Faulting in Banks Peninsula: Tectonic Setting and Structural Controls for Late Miocene Intraplate Volcanism, New Zealand

research-articleResearch Article169X10.1144/jgs2011-167U. Ring & S. HamptonBanks Peninsula Volcanism 2012

Downloaded from http://jgs.lyellcollection.org/ at Stockholm University on February 11, 2015 Journal of the Geological Society, London, Vol. 169, 2012, pp. 773 –785. doi: 10.1144/jgs2011-167.

Faulting in Banks Peninsula: tectonic setting and structural controls for late Miocene intraplate volcanism, New Zealand

UWE RING1,2* & SAM HAMPToN1 1Department of Geological Sciences, University of Canterbury, Christchurch 8140, New Zealand 2Present address: Department of Geological Sciences, Stockholm University, 106 91 Stockholm, Sweden *Corresponding author (e-mail: [email protected])

Abstract: An analysis of faulting in the late Miocene volcanic rocks of Banks Peninsula, South Island, New Zealand, shows that the formation of the volcanic edifice was largely controlled by NE–SW-striking dextral- oblique strike-slip faults. The data show a variable component of west–east- or NW–SE-oriented shortening and north–south or NE–SW extension. Synvolcanic faults reactivated Cretaceous normal faults and are interpreted to have formed a local pull-apart basin that controlled volcanism. Further east, the geometry of Akaroa Harbour is controlled by a north–south-striking oblique reverse fault. Limited fault-slip data collected from sub-recent loess deposits are not significantly different from the data collected in the volcanic rocks and appear to show that the kinematic field did not change significantly over the last c. 10 Ma. The overall kinematic field causing the recent series of earthquakes in the greater Christchurch region is also not fundamentally different from the one that controlled the eruption of the volcanic rocks. We conclude that the inherited Cretaceous faults controlled the development of the late Miocene volcanism on Banks Peninsula and largely provided a major anisotropy along which the recent faults ruptured.

Pre-existing basement structures are known to greatly influence the The aim of this paper is to explore the relationships between the location and eruptive sequence of volcanic centres (Toprak 1998; Canterbury basement faults, the emplacement of the Banks Peninsula Patanè et al. 2006; Lavallée et al. 2009). As lavas feeding volcanic volcanic rocks and faults activated during the recent earthquakes. We vents and dykes ascend subvertically, strike-slip faults and other argue that the basement faults controlled the late Miocene Banks subvertical structures are most effective in aiding volcanism. Many Peninsula volcanic complex and were also important for the recent strike-slip faults, such as the Great Sumatran fault zone, are indeed Christchurch earthquakes. Especially since the February 2011 earth- accompanied by volcanoes (Chakraborty & Khan 2009). Exten- quake, but also before, the aftershocks occurred along faults in and sional structures forming local pull-apart basins are common along along the margins of the Banks Peninsula volcanic complex. strike-slip faults (Sylvester 1988). In such areas, extensional fea- tures are often associated with single volcanoes, links between volcanoes and pull-apart structures are commonly observed (Aydin & Overview Nur 1982; Van Wyk De Vries et al. 1998), and a causal relationship Tectonic setting between extension and volcanism or intrusions has been widely envisaged (Moore 1979; Hutton & Reavy 1992). Pre-existing Up to the mid-Cretaceous New Zealand was part of a subduction structures are also known to play an important role in guiding system at the eastern margin of Gondwana (Bradshaw 1989). Most subsequent large-scale deformation (Ring 1994; Schumacher 2002; of the eastern part of the South Island is made up of accreted rocks Di Luccio et al. 2010). of this subduction system, the Torlesse greywacke (Mortimer The volcanic edifice of Banks Peninsula just south of 2004), which represent the basement. Subduction-related shorten- Christchurch, New Zealand (Fig. 1), serves as a natural laboratory ing caused east–west-striking thrusts and mélange belts (in present- for studying the effect that pre-existing structures have on the erup- day coordinates; it should be noted that the eastern part of the South tion of volcanic centres and perhaps also on guiding recent earth- Island rotated almost 90° counterclockwise in the Tertiary (Furlong quakes in this region. Christchurch and its surroundings was hit by & Kamp 2009; Fig. 1 inset)). The northern margin of the east–west-

four major earthquakes recently: the Mw 7.0 Darfield earthquake oriented Chatham Rise is inferred to be aligned with this Cretaceous east of Christchurch occurred on 4 September 2010 and was fol- subduction zone (Mortimer et al. 2006).

lowed by an Mw 6.1 event on 22 February, an Mw 6.0 earthquake on After subduction along the New Zealand sector of the east 13 June and an Mw 5.8 event on 23 December 2011 at the northern- Gondwana subduction system ceased at 110–100 Ma, widespread most margin of the volcanic edifice of Banks Peninsula in the extensional deformation affected the South Island. Normal faults southeastern suburbs of Christchurch (Fig. 1) (GeoNet website). developed at this time have variable strike, with the dominant direc- The earthquakes originated on different faults and it appears that tions being east–west and NE–SW (Fig. 1). The first phase of exten- pre-existing structures in the pre-mid-Cretaceous (Torlesse) base- sional tectonism was roughly north–south oriented, commenced at c. ment played an important role in focusing earthquake activity 110 Ma (Deckert et al. 2002; Gray & Foster 2004), and is recorded (Ghisetti & Sibson 2012; Sibson et al. 2012). Stramondo et al. by numerous roughly east–west-striking faults (Fig. 1, Fig. 2 inset), (2011) argued, using Coulomb stress triggering theory (King et al. which parallel the former subduction-related thrusts (Barnes 1994). 1994), that the mainshock on 4 September 2010 triggered subse- Arguably, north–south extension was followed by NW–SE exten- quent earthquakes in the region, which tend not to occur within the sion in the late Cretaceous at c. 85 Ma (Eagles et al. 2004; Mogg Banks Peninsula volcanic complex (Fig. 1). et al. 2008; Kula et al. 2009). Rifting ceased in the latest Cretaceous

773 Downloaded from http://jgs.lyellcollection.org/ at Stockholm University on February 11, 2015 774 U. RING & S. HAMPToN

N Lyttleton Zone 43.60 S Averaged 172.72 E current shortening direction Wellington Subduction 40-35 30

25 Hikurangi 35-30 2-0 20 10 mm/yr Pacific Plate slip vector (NUVEL-1A) 5 10 15 42 mm/yr 20 mm/yr

30 mm/yr

40 mm/yr Christchurch * ** Greendale Chatham Rise Alpine Fault Fault Banks Peninsula Australian Plate Port Hills Fault Southern Alps

limit of Canterbury Chatham Rise

Basin Bounty Trough

Dunedin

Active fault (thrust and strike slip)

