HAL Id: hal-02133783 https://hal.archives-ouvertes.fr/hal-02133783 Submitted on 19 May 2019 HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés. Peak metamorphic temperature and thermal history of the Southern Alps (New Zealand) O. Beyssac, S.C. Cox, J. Vry, F. Herman To cite this version: O. Beyssac, S.C. Cox, J. Vry, F. Herman. Peak metamorphic temperature and thermal his- tory of the Southern Alps (New Zealand). Tectonophysics, Elsevier, 2016, 676, pp.229-249. 10.1016/j.tecto.2015.12.024. hal-02133783
55
Embed
Peak metamorphic temperature and thermal history of the ...
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
HAL Id: hal-02133783https://hal.archives-ouvertes.fr/hal-02133783
Submitted on 19 May 2019
HAL is a multi-disciplinary open accessarchive for the deposit and dissemination of sci-entific research documents, whether they are pub-lished or not. The documents may come fromteaching and research institutions in France orabroad, or from public or private research centers.
L’archive ouverte pluridisciplinaire HAL, estdestinée au dépôt et à la diffusion de documentsscientifiques de niveau recherche, publiés ou non,émanant des établissements d’enseignement et derecherche français ou étrangers, des laboratoirespublics ou privés.
Peak metamorphic temperature and thermal history ofthe Southern Alps (New Zealand)
O. Beyssac, S.C. Cox, J. Vry, F. Herman
To cite this version:O. Beyssac, S.C. Cox, J. Vry, F. Herman. Peak metamorphic temperature and thermal his-tory of the Southern Alps (New Zealand). Tectonophysics, Elsevier, 2016, 676, pp.229-249.10.1016/j.tecto.2015.12.024. hal-02133783
Peak metamorphic temperature and thermal history of the 1
Southern Alps (New Zealand) 2
3
Beyssac O. (1),*
, Cox S.C. (2)
, Vry J. (3)
, Herman F. (4)
4 5
(1) Institut de Minéralogie, de Physique des Matériaux, et de Cosmochimie, UMR 6 CNRS 7590, Sorbonne Universités – UPMC, Muséum National d’Histoire 7 Naturelle, IRD, 4 place Jussieu, 75005 Paris, France 8
(2) GNS Science, Private Bag 1930, Dunedin, New Zealand 9 (3) Victoria University of Wellington, P O Box 600, Wellington, New Zealand 10 (4) Institute of Earth Surface Dynamics, University of Lausanne, Switzerland 11
14 Submitted to Tectonophysics. Word count ca 14400 all included, 12 Figures, 2 tables. 15
16
17
Abstract 18
The Southern Alps of New Zealand result from late Cenozoic convergence between 19 the IndoAustralian and Pacific plates, and are one of the most active mountain belts in 20 the world. Metamorphic rocks carrying a polymetamorphic legacy, ranging from low-21
greenschist to high-grade amphibolites, are exhumed in the hanging wall of the 22 Alpine Fault. On a regional scale, the metamorphic grade has previously been 23 described in terms of metamorphic zones and mineral isograds; application of 24 quantitative petrology being severely limited owing to unfavourable quartzo-25 feldspathic lithologies. This study quantifies peak metamorphic temperatures (T) in a 26 300 x 20 km area, based on samples forming 13 transects along-strike from Haast in 27 the south to Hokitika in the north, using thermometry based on Raman spectroscopy 28 of carbonaceous material (RSCM). Peak metamorphic T decreases across each 29 transect from ≥ 640°C locally in the direct vicinity of the Alpine Fault to less than 30 330°C at the drainage divide 15-20 km southeast of the fault. Thermal field gradients 31 exhibit a degree of similarity from southernmost to northernmost transects, are greater 32
in low-grade semischist than high-grade schist, are affected by folding or 33 discontinuous juxtaposition of metamorphic zones, and contain limited information on 34 crustal-scale geothermal gradients. Temperatures derived by RSCM thermometry are 35 slightly (≤ 50°C) higher than those derived by traditional quantitative petrology using 36
garnet-biotite thermometry and THERMOCALC modeling. The age of RSCM T 37 appears to be mostly pre-Cenozoic over most of the area except in central Southern 38 Alps (Franz Josef-Fox area), where the amphibolite facies schists have T of likely 39 Cenozoic age. The RSCM T data place some constraints on the mode of exhumation 40 along the Alpine Fault and have implications for models of Southern Alps tectonics. 41
models and nonideality parameters are based on Holland & Powell (1998) and Powell 364
& Holland (1999). The activity-composition relationships used for garnet 365
(CaMnFMAS), white mica and paragonite (NKFMASH), plagioclase (NCAS), biotite 366
(KFMASHTO), epidote (CaFe3+
ASH), and chlorite (MnFMASH) are as described in 367
Vry et al. (2008). The activity-composition model for magnetite (FTO) is from White 368
et al. (2000), and the ilmenite (MnFTO) is from White et al. (2005), but with non 369
ideality described by W(ordered ilmenite, pyrophanite) = 2 kJ, W(disordered ilmenite, 370
pyrophanite) = 2 kJ, and W(hematite, pyrophanite) = 25 kJ (R. Powell, personal 371
communication, 2012). Albite, rutile, sphene, quartz, and H2O were treated as pure 372
end-members. 373
374
4. Results 375
376
4.1. Graphitic carbon in the Southern Alps 377
12
Graphitic carbon is widespread in rocks of the Southern Alps, generally dispersed in 378
the mineral matrix and totally absent only at a few localities. The latter localities may 379
correspond to unfavourable lithologies or lithologies affected by intense fluid 380
circulation which may be responsible for bleaching of carbonaceous material in the 381
rocks. In addition, distinctive hydrothermal graphite has been found in association 382
