas with rhyolite & dacite lavas, obsidian, fine-grained sediments, black ignimbrite, dolerites to granitoids), poorly-sorted, accretionary lapilli-bearing, abundant vitric fine-ash matrix, contains a basal clast-supported (up to 1 m-size) breccia that grades normally to matrix supported ignimbrite (see Gravley, 2004; Gravley et al., 2007). * ~280 to Rhyolitic ignimbrite - Welded, pink to gray colored, vapor-phase altered, & crystal-poor (see Huka Mamaku Formation Rotorua Group 290 ka Milner, 2001; Milner et al., 2003). (see Grindley, 1965; Steiner, 1977) 1965; Steiner, (see Grindley,

Pokai Formation * ~300 ka Kapenga Rhyolitic ignimbrite - Unwelded base to welded columnar jointed top, brown colored with orange pumice, & crystal-poor (see Karhunen, 1993). Variable, but typically flow-banded to perlitic textured and crystal-poor (see Wilson et al., 1995) Variable, (see Browne et al., 1992; Arehart et al., 2002; Heise 2007) (see Browne et al., 1992; Variable, but typically high-alumina compositions (see Wilson et al., 1995) Variable, porphyritic texture in a microcrystalline groundmass (see Wilson et al., 1995) Typically (see Nairn, 2002; Leonard et al., 2010)

Chimp Formation Unknown Kapenga Rhyolitic ignimbrite - Unwelded, common crystal-poor pumice, cream colored matrix & pumice (see Karhunen, 1993).

Matahina Formation 322±7 ka Okataina † Rhyolitic ignimbrite - Variably welded, tan to pink to purplish-gray colored, columnar jointed, crystal-poor (plagioclase, quartz, hornblende, orthopyroxene), pumice-poor to -moderate (typically tan colored), contains abundant black slightly vesicular vitric clasts, & has a fine- grained vitric matrix (see Bailey and Carr, 1994; Grindley et al., 1994; Nairn, 2002). Maroa & Okataina Volcanic Plutonic rocks Predominantly lacustrine & fluvial sediments, & undefined volcanics Rhyolite lavas Basalt lavas Andesite & dacite lavas Center pyroclastic deposits Locally Pyroclastic deposit - Lithic-rich, poorly-sorted, vitrophyric (commonly altered) matrix with airfall & Unknown Rautawiri Breccia sourced water-laid material (see Grindley, 1970; Wood, 1983).

Paeroa Subgroup: (Te Kopia, Te Weta, Paeroa ignimbrites) 339±5 ka Paeroa † Rhyolitic ignimbrites - Variably welded, massive to eutaxitic, columnar jointed, crystal-rich (quartz, plagioclase, biotite, hornblende, pyroxene, magnetite/ilmenite), pumice-rich to -poor, lithic-rich to -poor (up to 4 m-size with ignimbrites, obsidian, rhyolite & andesite lavas), & has a fine-grained matrix (see Wilson et al., 1986; Keall, 1988; Brown et al., 1998). Paeroa Subgroup is typically more strongly welded than other Whakamaru Group ignimbrites. Group

Whakamaru (Manunui, Te Whaiti, Rangitaiki, Whakamaru ignimbrites, Rangitawa tephra) 349±4 ka Whakamaru

Waiotapu Formation 710±60 ka Kapenga † Rhyolitic ignimbrite - Strongly welded (locally silicified), strongly eutaxitic, devitrified, tan-pink to black colored, lithic-moderate (rhyolite & andesite lavas, ignimbrite, fine-grained sediments), crystal-poor (absent quartz), & fine-grained matrix (see Grindley et al., 1994).

Marshall Formation 950±30 ka Mangakino Rhyolitic ignimbrites - 1) dark colored unit with brown pumice, 2) pale gray colored, vapor-phase altered unit, & 3) dark gray colored unit. Variably welded (see Wilson, 1986). (see Gravley et al., 2006)

Akatarewa ignimbrite 950±50 ka Unknown Rhyolitic ignimbrite - Crystal-rich, is not correlative with any known surface ignimbrite. Stratigraphic position relative to the Marshall Formation is unclear (see Wilson et al., 2010). Group

Reporoa Ahuroa ignimbrite 1.18±0.02 Ma Mangakino Rhyolitic ignimbrite - Variably zoned, welded, & crystal-rich with sparse to absent quartz (see Wilson, 1986; Wilson et al., 2010).

Ongatiti Formation 1.21±0.04 Ma Mangakino Rhyolitic ignimbrite - Variably welded, commonly vitrophyric, pumice-rich, & crystal-rich (see Wilson, 1986; Wilson et al., 2010).

1.45 Ma ignimbrite 1.45±0.05 MaUnknown Rhyolitic ignimbrite - Welded & crystal-rich (see Wilson et al., 2010). Predominantly lacustrine & fluvial sediments, & undefined volcanics

Granitic- Slightly metamorphosed, massive, well-indurated, fine-grained, quartzofeldspathic sandstone Metasedimentary Torlesse Supergroup Late rhyolitic with a minor component of interbedded mudstone (see Mortimer, 1994; Wood et al., 2001; basement rocks (Kaweka Terrane) Jurassic provenance Adams et al., 2009).

Figure 3. Stratigraphic architecture, descriptions, and key for all deposits discussed within the text and shown on all maps, cross sections, and illustrations (unless otherwise indicated). The asterisk indicates new age dates discussed in the text, and the dagger symbol denotes our description of deposits from the Paeroa block. The thick boxes (Akatarewa ignimbrite, Chimp Formation) represent units that are mentioned in the text, but are not displayed in any ﬁ gures. See references noted by deposits for more detailed descriptions and information.

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dominated by rhyolitic volcanism, the central 2). The total estimated resource in this basin is delimited by the currently inactive Kaingaroa TVZ (Fig. 1) (Wilson et al., 1995). Structur- exceeds 2000 MW (Bibby et al., 1995). The Fault zone, which is discussed in more detail in ally, the axis of the rift is offset along strike rich database of drill hole stratigraphy obtained the following (Fig. 2). by transfer (accommodation) zones that align during more than 50 years of geothermal explo- Geothermal drilling demonstrates that the top with caldera margins, geothermal fi elds, and ration, and high resolution local- and regional- surface of the metasedimentary basement, in inferred deep-seated basement faults, some of scale geophysical imaging (aeromagnetics, which the basin has formed, is at ~1 to >3 km which align with faults of the north-northwest– gravity, resistivity, magnetotellurics) has facili- depth (Wood et al., 2001; Rae, 2007; Rosenberg trending Hauraki Rift (Rowland and Sibson, tated development of reservoir- (e.g., Rae, 2007; et al., 2010). Basin fi ll comprises predominantly 2001, 2004). Rosenberg et al., 2009) and crustal-scale geo- Quaternary rhyolitic ignimbrites interbedded The volcaniclastic fi lled basins typically are logical models (e.g., Heise et al., 2007, 2010). with lavas of all compositions, reworked vol cani- interpreted as developing gradually through However, the 3-D stratigraphic and structural clastic strata, lacustrine and fl uvial sediments, secular rifting (Villamor and Berryman, 2001; architecture of the TRB and its temporal evolu- and paleosols (Figs. 2, 3, and 4A–4C) (Leonard Nicol et al., 2006), which increases in magni- tion remain poorly resolved. et al., 2010). Silicic plutonic bodies have been tude from ~3 to >15 mm/yr from southwest to 2. The timing and rapidity of tectonism, proposed as being widely present (Evison et al., northeast, respectively, along the axis of the volcanism, and magmatism that strongly infl u- 1976; Stern, 1985); however, the only in-situ TVZ (Wallace et al., 2004). However, within the ence basin development within the TVZ are plutonic bodies identifi ed are in the Ngatamariki central TVZ, an additional, episodic basin-form- unknown (gradual versus punctuated; Villamor geothermal fi eld at 2.6 km or greater depth (Fig. ing process adds complexity to the structural and Berryman, 2001; Rowland et al., 2010). 4B) (Browne et al., 1992; Arehart et al., 2002; and stratigraphic history of the region. Over the Comprehensive mapping, reevaluation of drill Mighty River Power, 2013, written commun., past 1.6 m.y., the central TVZ has undergone at hole stratigraphy, and geochronology allow for drill hole data). least 25 large (30 to >1500 km3) rhyolitic erup- the evolution of the TRB to be understood in Quaternary strata within the TRB can be tions, resulting in the formation of at least eight terms of interconnected tectonic, volcanic, and divided into four packages, the fi rst three of calderas centers (Wilson et al., 2009). These cal- magmatic processes. which are relevant to our discussion (Figs. 3 and deras range in diameter from ~10 to 40 km, and 3. The TRB occupies an equivocal position in 5A–5D): (1) the Reporoa Group, which includes are superimposed upon, and in some cases per- the tectonic confi guration of the Hikurangi sub- deposits that unconformably overlie metasedi- haps intimately associated with the formation duction margin, between the TFB to the west mentary basement through to but not including of, fault-controlled rift basins (Gravley et al., and the right-lateral North Island fault system the 349 ± 4 ka Whakamaru Group (Gravley et al., 2007; Rowland et al., 2010; Allan et al., 2012). to the east. Whereas the TFB accommodates 2006); (2) the Whakamaru Group, which corre- The modern foci of rhyolitic volcanism are the near orthogonal extension, the latter feature sponds with a regionally extensive and distinc- Okataina and Taupo volcanic centers (Fig. 1) accommodates the margin-parallel component tive time horizon comprising ignimbrites dated (Nairn, 2002; Wilson et al., 2009). of oblique plate motion between the Pacifi c and at 349 ± 4 ka, and a geochemically and petro- Currently, the central TVZ is geographically Australian plates at the Hikurangi subduction logi cally similar but slightly younger (339 ± divided into two parallel northeast-elongate margin (Beanland and Haines, 1998; Villamor 5 ka) suite of ignimbrites, the Paeroa Subgroup basins. The northwestern basin is the Taupo and Berryman, 2001). It is not clear from cur- (Wilson et al., 1986; Brown et al., 1998; ages fault belt (TFB), which exhibits a classic rift rent studies whether the TRB is undergoing from Downs et al., 2013); (3) the Huka Group, morphology (Rowland and Sibson, 2001), is near orthogonal extension, or whether it is a which includes all strata between the Whaka- seismically very active (Bryan et al., 1999), and transtensional feature. The TRB’s position on maru Group and the 25.4 ± 0.2 ka Oruanui For- has subsided at a rate of 3 to 4 mm/yr since at the eastern margin of the TVZ, coupled with its mation pyroclastic deposits (Grindley , 1965; age least ~61 ka, based on studies of its paleoseis- extraordinary heat output, suggest the possibil- from Vandergoes et al., 2013); and (4) a surfi - mology (Villamor and Berryman, 2001). The ity that it is an incipient rift jump in the context cial cover sequence of negligible relief, which southeastern basin is the TRB (Fig. 2), which of a southeastward migrating arc. mostly comprises two young ignimbrite forma- in contrast to the TFB has very limited geomor- tions (~530 km3 Oruanui , ~35 km3 Taupo) and phic evidence for faulting and is seismically less Taupo-Reporoa Basin and their reworked equivalents (Manville, 2001; Wil- active (Bryan et al., 1999), but nonetheless has Bounding Features son, 2001; Manville and Wilson, 2004; Wilson subsided at an equivalent rate of 3 to 4 mm/yr et al., 2009). since at least ~1.8 ka, based on the mapping of As defi ned here and shown in Figure 2, the The modern (past ~61 k.y.) TRB is volcani- paleolake shoreline elevations (Manville, 2001). TRB extends along strike from the Taupo vol- cally quiescent, although it had an explosive The kinematics of the TFB are reasonably well canic center in the southwest to the Waiotapu past. In its northern part, the TRB encompasses understood in terms of orthogonal rifting, per- geothermal fi eld in the northeast. In the north- the Reporoa caldera, which formed in associa- haps with a minor component of strike-slip west, the TRB is delimited by the active (last tion with eruption of the Kaingaroa Formation (Rowland and Sibson, 2001; Acocella et al., rupture at 20 ka or later; GNS Science Active (Nairn et al., 1994) at 281 ± 21 ka (new age 2003). Little is known of the kinematics of the Faults Database, http:// data .gns .cri .nz /af/), and estimate discussed in the following). In the TRB, or its contribution to the tectonic and geomorphically well expressed ~25 km long south, the TRB spans the southeastern part of magmatic evolution of the TVZ; however, the Paeroa Fault, and its associated upstanding foot- the Whakamaru caldera, which formed during TRB is of considerable interest for the follow- wall block (Paeroa block; Fig. 2). Along strike eruption of the regionally extensive ignimbrites ing reasons. beyond the southwestern limit of the Paeroa of the Whakamaru Group at 349 ± 4 ka (Wil- 1. New Zealand’s high temperature (>250 °C) Fault the boundary is less obvious within more son et al., 1986). Stratigraphically important geothermal resources currently under explora- subdued topography, but is delimited by the proximal members of the Whakamaru Group tion or development are mostly located within, active Orakei Korako, Whakaheke, and Kaiapo (Paeroa Subgroup ) were erupted at 339 ± 5 ka or on the perimeter of, the TRB (Figs. 1 and faults (Fig. 2). On its eastern margin the TRB from a source close to the present-day Paeroa