Cretaceous high-angle normal fault

Basin Pacific Plate Cretaceous low-angle normal fault

Great South

Fig. 1. Plate-tectonic setting of the South Island of New Zealand showing the transition from the Hikurangi subduction zone in the north of the South Island to strike-slip faulting along the Alpine Fault. Also shown is the slip vector of the Pacific plate and the averaged shortening direction across the Southern Alps (from Ghisetti & Sibson 2012). Banks Peninsula and the recent Canterbury earthquake sequence in relation to the main plate boundary fault system and mid- to late Cretaceous extensional faults in the South Island of New Zealand are also shown. It should be noted that the strike of the faults on Banks Peninsula is subparallel to the strike of mid- to late Cretaceous high-angle normal faults in the Canterbury basin to the east and south of the peninsula, and that the Greendale Fault is also parallel to the strike of mid- to late Cretaceous normal fault offshore just east of Christchurch. Inset: motion of the Pacific plate for a point at Lyttelton in an Australia plate fixed reference frame. The direction of motion is given by the vector from the centre to the point (i.e. the azimuth of the point on the plot) and the velocity is the distance from the centre, so, for example, at 20 Ma the relative plate motion between Australia and Pacific at Lyttelton would be in a direction of 242° with a relative velocity of 23 mm a−1. The graph shows that the azimuth changed from about 240° at 15 Ma to about 246° at 8 Ma and relative plate motion increased by about 10 mm a−1. and is marked by an angular unconformity that separates synrift and that the azimuth changed from about 240° at 15 Ma to about 246° at post-rift sequences (Mogg et al. 2008). 8 Ma and relative plate motions increased by about 10 mm a−1. It is In the late Miocene the Southern Alps of New Zealand started important to note here that at this time Banks Peninsula was some to form in the collision zone between the two recent subduction 110 km east of the plate boundary. None the less, there would have systems (Fig. 1). Initially the orogen grew mainly to the west been a change in kinematics at this time, which largely coincided (Ghisetti & Sibson 2006). At about 5 Ma a distinct and sustained with the onset of mountain building in the Southern Alps. rainshadow effect set in. As a result of high erosion rates and associated large-scale removal of material from the western side of the Magmatism orogen, deformation in the west became largely pinned to the Alpine Fault and the orogen started to grow significantly to the Since New Zealand started to drift away from Gondwana in the late east (Chamberlain & Poage 2000; Ring & Bernet 2010). The prop- Cretaceous (84–82 Ma; Gaina et al. 1998), widespread products of agation of deformation to the east is now in the process of affect- intraplate volcanism formed nearly continuously throughout the ing the Canterbury Plains and the recent earthquakes are arguably late Cretaceous and Cenozoic. There are various expressions of this a manifestation of this process. volcanic activity throughout the South Island (Hoernle et al. 2006; The motions of the Pacific plate for a point at Lyttelton in an Timm et al. 2009), with large composite volcanoes, such as the Australia plate fixed reference frame are shown in Figure 1 Banks Peninsula volcanoes, being the most impressive type of (Furlong & Kamp 2009). The current tectonic regime in New volcanism (Weaver & Smith 1989). Zealand was established by about 28 Ma and then changed slightly Banks Peninsula is predominantly made up by the Lyttelton at about 10 Ma (Furlong & Kamp 2009) (Fig. 1). The graph shows and Akaroa composite volcanoes (Fig. 2), with smaller volumes Downloaded from http://jgs.lyellcollection.org/ at Stockholm University on February 11, 2015 BANKS PENINSULA VoLCANISM 775

