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An assessment of the alignments of vents based on geostatistical analysis in the Auckland Volcanic Field, New Zealand

Étude des alignements des cratères du Champ Volcanique d’Auckland en Nouvelle Zélande
Mark W. Von Veh et Károly Németh
p. 175-186

Résumés

La ville d’Auckland se situe sur un champ volcanique monogénétique actif. La planification des secours visant à pallier le risque lié à une future activité volcanique est rendue difficile en raison de l’incertitude relative à propos de la localisation probable de la prochaine éruption. Les alignements qui présentent plusieurs cônes volcaniques voisins indiquent que l’éruption dépend de la présence de zones de faiblesse dans la croûte sous-jacente. Une fissure orientée NE-SW et traversant le champ volcanique peut être identifiée à l’aide de l’analyse statistique du « voisin » le plus proche et en joignant les différentes directions. La transformée de Hough a été utilisée pour localiser les alignements majeurs qui ont une orientation NE-SW et NW-SE, i.e. sub-parallèle aux failles régionales ; ils indiquent l’existence de directions d’extension NW-SE et NE-SW pendant le Quaternaire. La probabilité d’une reprise de l’activité effusive est plus forte le long de la direction nord-ouest ainsi mise en évidence pour la seule faille active de cette région, celle de Wairoa Nord. Le long de cet élément structural sont également localisés les cônes adventifs les plus jeunes (environ 700 ans BP) du champ lavique de Rangitoto et Motukorea.

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Notes de la rédaction

Article soumis le 21 mars 2008, accepté le 11 mai 2009

Texte intégral

Funding for this research by Unitec New Zealand (MvV), FRST Post-Doctoral fellowship MAUX0405 (KN) and Massey University Research Grant (KN) are gratefully acknowledged. The authors wish to thank the journal referees (G. Groppelli and an anonymous reviewer) and journal editor (J.-C. Thouret) for their constructive comments, which have significantly lifted the quality of this report. French translations were made by the help of J. Lecointre.

Introduction

1Monogenetic volcanic fields consist of a large number of individual, commonly mafic volcanoes, which erupted only once with small volumes of magma (usually <0.01 km3; Walker, 1993). They take the form of scoria cones, tuff rings, maars or tuff cones. Scoria cones are the most common landformsand they show a great diversity in size, morphology and eruptive products (e.g., Valentine and Gregg, 2008). Volcanic fields traditionally viewed as monogenetic can be very complex. The volcanic edifices are commonly small in volume (in comparison with stratovolcanoes), but show clear evidence for recurrence and multiple activity (Walker, 1993). The nature of a volcanic field has implications for forecasting the timing of future volcanic eruptions, the locations of future vents, and perhaps the style, longevity, and intensity of eruptions. A common problem in studies of young volcanic fields is the lack of datable material. The use of morphometric parameters of scoria cones to establish their relative ages has proven to be a powerful tool (Wood, 1980a, 1980b; Hooper and Sheridan, 1998; Degai, 2004). Several lines of evidence have however demonstrated that due to the complexity of scoria cones their structure needs to be established before age estimates based on morphometry can be given (Németh et al., 2007).

2Scoria cones form due to magmatic moderate explosions in a vent and their typical products are scoriaceous ash and lapilli interbedded with bomb horizons (e.g., Valentine et al., 2005, 2007; Martin and Németh, 2006). They can also produce significant volumes of fine ash that can cover areas tens of km away from their source (e.g., Gregg and Williams, 1996). Scoria cones represent a “dry” end member of small-volume volcanic eruptions typical in a volcanic field, while tuff rings and maars are considered to be the “wet” equivalents (Lorenz, 1986). Tuff rings are generally low rimmed and wide cratered monogenetic volcanoes that form due to magma and surface and/or near surface aquifer interaction (Lorenz, 1986). Their eruptions are usually considered to be more violent and generate devastating pyroclastic density currents that may have a runout distance of few kilometres from their source (Lorenz, 2007). The distributional pattern of volcanic edifices in a volcanic field should be taken into consideration in establishing and refining its eruptive evolution. Such studies can help to establish the potential location and style of future eruptions in a field. Vent distribution studies have often identified vent clustering and/or vent lineaments (Yogodzinski et al., 1996; Toprak, 1998; Connor et al., 2000; Mazzarini, 2004). By combining studies of vent distribution pattern and volcano types it has also been possible to identify paleo-river valleys in Pinacate, Sonora (Gutmann, 2002). Non-homogeneous Poisson models have been applied to assess the probability of future eruptions, for example at Yucca Mountain (Connor et al., 1992; Connor and Hill, 1995). Accurate age determinations of individual eruptive centres are essential for probabilistic forecasting of eruptions (Cronin et al., 2001;Magill et al., 2005). Reliable radiometric dating for the Auckland Volcanic Field (AVF), the subject of this paper, is lacking, hindered by excess argon related to its young age (McDougall et al., 1969). In addition, lack of widespread marker beds such as tephra layers makes it difficult to determine relative ages (Magill et al., 2005).

3In this study we attempt to identify the fundamental zones of crustal weakness in the volcanic field, which may be the locus of future eruptions, based purely on a mathematical treatment of known vent locations. The result of this “blind-test” of lineament locations is then evaluated and compared with existing studies of fault distribution patterns in which alignments are recognized. The evidence for the vent alignment directions is then used together with available geological field information, such as the locations of faults and the morphological characteristics of volcanoes, to forecast a likely location for the next eruptive centre.

