Structural controls on the location, geometry and longevity of an intraplate volcanic system: the Tuatara Volcanic Field, Great South Basin, New Zealand

TL;DR: In this article, the authors use 2D seismic reflection data to characterize the 3D geometry of the Tuatara Volcanic Field located offshore New Zealand9s South Island and investigate its relationship with the pre-existing structure.

Abstract: Intraplate volcanism is widely distributed across the continents, but the controls on the 3D geometry and longevity of individual volcanic systems remain poorly understood. Geophysical data provide insights into magma plumbing systems, but, as a result of the relatively low resolution of these techniques, it is difficult to evaluate how magma transits highly heterogeneous continental interiors. We use borehole-constrained 2D seismic reflection data to characterize the 3D geometry of the Tuatara Volcanic Field located offshore New Zealand9s South Island and investigate its relationship with the pre-existing structure. This c. 270 km2 field is dominated by a dome-shaped lava edifice, surrounded and overlain by c. 69 volcanoes and >70 sills emplaced over 40 myr from the Late Cretaceous to Early Eocene (c. 85–45 Ma). The Tuatara Volcanic Field is located above a basement terrane boundary represented by the Livingstone Fault; the recently active Auckland Volcanic Field is similarly located along-strike on North Island. We suggest that the Livingstone Fault controlled the location of the Tuatara Volcanic Field by producing relief at the base of the lithosphere, thereby focussing lithospheric detachment over c. 40 myr, and provided a pathway that facilitated the ascent of magma. We highlight how observations from ancient intraplate volcanic systems may inform our understanding of active intraplate volcanic systems, including the Auckland Volcanic Field. Supplementary material: Interpreted seismic section showing well control on stratigraphic interpretation is available at https://doi.org/10.6084/m9.figshare.c.5004464

Summary

1 Introduction

• The authors propose the location and longevity of the Tuatara Volcanic Field was controlled by the underlying terrane boundary marked by the Livingstone Fault.
• In particular, the authors suggest changing relief of the lithosphere-asthenosphere boundary across the terrane boundary promoted local lithospheric detachment and melting, whilst the Livingstone Fault facilitated magma ascent.
• The structural setting of the Tuatara Volcanic Field is equivalent to that of the recently active Auckland Volcanic Field on the North Island (active 193 Ka -500 y BP; Hopkins et al., 2020; Lindsay et al., 2011) .
• The authors study of the Tuatara Volcanic Field, which highlights how pre-existing structure and sill-complex development can influence the location and longevity of volcanic activity, may offer important insights into the processes occurring at the Auckland Volcanic Field and intraplate volcanism elsewhere.

2.1 Regional geological evolution

• Late Cretaceous syn-rift strata within the Great South Basin, which were deposited unconformably onto the Permian-to-Triassic crystalline basement of the Caples and Dun Mountain-Maitai terranes, are dominated by siliciclastic rocks and coal measures of the Hoiho Group (Higgs et al., 2019; Killops et al., 1997; Sahoo et al., 2014) .
• Widespread deposition of deep marine mudstones and siltstones occurred across the majority of the Great South and Canterbury Basins during the Cenozoic, with some carbonate deposition in the Oligocene-Miocene (Fig. 1b ) (Bertoni et al., 2019; Chenrai and Huuse, 2020; Morley et al., 2017) .
• The Marshall Paraconformity forms the Oligocene-Eocene boundary across the area and is purported to be related to the onset of the Antarctic circumpolar current (Fulthorpe et al., 1996; Morley et al., 2017) .
• Much of the shallow stratigraphy across the Great South and Canterbury Basins has been reworked into contourite deposits (Fulthorpe et al., 1996; Lu and Fulthorpe, 2004) .
• At the present day, a series of steep-sided canyons traverse the seabed across the area, often eroding down to the Marshall Paraconformity surface .