with orogenic gold-quartz mineral deposits in Otago (Pitcairn et al., 2005; Henne and 383
Craw, 2012; Hu et al. 2015). We found no such carbonaceous material in our samples 384
or fieldwork in the central Southern Alps. Figure 3 depicts representative Raman 385
spectra from the Southern Alps and demonstrates the general gradient of 386
graphitization following increasing metamorphism from SE to NW towards the 387
Alpine Fault. In the greywacke east of the drainage divide (Main Divide), 388
carbonaceous material exhibits Raman spectrum with several defect bands (e.g. D1, 389
D2, D3 and D4). Such spectra are characteristic of very disordered graphitic carbon 390
that were transformed under low-grade metamorphism at temperature below 330°C 391
(Beyssac et al., 2002; Lahfid et al., 2010). In this study, the peak temperature has been 392
assumed to be less than 330°C in these samples (Figure 4). To the west, there is a 393
progressive increase of the degree of graphitization through semischist and schist 394
towards the Alpine Fault. Close to the Main Divide, semischist samples exhibit 395
spectra with an intense main defect band as well as a strong D2 defect band as a 396
shoulder on the G peak: such spectra correspond to disordered graphitic carbon in 397
which the tridimensionnal aromatic skeleton remains poorly developed. Going 398
towards the Alpine Fault, both D1 and D2 bands decrease progressively with 399
increasing metamorphic grade and finally completely disappear in some of the highest 400
grade schist samples near the Alpine Fault. This shows a progressive graphitization 401
process that is completely achieved on the Alpine Fault. Importantly, detrital graphitic 402
carbon has been found in samples at all metamorphic grades. It can be easily 403
distinguished from in situ graphitizing organic matter based on: (i) morphological 404
criteria - as it generally appears as isolated grains or flakes; and (ii) Raman spectra - 405
as it usually exhibits a high crystallinity except in very high grade samples where it is 406
difficult to distinguish from organic matter from the simple observation of the Raman 407
spectra. Detrital graphite spectra were not included in RSCM T determinations. Note 408
that the presence of detrital graphite throughout the sequence has been observed in 409
many other metamorphic belts because graphite is easily recycled during the 410
13
erosion/weathering cycle (see Galy et al., 2008), and occurs widely in sedimentary 411
rocks. 412
More specifically, we have also carried out detailed investigation of inclusions 413
of graphitic carbon in the garnet porphyroblasts studied by Vry et al. (2004), sample 414
MA2 (VU37559 in Table 1). Graphitic carbon provides a marker of the garnet zoning 415
as it is present in some zones and absent from others, matching the chemical zonation 416
of garnets. The Raman spectrum of all such inclusions is constant and representative 417
of highly crystalline graphite. Calculating the temperature for such spectra yields ca. 418
575°C (Table 1) in good agreement with the maximum T obtained for the external rim 419
of garnet (ca. 600°C) and late Cenozoic ages. We conclude that all graphitic carbon in 420
this sample recorded the maximum T while garnet composition was only equilibrated 421
in the rims and not in the core during increasing metamorphism. 422
423
4.2. RSCM temperatures in the Southern Alps 424
All RSCM T are listed in Table 1 with key parameters such as location, geological 425
information, the number of spectra per sample, the mean R2 ratio parameter and T 426
with associated uncertainties. For very disordered graphitic carbon that is found in 427
least metamorphosed rocks, we assign T<330°C which is the lower bound of the 428
calibration by Beyssac et al., (2002). RSCM T are plotted on a regional-scale map 429
(Figure 4) and on three local maps depicting the main textural zones, metamorphic 430
mineral zone ‘isograd’ boundaries, faults and folds along the northern (Figure 5, 431
Wanganui to Taramakau), central (Figure 6, Copland to Whataroa) and southern 432
(Figure 7, Haast to Karangarua) segments of the Alpine Fault hanging wall. The latter 433
figures also include T profiles against the structural distance (D) to the Alpine Fault, 434
together with some approximate thermal field gradients drawn manually through the 435
data points. Curve fitting was deemed unwarranted for these transect gradients, due to 436
the relatively small numbers of samples involved in each transect. In addition, some 437
of the RSCM T data are depicted along four geological cross sections (Figure 8) 438
representing variations in geology along the Southern Alps: Waitaha river – Rakaia 439
valley, Whataroa river – Havelock river, Franz Josef – Godley valley, Karangarua - 440
Mt Cook village. Last, all RSCM T were plotted in frequency histograms for the 441
various metamorphic and textural zones (Figure 9). 442
443
14
Based on the dataset, these figures and Table 1, we make the following general 444
observations: 445
- Highest RSCM T measurements were from K-feldspar zone mylonites and schist 446
beside the Alpine Fault, where rocks locally contain only perfect graphite and are 447
inferred to have reached a minimum T of 640°C. Such very high T values are 448
observed in the Haast and Moeraki transects in the south, but also in Copland, 449
Waikukupa and Whataroa transects. There are two samples from garnet-oligoclase 450