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south-southwest north-northeast A A ??? A′ Whakamaru caldera Kaimanawa & Reporoa caldera Oruahineawe 12 rhyolite lavas 1 THM14 TH11 TH TH17 +0.5 TH3 WT3 RK16 RK4 RK19 BR39 BR34 BR15 BR18 RP WT7 WT6WT5WT4

? ? –1 Deep strata unknown ?

km (RSL) ? –2 Deep strata unknown Dikes utilizing VE = fault systems 4× –3 0 5 10 15 20 25 30 35 40 45 km north-northwest south-southeast B B Kaimanawa & Oruahineawe B′ rhyolite lavas Kaingaroa Fault

4 zone RK8 OK4 NM6 +0.5 OK1 OKM1 NM4 NM9 NM5 RK OK2 NMM18 NM2 NM7 RK6 OK6 RK25

–1 ? ? km (RSL) –2 Deep strata unknown VE = –3 2× 0 5 10 15 20 25 km

Figure 4 (on this and following page). Cross sections showing rift-parallel to rift-perpendicular orientations, and displaying variations in block rotation and fault dips, as would be expected for each orientation from this rift setting (e.g., Villamor and Berryman, 2001; Lamarche et al., 2006). Dikes using faults as pathways are inferred. (A) Rift-parallel cross section displaying horsts and grabens inferred to be controlled by northeast-trending and oblique (northwest-trending) structures. RSL—relative sea level; VE—vertical exaggeration. (B) Rift- oblique cross section from Orakei Korako across the inactive Kaingaroa Fault zone.

Fault (Keall, 1988; Grindley et al., 1994; Downs >25 km along strike and rising to 500 m at its ing Paeroa block preserves some of the best et al., 2013). highest point above the adjacent TFB and TRB, exposures of Quaternary strata within the TRB. The TRB is generally interpreted as a sim- between which it forms the major divide (Fig. 2). The geology of the Paeroa block has received ple fault-angle depression (i.e., half-graben) It is back tilted ~7° eastward, presumably as a little new investigation since it was mapped at between westward dipping normal faults result of footwall rotation around a hori zontal a reconnaissance level in the 1950s and 1960s (Modriniak and Studt, 1959). However, the axis in response to slip on the westward fac- (Grindley, 1959, 1961; Healy et al., 1964; presence of calderas at its northern and southern ing Paeroa Fault (Berryman et al., 2008). The Leonard et al., 2010). Correlation of outcrops extents (Wilson et al., 1986; Nairn et al., 1994), Paeroa Fault is crustal in scale, strikes 040° to within the block with subsurface data from the the morphology and geological complexity 050°, undergoes pure normal dip slip, and has wider region is based on earlier work (Grindley of its western boundary, and numerous intra- slipped at a rate of 1.1 to 1.7 mm/yr, based on et al., 1994), and open to reinterpretation based basinal faults, as inferred from offsets (>100 m) paleoseismology investigations of its northern on improved geochronology (e.g., Wilson of stratigraphic contacts in geothermal drill splays and estimates derived from 550 ± 50 m et al., 2010). holes (Figs. 4A–4C) (Henrys and Hochstein, displacement of the 339 ± 5 ka Paeroa Subgroup 1990; Wood et al., 2001), suggest a more com- (Berryman et al., 2008). Although the elevation TRB Eastern Margin: Kaingaroa Fault Zone plex volcano-tectonic evolution. of the Paeroa block is generally attributed to The Kaingaroa Fault (Fig. 2), which deﬁ nes tectonically induced footwall uplift (Villamor the eastern margin of the TRB, is manifested as TRB Western Margin: Paeroa Block and Berryman, 2001; Berryman et al., 2008), a topographic scarp along most of its extent and The Paeroa block is the largest exposed structural resurgence has also been proposed coincides with the eastern margin of the TVZ as fault block within the central TVZ, extending (Healy, 1964). Whatever the cause, the upstand- deﬁ ned from vent locations (Wilson et al., 1995).

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west east ′ C Kaingaroa Plateau C C southern Paeroa block Kaingaroa Fault

+0.5 PaeroaFault zone OK2 BR38 BR15 BR7 BR16

–1 Deep strata unknown Dikes utilizing

km (RSL) fault systems –2

VE = 2× –3 0 5 10km 15 20 D A′ WT3 WT4 WT5 WT6 WT7

RP1 Figure 4 (continued). (C) Rift-perpendicular cross section from the southern Paeroa block to the Kaingaroa B Plateau displaying the Paeroa block horst and actively OK6 C OK2 subsiding Taupo-Reporoa Basin (TRB) and currently OK1 OKM1 OK4 inactive Kaingaroa Fault zone. (D) Map of the TRB showing the approximate locations of the cross sec-

NMM18 tions and geothermal drill holes. The cross sections were constructed and modiﬁ ed from stratigraphic and BR38 BR15 BR18 NM9 NM4 geophysical (white dashed lines) studies undertaken NM2 BR34

NM7 by Modriniak and Studt (1959), Grindley (1970),

BR39 BR7 NM5 BR16 Wood (1983), Henrys and Hochstein (1990), Nairn NM6 et al. (1994), Stagpoole (1994), Wood et al. (2001), Rae (2007), Rosenberg et al. (2009, 2010), Wilson et al. RK8 C′ (2010), Boseley et al. (2012), Mighty River Power RK6 RK25 RK19 (2013, written commun., drill hole data), and our new RK16 mapping in the Paeroa block. See Figure 3 for color RK4 legend .

TH12 4 TH17

TH11 TH3 A THM1

B′

It marks the western margin of the Kaingaroa of ~650 m, based on seismic reﬂ ection surveys the surface mapped Kaingaroa Fault is the east- Plateau, a broad (~30 km wide) area of low relief (Stagpoole, 1994). Interpretation of resistivity ernmost structure within a narrow belt of faults that abuts the axial ranges of the North Island and and gravity data across the topographic scarp of referred to here as the Kaingaroa Fault zone. the attendant North Island fault system (Figs. 1 the Kaingaroa Fault, in combination with geo- Limited observations (Wilson et al., 1986; Nairn and 2). The surface of the Kaingaroa Plateau is thermal drill hole stratigraphy from within the et al., 1994; Tanaka et al., 1996) and geophysical capped by rhyolitic ignimbrites that are correla- TRB, indicate that the basement is displaced by interpretations (Stagpoole, 1994; Bibby et al., tive with ignimbrites within the TRB (Figs. 4B, >2 km by several faults over a horizontal dis- 1998) of stratigraphic relationships across the 4C). These ignimbrites unconformably overlie tance of ~6 km (Henrys and Hochstein, 1990; Kaingaroa Fault are consistent with the notion metasedimentary basement at an inferred depth Bibby et al., 1998; Wood et al., 2001). Thus, that the easternmost fault, and presumably the

Geosphere, February 2014 191

Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/10/1/185/3333253/185.pdf by guest on 25 September 2021 Downs et al. eld MA Mangakino MA PLVZ Paeroa linear vent zone PLVZ KA Kapenga KA OH Ohakuri PLVZ RE Reporoa TA TaupoTA WH Whakamaru Calderas

block uplift Taupo-Reporoa Basin Taupo-Reporoa with uplift and tilting of theC) Huka Group northern Paeroa Huka Group

ctonic and volcanic features. TVZ—Taupo Vol canic Vol TVZ—Taupo ctonic and volcanic features.

