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Article Volume 11, Number 4 28 April 2010 Q04008, doi:10.1029/2009GC002705
AN ELECTRONIC JOURNAL OF THE EARTH SCIENCES Published by AGU and the Geochemical Society
ISSN: 1525‐2027
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Redefining the Waitemata Basin, New Zealand: A new tectonic, magmatic, and basin evolution model at a subduction terminus in the SW Pacific Phil Shane, Lorna J. Strachan, and Ian Smith SGGES, University of Auckland, Private Bag 92019, Auckland, New Zealand (
[email protected]) [1] The early Miocene Waitemata Basin has long been described as an interarc/intra‐arc basin, formed
between twin chains of arc volcanoes in Northland, New Zealand. However, deep marine, polymict, volcaniclastic conglomerates within the basin reveal tectonic and magmatic signals that are not evident from neighboring volcanic edifices. The conglomerates were deposited by high‐density turbidity currents and debris flows and include single sediment cycle megaclasts of lava. These basaltic lavas have ocean island basalt (OIB)‐like geochemical affinities and are precisely dated at 20 Ma by 40Ar‐39Ar methods. Their age and the occurrence of subordinate clasts derived from an ophiolitic nappe to the north indicate the basin postdates the initiation of collision in the wider region. Contemporaneous calc‐alkaline volcanism did occur some 250 km NW of the basin. However, the conglomerates lack clasts of calc‐alkaline/arc affinities indicating an absence of arc‐like volcanism in the vicinity of the basin. Recent tectonic models for the SW Pacific region and mantle tomography highlight the importance of wholesale slab detachment in driving early Miocene calc‐alkaline volcanism and basin development. Although such models provide a slab window for the eruption of nonarc OIB‐like magmas, they would not explain their localized occurrence at the proposed leading edge of the tear (Waitemata Basin), rather than progressively along the entire length of the detachment (Northland), as seen in other detachment settings. In addition, OIB‐like volcanism predates the adjacent calc‐alkaline volcanism on the margin of the basin, a transition that is the opposite of that found at other slab detachment settings. The occurrence of OIB‐like volcanism is better explained by a lateral slab termination in the vicinity of the Waitemata Basin that allowed asthenospheric‐derived magmas to erupt. The basin is inferred to have developed in response to asthenospheric upwelling and associated lithospheric extension or deformation associated with a slab termination zone. This study highlights the importance and potential of combined geochemical, geochronological, and sedimentological studies of conglomerates in reconstructing the geodynamic setting of a basin. Components: 12,200 words, 13 figures, 4 tables. Keywords: subduction; basalt; conglomerate; Waitemata Basin; geochemistry; geochronology. Index Terms: 8104 Tectonophysics: Continental margins: convergent (1031); 8185 Tectonophysics: Volcanic arcs (1031); 8169 Tectonophysics: Sedimentary basin processes (1031); 3613 Mineralogy and Petrology: Subduction zone processes (1031); 3022 Marine Geology and Geophysics: Marine sediments: processes and transport. Received 25 June 2009; Revised 24 November 2009; Accepted 1 March 2010; Published 28 April 2010. Shane, P., L. J. Strachan, and I. Smith (2010), Redefining the Waitemata Basin, New Zealand: A new tectonic, magmatic, and basin evolution model at a subduction terminus in the SW Pacific, Geochem. Geophys. Geosyst., 11, Q04008, doi:10.1029/2009GC002705.
