Tectonophysics 509 (2011) 165–180

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Large tectonic rotations in a wide zone of Neogene distributed dextral shear,northeastern South Island, New Zealand

Karen Randall 1, Simon Lamb ⁎, Conall Mac NiocaillDepartment of Earth Sciences, South Parks Road, Oxford, UK

⁎ Corresponding author at: Victoria University oWellington, New Zealand. Tel.: +64 4 472 1000.

E-mail address: Simon.Lamb@vuw.ac.nz (S. Lamb).1 Present address: Neftex, Abingdon, UK.

0040-1951/$ – see front matter © 2011 Elsevier B.V. Aldoi:10.1016/j.tecto.2011.05.006

a b s t r a c t

a r t i c l e i n f o

Article history:Received 17 December 2010Received in revised form 17 May 2011Accepted 19 May 2011Available online 27 May 2011

Keywords:Tectonic rotationsPaleomagnetismContinental dextral shearSubductionNew Zealand plate-boundary zoneCenozoic deformation

The New Zealand plate-boundary zone in the northern part of South Island, New Zealand, comprises a series ofactive dextral strike-slip faults, referred to as the Marlborough Fault Zone (MFZ), which link obliquecontinental convergence farther south, along the Alpine Fault, to subduction of the Pacific plate along theHikurangi margin. New paleomagnetic data are presented which, together with previous studies, suggestcoherent Neogene clockwise rotation up to ~130° about a vertical axis of crustal blocks in the MFZ, starting at~20 Ma. There is a striking correlation between the amount of observed rotation and the trend of Mesozoicbasement structures, creating this part of the New Zealand orocline in a zone of distributed dextral shear,about 100 km wide, which has persisted for about 20 Ma. In the southern part of the MFZ, crustal blocksappear to be on a scale of 1–10 km, with boundaries that are subparallel to the basement strike. This way,deformation is accommodated by a combination of fault slip and rotation in a zone of more pervasive dextralshear, resulting in coherent bending of the basement structure. In the central part of the MFZ, this rotationappears to be that of elongate crustal blocks, ~50 km×10 km in size, rotating ~80° clockwise at an averagerotation rate ~4°/Ma. In detail, the rotation rate has decreased towards the present, from ~6°/Ma at ~20 Ma tob2/Ma today. The overall effect of all this deformation is a ‘straightening-out’ of the major faults, so that thenorthern segments have evolved from ~NW trending thrusts in the early Miocene to ~NE dextral strike-slipfaults today. A zone of higher shear strain farther north has resulted in an additional clockwise rotation 40–50°, with a total of ~130° clockwise rotation. The combination of rotation and fault displacement has acted as ahinge at the southern end of the Hikurangi margin, accommodating a drastic swing in trend from ~NW in theearly–middle Miocene to ~NE trend.

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© 2011 Elsevier B.V. All rights reserved.

1. Introduction

Relative motion across plate-boundary zones through continentallithosphere results in displacement, strain and rotation in a broadzone (Lamb 1987, 1994, McKenzie and Jackson, 1983), up to severalthousand kilometers wide, which can lead to profound and rapid re-orientation of earlier-formed geological features. It is only possible todetermine the tectonic evolution by reconstructing the deformingzone back through time, but this remains difficult in most plate-boundary zones, because it requires a detailed knowledge of allcomponents of the deformation.

New Zealand, which straddles the obliquely convergent boundarybetween the Australian and Pacific plates (Fig. 1a, DeMets et al., 1994),is probably unique in this respect. There are both well-constrainedNeogene plate reconstructions of this region based on sea floormagnetic anomalies, and a wealth of paleomagnetic evidence for

rotations about a vertical axis of crustal blocks. In addition, the entirewidth of the plate-boundary zone is exposed on-land in a zone~250 km wide, and prominent geological and bathymetric featurescan be matched and traced right across the deforming zone. Thesedata require significant displacement and re-orientation of the plate-boundary zone, so that virtually the entire Hikurangi subduction zone,extending for over 500 km along strike, swung round from a ~NWtrend in the early Miocene to its present ~NE trend (Cande and Stock,2004, King, 2000, Walcott, 1984, 1987, 1989, Walcott et al., 1981).Understanding the detailed kinematics of deformation structures thathave accommodated this drastic and rapid re-orientation providesinsights into the more general behavior of continental lithosphereadjacent to subduction zones.

An important clue to the re-orientation of the New Zealand plate-boundary zone is an understanding of how displacement, such as thewell-documented ~450 km Cenozoic dextral displacement along theAlpine Fault (Fig. 1a, Sutherland, 1999), is accommodated farthernorth as both displacement and rotation. At the northern end ofSouth Island, motion along the Alpine Fault is transferred to obliquesubduction along the Hikurangi margin by displacement in theMarlborough Fault Zone (Bibby, 1981, Walcott, 1998). This region is

-2000 m

170oE 180oE

170oE 180oE

45oS45oS

35oS35oS

37 mm/yr

Pacific Plate

Australian Plate

Esk Headsubterrane

StokesMagneticAnomaly H

ikur

angi

Tre

nch

Alpine

Fault MFZ

(a)

(b)

Fig. 2

Structural grain

Subduction deformation front

37 mm/yrPAC-AUS

P.Mag Locality > 17 Ma

P.Mag Locality < 8 Ma

BIR HIN

Eskhead Terrane

Wairau F.

Clarence F.

Hope F.

KF

SMD

NMD

End of trench

Basement~45° kink axis

Edge of subductedslab

Awatere F.

Fig. 1. (a) Map showing the boundary between the obliquely converging (37 mm/yr, DeMets et al., 1994) Pacific and Australian plates running through the New Zealand region. Inthe north, oceanic Pacific plate is being subducted beneath Australian plate along the Hikurangi subduction zone. Farther south, at the southern end of the subduction zone, obliqueconvergence is transferred through the dextral strike-slip Marlborough Fault Zone (MFZ) to a zone of oblique continental collision along the Alpine Fault. The orientation ofprominent geological features such as the Stokes Magnetic Anomaly (outcropping as the Dun Mountain Ophiolite belt, Hunt, 1978) and Eskhead subterrane define the New ZealandOrocline. Box defines area in (b). (b) More detailed map in the vicinity of the MFZ, showing the major faults, both onshore and offshore. Also shown are the locations ofpaleomagnetic sample sites in this and previous studies, the outcrop of the Eskhead terrane, and the boundary (heavy dashed line) between the southern (SMD) and northernMarlborough (NMD) domains (Lamb, 1988), defined by the marked change in trend of the Marlborough Faults (see also Fig. 2). KF: Kekerengu F.

166 K. Randall et al. / Tectonophysics 509 (2011) 165–180

essentially contiguous with the stable Pacific Plate, effectively actingas a hinge to the rotating Hikurangi margin in North Island (Lamb,1988, Lamb and Bibby, 1989, Vickery and Lamb, 1995). Therefore,the nature and timing of deformation here place constraints on thetiming and accommodation of rotation of the whole margin. Inparticular, the tectonic evolution of this region reveals the behaviorof continental crust in a transform zone that cuts through bothcontinental lithosphere and where the crust is underlain by thesubducted plate.

1.1. Marlborough Fault Zone (MFZ)

The Marlborough Fault Zone (MFZ) comprises five major andactive dextral strike-slip faults dipping 60°NW to vertical (Figs. 1b, 2,Wairau, Awatere, Clarence, Kekerengu and Hope Faults). A markedchange in trend of the faults from ~070° in the south to ~055° farthernorth defines the boundary between the southern and northernMarlborough domains (Lamb, 1988), and the crust in these domainspartly rests on the seismically active subducted slab (Fig. 1b).

Estimates of Late Quaternary slip rates vary from 20 to 30 mm/yr ontheHope Fault (Cowan, 1990; VanDissen andYeats, 1991) to 4–6 mm/yron the Wairau Fault (Berryman et al., 1992), with a cumulative slip rateclose to the current relative plate motion (Bourne et al., 1998; Holt andHaines, 1995, Wallace et al., 2007). The finite offsets on individual faults(except theWairau) are less than ~35 km (Little and Jones, 1998), with aprobable cumulative offset of ~60 km (Reay, 1993;Wood et al., 1994). Attheir present rates ofmotion, the strike-slip faults in theMFZ could easily

have achieved their finite offsets in much less than 5 Myrs. However,dextral shear, especially in the southern Marlborough domain, may alsobe accommodated by rotation of crustal blocks between the faults. Inaddition, there is evidence for widespread folding and thrusting in thenorthern Marlborough domain, which is most plausibly related to Earlyto Middle Miocene subduction along the Hikurangi margin (Lamb andBibby, 1989; Rait et al., 1990).

The stratigraphy of the region consists of Mesozoic basement rock,referred to as the Torlesse terrane or supergroup (MacKinnon, 1983),including the Eskhead subterrane,which is unconformably overlain by awell-dated Late Cretaceous to recent cover sequence, outcropping in aregion extending from south of the Wairau Fault to south of the HopeFault (Fig. 2, Reay, 1993). The cover sequence consists locally of basalterrestrial volcanics (Lookout volcanics and Gridiron Formation), whichpass up into fine-grained and deep water marine sediments, includingthe Paleocene–Eocene Amuri limestone. The sediments becomemarkedly coarse-grained in the Early Miocene (~20–16 Ma) when theGreat Marlborough Conglomerate was deposited in an unstable near-shelf submarine fan (Lewis et al., 1980), containing reworked clasts ofolder parts of the cover sequence. This most likely marks the onset ofsubduction in the Marlborough region in Early Miocene times (Lamband Bibby, 1989; Rait et al., 1990). Above the Great MarlboroughConglomerate, there is a sequence of Late Miocene to Pliocene deepwater marine sandstones, siltstones and mudstones (Medway, Uptonand younger formations and equivalents), which become shallowerhigher up in the section, and are eventually overlain by fluvial andlacustrine facies.