Taupo fault belt fault Taupo with a geothermal ﬁ Group deposits. (A) Reporoa WH

aikato River aikato Whakamaru Group W Reporoa Group as being close to source. inferred ed within geothermal drill holes that are TA Faults Whakamaru Group (349 to 339 ka) modern hydrothermal activity Proto- Lake Taupo Lake Taupo

B D

38°20’S 38°30’S 38°40’S TVZ boundary Sinter Kaingaroa Plateau RE Buried lavas

+ Kaingaroa Fault zone Fault Kaingaroa TILT A

KA K KA tilting block

Inferred calderas (inactive) southern Paeroa southern Trace of future Trace River Waikato 176°10’E Maroa centre volcanic Inferred vent (active)

t t

l l

e e

B B

e e

m m

o o

D Geothermal fields

n n

r r

e e t t

s s e e

Western Dome Belt Belt Dome Dome Western W W Western Huka Group (339 to 25.4 ka) Reporoa Group (~2 Ma to 349 ka) Future Lake Taupo Proto- Lake Taupo 175°50’E Key MA A C inferred from Arehart et al. (2002). (B) Whakamaru Group and Paeroa Subgroup emplacement, and development of the TRB and TFB. ( TRB and emplacement, and development of the Subgroup and Paeroa Whakamaru Group et al. (2002). (B) Arehart from inferred buried lavas identiﬁ The plus symbols represent activity. block. (D) Modern hydrothermal southern Paeroa Zone. In key, groups are differentiated: dark colors represent lavas and light colors represent volcaniclastic and sedimentary lavas and light colors represent dark colors represent differentiated: are groups Zone. In key, Figure 5. Plan view stratigraphic time reconstruction of the Taupo-Reporoa Basin (TRB) and Taupo fault belt (TFB) displaying te Taupo Basin (TRB) and Taupo-Reporoa of the 5. Plan view stratigraphic time reconstruction Figure

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entire fault zone, has been inactive since at least time frame from post–Paeroa Subgroup (339 ± mill and plagioclase concentrates were pre- 281 ± 21 ka, that is since eruption of the Kainga- 5 ka) to 239 ka within the central TVZ, which pared by hand-picking and using a LB-1 Bar- roa Formation (Fig. 2). spans ignimbrite ages within the major part of rier Frantz magnetic separator. Separates were the Huka Group stratigraphy studied here, there etched with 0.1 M hydrofl uoric acid to remove METHODS is an overlap in major and trace element com- any adhering glassy material, then washed in positions, and petrographic characteristics for acetone and deionized water. Final separates Development of a basin-wide evolutionary almost all deposits (Karhunen, 1993; Beresford, for irradiation were hand-picked to remove any model for the TRB necessitated defi nition of a 1997; Milner, 2001; Gravley, 2004). We have crystals containing inclusions or with remaining new and high resolution stratigraphic architec- thus undertaken petrography to identify min- adhering minerals or glass. Encapsulated pack- ture. To achieve this we used the following four- eral assemblages and X-ray fl uorescence (XRF) ets of ~200 mg of plagioclase were irradiated for step process. analyses to identify major and trace element 1 h in the central thimble of the U.S. Geologi- First, a comprehensive review and evaluation trends, but such data are only used to comple- cal Survey TRIGA reactor in Denver, Colorado of lithologic descriptions from geothermal drill ment and further support lithological descrip- (methods described in Dalrymple et al., 1981). cores and cuttings was undertaken to construct tions for correlating strata. Samples were shielded from thermal neutrons the stratigraphic framework and structural archi- Fourth, age control was established for the and neutron fl ux was measured using Taylor tecture of the TRB (Figs. 3 and 4A–4D). The new stratigraphic architecture by supplement- Creek sanidine (TCR-2) fl uence monitors with drill hole logs provide virtually the only strati- ing existing data (Houghton et al., 1995; Wilson an assigned age of 27.87 Ma (Dalrymple and graphic record for the basin interior because et al., 2009, 2010) with new 40Ar/39Ar age dates Duffi eld, 1988). The reactor vessel was rotated young (25.4 ± 0.2 ka and younger) ignimbrites on selected lavas and pumice clasts obtained continuously during irradiation to avoid lat- and sediments mask older units except for rare from the Paeroa block and rare surface expo- eral neutron fl ux gradients. Fluence monitors surface exposures of 100 ka and older rhyolite, sures within the TRB. were analyzed using a continuous laser system dacite, and andesite lavas (Fig. 2). Regard- and a MAP 216 mass spectrometer (methods less of location within the basin, or degree of XRF Techniques described in Dalrymple, 1989). hydrothermal alteration, the four packages of Argon was extracted from the plagioclase Quaternary strata defi ned earlier can be readily Rhyolite lavas and juvenile pumice clasts (of separates using a Molybdenum crucible in a discerned in the drill hole records. In particu- at least 4 cm) obtained from exposed rhyolitic Staudacher-type custom resistance furnace lar, the Whakamaru Group is the most distinc- ignimbrites and lavas were used for individual attached to the mass spectrometer. Heating tem- tive marker horizon throughout the region and XRF analyses. Samples were washed in deion- peratures were monitored with an optical fi ber as a result, along with the Reporoa and Huka ized water to remove attached matrix or foreign thermometer and controlled with an Accufi ber Groups, it is well located at depth throughout material, and dried in an oven at 100 °C for sev- Model 10 controller. Gas was purifi ed continu- the TRB (Figs. 4A–4C). Although hydrother- eral days before crushing. Clasts were crushed ously during extraction using two SAES ST-172 mal alteration limits geochemical and geochro- and altered material was removed. Samples getters operated at 4A and 2.5A. nological fi ngerprinting of units sampled from were powdered using a tungsten carbide Tema Detailed step-heating experiments were under- geothermal drill holes, further refi nement of mill. Powder (5 g) from each sample was dried taken to yield plateau age spectra and isochron the stratigraphic framework is possible using at 110 °C for 24 h to remove meteoric water. ages with regression intercepts (York, 1968) petrography and zircon age spectra to aid in Samples were then ignited at 850 °C for 12 h to within error of the atmosphere. Degassing was identifi cation and correlation of altered units determine loss on ignition by removing volatiles; done to 650 °C and steps utilized started at (Wilson et al., 2010). 2 g of ignited sample were mixed with 6 g of 700 °C. Signifi cant 39Ar came off at 1400 °C so Second, the Paeroa block on the northwest- 12:22 fl ux and fused into glass discs. Major and this was analyzed as a last step for calculating ern margin of the TRB was mapped to identify trace element geochemistry (Table 1; Table DR1 plateau ages. Analytical protocols for determin- stratigraphic units and determine deposit geom- in the Supplemental File1) on glass discs was ing furnace blanks and mass discrimination etries in the best exposed part of the TRB. Dur- determined with a Siemens SRS 3000 sequential followed those detailed in Calvert and Lan- ing mapping, comprehensive descriptions of X-ray spectrometer with an Rh tube at the Uni- phere (2006). All ages are reported with 1σ units were compiled (Fig. 3) and used to inter- versity of Auckland. All major oxides are recal- errors including errors in neutron fl ux, but not pret volcanic sources, transport processes, and culated to total 100% anhydrous to account for including errors in decay constants or monitor depositional environments. Juvenile pumice and differences in hydration between samples. minerals. Details of the experimental results are lava samples were collected for 40Ar/39Ar age summarized in Table 2 and Figures 6 and 7 (see dating and geochemical analysis, and lithic clast 40Ar/39Ar Dating Techniques Figs. DR1–DR11 in the Supplemental File [see componentry was undertaken for comparison footnote 1] for complete age profi les). with previously described ignimbrites. Samples for age determinations were col- Third, Paeroa block strata were correlated lected from seven rhyolite lavas and three RESULTS where possible, with units identifi ed at depth rhyolitic ignimbrites exposed within the TRB within geothermal drill holes and exposed along (Table 2). Samples were crushed using a disc Stratigraphic Summary the Kaingaroa Fault scarp. Correlations were based predominantly on lithological descrip- 1Supplemental File. Pumice and rhyolite lava geo- Deposits mapped within the Paeroa block tions and juvenile clast petrology. Pumice is chemistry (Table DR1) and complete 40Ar/39Ar age are shown in Figure 2. All are correlative with the juvenile magmatic component from ignim- profi les (Figs. DR1–DR11). If you are viewing the previously mapped or described formations. PDF of this paper or reading it offl ine, please visit brites, and pumice petrography and geochem- http:// dx .doi .org /10.1130 /GES00965 .S1 or the full- The overall stratigraphic architecture, ages, istry are widely used in correlating ignimbrites text article on www.gsapubs.org to view the supple- volcanic sources, and deposit descriptions are (Hildreth and Mahood, 1985). However, for the mental fi le. summarized in Figure 3. Strata of the Reporoa,

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Whakamaru, and Huka Groups are all exposed 36 68 22 33 57 29 12 within the Paeroa block, and identiﬁ able in the Lava Mac1 Kairuru subsurface of the TRB. The spatial distribution of each group, their dominant formations, and the geometric relationships between them are Lava 154a outlined in the following. Pukekahu Reporoa Group strata are poorly exposed throughout the TRB (Figs. 2 and 5A). Out- Hill 10 9 10 crops are limited to two rhyolite lavas and the Lava 153a

Deer 710 ± 60 ka Waiotapu Formation in the northern Paeroa block, and a 712 ± 27 ka andesite cone

8 at Rolles Peak (Houghton et al., 1995; Tanaka 0.05 0.034.82 0.02 0.46 0.03 0.59 0.46 0.251.32 0.14 1.17 0.15 1.17 0.10 0.76 lag et al., 1996). The rest of the Reporoa Group is Pumice Pumice Cd110e Cd110e delineated from hydrothermally altered geothermal drill cores and cuttings (Gravley et al., 2006). Zircon age spectra recovered from Mihi MR1g Pumice Pumice rhyolitic ignimbrites (Fig. 3; 1.45 ± 0.05 Ma unnamed ignimbrite, 1.21 ± 0.04 Ma Ongatiti Formation, 1.18 ± 0.02 Ahuroa ignimbrite, Mihi