Copyright 2010 by the American Geophysical Union
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1. Introduction [2] Convergent plate boundaries are the locus of intense tectonism, volcanism and sediment production that extend on length scales of the order of 102–103 km. Hence, these regions are important in plate tectonic reconstructions and in developing geodynamic models of basin evolution and magma generation. Convergent boundaries typically exhibit complex geologic histories. Arc‐continent collisions [e.g., Clift et al., 2003] or subduction of mid ocean ridges [e.g., Cole and Stewart, 2009] can result in subduction termination, ophiolite obduction, flips in subduction polarity, and production of microplates. All of these processes can be expressed as surface manifestations of volcanism and sedimentation that can overprint and obscure previous events. During the evolution of subduction systems, laterally propagating slab detachment can result from the attempted subduction of young lithosphere that is more buoyant than the slab already subducted [e.g., Wortel and Spakman, 2000]. The resulting asthenospheric upwelling through the detachment zone influences the nature of magmas erupted [e.g., Ferrari, 2004], the overlying regional topography [e.g., Rogers et al., 2002], and migration of depocenters [e.g., van der Meulen et al., 1998]. [3] Identifying such discontinuities in the slab is a challenge to understanding the geodynamic setting of coeval sedimentary and volcanic systems. Reconstructing convergent plate boundaries from ancient terranes, where plate boundary configurations are no longer obvious, relies heavily on volcanic rock geochemistry, isotopic ages and sedimentary architecture. Commonly, chains of temporally associated calc‐alkaline volcanic edifices or their erosional products mark ancient subduction systems. Basalts of the arc and back‐arc regions are distinguished by characteristic geochemical signatures of slab‐derived fluids in the suprasubduction mantle. However, nonarc magmas with ocean island basalt (OIB)‐like and mid ocean ridge basalt (MORB)‐like affinities are known to erupt through slab windows and at slab terminations [e.g., Hole et al., 1991; Ferrari, 2004; Portnyagin et al., 2005; Cole and Stewart, 2009]. Such occurrences make reconstructions of ancient terranes more difficult, but also provide insight to the nature of subduction processes. [4] The aim of this study is to demonstrate the value of a combined petrological, geochronological and sedimentological investigation of volcaniclas-
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tic conglomerates in reconstructing the geodynamic setting of a basin where geological and geophysical features of subduction such as the fore‐arc region, trench or slab can no longer be recognized, and volcanic edifices have been obliterated. We illustrate this approach with the early Miocene Waitemata Basin [Ballance, 1974] in Northland, New Zealand (Figures 1 and 2). Despite more than 30 years of study, the origin of formation of this basin remains enigmatic. Nevertheless, it has been inferred to be an intra‐arc basin developed between two calc‐ alkaline volcanic arcs, which today bound the basin [Ballance, 1974; Hayward, 1979, 1993; Hayward et al., 2001]. [5] The Northland region is important in the numerous and often conflicting plate tectonic reconstructions of the SW Pacific, because it defines the southern end of an Eocene to early Miocene subduction system that extended from north of New Caledonia and was the locus of extensive ophiolite emplacement and slab rollback episodes between the Pacific Plate and the eastern margin of Australian Plate [Schellart et al., 2006; Whattam et al., 2008] (Figure 1). Recently, wholesale slab detachment in the early Miocene has been proposed to explain the widespread calc‐alkaline volcanism in Northland and the formation of Waitemata Basin [Schellart, 2007]. [6] We have used the sedimentology, petrology, geochemistry and 40Ar‐39Ar ages of clasts in volcaniclastic conglomerates of the Waitemata Basin to determine the transport, provenance and age of the deposits, and infer the type of magmatism (i.e., intraplate versus subduction‐related) that occurred adjacent to or within the basin. The age and type of magmatism and the terranes that supplied the sediments provide insight to the tectonovolcanic setting of the basin and thus an opportunity to test tectonic models of the basin and the wider SW Pacific region.
2. Geological Setting 2.1. Northland Region [7] The Waitemata Basin is located in the southern part of Northland (Figure 2), a region that has undergone a history of alternating convergent boundary and passive margin tectonism. The basement comprises Permian to Jurassic metagreywacke terranes accreted to the margin of Gondwana in the Cretaceous. During most of the late Cretaceous to Oligocene, the region was a passive margin of the 2 of 23
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Figure 1. Map of the SW Pacific showing features associated with the Eocene–early Miocene subduction boundary adapted from Schellart [2007], location of the Waitemata Basin (W) in New Zealand, and present‐day physiographic features. The Eocene–early Miocene subduction boundary is thought by some workers to represent the east dipping subduction of the South Loyalty Basin slab (see Table 1). Box outline shows the location of the map in Figure 2.