The Auckland Volcanic Field (AVF)

4The city of Auckland on the North Island of New Zealand is built on an active monogenetic volcanic field containing 49 discrete volcanic vents (fig. 1). A wide range of eruption styles from phreatomagmatic, Strombolian to Hawaiian eruptions and effusive volcanism are represented in the AVF (Smith 1989 ; Allen et al., 1996 ; Houghton et al., 1996 ; Rogan et al., 1996 ; Homer et al., 2000 ; Affleck et al., 2001 ; Cassidy and Locke, 2004 ; Houghton et al., 2006 ; Cassidy et al., 2007). Phreatomagmatic eruptions produced tuff rings with wide craters surrounded by relatively thin crater rim deposits with gently dipping beds (Allen et al., 1996 ; Marra et al., 2006). Maar volcanoes are also present (fig. 2A ; Hayward et al., 2002 ; Cassidy et al., 2007), although they are infrequent and cannot be readily distinguished from broad tuff rings (Németh et al., 2009 ; Cronin et al., 2009) due to the flat and shallow volcanic edifices, similar to those in Eastern Oregon (Heiken, 1971) and the western Pannonian Basin (Martin and Németh, 2005). Tuff rings are commonly overlain by scoriaceous pyroclastic deposits (fig. 2B) resulting from Strombolian-style eruptions as the water supply was exhausted in the course of the eruptions (Searle 1959, 1962 ; Allen et al., 1996). The phreatomagmatism in the AVF produced pyroclastic density currents, which deposited typical fine-grained cross-bedded ash layers with antidunes(fig. 2C) interbedded with coarse-grained tephra layers from phreatomagmatic falls (Allen et al., 1996). The volume of accidental lithic fragments (fig. 2D) and/or free crystals derived from country rocksin the accumulated pyroclastic deposits are typical for tuff rings and broad maars formed on a unconsolidated (“soft”) substrate of sand and mud (e.g., Auer et al., 2007). Such bedrock is believed to enhance lateral quarrying of the vent site, that lead to create wide craters, as well as broad tuff rings and broad and shallow maars that are difficult to distinguish (Auer et al., 2007). The shallow-level pre-volcanic rock units at Auckland consist of mud, sand and silt beds of the Miocene Waitemata Group. The state of water saturation and the presence of surface water together are inferred to be the main controlling parameters of the resulting types of volcanoes in AVF (e.g., effusive, explosive, and phreatomagmatic). The phreatomagmatic pyroclastic rock units have a matrix rich in sand and silt sourced from the substrate Waitemata Group sediments (Cronin et al., 2009; Németh et al., 2009). Larger juvenile bombs and lapilli are commonly cauliflower shaped, host thermally altered sand and silt, and are at times coated by mud. This demonstrates an occasional intimate interaction of intruding magma with a muddy impure coolant (e.g., Martin and Németh, 2007) formed from milling of the Waitemata Group sediments.

Fig. 1 – Map of the Auckland Volcanic Field showing the known volcanic centers (after Kermode, 1992).
Fig. 1 – Carte du champ volcanique d’Auckland montrant l’emplacement des centres éruptifs identifiés jusqu’à présent (d’après Kermode, 1992)

Fig. 1 – Map of the Auckland Volcanic Field showing the known volcanic centers (after Kermode, 1992). Fig. 1 – Carte du champ volcanique d’Auckland montrant l’emplacement des centres éruptifs identifiés jusqu’à présent (d’après Kermode, 1992)

A refers to basalt lava flows and B to pyroclastic rocks on surface. The numbers refer to individual eruptive centres: 1: Auckland Domain; 2: Albert Park; 3: St Heliers; 4: Onepoto; 5: Tank Farm; 6: Lake Pupuke; 7: Mt Cambria; 8: Mt Victoria; 9: North Head; 10: Mt Albert: 11: Mt Roskill; 12: Panmure Basin; 13: McLennan Hills; 14: Mt Richmond; 15: Robertson Hill; 16: Pukekiwiriki; 17: Waitomokia; 18: Maungataketake; 19: Pukeiti; 20: Otuataua; 21: Matakarua; 22: Ash Hill; 23: Kohuora; 24: Crater Hill; 25: Wiri Mountain; 26: Puketutu; 27: Pigeon Mountain; 28: Taylor Hill; 29: Pukaki; 30: Mt Smart; 31: Styaks Swamp; 32: Green Hill; 33: Otara Hill; 34: Hampton Park; 35: Mangere Lagoon; 36: Mt Mangere; 37: One Tree Hill; 38: Three Kings; 39: Hopua; 40: Te Pouhawaiki; 41: Mt Eden; 42: Mt St John; 43: Mt Hobson; 44: Little Rangitoto; 45: Orakei Basin; 46: Purchas Hill; 47: Mt Wellington; 48: Motukorea; 49: Rangitoto.
A dénote l’emplacement des coulées de lave basaltique et B représente les tufs pyroclastiques. Les numéros correspondent aux différents centres éruptifs : 1 : Auckland Domain; 2 : Albert Park; 3 : St Heliers; 4 : Onepoto; 5 : Tank Farm; 6 : Lake Pupuke; 7 : Mt Cambria; 8 : Mt Victoria; 9 : North Head; 10 : Mt Albert; 11 : Mt Roskill; 12 : Panmure Basin; 13 : McLennan Hills; 14 : Mt Richmond; 15 : Robertson Hill; 16 : Pukekiwiriki; 17 : Waitomokia; 18 : Maungataketake; 19 : Pukeiti; 20 : Otuataua; 21 : Matakarua; 22 : Ash Hill; 23 : Kohuora; 24 : Crater Hill; 25 : Wiri Mountain; 26 : Puketutu; 27 : Pigeon Mountain; 28 : Taylor Hill; 29 : Pukaki; 30 : Mt Smart; 31 : Styaks Swamp; 32 : Green Hill; 33 : Otara Hill; 34 : Hampton Park; 35 : Mangere Lagoon; 36 : Mt Mangere; 37 : One Tree Hill; 38 : Three Kings; 39 : Hopua; 40 : Te Pouhawaiki; 41 : Mt Eden; 42 : Mt St John; 43 : Mt Hobson; 44 : Little Rangitoto; 45 : Orakei Basin; 46 : Purchas Hill; 47 : Mt Wellington; 48 : Motukorea; 49 : Rangitoto.