zone rocks that contain only highly crystalline graphitic material yielding high T, 451
one from just south of Karangarua and the other at Wanganui river quarry. 452
- Lowest RSCM T measurements, where values have been assigned 330°C to 453
represent the current lower bound of the Beyssac et al. (2002) calibration were 454
observed in the vicinity of the Main Divide or to the southeast. Here the rocks are 455
TZ2a cleaved greywacke or textural zone TZ2b semischist, metamorphosed to 456
either sub-greenschist or chlorite zone. 457
- Plotting all RSCM T data versus metamorphic or textural zones shows the general 458
systematic T increase with increasing metamorphic grade and deformation (Figure 459
9). We note that some metamorphic (e.g. biotite or garnet-oligoclase) or textural 460
(e.g. TZ3 or TZ4) zones are characterized by a relative clustering of the T data, 461
whereas other zones (e.g. chlorite zone or textural zone 2B) exhibit a significantly 462
wider range of RSCM T measurements. This in part reflects the presence of some 463
relatively undeformed TZ2b rocks of the Aspiring lithologic association between 464
Waitaha-Arahura rivers, that are unusual in that they have reached amphibolite 465
facies metamorphism and yet retain remnants of original sedimentary structures 466
(Cox and Barrell, 2007; Cooper and Ireland, 2013). 467
- The RSCM T profiles exhibit a degree of similarity from the southernmost to the 468
northernmost transects. Plots of T as a function of D (Figures 5,6,7) nearly all 469
show RSCM thermal field gradients through the sub-greenschist to greenschist 470
facies (chlorite and biotite zone) rocks that are higher (>35°C/km) than field 471
gradients through amphibolite facies (garnet and K-feldspar zone) (<20°C/km). 472
473
In detail, the RSCM T data along the four geological sections of Figure 8 yields some 474
insight on the thermal evolution of the Alpine Schist. To the south, along the 475
Karangarua – Mt Cook village section (Figure 8d) there is extensive exposure of TZ3 476
and TZ4 rocks, corresponding to biotite and garnet zones, which comprise a map 477
15
thickness of more than 10 km. The rocks exhibit a fan-like structure, marked on a 478
broad scale by the opposite vergence of the Alpine Fault, which dips towards the SE, 479
and faults in the southeast, including the Main Divide Fault Zone (Cox and Findlay, 480
1985), which dip NW. This fan shape structure is mimicked by the main S3 481
crenulation cleavage and schistosity, which dips towards the SE close to the Alpine 482
Fault, is nearly vertical in the garnet zone and then progressively changes to dip 483
towards the NW in the garnet and biotite zones. The enveloping surface of folded, 484
early (S1/2) fabric and lithological variation dips gently southwest. Along this 485
geological section and nearby profiles (e.g. Copland or Karangarua), the field 486
gradients appear systematic with progressive increase in RSCM T towards the Alpine 487
Fault (Figures 6, 7). Yet in detail metamorphic sequences in pumpellyite-actinolite to 488
biotite zone rocks (TZ2a-3) near the Main Divide are locally inverted by juxtaposition 489
on NW-dipping faults (Cox et al., 1997; Craw, 1998) – a brittle juxtaposition which 490
results in steep thermal field gradients. Along the Moeraki profile there are also 491
kilometer-scale late- or post-metamorphic antiform and synform structures plunging 492
10-20° SW that fold the S2 surface (Rattenbury et al., 2010). Here strong T reversals 493
are present in RSCM measurements of garnet zone rocks (Figure 7). Other smaller 494
temperature reversals occur locally in data from the Otoko and Haast profiles, which 495
can also be attributed to late- or post-metamorphic folds (Figure 7; Cooper 1974; 496
Rattenbury et al., 2010). Where such folds are prominent, resulting thermal field 497
gradients are low. The Otoko profile also crosses an area of garnet-biotite-albite zone 498
samples within the greenschist facies, that contain very fine-grained (<1 mm) 499
grossular-spessartine garnets that are thought to be a remanent of Mesozoic 500
metamorphism exposed more widely in Otago (Figure 1; Mortimer, 2000; Rattenbury 501
et al., 2010). There do not appear to be any distinct steps or obvious thermal effects in 502
the dataset associated with this zone, which is distinguished by the ‘garnet-1 isograd’ 503
in Figure 7. 504
In the Franz Josef – Fox area, the high-grade TZ3 and TZ4 schist units 505
metamorphosed to biotite zone and above, are much narrower (8 km, Figure 6), and 506
have been juxtaposed against semischist sequences by the BDTZ with escalator-like 507
component of dip-slip motion (Figures 6, 8c; Little et al., 2002b; Wightman and 508
Little, 2007). Here the distance between the Alpine Fault and the Main Divide is also 509
the smallest, being the region where the late Quaternary dip-slip rates are the greatest 510
on the Alpine Fault (>12 mm/year – Norris and Cooper, 2001), and uplift and 511
16
exhumation rates of schist also appear to be the highest along the plate boundary 512
(Little et al., 2005). The high grade schist units contain an S3 crenulation cleavage 513
dipping towards the SE, weakly oblique to the Alpine Fault, which steepens with 514
distance from the fault (Little et al., 2002a). The geological section across the schist 515
units contains upright SE plunging folds and a tight fan-like structure, with an abrupt 516
truncation of the structural trend towards the NW at the BDTZ. Along this section, 517