Fig. 2. Simplified volcanic geology of Banks Peninsula after Weaver & Sewell (1986) and Sewell et al. (1992). The inferred faults, the various volcanic vents (Hampton 2010) and localities for fault-slip analysis are shown. Inset: tectonic setting of Banks Peninsula and Canterbury Plains, showing recent faults (fine continuous lines), late Miocene to recent(?) faults on Banks Peninsula (bold continuous lines on the peninsula) and late Cretaceous normal faults (bold dashed lines), the last of which caused regional gravity anomalies (Bennett et al. 2000). The gravity anomalies delineate the Mid Canterbury Horst, a first-order structure that focused late Miocene volcanism (Ghisetti et al. 2012). erupted from intervening and late-stage volcanism (Mt Herbert that the volcanoes had original outcrop area of c. 35 km2 for Volcanic Group and Diamond Harbour Volcanic Group respec- Lyttelton and c. 50 km2 for Akaroa, and that the volcanic rocks tively; Sewell et al. 1992). Drilled lavas offshore of Banks were more than 2–3 km thick (Weaver & Smith 1989). In a revi- Peninsula and beneath the surrounding Canterbury Plains suggest sion of all age datasets, periods of volcanism on Banks Peninsula Downloaded from http://jgs.lyellcollection.org/ at Stockholm University on February 11, 2015 776 U. RING & S. HAMPToN are Lyttelton Volcanic Group 12.4–9.7 Ma, Mt Herbert Volcanic Fault zones on Banks Peninsula Group 9.7–8.0 Ma, Akaroa Volcanic Group 9.4–8.0 Ma and Diamond Harbour Volcanic Group 8.1–5.8 Ma (Stipp & McDougall In general, the fault zones in the volcanic rocks of Banks Peninsula 1968; Sewell 1985, 1988; Sewell et al. 1992; Forsyth et al. 2008; are not well exposed and the fractured rocks are severely weathered. Timm et al. 2009). The onset of volcanism largely coincides with In Figure 2 we have distinguished from west to east the NE–SW- the onset of the Southern Alps orogeny but it is unclear whether striking Gebbies Pass fault system (consisting of the Gebbies Pass there is a genetic relationship between the two events. Fault in the west and two minor faults that are subparallel to it), and Volcanism on Banks Peninsula has been primarily basaltic, with the Mt Herbert, Port Levy, Little River and Le Bons faults. on the limited trachytic intrusive rocks. Hampton & Cole (2009) and western side of Akaroa Harbour occurs the north–south-striking Hampton (2010) identified seven main eruptive centres within the Akaroa Fault. At the southern end of Akaroa Harbour this north– Lyttelton Volcano. Further eruptive centres on Banks Peninsula south-striking fault either swings into a NNW–SSE orientation or is have been identified from mapped vents and plugs, and strati- cut by a NNW–SSE-striking fault. There are a few north–south- graphic relationships (Fig. 2; Sewell et al. 1992). striking minor faults between the main NE–SW-striking faults. We The Banks Peninsula volcanoes sit on Mesozoic greywacke of did not observe robust cross-cutting relationships between the fault the Torlesse accretionary wedge and, to a much lesser degree, on sets in the field. In addition, there are a few east–west-striking faults intermediate to silicic, late Cretaceous volcanic rocks (Fig. 2, mainly in the Lyttelton Harbour area. We also show an inferred inset). This basement forms a local NE–SW-trending horst that is east–west-striking fault in the south of the volcanic edifice. characterized by the strongest positive gravity anomaly in the The fault zones are characterized by anastomosing zones of region (Bennett et al. 2000). Another, less well-expressed gravity gouge, cataclasite, breccia and hematite-clay-coated fractured rock high occurs in the centre of the Akaroa Volcano (Fig. 2, inset; (Fig. 3). In the centres of the fault zones grain-size reduction com- Bennett et al. 2000). monly produced fine-grained layers of non-cohesive rock, which To constrain the tectonic boundary conditions of the emplacement we refer to as gouge zones. Between the gouge zones fractured and of the Banks Peninsula volcanic rocks, fault-slip data have been col- partly brecciated volcanic rocks occur, which we refer to as cata- lected from the western part of Banks Peninsula, especially around clasite. The fault zones are interlinked sets of gouge zones with Lyttelton Harbour, and from the eastern Banks Peninsula (Fig. 2). minor step-over faults and small intervening blocks of intact volcanic rocks. Subsidiary fault zones show a relatively simple pro- Structure of Canterbury Plains and Canterbury Basin gression from non-fractured country rock into severely fractured cataclasite and thin gouge zones (less than c. 2 m) in their centres. The mid- to late Cretaceous extensional deformation left a distinct Heterogeneous brittle deformation produced arrays of blocks structural grain in the greywacke basement (Figs 1 and 2). We will whose surfaces have a wide distribution of orientations. Single argue below that this inherited fault pattern was important for the blocks are separated by thin, slickensided surfaces with fibres and extrusion of the Banks Peninsula volcanic rocks. striae, which make these fault zones suitable for brittle strain analy- one major structure is the Chatham Rise and the Bounty Trough sis. The slickensided surfaces occur in the vicinity of the main fault in the offshore region to the east, both of which strike east–west zones and increase in number towards them. This spatial relation- (Fig. 1). The Bounty Trough is a late Cretaceous failed rift (Eagles ship is considered to imply a genetic connection of the mesoscale et al. 2004). The rifting process created numerous east–west- to fault zones with the mapped main fault zones. Therefore, fault-slip WNW–ESE-striking normal faults (Grobys et al. 2007). The analysis of the mesoscale fault zones allows us to infer the kinemat- Chatham Rise narrows westward and gravity data (Bennett et al. ics of the main fault zones. 2000) suggest that part of the Chatham Rise continues onland as an east–west-striking basement horst just south of Christchurch, the Fault-slip analysis Mid Canterbury Horst (Ghisetti & Sibson 2012; Fig. 2, inset). This horst is internally segmented by east–west-striking normal faults To evaluate the kinematics of a fault, the orientation of primary and and is bounded to the north and south by Cretaceous–Tertiary secondary fault planes, the trend and plunge of striations, and the basins, which are covered by a few hundred metres of late sense of relative displacement on these planes have been mapped. Pleistocene gravels (Mogg et al. 2008). The narrowing of the gen- In this study a simple graphical method is used to determine prin- eral horst structure would have been accommodated by ENE– cipal strain axes, using the program ‘Fault Kinematics’ written by WSW- or NE–SW-striking faults. Allmendinger (Marrett & Allmendinger 1990). This method graphi- It is the east–west-striking Chatham Rise–Mid Canterbury horst cally constructs the principal incremental shortening and extension structure that focused late Cretaceous to Recent magmatism in the axes for a given population of faults. Each pair of axes lies in the region (Mortimer et al. 2006; Panter et al. 2006; Ghisetti & Sibson movement plane of the fault (a plane perpendicular to the fault plane 2012). on Banks Peninsula the late Cretaceous Mt Somers volcanic that contains the unit vector parallel to the direction of accumulated rocks erupted and later the late Miocene Banks Peninsula volcanic slip and the normal vector to the fault plane). Furthermore, each pair rocks formed within the horst block. of axes is at an angle of 45° to each of the vectors (Fig. 3, inset). To South of Banks Peninsula, the NE–SW-trending Canterbury distinguish between the shortening and extension axes it is necessary Basin formed in the mid- to late Cretaceous. Its formation is largely to have information on the sense of slip. Bingham distribution statis- concurrent with the much larger Great South Basin further south tics for axial data will be used to optimize clusters of kinematic axes (Eagles et al. 2004; Kula et al. 2009). The formation of these basins of a fault array (Mardia 1972). The linked Bingham distribution is left strong NE–SW-oriented anisotropies. This structural grain is equivalent to an unweighted moment tensor summation (a moment locally modified by north–south- and WNW–ESE-striking graben tensor sum in which all faults are weighted equally). just east of Banks Peninsula (Figs 1 and 2; Mogg et al. 2008). The direction and sense of shear on these surfaces have been In the foothills of the Southern Alps west of Banks Peninsula, deduced from the orientation of fibres, striae and fractures associ- NE–SW-striking thrusts and anticlines formed (Fig. 2, inset) as a ated with the fault (Hancock 1985). Fibre and striae orientations on result of Pliocene to Recent WNW–ESE-oriented shortening slickensides from the subsidiary faults are usually simple and con- (Fig. 1). In addition, ENE–WSW- to east–west-striking dextral sistent, and are readily interpretable with the geometry of the strike-slip faults occur (Ghisetti & Sibson 2012). mapped faults at a regional scale. Downloaded from http://jgs.lyellcollection.org/ at Stockholm University on February 11, 2015 BANKS PENINSULA VoLCANISM 777

Principal strain directions a for a fault

Z X Trachyte Movement dike plane Y

EW50 cm Fault zones b E W c SW NE Fig. 3. Photographs of fault zones along the Gebbies Pass Fault. (a) Several Riedel meso-scale fault zones cutting through shear Lyttelton volcanic rocks near Bridle Pass, Summit Road of Port Hills. Crosscutting trachyte dyke at the left is hardly faulted. (b) Close-up of the fault zone shown in (a), showing pronounced cataclasis of E basalt in fault zone. The development of near-vertical secondary Riedel shears associated with normal faulting should be noted. (c) Small-scale normal faulting in basalt, Summit Road of Port Hills. Normal The inset in the upper right of (a) shows fault a graphical construction of the principal incremental shortening and extension axes for a given fault; the movement plane of the fault is perpendicular to the fault plane and contains the unit vector parallel to the direction of accumulated slip and the normal vector to the fault plane; the Fault zone 20 cm 20 cm shortening and extension axes are at 45° to the fault plane.

