Ultra-fast early Miocene exhumation of Cavalli Seamount ...

New Zealand Journal of Geology & Geophysics, 2008, Vol. 51: 29-420028-8306/08/5101-0029 © The Royal Society of New Zealand 2008

29

Ultra-fast early Miocene exhumation of Cavalli Seamount, Northland Plateau,Southwest Pacific Ocean

N. MORTIMER

GNS SciencePrivate Bag 1930Dunedin 9054, New [email protected]

W. J. DUNLAP

Research School of Earth SciencesAustralian National UniversityCanberra, ACT 0200, AustraliaPresent address Department of Geology & Geophysics,

University of Minnesota, Minneapolis MN 55455, USA

J. M. PALIN

Department of GeologyUniversity of OtagoPO Box 56Dunedin 9054, New Zealand

R. H. HERZER

GNS SciencePO Box 30368Lower Hutt 5040, New Zealand

F. HAUFF

IFM-GEOMAR Leibniz Institute for Marine SciencesWischhofstrasse 1-3D-24148 Kiel, Germany

M. CLARKNIWAPrivate Bag 14901Kilbirnie, Wellington 6241, New Zealand

Abstract We present new photographic, petrological,geochronological, and isotopic data for gneissic and graniticrocks obtained from six sample stations on Cavalli Seamountduring two cruises in 2002. These data lead to revision ofearlier conclusions based on two dredges of schist in 1999.Based on c. 100 Ma ages of zircon cores, and whole rockpetrochemistry and tracer isotopes, we interpret the protolithsof paragneisses and orthogneisses to probably have beensedimentary and plutonic correlatives of the Late CretaceousHouhora Complex. U-Pb dating of low Th/U zircon rimsconfirms an earliest Miocene high-grade metamorphicepisode. A cooling history based on Ar-Ar K-feldspar datingindicates ultra-rapid cooling (c. 2000°C/m.y.) and verticalexhumation (c. 100 mm/yr) of the rocks at 19.9 Ma. Ourpreferred tectonic model relates the amphibolite faciesmetamorphism to Northland Allochthon emplacement and

G07013; Online publication date 28 February 2008Received 28 June 2007; accepted 18 December 2007

the rapid exhumation to dextral transtension along the VeningMeinesz Fracture Zone system and/or a rapidly retreatingPacific trench.

Keywords New Zealand; Southwest Pacific Ocean; North-land Plateau; Vening Meinesz Fracture Zone; NorthlandAllochthon; tectonics; exhumation; strike-slip faulting; geo-chronology; Miocene

INTRODUCTION

Cavalli Seamount is a prominent, flat-topped, c. 40 x 20 x1 km feature located c. 100 km ENE of North Cape (Fig. 1,2) (Mitchell & Eade 1990). Geomorphically it lies off thewell-defined Northland continental slope, near the edge ofthe Northland Plateau (Herzer et al. 2000). In a cruise in1999, samples of schist were recovered from two dredgeson the east slopes of Cavalli Seamount. Post-cruise analysisand interpretation of those rocks led Mortimer et al. (2003) topropose a Miocene metamorphic core complex model. Keyresults leading to this model were establishment of (1) LateCretaceous-Paleogene sedimentary protolith and (2) rapidearly Miocene exhumation.

This paper is an update of Mortimer et al. (2003) and isbased on new, previously unpublished results from two cruisesmade in 2002 (Clark et al. 2002; Herzer et al. 2004) (Table 1).One, a dedicated geological cruise by GNS Science, made onerock dredge up the south flank of the western part of Cavalli.On a separate biological cruise by the National Instituteof Water and Atmospheric Research (NIWA), rocks wererecovered as "by-catch" at several stations on the seamount'sflat top, and bottom camera traverses were made of the seabed.Our new samples, analyses, and images give more confidenceto, and supplement, the interpretations made by Mortimer etal. (2003).

With the publication of dredging results from the NorthlandPlateau (Mortimer et al. 2007), it is now possible to viewCavalli in an onland-offshore geological context (Fig. 1).Much of the Northland continental shelf edge (<1500 m waterdepth) is underlain by the Vening Meinesz Fracture Zone(VMFZ) system of faults. Cavalli Seamount and the NorthlandPlateau (a broad region of mainly Miocene volcanoes) liebetween the VMFZ and the edge of the Kupe Abyssal Plainof the South Fiji Basin (c. 3000 m water depth). CavalliSeamount separates the Knights and Whangaroa Basins.The autochthonous basement geology of onland Northlandcomprises Waipapa Terrane (Permian-Jurassic greywacke-argillite association with minor basalt-chert-limestone) andHouhora Complex (Late Cretaceous intercalated submarinebasalt, andesite, dacite, rhyolite, tuff-breccia, conglomerate,sandstone, and mudstone) (Isaac et al. 1994; Isaac 1996).Houhora Complex igneous rocks have given U-Pb zircon agesof c. 102 Ma (GNS unpubl. data). Late Cretaceous-Oligocenesedimentary rocks rest unconformably on the basement.

30 New Zealand Journal of Geology and Geophysics, 2008, Vol. 51

175°E

Kupe Abyssal Plain

t»/ \ Miocene volcano

I Houhora ComplexED Waipapa Terrane

100 km

Fig. 1 location of Cavalli Seamount (1200 m isobath shown) inrelation to neighbouring geological features of onland and offshoreNorthland. only onshore and western parts of the allochthon areshown. late Cretaceous-Pleistocene sedimentary cover are notshown. 3Ki = Three Kings islands, NC = North Cape, K = KarikariPeninsula. Rectangle is area ofFig. 2. after isaac et al. (1994), Herzer& Mascle (1996), and Mortimer et al. (2007).

Tectonically overlying the autochthonous rocks is a now-southwest-tilted thrust sheet, the Northland allochthon, whichwas emplaced in the Waitakian (early Miocene, 25-22 Ma).Following allochthon emplacement, early Miocene volcanoesof the Northland arc erupted through and onto the allochthonand, probably in the same early-middle Miocene time interval,the dextral VMFZ juxtaposed rocks of the Northland Plateau,including Cavalli Seamount, against the Northland continentalmargin (Fig. 1).

METHODS

With a few exceptions, X-ray fluorescence (XRF), inductivelycoupled plasma mass spectrometry (iCP-MS), ar-ar and u-Pbdating, and Sr, Nd and Pb isotopic analytical methods areidentical to those reported in Mortimer et al. (2006, appendixDR1). after analysing the whole rocks by iCP-MS with aciddissolution, it was realised that there were unrealistically lowconcentrations of Zr and rare-earth elements, presumably dueto incomplete sample dissolution of minerals such as zircon.

ROCK DREDGE SITES

• Sedimentary cover

• Plutonic-metamorphic 4°20'S -

Fig. 2 location of dredge sites, and plutonic-metamorphic rocktypes obtained from Cavalli Seamount on GNS Tangaroa cruisesSF9901 and TaN0210, and on NiWa Kaharoa cruise KaH0204.Kaharoa camera station tracks 5 and 25 also shown. Bathymetryfrom Mitchell & eade (1990). For sample details see Table 1.