high RSCM T (graphitic material at >640°C) was observed in the Alpine Fault 518
mylonite zone, but RSCM T are relatively constant in the range 525-570°C in garnet 519
zone and biotite zone rocks structurally below the BDTZ (see Fox, Waikukupa, Franz 520
profiles on Figure 6). Structurally above the BDTZ, where foliation and bedding have 521
a predominantly NW dip, the thermal field gradients are much higher (>35°C/km, 522
potentially reaching ~90°C/km). Field gradients are also steep through chlorite and 523
biotite zone rocks of the Whataroa profile (Figure 6), which we interpret to be a 524
combination of the effects of topography, flat lying S2 fabric, and fault juxtaposition 525
of different metamorphic blocks (Figure 8b). 526
Features observed in the southern and central profiles are also present in the 527
northern region (Figures 5, 8a,b). The high-grade TZ3 and TZ4 schist units widen to 528
14 km in both map view and geological section, and there are a number of mapped 529
kilometer-scale late- or post-metamorphic antiform and synform structures which 530
appear to fold isotect and isograd boundaries between Wanganui and Hokitika rivers. 531
Such folds are relatively tight with steep limbs in foliated schist sequences, but 532
typically open in the semischists (Figure 8a). The result appears to be a relatively flat-533
lying S2 enveloping surface, similar to the Whataroa-Havelock section (Figure 8b), 534
plunging gently-moderately to the northeast, which produces temperature reversals in 535
the Waitaha profile (Figure 8a). TZ2a and TZ2b semischist sequences (greenschist 536
facies) are relatively flat-lying, with structural blocks juxtaposed by subhorizontal 537
folding or faults (Andrews et al., 1974). Thermal field gradients are high (>45°C/km) 538
in the sub-greenschist to biotite zone rocks, and very low (<10°C/km) across schist 539
sequences. 540
541
4.3. Petrological constraints 542
Petrological data were collated for samples from the Franz Josef-Fox area, including 543
older studies (Grapes and Watanabe, 1992; Grapes, 1995). Traditional garnet-biotite 544
geothermometry, and temperature estimates based on results of the P-T pseudosection 545
17
calculations new for this study are presented in Table 2. An example of pseudosection 546
calculation is given as supplementary material. These data are shown on Figure 10 547
along with P-T estimates extracted from Figure 4 in Grapes and Watanabe (1992), and 548
shown in map view on Figure 11. Note that garnet-biotite temperatures in our study 549
are generally higher than those from Grapes and Watanabe (1992), based on the same 550
mineral analyses. In our study, we used the calibration by Hodges and Spear (1982) 551
based on rim analyses using pressure estimates based on results of garnet-biotite-552
muscovite-plagioclase barometry (Hoisch, 1990, Fe-endmember). Grapes and 553
Watanabe (1992) used the calibration by Ferry and Spear (1978), as modified for Ca 554
content by Hodges and Spear (1982) and Hoinkes (1986), and the garnet-biotite-555
muscovite-plagioclase geobarometer of Ghent and Stout (1981), with modification by 556
Hodges and Crowley (1985). The P and T uncertainties (± 1 kbar and ± 50°C; Grapes 557
and Watanabe, 1992), as estimated from standard deviations, apply for both studies. 558
559
5. Discussion 560
561
5.1. Comparison of RSCM data with petrological constraints 562
At some localities close to the Alpine Fault, in the high-temperature range, there is 563
good agreement between RSCM thermometry and the peak T estimated by petrology. 564
Nearly pure crystalline graphite is present yielding RSCM T above 580°C close to the 565
Alpine Fault, and in many places perfect graphite is found in the central and southern 566
areas yielding RSCM T above 640°C, the higher limit of the calibration by Beyssac et 567
al. (2002). This is in agreement with the T estimates by Vry et al. (2004) who showed 568
that the maximum T recorded by a garnet porphyroblast in Hokitika area (Mac’s 569
Creek) is ca. 600°C using garnet-biotite thermometry. The lowest RSCM T occur 570
from the Main Divide southeastwards, where most samples have been assigned 571
<330°C based on the lower limit of the Beyssac et al. (2002) calibration. There are no 572
quantitative petrological estimates of T for quartzofeldspathic lithologies 573
metamorphosed to such low temperatures. However the observed low temperatures 574
are in agreement with: 1) the observed metamorphism of rocks at prehnite-575
pumpellyite and pumpellyite-actinolite facies. For instance, prehnite occurring 576
together with pumpellyite typically indicates temperatures in the range 250-300 °C 577
and pressures below about 2 kbar (e.g. Willner et al., 2013); 2) the presence of 578
18
partially annealed fission tracks in zircons, with zircon ages which suggest rocks had 579
exceeded 240°C closure temperatures and have been uplifted from partial annealing 580
zone or deeper (Batt et al., 2000; Cox and Findlay 1995; Tippett and Kamp 1993a,b); 581
3) K-Ar ages that suggest the rocks have partial retention of gas and remained below 582
temperatures of ca. 300°C (Batt et al., 2000). 583
All methods record the decrease in peak temperature away from the Alpine 584
Fault towards the southeast (Figure 11), but RSCM T are generally higher than any 585
related estimate from petrology and yield significantly lower apparent field 586
metamorphic gradients through the high-grade schist. This is most-clearly illustrated 587