Fault-slip data The fault-slip data from the Lyttelton Harbour area (Fig. 5) between the Gebbies Pass and Mt Herbert faults in general show We present our fault-slip data for the volcanic rocks of Banks patterns that are compatible with dextral oblique-slip faulting along Peninsula in three regions defined according to the predominant both faults. Datasets H2 and H4 show a well-defined NNE–SSW- volcanic group, age and eruptive footprint: western Banks Peninsula oriented extension direction resulting from a pronounced compo- (Lyttelton Volcanic Group), central Banks Peninsula (Mt Herbert nent of normal faulting (Fig. 5d). The other two datasets are more and Diamond Harbour Volcanic Groups), and eastern Banks mixed and show a strong reverse faulting component resulting Peninsula (Akaroa Volcanic Group). It should be noted that within from NW–SE- to WNW–ESE-oriented shortening. Taken together, this analysis there is some overlap of volcanic groups. the data suggest a mixture between strike-slip faulting and almost pure normal faulting. Western Banks Peninsula: Lyttelton Volcanic Group (12.4–9.7 Ma) Central Banks Peninsula: Mt Herbert and Diamond Harbour Volcanic Groups (9.7–5.8 Ma) The fault-slip data from various outcrops along the Gebbies Pass Fault are dominated by roughly east–west-striking small-scale nor- The data from the Mt Herbert Fault depict a slightly less pro- mal faults resulting from north–south extension (Fig. 4). Some data- nounced normal faulting component compared with the data from sets show a strong east–west shortening component (Fig. 4e). the Gebbies Pass Fault (Fig. 6). The extension directions of the overall, the fault-slip data from mesoscale faults yielded fairly con- various datasets are relatively consistent and trend NE–SW, with sistent results, which are compatible with dextral oblique-slip (tran- NW–SE-trending shortening axes. overall, the data again suggest stensional) faulting along the NE–SW-striking Gebbies Pass Fault. dextral strike-slip faulting along the Mt Herbert Fault. Compared Downloaded from http://jgs.lyellcollection.org/ at Stockholm University on February 11, 2015 778 U. RING & S. HAMPToN Gebbies Pass Fault Fig. 4. Fault-slip data from the Gebbies Pass a b c Fault. The diagrams show great circles of fault N G1 G2 X G3a planes and the projected trace of the associated n=8 n=16 n=14 slickenside lineation in a lower-hemisphere equal-area projection. The principal strain axes (X > Y > Z) are shown. The deduced extension Y directions (X) are indicated by white arrows, and Z Z the shortening directions (Z) by grey arrows. Y Z Y The outcrop number is indicated at the upper left and locations are shown in Figure 2. (a) outcrop G1 is dominated by east–west-striking normal faults; consequently, the calculated extension X X direction is north–south. (b) G2 contains more d e f data but shows similar characteristics to G1; in G5 addition, a few north–south-striking sinistral G3 G4 Y n=9 n=8 n=6 X strike-slip faults occur. (c) outcrop G3a is again similar to G2. (d) G3 has a limited dataset that again shows north–south extension. (e) outcrop Y 4 shows a number of north–south-striking Z Z reverse faults and a few NW–SE-striking

X Y Z oblique-slip normal faults. The fault pattern resulted from east–west shortening; there is minor north–south extension in the Y direction of the strain ellipsoid; the maximum extension X direction X is subvertical. (f) outcrop G5 shows g h i a mixture of east–west-striking normal faults G6 G7 G8 and north–south-striking reverse faults. n=8 n=10 n=10 Z (g) outcrop G6 (see also (a) and (b)) depicts

X a more complicated dataset with east–west- Z striking normal and oblique normal faults, as well as NNE–SSW-striking oblique reverse Z Y faults; one NNE–SSW-striking fault is a normal Y fault. (h) outcrop G7 shows a number of NNW–

X SSE-striking faults, which have either reverse

Y slip kinematics (NE-plunging striations) or

X normal kinematics (ENE-plunging striations). In addition, strike-slip faults and a NE–SW-striking normal fault occurs. (i) G8 shows also a mix of reverse, normal and strike-slip faults that overall combine to a kinematic pattern of shortening and extension axes similar to that of G7. with the Gebbies Pass Fault, the Mt Herbert Fault appears to have extension. Further along strike of the Akaroa Fault, the Pigeon Bay a less pronounced oblique normal faulting component. Fault records a relatively strong NNW–SSE-directed shortening com- The fault-slip data from the Port Levy Fault (Fig. 7) are of lim- ponent and minor ENE–WSW extension (Fig. 11). ited use as most outcrops did not expose many fault planes. However, the data are compatible with the data from the Gebbies Data from loess deposits Pass and Mt Herbert faults and support dextral strike-slip kinemat- Large parts of Banks Peninsula are covered with several metres of ics along the NE–SW-striking faults in western Banks Peninsula. loess deposits. The loess is not well dated but appears to be 40– 60 ka in age (Goh et al. 1977; Berger et al. 2001). The loess covers Eastern Banks Peninsula: Akaroa Volcanic Group the mapped fault zones and is commonly unfaulted. However, at a (9.4–8.0 Ma) few outcrops the loess covering the fault zones has been faulted, attesting to a recent reactivation of the faults. The fault-slip data from the Akaroa Volcanic Group are not funda- We collected fault-slip data from the sub-recent loess deposits mentally different from those of western and central Banks (Fig. 12) at two outcrops along the northern part of the Akaroa Peninsula but overall appear to be characterized by a more pro- Fault. The kinematic data derived from the loess datasets are simi- nounced NW–SE shortening component. lar to the data from the Pigeon Bay Fault and thus dominated by The two NE–SW-trending faults in Akaroa Volcanic Group have WNW–ESE- to NW–SE-trending shortening. almost identical fault sets to the NE–SW-trending faults in the older Lyttelton Volcanic Group. The Little River Fault (Fig. 8) and the Le Interpretation of fault-slip data Bons Fault (Fig. 9) both have mixed datasets dominated by strike-slip and oblique-slip faults that resulted from NW–SE-directed shortening The NE–SW-striking faults dominate the structure of Banks and NE–SW extension. The Akaroa Fault is a north–south-striking Peninsula, especially the western part of it. It appears that these oblique reverse fault. The fault-slip data for small-scale faults indicate faults are largely dextral strike-slip faults that have a variable com- a pronounced WNW–ESE- to NW–SE-trending shortening direction ponent of roughly north–south to NE–SW extension. This extension (especially datasets AB1 and AB2 in Fig. 10) with minor NNE–SSW component is most pronounced for the Gebbies Pass Fault bordering Downloaded from http://jgs.lyellcollection.org/ at Stockholm University on February 11, 2015 BANKS PENINSULA VoLCANISM 779 Lyttelton Harbour area Mt Herbert Fault a N b a N b K1 K2 H1 Z H2 X n=7 n=11 n=6 X n=6 Y