MR1a 950 ± 50 ka Akatarewa ignimbrite) within sev- Pumice Pumice eral geothermal ﬁ elds have helped unravel strati-

d totals are given.d totals graphic relationships and lateral extents that

108 89 112 124 113were 106 previously unknown (Wilson et al., 2010). Mihi MR9h Pumice Pumice The Whakamaru Group is a useful marker horizon with which to spatially and temporally 9989

63 divide the geological history of the TVZ for two 125150 129744 210 136 770 138 116 651 221 260 676 149 132 706 232 132 754 209 122 770 147 786 0.05 0.053.13 0.05 0.66 0.07 3.22 3.40 0.140.80 0.29 1.24 0.14 1.06 0.21 1.27 Mihi reasons. First, although subdivided into at least Cd22b Pumice Pumice seven locally distinct deposits, these ignimbrites share easily identiﬁ able petrographic and 11 86 99.87 99.85 99.89 99.86 99.86 99.85 99.85 99.86

137 212 783 lithological characteristics (Keall, 1988; Brown 407d Pumice Pumice et al., 1998). Second, the older (349 ± 4 ka), Kaingaroa voluminous (>1500 km3) ignimbrites within the Whakamaru Group are widespread throughout

072b the central North Island. Within the study area, Pumice Pumice Kaingaroa these older ignimbrites blanket the Kainga- roa Plateau and are commonly penetrated by drilling in TRB geothermal ﬁ elds (Figs. 2 and 183c

Pumice Pumice 4A–4C). The thicknesses and lithic clast distri- Kaingaroa butions in the ignimbrites, coupled with structural and geophysical evidence, were used to

183b propose a source Whakamaru caldera that over- Pumice Pumice Kaingaroa laps much of the southern TRB (Wilson et al., 1986; Brown et al., 1998). In detail, aspects of 9111011 the distribution, thicknesses of, and depths to 347b Pumice Pumice Ohakuri the Whakamaru ignimbrites in geothermal drill holes are complex (Figs. 4A–4C). The Whaka- 9111824131423171314181010

10 maru Group units are apparently missing in

335c some locations within the subsurface (e.g., over Pumice Pumice Ohakuri TABLE 1. REPRESENTATIVE MAJOR AND TRACE ELEMENT ANALYSES OF JUVENILE PUMICE CLASTS AND RHYOLITE LAVAS OF JUVENILE PUMICE CLASTS ANALYSES TRACE ELEMENT AND MAJOR 1. REPRESENTATIVE TABLE a buried andesite cone at Rotokawa), and this absence may either refl ect non-deposition, rapid 13 erosion, or that the deposits are so thin that they 120h Pumice Ohakuri are not recognized in drill hole cuttings. Given the scale of the Whakamaru eruption, the latter two options are more likely, and can be readily 2.13 1.720.02 2.17 0.01 2.14 0.04 2.23 0.03 1.92 0.01 1.91 0.01 1.63 0.03 1.48 0.03 1.90 0.01 1.54 0.02 1.94 0.01 1.56 0.02 1.38 0.02 1.20 0.02 1.28 0.03 0.01 0.33 0.18 0.23 0.25 0.18 0.18 0.18 0.17 0.14 0.22 0.11 0.19 0.11 0.25 0.21 0.14 047a 13.97 13.18 13.35 13.91 14.85 14.79 14.34 13.98 13.26 12.97 12.77 13.16 12.94 13.58 13.34 12.57 74.87 76.57 75.39 74.47 74.84 75.01 75.49 75.96 76.53 75.44 76.58 75.56 75.72explained 75.67 76.04by the 77.31 existence of high ground dur- Pumice Ohakuri ing emplacement. Such an interpretation may provoke reconsideration of the southeastern Analyses were performed by X-ray fluorescence. fluorescence. an loss but (LOI) on original ignition are elementsby performedrecalculated were Major X-ray anhydrous, Analyses limit of the Whakamaru caldera. 3 3 5 O 3.11 2.88 3.39 3.60 2.28 2.82 2.42 2.66 3.99 4.19 3.93 4.08 1.43 4.02 4.12 4.18 O 2 2 O The younger (339 ± 5 ka) Whakamaru Group 2 O O 3.73 3.97 3.62 3.73 4.07 3.82 4.24 4.40 3.60 3.69 3.80 3.51 6.60 3.75 3.73 3.62 2 Note: 2 2 2 K ppm Al Fe P Na Zn 70 50 56 52 62 53 50 51 45 42 39 54 44 31 28 SrNbLa 130 107 9 29 116 10 27 133 27 92 29 107 108 28 93 24 30 28 29 28 27 24 22 23 35 Ba 700 762 717 714 699 724 695 Sample Sample Rock type SiO TiO RbYZr 293332303231313433272933322127 133CePbTh 228 132 210 126 44 23 18 232 125 39 21 271 138 53 20 240 137 53 18 232 153 42 239 22 47 22 37 23 56 24 57 23 42 23 44 15 50 17 44 13 42 19 34 21 MgOCaO 0.23Total 1.55 0.27 1.16 99.85 0.31 1.44 99.86 0.30 1.51 99.85 0.39 99.84 1.11 0.24 99.87 1.16 99.84 0.20 1.14 99.87 0.12 1.00 99. wt% MnO 0.05LOI 0.06 0.06 4.12 0.07 4.64 0.04 4.73 0.04 4.21 0.05 5.09 0.05 4.64 4.52 4.77 Deposit ignimbrites are delineated separately as the

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Paeroa Subgroup, and are only exposed within the Paeroa and Te Weta blocks (Fig. 2). Despite petrographic and lithologic similarities to the (ka) Total Total

gas age older Whakamaru Group ignimbrites, the thickness of the Paeroa Subgroup (>500 m along the Paeroa Fault scarp) strata and the sizes (to ~4 m † i

Ar across) and distribution of lithic clasts indicate 39 that they were erupted from a source closer to Ar/ 40 the present-day position of the Paeroa Fault, not from vents within the >20 km distant Whaka- maru caldera (Keall, 1988; Grindley et al., 1994; Downs et al., 2013). The Huka Group encompasses the best exposed deposits within the TRB and TFB, Isochron (ka) MSWD Age particularly in the Paeroa and Te Weta blocks (Fig. 2). Individual formations within the Huka Group comprise mostly ignimbrites and lacustrine sediments. Units of the Huka Group are Ar

39 exposed extensively throughout the southern % (steps) Paeroa block, south of an east-west–striking fault contact with the Paeroa Subgroup, and to the north where they overlie the Paeroa Sub- group. Elsewhere within the TRB, the Huka Group is mostly buried by younger (25.4 ± 0.2 ka and younger) surﬁ cial deposits. High standing rhyolitic lava domes and dacitic cones (ka)

281 ± 9 (10 of 10) 100 281 ± 9 1.0 341 ± 16 338 ± 3 provide rare surface exposures of Huka Group

Interpreted age formations within the low relief interior of the TRB. Although geothermal drilling within the basin often intercepts units of the Huka Group, discriminating between individual formations is difﬁ cult because cores and cuttings rarely pro- used Ar TECHNIQUES ON RHYOLITIC IGNIMBRITES AND LAVAS TECHNIQUES ON RHYOLITIC IGNIMBRITES Ar 39 isochron vide sufﬁ cient material or context for correlation Ar/

40 (Fig. 4A). We therefore focus on exposures of Huka Group formations within the Paeroa block (ka) MSWD Age used that reveal stratigraphic and geometric relation- isochron Plateau ships in sufﬁ cient detail to provide insight into basin-forming processes and rates.

Ar Huka Group formations exposed within the 39 used % (steps)

isochron southern Paeroa block consist of poorly sorted 98 (9 of 10) 264 ± 4 1.3 264 ± 4 as for plateau 258 ± 12 1.4 298 ± 13 266 ± 4 46 (3 of 10) 142 ± 4 1.1 142 ± 4 as for plateau 158 ± 11 0.4 290 ± 9 153 ± 2 55 (3 of 10) 103 ± 6 0.3 103 ± 6 as for plateau ± 28 115 85 (7 of 10) 0.0 ± 4 312 64 (6 of 10) 287 ± 39 ± 3 490 0.4 131 ± 7 0.9 312 ± 4 490 ± 3 for plateau as 302 ± 12 for plateau as 494 ± 7 0.3 300 ± 13 0.8 291 ± 15 322 ± 4 502 ± 3 90 (8 of 10) 239 ± 3 1.454 (4 of 11) 249 ± 2 2.2 239 ± 3 263 ± 5 as for plateau 243 ± 12 1.5 as for plateau 263 ± 5 293 ± 12 0.1 236 ± 3 284 ± 9 ± 1 242 52 (3 of 8) 965 ± 8 3.4 965 ± 8 for plateau as 976 ± 53 6.5 288 ± 88 964 ± 3 100 (10 of 10) 247 ± 2 1.0 247 ± 2 for plateau as 252 ± 5 1.0 292 ± 7 ± 2 246 100 (10 of 10) ± 3 298 1.1 281 ± 21 as for plateau ± 5 294 lapilli-rich 1.1 298 ± 6pumice 299 ± 3 breccias interbedded with ﬁ nely laminated sands and silts, which are indicative of a lacustrine environment (Grindley, 1965; Steiner, 1977); the breccias have charac- TABLE 2. AGE DETERMINATIONS BY BY AGE DETERMINATIONS 2. TABLE Pumice: Pumice: Pumice: Pumice: teristics (e.g., welding, petrography, sedimen- plagioclase plagioclase plagioclase plagioclase plagioclase plagioclase plagioclase plagioclase plagioclase plagioclase groundmass Rhyolite lava: Rhyolite lava: Rhyolite lava: Rhyolite lava: Rhyolite lava: Rhyolite lava: Rhyolite lava: . tary structures, geochemistry) that allow for σ correlation with previously named ignimbrites (Fig. 3, and references listed therein). In detail from west to east, the Ohakuri For- rhyolite rhyolite rhyolite rhyolite rhyolite rhyolite rhyolite breccia Breccia Breccia Formation Ar, which is 2 Ar, mation (Figs. 8A–8C) is the stratigraphically 39