SV

BB2

CC

SS

*

KAI

*

* Hope F

(* this study)

BOU

BIG

WHP

SEY

LYF

LOT

*

*

*

*

DEE*CAV

NorthernMarlboroughDomain

SouthernMarlboroughDomain

Average basement structural trend,reference azimuth = 290 - 310˚(Hall et al. 2004) - See Fig. 9

Mio

cene

Low

erTe

rtia

ryU

pper

Cre

tace

ous

AmuriLimestone

Shale

MesozoicBasement

Alluvium

Conglomerate

Siltstone

Siltst./Mudst.

Tectonic rotation

Assumingprimary

magnetisation

HC2HC3

DS

Hinge of Little & Roberts (1997)

174˚E173˚E

42˚S

42.5˚S42.5˚S

42˚S

174˚E173˚E

* Paleomagnetic samples in this study

*

*

*

*

Volcanics

VolcanicsMa

10

20

30

40

50

60

70

80

90

100

Cenozoic stratigraphy

Pliocene

3.3

418

17

60

30

8

3.9

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4.2

9

~6

~6

95

5.4

54

~50

66

54

30

30

95

18

20

WH 60WC3

WC1-2

9

(with age in Ma)

Wairau F.

4.2

Awatere F.

Clarence F.

Fig. 3e

Fig. 3b

BR

RB

UB

FC

WB

NC

WD1WD2CS

PUH

CLA

SW

MOT

Fig. 3cFig. 3d

50 km

WV

BS

290˚

310˚

Keke

reng

u F.

Fig. 3a

U

Fig. 2. Detailed map of the Marlborough Fault Zone showing the general stratigraphy (see text—gray shade on map represents cover sequences, white is basement), basementstructural trends (thin dashed lines), and paleomagnetic localities (with ages in Ma) with results in this study (WHP, BIG, STR, BOU, CAV, DEE, SEY, LYF, LOT) and previous studies(summarized in Lamb, this volume). Note the prominent swing in the basement structural trend (after Hall et al., 2004) and hinge of Little and Roberts (1997), defining part of theNew Zealand Orocline. Boxes show location of more detailed locality maps in Fig. 3.

167K. Randall et al. / Tectonophysics 509 (2011) 165–180

1.2. Tectonic rotations

Numerous paleomagnetic studies of Cenozoic sedimentary rocks andvolcanics have documented clockwise rotations of crustal blocks up to~130° in the Marlborough Fault Zone and adjacent regions (Figs. 1b, 2,Walcott et al., 1981, Mumme andWalcott, 1985, Roberts, 1992, Vickeryand Lamb, 1995, Little and Roberts, 1997, Townsend, 2001, Hall et al.,2004). However, the distribution, timing, and significance of this rotationremain controversial, mainly because sampling localities are largelyconfined to the more easily accessible coastal regions, so that rotationfarther inlandhas been inferred froman extrapolation of the coastal data.Thus, Hall et al. (2004) argued that this rotation since the early Miocenereflects the formationof part of theNewZealandOrocline,with the large-scale bending of Paleozoic–Mesozoic basement terranes (Fig. 1a, b).However, Bradshaw et al. (1996) considered that this bending was aMesozoic feature, and Little and Roberts (1997) and Rowan and Roberts(2008) suggested that the paleomagnetically determined rotations hereare LateMioceneandyounger, andonlyof local significance, applying to asmall region at the northern end of the Marlborough Fault Zone.

In this study, we present new paleomagnetic data for the inlandportion of the Marlborough Fault Zone. This makes it possible for the

first time to show that Cenozoic relative plate motion has resulted in acoherent clockwise rotation of this part of the plate-boundary zone by~80°. This way, early deformation structures have undergone a majorre-orientation, as first suggested by Lamb (1988) and Lamb and Bibby(1989), evolving from mainly thrust faults in the Early to MiddleMiocene, to predominantly strike-slip faults today.

2. Paleomagnetic analysis

Twenty-seven localities in all were sampled, from which 1534specimens were prepared and demagnetized, but only 15 of theselocalities yielded stable remanencemagnetizations (Fig. 1b). Six of theselocalities had been sampled in previous studies (Hall et al., 2004, Lamb,1988,MummeandWalcott 1985, Roberts 1992, Vickery and Lamb1995),providing a test of paleomagnetic reproducibility in these lithologies.

The main sampling strategy was to improve the distribution ofsample sites, both in space and time, targeting lithologies that previousexperience has shown to preserve primary remanence magnetizations.At each locality, a sufficient time span (≫10,000 years) was sampled inorder to average the effects of secular variation, with a minimum of 12samples (13–73 specimens in total) at each locality, and the sampling

168 K. Randall et al. / Tectonophysics 509 (2011) 165–180

procedure and analysis closely follow that reported in Vickery and Lamb(1995), with measurements made on a 2-G 3-axis cryogenic magne-tometer in amagnetically shielded room. Best fit magnetic componentsduring demagnetization were determined from linear segments on aZijderveld diagram, where the horizontal and vertical projections ofmagnetization at each stage (thermal or AF diamagnetisation) areplotted. A variety of magnetic tests were used to determine magneticcarriers and the nature of the magnetic remanence. Details of thelocalities in this and previous studies that yielded stable remanencemagnetizations are shown in Table 1 and Figs. 1b, 2, 3a, 4, 5.

2.1. Late Cretaceous volcanics

2.1.1. Geological settingA sequence of Late Cretaceous basaltic lavas (Lookout Volcanics),

100 s ofmeters thick, together with a regionally extensive basaltic dykeswarm, were sampled (including, in some cases, the baked margins).These outcrop in the middle Awatere Valley (Figs. 2, 3a, localities WIN,UPC, MID), and farther SE, in the Clarence Valley (locality SEY, GridironFormation). The Lookout volcanics overlie Mesozoic basement withangular unconformity (this study, Kundycki, 1996), but are themselvesfolded into a regional syncline, extending horizontally for ~10 km, withlimbs dipping up to 40° (Fig. 3a). K/Ar dates for the Lookout volcanicsand Gridiron Formation show that they are essentially contemporane-ous, with ages ranging between90.3 Ma and 99.9 Ma (Baker et al., 1994,Reay, 1993), whereas K/Ar whole rock ages for the dykes show a wideage range between 100 and 60 Ma (Grapes et al., 1992). However, thesimilar geochemistry of the dykes and lavas suggests that they are co-genetic (Baker et al., 1994), and field evidence shows that the dykesterminate and feed the overlying thick sequence of lavas, and so the

Table 1Paleomagnetic data in this study, mainly from the Marlborough Fault Zone, South Island N

Locality Age Bedding1 n/N2 Strat.3 Iex4 Dis

96–30 Ma:WIN 95±5 225/28 72/14 dyke −74±3 002UPC 225/28 111/21 v −74±3 331MID 029/43 42/1 v −74±3 126CAV (WIN+UPC+MID) /36 v −74±3 342SEY1 96±3 231/35 80/8 v −74±3 342SEY2 232/44 47/3 v −74±3 349SEY (SEY1+SEY2) /11 v −74±3 343DEE 54–53 224/50 /20 30 m −62±4 001LOT 30±4 210/69 86/ v −58±5 195LYF 30±4 230/31 54/ v −58±5 005N. Marlborough9 17–95 /12 099Amuri Lst9 50–65 /7 052

9–2.5 Ma:BIG 9±2 228/55 /38 80 m −61±2 017STR 9±2 214/47 /12 15 m −61±2 014BIG (BIG+STR)10 “ /50 95 m −61±2 016BIG** (BIG+STR)11 9±1 220/20 /50 “ −61±2 016BOU 9±2 233/50 /18 70 m −61±2 009BS (Roberts, 1992; = BOU) /14 88 m −61±2 353WHP12 6±2 Ma 245/15 /59 20 m −61±2 0326

WV12 (Roberts, 1992; = WHP) 6±2 Ma /39 100 m −61±2 0066

North Island localities:HIN 9±2 250/16 /24 20 m −61±2 023HIN (Walcott et al. 1981) /20 ? −61±2 015BIR 2.5±0.5 279/22 /44 20 m −62±2 007BIR (Lamb, 1988) ? ? −62±2 000

Notes: (1) mean bedding for locality (dip clockwise of strike); (2) number of specimens (n)sequence, v= volcanics; (4) expected inclination based on Pacific plate polar wander path (Can100% unfolding (Tauxe andWatson 1994); (6) other tests for primarymagnetization (R= reve(7) tectonic rotation relative to the Pacific Plate (ΔR) andflattening (ΔF) relative to expected angiven its age and location(seenote4); (8) average rotation rateassumingall rotationyounger thAmuri Lst in northernMarlborough domain; (10) BIG and STR treated as single locality; (11) asbase of ~9 Ma Upton Formation; (12) mainly normal polarities for WHP, but mainly reversed

younger ages obtained by Grapes et al. (1992) are most likely alterationages. Furthermore, themeanorientation of the dykes in the study area is109/78S, but is rotated to vertical (287/89 N) after rotation about themean strike of the overlying volcanics (225/28 N). For these reasons, weassume that all the dykes are coeval with the nearby lavas.

These localities potentially provide an important constraint on theCenozoic rotation in the interior parts of the Marlborough Fault Zone.The dykes were previously sampled by Vickery and Lamb (1995), buttheir results indicated markedly different tectonic rotation for closelyspaced localities (22±15° and 106±63° of clockwise rotationrespectively). The most likely explanation for this is that their tiltcorrections were poorly constrained.

2.1.2. DemagnetizationNatural remanence magnetization (NRM) intensities are variable,

ranging from 25 to 2650 mA/m. Both thermal and AF demagnetizationtechniques were used, in most cases yielding very clear demagnetiza-tion paths, with up to two components. The high temperature orcoercivity remanence component for all samples has a normal polarity,consistent with the age of eruption during the Late Cretaceous quietperiod. Coercivity and blocking temperature spectra suggest ti-magnetite as a primary carrier, often as multi-domains, with hematiteas a secondary carrier. Figs. 4 and 5 show typical Zijderfeld demagne-tization diagrams and equal area stereographic plots.

2.2. Paleocene–Eocene micritic limestones

2.2.1. Geological settingPaleocene to Eocene micritic limestones outcrop throughout the

coastal parts of the Marlborough region, as part of the cover sequence

ew Zealand.