Australian Plate now represented by both autochthonous and allochthonous, mostly marine, sedimentary and volcanic sequences (Figure 2). Allochthonous sequences including ophiolite igneous rocks, referred to as the Northland Allochthon or Northland ophiolitic nappe [Whattam, 2009], were emplaced onto regional basement terranes from the northeast at about 25–21 Ma [Ballance and Spörli, 1979]. The exact timing of the emplacement is imprecise and is
partly based on limited biostratigraphy in adjacent marine basins. High‐precision isotopic data on the ophiolite igneous rocks give eruption ages of 32– 26 Ma [Whattam et al., 2005, 2006]. [8] Evidence for subduction‐related volcanism in
the early Miocene is based on the occurrence of calc‐alkaline volcanic edifices exposed along the west and east coasts of Northland (Figure 2) vari3 of 23
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Figure 2. Sketch map of Northland, New Zealand, showing the location of Waitemata Basin and elements of Miocene geology discussed in section 2, adapted from Hayward [1993]. Volcanoes: T, Tokatoka; H, Hukatere; M, Manukau. Buried offshore volcanoes from S. Johnston (personal communication, 2009). Inset is a map showing sites in the Waitemata Basin examined in this study.
ously dated at 25–11 Ma [Hayward et al., 2001]. Despite the numerous K‐Ar ages of lavas from these volcanic edifices, some uncertainty remains concerning the initiation of volcanism as discordant ages have been obtained from some lavas and the selection of other ages have been based on concordance with biostratigraphy of interbedded sedimentary rock [Hayward et al., 2001]. Volcanic edifices delineated from geophysical evidence offshore of Northland (Figure 2) are also grouped as part of the volcanic arc [e.g., Herzer, 1995; Hayward et al., 2001]. However, their composition and age are unverified because they have not been sampled. [9] Coeval ophiolite emplacement, activation of major faults in New Zealand, and shifts in the Pacific plate pole of rotation, are all features of a major episode of plate boundary reorganization
during the Oligocene and early Miocene [e.g., Schellart et al., 2006; Whattam et al., 2008]. These authors suggest that continent‐arc collision or attempted subduction of Northland passive margin (eastern Australian Plate) was responsible for the ophiolite emplacement in Northland. An Eocene to early Miocene subduction system between the Australian and Pacific plates, and involving various microplates, extended from north of New Caledonia to a southern termination in the Northland region (Figure 1). However, numerous Cenozoic plate tectonic reconstructions have been proposed for Northland that differ in the number and orientation of dipping slabs and the cause of calc‐alkaline volcanism in the area (Table 1). The polarity of subduction is unclear because of the lack of geological or seismic evidence for a fore‐arc area, trench or subducted slab on either side of Northland. In addition, the geometry and continuity of 4 of 23
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Table 1.
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Summary of Early Miocene Subduction Systems Proposed for Northland Author
Slab Beneath Northland
Notes
Ballance [1976] and Mortimer et al. [2007] Brothers [1984] Herzer [1995] and Whattam et al. [2005] Crawford et al. [2003]
SW to west dipping Pacific slab
no time progression in volcanism
NW dipping Pacific slab SW to west dipping South Fiji basin slab SE dipping South Loyalty basin slab
Schellart et al. [2006; 2009] and Schellart [2007]
SE dipping South Loyalty basin slab; slab detachment
Malpas et al. [1992] and Whattam et al. [2006; 2008]
SE dipping South Loyalty basin slab (±detachment); flip to SW dipping South Fiji Basin slab
SE rollback and migration of volcanism no time progression in volcanism; subduction driven volcanism explains widespread SW Pacific ophiolite emplacement; extension driven volcanism explains widespread SW Pacific ophiolite emplacement; slab detachment driven volcanism explains widespread SW Pacific ophiolite emplacement; subduction and/or slab detachment driven volcanism
the arc north of Northland is complicated by Miocene dextral movement on the Vening Meinesz Fault (Figures 1 and 2) [e.g., Mortimer et al., 2007]. [10] The timing of the cessation of subduction in
Northland is uncertain, but generally placed at post–15 Ma on the basis of K‐Ar ages [Hayward et al., 2001].The post–10 Ma magmatic history of Northland (Australian Plate) is characterized by intraplate terrestrial basaltic fields. Two of these fields, the Auckland Volcanic Field (AVF) [Huang et al., 1997] and South Auckland Volcanic Field (SAVF) [Cook et al., 2005] were erupted through the southern margin of the Waitemata Basin in the last 1.6 Myr.