Samples were thus reanalysed by iCP-MS using fused beaddissolution, as indicated in Table 2. The analytical approachused in 40ar/39ar analysis is identical to that outlined inDunlap (2003), except for the following: correction factorsused are (37ar/39ar)Ca = 3.50 × 10–4, (39ar/37ar)Ca = 7.86 ×10–4, (40ar/39ar)K = 0.027. The individual step ages of theK-feldspar were calculated using the Fish Canyon sanidineneutron fluence (dose) monitor, which has an intercalibratedage of 28.1 ± 0.04 Ma (Spell & McDougall 2003).

al l samples have been lodged in the GNS Petrology "P"Collection, and associated sample and analytical data havebeen catalogued in the P e T l a B database (http://pet.gns.cri.nz).

SAMPLE DATA

During the KaH0204 cruise, 317 photographs were takenduring the course of 11 still-camera tows across the tops ofthe eastern and western parts of Cavalli Seamount. in mostcases the camera faced vertically down, perpendicular tothe seafloor. The camera was not deliberately oriented, thepurpose being to take biological images. We believe that thelong axis of the photographs was probably oriented parallelto the ship's track, although the possibility of cross-currentsand local sled disturbances means we cannot be entirely sureof this. In situ rock exposures (as opposed to sandy or muddybottom) were visible in about one-fifth of the photographs,and 21 photographs in nine different tows showed a significant

Table 1 Sample location and description data for Cavalli samples dredged in 1999 and 2002. f

3

GNSP# Deck* Sample size Rock names Notes

SF9901-1A, 23 Mar 99, -34.1050S, 174.1781E, 985-1229 m63144-48 D1A-1-4 <1 kg

SF9901-1B, 23 Mar 99, -34.1005S, 174.1668E, 577-778 m62586,62656-65,63139-63143 >100kgKAH0204-08,14 Apr 02, -34.1152S, 174.1450E, 610-640 m67667 D8-1 30x20x20 cm angular with broken face

KAH0204-14,15 Apr 02, -34.0957S, 174.1105E, 507-520 m67670 D14-1 35x25x10 cm slab

67671 D14-2 20x15x10 cm angular, irregular

KAH0204-22,16 Apr 02, -34.078S, 174.0787E, 550-610 m67673* D22-1 15x10x7 cm, angular with broken face67674 D22-2 20x20x7 cm angular

KAH0204-28,16 Apr 02, -34.0963S, 174.1148E, 490-515 m67676 D28-1 10x7x4 cm subangular block

KAH0204-32,17 Apr 02, -34.1620S, 173.9618E, 780-810 m67678* D32-1 4x3x2 cm angular

TAN0210-22,5 Aug 02, -34.2070S, 173.9784E, 1330-1750 m66836 D22-1A 5-50 cm subangular boulder

66837 D22-1B 5-50 cm subangular boulder

cataclastic schist

gt bi schist and calcsilicate gneiss

cataclastic biotite schist/gneiss

biotite paragneiss andorthogneisssillimanite biotite paragneiss

granodioritic orthogneissgranodioritic orthogneiss

pegmatite and amphibolite

cataclastic biotite paragneiss

banded biotite paragneiss

banded biotite paragneiss

6683866839

66840

6684166842

D22-1CD22-2A

D22-2B

D22-3D22-4

5-50 cm subangular boulder40x30x20 cm angular slab

5-50 cm subangular boulder

5-20 cm bored subrounded slab5-20 cm bored subrounded slab

migmatitic gneissgranitic orthogneiss

folded, veined biotite

soft sandy mudstonehard brown sandstone

See Mortimer et al. (2003).

See Mortimer et al. (2003).

Cataclastic textures prominent. Biotite retrograded to chlorite.Faults grade into open cracks with calcooze infilling.

Fine-grained biotite gneiss in contact with coarser perthitic biotite gneiss.

Quartz, oligoclase, biotite, garnet, sillimanite, FeTi oxide, titanite, muscovite.

Weakly foliated. Granodioritic composition from geochemistry.Weakly foliated. Identical to P67670.

Garnet hornblendite (inclusion?) in contact with (larger mass of?) granitepegmatite, median grain size 8 mm.

Grey-green hard rebrecciated and sediment-cemented cataclastic gneiss.

20% quartz, 60% oligoclase, 15% biotite, 5% anhedral garnet, FeTi oxide,titanite, graphite, apatite 0.3 mm median grain size. Cut by 1 cm irregularveins of granitic gneiss (see P66839) that are subparallel to, and crosscut butpredate, the main foliation.Similar to P66836 but with mica segregations spaced at 5 mm. Biotite ismore oxidised. Granitic veins are more intimately interbanded and the wholerock analysis is probbaly a mix of host gneiss and granite.Up to half of sample is irregular granitic veins. Forams in crack infill.50% quartz, 20% K-feldspar, 25% plagioclase, 5% chloritised biotite, FeTioxides, secondary muscovite. Weakly foliated.Similar to P66838. Irregular (pytgmatic, pre-main foliation) granitic veins up to2 cm wide cut gneiss. Accessory tourmaline in one granitic vein.Bioturbated, foram-bearing, some gneissic clasts.Some gneissic clasts.

I

*No thin section: hand sample description only.


Table 2 Whole rock analyses of Cavalli samples dredged in 2002.

GNSP#

Dredgerock

Sio2 (wt%)Tio2

A1AFe2o3T

Mno

MgoCaoNa 2 o

K 2 op ALOITotal

a s (ppm)