2.2 Intraplate igneous activity across Zealandia

• Volcanic activity is not compatible with a plume-related origin as such a long record of activity would require a static Zealandia relative to a 'fixed' mantle source; yet plate motion data indicate Zealandia has moved ~4000 km N/NW during the Cenozoic (Clouard and Bonneville, 2005; Hoernle et al., 2006; Sutherland, 1995; Wright et al., 2016) .
• Furthermore, aside from igneous activity related to back-arc rifting in the Taupo Volcanic Zone, Cenozoic magmatism across Zealandia is not related to lithospheric thinning and extension, which ceased during the Late Cretaceous (Acocella et al., 2003; Kula et al., 2007; Laird and Bradshaw, 2004; Mortimer et al., 2019) .
• As a driver for lithosphere detachment, it has been suggested that the lithosphere beneath Zealandia contains large amounts of garnet pyroxenites and eclogites following protracted subduction, creating a contrast between dense lower lithosphere and relatively less dense upper asthenosphere (Elkins-Tanton, 2005; Hoernle et al., 2006; Timm et al., 2009) .
• Similarly, increased mantle water content in a post-subduction setting may decrease mantle viscosity and lower the peridotite solidus, resulting in small-scale convection at the base of the lithosphere (Elkins-Tanton, 2005; Kaislaniemi et al., 2014) .
• A key component of this coupled lithosphere detachment and small-scale convection mechanism is that the magma source is not fixed in specific locations in the mantle or lithosphere, allowing individual intraplate volcanic systems to occur over widespread areas and long timescales.

3.1 Available data and seismic interpretation

• The authors use 2D seismic reflection data from three different surveys (OMV, DUN and HUN), with a total line length of >50,000 km.
• These datasets were acquired over a range of years (1972, 2006, and 2008) and, accordingly, have different acquisition and processing parameters.
• All seismic data are zero phase and displayed in normal polarity, such that a downward increase in acoustic impedance (e.g., the seabed) is represented by a peak (red) reflection, with a downward decrease in acoustic impedance represented by a trough (blue).
• The seabed across the study area is cut by multiple, steepsided, up to ~0.5 s TWT deep canyons, which often produce geophysical artefacts at deeper stratigraphic levels that partially obscure reflection configurations.
• The magnetic data used in this study are shown as reduced to pole in order to place the anomalies vertically above the magnetic source .

3.2 Seismic resolution

• No igneous features associated with the Tuatara Volcanic Field are penetrated by boreholes, so the authors do not know their composition or seismic velocity.
• Combined with a dominant seismic frequency of ~30 Hz within the depth interval of the Tuatara Volcanic Field, their inferred velocities correspond to limits of separability and visibility of ~25-50 m and 3-7 m, respectively.
• As the authors do not know the detailed velocity structure of the volcanic province and surrounding strata, particularly its lateral variability, they do not depth-convert the seismic reflection data and present measurements in time rather than depth to avoid additional errors.

3.3 Interpreting and dating volcano-magmatic features

• As only 2D seismic reflection data are available, many volcano-magmatic features are only observed on individual 2D lines, meaning the authors cannot assess or quantify their individual 3D geometry.
• Where volcano-magmatic features can confidently be mapped across several 2D lines, the authors utilise the seismic software to interpolate their 2D horizon interpretations and recover their approximated 3D structure (see Hansen et al., 2008) .
• Furthermore, the authors acknowledge that seemingly isolated features interpreted on different sections may in fact form part of a larger, singular structure.
• Quantitative measurements can therefore only be considered to represent minimum estimates.
• Finally, the authors note that some small volcanomagmatic features present in the study area may occur between and thus not be imaged by their 2D seismic grid.

Volcanic Field

• The authors recognise a variety of different intrusive and extrusive igneous features that they can differentiate based on their location relative a central structural high, which they term the 'Central edifice'.
• For clarity, here the authors sequentially describe and interpret the origin and age of: (i) high-amplitude reflectivity comprising the Central edifice; (ii) mound-like structures atop and beyond the lateral limits of the Central edifice; and (iii) intrusive features, and associated host rock structures, beyond the lateral limits of the main edifice.

4.1.1 Observations

• The authors also identify some areas of lower amplitude reflectivity within the reflection package, which typically correspond to mound-shaped features (see section 4.2), or reflections displaying a clinoformlike geometry (Figs 3, 4); i.e. they consist of gently dipping reflections with a sigmoidal geometry <100 ms TWT high.
• The top inflexion point of these sigmoidal reflections is typically horizontal across the reflection package .