on Figure 10 where all RSCM T data for the southern, central and northern profiles 588
are shown together with petrological T estimates from Grapes and Watanabe (1992) 589
and this study (see Table 2) from the Franz Josef – Fox area. The T estimates by 590
Grapes and Watanabe (1992) are definitely lower by several tens of degrees compared 591
to RSCM T (central Southern Alps profiles) except those in the direct vicinity of the 592
Alpine Fault where they converge towards ca. 600°C. THERMOCALC T estimates 593
are higher than those of Grapes and Watanabe (1992) and for the most-part are lower 594
than RSCM T. 595
A possible explanation of the difference between RSCM T and petrological 596
estimates is due the irreversibility of graphitization (Beyssac et al. 2002) such that 597
RSCM records the peak T and is not sensitive to the retrograde/exhumation path of 598
the rocks. By way of contrast, mineral assemblages can re-equilibrate their chemistry 599
during retrogression, modifying and erasing the peak T signal, especially when fluid 600
circulation and deformation are important. In the Alpine Schist, garnet compositions 601
are not simple Fe-Mg end-member mixtures, and can vary considerably from rock to 602
rock, and the biotite can be subject to re-equilibration and regrowth during uplift and 603
cooling. Fluid flow has locally affected some of the rocks, and the effects and timing 604
of this may pass unrecognized, and have not been quantified. The MnO contents in 605
the bulk rock compositions vary, and the first appearance of garnet in P-T 606
pseudosections is very sensitive to this, as well as the choices of activity-composition 607
models. In any case, we note that temperature estimates based on results of P-T 608
pseudosection calculations for rock samples that contain relatively high-grade mineral 609
assemblages (containing ilmenite, oligoclase, ± garnet), are typically only slightly 610
lower than, and within error of, the RSCM T obtained from nearby samples. 611
However, for the samples that contain lower-grade mineral assemblages with sphene, 612
19
there is a larger temperature difference, with higher T estimates being obtained from 613
the RSCM data (see Figure 10). 614
At some localities (see Table 2), the discrepancy between RSCM T and 615
petrological T estimates is, nonetheless, somewhat surprising. RSCM thermometry 616
has now been used in many various geological contexts and generally exhibits good to 617
excellent concordance with conventional petrology, including garnet-biotite 618
thermometry (e.g. see Plunder et al., 2012 and references therein). However, in most 619
available studies, RSCM T was applied to simple thermal histories, i.e. 620
monometamorphic history with one single thermal event, except the notable examples 621
of some internal units of Taiwan (Beyssac et al. 2007). But given the high quality and 622
consistency of the RSCM spectral data in the Alpine Schist, and considering that 623
minerals may not record the true peak metamorphic temperature, we consider that 624
RSCM thermometry yields a first quantification on a large scale of peak metamorphic 625
T. 626
627
5.2. RSCM T pattern in the Southern Alps 628
Figure 10 is a compilation of all local-scale profiles presented on Figures 5, 6 and 7 629
for southern, central and northern Alpine Schist. RSCM T data from all metamorphic 630
zones has also been projected onto an along-strike section parallel to the Alpine Fault 631
(Figure 12). Also shown are the approximate position of key metamorphic mineral 632
‘isograds’ (first appearance of biotite; garnet and K-feldspar), and the zone where 633
young (<6 Ma) 40
Ar-39
Ar and K/Ar ages have been obtained in the central Southern 634
Alps. It is useful compare the along-strike projection against Figure 10 that shows the 635
same data plotted perpendicular to the Alpine Fault. From Figures 10 and 12, it seems 636
there may be a decrease in peak T along the Alpine Fault in the K-feldspar zone going 637
towards the northeast, although this trend may in part be apparent due to the absence 638
of samples analysed from the northernmost area. Both the garnet-oligoclase and 639
biotite zones exhibit a relatively clustered pattern of peak T around 560°C and 520°C 640
respectively all along strike. There is dispersion in both cases around the mean values 641
for each zone especially in the central Southern Alps although this may also be due to 642
a denser sampling bias in this area. The chlorite zone all along the Alpine Fault covers 643
an extremely wide range of T from ca. 360°C reaching as high as ca. 550°C. This is a 644
very wide T range for a classical chlorite greenschist facies zone, especially towards 645
the high T. One possibility is that the lithology/bulk chemistry may not have allowed 646
20
growth of index minerals (garnet and/or biotite, see Vry et al., 2008), or the rocks 647
suffered complete retrogression obliterating some high-grade minerals, resulting in 648
the possibility that samples could have been ‘misclassified’ as being apparently of 649
lower metamorphic grade than the temperature they actually experienced. 650
Figure 10 shows that higher RSCM T extends farther from the Alpine Fault 651
along the southern and, to a lesser degree, the northern compared to the central 652
profiles. This is due to the wider map extension of high-grade units (biotite and garnet 653
oligoclase) in the south and north, whereas these units are far less thick and steeper in 654
