Thermo-tectonic Implications of Zircon and Apatite FT Data of the Marlborough Region , South Island , New Zealand

Zircon and apatite fission track (FT) reveal some of the thermo-tee- tonic features of the Marlborough Region, South Island, New Zealand. The very young FT ages (＜10 Ma) of zircon and apatite in the vicinity of the Alpine Fault bend and Seaward Kaikoura Range coincide with the recent rapid uplift/erosion. Four samples with reset zircon ages in the Alpine Fault bend reveal that the host rocks in this area cooled below the closure termperature of zircon (~240℃) in the late Miocene. Unlike these four zircon FT ages, most zircon FT ages are consistent with depositional ages. Annealed apatite and unannealed zircon FT ages show that the Marlborough did not experience exposure to the closure temperature of zone (PAZ) of apatite (~60-100℃). The host rocks in the north rather than those in the south passed through the lower part of apatite PAZ. In addition, most of the zircon samples with low P( X2) values (＜5 %) show that the samples have been slightly annealed, implying that the host rocks might have experienced the upper part of the partial annealing zone of zircon (~175℃) during the Mesozoic cooling.

FT data provide information about the tectonic and thermal history of host rocks.The age and cooling of host rocks may be constrained by FT analysis.FTs in U-bearing minerals such as zircon and apatite result from the spontaneous fission of 238U, and can be applied to thermo tectonic studies.Annealing of FTs is an important feature of FT analysis.If FT ages of host rocks were consistent with depositional ages, the distributions of single-grain FT ages would have higher P( X2) values (>5% ), indicating that the host rocks had not experienced the partial annealing zone (PAZ) of zircon (175°C -245°C) or of apatite (60°C -110°C).For the an nealed FT samples, the distributions of single-grain FT ages may have low P( X2) values (<5%).
"Closure temperature (CT)" is a concept that links the observed age to the temperature at which the FT age starts to accumulate (Dodson 1973;Hodges 1991).The closure temperature for zircon is about 240°C.The closure temperature for apatite ranges from l 10°C to 125°C, depending on apatite composition (Gleadow and Duddy 1981;Green et al. 1989b).When the host rocks experience a higher temperature (>CT), FTs will be totally annealed.
Although the annealing of zircon FTs is still unknown, the kinetic of annealing in apatite (Green 1986, Green et al. 1989b;Laslett et al. 1987;Duddy et al. 1988;Crowley et al. 1991) has been established and applied to thermo-tectonic studies.Thermal histories can be recon structed from forward modeling of time-temperature histories and comparison of predicted and measured FT ages and lengths of apatite.The aim of this article is to investigate the thermo-tectonic development of basement in Marlborough by compilation of zircon data and previously reported apatite data and the modeled thermal histories of apatite (Kao 2001).

GEOLOGIC SETTING
The Marlborough region (Fig. 1) is located within the Australian-Pacific plate boundary zone at a critical position between the southern end of the Hikurangi margin and the Alpine Fault section.The Torlesse Supergroup constitutes the basement of the Marlborough region.The depositional ages of basement rocks range from the Late Jurassic to Early Cretaceous, with Triassic successions in the far west.The exposure of the Alpine Schist mainly results from Neogene denudation and partly from Cretaceous denudation (Suggate 1978b).The ages of cover strata range from Cretaceous to Quaternary (Fig. 1).When Marlborough was part of a passive margin environment, early Cenozoic sequences accumulated during a tectonically quiet period which lasted from 90 to 25 Ma (Baker and Seward 1996).The Kaikoura Orogeny has followed the tectonic quiescence since the early Miocene, reflecting development of the modem Australia-Pacific plate boundary in the region (Browne 1995).Crustal shortening and strike-slip faulting are considered to have become increasingly important in this region since the Miocene (Carter and Norris 1976;Suggate 1978a;Baker and Seward 1996).

Sampling
The Marlborough region may be divided into four blocks by the Marlborough Fault System.They are: Wairau, Inland Kaikoura, Seaward Kaikoura, and Kahutara (Fig. 1).Eighty-eight samples (9414-1 to -88) were collected from outcrops along roads throughout Marlborough, or by helicopter from the Inland and Seaward Kaikoura Ranges.(Lensen, 1962).
The separation of apatite concentrates from rock samples (-4 kg), experimental procedures, and FT dating for apatite are stated in the paper of Kao (2001).The zircon concentrates from the same rock samples were obtained by the following methods: standard magnetic an d heavy liquid techniques.Zircon concentrates were mounted in PEP Teflon™ at -300°C and ground to reveal internal surfaces.Once ground, zircon separates were polished with alumina slurry and then 1-µm diamond paste.The zircon mounts were etched in molten KOH-NaOH eutec tic at -205°C.The time of etching ranged from 16 to 56 hours, Finally, zircon mounts were cleaned by placing them in dilute HF for 1 hour.
The experimental procedures were as follows: (a) all mounts were cut to 1 x 1.5 cm and cleaned with detergent and alcohol, (b) low-uranium mica external detectors were sealed di rectly in contact with the mounts by using envelopes of heat-shrink plastic, (c) pinpricks were made at the comers of each mount-mica sandwich for subsequent location, (d) all mounts were stacked vertically with dosimeter glass standards (CNl for zircons) placed at the top and bot tom of each stack for irradiation.Each dosimeter was also mounted with a mica detector.Afterwards, all stacks were packed into canisters and irradiated at the X-7 facility of the HIF AR reactor, New South Wales, Australia.The nominal fluence of thermal neutrons was 3 x 1015 neutrons cm-2 for zircon.
The external detector method was applied to the FT dating (Gleadow 1981).The Ff ages were determined by the zeta calibration method (Hurford and Green 1982;Green 1985).A chi-square statistic was used to assess the probability of grains counted in a sample belonging to a single population of ages (Galbraith 1981).The results of weighted mean zetas are re ported in Table 1: zircon weighted mean�== 140.2 ± 3.6 (CNl).

