Journal of Volcanology and Geothermal Research 195 (2010) 127–137

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Journal of Volcanology and Geothermal Research

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The Auckland volcanic field, New Zealand: Geophysical evidence for structural andspatio-temporal relationships

John Cassidy ⁎, Corinne A. LockeSchool of Environment, The University of Auckland, Private Bag 92019, Auckland 1142, New Zealand

⁎ Corresponding author. Tel.: +64 9 3737599x87824E-mail address: j.cassidy@auckland.ac.nz (J. Cassidy)

0377-0273/$ – see front matter © 2010 Elsevier B.V. Aldoi:10.1016/j.jvolgeores.2010.06.016

a b s t r a c t

a r t i c l e i n f o

Article history:Received 7 February 2010Accepted 25 June 2010

Keywords:Auckland volcanic fieldmonogeneticgeophysical dataaeromagnetic

Geophysical data from the monogenetic Auckland volcanic field reveal complex structural and spatio-temporal relationships at different scales. The volcanic field is coincident with regional magnetic and gravityanomalies that mark a major crustal suture and with a discontinuity marking a significant structural asperity.Here, the linear regional magnetic anomaly splays into a wide band of NNW-trending lineaments, arisingfrom serpentinised shear zones in the upper crust, that matches the extent of the volcanic field and that mayreflect a region of crustal weakness creating preferential permeability. However, there appears to be nosimple correlation between the locations of individual vents and these lineaments that might delineate moreshallow structural controls with this orientation, probably as a consequence of other structural influences.High-resolution aeromagnetic data over the volcanic field show that the volcanoes have a wide range ofmagnetic signatures indicating a variability of subsurface structure. Scoria cone volcanoes typically havestrong anomalies (up to several 100 nT) whilst tuff-ring volcanoes typically have weak anomalies (less than50 nT), though the surface geology is not always an indicator of the nature and extent of the subsurfacedeposits. Both cone and tuff-ring volcanoes in the Auckland field appear to be underlain by subsurface bowl-shaped bodies of basalt, implying that their eruption histories commonly involve lava ponding into earlyexcavated craters. The present geophysical data give no evidence for subsurface dyke-like structures or forsubstantial near-surface volumes of basaltic rocks where there are no known eruption centres or buriedflows. Aeromagnetic and palaeomagnetic data suggest that a number of adjacent vents with an impliedstructural linkage may be contemporaneous, though other examples occur where vents of clearly differentages exploit the same apparent structure. A unique feature of the Auckland field is that at least 5 widelyseparated and structurally unrelated volcanoes are contemporaneous. These observations highlight thespatial and temporal heterogeneities that can occur in monogenetic fields and have an important bearing onrecurrence rates and temporally linked eruptions, which are probably more common than is generallysupposed, and are key factors in statistically based hazard assessments.

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1. Introduction

Identifying the controls and drivers for monogenetic volcanism is akey to understanding the nature and evolution of monogenetic fields.Such fields are generally regarded as an expression of low magmasupply rate, probably coupled with regional extensional stress (e.g.,Walker, 1993). More recently, holistic models of monogeneticvolcanism, for example in terms of volcanic systems (Canon-Tapiaand Walker, 2004) and volume–time flux relationships (Valentineand Perry, 2007; Valentine and Gregg, 2008), have been developed.The volcanic systems approach seeks to explain interacting processesfrom magma generation to eruption in terms of a single coherentmodel, whereas volume–timemodels viewmonogenetic volcanism interms of steady-state systems with either time-predictable or

volume-predictable properties, depending on whether the controlsare primarily tectonic or magmatic, respectively. However, the factorsthat determine the spatio-temporal patterns of eruptions in suchfields are not well known despite their importance to hazardassessment, most notably where monogenetic fields are locatedclose to major cities and critical facilities (Connor and Conway, 2000;Houghton et al., 2006; Valentine and Perry, 2006; Keating et al., 2008).Recent structural, geochemical and geophysical studies haveaddressed the meaning of the term monogenetic, especially in regardto the complexity of eruption styles and magma chemistries ofindividual monogenetic centres (e.g., Reiners, 2002; Nemeth et al.,2003; Valentine and Keating, 2007), their eruption histories (e.g.,Wijbrans et al., 2007) and the temporal relationships between centres(e.g., Cassidy, 2006; Valentine and Perry, 2007).

The Auckland volcanic field, a small but classic example of aQuaternary monogenetic field, exemplifies complexity in magmagenesis, accumulation and eruption styles (e.g., Houghton et al., 1999;Smith et al., 2008) and in eruption timing (e.g., Shane and Hoverd,

128 J. Cassidy, C.A. Locke / Journal of Volcanology and Geothermal Research 195 (2010) 127–137

2002; Cassidy, 2006; Cassata et al., 2008; Molloy et al., 2009). Here inparticular, the patterns of eruption in space and time, and the controlson them (on all scales), are poorly known. Therefore, statisticalmodels for determining the likelihood of future eruptions in theAuckland field, and their associated risks, are not well constrained.This has important implications, since the volcanic field is coincidentwith Auckland City and its population of over 1 million people.

Temporal patterns of eruption in the Auckland field are largelyunknown because dating the young basalts has been problematic(Allen and Smith, 1994; Cassata et al., 2008). However, increasingevidence from tephra layers in sedimentary cores (Shane and Hoverd,2002) and palaeomagnetic studies (Cassidy, 2006) does reveal thatthe recurrence interval of eruptions is highly irregular. Also the field isonly young and the more recent volcanoes appear to be increasing involume (Allen and Smith, 1994), which suggests that the field is at anearly stage of its evolution and has not reached a ‘steady state.’ Time-predictable models (e.g., Valentine and Perry, 2007) therefore maynot be applicable to the Auckland volcanic field.

Spatial patterns in the distribution of volcanic centres within theAuckland field are either ambiguous or absent; though attempts havebeen made to demonstrate statistical alignments (Von Veh andNemeth, 2009), no dominant structural control is evident. Thiscontrasts with monogenetic fields elsewhere in which vent align-ments are commonly observed to reflect regional structural trends(Walker, 1993; Connor and Conway, 2000; Connor et al., 2000). Whatstructural and magmatic controls determine both the location of theAuckland volcanic field as a whole, and the location and timing ofindividual eruptions, are fundamental questions with regard tounderstanding its evolution but remain unanswered.