5Volcanic activity in the AVF probably began about 140000 a BP although absolute and relative ages are poorly constrained and available reliable ages are only from few places (McDougall et al., 1969; Law 1975; Smith 1989; Newnham and Lowe, 1991; Allen and Smith, 1994; Mochizuki et al., 2004, 2007). In spite of the problem of radiometric dating, some promising results were obtained recently (e.g., Mochizuki et al., 2004, 2007) using high precision K-Ar methods that have been tested in other places as well (e.g., Balogh and Németh 2005). Past eruptions have generally been small with volumes less than 0.05 km3 each eruption yet a trend of increasing volumes and number of eruptions in the last 20000 a has been suggested (Allen and Smith, 1994) but so far not fully supported. The most recent eruption of the Rangitoto volcano, dated at around 700 a BP, had a volume of more than 2 km3, which is about halfof the estimated total eruptive output of the AVF (Allen and Smith, 1994). Drilling into crater lake sediments recently identified significant volume of ash falls from local and distal eruption sites allowing to establish the preliminary eruption chronology of the AVF. This chronology demonstrates an average of at least one eruption in every 2500 a over the last 50000 a (Lowe et al., 2000; Sandiford et al., 2001; Lindsay and Leonard, 2007).

Fig. 2 – Volcanic landforms and deposits from the Auckland Volcanic Field.
Fig. 2 Principales formes du relief et produits du champ volcanique d’Auckland.

Fig. 2 – Volcanic landforms and deposits from the Auckland Volcanic Field.Fig. 2 – Principales formes du relief et produits du champ volcanique d’Auckland.

A: Orakei Basin maar with wide crater filled with estuary. The view is looking north; B: scoriaceous lapilli unit in the rim of the Panmure basin inferred to be remnant of an intra-maar scoria cone; C: dune-bedded lapilli tuff and tuff succession deposited from base surges of the Orakei Basin maar. Base surge transportation direction is marked by the arrow; D: accidental lithic rich lapilli tuff (sand/mud and sandstone/mudstone marked with arrows) in the basal pyroclastic units of the Orakei Basin maar. On C and D the spate is about 20 cm long.
A : large cratère de maar du Bassin d’Orakei comblé par un estuaire. La photo est orientée vers le nord ; B : niveau de lapilli scoriacés trouvé dans la bordure du bassin de Panmure, dépôt soulignant les restes d’un cône de scories érodé en position intra-cratérique ; C : niveau de tufs à lapilli riche en antidunes entrecroisées et dépôts de tufs de déferlantes basales (maar du Bassin d’Orakei). La direction de transport des déferlantes est indiquée par la flèche ; D : niveau de tufs à lapilli riche en xénolithes d’origine accidentelle (les flèches soulignent les blocs de sable/limon et grès/marnes calcaires) au sein des unités pyroclastiques basales du maar du bassin d’Orakei (photos C et D : l’instrument mesure 20 cm de long).

Volcanic hazard and risk at the Auckland Volcanic Field

6The impact of a future eruption on Auckland’s society and economic development is likely to be significant as the city is New Zealand’s economic powerhouse and home to more than one million inhabitants (Magill et al., 2006). The damage from cone building and tephra fallout could be substantial due to the high density of buildings and infrastructure (Magill and Blong, 2005a, 2005b; Magill et al., 2006a, 2006b). Emergency planning by the metropolitan authorities to manage risk has been hampered by the difficulty in predicting eruption locations because of the monogenetic character of the field (Magill et al., 2005; Houghton et al., 2006; Sherburn et al., 2007). A study of eruption scenarios for various locations around Auckland (Johnston et al., 1997; Paton, 1999) highlighted the extreme spatial variation in risk to communities and essential services and underscored the need for a better understanding of the spatio-temporal distribution of the volcanic centers in the AVF to identify and foresee future event locations.

Methods to identify vent alignments

7The two quantitative techniques (“Nearest-Neighbour Azimuth” and “Hough Transform”) for assessing the vent alignments are adapted here from similar studies in other volcanic fields comparable with the AVF by size, age, eruptive mechanism, and preservation (Wadge and Cross, 1988; Wadge and Cross, 1989; Connor et al., 1992; 2000; Mazzarini and D’Orazio, 2003; Mazzarini et al., 2008).

8The nearest-neighbour azimuth method for determining the preferred orientations in vent distributions is based on the assumption that a volcanic body associated with a fault zone is likely to have a nearest neighbour that lies along the same structure. Preferred trends in nearest neighbour directions provide evidence for structurally controlled emplacement. The likelihood of assessing adjacent bodies that share a common fault zone is increased by only considering neighbours that are closer together than would be expected by chance. A minimum or ‘threshold’ distance between nearest neighbours can be determined from the mean distance and standard error for a random distribution for the same area (Mitchell, 2005). The expected mean distance de for a random distribution is given by the equation:

9de = 0.5 / (√n/A)                                                 (1)

10where n is the number of features and A is the area (Clark and Evans, 1954). The standard error SE is given by:

11SE= 0.26136 / (√n2/A)                                                (2)

12The pairs of neighbouring vents that have distances between them that are less than the threshold distance for a given confidence level are then isolated and the trends of their joins are measured.