Zircon FT Results
The distribution of zircon FT ages is illustrated in Fig. 2. Except for a few samples of poor zircon concentrates, zircon FT results of the Marlborough samples are shown in Table 2 and Figs.3-6.Transects Tl, T2, T3, and T4 are located within four blocks, respectively (Fig. 2).Uncertainties of FT ages are reported at the ls level.Zircon ages •of the samples range from 6.8 ± 0.5 to 336.2 ± 45.3 Ma.

Wairau transect (Tl)
Upper Triassic-Jurassic sandstone (greywacke) constitutes mainly the basement of the Waiaru block.The cover strata are of the late Miocene and late Quaternary.Samples 9414-19, -20, -21 and-25 (Fig. 2), lying in the vicinity of the Alpine Fault bend, have very young FT ages ( <l 0 Ma) of zircon (Table 2), indicating that they have experienced the closure tem perature of zircon (-240°C).These FT ages can be correlated with recent rapid uplift/erosion in this area.Excluding these four samples, the zircon FT ages of thirteen samples (Table 2 and Fig. 3) in the Waiaru block are consistent with the depositional ages.The zircon FT ages (> 250 Ma) of the rest of the samples are older than the depositional ages, showing that they can be correlated with the source provenance.According to these FT data, the samples of the host rocks in this block have been slightly annealed and might have passed through the upper part of the zircon PAZ (-175°C) in the Mesozoic denudation.

Inland Kaikoura transect (T2)
Twelve samples of this transect were collected from the Inland Kaikoura Range which lies between the Awatere and Clarence Faults (Figs. 1 and 2).The stratigraphic units of the   2 and Fig. 4), implying that the host rocks have not experienced the PAZ of zircon (175°C -245°C).
In addition, most host rock samples in this block are low in P(X2) values (<5%) (Table 2), showing that they might have experienced the upper part of the zircon PAZ (-175°C) during the Mesozoic denudation.

� -•
;::  (Table 2 and Fig Eight samples (9414-61 to-68) were collected from south of the Hope Fault (Figs. 1 and 2), and belong to Jurassic depositional ages.Sample 9414-61 with a young zircon FT age ( -129 Ma) (Table 2 and Fig. 6) indicates that it experienced the PAZ of zircon.Except for Samples 9414-62 and -65, the zircon FT ages of the samples (Table 2 and Fig. 6) in the Kahutara block are consistent with the depositional age, reflecting that the host rocks have not passed through the PAZ of zircon.In brief, the samples in this block might be slightly annealed.The older zircon FT ages (>200 Ma) (Samples 9414-62 and-65) may be related to the source provenance of the sediments.

A p atite Results
Modeled thermal histories (Fig. 7) of apatite samples with good length data were reported in the paper of Kao (2001 ).The modeled thermal histories indicate two major cooling events: the earlier one occurring in the mid-Cretaceous (-100 Ma) and the later one lasting from the early Miocene (-20 Ma) to the present.Annealed apatite FT data show that the host rocks experienced the PAZ of apatite.The timing of the main Neogene uplift/erosion event was earlier (mid to late Miocene) in the W airau block than that in the place to the southeast of the Seaward Kaikoura Range (late Pliocene-Pleistocene).The modeled thermal histories of the samples collected from Kaikoura Ranges reflect the continuance of the cooling event from Pliocene to the present.In addition, the samples in the north Marlborough rather than those in the south experienced partial annealing in the lower part of the apatite PAZ.