Geophysical methods (especially gravity, magnetic and palaeomag-netic) have proven exceptionally useful for investigating the Aucklandvolcanic field and have illuminated several aspects of monogeneticvolcanism (e.g., temporal eruption patterns and subsurface volcanicarchitecture) that are relevant to similar fields elsewhere (Cassidy,2006; Cassidy et al., 2007). Because the Auckland field is young withspatially discrete centres intruded into contrasting low density, non-magnetic host rocks, it is an ideal geophysical target. In contrast, manymonogenetic fields elsewhere occur within more complex volcanicenvironments (e.g., Connor and Conway, 2000) making the applicationof geophysical methods (with the exception of palaeomagnetic) moredifficult. Furthermore, and especially relevant to this paper, there aresome striking geophysical markers in the basement rocks below theAuckland region which provide important information on deep-seatedstructures that may act as structural controls on the field.

This paper provides a new integration of geophysical data from theAuckland volcanic field. We analyse recently acquired regionalmagnetic data to better define deep crustal structure beneath theAuckland field; we present a new residual magnetic anomaly map ofthe field that is used to characterise the subsurface structure of theAuckland volcanic centres and investigate related near-surfacestructural features; finally we investigate the occurrence of spatio-temporal relationships between centres based on existing palaeo-magnetic and age data. These results represent one of the mostcomprehensive geophysical studies of a monogenetic field and shednew light on the nature of monogenetic volcanism, both in theAuckland volcanic field and elsewhere.

2. Auckland volcanic field and regional setting

The Quaternary Auckland volcanic field is located in an intraplatesetting, 400 km west and behind the present-day plate boundarythrough New Zealand (Smith 1989; Sporli and Eastwood, 1997). Thisfield is the youngest and most northerly of 4 adjacent intraplatebasaltic fields (Fig. 1) that increase in age to the south (SouthAuckland: 1.59–0.51 Ma; Ngatutura: 1.83–1.54 Ma; Okete: 2.69–1.80 Ma) and are spaced at about 35–38 km suggesting that the

mantle source has steadily migrated northwards at about 5 cm/year(Briggs et al., 1994).

The Auckland field consists of 49 small monogenetic volcaniccentres, most of whose ages are poorly known. The best-definedmaximum age of the field is 200 ka (Cassata et al., 2008), though it maybe at least 250 ka old (Shane et al., 2002). The youngest volcano(Rangitoto; Fig. 1b) is only several hundred years old and perhapssignificantly, is estimated to have produced about half the total eruptivevolume of the field (of total 4 km3) (Allen and Smith, 1994). The field issmall in comparison with many monogenetic fields elsewhere, whichcan beup to tenfold larger in termsof area andnumber of vents (Connorand Conway, 2000) and have correspondingly larger volumes, forexample the Eifel field (11.5 km3) (Schmincke et al., 1983), theSouthwestern Nevada field (N16 km3) (Valentine and Perry, 2006)and the Springerville field (300 km3) (Condit et al., 1989).

The volcanic rocks are dominantly alkali basalts or basinites withless common tholeiite, transitional basalt and nephelinite (Hemingand Barnet, 1986). There is a wide range of eruption styles in theAuckland field, from dominantly phreatomagmatic (producing maarsand tuff rings) to dominantly magmatic (producing Strombolian/Hawaiian scoria cones and lava flows) (Allen and Smith, 1994), typicalof many monogenetic fields elsewhere (e.g., Schmincke et al., 1983;Hayakawa and Koyama, 1992; Walker, 1993).

The Auckland volcanic field is hosted mainly by sedimentary rockscomprising sequences of alternating sandstones and mudstones ofMiocene–Eocene age, with occasional grit and conglomeritic layers atshallow levels (Edbrooke et al., 1998). These rocks are 0.5–1 km thickin the Auckland region, based on borehole and seismic evidence(Williams et al., 2006; Davy, 2008). Locally overlying the Miocenesequences, particularly in the south of the field and in palaeo-valleyswithin the Miocene rocks, are loosely consolidated Plio-Pleistocenesediments up to several tens of metres thick (Kermode, 1992).

The basement of the region consists of a number of Mesozoicmetasedimentary (greywacke) terranes trending NNW; a majorsuture through the length of New Zealand, which is exposed in theSouth Island (Bradshaw, 1989), passes directly beneath Auckland.This suture is marked by a prominent magnetic anomaly (the‘Junction Magnetic Anomaly’), traceable throughout the length ofNew Zealand, which is caused by a sheared zone of highly magnetic,serpentinised ultramafic ophiolite rocks (Hatherton and Sibson,1970). The suture, and its associated geophysical expression, isespecially complex in the Auckland region (Eccles et al., 2005;Williams et al., 2006). The basement is uplifted immediately to theeast as a consequence of Miocene–Quaternary extensional blockfaulting (striking NNW and ENE) that occurred throughout the region(Kermode, 1992; Hochstein and Ballance, 1993; Edbrooke, 2001),probably exploiting older inherited crustal structures (Sporli, 1987).There is some evidence that this extensional faulting has continued upto Recent times (Wise et al., 2003; Davy, 2008).

A shallow mantle source for the Auckland field (at 80–140 kmdepth) has been inferred from U–Th isotope studies (Huang et al.,1997) and attributed by Sporli and Eastwood (1997) to either a smallmantle dome or the result of extensional stress related to afundamental lithospheric asperity. More recently, a joint inversionof teleseismic receiver functions and surface wave phase velocitiesidentified a low-velocity zone in the upper mantle 70–90 km belowthe Auckland field which is interpreted as a region of 2–3% partialmelt (Horspool et al., 2006), corroborating the earlier estimates forthe depth of the melt source.

3. Geophysical data

3.1. Aeromagnetic data acquisition and processing

The aeromagnetic data described here consist of that previouslyreported by Eccles et al. (2005) and Cassidy et al. (1999), covering the

Fig. 1. (a) Simplified geology map of the Auckland region (after Edbrooke, 2001). (b) Simplified geology map of the Auckland volcanic field (after Kermode, 1992); area of b isapproximately that of Auckland City and is shown as dashed line in a. Inset shows the location of a (boxed) within northern New Zealand, the approximate trace of the JunctionMagnetic Anomaly is shown by long dashes, the extent of the Taupo Volcanic Zone (TVZ) is shown by short dashes, and the Quaternary basaltic fields of the Auckland-Northlandprovince are labelled: 1. Kaikohe-Bay of Islands, 2. Whangarei, 3. Auckland, 4. South Auckland, 5. Ngatatura, 6. Okete.