the central Southern Alps. Although the data are somewhat scattered, it seems that the 655
RSCM T pattern along both the central and northern profiles has nearly constant high 656
T within the first kilometers close to the Alpine Fault (ca. 6 km for the central and ca. 657
8 km for the north) followed by a steep decrease of T in the eastern biotite and 658
chlorite zones. In the south, the T profile is more continuous and progressive, with no 659
break in the slope, and projection toward Alpine Fault suggest rocks here may have 660
reached temperatures ~700°C. This is consistent with widespread occurrences of 661
pegmatite in the Mataketake Range and less commonly elsewhere between Haast and 662
Moeraki rivers, which reflect partial melting of the schist that occurred during the 663
Late Cretaceous (Chamberlain et al., 1995). The calculation of D for these data 664
assumes the Alpine Fault has a constant dip of 45° (Figure 2). Also shown in Figure 665
10 are the range of possible D values and field gradients that could occur if the fault 666
dipped as shallow as 30° or as steeply as 60°. 667
668
5.3. Age of RSCM temperatures in the Southern Alps 669
RSCM thermometry records the peak metamorphic T undergone by carbonaceous 670
material and the host rock during the burial history but it carries no age information 671
by itself. In the case of complex poly-phased metamorphic histories like in the Alpine 672
Schist with at least two major thermal events, either associated with tectonics on the 673
margin of Gondwana during Jurassic – Cretaceous or with evolution of the present 674
plate boundary during late Cenozoic, an important issue remains with regards to the 675
age of the recorded RSCM T. This is a significant question if one wants to use such 676
data to infer and constrain models of the thermal structure of the Southern Alps, or 677
rheology of Alpine Fault rocks at depth (e.g. Toy et al. 2010), as the T measurements 678
will be most directly relevant if they are late Cenozoic in age. Confirming the RSCM 679
T measurements as late Cenozoic is made more difficult by the complete absence of 680
21
Cenozoic-aged metasedimentary units in the Southern Alps that would only contain 681
the expression of late Cenozoic metamorphism. Age of mineral assemblages and of 682
peak P-T conditions recorded in the Alpine Schist have therefore long been discussed 683
in the literature (e.g. Chamberlain et al., 1995; Little et al. 2002a,b, 2005; Vry et al. 684
2004, 2008), but there is no real consensus as to the exact locations where peak P-T 685
conditions recorded by the rocks are late Cenozoic. There are a number of arguments 686
however that supports this to be the case, at least locally, for Alpine Schist between 687
Fox and Franz Josef, and potentially Copland – Whataroa rivers, as we discuss below. 688
On a large scale, the spatial distribution of RSCM T is strongly correlated with 689
the geometry of the main contacts and tectono-stratigraphic structures that were 690
acquired during formation of the Southern Alps. But the classical two-dimensional 691
model of exhumation by Wellman (1979), exhuming deep old rocks up a ramp formed 692
by the Alpine Fault, would yield the same results whether the peak T was late 693
Cenozoic, or a product of older Jurassic-Cretaceous metamorphism with passive 694
exhumation up the Alpine Fault ramp during the late Cenozoic. Instead, 695
thermochronologic data at least record when the rocks have cooled below some 696
threshold temperature (closure temperature), depending on the selected 697
thermochronometer. Except for U-Pb geochronology on zircon yielding 698
crystallization ages, the highest temperature thermochronometer so far applied has 699
been 40
Ar-39
Ar ages on hornblende. Between Franz Josef and Fox, hornblende from 700
mylonite beside the Alpine Fault has 40
Ar-39
Ar ages older than Neogene formation of 701
the plate boundary, yet garnet-oligoclase zone Alpine Schist been reset to < 6 Ma 702
when the Southern Alps were uplifted (Figures 4, 11; Chamberlain et al., 1995; Little 703
et al., 2005). The reset ages are distributed through a zone where measured RSCM T 704
range between 530-565°C (Figure 11). These RSCM temperatures are therefore 705
equivalent to, or higher, than the commonly assumed value for the closure 706
temperature of 40
Ar-39
Ar ages on hornblende which is ca. 500-550°C. At least, in this 707
zone it seems reasonable to assume that RSCM T represents a peak T which was 708
reached during the late Cenozoic. Further support is given by the close 709
correspondence between (i) peak T of ca. 600°C recorded by the most external rim of 710
garnet porphyroblasts dated to Cenozoic ages (Vry et al., 2004) and (ii) RSCM T of 711
ca. 575°C obtained on all graphitic inclusions on the same garnet from the core to the 712
rim. 713
22
To the south or north, however, there are no other places where a late 714
Cenozoic age for thermochronometers or RSCM T can be easily inferred. There are 715
also observations, as outlined above, showing that peak RSCM T values are 716
Mesozoic: (i) 40
Ar-39
Ar ages on hornblende are Miocene or older to the south, (ii) 717
RSCM T reversals occur on profiles where the regional S1/S2 foliation has been 718
folded and cross-cut by the S3 crenulation cleavage, suggesting the RSCM T data 719
record the peak metamorphism associated with this older fabric, and (iii) that 720
projection of RSCM temperatures on the Haast and Otoko profiles suggest the schists 721