Iis Dtc Itc α95 k Foldtest5

OtherTests6

ΔR7 ΔF7 ω8

−72 093 −70 6.2 41.7 M, bm 88±22 4±5−64 065 −83 3.4 88 M 60±28 −9±3−54 268 −82 2.7 65.6 M −97±23 −8±3−69 083 −79 3.8 41.4 Yes 78±22 −5±4 4.5°/Ma−41 013 −71 7.7 53.3 M 8±25 3±7−47 066 −72 20.7 36.5 M 61±? 2±17−42 026 −72 7.7 36.3 21±26 2±7 1.2°/Ma−50 076 −56 7.8 18.6 R, M 88±14 6±7 5.2°/Ma

30 219 62 5.1 9.9 Yes 51±10* −4±6 3.0°/Ma−40 042 −57 4.4 20.6 54±11 1±6 3.1°/Ma−38 117 −64 7.3 36.36 Yes 129 ± 7.6°/Ma−55 109 −68 11.1 30.3 Yes 121 ± 7.1°/Ma

−40 078 −46 6.9 13.4 R 82±9 15±6 9.1°/Ma−54 073 −45 8.4 28 77±10 16±7 8.6°/Ma−44 076 −46 5.6 14.1 R 80±8 15±5 8.9°/Ma−44 037 −49 5.6 14 41±8 12±5 4.6°/Ma−21 030 −57 14 7 34±21 4±11 3.8°/Ma−29 017 −63 7.5 29.4 21±13 −2±6 2.2°/Ma−61 062 −66 4 22.4 R 66±9 −5±4 11°/Ma−49 019 −61 2.6 78.5 23±6 0±3 3.8°/Ma

−53 045 −63 4.9 37.8 32±10 −2±5 3.6°/Ma−49 030 −63 4.7 48.4 17±10 2±5−40 005 −62 5.7 15.5 0±1113 0±6 2°/Ma−30 356 −52 3.8 ? −9±713 10±5

and epochs, flows or localities (N); (3) stratigraphic thickness of sampled sedimentaryde et al. 1995, 2000, DiVenere et al. 1994); (5) yes if it showsmost improved clustering forrsal test, M=consistentmagnetostratigraphy for age, bm=bakedmargin test for dykes);d observeddeclination and inclination ofmagnetization (with errors after Demarest 1983),an17 Ma; (9) all localities innorthernMarlboroughdomainwithΔRN60°, or all localities insuming remagnetization for BIG and STR acquired at time of formation of unconformity atat WV; (13) rotation relative to Australian Plate.

57

62

DEE

75

56

2937

40 32

42

26 42

74

28

6254

3875

3830

4040

69

56

20

52

40

50

Clarence Fault173° 44’

42° 00’

173° 03’

42° 31’Hope Fault

1 km

1 km

Amuri Lst

Split Rock Fm

Torlesse

Mesozoic Basement

Weka Pass Stone

CooksonVolcanics

Greta Fm

CooksonVolcanics

Waima Fm

Torlesse

1 km

30

3047

2335

20

N173° 11’

42° 30’

173° 30’

42° 00’

2 km

Awatere Fault

MID

WIN

UPC

Lookout Volcanics

Winterton Fm

Fold axis = 6/211

(a) (b)

(c) (d)

LYF

LOT

CAV

DEE

FuchsiaC

k

AwatereR iver

AwatereR iver

UptonBrook

Ric

hmon

dB

rook

Blind Rive

r

Med

way

Riv

er

Big

Gully

Boundary

Stream

Colonel G

ullyStream

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34

5620

29

47

1435

30

2818

20

22

3630

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<6 Ma

6 - 9 Ma

9 - 11 Ma

Mesozoic

Upton Fm

Medway Fm

Marine sediments

~8 Ma - 0 Hinge

20 - ~8 Ma

Hinge

Rotation boundary

(Little & Roberts 1997)

Paleomagnetic locality

2 km

Awatere Fault

Local relativeanticlockwise rotation in fault sliver?

Flaxbourne F.

Haldon F.

FC

BOU/BS

BIG

STR

BR2

173° 50’ 174° 00’

41° 45’

41° 50’

UB

Basement trends

Bas

emen

ttre

nds

Fig. 3.Detailed geological maps of themain paleomagnetic localities in this study. (a) Late Cretaceous basaltic lavas and dykes (Lookout volcanics) which unconformably overlie bothsandstones and siltstones of the Late Cretaceous Winterton Formation, and deformed Mesozoic Torlesse basement. (b) Cretaceous–Paleocene micritic limestones (Amurilimestones) outcropping along the footwall of the Clarence Fault. These sequences lie on the western limb of a regional anticline at the northern ends of the Clarence and KekerenguFaults. (c) and (d) Oligocene siltstones, sandstones and basaltic volcanics (Cookson Volcanics), forming part of dipping sequences extending up to 10 km south of the Hope Fault.(e) Late Miocenemarine mudstones, siltstones and sandstones outcropping at the northern ends of the Awatere and Clarence Faults, in theMedway Valley and lower Awatere Valley(after Maxwell, 1990), resting unconformably on folded Torlesse basement. Note the angular unconformity at the base of the 6–9 Ma Upton Formation, and the location of the hingesto a prominent kink in the subvertical basement structure.

169K. Randall et al. / Tectonophysics 509 (2011) 165–180

overlying Mesozoic basement, and these are well dated frommicrofau-nal assemblages (Hancock et al., 2003, Reay, 1993). They are tightlyfolded as part of a regional NE-trending and plunging anticline. The

anticline extends ~50 km along strike at the northern ends of theClarence and Kekerengu faults (Fig. 2), with a half-wavelength of~20 km and dips ranging between 30 and 90°. In the fold hinge, the

2 6

6

10

Magnetisation

350°C 630°C680°C

680°C

630°C

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LOT

200 100

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200

Magnetisation(mA/m)

YF2-AH2

LYF

400800

400

Magnetisation(mA/m)

800GLA-Z1

WIN

42

2

4

Magnetisation

100°C

300°C

400°C

UPC

10

270, UP

090, Down

3601809mT

3mT

25mT

9mT3mT

(A/m)

LOT-AC1

9mT

3mT

20mT

UPC-A3-2

(A/m)

270, UP

090, Down

360180

270, UP

090, Down

360180

270, UP

090, Down

360180

Fig. 4. Typical Zijderfeld plots for horizontal (filled circles) and vertical (open circles)projections of magnetization during thermal or Alternating Frequency (AF) demagne-tization for ~95 Ma (WIN, UPC, Lookout volcanics) and ~30 Ma basalts (LOT, LYFCookson volcanics). All samples show high NRM intensities, typical of these volcanics(N0.2 Am−1). See Figs. 2, and 3a, c and d for locations. A low temperature or coercivityunblocking component is observed up to ~350 °C or 9 mT, with most of thedemagnetization occurring in a higher temperature or coercivity unblocking compo-nent, up to 680 °C or 25 mT, with more-or-less straight line trajectories to origin. Thissuggests ti-magnetite±hematite as the main magnetic carriers.

170 K. Randall et al. / Tectonophysics 509 (2011) 165–180

limestones are repeated by a series of low angle thrusts, which are alsofolded by the regional anticline (Prebble, 1976, Lamb and Bibby, 1989,Townsend, 2001). The SE limb of this anticline has been extensivelysampled where the limestones have a distinct pink color (Townsend,2001, Vickery and Lamb, 1995). In this study, we sampled theNW-limb,outcropping along the footwall of the Clarence fault (Fig. 3b), in thesame section as a detailed biostratigraphy dated to 54–53 Ma (Hancocket al., 2003)—this age is supported by the magneto-stratigraphyobtained in this study (Fig. 5a). Thus, this study in combination withprevious work provides the basis for a rigorous fold test for severallocalities between the Clarence and Kekerengu faults (Table 1, seeSection 3.2).

2.2.2. DemagnetizationNRM intensities are very low (0.02–0.23 mA/m), typical of these

micritic limestones from previous studies, with very noisy demagneti-zation trajectories, especially during thermal demagnetization—howeverdemagnetization vectors could be defined for most samples with AFdemagnetization, usually with two distinct components. The highcoercivity component has both normal and reverse polarity, consistentwith themagneto-stratigraphy for this time period. IRM studies indicatetitanomagnetite as a carrier, but there seems to be no relation betweenthe strength of NRM—and hence amount of titanomagnetite—and astable magnetization. Figs. 5 and 6 show relevant Zijderveld demagne-tization diagrams and equal area stereographic plots.

2.3. Oligocene volcanics

2.3.1. Geological settingA sequence of Oligocene basaltic lavas and tuffs (Cookson volcanics)

outcrop throughout the Marlborough region, generally sandwichedbetween micritic limestones and smectitic shales (Reay, 1993). These

were sampled within 10 km south of the Hope Fault at two localities(Figs. 2, 3c and d, localities LOT, LYF),within folded sequenceswhichdip30–80° and extend for ~10 km along strike and several kilometersacross strike (Figs. 2, 3c and d). Ages in the range 30±4 Ma for thesevolcanics is given by stratigraphic ages of the sediments immediatelyabove and below (Reay, 1993, Warren, 1995), as well as a K/Ar age of27.6±1.1 for a volcanic plug outcropping in the central Clarence Valley(Reay, 1993,Warren, 1995). The lavas in the Lottery River (locality LOT)were sampled on both limbs of a tight anticline, with a half wavelengthof ~1 km (Fig. 3d).

2.3.2. DemagnetizationBoth AF and thermal demagnetization revealed up to three com-

ponents of remanence magnetization, with the highest temperatureor coercivity component carrying most of the NRM. The bulk ofdemagnetization occurs in the range 15–50 mT or 300–600 °C,indicating magnetite as the principal remanence carrier, but somemagnetization remained at temperatures N645 °C, indicating somehematite. Fig. 4 shows relevant Zijderveld demagnetization diagrams.