4.1.2 Interpretation

• Based on their location within and associated with the stacked lava sequences, the authors suggest the relatively low-amplitude, sigmoidal reflection packaes may also be igneous in origin.
• In particular, the authors consider these sigmoidal reflection packages correspond to lava or hyaloclastite deltas, as they appear similar in their geometry, structural setting, and seismic character to examples identified, and occasionally drilled, in other sedimentary basins (Planke et al., 2000; Wright et al., 2012) .
• Using the sequence-stratigraphic terminology applied to clinoforms, the sigmoidal reflection packages described here represent dominantly progradational sequences, indicative of transport away from areas of higher relief, with little to no aggradation .
• At shallow depths, the top Early Eocene horizon blankets the edifice indicating that it postdates its formation.
• Between these maximum and minimum age constraints, the top Paleocene horizon appears to represent the basal surface of a lava sequence in the upper parts of the high-amplitude reflection package , suggesting the Central edifice formed through at least two extrusive events in the Upper Cretaceous and towards the end Palaeocene.

4.2.1 Observations

• To the south of the Central edifice, the authors identify multiple high-amplitude, positive polarity, tuned reflection packages within the Lower Eocene succession .
• These high-amplitude reflections are situated beneath, but close to, the top Early Eocene horizon and are downlapped by Early Eocene-aged strata .
• In places, the high-amplitude reflections appear to cross-cut and/or truncate underlying reflections .

4.2.2 Interpretation

• The brightening of the lava flows away from the volcanic cone likely represents a decrease in thickness, and an associated increase in seismic tuning, towards their termination (Kallweit and Wood, 1982; Smallwood and Maresh, 2002; Widess, 1973) .
• The truncation of underlying strata by the lava flows may correspond to the formation of lava channels and the associated bulldozing and erosion of the underlying stratigraphy (Sun et al., 2019a) .
• The depression on the Marshall Paraconformity surface co-located above the Early Eocene volcano cone appears to have formed due to fluid expulsion following burial of the volcano, which focussed migrating, non-volcanic-related fluids (Holford et al., 2017; Sun et al., 2020) .
• Buried volcanoes elsewhere have been shown to act as conduits for later fluid flow in the subsurface (Holford et al., 2017; Sun et al., 2020) .
• The authors also interpret the sub-horizontal, high-amplitude reflection within the overlying differential compaction fold of one volcano as a 'flat-spot', likely corresponding to a trapped gas pocket .

4.3.1 Observations

• Many of these transgressive high-amplitude reflections are located beneath interpreted volcanoes, although some appear independent of extrusive features and occur up to 30-40 km from the Central edifice .
• The majority of these high-amplitude transgressive reflections are associated with clear anticlinal folds in the overlying strata, the outer inflection points of which directly overlie the lateral terminations of the high-amplitude reflection .
• These folds can be identified throughout Late Cretaceous-to-Early Eocene strata, with strata onlapping onto their margins .

4.3.2 Interpretation

• The authors interpret the folds overlying some of the sills as intrusion-induced forced folds that formed via overburden uplift to accommodate shallow-level magma emplacement (Hansen and Cartwright, 2006; Magee et al., 2013a; Reeves et al., 2018; Trude et al., 2003) .
• Recognition of overlying strata onlapping onto these folds indicates the top fold surface corresponded to the syn-emplacement palaeosurface , and can be used to date intrusion (e.g. Trude et al., 2003) .
• The authors acknowledge that simply using host rock age to establish relative emplacement timings is not ideal because the mapped sills could be significantly younger than the rocks they intruded.

6 Discussion

• The Tuatara Volcanic Field occupies a relatively localised (~270 km 2 ) setting above the Livingstone Fault, which marks the boundary between the Dun Mountain-Maitai Terrane to the south, and the Caples Terrane in the north (Mortimer et al., 2002; Tarling et al., 2019) .
• Here, the authors examine the controls on the localisation, geometry, and longevity of intraplate volcanic activity at the Tuatara Volcanic Field.
• The authors also discuss how their observations of the internal structure and plumbing system of the Tuatara Volcanic Field may relate to the Auckland Volcanic Field and enhance their understanding of intraplate volcanic systems generally.