2.2. Waitemata Basin [11] Much of the early Miocene history of North-
land is based on sediments of the marine Waitemata Basin that occupied the southern part of the region. The basin is thought to be an elongate and narrow extending ∼130 km southeastward from Northland with a width of ∼60 km (Figure 2). Previous workers have described it as an interarc or intra‐arc basin because remnants of calc‐alkaline volcanoes presently border its margins, and volcaniclastic debris occurs within the basin [Ballance, 1974; Hayward, 1979, 1993]. Ballance [1974] first suggested that the basin underwent rapid subsidence in its early history, and Schellart [2007] proposed that slab pull during detachment of an east dipping subducted slab was the driving mechanism. A maximum subsidence of about 1 km is indicated by vitrinite reflectance and apatite fission track studies [Raza et al., 1999]. Deposition in the basin resulted from turbidity currents, pelagic fallout, submarine debris flows, and siliciclastic and volcaniclastic slumping [Ballance, 1974; Strachan,
2008]. Proposed sediment provenances include greywacke basement on the eastern margin, ophiolitic nappe to the northwest and calc‐alkaline volcanoes to the west and east [e.g., Ballance, 1974; Hayward, 1993; Raza et al., 1999] (Figure 2). [12] The chronology of the basin is poorly estab-
lished because the turbidite facies that fill most of the basin are poorly fossiliferous. These comprise the Waitemata Group (Pakiri and East Coast Bays Formations) that include the volcaniclastic conglomerates examined here. The strata are dated younger than 22 Ma [Hayward et al., 2001] based on the occurrence of foraminifer Ehrenbergina marwicki, a facies‐restricted species found only in basal Waitemata Basin sediments on the eastern margin of the basin [Hayward and Brook, 1984]. The absence of foraminifer Globorotalia incognita that is estimated to appear at ∼20 Ma is taken to restrict much of the turbidite facies to older than ∼20 Ma [Hayward et al., 2001]. However, these endemic species and the New Zealand Miocene biostratigraphic divisions are poorly calibrated [Cooper, 2004], and lack isotopic age control. In addition, the lack of basin‐wide stratigraphic markers prevents the wide application of bioevents. Zones of intense synsedimentary deformation, some attributed to thrusting or sliding of allochthonous rocks into the northern margin of the basin [Hayward, 1993], has hindered the development of an internal stratigraphy. The overall westward tilt of the basin now exposes older sediments in the central and eastern parts of the basin. [13] The western margin of the basin is marked by
calc‐alkaline andesite volcanic edifices [Herzer, 1995], including the Manukau and Hukatere volcanic centers (Figure 2). Arc‐derived volcaniclastic debris and pillow lava interfinger with marine 5 of 23
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Table 2. Site 1 2 3 4 5 6 7 8c a
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Parnell Volcaniclastic Conglomerate Rock Samples, Site Locations, and Analyses Performed
Grid Referencea R09/705464 R09/644209 R10/637155 R10/731104 R10/664075 R10/674983 R10/684933 Q11/376843
Locality
Petrographic Analysis
Leigh (L) Opahi Bay (OB) Waiwera (W) Army Bay (A) Tarihunga Tor Bay (T) Campbell Bay (CB) Muriwai (Mu)
b
Geochemical Analysis
40
Ar‐39Ar Analysis
L1–2 OB1–10 W1–34 A1–13
L1 OB1–10 W1–9, 16–19, 22–30,3 2 A1–8, 10, 11
W1, W8 A1, A4
T1–4 CB1–7 Mu1–12
T1 CB1–7 Mu1
CB1 Mu1
Grid Reference from the NZ 1:50,000 metric map series. Field sample numbers. Site 8 is Muriwai lava, Manukau volcano, Waitakere Group.