Inter p retation
Significant thermo-tectonic implications can be discussed in the light of the apatite and zircon FT data presented above.
The extremely young zircon ages (-6.8-10.0Ma) of the samples (9414-19, -20, -21 and-25) lying in the vicinity of the Alpine Fault bend reveal that the FTs have been reset during the recent rapid uplift and experienced cooling from temperatures of -240°C, the closure tem perature of zircon.Except for these reset samples, most of the zircon ages are consistent with the stratigraphic ages.In addition, there are three samples (9414-61, -79 and -81) with zircon ages (124-129 Ma) younger than the stratigraphic ages, showing that the samples have been slightly annealed.The host rocks passed through the upper part of the partial annealing zone of zircon (l 75°C) during the Mesozoic denudation.
The results of apatite and zircon FT data (Figs.3-7) can model thermal histories of the

Correlation with Other Geologic Events
Apatite and Zircon FT data show that there were two major cooling events, the earlier one occurring in the mid-Cretaceous (-100 Ma) and the later one expanding from the early Mi ocene ( -20 Ma) to the present.
The earlier cooling event can be correlated with a magmatic and extension event that occurred around -100 Ma in this region, as supported by the following evidence: (a) the Rb-Sr and FT ages of an igneous pluton, forming the peaks of the Inland Kaikoura Range, range from 105 to 93 Ma (Baker and Seward 1996), (b) one formation (105-100 Ma) unconformably • overlies Torlesse basement (Reay 1993), and (c) terrestrial sediments (94-100 Ma) overlies a marine fan delta sequence, showing an end of a regress event (Reay 1993).The later cooling event ( -20 Ma) can be correlated with the sedimentation of the Great Marlborough Conglom erate (Browne 1995) which indicates rapid uplift and erosion during the early Miocene (-20 Ma).Thrusting and shortening deformation became dominant during the late Cenozoic Kaikoura Orogeny.The initial movements of the Marlborough Fault System can be related to the Great Marlborough Conglomerate (Browne 1995).
Apatite and Zircon FT ages ( <10 Ma) of the host rocks in the vicinity of the Alpine Fault bend coincide with the recent rapid uplift/erosion, related to the continuation of Kaikoura Orogeny.The temperature-time path in the Marlborough region modeled by zircon and apa tite FT data (Fig. 8) shows the thermo-tectonic characters of two major cooling events in the present region.

CONCLUSIONS
The young ages (<10 Ma) of zircon in the vicinity of the Alpine Fault bend can be corre Fig. I. Geological map of Marlborough, South Island, New Zealand(Lensen, 1962).
Fig. 2. Distribution of zircon fission track ages (given in Ma) in Marlborough.Four major blocks, designated I, II, III, and IV in the Marlborough re gion are shown.The main faults whose names are given in Fig. 1 are indicated by solid lines.Four transects (Tl, T2, T3, and T4) shown in long-dashed lines are located within the four major blocks, respectively.The sample numbers with their prefix 9414-omitted are shown in parentheses.

Fig. 4 .Fig. 5 .Fig. 6 .
Fig. 4. Zircon fission track ages versus distance from Sample 9414-47 along Transect T2; the area between two horizontal lines is the depositional age (from Triassic to Jurassic).The distance is arbitrarily designated as positive for the samples northeast from Sample 9414-47, and as negative for the samples southwest from Sample 9414-47.
. 5) are consistent with the depositional ages (from Triassic to Jurassic), re flecting that the host rocks have not passed through the PAZ of zircon.Samples 9414-79 and 81 with young zircon FT ages (-124-129 Ma) indicate that they have been annealed.Similar to those of the Inland Kaikoura transect, most host rock samples are low in P( X2) values (<5%), reflecting that they might have also passed through the upper part of the zircon PAZ (-l 75°C) during the Mesozoic cooling.4.1.4Kahutara transect (T4)

Fig. 7 .
Fig. 7. Modeled thermal histories of Marlborough, estab lished by the apatite fission track data (modified from Kao 2001).The area be tween two short-dashed lines is the apatite partial annealing zone (PAZ).

Fig. 8 .
Fig. 8. Modeled thermal histories of the Marlborough region, assessed by zircon and apatite fission track data in the present study.The solid and dotted lines indicate the modeled thermal histories of the northern and southern parts of the Marlborough region, respectively.Ap: apatite; Zr: zircon; PAZ: partial annealing zone.
lated with the recent rapid uplift/erosion in this area.Most of the zircon FT ages are consistent with depositional ages.Three samples, collected from the SE Marlborough (Seaward Kaikoura Range and Kahutara areas), have younger zircon ages(< depositional ages), indicating that the host rocks in SE Marlborough have experienced exposure to the temperatures close to the upper zone of partial annealing for zircon ( -17 5°C) and cooled in the late Mesozoic denudation.In addition, the zircon and apatite FT data show that the host rocks in Marlborough have not experienced exposure to the PAZ of zircon in the Mesozoic burial, but passed through the PAZ of apatite.Each of the four blocks in Marlborough has experienced different temperature-time paths since the late Mesozoic cooling.The host rocks in the north rather than those in the south passed through the lower part of apatite PAZ.

Table 1 .
Results of calibration of fission track age determinations by the zeta approach.
Zircon Mean I; 140.2 ± 3.6 Analyses of zircon age standards are by extemal detector method; track densities

Table 2 .
Zircon fission track data for Marlborough samples.