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northern two-thirds of the study area (Fig. 1a), plus new data fromthis study over the remaining part of the area, which together providea merged dataset covering the whole of the Auckland volcanic fieldand its regional setting. Data were collected over the entire area at430 m elevation (a.s.l.) with a flight-line spacing of 1 km and nominalbearing of 60° E, perpendicular to the regional structural trend. Overthe Auckland isthmus line spacing was reduced to 250 m (see Eccleset al., 2005) and south of the isthmus spacingwas 250–500 m over thevolcanoes. A sampling interval of 2 s provided data at 100 m spacingalong the flight lines; position was determined by differential GPS toan accuracy of ±2–5 m. Acquisition parameters for the new datawerethe same as those for the previous dataset (Eccles et al., 2005).

The merged dataset was processed as described in Eccles et al.(2005). The data were corrected for geomagnetic diurnal variations,recorded at a local base station, and the total magnetic field intensity(TMI) data were calculated by subtraction of the InternationalGeomagnetic Reference Field (IGRF). The resulting TMI for thewhole study area is shown as a shaded relief perspective image(Fig. 2a), together with a more detailed TMI contour plot for the areaof the Auckland volcanic field (Fig. 2b).

The regional and residual components of these data wereseparated, also following the approach described in Eccles et al.(2005), in order to isolate the magnetic anomalies associated with theAuckland volcanic field from the effects caused by the regionalgeology. Eccles et al. (2005) showed that because of some degree ofspatial and spectral overlap between these regional and local effects,analytical filtering of wavelengths (i.e. Fourier-domain spectral

wavelength filtering) resulted in an unacceptable loss of resolutionof the data therefore manual separation of the regional and residualcomponents of the signal was preferred. The extent of effects in themagnetic data resulting from the volcanoes was delineated using acombination of horizontal magnetic gradient, cross-line continuityand the mapped locations of volcanic rocks. The horizontal gradient inparticular is a distinctive feature which separates the effects ofspatially limited near-surface volcanic rocks from those arising fromdeep basement sources. Using INTREPID™ software, the regional datawere gridded using a trend spline (Fitzgerald et al., 1997).

The resulting regional magnetic field is shown in Fig. 3 as twoshaded relief images illuminated perpendicular to the regional trend,one with the regional gravity field superimposed and the other withinterpreted magnetic lineaments. Some probable flight-line parallelartifacts occur because of a loss of tie-line data during surveying,however these have minimal effect on the overall anomaly pattern.This regional surface was subtracted from the TMI data to give theresidual magnetic anomaly data shown in Fig. 4a; in addition areduced-to-the-pole (RTP) residual magnetic anomaly was alsocalculated (Fig. 4b) to aid interpretation (see Section 4).

3.2. Palaeomagnetic data

Palaeomagnetic data from a total of 23 of the 49 Aucklandvolcanoes are reported by Shibuya et al. (1992), Mochizuki et al.(2006), Cassidy (2006) and Cassidy and Hill (2009). These studies aresomewhat limited by a paucity of outcrop in the Auckland field.

Fig. 2. Total magnetic intensity (TMI) anomaly maps. (a) Perspective plot of the TMI anomaly in the Auckland region viewed from the southeast (130° E); survey area approximately asin Fig. 1a and defined by clipped area of Fig. 3a. (b) Contour plot of the TMI anomaly for the Auckland volcanic field; area as in Fig. 1b (marked as box in a). Contour interval is 50 nT,rainbow scale is clipped above 200 nT, white line denotes coastline, black triangles denote location of volcanoes. In the Auckland region the geomagnetic field has a declination of 21° Eand inclination of −60°.

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However, in conjunction with the aeromagnetic data, rock magneticdata (especially the present-day NRM values and the primarypalaeomagnetic directions) are invaluable in providing both con-straints for modelling volcanic structure and important evidence forthe relative timing of eruptions (see interpretation section later). Itshould be noted that because the volcanoes are probably short-lived,different lava flows and scoria cones within each volcano would beexpected to give similar palaeomagnetic results and therefore can beconsidered a single palaeomagnetic site for the purposes of statisticaltreatment.

4. Regional and residual aeromagnetic anomalies

The TMI (Fig. 2) clearly shows the short-wavelength, high-amplitude magnetic effects of the Auckland volcanoes, superimposedon a background of relatively subdued regional linear anomalies, andis striking in highlighting the close spatial association of the Aucklandvolcanic field with the key geophysical structural marker through theregion (the Junction Magnetic Anomaly). The regional component ofthe magnetic data (Fig. 3) more clearly defines the NNW trendingJunctionMagnetic Anomaly through the Auckland region. In the northand south of the mapped area, the Junction Magnetic Anomalygenerally forms a narrow (~6–8 km wide), continuous anomaly. Thisanomaly in the north of the region has been modelled as resultingfrom eastward-dipping serpentinite shear or melange zones, up to2 kmwide and extending to 15–20 km depth in the crust (Eccles et al.,2005). Towards the middle of the region the Junction MagneticAnomaly abruptly widens into a more complex feature (Fig. 3a),subdivided into multiple sub-parallel lineaments over a broad ~10–14 km wide zone (Fig. 3b). This major discontinuity in the JunctionMagnetic Anomaly implies the basement suture is similarly discon-tinuous and notably, this magnetic discontinuity coincides with a

significant positive Bouguer gravity anomaly (Fig. 3a). This latterfeature has been interpreted as resulting from a large block ofanomalously dense basement terrane, ascribed to non-shearedultramafic rocks associated with the Maitai terrane (Williams et al.,2006).