Ar/ lowest unit of the Huka Group mapped along the 40 Paeroa Fault scarp within the southern Paeroa block. It is as thick as 200 m and thins to the east. Farther west within the TFB, toward its inferred other than

σ caldera source, drill holes penetrate Ohakuri Formation material that is >400 m thick (Grav- ley et al., 2007). Directly overlying Ohakuri For-

MSWD—mean square of weighted deviates. mation on the Paeroa Fault scarp are reworked

All errors are 1 volcaniclastic and ﬁ nely laminated lacustrine † Note: Geodetic System 84. *Grid references are in World GL1084 38.55S, 176.26E Orakei MR1 38.47S, 176.25ECd110 Mihi 38.48S, 176.26E Lag GL1034 38.62S, 176.14E Aratiatia MR9 38.47S, 176.26E Mihi GL1089 38.36S, 176.36E 8566 Trig Sample number Grid reference* Unit Sample type M1M2GL1093 176.31E 38.44S, 38.46S, 176.35E153 Kairuru Pukekahu 38.49S, 176.34E Deer Hill CR1 38.44S, 176.17E Kaingaroa 245 38.35S, 176.33E Ngapouri sediments that dip 7° to the southeast, but that

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600 600 Aratiatia (sample GL1034) Deer Hill (sample 153) 500 500 WMPA = 103±6 ka 1400 WMPA = 264±4 ka 400 MSWD = 0.3 700 MSWD = 1.3 400

39 1250 39 300 55% Ar released 98% Ar released 300

1175 1275 1350 1050 1125 1200 1450 725 900 975 200 1325 800 200 1100 875 950 1025 100 100 800 600 600 Orakei (sample GL1084) Kaingaroa (sample CR1) 500 500 WMPA = 142±4 ka 400 MSWD = 1.1 400 700 39 900 975 1050 1275 1425 300 46% Ar released 1125 1200 1350 300 800 1175 1400 200 1250 WMPA = 298±3 ka* 200 1100 800 950 1025 1325 MSWD = 1.1 100 39 100 725 875 100% Ar released

600 600 Mihi (sample MR9) Lag breccia (sample Cd110) 700 500 500 WMPA = 239±3 ka 400 MSWD = 1.4 900 400 39 975 1050 1350 1450 90% Ar released 1125 1200 1275 300 300 700 1275 1350 800 900 975 1050 1125 1200 800 1450 WMPA = 312±4 ka

Apparent age (ka) 200 200 MSWD = 0.4 100 85% 39Ar released 100

600 600

Kairuru (sample M1M2) Trig 8566 (sample GL1089) 1325 725 500 1175 1250 500 950 1025 1100 WMPA = 247±2 ka 800 875 1400 400 MSWD = 1.0 400 100% 39Ar released 300 800 1425 WMPA = 490±3 ka 300 1275 1350 900 975 1050 1125 1200 MSWD = 0.9 200 64% 39Ar released 200 700 100 100

600 0 Pukekahu (sample GL1093) Ngapouri (sample 245) 1100 1400 500 1300 WMPA = 263±5 ka 1000 1100 1200 900 900 MSWD = 2.2 800 400 700 54% 39Ar released 300 550 1100 500 600 650 700 750 1050 800 850 WMPA = 965±8 ka 925 1000 200 MSWD = 3.4 300 39 700 52% Ar released 100 100 -100 0 0.0 0.2 0.4 0.6 0.8 1.0 0.2 0.4 0.6 0.8 1.0 Cumulative 39Ar released

Figure 6. Weighted mean plateau ages (WMPA) for all samples, except Mihi sample MR1, dated using 40Ar/39Ar techniques from the Taupo-Reporoa Basin (MSWD—mean square of weighted deviates). Gray boxes were used to calcu- late the WMPA, and black boxes were discounted. All samples have been scaled to an apparent age of 600 ka except for the Ngapouri rhyolite lava (sample 245). The WMPA for the Kaingaroa Formation (sample CR1) is displayed, but based on stratigraphic evidence an age of 281 ± 21 ka is considered more appropriate (see text for details).

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1350 are horizontal to sub-horizontal in other proxi- Mihi (sample MR1) mal locations within the southern Paeroa block. 700 1125 Gravley (2004) mapped a similar stratigraphic 1050 1200 sequence of thin lacustrine beds (≤10 k.y. of 600 1425 975

sedimentation) overlying the Ohakuri Forma- Ar tion within the TFB to the west. The lacustrine 36 500 1275 Ar/ 800

sediments within the Paeroa block are overlain 40 900 by ignimbrite of the Kaingaroa Formation (Fig. Age = 281±9 ka 8D). Within the southern Paeroa block, decreas- 400 700 MSWD = 1.0 40 39 ing lithic clast sizes (largest is >2 m across) and Ar/ Ari = 341±16 (±2σ) thinning of the Kaingaroa Formation to the west Air are consistent with it originating from a Repo- 0 100 200 300 400 500 39Ar/36Ar roa caldera source (Fig. 2). Both the Ohakuri and Kaingaroa Formations include accretionary Figure 7. Isochron age for one of the Mihi Breccia samples (MR1). This age is preferred lapilli–bearing beds, suggesting that abundant over the weighted mean plateau age, which comes back older than its stratigraphic position water was present during both eruptions (Beres- indicates. MSWD—mean square of weighted deviates. ford, 1997; Gravley, 2004). Overlying the aforementioned formations is the Mihi Breccia (Fig. 2), an enigmatic composite of interbedded lacustrine, volcaniclastic, and A B pyroclastic density current deposits (Figs. 9A, 9B). Prismatically jointed (breadcrusted) pumiceous clasts, soft-sediment deformation, and abundant lithic clasts of lacustrine sediments (Fig. 9B) all support the notion that the pyroclastic density currents ﬂ owed into, or erupted beneath, a lake system. The Mihi Breccia thus records numerous shifts from low energy lacustrine sedimentation to rapid accumulation of primary and reworked volcaniclastic material (Fig. 3). The Mihi Breccia is considered equivalent to, but not correlative with, numerous C subsurface rhyolite lavas and their pyroclastic equivalents that are interbedded with lacustrine sediments throughout the TRB (e.g., Rosenberg et al., 2009). Additional deposits within the Paeroa block include a pumice-rich breccia of limited extent (Fig. 8E), a clast-supported hydrothermal eruption breccia (Fig. 9C), and a stratiﬁ ed basaltic lapilli tuff (Mangamingi basalt; Fig. 9D); DE all derived from sources within the southern Paeroa block.

Geochemistry

A large database of XRF analyses exists for rhyolitic ignimbrites encompassed by the Huka Group time frame (Karhunen, 1993; Beresford, 1997; Milner, 2001; Gravley, 2004). However, since overlap in the values of all major and trace elements is common in deposits erupted during this time frame, analyses of juvenile clasts were Figure 8. Photographs of southern Paeroa block strata. Hammer used for scale is ~33 cm used only as a complement to lithologic descrip- long and shovel is ~1 m. All map references are in the World Geodetic System 84 reference tions for correlating deposits within the south- grid. (A) Crossbedded Ohakuri Formation consisting of pumice-rich, ash-rich, and accre- ern Paeroa block. Single pumice clast major and tionary lapilli–bearing beds (38.47S, 176.20E). (B) Massive Ohakuri Formation overlain by trace element geochemistry on the Ohakuri and crossbedded Ohakuri Formation (38.46S, 176.21E). (C) Basal clast-supported breccia of the Kaingaroa Formations overlap well with their Ohakuri Formation (38.46S, 176.18E). (D) Late-stage Kaingaroa Formation with recycled respective ﬁ elds, conﬁ rming correlations (Fig. black and red-pink Kaingaroa Formation (38.44S, 176.25E). (E) Graywacke pebble con- 10; Table 1). Slight variations outside of the glomerate and pumice-rich lag breccia (38.48S, 176.26E).