BaCeCoCr

CsCu

Dyer

eu

GaGdHf

Hol al i

l uMoNb

NdNiPbPr

RbSb

ScSmSn

Sr

TaTbThTlTm

UVY

YbZnZr

8 7 S r / 8 6 S r ± 2 σ

1 4 3 N d / 1 4 4 N d ± 2 σ

206Pb/204Pb ± 2 σ

207Pb/204Pb ± 2 σ

208Pb/204Pb ± 2 σ

P66836SF0202-22

biotite paragneiss

66.270.67

17.193.510.071.22

2.874.392.64

0.100.84

99.77

<1

101078.7

na21

2.5018

6.894.031.47

22.77.177.91

1.3934

na

0.643na

13.0

37.11330.1

9.5593.9

na117.74

na

511na

1.17

11.9na

0.6093.22

57.839.2

4.21

80.8267

0.7089220.000004

0.5126590.000002

18.8330.001

15.6280.001

38.736

0.001

Major elements and As, Cr, Cu, Ni,

P66837SF0202-22

biotite and granite gneiss

71.220.64

14.22

4.880.061.420.74

2.01

2.830.071.82

99.90

3

59956.2

50.544

3.0822

3.371.98

0.68818.983.897.52

0.70924.4

24.5

0.3600.6019.48

23.4

2615.66.32

102

0.184124.52

0.940161

0.1910.543

10.60.7310.309

2.3183.318.52.24

89.1254

0.7094830.000004

0.5124710.000002

18.8390.001

15.6290.001

38.7580.002

, Sc, and Zr by X-ray fluorei

P66839SF0202-22

granitic orthogneiss

73.790.10

14.84

0.700.01

0.271.41

4.183.94

0.100.52

99.86

1

70831.176.9<1

2.532

2.461.32

0.227

17.92.692.58

0.47315.44.54

0.202

0.1277.09

12.2<2

26.8

3.46128

0.093<2

2.672.03

1900.7950.396

7.770.632

0.199

2.908.46

15.3

1.4015.065

0.7068860.000004

0.5126430.000002

18.8330.001

15.6190.001

38.707

0.001

scence methods at Spec

P67673KaH0204-22

granodioritic orthogneiss

64.010.56

15.204.04

0.07

1.904.204.33

2.680.19

2.5399.69

<1

704

56.371.4

263.602

3.34

1.860.907

19.64.075.09

0.64927.422.1

0.3050.1935.97

24.81917.9

6.7689.30.043

12

4.732.14

534

0.4750.591

9.470.5810.280

1.7169.918.62.02

64.7175

0.7059130.000004

0.5125530.000002

18.841

0.00115.621

0.00138.798

0.001

ctrachem Analytical. Li, Co

P67674KaH0204-22

granodioritic orthogneiss

62.690.61

15.384.42

0.07

1.884.564.39

3.010.192.74

99.94

<1

81063.159.3

263.6073.74

2.090.953

20.84.397.110.732

30.825.2

0.341

0.1298.68

27.91516.9

7.5395.6

0.054

12

5.232.23

5300.3750.653

10.70.6530.317

2.0776.720.5

2.2364.6

199

0.7059190.0000050.5125530.000002

18.8380.001

15.6150.001

38.780

0.001

, Mo, Sb, Sn, Ta and Tl

by iCP-MS dissolution methods at university of Kiel. al l other elements by iCP-MS fused bead methods at university of Kiel. l o i =

loss on ignition, na = not analysed.

Mortimer et al.—Cavalli Seamount revisited 33

Fig. 3 KAH0204 camera images of Cavalli seafloor; field of view c. 2 m (see Fig. 2 for locations), inferred azimuths are speculative(see text for discussion). A, Station 5, photo 8928, showing moderately developed subhorizontal gneiss foliation cut by sediment-filledsubvertical joint which possibly strikes 170. B, Station 25, photo 9351, showing well-developed subhorizontal gneiss foliation with apro-nounced penetrative lineation that has a possible azimuth of 075. C, Station 26, photo 9369, showing rare subvertically dipping foliationwith possible strike of 040. D, Microscope image of dated granitic orthogneiss P66839, 5 mm wide, taken in plane polarised light.

measure of photogeological detail. in most of these, theshadows cast by rock mass edges revealed the presence ofa subhorizontally dipping planar anisotropic fabric, inferredto be metamorphic foliation (e.g,. Fig. 3a,B). in one case,steeply dipping, possibly isoclinally folded foliation appearedto be exposed on a subhorizontal joint face (Fig. 3C).

A summary of the hand specimen and petrographiccharacteristics of the igneous and metamorphic samplesis given in Table 1. including the two 1999 dredge sites,metamorphic and plutonic rocks have now been obtainedfrom eight sample sites up to 20 km apart on the easternand western parts of Cavalli Seamount (Fig. 2). in contrastto neighbouring seamounts on the Northland Plateau andcontinental shelf, which are extinct volcanoes (Fig. 1), it issignificant that no volcanic or volcaniclastic rocks have thusfar been obtained from Cavalli Seamount. This suggests thatthe entire seamount represents a significant submarine outcropof continental metamorphic rocks.

Compared with the rocks dredged in 1999, no calcsilicategneiss was sampled. The textures of the 2002 biotite-bearingmetamorphic rocks are more semischistose-granoblasticthan schistose, and have relatively poor fissility compared

to the 1999 rocks. Hence, in the absence of mesoscopic fieldcriteria, we describe the 2002 rocks as gneisses rather thanschists (Compton 1985) but acknowledge there is probably agradation between these rock types. The metamorphic mineralassemblages in the 2002 biotite gneisses are the same as inthe 1999 biotite schists, with the index mineral sillimanitebeing present in gneiss P67671 from the eastern seamounttop. layering and heterogeneity probably indicates that mostof the biotite gneisses had a metasedimentary protolith, andwe refer to them as paragneisses.

Plutonic rocks were entirely absent from the two1999 dredges. However, in the 2002 dredges, significantoccurrences of metaplutonic rocks (granitic and dioriticgneiss) were discovered on both the eastern and westernparts of the seamount (Fig. 2). These rock types occur bothas deformed granitic veins in gneiss (e.g., P67670, P66838)and as separate boulders of granitic and dioritic gneiss(e.g., P66839). Foliation in the metaplutonic rocks is muchweaker than in the aforementioned biotite gneisses, andbanding is absent (Fig. 2). a sample of pegmatite vein cuttingamphibolite (P67676) was obtained from the eastern top ofthe seamount.


-

-

-

-

-

-

-

-1

wt% Na2O+K2O

t+ +

/ . •/

1 1 1

+ + Jr-dti x

* ^ o ° o H

\Karikaris

1 1 r

++ o +x "tj.

D

A1 1

40 50 60

SiO2(wt%)70 SO

oo :

-

-

100:

-

-

ppm Rb

post-colllsional

xx^<>x c

Karikaris

volcanic arc

/X

+

within plate

>62wt%SiO2

B10 100

Y+Nb (ppm)

1000

1000

100

0.1

rock/primitive mantle- x - P67673 dioritic gneiss— P66839 granitic gneiss

iW5s;

Cs Rb Ba Th U Nb Th K La Ce Pb Pr Nd Sr P SmHf Zr Tl Eu GdTb Dy Ho Y Er Yb Lu

Element

1000

100

0.1

rock/primitive mantle

J u V W\/\\' \ //v w vv //

• Cretaceous Houhora Volcanics (n=7)

• Miocene Karikari Plutonics (n=21)

W IF3 D a

>62 wt%SiO2 D

Cs Rb Ba Th U Nb Th K La Ce Pb Pr Nd Sr P SmHf Zr Tl Eu GdTb Dy Ho Y Er Yb Lu

Element

10

8

6 •

4 •

2 •

0 •

-2-

-4

Nd Initial All points t = lOOMa exceptortnogneisses 20Ma and lOOMa

modem TaupoVolcanic Zone lavas

D

CD 20Ma OTO *

-60.702 0.704 0.706 0.708

87Sr/86Sr initial0.710

0.710-

0.709-

O.7O8-

0.707-

0.706-

0.705-

87Sr/86Sr

CD

• -Paragneisses & schists

O 30±56MaMSWD=5625

oOrthogneisses

47±l lMaMSWD=12

F0.0 0.4 0.8 1.2 1.6

17Rb/86Sr

2.0 2.4

REFERENCE DATAHouhora lavas +Houhora sediments xWaipapa sediments *

CAVALLI DATA1999 schist O2002 paragneiss/schist •2002 orthogneiss O

Fig. 4 Whole rock chemistry isotopic composition of Cavalli Seamount rocks compared with possible correlatives. A, Silica versus totalalkalies diagram. B, Siliceous igneous rocks trace element diagram (Whalen et al. 1987). C, D, Primitive mantle normalised multi-elementdiagram (Sun & McDonough 1989). E, Sr versus Nd tracer isotope diagram. F, Strontium isochron diagram (no useful age informationcan be derived from the isochron lines, they are plotted for reference only). Reference data from Ruddock (1990), McCulloch et al. (1994),Palmer et al. (1995), Nicholson et al. (2000), Nicholson & Black (2004), adams et al. (2005), and Mortimer et al. (1998,2003,2006).