The most obvious characteristic of the residual magnetic map ofthe Auckland volcanic field (Fig. 4a) is the occurrence of largelypositive anomalies, which correlate withmany of the volcanic centres.These anomalies are of the form that might be expected for highlymagnetic basaltic bodies erupted during the Brunhes normal chron.Generally, the strongest anomalies most clearly show the type bipolarform (i.e. with stronger positive component to the north of thevolcanic body) such as those associated with Mt Mangere andMaungataketake volcanoes (#37 and 43, Fig. 4a). At some centres,the weaker negative component of the bipolar anomaly is attenuated,possibly as a consequence of overlapping positive anomalies fromsurrounding lava flows; an example of an isolated anomaly from sucha lava flow can be seen extending west from Mt Wellington volcano(#25). It should be noted however that the negative component ofsome anomalies may be subdued as a consequence of a deeplyextensive source body. The RTPmap of the residual magnetic anomalydata (Fig. 4b) more clearly shows the correlation of anomalies withindividual volcanic centres as it has the advantage of removing anyanomaly skewness caused by the ambient magnetic field. However,given that remanent magnetisation is dominant in the Aucklandvolcanoes (and often reflects secular variation) the RTP calculationonly partially aligns anomalies. A compilation of the physical featuresof all the volcanic centres and their associated magnetic anomalies isgiven in Table 1.

Volcanoes with associated cones typically have distinct magneticanomalies that are strongly positive (e.g., One Tree Hill and MtWellington; #22 and 25, Fig. 4a and b); as expected, Rangitoto volcano

Fig. 3. (a) Regional TMI anomalymap (shaded colour relief) and Bouguer gravity anomaly (contours) in the Auckland region (area as in Fig. 1a) (Williams et al., 2006;Woodward, 1971).Contour interval is 20 g.u., location of volcanoes shownbywhite triangles. Colour image illuminated at 45° inclination from60° E. (b) Interpreted lineations (blue lines) based onmagneticanomaly maxima are shown on equivalent greyscale image of the same area as (a). Image illuminated at 60° inclination from 60° E, location of volcanoes shown by red triangles.

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(#1, Fig. 4a and b) has by far the highest amplitude and mostextensive anomaly (see also Table 1). The correlations between stronganomalies and the causative volcanic centres are particularly evidentin the RTP map (Fig. 4b). Volcanoes with both well-developed tuffrings and small central scoria cones typically have moderate magneticanomalies (e.g., Waitomokia and Robertson Hill; #40 and 36, Fig. 4a)(see also Cassidy et al., 2007). Volcanoes that have formed only tuffrings at the surface mostly have very weakmagnetic expressions (e.g.,Onepoto and Pukaki; #4 and 44, Fig. 4a) (see also Cassidy et al., 2007).Two exceptions to this are Panmure Basin and Pukekiwiriki volcanoes(#27 and 33, Fig. 4a and Table 1), which are both associated withsubstantial positive anomalies. Five volcanoes (Taylor Hill, MtRichmond, Puketutu, Crater Hill and Manurewa; #10, 35, 39, 45 and48, Fig. 4a) have atypical negative residual magnetic anomalies as aconsequence of having formed during a geomagnetic excursion (seeCassidy, 2006).

Other signatures in the residual magnetic map (Fig. 4a) include aseries of elongate anomalies mostly trending W or NW whichcorrelate with lava flows, most clearly seen at Mt Wellington andMt Roskill volcanoes (#25 and 20, Fig. 4a), but also associated withseveral volcanoes in the central isthmus where lava flows extendacross the isthmus into theWaitemata Harbour. No further significantelongate or linear anomalies are evident, other than those related toknown lava flows. All the significant (N50 nT) features in the residualmagnetic anomaly map are clearly associated with volcanoes and/orlava flows, except for one anomaly (~50 nT) on the east coast of theManukau Harbour (1.5 km west of Robertson Hill volcano; #36,Fig. 4a) which could be caused by unmapped volcanic deposits buriedby Pleistocene sediments but requires further investigation.

These aeromagnetic data can also be used to estimate the averagebulk magnetic declination of volcanic centres. This is especially useful

where there is limited or no suitable outcrop for palaeomagneticstudies and has the added advantage of not being subject to the samesampling limitations. Bulk magnetisation (which is equivalent to themean NRM) is a valuable parameter for Auckland volcanoes because itis a close approximation to the primary remanent direction, onaccount of the high Königsberger ratios and low magnetic overprinttypical of these rocks (Shibuya et al., 1992; Cassidy, 2006). Althoughbulk magnetic declinations for individual volcanoes are best deter-mined from modelling, they can be estimated for several centresdirectly from the aeromagnetic data (Fig. 4a) by the apparent azimuthof their bipolar anomalies. Such simple anomaly forms reflect anessentially symmetric subsurface geometry, as shown by previousmodelling (Rout et al., 1993; Cassidy et al., 2007). Unsurprisingly,where such azimuths can be estimated from the aeromagnetic data,they often differ from declinations reported by Shibuya et al. (1992)that are typically based on measurements from a single site (Table 1).

5. Interpretation

The regional geophysical data delineate major structural elementsin the upper crust below Auckland and hence point to possible deep-seated structural controls on the location and evolution of theAuckland volcanic field. The higher resolution residual magneticdata provide key information on the subsurface structure of thevolcanic centres and hence their eruptive histories, and also allownear-surface structural features associated with volcanism to beinvestigated. By contrast, palaeomagnetic and other magneticdeclination data bear on temporal aspects of the Auckland volcanicfield and together with structural considerations, provide newinformation on spatio-temporal relationships.

Fig. 4. Residual magnetic anomaly map (a) and reduced-to-the-pole (RTP) residual magnetic anomaly map (b) of the Auckland volcanic field. Area of a and b as for Fig. 1b. Contourinterval is 50 nT (note that the 0 nT contour has been omitted for clarity), rainbow scale is clipped above 200 nT. White outline shows coastline, black triangles show location ofvolcanoes as follows: 1: Rangitoto, 2: Pupuke, 3: Tank Farm, 4: Onepoto, 5: Mt Victoria, 6: Mt Cambria, 7: North Head, 8: Motukorea, 9: St Heliers, 10: Taylor Hill, 11: Orakei Basin, 12:Little Rangitoto, 13: Albert Park, 14: Domain, 15: Mt Hobson, 16: Mt St John, 17: Mt Eden, 18: Te Pouhawaiki, 19: Mt Albert, 20: Mt Roskill, 21: Three Kings, 22: One Tree Hill, 23:Hopua, 24: Mt Smart, 25: MtWellington, 26: Purchase Hill, 27: Panmure Basin, 28: Pidgeon Mt, 29: Styaks Swamp, 30: Green Hill, 31: Otara Hill, 32: Hampton Park, 33: Pukekiwiriki,34: McLennan Hills, 35: Mt Richmond, 36: Robertson Hill, 37: Mt Mangere, 38: Mangere Lagoon, 39: Puketutu, 40: Waitomokia, 41: Pukeiti, 42: Otuataua, 43: Maungataketake, 44:Pukaki, 45: Crater Hill, 46: Kohuora, 47: Matakarua, 48: Manurewa-Wiri, and 49: Ash Hill.