6.1 Controls on the location of the Tuatara Volcanic Field

• It is difficult to ascertain whether its individual intrusive or extrusive components were similarly structurally controlled.
• At shallow depths within the Tuatara Volcanic Field, the authors are unable to fully constrain the geometry of rift-related faults beneath the Central edifice, although faults typically strike NE-SW across the Great South and Canterbury Basins (Phillips and McCaffrey, 2019; Uruski et al., 2007; Uruski, 2010) .

6.2 Controls on the longevity of the Tuatara Volcanic Field

• Similarly, at the Banks Peninsula on the South Island, volcanic activity migrates along the NWtrending boundary between the Pahau and Rakaia terranes, albeit migrating in the opposite direction to that observed at the Tuatara Volcanic Field.
• In the case of Banks Peninsula, this may reflect progressive lithosphere detachment towards the southeast along the terrane boundary; as the lithosphere begins to anneal beneath the Lyttelton Volcano, further detachment occurs to the southeast leading to activity at the Akaroa Volcano (Timm et al., 2009) .
• The authors propose a similar mechanism of along-strike progressive lithospheric detachment focused along the expression of the Livingstone Fault at the lithosphereasthenosphere boundary to explain the longevity and age progression within the Tuatara Volcanic Field (Mortimer, 2004) .
• In particular, the authors suggest such localisation by pre-existing structures may result in a quasi-periodic 'dripping' of material from the base of the lithosphere, focussing magmatic upwelling and inhibiting the complete annealing of the lithosphere over prolonged (>10 Myr) periods.
• That monogenetic volcanic fields are typically characterised by smalldegree melts (Hoernle et al., 2006; Timm et al., 2009) suggests this quasi-periodic dripping is involves relatively small, but relatively frequent detaching of lithospheric material.

6.3 Implications for intraplate volcanism

• The authors document the 3D geometry and longevity of an intraplate volcanic system, highlighting that seismic reflection data can potentially provide important insights into the processes driving intraplate volcanism.
• In particular, the shallow-level plumbing system of the Tuatara Volcanic Field involves interconnected sills .
• The authors work also implies that extinct volcanoes may be buried beneath the current exposure of the Auckland Volcanic Field; these buried volcanoes could provide pathways for fluid/gas escape (e.g., Holford et al., 2017; Sun et al., 2020) .
• Finally, whilst the authors are unable to directly comment on the likely longevity of volcanic activity in the Auckland Volcanic Field, observations from the Tuatara Field described here suggest that volcanic fields partly controlled by pre-existing structures could periodically be rejuvenated.

7 Conclusions

• The geometry and structural setting of the Tuatara Volcanic Field resembles the Auckland Volcanic Field; the locations of both volcanic fields appear to be controlled by the pre-existing Livingstone Fault and terrane boundary.
• In particular, whilst magma transport is dominantly vertical throughout the lithosphere, the plumbing system of the Auckland Volcanic Field may include a sill-complex that could connect and control the distribution of volcanoes within the field.
• Furthermore, the authors postulate that the longevity and progression of activity at the Tuatara Volcanic Field could imply that the Auckland Volcanic Field is perhaps relatively early in its evolution and that activity may migrate along the pre-existing structure across geological time.
• The authors highlight how pre-existing crustal and lithospheric structure exert an important influence over the location and longevity of individual intraplate volcanic systems.
• The authors offer insights into the internal structure and plumbing system of these volcanic fields that may be applicable to other ancient and active intraplate systems.

Frequently asked questions

Q1. What are the contributions in this paper?

The authors use borehole-constrained 2D seismic 14 reflection data to characterise the 3D geometry of a volcanic field offshore New Zealand ’ s South 15 Island, termed the Tuatara Volcanic Field, and investigate its relationship with pre-existing structure. The authors suggest the Livingstone Fault controlled the location of the Tuatara Volcanic Field 21 by producing relief at the base lithosphere, thereby focussing lithosphere detachment over ~40 Myr, 22 and provided a pathway that facilitated magma ascent. The authors highlight how observations from ancient 23 intraplate volcanic systems may inform their understanding of active intraplate volcanic systems, 24 including the Auckland Volcanic Field.