13The Hough Transform method allows linear alignments of vents across a field to be located (Dudani and Luk, 1978). The method is based on the transformation of lines in a Cartesian x, y plane to points in a (ρ, θ) parameter plane, where ρ is defined as the length of a normal drawn between any line and the origin, and θ is the angle between that normal and the x axis (fig. 3A). For any (x, y) point in the Cartesian plane, a sinusoidal curve representing all possible lines passing through the point can be mapped onto the (ρ, θ) plane using the equation:

14ρ = x cos(θ) + y sin(θ)                                               (3)

15where x and y are the coordinates of the point and 0º ≤ θ ≤ 360º. Alignments are identified where more than two curves intersect at a common (ρ, θ) point (fig. 3B). The values for ρ and θ at this point are used to define the alignment in coordinate space. To group points that are nearly collinear as belonging to a common line, the scales of ρ and θ are quantized into discrete Δρ, Δθ bins and the number of contributing points in each cell of the accumulator array are counted (Wadge and Cross, 1988). Alignments are identified by the cells with high values.

Fig. 3 – Illustration of the Hough transformation of three colinear points on the (x, y) coordinate plane to corresponding curves in the (ρ, θ) plane. Detailed explanation is in the text.
Fig. 3 – Transformée de Hough de trois points colinéaires sur un plan de coordonnées (x, y) et leurs courbes correspondantes dans le plan (ρ, θ). L’explication détaillée de cette opération se trouve dans le texte.

Fig. 3 – Illustration of the Hough transformation of three colinear points on the (x, y) coordinate plane to corresponding curves in the (ρ, θ) plane. Detailed explanation is in the text.Fig. 3 – Transformée de Hough de trois points colinéaires sur un plan de coordonnées (x, y) et leurs courbes correspondantes dans le plan (ρ, θ). L’explication détaillée de cette opération se trouve dans le texte.

Results

16For the purpose of nearest-neighbour azimuth method, the outer boundary of the AVF was assumed to be defined by a best-fitting standard deviational ellipse with three standard deviations of the features from the mean centre of all the vents (fig. 4A). Approximately 99.7% of the vents can be expected to occur within this ~832 km2 area. The ellipse has a NNW-SSE trending major axis (seeSpörli and Eastwood, 1998). The expected mean distance between nearest neighbours for a random distribution of 49 volcanic features within this area is 2060 m (standard error: 153 m) from equations (1) and (2) above. The threshold distance below which features are not near each other simply by chance at a 95% confidence level is 1752 m. The observed mean length of nearest neighbour joins is 1465 m (median 1401 m; fig. 4A), which is less than the expected mean distance because of clustering within the centreof the study area. Short joins are uncommon, the shortest one having a length of ~410 m (fig. 4B). The joins that have lengths below the threshold distance tend to trend towards the ENE and NNE, and to a lesser extent to the NNW (fig. 4C).

17The families of lines passing through each of the 49 volcanic centres were transformed into curves on a (ρ, θ) plane applying Hough Transform Method. An accumulator array with a bin size of Δρ = 200 m and Δθ = 2° was then used to count the number of curves passing though each bin and the distribution of counts was contoured (fig. 5). The count maxima were located and transformed back to alignments in geographic space (fig. 6). Significant alignments consisting of five or more vents are highlighted. The results confirm the importance of ENE and NNE alignment trends, and to a lesser extent NNW to NW trends (fig. 7).

Fig. 4 – A: Nearest neighbour joins in the AVF.
Fig. 4 – A : La méthode du plus proche voisin appliquée aux segments dans le champ volcanique d’Auckland (CVA).

Fig. 4 – A: Nearest neighbour joins in the AVF. Fig. 4 – A : La méthode du plus proche voisin appliquée aux segments dans le champ volcanique d’Auckland (CVA).

Numbers refer to the identified vents named in fig. 1. The dashed ellipsoids define the locus of points that are 1, 2 and 3 standard deviations from a mean vent centre. The dotted lines joining vents are above the threshold distance and the solid lines are below the threshold distance. See text for further details. B: Histogram of the nearest neighbour (vent) distance of the AVF. C: Rose diagram showing the orientations of the joins that are not statistically near each other simply by chance. The count of join azimuths in 10º intervals is shown (n=17).
Les numéros correspondent aux centres éruptifs identifiés sur la figure 1. Les ellipses en tirets marquent l’emplacement des points correspondant à 1, 2 ou 3 déviations standard de la position moyenne de l’évent. Les lignes pointillées qui relient les cratères sont situées au-dessus de la distance seuil, contrairement aux lignes continues qui sont situées en dessous. Voir le texte pour les explications détaillées. B : Histogramme des distances des évents les plus proches pour le CVA. C : Diagramme en rose montrant les orientations des appareils apparemment associés et qui ne sont pas statistiquement proches l’un de l’autre par « simple chance ». Le décompte de l’azimuth des évents associés, par intervalles de 10º, est représenté (n =17).

Fig. 5 – Contour plot of the bins’ counts of sinusoidal curves that pass through discrete Δρ, Δθ.
Fig. 5 – Diagramme des contours représentant le nombre de courbes sinusoïdales passant par Δρ, Δθ distincts.