132 J. Cassidy, C.A. Locke / Journal of Volcanology and Geothermal Research 195 (2010) 127–137

5.1. Deep-seated structural controls on the Auckland volcanic field

The striking coincidence of the Auckland volcanic field with amajor crustal suture, marked by the Junction Magnetic Anomaly(Figs. 2 and 3a), raises the question of whether structures associatedwith this suture influence both the location of the field as a whole andmagma pathways to the surface. On the regional scale, Sporli andEastwood (1997) have suggested that the northward migration ofbasaltic volcanism along a propagating lithospheric fracture may havebeen arrested by its intersection with the suture. This intersectionpoint coincides with the anomalous large dense body of ultramaficrocks in the upper crust (Williams et al., 2006), whichmay represent astrong mechanical discontinuity causing a refraction of stress releaseinto the mantle source, allowing magma to be mobilised (Sporli andEastwood, 1997). An interesting question is whether future migrationof volcanism will continue in its northerly direction or if it will bedeflected northwest by this fundamental crustal suture and its effecton the stress distribution in the crust.

It is apparent from these new data that the Auckland volcanic fieldis centred at a significant discontinuity in the Junction MagneticAnomaly and hence crustal structure, marked by an abrupt constric-tion in the anomaly from north to south (Fig. 3a). This constrictionalso marks the southern end of the large gravity anomaly, around

which there is perhaps a hint that the vents are distributed. Alsonotable is that the spread of vents of the Auckland volcanic field isalmost exactly encompassed by the extent of the lineamentsassociated with the anomalously widened Junction Magnetic Anom-aly in the region (Fig. 3b). These observations suggest some strongstructural control on the rise of magma through deeper levels of thecrust. The shear zones marked by these magnetic lineaments mayform a wide region of crustal weakness and create preferentialpermeability that could be exploited by magma rising through thecrust. The influence of factors such as the strength of overlying crust,and in particular the effects of pre-existing structure, on the rise ofmelt from deep sources has been described for example by Canon-Tapia and Walker (2004) and modelled for the shallow crust byGaffney et al. (2007). However, there is no simple correlation betweenthe locations of individual volcanic vents and the interpretedlineaments (Fig. 3b) that might delineate more shallow structuralcontrols. Although the dominant Cretaceous NNW structural fabricwas reactivated in the Late-Quaternary, other younger structuraltrends might have similarly influenced the pathways of magma in theupper levels of the crust. In addition, the apparent overall N–Selongation of the field has been interpreted by Sporli and Eastwood(1997) to reflect the geometry of the source region in the uppermantle.

Table 1Summary of physical dimensions, aeromagnetic anomalies and magnetic parameters of the Auckland volcanoes.

Volcano groupand name

Tuff ring diametera

(m)Cone diameter/heighta

(m)Deposit typeb Residual magnetic anomaly

(maximum amplitude, nT)Apparent bipolar azimuth(°)

Palaeomagnetic declinationc

(°)Tuff(t)Scoria(s)Lava(l)

Tuff rings onlyTank Farm (3) 800 – t +30 – –

Onepoto (4) 700 – t c.0 – –

St Heliers (9) 600 – t +10 – –

Orakei Basin (11) 1100 – t +20 – –

Hopua (23) 500 – t c.0 – –

Panmure Basin (27) 1200 – t +220 ~20E –

Styaks Swamp (29) 350 – t c.+50 – –

Pukekiwiriki (33) 600 – t +160 ~10E –

Pukaki (44) 800 – t +10 – –

Kohuora (46) 650 – t +30 – –

Ash Hill (49) 250 – t +40 – –

Tuff rings with cones (or lava flows)Pupuke (2) 1400 – t, l +40 2 WMotukorea (8) 700 400/60 t, s, l +250 ~20 W 0Taylor Hill (10) 700 350/25 t, s, l –40 – 18 W (EX)Albert Park (13) c.500 – t, s, l +240 – –

Domain (14) 700 80/20 t, s, l +280 – –

Te Pouhawaiki (18) c.800 c.100/20 t, s c.+50 – –

Pidgeon Mt (28) 700 300/40 t, s, l +100 10E –

Green Hill (30) c.700 300/45 t, s, l +360 30E 17 WOtara Hill (31) c.700 400/60 t, s, l – – 111 W (EX)Hampton Park (32) 400 100/20 t, s, l +180 – 96 W (EX)Mt Richmond (35) 700 400/35 t, s –70 – 15 W (EX)Robertson Hill (36) 700 c.300/15 t, s +90 – –

Mangere Lagoon (38) 700 100/10 t, s c. +50 – –

Waitomokia (40) 700 c.300/40 t, s +230 0 –

Crater Hill (45) 750 c.80/15 t, s, l −120 8 W (EX)

Cones only (mostly with lava flows)Rangitoto (1) – 400/70 s, l +1200 10E 1EMt Victoria (5) – 350/50 s, l +100 30 W 4 WMt Cambria (6) – 150/15 s c.+50 – –

North Head (7) – 400/55 t, s, l +70 – –

Little Rangitoto (12) – 150/25 s, l +30 – –

Mt Hobson (15) – 450/55 t, s, l +350 – –

Mt St John (16) – 400/45 s c.+150 – –

Mt Eden (17) – 550/95 s, l +250 – 31 WMt Albert (19) – 600/75 t, s, l +380 – –

Mt Roskill (20) – 450/50 t, s, l +210 – 7 WThree Kings (21) – 650/60 t, s, l +480 – 3EOne Tree Hill (22) – 650/80 s, l +620 – 22EMt Smart (24) – 450/60 s, l +110 ~30 W 29EMt Wellington (25) – 550/95 t, s, l +300 30 W 10EPurchase Hill (26) – 200/15 s c.+200 – –

McLennan Hills (34) – 400/15 t, s, l +70 – 166E (EX)Mt Mangere (37) – 650/70 s, l +470 25 W 2 WPuketutu (39) – 650/55 t, s, l –120 – 3E (EX)Pukeiti (41) – 100/15 s, l c.+20 – –

Otuataua (42) – 200/40 s, l +40 – 2EMaungataketake (43) – 500/65 t, s, l +500 0 2EMatakarua (47) – 350/50 s, l +90 20E 8EManurewa/Wiri (48) – 300/50 t, s, l –40 – 5 W (EX)

a Determined from Searle (1981), note some original cone heights have been reduced by quarrying.b From Allen and Smith (1994).c Available data from Shibuya et al. (1992) and Cassidy (2006), note these directions are similar to NRM directions and so relate to apparent bipolar azimuth, except in the case of

excursion declinations (denoted EX).