b c
2.3. Early Miocene Volcaniclastic Conglomerates
Waitemata Basin. Parnell Grits have been reported from throughout Waitemata Group successions encompassing basal and central stratigraphic formations [Ballance, 1974; Ballance and Gregory, 1991; Allen, 2004]. They range in thickness from 0.5 m to 20 m. Their lateral extent is yet to be determined and consequently very few units have been laterally correlated [Allen, 2004]. The stratigraphic usage of the term “Parnell Grit Member” is now obsolete [e.g., Allen, 2004], and the descriptor “grit” is not used in modern grain size schemes, making the term redundant. This study uses the descriptive term Parnell Volcaniclastic Conglomerate (PVC) for the conspicuous basalt dominated– volcaniclastic polymict pebble to boulder conglomerates and pebbly sandstones examined here.
[14] Conspicuous
[16] PVCs have been described as submarine debris
sediments (Waitakere Group) that are assumed to overlie Waitemata Group sediments [e.g., Hayward, 1979, 1993]. Available K‐Ar data fail to demonstrate that the volcanic edifices are contemporaneous with volcaniclastic conglomerates within the Waitemata Basin studied here, and limited biostratigraphic data suggests calc‐alkaline volcanism postdates much of the basin fill (see section 7.2). Uplift and terrestrial sedimentation is estimated to have occurred at ∼17 Ma, based on microfaunas restricted to the Waitakere Group [e.g., Hayward, 1993].
volcaniclastic conglomerates, referred to in previous work as “Parnell Grits,” that occur within turbidite successions of the Waitemata Basin have been used to validate an intra‐arc/ interarc basin setting [e.g., Ballance, 1974; Hayward, 1979, 1993; Ballance and Gregory, 1991; Allen, 2004]. A critical aspect of this interpretation was that clasts within these event beds correlate with calc‐alkaline andesite volcanic edifices to the west of the basin, suggesting western and northwestern sediment sources. Such inferences were made with limited paleocurrent data (two measurements [Ballance, 1974]), and were contrary to reconnaissance geochemical analyses of the clasts that revealed low‐silica basalt compositions dissimilar to rocks of the andesite volcanic edifices [Smith et al., 1989]. Yet with such limited and contradictory evidence, a westerly volcanic arc source for the conglomerates continues to dominate most Waitemata Basin models [Ballance and Gregory, 1991; Hayward et al., 2001; Allen, 2004]. [15] The focus of this study is a detailed examination
of a small number of the coarsest volcaniclastic conglomerate horizons within the turbidite‐bearing
flow deposits, dominated by traction carpet processes, where laminar flow dominated and deposition was via frictional en masse freezing [Ballance and Gregory, 1991], and alternatively by complex flows composed of a lower debris flow, overlain by a slurry flow [Allen, 2004]. Trigger mechanisms for these flows include large‐scale sector collapse from terrestrial or shallow marine stratovolcanoes [Ballance and Gregory, 1991] or gravitational failure of volcaniclastic ring plain deposits in coastal settings [Allen, 2004].
3. Methods [17] Sedimentary logging, paleocurrent measure-
ments, rock sample collection and detailed field observations were made at seven PVC sites (Figure 2). Samples of igneous clasts for petrographic and geochemical analysis were collected from six sites (Table 2). Forty‐eight clasts were selected for geochemical characterization by X‐ray fluorescence and laser ablation ICPMS (Appendix A). Reconnaissance crystal chemistry was undertaken 6 of 23
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Table 3.
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Major Parnell Volcaniclastic Conglomerate Clast Types and Their Characteristics Lithology
Size (cm)
Form
Porphyritic basalt Aphyric basalt Red porphyritic scoria
Type 1 Subangular Basalt