Y Y X Y

X Z

Z Z

c c d d K3 K4 H3 H4 n=14 n=7 Y n=5 n=5 X

Y X Z X

Z Y Z Z

Fig. 5. Fault-slip data from the Lyttelton Harbour region. (a) outcrop Fig. 6. Fault-slip data from the Mt Herbert Fault. (a) outcrop K1 is H1 is dominated by ENE–WSW-striking reverse faults; in addition dominated by east–west-striking dextral oblique normal faults and two two north–south-striking normal faults occur. The NW–SE-trending east–west-striking dextral oblique reverse faults. The fault pattern is shortening axis and the near-vertical extension axis for this fault pattern compatible with dextral strike-slip faulting along the NE–SW-striking Mt is well defined. The intermediate Y-axis is oriented ENE–WSW. (b) H2 Herbert Fault (Fig. 2) and a NNE–SSW-trending extension and a NW–SE- shows a straightforward pattern of normal and oblique normal faults trending shortening axis. (b) K2 shows a rather scattered dataset with three that strike roughly east–west. Consequently, the NNE–SSW-trending NE–SW-striking reverse faults and two NE–SW-striking oblique normal extension direction is well constrained. (c) outcrop H3 has three reverse faults; in addition, one NW–SE-striking sinistral strike-slip fault occurs. faults, one normal fault and one NNE–SSW-striking sinistral normal The fault pattern resulted from NW–SE-trending shortening; the X- and fault. The fault pattern results basically from WNW–ESE shortening and Y-axes have very similar positive eigenvalues indicating a flattening strain near-vertical extension. (d) H4 has a limited dataset that resembles H2, type with two extension directions. (c) outcrop K3 has a larger dataset than with a well-defined NNE–SSW-oriented extension direction. K2 but the data are similar indicating WNW–ESE-directed shortening and NNE–SSW extension. (d) K4 has a limited dataset with oblique reverse and oblique normal faults that resulted from ENE–WSW extension. Lyttelton Harbour at its NW side. Further SE, the geometry of Akaroa Harbour is controlled by the north–south-striking sinistral-reverse Akaroa Fault. overall, there is a general tendency It is proposed that the entire fault-slip dataset from the Lyttelton from dextral transtensional faults in the west to faults with a more Volcano area represents a dextral pull-apart basin. Figure 13 shows transpressional component in the Akaroa Harbour area in the east. east–west-striking releasing bend faults in the Lyttelton Harbour The few collected fault-slip data from the sub-recent loess area, which transfer the extensional strain between the major dex- deposits do not show significantly different kinematics from the tral oblique normal faults. data from the volcanic rocks, especially not from those collected in the Akaroa Volcanic Group. We interpret this to indicate that the overall kinematic field did not change to any significant degree Tectonovolcanic interactions since c. 10 Ma. The fact that there is hardly any evidence for dis- Spatial relationships of faults and vents placements of more than 50–100 m of the volcanic rocks suggests that the young reactivation was modest. The age of fault activity is not directly constrained. The extrusion of the volcanic rocks took place between 12.4 and 5.8 Ma (see Dextral pull-apart model for Lyttelton Harbour above). The faults in the volcanic rocks appear to be largely synvolcanic as they occur in the volcanic rocks but have, in general, not Figure 13 summarizes the inferred tectonic model for western significantly displaced them. on the other hand, the mapped Banks Peninsula, Lyttelton Volcano. The fault-slip data are con- Gebbies Pass and Mt Herbert faults do not show up prominently on sistent with dextral strike-slip faulting along the NE–SW-striking aerial photographs and are, at least in part, masked by volcanic faults in western Banks Peninsula. The Gebbies Pass Fault shows a flows. These relationships suggest that faulting was largely concur- relatively strong north–south- to NE–SW-trending extension com- rent with the volcanic activity and has probably controlled the loca- ponent associated with dextral strike slip. Between the dextral tion of the volcanic vents (Fig. 2). Gebbies Pass and Mt Herbert faults are segments, near Diamond As previously indicated there is differing fault kinematics within Harbour for instance, which are characterized by almost pure nor- the various sectors of Banks Peninsula, defined by its volcanic mal faulting owing to NNE–SSW-oriented extension (Fig. 13). region. The vents of Lyttelton Volcano concentrate along the Downloaded from http://jgs.lyellcollection.org/ at Stockholm University on February 11, 2015 780 U. RING & S. HAMPToN Port Levy Fault Fig. 7. Fault-slip data from the Port Levy a N b c Fault. (a) L1 shows two NE–SW-striking L1 L2 L3 reverse faults and two oblique normal faults. n=4 n=6 n=5 The strain field is flattening with extension in X X and Y directions; the well-defined shortening X X axis is NW–SW-trending. (b) L2 shows a

Z heterogeneous dataset with two NNW–SSE- striking normal faults, a NW–SE-striking Z dextral strike-slip fault, and two WNW–ESE- Z striking oblique reverse faults. The pattern Y resulted from east–west shortening and Y Y north–south extension. (c) outcrop L3 has four normal and oblique normal faults and one d e f PB3 PL1 single reverse fault. The pattern is resulting n=5 PL2 n=5 n=5 from NE–SW extension. (d) PB3 is again Y dominated by normal and dextral-oblique Z X normal faults resulting from dominantly Z NNE–SSW extension. (e) PL1 shows mainly north–south- to NE–SW-striking reverse faults Y Z plus a WNW–ESE-striking normal fault. X This fault set is due to WNW–ESE-directed shortening. (f) outcrop PL2 has three NNW– SSE-striking normal faults and a NNE–SSW-

X Y striking sinistral-oblique normal faults. This fault pattern indicates ENE–WSW-directed extension and subvertical shortening.