Fig. 5 – Contour plot of the bins’ counts of sinusoidal curves that pass through discrete Δρ, Δθ.Fig. 5 – Diagramme des contours représentant le nombre de courbes sinusoïdales passant par Δρ, Δθ distincts.

A refers to the shading used for bins with counts of 3-5 and B to the shading for counts greater than 5. The numbered labels in the figure refer to the point maxima that were used to derive the lineaments in fig.6.
A correspond à la teinte utilisée pour les cases de valeur 3-5 et B à la teinte utilisée pour les cases de valeur >5. Les étiquettes numérotées se réfèrent aux points maxima qui ont été utilisés pour déduire les alignements dans la fig. 6.

Fig. 6 – Map of lineaments derived from the Hough method.
Fig. 6 – Carte des alignements déduits par la méthode de Hough.

Fig. 6 – Map of lineaments derived from the Hough method. Fig. 6 – Carte des alignements déduits par la méthode de Hough.

A: lineaments defined by the alignment of up to 5 centres; B: lineaments defined by more than five volcanic centres; C: inferred fault. The lineament labels correspond to the point maxima labels shown in fig. 5.
A : alignements définis par cinq évents au maximum ; B : alignements définis par plus de cinq évents ; C : faille supposée. Les références utilisées pour les alignements correspondent aux étiquettes des points maxima présentées sur la figure 5.

Discussion

18The Auckland region is crossedby numerous normal faults of Miocene age that trend to the northeast and northwest (fig. 7). Activity of the northeast-trending faults is believed to mostly predate the north-westerly trending faults, indicating a mid- to late-Miocene change from northwest-southeast extension to northeast-southwest extension (Spörli, 1989 ; Spörli and Eastwood, 1997). The similarity in the fault and lineament orientations indicates that the lineaments represent zones of crustal weakness along which intrusion took place during phases of crustal extension. The location of vents along the intersections of lineaments is notable in this regard. The importance of the northeasterly trend for the vent alignments indicates that northwest-southeast extension prevailed well into the Quaternary period.

Fig. 7 – Map of regional faults (after Kermode, 1992).
Fig. 7 – Carte des failles régionales (d’après Kermode, 1992).

Fig. 7 – Map of regional faults (after Kermode, 1992).Fig. 7 – Carte des failles régionales (d’après Kermode, 1992).

A: fault; B: inferred fault. Lineaments in fig. 8 lie in the elliptical area. Inset: Rose diagram of fault traces in 10º sector intervals. Radii of the sectors are proportional to the total fault length within that sector (total length= 2177 km).
A : faille reconnue ; B : faille supposée. Les alignements de la fig. 8 sont regroupés dans la zone elliptique. Encart : diagramme en rose des traces de faille par secteurs de 10º. Les rayons de ces secteurs sont proportionnels à la longueur totale de faille au sein de chaque domaine angulaire (longueur totale = 2177 km).

19If volcanic activity occurred contemporaneously along the length of individual lineaments it is to be expected that similar dates would be recorded for the volcanoes along them. Dating of the lineaments is constrained by the lack of age data and complicated by the likelihood of repeated rejuvenation of volcanism along them and coeval intrusion along separate lineaments. An aeromagnetic study of the AVF by J. Cassidy and C.A. Locke (2004) revealed an anomalous remanent magnetization direction and a likely temporal link for widely separated volcanoes. Recurrence of volcanism and complex edifice growth at individual volcanoes have commonly been recognised in many of the so-called monogenetic volcanic fields (e.g., Houghton and Schmincke, 1986, 1989; Auer et al., 2007). Especially large volcanic complexes with complex eruptive products are good candidates to be eruptive sites where volcanism recurred in the same place over a long time period. The present knowledge of the volcanoes in the AVF excludes the presence of such sites.

Conclusion

20The existence of field-wide vent alignments representing zones of crustal weakness has implications for forecastof future volcanic events in the AVF. A recent probabilistic assessment of likely vent locations by C.R. Magill et al. (2005) was based on the assumption that the distance from the most recent volcano, Rangitoto, to the next vent can be estimated from distances between centres of historical consecutive events and that no preferred azimuthal direction exists between successive events. The evidence presented here points to future vents having a higher probability of occurring along northeast-southwest or northwest-southeast lines. The alignment of Rangitoto with the nearby ~10000 a-old Motokorea vent as well as the only active fault in the field, the Wairoa North fault (Wise et al., 2003) indicate that the risk of future activity may be higher along this northwest-southeast line (fig. 8). Future eruptions along this line are likely to be phreatomagmatic at least in their initial stage, as was the case for the Motokorea volcano. Inforecastingand preparing for a future eruption in Auckland, consideration needs to be given to the effects of scoria cone formation and associated lava flow effusion, but also importantly to the impact of offshore phreatomagmatism on its people, buildings and infrastructure, at least in the near shore areas.

Fig. 8 – Map of the northwestern AVF showing possible extension of the North Wairau Fault towards Rangitoto.
Fig. 8 – Carte du secteur nord-ouest du champ volcanique d’Auckland montrant l’extension probable de la Faille de Wairau en direction de l’île de Rangitoto.