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A B within the Huka Group time frame (e.g., Grav- ley et al., 2007). Our new 40Ar/39Ar age determination for the Kaingaroa Formation, sampled at the southwest end of the Paeroa Fault scarp, is 298 ± 3 ka (Fig. 6); this contrasts with the pre- vious determination of 230 ± 10 ka (Houghton et al., 1995). However, numerous fall deposits, at least fi ve caldera-related ignimbrites (Matahina, Chimp, Pokai, Mamaku, Ohakuri), lacustrine sediments, and multiple paleosols (Karhunen, 1993; Manning, 1996; Gravley et al., 2007) occur between the Paeroa Subgroup at 339 ± CD 5 ka and the deposit sampled as the Kaingaroa Formation (Fig. 12). A time span of ~40 k.y. for such a complex and prolonged stratigraphic record coupled with existing age data (Hough- ton et al., 1995; Gravley et al., 2007) imply that an age of 298 ± 3 ka for the Kaingaroa Forma- tion is unrealistic, and we therefore use it only as an upper age constraint on the Kaingaroa eruption. The three exposed post-Kaingaroa rhyolite lavas on the edge of, or just south of, the Repo- roa caldera provide minimum age limits. The 264 ± 4 ka Deer Hill lava provides the lower age constraint. An age of 281 ± 21 ka, which is Figure 9. Photographs of southern Paeroa block strata. Shovel used for scale is ~1 m long, halfway between 264 ± 4 and 298 ± 3 ka, plus hammer head is ~16 cm, and hammer is ~33 cm. All map references are in the World Geo- or minus half the total difference including their detic System 84 reference grid. (A) Contact between lacustrine sediments and Mihi Breccia uncertainties, is thus adopted here for the age of (38.47S, 176.26E). (B) Entrained lacustrine clasts within Mihi Breccia (38.48S, 176.26E). the Kaingaroa Formation. This age is identical (C) Hydrothermal eruption breccia consisting of fi ne-grained laminated lacustrine sediment within uncertainty to the ~285 ka value, based boulders (38.44S, 176.23E). (D) Mangamingi basalt (38.49S, 176.22E). on tephrostratigraphy and oxygen isotopic stratigraphy, for the age of the Kaingaroa Formation airfall deposit reported by Manning (1996). Gravley et al. (2007) proposed an Ohakuri defi ned fi elds are inferred to be related to weak 40Ar/39Ar Geochronology Formation age of 240 ± 11 ka, based on its hydrothermal alteration of clasts, or to primary stratigraphic position and 40Ar/39Ar age deter- variations that have previously been undetected. Age determinations using 40Ar/39Ar tech- minations relative to the Mamaku Formation. Three pumice types were geochemically identi- niques are displayed in Table 2 and Figures 6 However, Ohakuri Formation ages are scattered fi ed by Gravley (2004) within the Ohakuri For- and 7. We have dated the two surfi cial Reporoa beyond analytical uncertainties. The Ohakuri mation; one is minor and has not been identi- Group rhyolite lavas within the northern Paeroa and Mamaku Formations are of geologically fi ed here. block that yield ages of 965 ± 8 ka (at Ngapouri) identical ages, as demonstrated by Gravley Geochemical compositions of pumice and and 490 ± 3 ka (at Trig 8566; Fig. 2). We use the et al. (2007), and underlie the Kaingaroa For- rhyolite lava clasts from the Mihi Breccia are age of 710 ± 60 ka from Houghton et al. (1995) mation, and therefore are older. It is unknown delineated into two groups that correspond to for the Waiotapu Formation within the northern how much older, but Gravley (2004) estimated signatures from nearby rhyolite lavas exposed Paeroa block. Five members of the Whakamaru that lacustrine sediments overlying the Ohakuri within the TRB (Kairuru, Pukekahu, Deer Hill; Group and Paeroa Subgroup have been dated, Formation in the TFB represented a ≤10 k.y. Fig. 2). The fi rst is a high silica group closely and the results are discussed in Downs et al. time frame, based on thicknesses and sedimen- corresponding in composition to Kairuru, and (2013). The oldest Huka Group eruptive iden- tation rates. We have identifi ed stratigraphi- the second is a lower silica group that encom- tifi ed within the southern Paeroa block is the cally equivalent lacustrine sediments within the passes Pukekahu and Deer Hill rhyolite lavas localized pumice-rich breccia (Fig. 8E), which Paeroa block, and thus estimate the eruption (Fig. 11; Table 1). Mihi Breccia juvenile clasts yields an age of 312 ± 4 ka from a juvenile clast ages of the Ohakuri and Mamaku Formations are texturally and petrographically similar to (Fig. 6). However, this unit has only been identi- to between ~280 and 290 ka. This age range those exposed rhyolite lavas. In addition, a bur- fi ed in one locality and there are no stratigraphic falls within uncertainty of the oldest Ohakuri ied dome complex inferred to correlate with or geochemical correlatives, and so we presume age (275 ± 16 ka) reported by Gravley et al. Kairuru is interpreted to be beneath the southern that this is a localized eruption. (2007). The Pokai Formation (Fig. 3) within the Paeroa block, based on magnetic anomaly data Of central interest to this study are the ages TFB has been dated at 275 ± 10 ka (Gravley (Soengkono and Hochstein, 1996). This buried of three closely spaced caldera-forming erup- et al., 2007), but its top surface contains a well- dome complex, or other nearby rhyolitic com- tions (Ohakuri, Kaingaroa, Mamaku, the last of developed paleosol with Mamaku and Ohakuri plexes, represents possible vent sources for the which has not been identifi ed within the TRB) units overlying it. The Pokai Formation overlies Mihi Breccia. and their timing relative to other ignimbrites two airfall tephra sequences that are interbedded

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that transects the Paeroa block, the Paeroa Sub- 300 Ohakuri Formation group is exposed and there is a lack of younger pumice field Kaingaroa Formation pumice field cover material. The interpretation of the southeastward slope of the surface of the northern

200 Paeroa block is equivocal; it could represent a Sr (ppm) Sr primary depositional feature of an ignimbrite fan (our data), a structural resurgence related to 100 the Paeroa Subgroup magmatic system (Healy, 1964), tectonic tilting (Berryman et al., 2008), 0 or some combination of all three processes. 400 Eruptive activity proximal to this part of the block, particularly from the Reporoa caldera accompanying the Kaingaroa Formation, would 300 most likely have resulted in emplacement of an

Zr (ppm) Zr ignimbrite sequence overlying the Paeroa Sub- 200 group. The absence of such material, with the implication that any deposits were thin and con- 100 sequently eroded away, leads us to infer that the northern Paeroa block has been a topographic 0 high since Paeroa Subgroup emplacement at 4.0 Minor 339 ± 5 ka, in marked contrast to the southern outlying pumice Paeroa block (Fig. 13C). 3.0 The thicknesses and degree of preservation of younger formations within the southern Paeroa block (100 to 500 m thick), interior of the TRB 2.0 Rb/Sr (generally >1000 m thick), and TFB (unknown thickness) indicate that these areas provided 1.0 the main post–Paeroa Subgroup accommodation space (Fig. 4C). Although all three areas 0.0 may have had a shared fl ux of volcaniclastic 65 70 75 80 70 75 80 materials, the role of the southern Paeroa block SiO2 (wt%) SiO2 (wt%) in accommodating these materials was shorter Figure 10. Selected trace element Harker variation diagrams of single pumice clasts (fi lled lived. Exposed stratigraphic positions and deposit symbols) used to confi rm correlations of southern Paeroa block deposits with the Ohakuri geometries indicate that the Ohakuri Formation and Kaingaroa Formations. Ohakuri and Kaingaroa fi elds (gray shaded areas) are from was emplaced into this southern Paeroa basin

data collected by Gravley (2004) and Beresford (1997), respectively. SiO2 values are recalcu- early, and was immediately covered by lacustrine lated to totals of 100%, after accounting for loss on ignition. sediments (Fig. 13D). The westward thicken- ing wedge of Ohakuri Formation and the ~7° southeast dip of overlying lacustrine sediments show that these units underwent fault-induced with the Chimp Formation, all of which overlie surface have been dated at 142 ± 4 ka (at Orakei) uplift and tilting along the present-day southern the 349 ± 4 ka widespread ignimbrites of the and 103 ± 6 ka (at Aratiatia; Fig. 6), although any Paeroa Fault. In contrast, similar aged lacus- Whakamaru Group (Figs. 3 and 12); thus, the correlative pyroclastic deposits are buried. trine sediments also located proximal to the Pokai Formation age is likely to be somewhat Paeroa Fault are horizontal to sub-horizontal. older than the Mamaku and Ohakuri Forma- DISCUSSION We use such evidence to interpret that most of tions, that is ~300 ka. the uplift and tilting of the southern Paeroa block Mihi Breccia ages were determined on two Evolution of the Paeroa Block occurred prior to emplacement of the overlying stratigraphic units, although their spatial com- 281 ± 21 ka Kaingaroa Formation (Fig. 13E). plexity within the Paeroa block makes their exact The pattern and distribution of strata within This notion is supported by the observation that stratigraphic positions difﬁ cult to determine. The the TRB and surrounds vary along and across Kaingaroa Formation is notably absent at higher two ages from the Mihi Breccia give a minimum the strike of the predominantly northeast-south- elevations along much of the southern Paeroa time range of 281 ± 9 ka to 239 ± 3 ka (Figs. west tectonic fabric. In particular, the geology block: a high-standing physical barrier is inferred 6 and 7). This range overlaps with the ages of of the Paeroa block varies abruptly along strike, to have been in place at the time to obstruct and the three post-Kaingaroa rhyolite lavas of Deer and attests to a contrasting evolutionary history limit the westward ﬂ ow of the parent pyroclas- Hill at 264 ± 4 ka, Pukekahu at 263 ± 5 ka, and from north to south. The mapped distribution of tic density currents. Thus, based on the age dates Kairuru at 247 ± 2 (Fig. 6), lending credence formations within the southern Paeroa block is discussed herein, the Ohakuri and Kaingaroa cal- that these, and related geochemically (Fig. 11) as expected for a tilted and somewhat eroded dera-forming eruptions, sedimentation of lacus- and petrographically similar subsurface rhyolite fault block: broadly, the units are elongate to the trine beds between these eruptions, and uplift lavas, are probable sources of the Mihi Breccia. northeast and young to the southeast (Fig. 2). of the southern Paeroa block is inferred to have Farther south, two rhyolite lavas exposed at the However, north of the east-west–striking fault occurred within a time frame of ≤10 k.y.

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300 Signiﬁ cance of Lacustrine Sediments Symbols The deposits of ephemeral and longer lived Lag breccia pumice clasts lakes have been documented within the central

200 Rb (ppm) TVZ, and are likely a common phenomenon Mihi Breccia pumice clasts given the region’s temperate climate and pro- Kairuru rhyolite pensity for drainage networks to be occasionally lava field 100 blocked by eruptive products (Smith et al., 1993; Deer Hill & Pukekahu Manville, 2001; Manville and Wilson, 2004). rhyolite lavas field Our age determinations indicate that lacustrine 3.0 0 sedimentation within the Huka Group of the Paeroa block spans >50 k.y., but it is unknown if the deposits are the products of one large,

2.0 200 Sr (ppm) long-lived lake or several smaller short-lived lake systems. The evidence for phreatomag- matic activity during the Ohakuri eruption (e.g., accretionary lapilli) and the widespread distri- CaO (wt%) CaO 1.0 100 bution of lacustrine sediments within the TFB and southern Paeroa block after emplacement of the Ohakuri Formation are taken to indicate 0.0 0 that a regional-scale lake system existed prior to 400 and subsequent to emplacement of the Ohakuri 3.0

Zr (ppm) Formation (Gravley, 2004; Gravley et al., 2007). 300

The scale of this lake, and its longevity (at least (wt%)

3 2.0 10 k.y.), would have required the existence of O

2 200 a long-lived dam across the major central TVZ Fe 1.0 drainage, the Waikato River (Figs. 13D, 13E). 100 Although the course of the Waikato River has undoubtedly changed over time (Manville and 0.0 0 Wilson, 2004), it has drained the central TVZ to the west via the Ongaroto Gorge (Fig. 2) for at 2.0 1000 least 349 ± 4 k.y., based on the coincident loca-