ANALYTICAL DATA

Petrochemistry

Paragneisses

Mortimer et al. (2003) discussed the usefulness and limitationsof petrography and of whole rock geochemical and isotopicanalyses in helping correlate the metasedimentary rockswith specific geological units. Basically, the banded biotitegneisses in this study have broadly similar compositions tothe 1999 biotite schists, but overall are slightly more silica-rich. Whole rock and tracer isotope data are compatible withany one of a number of local sources from Waipapa Terraneand Houhora Complex basement, to cover sequences erodedtherefrom (Fig. 4). Dating of detrital zircons (see below) ismore informative for stratigraphic correlation.

Orthogneisses

Three analyses of the weakly foliated metaplutonic rockswere made. The two samples from KaH0204-22 are similarin composition to each other and are essentially duplicatesamples. They are granodiorites according to the petrochemicalscheme of Middlemost (1994). The third sample, P66839, isgranitic (Table 1, 2; Fig. 2B) and has 1.3 wt% corundum inthe norm (i.e., it is peraluminous).

A silica versus total alkalies diagram (Fig. 4 a ) haslimitations for metamorphic rocks. However, in overall bulkcomposition, the granodioritic orthogneisses are somewhatsimilar to the paragneisses (Fig. 4a,B) but the two sets ofgneisses plot on different arrays on an Rb-Sr isochron diagram,the paragneisses being more radiogenic (Fig. 4F). in termsof whole rock geochemistry, possible Northland correlativesof the orthogneisses are the Cretaceous (c. 100 Ma) HouhoraComplex (isaac et al. 1994; isaac 1996; Nicholson & Black2004) and the early Miocene (c. 20 Ma) Karikari plutonics(Ruddock 1990; isaac 1996) which are part of the earlyMiocene Northland arc. alternatively, they could be a newsuite of igneous rocks that have no onland equivalent. on asilica versus total alkalies diagram (Fig. 4a), both Houhoraand Karikari suites are plausible correlatives for the Cavalliorthogneisses. in terms of trace element contents in the samesilica range, the Cavalli orthogneisses have the distinctivehigh large ion lithophile element content ( l i l e , e.g., Cs,Rb, Ba, Th, u , K) of the Houhora Complex. Concentrationsof high field strength (HFS, e.g., Y, Nb, Zr) and rare-earthelements (Ree) are slightly lower and fall between therange of known Houhora Complex and those reported forthe Karikari Plutonics. Part of this poor match may resultfrom the fact that there exists a paucity of published traceelement data on the Houhora and Northland arc rocks, andthe Cavalli samples number only three. Sr and Nd isotopicdata are likewise not distinctive (Fig. 4e), but if the HouhoraComplex reference set contained dacites and rhyolites (likethat shown for Taupo Volcanic Zone), then a better matchmight be obtained.

U-Pb dating

Zircon grains were separated from two samples, P66836biotite paragneiss and P66839 granitic orthogneiss, usingstandard mineral separation techniques. Before dating,cathodoluminescence images of grains were used to identifytarget spots on cores and rims where possible. a total of 36dates were obtained from 29 spot analyses for P66836 and39 dates from 39 spots for P66839 (Tables 3,4).

Both samples gave broadly similar results: most of the datafall into two distinct age populations, 20-23 Ma and 98-99Ma, with scattered intermediate ages and several inheritedgrains (Fig. 5). For orthogneiss P66839, the principal agepopulations are 19.7 ± 0.8 Ma (2 σ error, n = 5, MSWD =2.05) and 98.3 ± 1.6 Ma (2 σ error, n = 13, MSWD = 1.16).For paragneiss P66836, the principal age populations are22.9 ± 0.6 Ma (2 σ error, n = 7, MSWD = 1.72) and 99.2 ±1.4 Ma (2 σ error, n = 14, MSWD = 1.31). a large numberof grains in the orthogneiss P66839 gave a range of agesintermediate between the two principal age populations. Withthe relatively low precision of the la-iCP-MS method, someof these intermediate ages appear concordant (Fig. 5), but208Pb/232Th data indicate they are not. a total of six grains inthe paragneiss and one grain in the orthogneiss had 206Pb/238uages of 120-610 Ma, which we interpret as inherited detritalcores. The ages of these inherited grains broadly match thoseof detrital zircons from the Waipapa Terrane basement ofonland Northland (adams et al. 2007). al l spots that yieldedearly Miocene ages (resolved rims in the case of grains fromP66836) have high u concentrations and correspondingly lowTh/u ratios compatible with a metamorphic origin (Rubatto2002; Hoskin & Schaltegger 2003). in contrast, spots withlate Cretaceous ages have u concentrations and Th/u ratiosmore consistent with an igneous paragenesis.

Ar-Ar and K-Ar dating

K-feldspar is plentiful in the weakly foliated graniticorthogneiss P66839 (Fig. 3D). a step heating experimentreveals an essentially flat plateau from which a very preciseage of 19.89 ± 0.05 Ma (2 σ) can be obtained (Fig. 6, Table5). There is no statistically significant slope on this plateau.Thermal modelling of the spectrum to obtain a 350-150°Ctime versus temperature cooling curve was not done, as theresult would simply be a line parallel to the temperature axisat 19.9 Ma. it is clear that P66839 has cooled exceptionallyrapidly for a holocrystalline high grade metaplutonic rockwith a 1—2 mm grain size. Taking the error limits literally,this would amount to some 200°C in 100 000 yr for thispart of West Cavalli Seamount, a result more than threetimes the maximum cooling rate inferred for part of eastCavalli by Mortimer et al. (2003). assuming a geobarometricdepth of c. 10 km for the rocks (Mortimer et al. 2003), thisconverts to a vertical exhumation rate of 100 mm/yr. TheCavalli rocks are thus at the extreme upper limit of knowncooling and exhumation rates from a wide range of geologicalsettings (Ring et al. 1999; Dunlap 2000; Rubatto & Herrmann2001).

Biotite from granodioritic orthogneiss P67674 gave aK-ar age of 19.3 ± 0.5 Ma (K = 6.72 wt%, 40Ar* = 2.22 ×10-10mol/g, %40ar* = 46.5).

DISCUSSION

Protoliths

Paragneiss

The dominance of zircon grains of c. 100 Ma age with igneousparentage in the paragneiss suggests a strong detrital influencefrom rocks similar in age to the c. 100 Ma Houhora Complex(isaac et al. 1994; isaac 1996). We cannot, on existing data,firmly establish if the protoliths were actually Cretaceous


0 20 40 60 80 100 120Age (Ma)

P66839 granitic orthogneiss

0.03 '

0.02 •

0.01 •

M S P b /

jA

2 3 B u P66836

/ ~ ^

2 ° 'Pb/ 2 3 5 U

0.05 0.1 0.15 0.2

0 20 40 60 80 100 120Age (Ma)

1.2

0.8

0.4

0.0

Th/U

1

• P66836• P66839 •

0 20 40 60 80 100 120Age (Ma)

0.02 •

0.01 •

2 K P b /

*°"

2 3 8 U

?