2.4. Miocene siltstones and mudstones

2.4.1. Geological settingLate Miocene–Pliocene marine conglomerates, sandstones, mud-

stones and siltstones, referred to as the Awatere Group, outcrop in thecoastal regions and as fault-bound tilted blocks in the northern part ofthe MFZ, and have been the focus of much previous work (Figs. 1b,2, 3e, Roberts, 1992, Little and Roberts, 1997). In this study, sequencesclosest to the interior of the MFZ were targeted, outcropping betweenthe Wairau, Awatere and Clarence Faults, and comprising the deepwater Medway Formation and equivalents (Figs. 2, 3e, localities STR,BIG, BOU—equivalent to BS of Roberts, 1992). These sediments areassigned to the New Zealand lower Tongaporutuan stage based onmicrofossil dating (9.5±1.5 Ma, Maxwell, 1990, Roberts, 1992). TheMedway Formation is folded, with dips up to ~50° NW, and cut byreverse faults (Fig. 3e, Maxwell, 1990). It is overlain with angularunconformity by the more gently dipping (~20° NW) upperTongaporutuan (7±1.5 Ma) Upton Formation (Fig. 3e). YoungerMio-Pliocene fresh water mudstones were sampled in the WairauValley (locality WHP—equivalent to WV of Roberts, 1992), with apoorly determined age that is most likely similar to that of terrestrialsequences overlying the Upton Formation (6±2 Ma).

Two localities of gently dipping mudstones and siltstones, withLate Miocene and Late Pliocene ages respectively (Fig. 1b, locality HIN,Bells Creek Formation, 9±2 Ma, and locality BIR, MangaopariFormation, 2.5±0.5 Ma, after Vella and Briggs, 1971, Vella and Collen,1998), were also sampled in the southern part of the North Island—microfossils suggest a deep marine environment for both theseformations (Vella and Briggs, 1971).

2.4.2. DemagnetizationAll specimens have very weak magnetizations (Fig. 6b, NRM

b0.6 mAm−1), typical of these lithologies along the east coast of theNorth Island (Rowan and Roberts, 2008), with both normal and reversepolarity secondary remagnetizations carried by low temperature, buthigh coercivity remanence minerals such as greigite. Identification ofany remagnetization is essential in order to identify primary magneti-zations with confidence. Fig. 7 shows equal area stereographic plots forhigh coercivity or temperature unblocking components.

Framboidal pyrite was detected in high resolution SEM images formudstones at all localities except WHP. Very fine-grained sulfidealteration at the margins of the pyrite has compositions similar togreigite. For this reason, greigite remagnetization was suspected at alllocalities. High temperature susceptibility analysis showed that thermalalteration of thesemudstones occurs above ~400 °C. Therefore, in orderto remove themagnetic effects of greigite, samples were first subjected

DEEN

CAVNN

(a) (b)

In situ In situTilt corrected

normal polarityDyke and flowmean vectors

WINUPCMID

Tilt corrected

Tilt corrected

Paleogene Amuri Limestone

Cretaceous to Early Miocene Localities

97 - 118% unfolding

% unfolding

τ 1 p

rinci

pal c

ompo

nent

103 - 112% unfolding(c) (d)1.0

0.8

0.6

0.4

0.2

0.0

τ1 p

rinci

pal c

ompo

nent

1.0

0.8

0.6

0.4

0.2

0.0-20 0 20 40 60 80 100 120 140 160

% unfolding-20 0 20 40 60 80 100 120 140 160

Fig. 5. Equal area lowerhemisphere stereographic plots ofhigh temperatureor highcoercivity remanencemagnetization components (in situand after tectonic tilt correction). (a) Plots forbasaltic lavaflows (localitiesUPCandMID) anddykes fromthe~95 MaLookoutVolcanics in theCentral AwatereValley (locality CAV inFig. 2), showing specimens andflowordykemeans;allmagnetizationshavenormal polarity, consistentwith theCretaceous quiet period. (b) Plots for ~53 Mamicritic Amuri limestone in theDee River, adjacent to theClarence Fault (localityDEE in Figs. 2, 3b); both normal and reversed components are present, passing a reversal test and consistentwith themagnetostratigraphy for this period (Fig. 6a). (c) and (d) Fold tests forgroupings of localities from the Marlborough Domains, providing strong evidence that the magnetizations are primary, pre-dating folding and acquired when these sediments were laiddown.A groupingof all localities from theMarlboroughDomain (17–95 Ma) that showrotationsN80° havemaximumclustering for 100%unfolding,with a ~80% increase in themaximum(τ1) principal component compared to in situ co-ordinates. Paleogene localities in Amuri Limestone in the northern Marlborough domain, where it is deformed by tight km-scale folds,show maximum (τ1) clustering for 100% unfolding, with a ~50% increase in the maximum principal component compared to in situ co-ordinates.

171K. Randall et al. / Tectonophysics 509 (2011) 165–180

to progressive thermal demagnetization up to 350 °C, followed byprogressive AF demagnetization. In many cases, this isolated stableremanence magnetization during AF treatment (Figs. 6b, 7). At nolocalities were a set of low temperature remanence magnetisationsdiscovered with coherent directions that were substantially differentfrom those carried by high temperature phases. Magnetizations carriedby greigite are therefore either randomly oriented, or consistent withthose carried by magnetite/hematite, implying acquisition at the sametime. FirstOrder Reversal Curveanalysis (FORCanalysis, Pikeet al., 1999,Roberts et al., 2000) shows little sensitivity to demagnetizationbehavior, with very similar distributions obtained from almost allsamples. These are dominated by superparamagnetic grains, with mostsamples probably containing some multi domain magnetite, andoccasionally single domain magnetite. High coercivities are notcommonly observed, suggesting that greigite may be less important asa remanence carrier than indicated by SEM observations.

3. Tectonic rotations

The mean magnetizations for the localities described above areassumed to represent the Earth's axial dipole geomagnetic field at the

average timewhen themagnetizationswere acquired, so that the effectsof secular variation have been averaged out. This conclusion issupported by the angular dispersion for magnetic means, which areconsistentwith that expected for secularvariation (~15° atNewZealandlatitudes, Butler, 1992). Here, we use the following criteria, listed inSection 3.1, to determine the age of acquisition of the remanencemagnetization. Localities that satisfy two or more of these criteria arelikely to contain magnetization acquired close to the deposition oremplacement of the rocks (see further discussion, Table 1).

3.1. Tests for primary remanence magnetization

(1) In any sample, the high temperature (350–700 °C) or highcoercivity (N10mT) component of remanencemagnetization (orthe high coercivity component after thermal demagnetization upto 350 °C in the cases of samples where the presence of alterediron sulfides, such as greigite, was suspected), withmagnetite orti-magnetite as the principal carrier, is most likely to representprimary magnetic remanence. This is corroborated by thepresence of lower temperature or coercivity magnetizations,

270, Up

090,Down

180 360

0.3

0.2

Magnetisation(mA/m)

0.1 0.30.2100°C

0.1

250°C100°C

250°C10mT

BOU-AG1

35mT

BOU

270,Up

360

090, Down

1800.3 0.2 0.1

0.1

0.2

0.3

Magnetisation(mA/m)

5mT

80mT

DEE-A

DEE

270,Up

090, Down

180 360

Magnetisation(mA/m)

100°C

0.2

0.4

0.20.4

200°C

15mT

100°C

200°C

WHP-AD1

0.6

0.6

50mT

WHP270,Up

090,Down

180 360

5mT

5mT

0.1

0.2

0.3

Magnetisation(mA/m)

0.10.20.3

20mT

50mT 20mT

STR-K1

STR

270,Up

090, Down

180 360100°C

Magnetisation(mA/m)

0.1

0.2

0.10.2

250°C350°C

100°C250°C

350°C

BIG2-A2

BIG

a)

58

56

54

52

48

50

C25rC25n

C24r

C24n.3nC24n.1nC23rC23n.2n

C22r

C22nC21r

b)30

25

20

15

10

5

100-100 0

Inclination (°)

Str

atig

raph

ic th

ickn

ess

(m)

Age

Ma

(a)

(b)

25mT

5mT

Fig. 6. Typical Zijderfeld plots for horizontal (filled circles) and vertical (open circles) projections of magnetization during thermal or Alternating Frequency (AF) demagnetization forsedimentary rocks in this study (see Figs. 2 and 3 for locations). All samples show very low NRM intensities (b0.6 mAm−1) typical of these sediments. (a) Alternating Frequencydemagnetization of micritic limestone (~53 Ma Amuri limestone) from locality DEE (Figs. 2, 3b), showing a single normal unblocking component above 5 mT. When samples areplaced in stratigraphic position, both normal and reversed polarities are consistent with magnetostratigraphy for the period 53 Ma–54 Ma, based on biostratigraphy for this section(Hancock et al. 2003) resolving chron and subchrons 24n and 24r. (b) Thermal and/or alternating frequency demagnetization of Miocene mudstones from the northern part of theMarlborough Fault Zone (see Figs. 2, 3e). Unblocking components tend to be noisy, with complete unblocking by ~350 °C or 50 mT. In order to remove the effects of a high coercivityalteration magnetization from iron sulfides like greigite, samples were subjected to thermal demagnetization up to 350 °C, followed by AF demagnetization (for example, sampleBOU and WHP). This way, clear higher coercivity components carried by ti-magnetite could be recognized in many cases.

172 K. Randall et al. / Tectonophysics 509 (2011) 165–180

which are evidence for secondary components of remanencemagnetization.

(2) Remanence magnetizations, either at a single locality orgrouping of localities (Table 1), have a high probability ofbeing acquired before folding if there is maximum clusteringafter a tilt correction with 100% unfolding (Tauxe and Watson,1994). In some cases, we also consider the possibility of well-grouped magnetization acquired during folding (for example,in Miocene mudstones and siltsones), as also found by Rowanand Roberts (2008).

(3) The existence of normal and reverse polarities in a stratigraphicsection, together with a consistent magneto-stratigraphy of thesection, is regarded as evidence for primary remanence magne-tization. However, reversed polarities are not necessarily diag-nostic on their own, because remagnetizations have occurredduring both normal and reversed periods of thefield (Rowan andRoberts, 2008).