Fig. 8. Fault-slip data from the Little River Little River Fault c abN Fault. (a) outcrop LR1 is characterized by LR1 AB6 PB2 n=9 n=6 n=6 roughly east–west-striking oblique normal X faults; in addition, one NNE–SSW-striking reverse fault occurs. The NNE–SSW-trending Z shortening axis is well defined. (b) AB6 Y X Z shows NNW–SSE-striking sinistral faults Y and SE–NW-striking dextral faults resulting from NW–SE shortening and NE–SW

Z extension. (c) PB2 has a set of north–south- striking oblique normal faults resulting from X Y ENE–WSW extension and near-vertical shortening.

Gebbies Pass and Mt Herbert faults, and associated faults that A similar horst setting controls eruptive centre sites on Mt Etna in delineate the envisaged Lyttelton Harbour pull-apart (Fig. 13). In southern Italy (Patanè et al. 2006). overlaying vent locations onto this dextral pull-apart basin model a on Banks Peninsula an eastward trending migration of activity is relationship between intersection of faults and sources of volcan- recognized (Shelley 1987; Sewell 1988). Subtle differences between ism can be proposed, indicating the structural control of magma- the mapped faults and vent distributions supply some constraints on tism (compare Figs 2 and 13). The present topography of Lyttelton the timing of faulting and migration of volcanism. In examining age Harbour is erosionally formed (Hampton & Cole 2009; Hampton relationships, periods of volcanism are contemporaneous or overlap et al. 2012) but erosion has been influenced by the underlying vol- (i.e. Mt Herbert Volcanic Group and Akaroa Volcanic Group), sug- canic and faulting relationships. gesting related but separate controls on magmatic source and erup- The vents of Mt Herbert Volcanic Group primarily occur along tive location. Here we propose that the control on the location of the Mt Herbert Fault, whereas the Akaroa Volcanic Group eruptive eruptions on Banks Peninsula is through regional fault kinematics, vents are focused along the Akaroa and Little River faults. Diamond which changed through time. The magmatic plumbing system is Harbour Volcanic Group formed as discrete vents almost over the proposed to follow pathways and fault intersections within the east– entire peninsula and these vents appear to align along north–southwest-striking Mid Canterbury Horst (Fig. 2). Tibaldi (2008) showed striking faults (Fig. 2). These faults yielded oblique reverse fault that dykes and fractures feeding magma to the surface align perpen- kinematics in Akaroa Harbour. We argue that the younger age rela- dicular to the regional extension direction, an aspect highlighted in tionship of the Diamond Harbour vents reflects the subtle changes the NE–SW elongation of Banks Peninsula (volcanic surface in the regional kinematic field. expression indicating the extent or morphology of the feeding system beneath) and the regional tectonic system. Regional tectonics and volcanism Stages of Banks Peninsula volcanism with specific faulting kinematics can be highlighted within the development of the plate We argue that the entire Banks Peninsula volcanism was controlled boundary in the South Island. Lyttelton volcanism occurred during by a regional horst structure crosscut by faults, with intersec- a stage of transtensional fault kinematics (c. 12–10 Ma). The onset tions becoming the focus of volcanic activity and eruptive vents. of the Mt Herbert and Akaroa volcanism (9.7 and 9.4 Ma) coincides Downloaded from http://jgs.lyellcollection.org/ at Stockholm University on February 11, 2015 BANKS PENINSULA VoLCANISM 781 Le Bons Bay Fault The 22 February 2011 earthquake occurred on the blind Port Hills Fault in the south of Christchurch (Fig. 14). The deduced N shortening axis was N c. 125°E (USGS website). The slip was a OK1 mixture of reverse faulting and right-lateral strike slip. The n=4 deeper part of the fault and the westernmost 5–6 km of the fault show predominantly strike-slip faulting, whereas the eastern and shallower parts of the fault show a greater proportion of reverse faulting (Beavan et al., 2011). These relationships suggest a curving fault trace. Intense aftershock activity was con- Z centrated in the hanging wall of the Port Hills Fault, extending Y especially out to sea to the north and east of Banks Peninsula (Sibson et al. 2012). The 13 June 2011 earthquake was a sinistral transpressive event at the eastern end of the inferred rupture of the February earthquake. The deduced shortening axis was N c. 115°E (USGS website). We refer to this fault here as the Godley Head–Port Levy Fault. The most recent major earthquake occurred on 23 December 2011 and was almost a pure thrust event on an offshore ENE– WSW-striking north-dipping fault. The shortening axis was N c. 122° (USGS website). X Between the Greendale Fault and the faults in Christchurch further east near Rolleston is a diffuse corridor with no identified or assumed fault(s) (Fig. 14). Geometrically this corridor would clas- sify as a releasing step-over and most of the focal-plane solutions are compatible with extension (GeoNet website). Sibson et al. (2012) speculated that this corridor is controlled by a subvertical 145 ± 5° striking sinistral strike-slip fault, the Norwood–Doyleston– Ellesmere lineament, which follows the approximate trend of a dis- Fig. 9. Fault slip data from the Le Bons Bay Fault. outcrop oK1 has tinct aftershock lineament south of the Greendale rupture running NW–SE-trending sinistral strike-slip faults resulting from NW–SE SE towards the mouth of Lake Ellesmere. shortening and NE–SW extension. The c. 150° striking Norwood–Doyleston–Ellesmere lineament and Godley Head–Port Levy Fault both have sinistral kinematics with a subtle change in Pacific plate motion (Furlong & Kamp and appear to act as accommodation structures between the 2009) (Fig. 1, inset), and the termination of the Mt Herbert and Greendale and Port Hills faults, both of which have different strikes Akaroa volcanism at 8.0 Ma coincides with a more significant and dips and have no common origin. The NW–SE-striking seg- change in the rate of plate convergence and plate motion (Furlong ment near the hamlet of Greendale strikes similarly but has totally & Kamp 2009). different kinematics. The two sinistrally displacing faults fit per- The Diamond Harbour Volcanic Group did not follow the previ- fectly with the current kinematic field (Sibson et al. 2012), but the ous trend of large-scale eruptive series from localized sources, with dextral fault near Greendale does not. It thus appears that dextral eruptive vents occurring all over the peninsula. This stage of volcan- strike-slip faulting along the NW–SE-striking segment is inhibiting ism occurred during a time when the developing plate boundary propagation of faulting along the dextral Greendale Fault to the underwent a change towards greater transpressional kinematics. This west. This view is consistent with the fact that the majority of the is hypothesized to have two effects: (1) the propagation of magma aftershocks, including many of the larger events, are concentrated along reactivated reverse faults, intersecting at low points within the at the eastern end of the Greendale Fault (Gledhill et al. 2011) and structure of Banks Peninsula, resulted in discrete eruptive locations, that subsequent earthquake activity progressed eastwards. The seis- a trend that follows that proposed and modelled by Galland et al mically active faults are avoiding the main part of the Banks (2007) and Tibaldi (2008); (2) as the plate boundary further devel- Peninsula volcanic edifice (Fig. 14). oped, transpression limited propagation of magma to the surface, and The deduced kinematic axes for the recent earthquakes are simi- resulted in the complete shut-off of Banks Peninsula’s volcanism. lar to the kinematic axes we obtained from our fault-slip analysis in Banks Peninsula (Fig. 14). This indicates that the kinematic field has not changed in any significant way since c. 10 Ma. Earthquake data