/

-o-

P66839

x/

2 0 7 p b / 2 3

/

5u

Fig. 5 u-Pb ages and Th/uratios of zircon grains fromgarnet biotite paragneiss P66836and granitic orthogneiss P66839.Black symbols are concordantanalyses that gave pooled earlyMiocene and late Cretaceousage populations. White symbolsare discordant analyses excludedfrom the early Miocene and lateCretaceous age populations.

0.05 0.1 0.15 0.2

24

strata or, like the schist dated by Mortimer et al. (2003),probable Cenozoic strata whose detrital mineralogy wasderived from local basement (in this case mainly HouhoraComplex with minor Waipapa Terrane). if a Cretaceous agefor the intruding orthogneisses is accepted (see below) thenthe West Cavalli paragneiss would indeed have a Cretaceousdepositional age.

Orthogneiss

on balance we interpret the geochemical, tracer isotope,and u-Pb zircon data to support a Cretaceous rather thanMiocene age of igneous rock formation. evidence in supportof a Cretaceous age is the contrast in Th/u of the zircons(Cretaceous = igneous values, Miocene = metamorphicvalues, Fig. 5), the high l i l e content of the rocks (similarto Houhora Complex, Fig. 4D), and the initial 87Sr/86Sr andεNd ratios which should be more similar to the paragneisses ifthe granite was a Miocene melt from the host rock (Fig. 4e,F).We attribute the imperfect match in the petrochemistry ofthe Cavalli samples with the onland Houhora samples to(1) aforementioned small datasets, and (2) the presencebetween onland Northland and Cavalli Seamount of thestrike-slip VMFZ. This means that Cavalli Seamount mayhave originated up to 500 km northwest of its present position(Fig. 7) (Mortimer et al. 2007).

23-

22-

17-

16

Age (Ma) P66839 K-feldspar, granitic orthogneiss

Plateau age 19.89±0.05 Ma (2 sigma)

MSWD = 1.2, 39/43 steps, 99% gas

Error weighted plateau slope 0.10±0.19

0.0 0.2 0.4 0.6 0.8 1.0

O.O1O

0.005-

0.000

300

200

100

0.2 0.8 1.00.4 0.6

Fraction 39Ar released

Fig. 6 40ar/39 a r age spectrum (age versus fraction of 3 9ar released)and accompanying K/Ca and K/Cl ratios for K-feldspar from graniticorthogneiss P66839.

Table 3

Spot

U-Th-Pb isotope data for Cavalli paragneiss P66836.

Pb*ppm

Uppm

Th/Uatomic ^ P b / 2 3 ^ ± l a

Metamorphic population pooled age (« = 7, MSWD =02-r05-rOlb-r17b-r04-r08-r28-r

11.258.954.664.623.307.775.35

3374279413691165929

22321449

0.040.020.060.030.020.030.05

Igneous population pooled age (n =29b-c1124Ola-c0322093012a-c26a-c18a-c27a-c1315

2.393.664.889.117.499.029.455.462.417.586.739.145.252.86

157216285541436523507310146451416494303158

0.450.680.720.630.710.741.050.660.460.530.420.920.590.71

0.003530.003500.003580.003870.003780.003720.00386

1.52.12.36.05.22.73.0

= 14, MSWD = 1.310.014590.015220.015350.015420.015480.015410.015480.015850.015810.015810.015780.015790.015940.01639

2.03.21.01.41.10.80.91.01.61.51.61.21.81.7

Spots showing intermediate dates and/or inheritance19-r14-r20-r18b-r12b-r27b-r26b-r25-r29a-r17a-c1007160621

7.664.027.575.052.655.563.266.163.485.059.684.348.25

83.3359.77

1994102219041207555772477901425235362115187

1364613

0.080.070.070.080.100.360.170.170.200.640.760.490.720.270.21

0.003940.004060.004150.004380.004800.006700.006960.006910.008190.019630.023690.036000.039570.059770.09964

3.92.62.85.24.34.13.42.62.01.80.81.11.911

0.5

207pb/235U ± l a 207pb/206pb ± l a

1.72,1 a absolute external error)0.02720.02370.02830.05800.04060.02900.0237

6.98.1

1320251312

0.05580.04920.05740.10880.07790.05660.0445

, 1 a absolute external error)0.09600.11110.12520.11150.10820.09960.10260.13320.12640.11120.10790.10620.10180.1220

0.02700.03180.03090.04400.05140.06880.08820.05600.06030.18010.17690.24320.27420.57280.8434

13164.46.04.24.04.25.49.06.46.45.17.99.2

27191212171429109.49.93.45.46.3

121.5

0.04770.05300.05920.05250.05070.04690.04810.06090.05800.05100.04960.04880.04630.0540

0.04980.05670.05410.07290.07780.07440.09180.05880.05340.06650.05420.04900.05030.06950.0614

6.77.9

1219251212

13164.35.84.14.04.15.38.96.26.25.07.79.1

27191210171429

9.99.29.73.35.36.02.61.4

208pb /232T h

0.006130.001920.004620.030070.015970.007370.00606

0.004810.005660.005170.005310.005020.004910.004880.006130.005770.005440.004980.005180.005620.00520

0.005560.006040.004710.004700.006790.004960.005620.005200.005410.007070.007900.011440.012920.037690.03153

± l a

9.0172213191316

106.22.75.02.12.01.53.05.04.14.21.94.14.0

2010201337

5.7178.85.4

101.93.13.04.11.4

20«p b* /238U

age (Ma)

22.522.522.723.023.423.624.9

93.496.896.898.198.798.799.099.899.9

100.7100.8100.9102.2104.1

25.225.826.427.329.741.642.343.752.2

122.6150.0228.5250.5367.4611.5

i l u

0.40.50.61.51.40.70.8

2.03.21.01.41.10.80.91.11.71.51.71.31.81.8

1.10.80.81.41.41.82.01.21.12.31.32.64.8

40.63.0

^ P b / ^ Uage (Ma)

22.722.523.024.924.323.924.8

93.497.498.298.699.098.699.0

101.4101.1101.1101.0101.0102.0104.8

25.326.126.728.230.843.044.744.452.6

125.3150.9228.0250.2374.3612.3

± l a

0.30.50.51.51.30.60.7

1.93.11.01.41.10.80.91.01.61.51.61.21.81.7

1.00.70.71.51.31.81.51.11.02.21.22.54.7

41.32.9

^ P b / 2 3 ^age (Ma)

27.223.828.457.240.429.023.7

93.0107.0119.8107.4104.396.499.2

127.0120.8107.0104.0102.598.5

116.9

27.131.730.943.850.967.585.855.459.5

168.2165.4221.0246.1459.8621.0

± l a

1.91.93.5

11.310.03.62.9

11.516.15.06.14.23.73.96.4

10.36.56.35.07.4

10.2

7.35.93.75.08.69.3

23.65.55.4

15.35.1

10.713.743.1

6.9

Percentconcord

839581436082

105

10091829295

1021008084949799

10490

9482866461645280887591

1031028199

O/Q 2 0 6 p b

common

1.160.331.368.304.031.25

-0.25

-0.020.621.400.550.33

-0.130.011.611.230.370.190.09

-0.210.73

0.401.260.943.353.993.515.861.490.792.260.63

-0.21-0.12

1.920.14

SpotMSWD

1.001.390.762.071.091.882.33

0.511.610.840.661.510.991.241.100.760.981.531.011.210.86

1.680.531.401.570.761.760.901.711.170.541.210.941.96994

2.21

Age(Ma)

22.922.522.522.723.023.423.624.9

99.293.496.896.898.198.798.799.099.899.9

100.7100.8100.9102.2104.1

iiiiiiiii

122.6150.0228.5250.5367.4611.5

± l a

0.30.40.50.61.51.40.70.8

0.72.03.21.11.51.20.91.01.21.81.61.71.31.91.9

2.31.32.64.8

40.63.0

Mortim

a

IBo

a.