(4) Magnetic inclinations should be consistent with plate polarwander paths, given their age. However, we note that inclinationflattening as a consequence of sedimentary compaction is typicalof fine-grained sediments, where the magnetization from othercriteria is deemed to be primary.

(5) Consistent magnetization in both an intrusive dyke and thebakedmargin is good evidence for acquisition of magnetizationduring cooling of the dyke, soon after intrusion.

We determine rotation and flattening anomalies by comparing thedirection of magnetization, given its age, with that predicted for thePacific plate (or Australian plate) at that time and location (Table 1,DiVenere et al., 1994, Cande et al. 1995, 2000). The Pacific plate showsnegligible Cenozoic rotation, but a small net rotation either b5°

clockwise or b12° anticlockwise, depending on age, since the LateCretaceous, whereas the Australian Plate shows clockwise rotation of~40° since the inception of the New Zealand plate-boundary zone,rotating on average about 1°/Myrs (DiVenere et al., 1994, Cande et al.,1995). A rotation anomaly is interpreted to be the result of rotationabout a vertical axis of a rigid crustal block. The results can be con-veniently divided into those for Late Cretaceous to Oligocene localities(95–30 Ma) and Late Miocene to Pliocene localities (11–2.5 Ma).

3.2. Late Cretaceous to Oligocene localities (95–30 Ma)

High temperature or coercivity components of remanence mag-netization obtained in this study, after correction for the stratal tilt,have mean declination anomalies in the range 50°–90° clockwise(except for locality SEY), regardless of age (Table 1, Figs. 5a and b, 8aand b). Remanence magnetizations from individual localities, orvarious combinations of adjacent localities (sometimes spanningdifferent parts of a fold structure), show improved clustering with100% unfolding at the 95% confidence level (Fig. 5c and d, Table 1).This provides strong evidence that this remanencemagnetization pre-dates folding. In addition, coherent magneto-stratigraphies from theGridiron Formation volcanics (together with baked margin tests), orPaleocene–Eocene Amuri limestones (Fig. 6a), support primaryremanence magnetizations acquired at the time of deposition oremplacement. Inclinations are identical within error to that expectedfor Pacific Plate at these localities, given the age (Table 1). For thesereasons, there is a good case that all thesemagnetizations are primary,representing the Earth's averaged geocentric axial dipole field at thetime of formation of the sediments or volcanics.

The rotation anomalies indicate that regional clockwise tectonicrotation in the range 40°–90° relative to the Pacific plate (except for

BOU BIG + STRNN

BIGSTR

a) N

WHP

NAF onlyThermal StageAF Stage

Tilt Corrected

BIR

HIN

In situ In situ

Tilt corrected

In situ Tilt corrected Tilt correctedIn situ

In situ

Tilt corrected

BOU

BIG+STR

Fig. 7. Equal area lower hemisphere stereographic plots of high temperature or high coercivity remanence magnetization components (in situ and after tectonic tilt correction) forMiocenemudstones from the northern part of theMarlborough Fault Zone (localities BOU, BIG, STR, andWHP, see Figs. 2, 3e) and southern part of the North Island (HIN, BIR, Fig. 1b).In many cases, samples were subjected to thermal demagnetization up to 350 °C, followed by AF demagnetization, in order to remove the effects of alteration iron sulfides such asgreigite, which have high coercivities but low unblocking temperatures. Also plotted are magnetizations obtained by Walcott et al. (1981) for locality HIN.

173K. Randall et al. / Tectonophysics 509 (2011) 165–180

locality SEY), right across the Marlborough Fault Zone. The timing anddistribution of this rotation, taken together with the results inprevious studies, are described in detail in Section 4.

3.3. Late Miocene to Pliocene localities (11–2.5 Ma)

Rocks in the age range ~11–15 Ma have not been confirmed in theMarlborough region, so the younger paleomagnetic data are confined tothe period since ~11 Ma (Tongaporutuan and younger New Zealandstages). However, despite extensive outcrops of Late Miocene–Pliocenesandstones, siltstones and mudstones, it has proved difficult tounambiguously identify primary magnetizations. This is partly becauseone cannot rule out the possibility of local but pervasive remagnetiza-tions by secondary growth of greigite (Rowan and Roberts, 2008, thisstudy)—patchy remagnetisations may also explain the difficulty ofreplicating some previous studies.

The high temperature or coercivity demagnetization componentsfor the five Late Miocene to Pliocene mudstone localities analyzed inthis study (Figs. 1b, 2, localities BIG, STR, BOU, WHP, HIN), after tilt

corrections, reveal declination anomalies in the range 30°–78°. TheLate Pliocene locality BIR in the southern part of the North Island,suggests negligible rotation since ~2.5 Ma. The declination anomaliesfor HIN and BIR, determined in this study, are identical within error tothose of Walcott et al. (1981) and Lamb (1988) for the same age rocksat adjacent sample sites.

The declinations for the Late Miocene localities BIG, STR andWHP aresignificantly greater than those observed in previous studies for similarlyaged or slightly younger siltstones and mudstones in the same region(Roberts, 1992), with higher flattening anomalies (~15°). In addition, weobtain predominantly normal polarities at localityWHP, whereas Roberts(1992) observed predominantly reversed polarities at essentially thesame outcrop (Fig. 2, locality WV, Roberts, 1992). One possibility is thatsome or all of these magnetizations were acquired during folding. Forexample, a pronounced angular unconformity between the Medway andoverlying Upton Formations in the lower Awatere Valley (Figs. 2, 3e),which sits structurally within a few hundred meters of the BIG and STRlocalities, suggests that pervasive remagnetization could have occurredafter initial tilting and erosion of the Medway Formation. This would

174 K. Randall et al. / Tectonophysics 509 (2011) 165–180

have taken place within ~1 Ma of deposition of the Medway Formationitself, but prior to the ~20° NW tilting of the overlying Upton Formation(localities BIG and STR analyzed this way are referred to as BIG** inTable 1). We prefer this explanation, because it is consistent with thesmaller and well-documented paleomagnetically-observed rotations(~35° clockwise) in slightly younger Late Miocene to Pliocenesequences at the northern end of the Clarence and Awatere Faults(Section 4, Fig. 8c, 10, 11a, Roberts, 1992).

We leave open the question of the timing of more poorlyconstrained magnetization at WHP (Table 1), for which an estimatedclockwise rotation ranges between ~30 and 66°, depending on whenthe magnetization was acquired during tilting.

0

50

100

150

200

20 40 60

Rot

atio

n (°

)

SW

MOT

CLA

KAI

HOPE F

20 - 17 Ma

95 - 30 Ma

Locality Age

-20

0

20

40

60

80

100

120

20 40 60MOT

CLA

KAI

LYF

LOT

PUH D

SE

Rot

atio

n (°

)

Locality Age

~95 Ma

60 - 30 Ma

~9 Ma HOPE F. C

-4

-2

0

2

4

6

8

10

12

14

10 20 30 40 50

3 - 6 Ma

6 - 10 Ma

95 - 30 Ma (20 Ma)

Locality Age

Ave

rage

Rot

atio

n R

ate

(°/M

a)

SW

CAV DEE

BIG STR

BIG**

FC BSBOU

Little & Roberts 1997

This Study: CAV, DEE, BIG, STR, BIG**,BOU

RSlower regional rotation (~4°/Ma) Transition

zone

(a)

(b)

(c)

?

~9 Ma

Fault

Structural continuity

Secondary magnetisations?

R

Region 1

Fig. 8. Plots of paleomagnetically determined rotation for rocks N17 Ma, with respect to tprojected onto the line of the traverse (shown in Fig. 9b). (a) Traverse 1 along the coast frRotations increase smoothly towards the NE to a maximum of ~140°. (b) Traverse 2 perpendof the low rotation at SEY is unclear, but may reflect local deformation. Note the structural copaleomagnetically-determined rotation. (c) Traverse 3 parallel to the Awatere Fault in the nolocalities N20 Ma are calculated assuming all rotation has occurred in the last 20 Ma. Rotatioincreasing in a transition zone across a pronounced basement kink (see Figs. 2, 9b). Also sh

4. Distribution and timing of rotation

4.1. Spatial variation of rotation

Fig. 8a and b show paleomagnetically observed rotation relative tothe Pacific plate for localities greater than ~17 Ma (except for ~9 Malocality MOT), in this and previous studies, plotted against position inthe Marlborough Fault Zone. In Fig. 8a, rotation is plotted againstdistance along the coast from Motanau (Locality MOT, Fig. 2), whichlies on the southern margin of the plate-boundary zone. MOT is takenas the southern limit of rotations, because b5° of rotation has beenobserved here in the last ~9 Ma (Walcott et al., 1981), with b20° (at

80 100 120 140 160Distance along coast (km) NE

PUH

SS

DS

HC2

HC3

WC1

WC2

WC3

WH

BB2

AULT KEKERENGU FAULT

80 100 120 140

EE

CAV

Y

Distance normal to Hope Fault (km)

LARENCE F.AWATERE F.

WAIRAU F.

60 70 80 90 100

Distance along Awatere Fault (km)

NEUB8

UB1

BR4

BR1

BB2

CC

SV

NC RBLamb (1988)

Average Hikurangi Margin(Lamb, this volume)

GPS: Wallace et al. 2007

apidly rotating ‘block’ at northern end of Clarence Fault (~7°/Ma)

Fault

Fault

Local basement structural rotation(reference trend = 300°)

egion 1 Region 2

Region 1

Region 2

he Pacific Plate, against distance. See Fig. 2 and 9b for locations. Localities have beenom Motanau (locality Mot), taken as the southern edge of the Cenozoic rotating zone.icular to the Hope Fault. Rotations increase smoothly to the Hope Fault. The significancentinuity of individual localities, and also that the swing in basement strikes mirrors therthern Marlborough domain. Here, average rotation rates are shown—rotation rates forn rates are significantly higher at the northern ends of the Awatere and Clarence Faults,own are patterns of rotation rate suggested in previous studies.

175K. Randall et al. / Tectonophysics 509 (2011) 165–180

the 95% confidence level) of clockwise rotation for locality CLA about30 km farther NE since 54 Ma (Hall et al., 2004). There is a progressiveincrease in rotation across the Marlborough Fault Zone towards theNE, reaching ~130° near Cook Strait.