Figure 14 shows the mapped and assumed fault planes that caused Importance of inherited structures the earthquake sequence in Canterbury (Beavan et al. 2010; Quigley et al. 2010; Beavan 2011; Sibson et al. 2012; and GeoNet website). In the following we argue that the inherited mainly Cretaceous The deduced shortening axis for the 4 September 2010 Darfield structural grain played a dominant role in guiding late Miocene vol- earthquake was N c. 135°E and the corresponding extension direc- canism on Banks Peninsula and in turn also controlled the recent tion would be N c. 45°E (USGS website). Near the hamlet of earthquake activity in the greater Christchurch area. Greendale, the dextral Greendale Fault is terminated in the west by a NW–SE-striking segment (Fig. 14). Given N c. 45°E-trending exten- Pre-existing faults and their possible control on sion the orientation of this NW–SE-striking fault would suggest volcanism and synvolcanism faulting almost pure normal slip on this segment. However, global position- ing system (GPS) data indicate that displacement on the NW–SE- The major faults on Banks Peninsula strike NE–SW. These faults striking segment is dextral strike slip as well (B. Duffy, pers. comm.). are parallel to the Cretaceous normal faults in the Canterbury Basin Downloaded from http://jgs.lyellcollection.org/ at Stockholm University on February 11, 2015 782 U. RING & S. HAMPToN Akaroa Harbour Fault a N bc AB1 X AB2 AB3 Fig. 10. Fault-slip data from the Akaroa n=9 n=13 n=7 Harbour Fault. (a) AB1 depicts a straightforward dataset with NE–SW-striking reverse and oblique reverse faults resulting X Z X from WNW–ESE shortening. (b) AB2 is Z rather similar with north–south- to NE–SW- striking reverse and oblique reverse faults Z Y resulting from WNW–ESE shortening. (c) AB3 shows a more heterogeneous dataset Y Y with NE–SW-striking reverse faults, a single d e dextral strike-slip fault and east–west- AB4 AB5 striking normal faults. overall, the pattern n=7 n=6 X again resulted from WNW–ESE shortening. X (d) North–south-striking reverse and NW– Z SE-striking normal faults characterize the Y AB4 outcrop. A NW–SE-oriented shortening direction is apparent. (e) AB5 is dominated by strike-slip faults, with NNW–SSE-striking ones being sinistral and NW–SE-striking Y Z ones being dextral. one WSW–ENE-striking fault is also dextral. As for AB3 and AB4, the shortening direction is roughly NW–SE with a NE–SW-trending extension component.

Pigeon Bay Fault Subrecent faulting in loess a N b a N b Z PB5 PB4 PB1 PB6 n=8 n=6 n=6 n=5

X Z Y Y

X X X Y

Z Z Y

Fig. 11. Fault-slip data from the Pigeon Bay Fault. (a) PB5 has NW–SE- Fig. 12. Fault-slip data from loess deposits. (a) PB1 shows a simple striking sinistral and NW–SE-striking dextral strike-slip faults, and roughly set of east–west- to NE–SW-striking oblique reverse faults resulting NNE–SSW- and east–west-striking oblique reverse faults. The fault set from NW–SE shortening. (b) outcrop PB6 has mainly NNE–SSW- resulted from NNW–SSE-directed shortening. (b) PB4 shows NW–SE- striking sinistral-oblique reverse faults resulting from NW–SE-directed striking oblique reverse faults and NNW–SSE-striking oblique normal shortening. faults resulting from east–west shortening and north–south extension. to the south. The north–south-striking oblique reverse faults paral- result. The latter faults might simply be rotated and reactivated east– lel Cretaceous faults mapped just east of Banks Peninsula. We west faults and/or newly formed late Miocene faults. therefore suggest that the faults that controlled the volcanic vents The NE–SW-striking faults are oblique to the trend of the elon- are largely inherited basement faults that were reactivated in the gate structure of Banks Peninsula (Fig. 13). Barnes (1994) and late Miocene and controlled the emplacement of the Banks Mogg et al. (2008) showed roughly east–west-striking inherited Peninsula volcanic rocks. faults to the east of the peninsula all along the Chatham Rise. East– A Cretaceous east–west-striking fault system is aligned along the west-striking faults are also inferred on-land from gravity data Chatham Rise and controlled the opening of the Bounty Trough (Bennett et al. 2000) (Fig. 2, inset). We argue that the intersection (Grobys et al. 2007). Barnes (1994) argued that at least some of the of these two inherited fault trends controls the location of the erup- east–west-striking faults were extensionally reactivated in the late tive centres (Fig. 13), similar to what has been observed in the Miocene. This late Miocene extensional reactivation is related to the Cappadocian Volcanic Province. encroachment of the incoming Chatham Rise, as part of the Pacific plate, against the evolving transpressive plate boundary zone. The Pre-existing structures and the recent earthquakes advance of the Chatham Rise would have been associated with an anticlockwise rotation and would have and possibly still is interact- Ghisetti & Sibson (2012) and Sibson et al. (2012) have argued that ing with the North Island. overall, this scenario caused a complex the recent earthquakes were, at least in part, occurring along faults kinematic regime and the ENE–WSW-striking faults formed as a that reactivated older structures. For some of the new faults it is Downloaded from http://jgs.lyellcollection.org/ at Stockholm University on February 11, 2015 BANKS PENINSULA VoLCANISM 783