* = radiogenic component only (207Pb-based common-Pb correction), a/b = two ages resolved from progressive drilling of a single zoned single spot, r = rim , c = core analysis where zonation is evident, i = intermediatedate of no geological significance.

Table 4 U-Th-Pb isotope data for Cavalli orthogneiss P66839.

Spot Pb* U Th/U ^ P b / ^ U ±1 a 2<r7Pb/B5U ±1 a ™PbP>*Pb ±1 appm ppm atomic

± l a ^Ph* / 2 3 ^ ± l a ^ P b / ^ U ±1 a 207Pb/B5U ±1 a Percent %206Pb Spot Ageage (Ma) age (Ma) age (Ma) concord common MSWD (Ma)

Metamorphic population pooled age (« = 5, MSWD =0214201321

14.6518.264.481.673.63

505261241469582

1180

0.240.310.280.050.07

Igneous population pooled age (n :

1612-c150917-c3626370603192423

4.8911.174.97

10.179.72

12.055.948.304.54

13.767.30

24.048.51

316644270600595711360499277791425

1319All

0.530.851.150.700.570.600.570.590.510.700.640.831.24

0.002960.002990.003090.003100.00328

= 13, MSWD0.014520.014880.014920.015290.015300.015460.015480.015470.015530.015710.015760.015950.01610

0.60.50.91.31.0

= 2.05,1 a absolute external error)0.01840.01890.02280.02260.0207

2.22.04.25.85.1

0.04500.04590.05360.05290.0457

= 1.16,1 a absolute external error)0.81.31.00.81.60.70.80.90.90.60.70.60.7

Spots showing intermediate dates and/or inheritance34-r0131-r042910081822071139323038283340253527-c

7.954.066.694.229.81

14.1011.4511.029.11

12.5517.2813.7211.4528.6012.0410.6815.294.22

11.9522.2612.64

220110891489708

1620240619301407860

106614811175818

2227926804

1003328829

1152388

0.090.120.130.220.380.240.180.220.450.530.520.501.170.650.680.681.120.440.711.591.34

0.003760.003880.004600.005810.005520.005790.005970.007990.009960.010820.010790.010940.011000.011530.011720.011880.012250.012340.012760.014200.02520

2.01.12.42.10.90.90.82.92.41.40.81.30.91.20.80.80.81.50.90.80.6

0.09910.10640.10260.10580.10220.11150.11190.10240.09950.10540.10420.10780.1168

0.02440.02870.03140.07340.03670.03790.03940.05490.06570.08040.07190.07540.07180.07880.08030.08140.08190.07970.08490.09210.1696

4.05.83.93.26.83.23.63.24.22.23.21.63.3

7.24.46.36.12.92.73.24.03.82.82.52.63.42.03.32.73.44.82.73.44.0

0.04950.05190.04990.05020.04850.05230.05240.04800.04650.04870.04800.04900.0526

0.04710.05360.04950.09150.04830.04750.04780.04980.04790.05390.04830.05000.04730.04960.04970.04970.04850.04680.04830.04710.0488

2.21.94.15.75.0

3.95.73.73.16.63.13.53.14.12.23.11.53.2

6.94.35.85.82.72.53.12.72.92.42.42.23.31.63.22.63.34.62.53.34.0

0.001020.000990.001060.001580.00192

0.005110.005400.004790.004990.004750.005890.004970.004940.004970.005060.004990.005070.00547

0.003260.002880.003850.005480.004110.003570.004210.003510.004330.004810.004500.004400.004210.004540.004150.004440.004370.004660.004720.004510.00812

1.41.63.19.45.3

2.12.01.51.53.72.21.81.52.01.21.60.91.3

5.93.75.06.61.51.91.62.91.71.41.41.11.31.01.31.61.22.11.31.31.5

19.119.319.719.821.1

92.894.895.397.597.898.398.599.099.5

100.4100.8101.9102.4

24.224.729.535.335.437.238.351.163.868.869.170.070.573.774.975.978.479.281.791.0

160.5

0.10.10.20.30.2

0.81.31.00.81.60.70.80.90.90.60.70.60.8

0.50.30.70.80.30.30.31.51.51.00.60.90.60.90.60.60.61.20.70.81.1

19.119.219.920.021.1

92.995.295.597.897.998.999.099.099.3

100.5100.8102.0103.0

24.224.929.637.435.537.238.451.363.969.469.270.270.573.975.176.178.579.181.790.9

160.5

0.10.10.20.30.2

0.81.30.90.81.60.70.80.80.90.60.70.60.7

0.50.30.70.80.30.30.31.51.51.00.60.90.60.90.60.60.61.20.70.81.0

18.519.022.922.720.8

95.9102.799.2

102.198.8

107.3107.799.096.3

101.8100.6103.9112.2

24.528.731.471.936.637.839.254.364.778.570.573.970.477.078.479.579.977.982.889.5

159.1

0.40.40.91.31.1

3.65.73.73.16.43.23.63.03.92.23.01.63.5

1.71.31.94.31.01.01.22.12.42.11.71.82.31.52.52.02.63.62.13.05.9

1031018788

102

97939696999292

10010399

1009892

998794529799989599889895

10096969698

10299

102101

-0.18-0.06

0.890.80

-0.09

0.200.490.240.270.060.540.550.00

-0.190.08

-0.010.120.56

0.070.880.355.840.190.090.130.340.070.800.110.32

-0.010.260.270.270.11

-0.090.08

-0.09-0.05

1.771.521.231.071.09

1.210.971.501.591.611.451.372.031.261.571.072.301.25

1.801.861.844.252.762.043.09

33.5219.678.831.856.021.68

11.831.532.261.633.132.981.290.62

19.719.119.319.719.821.1

98.392.894.895.397.597.898.398.599.099.5

100.4100.8101.9102.4

iiiiiiiiiiiiiiiiiiii

160.5

0.40.10.10.20.30.2

0.80.81.31.00.81.60.70.80.90.90.60.70.60.8

1.1

21

N&1o

sooi.o*Ba.Oo&*

o 'to

8OO

* = radiogenic component only (2<r7Pb-based common-Pb correction), r = rim, c = core analysis where zonation is evident, i = intermediate date of no geological significance.

Table 5 Ar-Ar data for K-feldspar from P66839. X40 K = 5.5430E-10, J = 2.9195E-4.