800 Ba (ppm) tion of a paleo–Waikato River gorge ﬁ lled with

600 Whakamaru Group ignimbrites (Martin, 1965).

Rb/Sr 1.0 The Ohakuri Formation is of sufﬁ cient volume 400 (~100 km3) to have overwhelmed drainage networks, and may have blocked the Ongaroto 200 Gorge or upstream tributaries of the Waikato 0.0 0 River. However, dams constructed from the 72 74 76 78 80 74 76 78 80 unwelded Ohakuri Formation are unlikely to SiO2 (wt%) SiO2 (wt%) have prevailed for more than a few decades (e.g., Figure 11. Selected major and trace element Harker variation diagrams of Mihi Breccia Manville, 2001; Manville and Wilson, 2004). juvenile pumiceous clasts and broadly coeval rhyolite lavas. Mihi Breccia juvenile clasts The Ongaroto Gorge walls consist of rhyolite have a broad overlap with rhyolite lavas exposed within the interior of the Taupo-Reporoa lavas of the Western Dome Belt (Fig. 2), and Basin. Major oxide data are recalculated to total 100% anhydrous. have K-Ar age determinations ranging from after 349 ± 4 to 187 ± 14 ka (Houghton et al., 1991; Leonard et al., 2010). We think that a more The construction of the entire Paeroa block considerations demonstrate that a minimum likely control on base level involved such lavas, from emplacement and uplift of the Paeroa slip rate of at least ~11 ± 6 mm/yr is required which presumably blocked the paleo–Waikato Subgroup in the northern block (339 ± 5 ka), to account for the maximum observed throw on River at the present-day Ongaroto Gorge to pro- through subsidence, lacustrine sedimentation, the fault (500 ± 50 m; Grindley et al., 1994); vide a long-lived (>10 k.y.) barrier (e.g., Crow and uplift in the southern block (281 ± 21 ka), however, this is an extraordinarily high rate for et al., 2008), or several barriers, in the appropri- is thus inferred to have occurred over 58 ± 26 rifting (Nicol et al., 2006). A more likely sce- ate location to allow formation of a regional lake. k.y. Despite the age uncertainties, this line of nario is fault displacement in association with Rhyolite lavas proximal to the Ongaroto reasoning draws into question the validity of eruptive episodes (Rowland et al., 2010). The Gorge have elevations of ≥500 m above sea only using modern fault slip rates, as deﬁ ned evacuation of hundreds to thousands of cubic level, consistent with the highest elevations from offset surfaces and displacement of 61 ka kilometers of volcanic material in association of exposed lacustrine sediments within the or younger tephras (Villamor and Berryman, with caldera-forming eruptions provides a plau- southern Paeroa block. Since uplift and tilting 2001; Berryman et al., 2008), to understand sible mechanism for transient and anomalous are interpreted to have occurred prior to 281 ± landscape-forming processes within highly pro- slip on pre-existing faults (e.g., Wilson, 2001; 21 ka, the southern Paeroa block has remained ductive magmatic rifts. Our chronostratigraphic Gravley et al., 2007; Allan et al., 2012). relatively stable with respect to the inferred

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Approximate basin Evolution of the Kaingaroa Fault Zone divide position northwest southeast Faults composing the Kaingaroa Fault zone Deer Hill are considered to be former strike-slip faults 264±4 ka rhyolite lava of the North Island fault system reactivated as dominantly normal faults to accommodate Kaingaroa Formation TVZ extension (Stagpoole, 1994; Bibby et al., (100 km3) 281±21 ka 1998). Some have proposed that these faults are periodically reactivated on a time frame used Lacustrine sediments to infer gradual migration of the linked eastern Ohakuri Formation TRB and TVZ margin (e.g., Stern, 1987). How- 3 ever, we argue that this fault zone reactivates on (100 km ) Between 280 a punctuated and episodic basis over limited line Mamaku Formation & 290 ka length, for the following reasons. (145 km3) The relatively uniform thicknesses of the Reporoa and Huka Groups (commonly ~1000 m Thick paleosol for each) within individual geothermal drill Intercalated ignimbrite, holes throughout the TRB (Figs. 4A–4C) sup- Pokai Formation airfall (5 tephras over port the concept of a long-lived subsiding basin. 3 ~300 ka If a gradual eastward migration of the linked (100 km ) the Kaingaroa Plateau TRB and TVZ margin were occurring, then & 16 at Bay of Plenty progressively younger sediments and a thinner coastline), paleosol, & Quaternary cover sequence would be expected 2 paleosols & in an eastward direction across the TRB, but

Taupo fault belt & Taupo loess deposits (e.g., multiple airfall this pattern has not been observed in drill hole

Taupo-Reporoa Basin Taupo-Reporoa Onuku Subgroup & deposits records (Figs. 4A–4C). Pikowai beds) between Sparse age constraints for movement along Matahina & Kaingaroa the Kaingaroa Fault zone are used to infer that Chimp Formation formations the age of fault reactivation youngs to the northeast, and that the easternmost fault has been (50 km3) Matahina Formation inactive since at least 281 ± 21 ka Kaingaroa Formation emplacement (Stagpoole, 1994). In 3 322±7 ka Multiple airfall & (150 km ) the southern part of the TRB, the 712 ± 27 ka paleosol deposits, Interbedded lacustrine Rolles Peak andesite lava (Fig. 2) has not been & erosional displaced by the fault zone. North along the sediments, paleosols, fault zone, the 349 ± 4 ka Whakamaru Group is surfaces present & tephras exposed on the scarp near the Ohaaki geothermal fi eld, but is displaced to depths of ~800 to Paeroa Subgroup 339±5 ka 1300 m within the fi eld (Fig. 4C). Farther north, (>110 km3) the 281 ± 21 ka Kaingaroa Formation mantles the easternmost fault, as defi ned geophysically Whakamaru Group 349±4 ka (Stagpoole, 1994), and is displaced to ~1250 m (>1500 km3) depth within the adjacent Reporoa caldera (Fig. 4A). The Kaingaroa Formation is the youngest basin

Single displaced unit along the fault zone; therefore, Reporoa Group the eastern margin of the TRB and TVZ has not migrated for at least the past 281 ± 21 k.y. Figure 12. Simplifi ed stratigraphic column with a summary of caldera-forming ignimbrites (Fig. 2). within the northwestern (Taupo fault belt) and southeastern (Taupo-Reporoa Basin) parts Thus, movement along the Kaingaroa Fault of the central Taupo Volcanic Zone, including the deposits that divide the ignimbrites, zone appears to be closely coincident with and relevant age determinations and estimates. Compiled from fi eld studies by Karhunen caldera-related eruptions (Whakamaru Group, (1993), Manning (1996), Gravley et al. (2007), and this study. Kaingaroa Formation), and we defi ne the fault zone as a composite volcano-tectonic feature.

Ongaroto Gorge barrier. Farther east in the TRB correlations tenuous, it is clear that the similarly Evolution of the Taupo-Reporoa Basin interior, geothermal drill holes commonly pen- aged lacustrine sediments within the TRB have etrate equivalent lacustrine sediments at depths subsided compared with those in the southern Prior to uplift of the Paeroa block, the central to 900 m (although to 1300 m depth at Wairakei- Paeroa block, and are continuing to subside at TVZ consisted of a single basin that extended Tauhara; Rosenberg et al., 2010) (Fig. 4A). 3 to 4 mm/yr, as estimated for the past ~1.8 k.y. from near the present-day Kaingaroa Fault zone Despite hydrothermal alteration making speciﬁ c (Manville, 2001). to the western margin of the old TVZ bound-

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Predominantly andesitic volcanism

A B

lake Kaingaroa Plateau Kaingaroa Plateau

Inactive pre-TVZ structures, periodically reactivated within the volcanic zone NE NW

SE northern Paeroa block uplift southern Paeroa block tilting SW Whakamaru caldera collapse C & Paeroa linear vent zone eruptions D Ohakuri caldera collapse

Huka Lake Kaingaroa Plateau Kaingaroa Plateau Taupo fault Taupo fault belt belt Taupo-R Taupo-Reporoa eporoa Basin Basin

Reporoa caldera collapse F E Ongarotolava rhyolitedam

Huka Lake

Kaingaroa Plateau Taupo faul Kaingaroa Plateau Taupo fault t belt belt Taupo-Reporoa Taupo-Reporo Basin Basi a n Figure 13. Time-series reconstruction of the central Taupo Volcanic Zone (TVZ). (A) Emplacement of abundant andesitic lavas (older than 1.9 Ma) and some rhyolitic ignimbrites in a single basin. (B) Continued ﬁ lling of the basin with Reporoa Group strata represented by numerous rhyolitic ignimbrites (i.e., Waiotapu Formation) and reworked (i.e., lacustrine, ﬂ uvial) equivalents. (C) Whakamaru caldera collapse at 349 ± 4 ka and Paeroa Subgroup emplacement at 339 ± 5 ka with subsequent uplift of the northern Paeroa block. (D) Ohakuri caldera collapse with subsequent tilting of the southern Paeroa block. (E) Reporoa caldera collapse at 281 ± 21 ka and continued lacustrine sedimentation from a rhyolite lava dam formed within the Ongaroto Gorge. (F) Continued basin development with episodes of rifting, effusive and explosive eruptions, and lacustrine sedimentation. See Figure 3 for color legend.

ary as deﬁ ned by Wilson et al. (1995) (Figs. Wilson et al., 1986). Rapid development of the (Lamarche et al., 2006). The shift in the locus of 5A and 13A, 13B). Parts of this basin are inter- Paeroa block, dividing the single central TVZ tectonism in the northern TVZ ~370 ka (Grav- preted to be superimposed upon a region of basin into TFB and TRB, was broadly contem- ley et al., 2010) predates the 349 ± 4 ka onset of earlier subsidence, as indicated by a deepening poraneous with an eastward focusing of sub- major caldera-forming volcanism (>3000 km3 of metasedimentary basement to the southwest. sidence within the northern TVZ, as interpreted of erupted material in eight caldera-forming This earlier period of subsidence appears to from stratigraphic relationships (Gravley et al., events within a 68 ± 25 k.y. time span; Fig. 12) be aligned with the ~7 Ma Hauraki Rift, par- 2010), drill hole stratigraphy at the Kawerau within the central TVZ. The close timing (~21 ticularly in the Whakamaru caldera area (Figs. geothermal ﬁ eld (Fig. 1) (Milicich et al., 2013), k.y.) between the onset of major caldera-form- 1 and 4A) (e.g., Modriniak and Studt, 1959; and patterns of fault growth in the offshore TVZ ing volcanism and rift localization and basin