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5.2. Shallow-seated volcanic structure and magmatism

The higher resolution residual magnetic and RTP maps of theAuckland volcanic field (Fig. 4a and b) reveal aspects of the subsurfacestructure and early evolution of the volcanic centres that are notalways apparent from surface mapping. Whilst volcanoes with largescoria cones have magnetic anomalies that generally correlate inmagnitude to cone diameter (Table 1), gravity and magneticmodelling studies (e.g., Cassidy et al., 2007) invariably show thatthe high-standing edifices themselves (i.e. the scoria cones) account

for only a small part of the anomaly. This is further illustrated atcentres where the cones have been removed by quarrying (e.g., #40and 43, Fig 4a) but substantial magnetic anomalies remain. Theseanomalies are mostly attributable to large subsurface volumes ofbasalt within bowl-shaped bodies beneath the cones. This suggeststhat either the development of scoria cones was often preceded bycrater-forming events with ponding of lavas, and/or that extensiveshallow relict feeder systems occur below the cones.

Conduit geometries mapped at deeply eroded monogeneticcentres elsewhere (e.g., Valentine and Gregg, 2008) show that feeder

134 J. Cassidy, C.A. Locke / Journal of Volcanology and Geothermal Research 195 (2010) 127–137

systems typically flare out at shallow levels to c. 100 m in lateralextent. This is considerably less than the widths of bodies modelledbelow the present centres (Cassidy et al., 2007), implying thatponding of greater extent occurred below the Auckland cones. Suchearly ponding of lavas during the building of cone volcanoes has beendescribed from the Eifel field, Germany (Houghton and Schmincke,1998), though it is more commonly observed in the late-stagedevelopment of maars, for example in northern California (Lavine andAalto, 2002) and western Hungary (Nemeth and Martin, 2007).However, the occurrence of narrow solidified central conduit systems(as described from the southern Nevada field by Valentine et al., 2007)would also contribute to the observed geophysical anomalies. Suchconduits would have originally fed the ponded lavas, in the mannerdescribed by Nemeth and Martin (2007) for phreatomagmaticvolcanoes in the Pannonian Basin, Hungary.

Similarly, the form of the large anomaly associated with Rangitotovolcano probably reflects a substantial subsurface body of solidifiedmagma, rather than being due to its extensive carapace of a–a lavaflows. Interestingly, the magnetic anomaly shows a double positivepeak aligned parallel to and coincident with one of the magneticlineations. These anomaly peaks also correlate with mapped northand south sub-cones (Needham, 2009) and therefore probably resultfrom two major feeder conduits in the vicinity of the summit.

In contrast, the association of simple tuff-ring volcanoes (i.e. thosewithout outcropping effusive deposits) with typically very weak (ifany) magnetic anomalies (Table 1) implies that magma supply waslimited and that all or most of the available magma interactedexplosively with groundwater and the ensuing phreatomagmaticeruption ejected only fragmented pyroclasts. However as notedearlier, two exceptions to this (#27 and 33, Fig. 4) have stronganomalies indicating a substantial volume of near-surface basalt. Thisdeduction has been recently corroborated for the Panmure Basinvolcano (#27) by drilling which encountered basaltic scoria atshallow levels (Cronin et al., 2009), consistent with the occurrenceof a deeper basaltic body at depth. In these latter cases magma supplyprobably exceeded groundwater supply leading to a cessation ofphreatomagmatic activity (as described for other monogeneticeruptions by Houghton et al., 1999), without leading to thedevelopment of high-standing scoria cones, but probably resultingin ponding of lavas in the deeper parts of the crater which are nowburied. In the case of those tuff-ring volcanoes where magma supplywas sufficient to create high-standing, though small, central scoriacones (and sometimes produce lavas) the magnetic anomalies arevery variable and sometimes large (Table 1), probably also reflectingponding of lavas in a previously formed crater, together with a relictfeeder system. Notably, most of these craters in particular have verysimilar diameters, perhaps suggesting some commonality in eruptioncontrols in terms of magma supply, groundwater conditions and/ormagma–groundwater interactions.

The residual magnetic map of the Auckland volcanic field provideslittle direct evidence for near-surface structural controls on thelocation of the volcanic centres. For example, there appear to be noelongate anomalies associatedwith centres (apart from those that canbe attributed to known lava flows), or any hint of subsurface basaltbodies connecting centres that are adjacent (especially those alignedalong supposed regional structural trends), that might indicatemagma intrusion and eruption along dyke-like conduits. Mapping ofsuch shallow dykes elsewhere (e.g., Valentine and Krogh, 2006)shows typical widths of up to several metres for lengths up to akilometre or more. Of course, narrower subsurface dykes (less than afew meters in width), or deeper-seated dykes (greater than a coupleof hundred meters depth) would be barely resolvable with thepresent aeromagnetic data; however it should be noted that ground-based surveys around many of the centres (unpublished data by theauthors) have never revealed any such features. Dykes and sills inmonogenetic fields elsewhere are commonly associated with a well-

developed pre-existing structure (e.g., Valentine and Krogh, 2006) butsuch a structure is not evident in the Auckland field.