may have reached maximum T ~700°C, where partial melting that produced 722
pegmatites have been dated as late Cretaceous. This does not mean that all RSCM T 723
are Mesozoic, as for instance the coverage for high T dating systems is restricted to 724
the Franz-Josef Fox region. In the south there are also some local complications that 725
are yet to be fully mapped and understood, such as inversion of the metamorphic 726
sequence in the mylonite zone caused by distributed shear on the Alpine Fault 727
(Cooper and Norris, 2011). 728
729
5.4. Some tectonic implications 730
The belt of schist along the Southern Alps has a seemingly continuous westward 731
increase in metamorphic grade toward the Alpine Fault. It has long been considered 732
(e.g. Suggate, 1963) to be due to an increase in depth of exhumation, with some 20-30 733
km of exhumation adjacent to the fault. It is commonly assumed that the metamorphic 734
and textural boundaries in the schist were once sub-horizontal, as observed in the 735
Otago region of southeast New Zealand, and have been rotated during Neogene 736
tectonics. The notion of rock uplift or exhumation and cooling in the Neogene has 737
been corroborated by thermochronologic studies, which demonstrated young ‘reset’ 738
ages adjacent to the Alpine Fault (e.g. Tippett and Kamp 1995; Batt et al. 2000; Little 739
et al. 2005; Herman et al. 2007; 2009). Structural models of the Southern Alps now 740
nearly all infer some form of deformation and rotation of upper crustal material within 741
the Southern Alps, as the Pacific Plate is delaminated and translated Alpine Fault 742
ramp (see Okaya et al., 2007 for a review). Petrological and field observations 743
provide evidence the Alpine Fault hanging wall has been tilted southeastward in the 744
central Southern Alps (Figure 2). Cumulative vertical displacements on an array of 745
fractures in the BDTZ effect a 22 ± 8º of bulk SE tilt (Wightman and Little, 2007; 746
Little et al. 2007). Structurally below the BDTZ, garnet zone rocks also preserve 747
23
microstructural evidence of distributed ductile shear strain (Ɣ = 0.6, down to the east) 748
with sufficient magnitude that could account for ~32° SE tilt of the schist sequence 749
(Holcombe and Little, 2001; Little et al., 2002a). 750
Although seemingly continuous at a regional-scale, in detail the pattern of 751
metamorphic zones represents a complex disrupted metamorphic pile, which involves 752
slices that are variably affected by folding, and transitions (‘isograd’ boundaries) that 753
involve juxtaposition on faults and shear zones. The presence of the BDTZ in the 754
Franz Josef-Fox area, where an escalator-like back shearing process has occurred, is 755
an important observation and reference frame (Wightman and Little, 2007; Little et al. 756
2007). Structurally highest chlorite and biotite zone rocks near the Main Divide, 757
remained above the brittle-ductile transition, so only record a brittle expression of the 758
modern phase of oblique convergence (e.g. Cox and Findlay 1995; Cox et al., 1997). 759
Below the BDTZ, ductile deformation resulted in constructive reinforcement of pre-760
existing fabrics rather than superposition of a new foliation, and the SE tilt of the rock 761
sequence. Clearly, field metamorphic gradients measured across major boundaries 762
such as the BDTZ will contain little or no information about crustal-scale geothermal 763
gradients, or pressure-temperature relationships during metamorphism. Measured 764
field gradients might also be expected to be very different either side of the BDTZ 765
due to the difference in nature and style of deformation either side of the boundary. 766
We observed high (>35°C/km) RSCM thermal field gradients through the sub-767
greenschist to greenschist facies (chlorite and biotite zone) rocks and low (<20°C/km) 768
field gradients through amphibolite facies (garnet and K-feldspar zone) (Figures 769
5,6,7,10). 770
The flux of Pacific Plate rock through the deforming zone has been suggested 771
to have two distinct domains/stages, with pure-shear style motion and thickening in 772
the eastern outboard domain, then inclined out-of-plane non-coaxial simple shear in 773
the inboard domain (Little, 2004; Cox et al., 2012). The metamorphic and textural 774
transition in the Franz Josef-Fox area (central Southern Alps) is considerably 775
narrower in map view relative to schist than further to the north and south along the 776
Alpine Fault (Figure 2), potentially the result of differing geometry of the fault at 777
depth (Little et al., 2005). Schist and semi-schist sequences in the central Southern 778
Alps appear to have been thinned relative to the north and south, but although 779
condensed, the westward prograde metamorphic mineral zonation has remained in 780
sequential order. There have been arguments for structural thinning (Grapes and 781
24
Watanabe 1992) and ‘extrusion’ of lower crustal material (Walcott, 1998) associated 782
with uplift of the Alpine Fault hanging wall. On the basis of geobarometry (garnet-783
biotite-muscovite-plagioclase) and geothermometry (garnet-biotite) of high-grade 784
schists, Grapes and Watanabe (1992) argued the crustal section in the central 785
Southern Alps has been thinned to one third its original thickness. However, because 786
there are significant uncertainties (at least ±1 kbar) in each pressure estimate, the 0.33 787