T(°C)

45045050050055055060060065065070070075075080080085085090090095095095010001000105010501050110011001100110011001100110011001200123012601290132013501450

Total

36Ar (mol)

9.853E-161.249E-161.676E-161.043E-162.603E-166.069E-172.255E-165.624E-172.059E-165.305E-171.830E-163.972E-171.231E-164.144E-171.360E-164.437E-171.455E-166.505E-171.751E-169.178E-171.856E-161.666E-162.524E-162.878E-162.847E-164.791E-164.380E-163.958E-162.983E-162.994E-163.629E-164.463E-165.234E-166.819E-168.628E-161.520E-155.165E-161.708E-161.537E-161.223E-161.694E-163.118E-164.313E-15

1.653E-14

37 Ar (mol)


4.091E-15

38Ar (mol)


3.789E-14

39Ar (mol)

3.404E-161.311E-153.575E-164.146E-168.740E-169.125E-161.542E-151.767E-152.634E-153.374E-154.370E-155.555E-155.809E-157.087E-156.225E-156.898E-155.221E-155.905E-154.894E-156.165E-155.030E-156.138E-156.767E-154.074E-156.126E-157.271E-159.986E-151.096E-146.423E-159.397E-151.193E-141.303E-141.23 IE-141.234E-148.904E-158.354E-156.069E-157.738E-156.179E-152.605E-159.846E-164.112E-166.672E-16

2.354E-13

""Ar (mol)


1.380E-11

"/o^Ar*

8.13.9

23.036.831.665.546.879.662.589.075.494.785.795.685.495.281.892.178.189.678.082.477.564.773.565.874.578.373.680.380.978.975.169.957.241.560.285.283.872.640.314.72.0

40Ar*/39Ar(K)

75.791.14441.4243.2740.6837.3938.0236.8338.4537.8137.9238.0837.8037.8237.8538.2037.0638.2637.8338.1538.7637.7338.0938.2338.2237.5137.9238.4838.3538.3538.2537.9638.0037.8738.3638.1337.9937.6638.1236.8734.3038.7139.22

37.88

Cum 3 9Ar

0.10.70.91.01.41.82.43.24.35.87.610.012.415.418.121.023.225.827.830.432.635.238.139.842.445.549.754.457.161.166.271.777.082.286.089.592.195.498.099.199.599.7100.0

Age

39.480.60

21.6922.6421.3019.5919.9219.2920.1419.8119.8619.9519.8019.8119.8320.0119.4120.0419.8219.9820.3019.7619.9520.0220.0219.6519.8620.1620.0920.0920.0319.8819.9019.8420.0919.9719.9019.7219.9619.3217.9820.2720.54

19.84

±

±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±±

±

la

8.241.811.720.980.840.430.450.200.340.140.160.100.140.080.160.120.160.180.180.130.200.160.120.200.160.210.140.140.240.160.130.110.130.210.310.370.200.150.200.441.554.84

23.78

0.29

Ca/K


Cl/K

3.43E-025.33E-047.39E-035.47E-037.33E-032.23E-033.66E-031.93E-033.05E-032.27E-032.03E-031.81E-031.95E-031.86E-032.11E-031.47E-031.51E-031.84E-036.34E-041.92E-032.15E-031.99E-032.25E-032.32E-032.15E-031.92E-032.23E-032.12E-032.51E-032.39E-032.54E-0

2.10E-032.56E-032.69E-032.15E-032.10E-032.27E-031.96E-031.78E-033.49E-032.63E-033.87E-037.11E-03

I

3

I


100-30 Ma

BNAIIoch

Model 1:NE-dippingsubduction 25-22 Ma

NAIIoch

Model 2:SW-dippingsubduction 25-22 Ma

A Active arcS Active shoshonites

Remnant arc• BABB (age in Ma)" ^ Emplaced allochthon •-«$• Houhora Complex

Fig. 7 Schematic cross-sections and map showing tectonic context of Cavalli Seamount (C) in the oligocene-Miocene. Cross-sectionsare based on Malpas et al. (1992) and oriented approximately northeast-southwest across the continental margin between New Zealand(NZ) and the Reinga Ridge (RR). Arrows emphasise significant vertical and horizontal motion. A, late Cretaceous Cavalli protolithsdeposited near bottom of sediment wedge on older Mesozoic orogen. B, Northeast-dipping subduction model for Northland allochthonemplacement. C, Southwest-dipping subduction model for Northland allochthon emplacement. D, Rapid post-allochthon, early Mioceneexhumation to sea level of Cavalli Seamount can be related to dextral (and possibly transtensional) strike-slip movement on the VeningMeinesz Fracture Zone (VMFZ), Pacific trench rollback, an opening Kupe Abyssal Plain back-arc basin, and widespread subduction-relatedvolcanism and shoshonitic rift-related volcanism. Twin subducted slabs are derived from different along-strike portions of the same Pacificslab on different sides of the VMFZ. Cross-section line d-d is offset on VMFZ. E, Map view showing features described in panel D. NZ= New Zealand, NC = New Caledonia, NP = Northland Plateau, VMFZ = Vening Meinesz Fracture Zone, CFZ = Cook Fracture Zone,NB = Norfolk Basin, MaP = Minerva abyssal Plain, KaP = Kupe abyssal Plain, NR = Norfolk Ridge, 3KR = Three Kings Ridge, TKR= Tong-Kermadec Ridge, l R = loyalty Ridge, BaBB = back-arc basin basalt dredge site.

We interpret the dated orthogneiss sample and smallergranitic veins (Table 1) to have been parts of subvolcanicdikes and plutons that intruded a la te Cretaceous silicicvolcanic-volcaniclastic carapace, similar to the case of theHouhora Complex (isaac et al. 1994; isaac 1996). a s such,the broad similarity between orthogneisses and paragneissbulk rock compositions need not be coincidence. We do notentirely rule out some remobilisation and/or migmatisationof the Cretaceous protoliths in the early Miocene (e.g., at thetime of intrusion of the Karikari Plutonics).

Metamorphism and exhumationMortimer et al. (2003) were unable to constrain the age ofprograde amphibolite facies metamorphism of the schistsand gneisses. The new u-Pb data presented in Fig. 5 indicatemetamorphic zircon growth possibly as early as 23 Ma andpossibly as late as 20 Ma. This definitely seems to link theprograde metamorphism with other early Miocene events inonland and offshore Northland (Mortimer et al. 2003,2007),rather than to some Cretaceous or Paleogene event. The ar -ardating of a single separate of K-feldspar from West Cavalli(P66839) provides a simpler and less ambiguous coolinghistory than the multiple minerals and methods used byMortimer et al. (2003) for the east Cavalli dredge site. P66839

underwent exceptionally rapid cooling rates of c. 2000°C/Ma through the 350-150°C temperature range at 19.9 Ma.The cooling interval of 23—21 Ma cited by Mortimer et al.(2003) was arrived at by the need to accommodate a 25 ± 4Ma zircon fission track age. While different parts of Cavallimay have been exhumed at different times (the 1999 and2002 sample sites are 20 km apart), the current study wouldappear to provide a more precise and consistent result, withwhich all thermochronological and micropaleontological dataof Mortimer et al. (2003), except the zircon fission track age,are entirely compatible. The inferred vertical exhumation rateof c. 100 mm/yr is comparable to moderately fast horizontalplate tectonic rates and matches the inferred 67-100 mm/yrrate calculated for the VMFZ from the alignment of Kupeabyssal Plain hotspot tracks (Mortimer et al. 2007).