Key Data Interpretation 5 km G8 SHmax

G7 N SHmin H4 G5 G6

Mt Herbert Fault H3 K3

Port Levy Fault H1 L1

G1 K4 G4 G2 G3

G3a

Gebbies Pass

Fault System

K2 K1

Fig. 13. Digital elevation model of the northeastern part of Banks Peninsula showing the major faults and the maximum horizontal extension direction inferred from the fault slip data shown in Figures 4–7. In those cases where the maximum shortening direction is near-horizontal, the shortening direction has been projected into the horizontal and is also shown. It should be noted that both the maximum extension and the shortening directions have been projected into the horizontal (SHmax and SHmin) and therefore do not coincide with the strain axes shown in Figures 4–7. Also shown is our interpretation of the kinematics of the major faults along which the data have been collected. The envisaged releasing bend model for the formation of Lyttelton Harbour is indicated by the large white arrows. The east–west-oriented depocentre is interpreted as a result of a complex dextral releasing bend structure between the NNE–SSW-striking dextral strike-slip to oblique-slip faults. hard or impossible to show that they were reactivating older struc- new fault propagates, at least in part, in its own way and would use tures as most of them are blind structures or cut through late the pre-existing fault-parallel and vertical fractures wherever con- Pleistocene gravels that had not previously been deformed. Here venient. we provide some tentative arguments on possible reactivation We suggest that a similar mechanism operated in Canterbury based mainly on geometric grounds. when the Greendale Fault formed. The initial anisotropy may have The east–west-striking Greendale Fault formed right above a been provided by Jurassic–Cretaceous thrusts and reverse faults in late Cretaceous normal fault in the subsurface that has been robustly the Torlesse accretionary wedge that guided the development of the inferred from the gravity data. This late Cretaceous fault is proba- late Cretaceous normal faults (Barnes 1994), which then in turn bly dipping at about 60° to the south, whereas the largely dextral helped the nucleation of the recent faults. Greendale Fault is subvertical with a south-side-up component of South of Christchurch marks an area where the recent strike-slip movement (Quigley et al. 2011). Similar cases where the strike of deformation is changing into oblique thrusting along ENE–WSW- a new fault has been inherited from older structures but the dip has striking faults and the new Port Hills Fault developed in the late not have been reported by Ring (1994) from the East African Rift. Pleistocene gravel along the northern interface of the volcanic According to Ring (1994), the new fault needs a nucleation point, rocks and gravel. We mapped similarly ENE–WSW-striking faults which is provided by the pre-existing structure. Above and below in and near Lyttelton Harbour (Fig. 13) and argued above that they the pre-existing structure are numerous fracture surfaces that are either are rotated east–west faults or formed when the Chatham subparallel to the pre-existing fault and also subvertical extension Rise encroached. This reasoning would again suggest that older fractures. After having nucleated along a pre-existing structure the faults may have guided the recent faults and we propose a similar Downloaded from http://jgs.lyellcollection.org/ at Stockholm University on February 11, 2015 784 U. RING & S. HAMPToN

Shortening and extension components of recent faults Fig. 14. Local setting of the Christchurch earthquake sequence. Surface rupture along the Greendale Fault is from Shortening and extension Quigley et al. (2011) and other faults components of late Miocene faults with inferred kinematic axes are from 22 Feb 2011 23 Dec 2011 Sibson et al. (2012). Also shown are Darfield Port Hills Fault main faults in Banks Peninsula with 13 Jun 2011 4 Sep 2010 * kinematic axes inferred from the data * Godley Head- presented in Figures 4–12. Because the * * Port Levy Fault Greendale Fault method used for constructing principal shortening and extension axes for a given population of faults is similar to that used Greendale to determine infinitesimal strain directions for earthquake focal mechanisms (P- and

Norwood-Doyleston- Ellesmere lineament T-axes, respectively), the kinematic axes can be compared. Fault pattern and deduced kinematic axes in Banks Peninsula do not differ significantly from those for Earthquake epicenter the recent faults. The basement in shown * with date of main shock in dark grey and the Tertiary sediments in light grey. nucleation mechanism to that envisaged for the Greendale Fault. Canterbury earthquakes. Rupturing that caused the earthquakes did We further speculate that the upward propagation of the Port Hills not occur parallel to pre-existing faults but the latter rather served Fault was, at least in part, controlled by the contact between the as nucleation points for the recent faults. In addition to the east– volcanic rocks and the basement and late Cretaceous–Tertiary sed- west-striking faults, ENE–WSW-striking faults were important for iments into which the volcanic rocks intruded. earthquake generation. The ENE–WSW-striking faults also occur The sinistral Godley Head–Port Levy Fault also seems to have in the Lyttelton Harbour area and are considered to be either rotated reactivated older structures as it is dipping at 70° and not vertical late Cretaceous east–west faults or faults that formed in the late (R. Sibson, pers. comm.). However, these NW–SE-striking accom- Miocene when the encroaching Chatham Rise interacted with the modation structures do not run parallel to any obvious pre-existing developing plate boundary zone on the South and North Island of structural grain, so further speculations on possible reactivation New Zealand. mechanisms are not warranted. The loading of the about 1000 km3 volcanic edifice of Banks We thank two anonymous reviewers and editor R. Phillips for helpful Peninsula elevates the vertical stress and therefore increases the comments, K. Furlong for providing the sketch showing the relative plate strength on any non-vertical fault. This might be the reason why the motions, and F. Ghisetti and R. Sibson for discussions. Port Hills Fault slipped at the northern periphery of the volcanic edifice, where the latter is covered by gravel and its thickness is signifi- References cantly reduced. We also note that most of the reverse slip on the Port Aydin, A. & nur, A. 1982. Evolution of pull-apart basins and their scale inde- Hills Fault occurred near the surface and along the eastern segment pendence. Tectonics, 1, 91–105. of the fault (Beavan et al., 2011), where the vertical stress was less. BArnes, P.M. 1994. Continental extension of the Pacific Plate at the southern termination of the Hikurangi subduction zone: the North Mernoo Fault Zone, Conclusions offshore New Zealand. Tectonics, 13, 753–754. BeAvAn, J. 2011. Preliminary report on the location and slip distribution of the We have shown that the eruption of the early volcanic rocks fault that caused the February 2011 Christchurch earthquake, as derived from geodetic data. GNS Science, Lower Hutt, internal report. (Lyttelton Volcanic Group) on Banks Peninsula was controlled by BeAvAn, J., sAmsonov, S., motAgh, M., WAllAce, L., ellis, S. & PAlmer, N.

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Received 3 January 2012; revised typescript accepted 25 June 2012. Scientific editing by Richard Phillips.