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reorganization in the central and northern TVZ, all compositions overlap spatially and tempo- mation and the oldest rhyolite lava, a minimum respectively, suggests interactions between tec- rally, and attest to a complex interplay between sedimentation rate of ~31 mm/yr is estimated tonic and magmatic processes over considerable the silicic and mafi c magmatic systems at mid- for the caldera fi lling rate. Considering the age distances (Rowland et al., 2010). Could such crustal depths, and secular rifting (Charlier uncertainties present, fi lling may have occurred phenomena relate to migration of the arc? et al., 2005; Rowland et al., 2010). These inter- more quickly. In contrast, a signifi cant part of The locus of modern magmatism related to actions are non-linear and may result in episodic the stratigraphic architecture within the TRB subduction extends along strike from the ande- landscape forming volcano-tectonic events from refl ects low energy lacustrine sedimentation sitic cone of Ruapehu in the southwest to White time to time (cf. Rowland et al., 2010). with occasional localized small-scale effusive Island in the northeast (Fig. 1), and manifests and explosive events (Fig. 13F). Shifts from within the central TVZ as rhyolitic volcanism at Basin Filling Rates dominantly pyroclastic and volcaniclastic depo- the Taupo and Okataina volcanic centers (Nairn, The interplay between tectonism, volcanism, sition to low energy lacustrine deposition are not 2002; Wilson et al., 2009), and the >2000 MW and magmatism has resulted in rapid changes well established due to the scarcity of continu- geothermal output within the TRB (Bibby to accommodation space throughout the evo- ous core (as opposed to cuttings) in geothermal et al., 1995; Rowland et al., 2010). Stern (1987) lution of central TVZ basins, and so basin fi ll- drill holes and the diffi culty in distinguish- described the spatial and temporal distribution ing rates are likely to have varied by orders of ing boundaries between low energy and high of arc front andesitic volcanoes, synthesiz- magnitude over their lifetimes. A thick (at least energy sedimentary regimes on a lithological ing the available geochronology, to postulate 2 to 3 km) sequence of lavas, and volcaniclastic basis (e.g., Rosenberg et al., 2009). Nonethe- migration of an andesitic arc front across the and sedimentary strata in TRB geothermal drill less, a long-term average rate of background central North Island and infer a secular rate of holes, with abundant age dates (Fig. 3), allows lacustrine sedimentation for central TVZ lakes arc rotation. Inherent in Stern’s (1987) model for upper limits on fi lling rates within the TRB has been estimated as 0.28 mm/yr (Smith et al., is the notion that the migration of the andesitic to be estimated. The oldest dated deposit at 1993). These varying sedimentation rates refl ect arc front is gradual, and can be distinguished Waiotapu is the 1.45 ± 0.05 Ma ignimbrite pen- the processes that have resulted in basin forma- on the millennial scale. However, although the etrated at ~1000 m depth (Wilson et al., 2010). tion and fi lling, and are intimately linked with TRB appears to be currently aligned with the Gravity studies have been used to interpret the tectonic, volcanic, and magmatic processes. arc front, many of the thickest andesite lavas presence of basement rocks beneath Waiotapu at intercepted by drilling closely overlie metasedi- ~2000 m depth (Modriniak and Studt, 1959). If CONCLUSIONS mentary basement rocks, indicating that one it is assumed that the 1.45 ± 0.05 Ma ignimbrite or more andesitic composite cones contributed and basement rocks are separated by ~1000 m, New mapping within the Paeroa block, coupled some of the fi rst volcanic extrusions to the TRB then either basin fi ll started to accumulate sig- with drill hole records and age dates within the (e.g., at Rotokawa; Browne et al., 1992). An nifi cantly earlier than ~1.45 Ma, or fi lling of the TRB, have allowed for a basin-wide evolution- andesitic lava (Ngakoro andesite) in the Waio- basin occurred at a rapid rate prior to that time. ary model to be developed. While questions tapu geothermal fi eld (drill hole WT4; Fig. 4D) Although in the TRB, the 1.45 ± 0.05 Ma ignim- remain on the nature of tectonic, volcanic, and is bracketed by a 1.45 ± 0.05 Ma ignimbrite and brite has only been identifi ed at Waiotapu, the magmatic relationships across the TRB and the 1.21 ± 0.04 Ma Ongatiti Formation (Fig. thickness of Reporoa Group strata penetrated by TVZ, these processes are interconnected in con- 4A) (Wilson et al., 2010). At Ngatamariki and drill holes throughout the TRB (Fig. 4A) would trolling basin development and its stratigraphic Rotokawa, early andesites are overlain by rhyo- normally imply that signifi cant time would be architecture. The evolution of the TRB involves litic pyroclastic deposits of the Reporoa Group, required for accumulation. This is supported by the following. which return U-Pb inferred eruption ages of the 1.89 Ma and younger ages recorded from 1. A single basinal structure spanning the 1.89 Ma and younger recorded from zircons deep tuffs (at depths varying from 1.6 to 2.5 km) central TVZ began to develop during the onset of (age data of Eastwood et al., 2013). Further- at Ngatamariki and Rotokawa geothermal fi elds TVZ rifting and volcanism from ~2 Ma onward. more, there is broad overlap in the known ages (age data of Eastwood et al., 2013). These dates, The deposits in this early manifestation of TVZ of andesites from the western side of the TVZ to when coupled with other age information from basin development are represented by Reporoa those on the east (Wilson et al., 1995, and refer- shallower lithologies, imply that these areas Group strata, deposited until eruption of the ences therein). Thus, despite the intuitive appeal have never formed part of a caldera collapse 349 ± 4 ka Whakamaru Group ignimbrites. This of a gradually migrating andesitic arc front, any area, and that average subsidence rates in these group includes thick accumulations of andesitic such arc front does not appear to have migrated areas are only on the order of 0.8 to 1.4 mm/yr lavas, indicating that composite cones existed for >1.9 m.y. (cf. Waiotapu; Wilson et al., 2010). throughout the TRB from before 1.9 Ma, and A more likely explanation for the rapid Eruption of the Kaingaroa Formation depos- into the Huka Group time frame. change in basin confi guration within the cen- its and accompanying formation of the Repo- 2. Whakamaru caldera collapse accompanied tral and northern TVZ is the interplay between roa caldera at 281 ± 21 ka (Nairn et al., 1994; emplacement of the voluminous and regionally secular rifting and the assembly and evacuation Beresford and Cole, 2000) can be used as an extensive ignimbrites defi ning the older part of of large rhyolitic magma bodies (e.g., Rowland example of rapid fi lling within a localized basin the Whakamaru Group at 349 ± 4 ka, providing et al., 2010; Allan et al., 2012). Although a paired formed within the northern TRB during caldera a useful time horizon. andesitic arc front, rhyolitic back-arc system is collapse. The intracaldera Kaingaroa Forma- 3. Emplacement of the Paeroa Subgroup often promoted to explain the distribution and tion has a top surface penetrated by drill hole (the younger part of the Whakamaru Group) range of volcanism and gas chemistry within the at ~1250 m depth (Nairn et al., 1994). However, occurred at 339 ± 5 ka from a source near the central TVZ (Stern, 1987; Giggenbach, 1995), the three surfi cial post-Kaingaroa rhyolite lavas present-day Paeroa Fault. Closely coincident the available geochronology of all volcanic dated by us record ages of 264 ± 4 ka (Deer with this eruption, the northern Paeroa block compositions within the TRB and wider TVZ Hill), 263 ± 5 ka (Pukekahu), and 247 ± 2 ka underwent rapid uplift, and/or the Te Weta block does not justify such a discrimination. Vents of (Kairuru). Using the ages of the Kaingaroa For- was downthrown. This abrupt event resulted in

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U-Pb dating of subsurface pyroclastic deposits (Taho- ACKNOWLEDGMENTS Black, P.M., Briggs, R.M., Itaya, T., Dewes, E.R., Dunbar, rakuri Formation) at Ngatamariki and Rotokawa geo- H.M., Kawasaki, K., Kuschel, E., and Smith, I.E.M., thermal fi elds: Proceedings of the 35th New Zealand We thank Jim Cole, Darren Gravley, Alan East- 1992, K-Ar age data and geochemistry of the Kiwi- Geothermal Workshop, Rotorua, New Zealand, 8 p. wood, Pat Browne, Greg Bignall, and Isabelle tahi Volcanics, western Hauraki Rift, North Island, Evison, F.F., Robinson, R., and Arabasz, W.J., 1976, Micro- Chambe fort for helpful discussions, John Wilmshurst New Zealand: New Zealand Journal of Geology and earthquakes, geothermal activity and structure, central for X-ray fl uorescence analyses, and Mighty River Geophysics, v. 35, p. 403–413, doi:10.1080 /00288306 North Island, New Zealand: New Zealand Journal .1992 .9514535. of Geology and Geophysics, v. 19, p. 625–637, doi: Power for access to unpublished drill hole records. Boseley, C., Bignall, G., Rae, A., Chambefort, I., and Lewis, 10.1080 /00288306 .1976 .10426311. Financial support was provided by the New Zealand B., 2012, Stratigraphy and hydrothermal alteration Giggenbach, W.F., 1995, Variations in the chemical and iso- Ministry of Science and Innovation. Helpful reviews encountered by monitor wells completed at Ngatamariki topic composition of fl uids discharged from the Taupo by Jeff Amato, Cathy Busby, and an anonymous and Orakei Korako in 2011: Proceedings of the 34th New Volcanic Zone, New Zealand: Journal of Volcanol- reviewer and editorial handling by Tim Wawrzyniec Zealand Geothermal Workshop: Auckland, New Zea- ogy and Geothermal Research, v. 68, p. 89–116, doi: greatly improved this manuscript. land, University of Auckland Geothermal Institute, 8 p. 10.1016 /0377 -0273 (95)00009-J. 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