Fig. 8 – Map of the northwestern AVF showing possible extension of the North Wairau Fault towards Rangitoto.Fig. 8 – Carte du secteur nord-ouest du champ volcanique d’Auckland montrant l’extension probable de la Faille de Wairau en direction de l’île de Rangitoto.
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Annexe

Version française abrégée

La ville d’Auckland en Nouvelle Zélande est construite sur un champ volcanique monogénétique appelé Champ Volcanique d’Auckland (CVA), qui englobe 49 centres volcaniques distincts (fig. 1). Une large gamme de styles éruptifs, soit phréatomagmatique, strombolien ou hawaiien, soit effusif, est représentée dans ce champ volcanique par la présence de nombreux édifisces, maars et anneaux de tufs, cônes de scories ou de spatter, ainsi que par de vastes champs de lave simples ou composés (fig. 2 ; Smith, 1989 ; Allen et al., 1996 ; Houghton et al., 1996 ; Rogan et al., 1996 ; Homer et al., 2000 ; Affleck et al., 2001 ; Cassidy et Locke, 2004 ; Houghton et al., 2006 ; Cassidy et al., 2007). L’activité volcanique a produit peu de magma (< 0,05 km3) dans le champ volcanique d’Auckland. Les éruptions débutèrent probablement il y a 140.000 ans BP mais les âges absolus et relatifs restent encore à confirmer (McDougall et al., 1969 ; Law, 1975 ; Smith, 1989 ; Newnham et Lowe, 1991 ; Allen et Smith, 1994 ; Mochizuki et al., 2004, 2007). L’éruption la plus récente, qui est celle du volcan Rangitoto, date de 700 ans BP environ (Allen et Smith, 1994).

À travers cette étude, nous tentons d’identifier les zones principales de faiblesse crustale dans le champ volcanique d’Auckland, qui peuvent être le lieu de futures éruptions, d’après les résultats du traitement statistique de la localisation des 49 évents ou cratères. Le résultat du test à l’aveugle de l’alignement des sites des évents est ensuite évalué et comparé avec d’autres études de la distribution de réseaux de failles dans lesquels des alignements ont été reconnus. Les deux techniques quantitatives (« Azimut du plus proche voisin » et « Transformée de Hough ») servant à mettre en lumière les alignements des évents sont adaptés ici sur la base d’études similaires conduites dans d’autres champs volcaniques comparables à celui d’Auckland en termes de taille, d’âge, du mécanisme éruptif et de l’état de préservation (Wadge et Cross, 1988 ; Wadge et Cross, 1989 ; Connor et al., 1992, 2000 ; Mazzarini et D’Orazio, 2003 ; Mazzarini et al., 2008). La recherche de l’azimut par la méthode du plus proche voisin, servant à déterminer les orientations préférentielles dans la distribution des réseaux d’évents, est fondée sur l’hypothèse selon laquelle un édifice volcanique associé à une zone de failles a probablement un plus proche voisin qui se situe sur la même structure tectonique. Des regroupements statistiques préférentiels dans les directions des plus proches voisins fournissent des arguments pour une mise en place contrôlée par une structure tectonique.

La probabilité d’identifier des édifices volcaniques contigus qui partagent la même zone tectonique s’accroît si l’on considère uniquement les voisins qui seraient plus proches les uns des autres que ce que le hasard permet d’attendre. Une distance minimum ou seuil entre les plus proches voisins peut être déterminée d’après la distance moyenne et l’écart-type d’une distribution aléatoire pour une même région (Mitchell, 2005). La méthode de la Transformée de Hough (fig. 3) permet de localiser des alignements linéaires d’évents dans un champ volcanique (Dudani et Luk, 1978). Afin d’utiliser la méthode de l’azimut du plus proche voisin, la limite extérieure du champ volcanique d’Auckland a été définie à titre d’hypothèse en adaptant au mieux une ellipse de déviation standard avec trois écart-types pour les objets étudiés à partir du centre moyen de tous les évents (fig. 4A ; cf. Spörli et Eastwood, 1998). La distance moyenne espérée entre les plus proches voisins pour une distribution aléatoire de 49 édifices au sein de l’ellipse ainsi représentée est de 2060 m. La distance seuil en dessous de laquelle les édifices ne sont pas plus proches autrement que par pur hasard est de 1752 m (avec un intervalle de confiance de 95 %). La longueur moyenne observée du segment unissant un cratère au plus proche voisin est de 1465 m (médiane 1401 m ; fig. 4A), ce qui est inférieur à la distance moyenne attendue à cause de la concentration des évents au centre de la zone d’étude. Les segments courts sont rares, le plus court ayant une longueur d’environ 410 m (fig. 4B). Les segments qui ont des longueurs inférieures à la distance seuil ont tendance à montrer une direction vers l’ENE et le NNE et, dans une moindre mesure, vers le NNW (fig. 4C).

Les familles de lignes passant à travers chacun des 49 centres volcaniques ont été transformées en courbes sur un plan (ρ =θ) en appliquant la méthode de Hough. Un « ordre de classification cumulée » selon une taillede tri (bin size) de Δρ a été ensuite utilisé pour compter le nombre de courbes passant à travers une taille de tri (bin size) de Δρ  = 200 m et Δθ  = 2°, et la répartition des azimuts des segmentscomptés a été indiquée sous forme de ligne (fig. 5). Les azimuts de longueur maximaleont été localisés et transformés en alignements dans un espace géographique (fig. 6). Les alignements significatifs, c’est-à-dire ceux qui regroupent au moins cinq évents, ont été mis en valeur. Les résultats confirment l’importance des alignements orientés ENE et NNE et, dans une moindre mesure, NNW à NW (fig. 6). La région d’Auckland est traversée par de nombreuses failles normales d’âge miocène dont l’orientation est NE à NW (fig. 7). L’activité des failles de direction NE aurait précédé celle des failles orientées vers le NW, indiquant ainsi que la contrainte de l’extension a varié, d’abord NW-SE puis NE-SW, du Miocène moyen au Miocène supérieur (Spörli, 1989 ; Spörli et Eastwood, 1997). Le parallélisme des orientations des failles et des linéaments suggère que les alignements d’évents situés sur des linéaments coïncident avec des zones de distension dans la croûte supérieure, le long desquelles des intrusions ont pris place durant des phases d’extension. L’importance de l’orientation NE des alignements des évents indique que la distension orientée NW-SE a encore prévalu durant le Quaternaire. L’existence d’alignements d’évents représentant des zones de faiblesse crustale a des implications en termes de prévision des futures éruptions dans le champ volcanique d’Auckland. Les arguments présentés ici montrent qu’il existe une forte probabilité pour que les futurs cratères se situent sur des lignes NE-SW ou NW-SE (fig. 8).