In Fig. 8b, rotation is plotted against distance perpendicular to theHope Fault, which is the southernmost major fault in theMarlboroughFault Zone, and the most active today with a dextral slip rate of 20–30 mm/yr (Wallace et al., 2007 and references therein). A progressiveincrease in clockwise rotation towards this fault is observed, reaching~80° north of the fault. The significance of the low rotation at SEY isunclear, but may in part reflect local deformation. The structuralcontinuity of the sample localities show that rotations are coherent ona length scale of at least ~10 km, and do not merely reflect local shearadjacent to the faults—this structural continuity requires the rotatedblocks here to have dimensions on the tens of kilometers scale (seeSection 5). In fact, taking Fig. 8a and b together, it is clear that themajor faults play only a small role in controlling the distribution ofrotation, with a maximum of 20–30° step across the Hope Fault. Thus,there is a coherent ~80° clockwise rotation of crustal blocks for mostof the Marlborough Fault Zone north of the Hope Fault, with anenhanced ~130° clockwise rotation of blocks at the northeastern endsof the major faults.

The important structural feature of the northern part of the SouthIsland is the geometry of bedding in the Mesozoic basement, whichgenerally dips steeply (N60°). In detail, there is marked local variability,butwhen averaged over subregions on a scale up to10 km(Fig. 9b, Littleand Roberts, 1997, Hall et al., 2004), a pronounced 80°±10° swing instrike becomes apparent (with subvertical average dips), from anazimuth of 300°±10° in the SE to ~020° farther NW (Figs. 2, 9b),described in detail by Hall et al. (2004). The greatest curvature is in thevicinity of the Hope Fault, where there is the greatest gradient inpaleomagnetically-determined rotation (Fig. 8b). The large-scale base-ment structure is also reflected in the trend of the Eskhead subterrane,

0

50

100

150

200

20 40 60 80 100 1

Local structural rotation (°)

Pal

eom

agne

tic R

otat

ion

(°)

CLASEYKAI

PUHDEE

CAVLOT

LYF

WC1WC2

WC3

SS

DS

95 - 30 Ma

20 - 17 Ma

Locality Age

290° 310° 350° 030°

Unity

(a)

Fig. 9. (a) Plot of paleomagnetically determined rotation, with respect to the Pacific Plate, forFig. 2 and inset map in (b), Table 1), against the local structural trend inMesozoic basement ((Fig. 1). Error bars showwhere there is marked local variation in basement strike. Rotation obasement trend, taken as that for unrotated Pacific plate rocks south of the Marlborough Faulthe interior of Marlborough Fault Zone, and paleomagnetically determined rotations up to 12close correlation between basement trends and paleomagnetically observed rotations strongoccurring since ~20 Ma. (b) Detailed map of basement structure, from Hall et al. (2004), sh

defining theeasternpart of theNewZealandOrocline (Section2.2, Fig. 1,Sutherland, 1999, Hall et al., 2004). A pronounced kink in the north addsa further ~45° to the basement bending, coinciding with Region 2 inFig. 10 (Little and Roberts, 1997), making ~130° of total bending in thebasement structure (Figs. 2, 9b).

Fig. 9a shows rotation anomalies for Late Cretaceous to MiddleMiocene localities, plotted against the average local deviation in strike ofthe underlying basement from the reference direction in the south—notetheuncertainty inbasement structural trend/rotationwhere there is localmarked structural complexity, especially for localities where rotationsN120° clockwise are observed (see also Fig. 8b). The paleomagneticrotations are clearly mirrored in the regional bending of basementstructure, which can now be dated to the Neogene deformation.

4.2. Rotation history

Fig. 10a shows a plot of the amount of rotation against age,regardless of location, for all paleomagnetic localities in this andprevious studies. Rotations in the northern Marlborough region arelarge, exceeding 120° clockwise in rocks as young as 17 Ma (Fig. 10a).If rotation has occurred at a constant rate since ~20 Ma, when bothstructural and stratigraphic data show that deformation in this part ofthe plate-boundary zone was initiated (Lamb and Bibby, 1989, Raitet al., 1990), then this implies average long term regional rotationrates in the range 4–7°/Ma (Fig. 8c, 10b).

In detail, the higher rotation rates are found in the northernMarlborough domain, at the northern ends of the Marlborough Faults,defined by Region 2 in Fig. 10b, coinciding with a ~45° kink inbasement trend. This is well illustrated in Fig. 8c, where averagepaleomagnetically-determined rotation rates are plotted againstdistance along the length of the fault block between the Awatereand Clarence faults—average rotation rates for localities N20 Ma havebeen calculated assuming all rotation has occurred in the last 20 Ma.

20 140

or Azimuth

Reference azimuth = 290°

Reference Azimuth = 310°BB2

WH

HC2

HC3

070°

Uncertainbasementtrend

110°

Average structural measurement

Strike ridge

50 km

N

173°E

42°S

174°E

43°S

Kaikoura

Hope F.

Clarence F.

Awatere F.

Kekerengu F.

Average basement trend(reference = 290° - 310°)

(b)

Wairau F.

PaleomagneticLocality

Profiles

(1)

(3)

Basement kink

MOT

(2)

localities in the Marlborough Fault Zone (Marlborough Domains) regardless of age (seefrom Little and Roberts 1997, Hall et al. 2004), defining part of the New Zealand Oroclinef basement structural trends is with respect to the reference WNW (290°) or NW (300°)t Zone (Hall et al. 2004, Rattenbury et al. 2006). Localities such as DEE and CAV are from0° have been observed in rocks as young as 17 Ma (Figs. 10a, 11a, Table 1). Therefore, thely suggests that all of the bending in this part of the New Zealand Orocline is Neogene,owing location of paleomagnetic localities plotted in (a).

-50

0

50

100

150

200

20 40 60 80 100 120

Age (Ma)

Clo

ckw

ise

Rot

atio

n (°

)

This study

Previous work

10°/Ma

4°/Ma

173° 174° 175°

-41°

-42°

-43°

100km

> 17 Ma

< 11 Ma

Paleomagneticlocalities

(a)

(b)

~4°/Ma

~7°/Ma

Small blockrotation

12

Region 2

Region 1

Southern limit

of rotationAverage

clockwise rotation ratesince 20 Ma

Fig. 11

~80°

120° - 140°

?Initiation of deformationin Marlborough Fault Zone

2°/Ma

6°/Ma

Secondary magnetisation?

Regions

Fig. 10. (a) Plot of paleomagnetically determined rotation, with respect to the Pacific Plate, for localities in the Marlborough Fault Zone (MFZ) and southern part of the North Islandagainst stratigraphic age of the samples (see Fig. 2, this study and previous work). A more detailed plot for the last 20 Ma is shown in Fig. 11. Since ~20 Ma, clockwise rotation ofcrustal blocks has occurred at an average rate of 5–10°/Ma. In detail, the spatial rotation pattern suggests two main regions, characterized by average rotation rates since ~20 Ma,labeled Regions 1 and 2 and defined in (b). Finite plate reconstructions suggest that the whole Hikurangi margin has rotated at ~4°/Ma since 20 Ma. (b) Map of the southern end ofthe Hikurangi margin, showing the Regions 1 and 2, defined by average clockwise rotation rates since ~20 Ma. The higher rotation rates (~7°/Ma) are confined to the extremenorthern end of the Marlborough Faults. In addition, the boundary between the northern and southern Marlborough domains (heavy dashed line) marks a change from rotation ofsmall blocks (km-scale) in the south, to larger blocks (10 km-scale) farther north.

176 K. Randall et al. / Tectonophysics 509 (2011) 165–180

The average rotation rates (6–8°/Ma) for Pliocene localities at thenorthern ends of the Awatere and Clarence Faults are the same as thatfor the nearby ~30 Mamicritic limestones at locality BB2 (Vickery andLamb, 1995). However, on a more regional scale, average rotation ratefor Late Miocene or older localities is ~4°/Ma, and similar to thatdeduced from paleomagnetic studies elsewhere along the Hikurangimargin (Lamb 2011-this volume).

Our new paleomagnetic data, when combined with previousstudies (Little and Roberts, 1997, Roberts, 1992, Vickery and Lamb,1995), indicate that rotation rates have varied through time. Region 2,defined in Fig. 10b, has the best documented rotation history. Here,magnetostratigraphic studies in the Lower Awatere Valley (Roberts,1992) show that the rotation rate in the Late Miocene, between~10 Ma and ~4 Ma was 1–2°/Ma, but much higher in the Plio-Pleistocene, with ~35° of clockwise rotation (Fig. 11a). This requires arapid rotation rate in the early–middle Miocene (20–10 Ma) toachieve a total Neogene clockwise rotation of 120–140° (Fig. 11a). InRegion 1 (Fig. 10b), an interpretation of the rotation history dependson howmuchweight is given to the results for Fuschia Creek (LocalityFC, Little and Roberts, 1997). This has a large uncertainty (0°±17°)and lies in a fault sliver close to the Awatere Fault (Fig. 3e) and so maynot reflect the more regional rotation in Region 1. If we give littleweight to this locality, then a constant rotation rate of ~4°/Ma since20 Ma is consistent with the other paleomagnetic data (Fig. 11a). Butthe paleomagnetic data for Region 1 can also be fitted within errorwith a marked decrease in rotation rate, from ~6°/Ma in the early tomiddle Miocene to b2° since 8 Ma (Fig. 11a), similar to that observedfor the rotation history prior to the Pliocene (~4 Ma) in Region 2(Fig. 11a).