Throughout the Auckland volcanic field there is no evidence in themagnetic data for substantial near-surface volumes of basaltic rockswhere there are no known eruption centres or buried flows (apartfrom the one possible case noted earlier). Stalling of magma atshallow depths to form substantial sills (of up to a kilometre extentand 100–200 m thickness) has been described from other monoge-netic fields, especially where there is a thick overburden of soft rocksas in the Pannonian Basin (Nemeth and Martin, 2007), or a strongrheological boundary as in the southern Nevada field (Valentine andKrogh, 2006). Neither of these situations are pertinent to Auckland,and any such bodies would have been detected by the presentgeophysical data. Therefore any magma reaching shallow levels in theAuckland field appears to have been erupted; if there are shallow-seated intrusive bodies that were not accompanied by eruption, theymust be of very small volume. Finally, it is perhaps possible that dyke-like intrusions occur coincident with the strong Junction MagneticAnomaly lineaments, such dykes would be more difficult to resolveduring the regional-residual separation process described earlier;however, short-wavelength components within these broader scalelineaments are not evident on close inspection of the data.

5.3. Spatio-temporal relationships

A unique aspect of themagnetic record from the Auckland volcanicfield is that five volcanoes have recorded almost identical butanomalous primary magnetisation directions (Cassidy, 2006). Atleast three of these have recently been dated unequivocally as beingassociated with the Mono Lake excursion at 31.6±1.8 ka (Cassata etal., 2008) and therefore it is highly likely (as originally argued byCassidy, 2006) that all five share a common age. Further palaeomag-netic analysis by Cassidy and Hill (2009) shows that one of the groupof five has a very slightly different palaeodirection and therefore mayhave erupted at a slightly different time. It is unknown if any othercentres in the Auckland volcanic field erupted during this sameperiod, however a further two centres (#31 and 32, Fig. 4) withanomalous (though different) magnetisation directions have less wellconstrained ages that overlap the Mono Lake date (Shibuya et al.,1992; Cassata et al., 2008).

Based on the short duration of the Mono Lake excursion and thecorrespondingly high rates of change of field direction, eruptions fromthese five volcanoes have been estimated to have occurred within atime period of the order of 100 years or less (Cassidy, 2006) andtherefore may, in fact, have been contemporaneous. The widedistribution of these five centres across the field makes a commonstructural control on magma pathways through the crust unlikely. Interms of magma generation and emplacement, given the assumedrapid ascent rate for the magmas from source to surface (Johnstonet al., 1997), this distribution implies concurrent melting across themantle reservoir. Also, the disparate trends in major and traceelement geochemistry of most of these five centres (L. McGee, pers.comm.) suggests a lack of melt interconnectivity in the sourcereservoir (cf. Canon-Tapia and Walker, 2004). In this case, a transientincrease in regional extension rate, perhaps associated with nearbyrifting to the east (c.f. Hochstein and Ballance, 1993), could haveinitiated relatively rapid decompression melting across the sourceregion, coupled with fracturing in the overlying lithosphere.

The above five volcanoes were identified using a combination ofpalaeomagnetic and aeromagnetic data which revealed five distinctnegative anomalies (Cassidy, 2006). However, several maar-typecentres do not have sufficient volumes of solidified magma (eithersurface or subsurface) to be adequately sampled by these methods,though it is clear from the dominance of positive anomalies in Fig. 4a(and most clearly in Fig 4b) that the majority of centres were eruptedduring times of normal geomagnetic polarity.

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The declination of magnetisation of the volcanic rocks providesfurther information on the relative timing of eruptions. Wherevolcanoes are associated with significantly different declinationsthey must have been erupted at different times, whereas if they aresimilar then it is possible they erupted at similar times, especially ifother information suggests that they may be related spatially ortemporally. For example, the two volcanoes in the SE of the fieldpreviously mentioned (#31 and 32, Fig. 4a), share a common andanomalous palaeomagnetic declination (and inclination in this case),strongly implying contemporaneity; notably they are also closelyadjacent and aligned on a NNE regional trend. Similarly, two othervolcanoes (#42 and 43, Fig. 4a) that are also closely adjacent and lieon this trend share near-identical measured magnetic declinations(Shibuya et al., 1992), which although not definitive, may also reflectcontemporaneity. Conversely, three volcanoes (#47, 48 and 49,Fig. 4a) in the south of the field that are aligned on the ENEQuaternary trend, have quite different bulk magnetic directions (sincevolcano #48 records an excursion as noted earlier) and are thereforenot temporally related though, of course, they may have exploited acommon structural pathway.

Of the dozen or so volcanic centres that have discernible bipolarresidual aeromagnetic anomalies (Fig. 4a and Table 1), about half haveapparent declinations significantly different from zero (with someover 20°). The fact that these bulk declinations are other than N–Sindicates that their total effusive and intrusive lifetimes (includingcooling times) must have occurred over a period considerably shorterthan the cycle of geomagnetic secular variation at the time, i.e.considerably less than, say, 1000 years. It is generally understood thatsuch monogenetic volcanoes have very short eruption lifetimes (i.e.weeks–years) (Kienle et al. 1980; Houghton et al., 1999; Valentineet al., 2006) and the aeromagnetic data corroborate this. A possibleexception in the Auckland field is Rangitoto volcano, which from amore detailed palaeomagnetic secular variation study of lavas(Robertson, 1986), exhibits a range of declinations; taking intoaccount the statistical error, these data would imply a period ofactivity spanning as much as 100–200 years.

6. Discussion

Understanding the eruption histories of volcanoes within mono-genetic fields is based largely on evidence from surface deposits,especially in young fields where the different styles of eruption can bedirectly observed, but also from within eroded fields where thedeeper parts of the volcanic systems have been exposed. To be able toobserve the complete volcanic suite in any single field (as in the HopiButtes field for example;White, 1991) is less common, particularly forindividual centres, and therefore geophysical investigations of thesubsurface, as part of an integrated study of monogenetic fields, areinvaluable.

For example, at some Aucklandmaars which aremapped as simplesediment-filled craters and tuff rings, the geophysical data revealsubstantial effusive (and probably intrusive) bodies at depth (asnoted earlier) indicating a more complex eruption history than isapparent at the surface. Conversely, the characteristic magnetic andgravity anomalies associated with many Auckland scoria cone centresindicate the occurrence of early crater-forming phreatomagmatism,the geological evidence for which is often completely buried by laterdeposits. Similar complexity has been observed (or inferred) forexample at some individual monogenetic centres in the PannonianBasin, which although predominantly phreatomagmatic in eruptionstyle (Németh et al., 2001), in some instances also show evidence forintrusive and/or effusive activity (Martin and Németh, 2007).However, it is generally difficult to directly observe all the stages inthe evolution of such a volcanic centre (unless borehole data areavailable).