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Table des illustrations

Titre Fig. 1 – Map of the Auckland Volcanic Field showing the known volcanic centers (after Kermode, 1992). Fig. 1 – Carte du champ volcanique d’Auckland montrant l’emplacement des centres éruptifs identifiés jusqu’à présent (d’après Kermode, 1992)
Légende A refers to basalt lava flows and B to pyroclastic rocks on surface. The numbers refer to individual eruptive centres: 1: Auckland Domain; 2: Albert Park; 3: St Heliers; 4: Onepoto; 5: Tank Farm; 6: Lake Pupuke; 7: Mt Cambria; 8: Mt Victoria; 9: North Head; 10: Mt Albert: 11: Mt Roskill; 12: Panmure Basin; 13: McLennan Hills; 14: Mt Richmond; 15: Robertson Hill; 16: Pukekiwiriki; 17: Waitomokia; 18: Maungataketake; 19: Pukeiti; 20: Otuataua; 21: Matakarua; 22: Ash Hill; 23: Kohuora; 24: Crater Hill; 25: Wiri Mountain; 26: Puketutu; 27: Pigeon Mountain; 28: Taylor Hill; 29: Pukaki; 30: Mt Smart; 31: Styaks Swamp; 32: Green Hill; 33: Otara Hill; 34: Hampton Park; 35: Mangere Lagoon; 36: Mt Mangere; 37: One Tree Hill; 38: Three Kings; 39: Hopua; 40: Te Pouhawaiki; 41: Mt Eden; 42: Mt St John; 43: Mt Hobson; 44: Little Rangitoto; 45: Orakei Basin; 46: Purchas Hill; 47: Mt Wellington; 48: Motukorea; 49: Rangitoto.A dénote l’emplacement des coulées de lave basaltique et B représente les tufs pyroclastiques. Les numéros correspondent aux différents centres éruptifs : 1 : Auckland Domain; 2 : Albert Park; 3 : St Heliers; 4 : Onepoto; 5 : Tank Farm; 6 : Lake Pupuke; 7 : Mt Cambria; 8 : Mt Victoria; 9 : North Head; 10 : Mt Albert; 11 : Mt Roskill; 12 : Panmure Basin; 13 : McLennan Hills; 14 : Mt Richmond; 15 : Robertson Hill; 16 : Pukekiwiriki; 17 : Waitomokia; 18 : Maungataketake; 19 : Pukeiti; 20 : Otuataua; 21 : Matakarua; 22 : Ash Hill; 23 : Kohuora; 24 : Crater Hill; 25 : Wiri Mountain; 26 : Puketutu; 27 : Pigeon Mountain; 28 : Taylor Hill; 29 : Pukaki; 30 : Mt Smart; 31 : Styaks Swamp; 32 : Green Hill; 33 : Otara Hill; 34 : Hampton Park; 35 : Mangere Lagoon; 36 : Mt Mangere; 37 : One Tree Hill; 38 : Three Kings; 39 : Hopua; 40 : Te Pouhawaiki; 41 : Mt Eden; 42 : Mt St John; 43 : Mt Hobson; 44 : Little Rangitoto; 45 : Orakei Basin; 46 : Purchas Hill; 47 : Mt Wellington; 48 : Motukorea; 49 : Rangitoto.
URL http://journals.openedition.org/geomorphologie/docannexe/image/7664/img-1.jpg
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Titre Fig. 2 – Volcanic landforms and deposits from the Auckland Volcanic Field.Fig. 2 Principales formes du relief et produits du champ volcanique d’Auckland.
Légende A: Orakei Basin maar with wide crater filled with estuary. The view is looking north; B: scoriaceous lapilli unit in the rim of the Panmure basin inferred to be remnant of an intra-maar scoria cone; C: dune-bedded lapilli tuff and tuff succession deposited from base surges of the Orakei Basin maar. Base surge transportation direction is marked by the arrow; D: accidental lithic rich lapilli tuff (sand/mud and sandstone/mudstone marked with arrows) in the basal pyroclastic units of the Orakei Basin maar. On C and D the spate is about 20 cm long.A : large cratère de maar du Bassin d’Orakei comblé par un estuaire. La photo est orientée vers le nord ; B : niveau de lapilli scoriacés trouvé dans la bordure du bassin de Panmure, dépôt soulignant les restes d’un cône de scories érodé en position intra-cratérique ; C : niveau de tufs à lapilli riche en antidunes entrecroisées et dépôts de tufs de déferlantes basales (maar du Bassin d’Orakei). La direction de transport des déferlantes est indiquée par la flèche ; D : niveau de tufs à lapilli riche en xénolithes d’origine accidentelle (les flèches soulignent les blocs de sable/limon et grès/marnes calcaires) au sein des unités pyroclastiques basales du maar du bassin d’Orakei (photos C et D : l’instrument mesure 20 cm de long).
URL http://journals.openedition.org/geomorphologie/docannexe/image/7664/img-2.jpg
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Titre Fig. 3 – Illustration of the Hough transformation of three colinear points on the (x, y) coordinate plane to corresponding curves in the (ρ, θ) plane. Detailed explanation is in the text.Fig. 3 – Transformée de Hough de trois points colinéaires sur un plan de coordonnées (x, y) et leurs courbes correspondantes dans le plan (ρ, θ). L’explication détaillée de cette opération se trouve dans le texte.
URL http://journals.openedition.org/geomorphologie/docannexe/image/7664/img-3.jpg
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Titre Fig. 4 – A: Nearest neighbour joins in the AVF. Fig. 4 – A : La méthode du plus proche voisin appliquée aux segments dans le champ volcanique d’Auckland (CVA).