The decrease in rotation rate in Region 1 is consistent with theinferred displacement history of the Marlborough faults (Little andJones, 1998), so that since the Late Miocene more of the dextral shearin the Marlborough Fault Zone has been taken up by slip on the faults,

and less by clockwise rigid body rotation of the intervening faultblocks (Hall et al., 2004). It is also supported by an analysis of theactive deformation on time scales of 10 kyr to 10 years (GPSmeasurements), which shows that, within the uncertainties, almostall the dextral shear could be currently taken up by slip on the faults,without any rotation of the intervening fault blocks relative to thePacific Plate (Fig. 8c, Wallace et al., 2007). However, given the totalobserved offset of these faults (~60 km, Fig. 1b), these rates of faultslip can only extend back b2 Myrs. Also, the model of activedeformation from GPS, described above, assumes the faults definevery long and thin blocks, 10–30 km wide and 200–250 km long. Ifthere is a more complicated pattern of faulting, definingmuch smallerfault blocks (Lamb 1989, 1994), then rotation up to 4°/Ma cannot beruled out in the Marlborough domains (Lamb 2011-this volume).

4.2.1. Evolution of northern basement kinkThe paleomagnetic, structural and stratigraphic data point to a

long term evolution, since 20 Ma, of the basement kink (Fig. 2, 3e,Little and Roberts, 1997 hinge) that has accommodated the relativerotation between Regions 1 and 2. The deformation of Late Miocenesequences that unconformably overlie parts of the basement kinkshow that it has had a complicated history. Thus, all ~45° of the kinkbend exposed between the Awatere and Wairau Faults must haveoccurred prior to ~6 Ma, because ~10° dipping sedimentary se-quences of this age unconformably overlie both limbs (Fig. 2,Rattenbury et al., 2006). About 20° of the kinking, where it is exposedbetween the Awatere and Clarence Faults, occurred before ~9 Ma,because sequences of this age truncate an already folded structure(Fig. 3e, Randall, 2007). Also, the axis of kinking seems to havemigrated northwards here, so that relative rotation between Region 1and 2 since 4 Ma was accommodated by thrust faulting along theFlaxbourne and Halden Faults (Fig. 3e, Lamb and Bibby, 1989, Roberts,

-50

0

50

100

150

200

5 10 15 20 25 30Age (Ma)

Rot

atio

n (°

)

Middle Awatere Block (Region 1) ~80° since ~20 Ma

Northern Awatere/Clarence Block (Region 2) ~130 since ~20 Ma

Secondary magnetisations?

BIG

STR

BIG**

BOU

FC

WHP

Elongate block in shear zone: decreasing rotation rate toward present day

Higher shear strain

Lower shear strain

Creation of N. Awatere/Clarence equidimensional fault block

Elongate blocks

Higher Shear

Lower shear strain

ShearZone(W)

Alpine Fault

(a)

(b) DD/W = 5

D/W = 2.5

~35° Rotation

Equidimensional block

~130°

~80°

8°/Ma

6°/Ma

4°/Ma

2°/Ma

Region 1

Region 2

Fig. 11. (a) Plot of paleomagnetically determined rotation since 20 Ma, with respect to the Pacific Plate, for localities in the Marlborough Fault Zone (MFZ) and southern part of the NorthIsland against stratigraphic age of the samples (see Figs. 2, 10). Rotation histories for Region 1 in the central Awatere block (circles) and Region 2 in the northern Awatere/Clarence block(squares) are shown. Paleomagnetic data for Region 2 require amarked reduction in rotation rate in the LateMiocene, compared to that in the early–middleMiocene. Rapid rotation heresince 4 Ma is the result of the creation of an equidimensional block in the northern part of the shear zone. Paleomagnetic data in Region 1 is also consistent with a marked reduction inrotation rate toward the present, but this is largely constrained by the uncertain result for one LateMiocene locality (Locality FC, Figs. 2, 3e, Little and Roberts 1997), and a constant rate of~4°/Ma since ~20 Ma cannot be excluded. (b) Shear zone model (see text) for the northern Marlborough domain, describing the rotation of elongate crustal blocks between the majorfaults. A zoneof higher shear strain in the north is required to explain the greater rotation here. Thismodel also predicts amarkeddecrease in rotation rate towards thepresent for elongateblocks (shown by curves in (a), assuming constant displacement rate since ~20 Ma), as they rotate into a direction more nearly parallel with the shear zone margins.

177K. Randall et al. / Tectonophysics 509 (2011) 165–180

1995, Townsend and Little, 1998). It is unclear if these faults are stillactive, and so it is possible that relative rotation here is also not active.

5. Neogene tectonic evolution of southern end of theHikurangi Margin

5.1. Neogene reconstruction of southern Hikurangi margin

An essential step in using the paleomagnetic data to unravel theeffects of Cenozoic deformation at the southern end of the Hikurangimargin is to define crustal blocks that have undergone coherentrotation, then attempt to reposition them back through time, con-strained by the nature and amount of deformation at their boundaries.The finitemotion of the bounding plates determines the total amount ofdeformation at any stage. These reconstructions should not be regardedas definitive restorations, but rather reconstructions that highlight thestructural implications of accommodating the paleomagnetically-observed rotations, rather than attempting to deduce these rotationsfrom other observations.

We focusonseveral keyblocks in thenorthernMarlboroughdomain,defined by the Wairau, Awatere and Clarence faults, and the basementkink at their northern ends (Fig. 12a). The internal integrity of the blocksis constrained by the structural continuity of the Cenozoic coversequences. Here, the block rotation history is given by the paleomag-netic data or basement trends. The remaining blocks have beenpositioned tominimize any displacementwith these ‘key’ blocks, takingaccount of basement trends. Fig. 12b and c show ‘cut-out’ blockreconstructions for two stages in the Neogene evolution of the southernHikurangimargin, at 10 Maand20 Ma, basedon thepaleomagnetic datacombinedwith structural and stratigraphic constraints, in the context ofthe relative plate motions (Cande and Stock, 2004). Rotation of NorthIsland is based on paleomagnetic data in this study (Locality HIN,Table 1) and paleomagnetic data summarized in Lamb (2011-thisvolume). Gaps between the blocks represent crustal shortening, withthe main shortening along the subduction thrust itself in the east, andsince ~10 Ma in the Southern Alps, along the western margin of SouthIsland—the crustal root beneath the Southern Alps requires at least

~50 kmof shortening (Stern et al., 2007). In addition, crustal thickeningbeneath western North Island suggests ~100 km shortening in theMiocene (Stern et al., 2006).

The southernMarlborough domain is treated as a zone of distributeddextral shear (Section 5.2), trending ~070°, althoughwith up to ~60 kmof fault displacement since the Late Miocene. Dextral shear in thesouthern Marlborough domain is accommodated by block rotation andfault displacement in the northern Marlborough domain, similar to therotating block and trellis model proposed by Lamb (1988). The longerterm ~80° rotation of the northern Marlborough domain, required bythe paleomagnetic data in this study, supports previous Late Cretaceousto early Neogene palinspastic restorations of this part of New Zealandinvolving rotation of a series of large rigid blocks (Crampton et al., 2003,Lamb and Bibby 1989, Townsend, 2001). The kinematics of Neogenedeformation at the southern end of the Hikurangi margin, required bythe block reconstructions in Fig. 12, are summarized in Fig. 13.

5.2. Shear zone model

Hall et al. (2004) used the available coastal data to infer rotation inthe interior parts of theMarlborough Fault Zone, and showed that thisrotation could be modeled in terms of the response of elongate rigidblocks in a zone of distributed dextral shear (Fig. 13). The blocks caneither be viewed as embedded in the distributed shear, or passively‘floating’ on an underlying distributed shear (Fig. 11b, Lamb 1987,1994). For a shear zone of widthW, and displacement D through timeacross the shear zone, the total amount of rotation at any particulartime, given the initial orientation ϕi of the long axis of the block, isdetermined by the ratio D/W, and the aspect ratio k (long axis to shortaxis) of the block (Fig. 11b, Lamb, 1987). For an aspect ratio kN5, theblock rotation in this model can be approximated within 10% accuracyto that of a passive marker line (Fig. 13c, Lamb 1987, 1994):

θ = 90˚−ϕi + atan D =W– tan 90˚−ϕi� �� � ð1Þ

Eq. (1) shows that as displacement D increases, the amount ofrotation approaches a constant value, and the rate of rotation tends to

(b)AUSTRALIAN

PLATE

(c)

AUSTRALIAN

PLATE

100 km

(a)

10 Ma

Edge of subducted slab

Proto

Southe

rn

Alps

Alpine

F.

20 Ma

Alpine

Fau

lt

PACIFICPLATE (fixed)

PACIFICPLATE (fixed)

Eskhead terrane

Today

PACIFICPLATE (fixed)

AUSTRALIANPLATE

N

Eskhead terrane

‘Key’ blocks

Paleomagnetic North

Edge of subducted slab

Edge of subducted slab

Fig. 12. Block reconstructions of the southern part of the Hikurangi margin since~20 Ma, based on paleomagnetically observed rotations in ‘key’ crustal blocks,combined with the pattern of faulting, bending and offset of prominent basementtrends or terranes, and relative plate motions determined from sea floor magneticanomalies (see text). Reconstruction for North Island is based on paleomagnetic data atLocality Hin (Fig. 1b, this study) and data summarized in Lamb (this volume). Thedimension of individual crustal blocks for which the paleomagnetic rotation applies isbased on pattern of Neogene faulting (after Rattenbury et al. 2006). Gaps betweenblocks (defined in (a)) imply subsequent crustal shortening. Dashed line shows theposition of the seismically active slab, as defined today. (a) At 10 Ma, the major faults inthe northern Marlborough domain have a nearly NNE trend, with a greater componentof thrusting, accommodating dextral shear on the ENE trending major dextral strike-slip faults in the southern Marlborough domain. Shear in the southern Marlboroughdomain ismainly accommodated by small scale block rotation, accommodating bending ofbasement terranes, whereas deformation in the northern Marlborough domain isdominantly thrusting, with clockwise rotation of the intervening thrust sheets. Localizedshear adjacent to the northern end of the Alpine Fault at this time may have resulted infurther bending of basement terranes, defining Little and Roberts (1997) basement hingein the northern part of theMFZ (see Figs. 2, 3e). (b) At 20 Ma, faulting is predominantly ona series of thrusts parallel to the subduction margin at this time, with no offset of theEskhead subterrane across the Alpine Fault.