It appears therefore that a whole range of eruption styles iscommon to the evolution of individual monogenetic centres, as isbecoming increasingly recognised from detailed studies of Quaternarymonogenetic volcanoes elsewhere, such as in southern Nevada(Valentine et al., 2007), the Mexican volcanic belt (Martin andNémeth, 2006; Carrasco-Núñez et al., 2007) and southern Europe (DiTraglia et al., 2009; Sottili et al., 2009). Provided both a sufficientmagma supply and groundwater source is available, then suchcomplexity of eruption styles atmonogenetic centres can be expected.

Further, because the subsurface volcanic architecture may not bereflected in the surface geology, the volume of subsurface basalt canbe underestimated. Along with eruption style, the total eruptivevolume at centres is an important component of quantitative hazardassessment in monogenetic fields, both for individual volcanoes (e.g.,Ho et al., 1991) and for studying more generic trends such as volume–time relationships (e.g., Valentine and Perry, 2007). A betterdelineation and quantification of the subsurface volumes of basaltflows based on geophysical data and equally importantly, theidentification of previously unknown buried vents, would allow amore accurate statistical analysis of volcanic hazard and provide aquantitative basis for comparisons between fields.

It is well documented that crustal structure can have a stronginfluence on the location of monogenetic volcanism, either incontrolling the overall location of fields, as noted for example atEifel (Schmincke et al., 1983) and in southwestern Nevada (Connoret al., 2000), or in controlling vent alignments, for example in theSpringerville field (Connor et al., 1992). Such vent alignmentshowever, may only be apparent after statistical analysis (e.g., Lutzand Gutmann, 1995; Paulsen and Wilson, 2009). Auckland appears tobe an example where strong structural control on the location of thefield as a whole is indicated, but vent alignments (either along thebasement lineaments mapped in this study or along younger regionalstructural trends) are neither clearly systematic or convincinglydemonstrated by statistical analysis.

There are a number of possible reasons why a consistent pattern ofvent alignments may not be apparent, in both this and othermonogenetic fields. For the case of Auckland, there are few knownfaults in the immediate vicinity of the field, especially within the hostMiocene sedimentary rocks; this contrasts for example with thesetting of the Yucca Mountain field (Nevada) which is highly faultedand where vents clearly align along faults (Connor et al., 2000). Thecombination of weak regional stress fields and older inheritedstructures (with different orientations) could be responsible for theobserved lack of consistent vent alignments. For example, in additionto apparent ENE and NNW vent alignments (e.g., Magill et al., 2005)that may relate to structures reactivated in the Quaternary, NNE ventalignments are also indicated (e.g., Houghton et al., 1999) whichcorrespond to current extensional stress orientations in the CentralNorth Island (Villamor and Berryman, 2001). It is also possible thatthese different structural controls dominate at different depths in thecrust, in the manner discussed by Valentine and Perry (2007) for theCentral Basin and Range Province.

It is noteworthy that in the South Auckland field (30 km to thesouth), vents do coincide with mapped NNW and ENE trendingQuaternary basement faults (Briggs et al., 1994). It could be thereforethat even if similar basement faults are present below the Aucklandvolcanic field, the thick (up to 1 km), relatively weak Miocene coverrocks are refracting magma pathways in the topmost crust so thatspatial patterns are disrupted; such mechanical layering has beenshown by modelling to significantly affect magma pathways to thesurface elsewhere (e.g., Bonafede and Rivalta, 1999; Gudmundssonand Loetveit, 2005).

The spatio-temporal relationships between volcanic centresdescribed in this study may be more common in monogenetic fieldsthan is generally supposed.Whilst adjacent centres lying on structuralalignments are sometimes known to have erupted at similar times

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(e.g., Connor and Conway, 2000), no monogenetic field other thanAuckland is known to have had several contemporaneous eruptions atdistant, structurally unrelated locations (Cassidy, 2006). Becausepalaeomagnetic data can provide a relative timing index of greaterresolution than conventional dating methods (especially for strati-graphically separate centres), wider application of these methods iswarranted. A particularly valuable technique that could be applied toother late-Quaternary fields is the use of a combination of precise40Ar/39Ar dating and magnetic palaeodeclination studies to investi-gate temporally related centres. It is also quite possible that centres inrelatively productive monogenetic fields have recorded excursions,which would make such studies even more valuable.

The recognition of contemporaneous eruptions obviously has animportant bearing on hazard assessment since a new eruption eventcould have a bigger and far more widespread impact than previouslyexpected. Probabilistic hazardmodels for monogenetic fields reportedin the literature (e.g., Ho et al., 1991; Connor and Hill, 1995) have nottaken account of the possiblity of multiple eruptions, especially fromwidespread centres (apart from in the Auckland field). Therefore thedefinition of an ‘event’ in such models, and hence any predictivecalculations, arguably needs to be revised. As noted earlier however,predictive models for the Auckland volcanic field itself (in the mannerof Valentine and Perry, 2006) would be problematic.

A similar consideration relates to the observation that 8 out of 49centres in the Auckland volcanic field have recorded geomagneticexcursions. At first sight this might imply a correlation betweeneruptions and excursions, since statistically, given the age of theAuckland volcanic field (Cassata et al., 2008) and the frequency andduration of excursions (e.g., Gubbins, 1999; Laj et al., 2000), onlyabout 2 centres would be expected to have recorded excursions.However, the available palaeomagnetic and age data suggests thatthese 8 centres do in fact represent only 2 (or at most 3) multipleevents.

This unique geophysical case study provides an integratedperspective of the Auckland volcanic field. The results highlight thecomplexity of structural and temporal relationships that can occur atdifferent scales in such monogenetic fields and have significantimplications for studies of monogenetic volcanism elsewhere.

Acknowledgements

We thank the University of Auckland Research Committee, theMarsden Fund (Royal Society of New Zealand) and the EarthquakeCommission Research Foundation for financial support, Colin Yong fortechnical support and Louise Cotterall for assistance with manuscriptpreparation. Richard Blakely and Karoly Nemeth are thanked forproviding thorough and insightful reviews of the manuscript.

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