Tectonic modelFrom the combined results of the 1999 and 2002 dredges,Cavalli Seamount protoliths are now known to consist ofcorrelatives of late Cretaceous-Paleogene sedimentary coverderived from Waipapa Terrane (Mortimer et al. 2003) and alsolate Cretaceous sedimentary and plutonic rocks similar toHouhora Complex (this study) (Fig. 7 a ) . Both these rock unitsare present in autochthonous basement of present day onland


Northland, although the Cavalli rocks may actually have beenripped off the continental Reinga Ridge many hundreds ofkilometres to the northwest where Houhora-like rocks areknown to occur (Mortimer et al. 1998). This arises as a simpleconsequence of the position of Cavalli Seamount oceanwardof the VMFZ (Fig. 1), and of closing the Norfolk Basin, andperhaps parts of Kupe Abyssal Plain, along the VMFZ (Fig. 7)(Herzer & Mascle 1996; Mortimer et al. 2007).

The Northland Allochthon was emplaced in the interval25-22 Ma, thus it is plausible to associate the 23-20 MaCavalli high grade metamorphism with crustal thickeningassociated with allochthon emplacement, similar to modelsof Eocene allochthon emplacement and metamorphism inNew Caledonia (Cluzel et al. 2001; Crawford et al. 2003).The recovery of plutonic and metamorphic rocks of similar,amphibolite facies, metamorphic grade from a wide area onCavalli Seamount (Fig. 2) would seem to argue against therocks occurring as a thin sub-allochthon metamorphic sole(one option considered by Mortimer et al. 2003) but wouldsuggest a distribution throughout a more significant crustalvolume. Two different scenarios for tectonic burial of theCavalli rocks to amphibolite facies depths in thickened crustbeneath the Northland Allochthon are presented in Fig. 7B,C(after Malpas et al. 1992). Neither the present study, nor thatof Mortimer et al. (2007), is able to provide a test of thepolarity of emplacement of the Northland Allochthon.

The early Miocene was a time of intense tectonomagmaticactivity and change in and near northern New Zealand.Mortimer et al. (2007) attributed this to tandem openingof the Norfolk and Kupe back-arc basins behind a rapidlyeast-retreating Pacific trench, separated from the continentalcrust of Zealandia by the VMFZ (Fig. 7D,E). Between thesouthern Norfolk Basin and offshore Northland, the VMFZis known to be a major dextral strike-slip fault system (Isaacet al. 1994; Herzer & Mascle 1996).

The exhumation event in which Cavalli schists and gneissescame from c. 10 km depth to exposure and planation at sealevel by the Altonian (19-17 Ma) (Mortimer et al. 2003) isnow more accurately and precisely dated at 19.9 Ma andinferred to have had an exceptional vertical exhumation rateof c. 100 mm/yr. Exhumation clearly followed the 25-22 Maemplacement of the Northland Allochthon. The images ofstretching lineations and sediment-filled cracks (Fig. 3A,B)provide outcrop information that was unavailable to Mortimeret al. (2003) that also can be fitted into a model. Unfortunately,we do not know if the Cavalli rocks are part of a longer linearbelt, or are a unique point occurrence.

Mortimer et al. (2003) noted that the rapid early Mioceneexhumation of Cavalli Seamount was consistent with it beingin the lower plate of an early Miocene metamorphic corecomplex. Our new data still support this conclusion but wecan now better identify the tectonic controls on exhumation.Fast rates of vertical exhumation of metamorphic rockshave been associated with extensional metamorphic corecomplexes where oceanic spreading ridges propagate intocontinental borderlands (e.g., >10 mm/yr exhumation rate,Baldwin et al. 2004), with continent-continent collision zones(e.g., 34 mm/yr, Rubatto & Hermann 2001; 6-9 mm/yr,Little et al. 2005) and in complex back-arc basin settings(e.g., >4 mm/yr, Thomson et al. 1998; Ring & Reishmann2002). Core complex formation and exhumation has alsobeen described from intracontinental strike-slip regimes(e.g., Whitney et al. 2007). Although the 10 km paleodepthestimates for the Cavalli gneisses is not well constrained,

the inferred exhumation rate far exceeds the aforementionedestimates which are generally considered to be fast (seealso Ring et al. 1999). We believe that the extreme Cavalliexhumation rate is a result of the coincidence in space andtime of multiple tectonic conditions favouring local verticalmotion. Contributing factors to the rapid exhumation wereprobably: (l)upto 100 mm/yr dextral motion on the adjacentVMFZ; (2) rapid Pacific trench rollback; (3) possible softlink to a coeval rapid back-arc spreading in the Kupe AbyssalPlain; and (4) buoyancy-assisted uplift of an overthickenedcontinental crust fragment against the VMFZ.

Figure 7D,E shows Cavalli Seamount in this regionalcontext of dextral transform faulting with subduction- andrift-related volcanoes erupting on both sides of the VMFZ.Improved understanding of the regional kinematic significanceof Cavalli Seamount must await seismic surveys of theNorthland continental margin to better define controllingstructures, and local multibeam bathymetric mapping ofCavalli Seamount to enable matching of our hand sample andoutcrop scale data with regional scale fault patterns.

CONCLUSIONS

Two cruises to Cavalli Seamount in 2002 added to the materialobtained on an earlier cruise in 1999 (Mortimer et al. 2003).The 2002 cruises yielded outcrop photographs, more samplesof high grade metasedimentary rocks, and, for the first time,metaplutonic rocks. Our analytical work indicates that LateCretaceous protoliths similar to Houhora Complex arepresent on West Cavalli, as well as the previously establishedWaipapa Terrane-derived sedimentary cover rocks on EastCavalli. U-Pb dating of zircons indicates amphibolite facies(sillimanite grade) metamorphism to have taken place inthe interval 20-23 Ma, and is plausibly related to crustalthickening associated with emplacement of the NorthlandAllochthon. Ar-Ar dating of K-feldspar indicates that ultra-fast exhumation took place at 19.9 Ma. Extremely rapidmid-crustal exhumation took place in a regime of intra-arcdextral transtension between the VMFZ and a retreatingPacific trench.

ACKNOWLEDGMENTS

We are grateful to the Captain and crew of the RVs Tangaroa andKaharoa for successful cruises in 2002. Neville Orr, John Simes,Belinda Smith Lyttle, and John Hunt provided their usual highquality technical support. Comments from two anonymous refereeshelped improve the content of the paper. We also thank the AustralianInstitute of Nuclear Science and Engineering (AINSE) and theAustralian Nuclear Science Technology Organisation (ANSTO)for facilitating irradiation of samples. Funded by the New ZealandFoundation for Research Science and Technology (FRST) ContractCO5X0703 (Physical Resources of the Oceans programme). NIWA'scruise was funded by FRST Contract CO1X0508 (SeamountBiodiversity and Processes).

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