178 K. Randall et al. / Tectonophysics 509 (2011) 165–180

zero, as blocks becomemore nearly parallel to the shear zonemargins.However, if blocks are more-or-less equidimensional (k~1), rotationis directly proportional to displacement (Lamb, 1987):

θ = 0:5D=W ð2Þ

5.2.1. Shear zone in the southern Marlborough domainThe southern Marlborough domain forms a distributed zone of

dextral shear through continental lithosphere (Fig. 13a and c). The

rotation of basement structure or terrane boundaries, such as theEskhead terrane, can be modeled as that of numerous small elongateblocks, defining strike segments, with a bulk behavior which isessentially that of a passive marker line. In this case, there has beensignificant slip parallel to bedding, which is a style of faulting that isvery difficult to detect from geological mapping where there are nocoherent or extensive cover sequences (Hall et al., 2004, Lamb 1989,1994, Rattenbury et al., 2006). From Eq. (1), this model predicts thatthe rate of rotation for elongate blocks decreases with displacementacross the shear zone, and the long axes of the blocks become morenearly aligned with the trend of the shear zone. Equidimensionalblocks will continue to rotate at a constant rate (Eq. 2).

The major faults in the southern Marlborough domain, takentogether with the swing in basement trends, defines a shear zone~100 km wide, with a southern margin parallel to the Hope Fault(trending ~070°). Fig. 13c and d illustrate the rotation history of apassive marker line in the shear zone, with an original azimuth of~300°. A total displacement of 200–250 km is required to explain the~90° swing in trend. If this displacement has occurred over the last20 Ma, at a constant rate, then this would mean rotation ratesdecreased from ~6°/Ma at 20 Ma, to ~4°/Ma at 10 Ma to b2°/Ma today(Fig. 13d).

5.2.2. Shear zone in the northern Marlborough domainThe shear zone here appears to be affected by the presence of the

underlying subduction zone, with a component of thrusting on themajor faults, which divide up the region into a series of elongateblocks (constrained by the structural continuity of Cenozoic coversequences, defining blocks with dimensions 10–20 km×50 km),resting on the subducted slab—the trench forms the eastern boundaryof this deformation (Fig. 13a and b). If these blocks (aspect ratio 5:1)have rotated in response to the same shear as that in the southernMarlborough domain, then theywill also behave essentially as passivemarkers given their present orientation (Lamb, 1987, 1994), followingEq. (1) and rotating overall ~80° since 20 Ma (Fig. 13e and f). Again,there is a decrease in rate from ~6°/Ma during the early stages of shearto b2°/Ma today (Fig. 13f). As this happened, the faults between theblocks evolved from essentially pure thrust structures in the earlyMiocene, to predominantly strike-slip faults with a small componentof shortening today (Figs. 12a and c, 13a and b).

The northern kink in basement structures could be explained as aresponse of small elongate blocks to higher shear strain in a narrowerzone, at the northern boundary of the shear zone, with a more rapidrotation history, but with rates again decreasing toward the present(Fig. 11a and b). If the more equidimensional block, at the northernends of the Awatere and Clarence Faults, was subsequently created inthis shear zone, then this explains the rapid rotation (~7°/Ma) in thelast 4 Ma (Fig. 11a and b). However, it is unclear if this rapid rotation isactive today.

6. Summary and discussion

The paleomagnetically observed rotations show that since ~20 Ma,the entire width of the Marlborough Fault Zone has been a zone ofdextral shear, ~100 km wide, forming the southern end of theHikurangi margin. The shear zone spans the transition fromsubduction, accommodating ~90° swing in trend of the subductionzone during the Neogene (Walcott 1987, 1989, Vickery and Lamb,1995, Rowan and Roberts, 2008, Lamb, 2011-this volume).

In the western part of the shear zone (southern Marlboroughdomain), where it passes through continental lithosphere, shear inthe Miocene has resulted in 70–90° of coherent rotation of basementstructures, and the crust has broken up into small elongate blocks,parallel to bedding and on a scale (1–10 km) smaller than thedetectable bending. In the eastern part of the shear zone, where thecrust is underlain by the subducted slab, it has formed large elongate

Australian Plate

Pacific Plate

~4°/Mawrt Pac

Compression

Distributed

Shear

Rotation of passive marker (many small elongate blocks) = basement structure

Rotation of large elongatefault blocks 2° -6°/Ma

Enhanced rotation of faultsand basement structure~7°/Ma

Northern Marlborough Domain (subduction dominated)

Southern Marlborough Domain (shear dominated)

20 - 4 Ma

0

60

120

0 100 200 300

~2°/Ma~6°/Ma

ShearZone

Rotation ofpassive marker line

Width W

Displacement D

θi

φ (°)

Shear rotation model: Displacement (km)

(c) (d)

Australian Plate

Pacific Plate

~3.5°/Mawrt Pac

Compression

Rapid rotation of equidimensionalblocks ~7°/Ma

Slow rotation of main faults <2°/Ma

Shear taken up mainly by faulting

4 - 0 Ma

W ~ 100 km

0 200 km

(a) (b)

~80°

0

60

120

0 100 200 300

2°/Ma

6°/MaShearZone Rotation of

elongate blocks

Width W

Displacement D

θi

φ = 90°- θi + tan-1(D/W - tan(90-θi))

φ (°)

Shear rotation model:Displacement (km)

(e) (f )

Shear Zone azimuth = 070°

~80°

Rotation rate decreasing with displacement/time

Rotation rate decreasing with displacement/time

Shear

Zone

100 km

φ = 90°- θi + tan-1(D/W - tan(90-θ i)) ~20 MaApproximate timescale

~20 Ma

Approximate timescale

0 0

Fig. 13. Cartoon diagrams summarizing the Neogene fault and rotation kinematics at the southern end of the Hikurangi margin, in theMarlborough Fault Zone. Here, deformation hasoccurred in a dextral shear zone, ~100 kmwide, spanning the transition from transpression in continental lithosphere to subduction, with the trench forming the eastern limit of theshear zone. Thus, the mode of deformation in the southern Marlborough domain is shear dominated, whereas the effects of subduction are dominant in the northern Marlboroughdomain. The kinematics of deformation for North Island is taken from Lamb (this volume). (a) Between 20 and 4 Ma, distributed shear in the southern Marlborough domain hasresulted in 70–90° bending of basement structure, including the Easkhead Terrane. Faults and elongate crustal blocks in the northern Marlborough domain have rotated 60–80°clockwise, forming mainly thrusts early in their evolution, and then ‘straightening-out’ and becoming principally strike-slip faults. There has been a further ~20° of rotation inbasement structures at their northern end. (b) Since 4 Ma, dextral shear has been mainly accommodated by dextral slip on the major Marlborough Faults, with slow rotation(b2°/Ma) of the faults in the northern Marlborough domain. However, the creation of an equidimensional block has allowed rapid rotation at the northern ends of the Clarence andAwatere Faults. (c)–(f) If rotation is modeled as that of a passive marker (numerous small elongate blocks in the southern Marlborough domain), or highly elongate blocks (severallarge elongate blocks in the northern Marlborough domain), in a zone of distributed dextral shear (widthW and a total displacement D), then the amount of rotation will depend onthe ratio of displacement to width (D/W). If the displacement rate is constant, then the rate of rotationwill progressively decrease through time, as blocks rotate into parallelismwiththe margins of the shear zone, as is observed from the paleomagnetic data.

179K. Randall et al. / Tectonophysics 509 (2011) 165–180

blocks, on a 10–50 km scale, that have also rotated 60–80° clockwise.Faults here have evolved from predominantly thrusts in the earlyMiocene, parallel to the subducting margin at that time (Fig. 12c), topredominantly strike-slip today (Fig. 1b), so that, overall, theMarlborough faults have ‘straightened-out’ into their present orien-tation. The eastern limit of this shear zone is the trench, which hasrotated with the crust in the shear zone. More localized shear on thenorthern margin of the shear zone, marking the northern extension ofthe Alpine fault, has resulted in enhanced rotation.

It is remarkable that shear in the early stages in the evolution ofthe plate-boundary zone, in the southern Marlborough domain, istaken up in such a pervasive manner, rather than by displacement ondiscrete faults (Figs. 8a and b, 13a). This behavior supports theconclusions of shear wave splitting studies, which suggest long liveddistributed deformation in a wide zone of the underlying mantle(Molnar et al., 1999, Savage et al., 2007). It seems that it is not untilmuch later, in the Plio-Pleistocene, when the early thrusts in thenorthern part of theMarlborough region had rotated to an orientationmore favorable to strike-slip, that the major through going strike-slipMarlborough Faults developed, with the creation of a rapidly rotatingand more equidimensional crustal block at the northern ends of theAwatere and Clarence faults (Fig. 13b). These conclusions differ fromthose in previous studies (Barnes and Audru, 1999, Little and Roberts,1997, Wallace et al., 2007) where it is assumed that the faults have

always maintained their present orientation, but have progressivelymigrated southwards since the Late Miocene.

The boundary between the northern and southern Marlboroughdomains, presently defined by the marked change in orientation of theMarlborough Faults (Figs. 1b, 2), may represent a long term contrast inlithospheric strength. Thus, the southern Marlborough domain hasremained largely in ‘warm’ and weak continental lithosphere, awayfrom the effects of subduction and breaking up into relatively smallcrustal blocks. In contrast, the northern Marlborough domain hasremained relatively ‘cool’, consisting of relatively large crustal blocksthat have for the most part rested on the subducted slab, ever sincesubduction initiated in this part of the plate-boundary zone in the earlyMiocene (Fig. 12, Bibby, 1981, Lamb, 1988, Lamb and Bibby, 1989). Inthis case, the southern edge of the subducted slab in the Hikurangimargin has exerted a fundamental control on the mode of crustaldeformation (Fig. 12, 13, Bibby, 1981).

Acknowledgements

This researchwas carried out as part of a NERC research studentship,supported by theBurdett-Coutts fund, andheldbyKarenRandall. SimonLamb and Conall MacNiocaill received support from a Royal Society ofLondon travel grant. We thank two anonymous reviewers for helpfulcomments that have improved the manuscript.

180 K. Randall et al. / Tectonophysics 509 (2011) 165–180

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