Légende Numbers refer to the identified vents named in fig. 1. The dashed ellipsoids define the locus of points that are 1, 2 and 3 standard deviations from a mean vent centre. The dotted lines joining vents are above the threshold distance and the solid lines are below the threshold distance. See text for further details. B: Histogram of the nearest neighbour (vent) distance of the AVF. C: Rose diagram showing the orientations of the joins that are not statistically near each other simply by chance. The count of join azimuths in 10º intervals is shown (n=17).Les numéros correspondent aux centres éruptifs identifiés sur la figure 1. Les ellipses en tirets marquent l’emplacement des points correspondant à 1, 2 ou 3 déviations standard de la position moyenne de l’évent. Les lignes pointillées qui relient les cratères sont situées au-dessus de la distance seuil, contrairement aux lignes continues qui sont situées en dessous. Voir le texte pour les explications détaillées. B : Histogramme des distances des évents les plus proches pour le CVA. C : Diagramme en rose montrant les orientations des appareils apparemment associés et qui ne sont pas statistiquement proches l’un de l’autre par « simple chance ». Le décompte de l’azimuth des évents associés, par intervalles de 10º, est représenté (n =17).
URL http://journals.openedition.org/geomorphologie/docannexe/image/7664/img-4.jpg
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Titre Fig. 5 – Contour plot of the bins’ counts of sinusoidal curves that pass through discrete Δρ, Δθ.Fig. 5 – Diagramme des contours représentant le nombre de courbes sinusoïdales passant par Δρ, Δθ distincts.
Légende A refers to the shading used for bins with counts of 3-5 and B to the shading for counts greater than 5. The numbered labels in the figure refer to the point maxima that were used to derive the lineaments in fig.6.A correspond à la teinte utilisée pour les cases de valeur 3-5 et B à la teinte utilisée pour les cases de valeur >5. Les étiquettes numérotées se réfèrent aux points maxima qui ont été utilisés pour déduire les alignements dans la fig. 6.
URL http://journals.openedition.org/geomorphologie/docannexe/image/7664/img-5.jpg
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Titre Fig. 6 – Map of lineaments derived from the Hough method. Fig. 6 – Carte des alignements déduits par la méthode de Hough.
Légende A: lineaments defined by the alignment of up to 5 centres; B: lineaments defined by more than five volcanic centres; C: inferred fault. The lineament labels correspond to the point maxima labels shown in fig. 5.A : alignements définis par cinq évents au maximum ; B : alignements définis par plus de cinq évents ; C : faille supposée. Les références utilisées pour les alignements correspondent aux étiquettes des points maxima présentées sur la figure 5.
URL http://journals.openedition.org/geomorphologie/docannexe/image/7664/img-6.jpg
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Titre Fig. 7 – Map of regional faults (after Kermode, 1992).Fig. 7 – Carte des failles régionales (d’après Kermode, 1992).
Légende A: fault; B: inferred fault. Lineaments in fig. 8 lie in the elliptical area. Inset: Rose diagram of fault traces in 10º sector intervals. Radii of the sectors are proportional to the total fault length within that sector (total length= 2177 km).A : faille reconnue ; B : faille supposée. Les alignements de la fig. 8 sont regroupés dans la zone elliptique. Encart : diagramme en rose des traces de faille par secteurs de 10º. Les rayons de ces secteurs sont proportionnels à la longueur totale de faille au sein de chaque domaine angulaire (longueur totale = 2177 km).
URL http://journals.openedition.org/geomorphologie/docannexe/image/7664/img-7.jpg
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Titre Fig. 8 – Map of the northwestern AVF showing possible extension of the North Wairau Fault towards Rangitoto.Fig. 8 – Carte du secteur nord-ouest du champ volcanique d’Auckland montrant l’extension probable de la Faille de Wairau en direction de l’île de Rangitoto.
URL http://journals.openedition.org/geomorphologie/docannexe/image/7664/img-8.jpg
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Mark W. Von Veh et Károly Németh, « An assessment of the alignments of vents based on geostatistical analysis in the Auckland Volcanic Field, New Zealand »Géomorphologie : relief, processus, environnement, vol. 15 - n° 3 | 2009, 175-186.

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Mark W. Von Veh et Károly Németh, « An assessment of the alignments of vents based on geostatistical analysis in the Auckland Volcanic Field, New Zealand »Géomorphologie : relief, processus, environnement [En ligne], vol. 15 - n° 3 | 2009, mis en ligne le 01 octobre 2011, consulté le 28 mars 2024. URL : http://journals.openedition.org/geomorphologie/7664 ; DOI : https://doi.org/10.4000/geomorphologie.7664

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Auteurs

Mark W. Von Veh

School of Natural Sciences, Unitec, Auckland, New Zealand

Károly Németh

Volcanic Risk Solutions, Massey University, PO Box 11 222, Palmerston North, New Zealand, email: k.nemeth@massey.ac.nz

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