thinning ratio carries high uncertainty. If tilting of high-grade Alpine Schist involved 788
thinning or shortening perpendicular to the Alpine Fault, we might expect to see it 789
represented in metamorphic peak T field gradients recorded by the RSCM T data. For 790
example, if thinning has been as substantial as the 0.33 ratio suggested by Grapes and 791
Watanabe (1992), the field gradients might be expected to greatly exceed ‘normal’ 792
crustal geothermal gradients of ~20-40°C/km. Instead, the RSCM T field gradients 793
observed for garnet-oligoclase and K-feldspar zone schist in this study were only 794
~20°C/km perpendicular to the Alpine Fault in the central Southern Alps, and around 795
10°C/km or lower to the north and to the south (Figures 5,6,7). 796
Local temperature reversals in the Otoko and Waitaha profiles clearly reflect 797
upright antiform and synform structures folding the S2 foliation surface and lowering 798
observed field gradients. Here the enveloping surface of the isograds must have a 799
relatively shallow SW dip and is near horizontal when considered perpendicular to the 800
Alpine Fault. Importantly, there is little evidence in the RSCM T data anywhere for 801
substantial thinning of the high-grade (garnet and K-feldspar zone) Alpine Schist 802
sequence perpendicular to the fault, whether the RSCM T data represent Mesozoic or 803
late Cenozoic peak temperatures. A thinning ratio of 0.33 would increase the apparent 804
geothermal gradient by a factor of 3, although the 0.9 thinning ratio proposed in a 3-D 805
kinematic model (Little, 2004) only requires a 1.11 increase, and is potentially 806
supported by RSCM-T. We suggest any Neogene deformation that occurred to the 807
high-grade schist sequence below the brittle-ductile transition is unlikely to have 808
involved more than 0.5 thinning relative to a fixed Alpine Fault reference frame 809
dipping 45°. Deformation by inclined simple shear would meet such criteria. By way 810
of contrast, chlorite and biotite zone semischist sequences show moderate to high (40 811
- 90°C/km) RSCM T field gradients. Here the rocks have remained above the brittle-812
ductile transition and Neogene deformation was accommodated by oblique dip-slip 813
faulting (backthrusts), imbricated duplex-like stacking and local reversal of 814
metamorphic grade (Cox and Findlay, 1995; Cox et al., 1997; Craw, 1998). The 815
25
RSCM T field gradients in semischist sequences appear boosted by juxtaposition of 816
metamorphic zones and we believe they are unlikely to represent any true geothermal 817
gradients in the crust. 818
Our observations are presented using a calculated structural distance (D) with 819
regard to an assumed Alpine Fault reference frame, fixed at 45° dip. What was 820
perhaps surprising was how low the resulting thermal field gradients were in the high-821
grade Alpine Schist, particularly that nowhere can they be considered to have 822
exceeded 20°C/km. Had a 60° Alpine Fault dip been selected, it would have resulted 823
in even lower calculated field gradients (to about 10°C/km, depending locally on 824
topography) and at 30° dip the field gradients would still not have exceeded 30°C/km. 825
Since equally low (5-10°C/km) thermal gradients have been independently predicted 826
during evolution of the Alpine Fault mylonite zone (Toy et al., 2010; Cross et al., 827
2015), it encourages us to think the low RSCM T field gradients might actually reflect 828
the thermal state of high-grade Alpine Schist prior to uplift and exhumation. If not 829
real, then such low geothermal gradients near the Alpine Fault can alternatively be 830
explained by vertical thickening (Little, 2004), a crustal drag structure (Little et al., 831
2005) or imbricated reversals associated with distributed oblique-slip (Cooper and 832
Norris, 2011). Perhaps the simplest alternative explanation is that the RSCM T field 833
gradients dominantly represent an oblique slice through the Mesozoic crustal pile, and 834
that the enveloping surface of Mesozoic isograds is much shallower than the Alpine 835
Fault. A corollary is that the degree of rotation of the hanging wall was limited and it 836
has not been completely rotated into parallelism with the Alpine Fault (as suggested 837
by Wellman fig. 4a,c 1979, or Walcott fig 15, 1998). There are various ways this 838
could be achieved. One is that the dip-slip displacement distributed on backthrust 839
structures almost matches that on the Alpine Fault, so that blocks bound by faults and 840
shear zones are only weakly rotated as they are exhumed up the Alpine Fault ramp 841
(Wellman figure 4b, 1979). A corollary is that faults in the Southern Alps hanging 842
wall must be active and have relatively high cumulative slip rates, which is important 843
for seismic hazard assessment (see Wallace et al., 2007; Cox et al., 2012). An 844
alternative hypothesis, that is not easily addressed with RSCM T data and is beyond 845
the scope of our study, is that the Alpine Fault could itself have been rotated, or 846
evolved to a shallower dip during the Neogene and Quaternary (Koons et al., 2003). 847
848
26
6. Conclusions 849
In this study, we present a dataset of peak metamorphic temperatures experienced by 850
Alpine Schist, semischist and greywacke now exhumed in the hangingwall of the 851
Alpine Fault. Carbonaceous material has been analysed in 142 samples, from 13 low- 852
to high-grade transects, in which peak metamorphic temperatures decrease from ca. 853
650-700°C near the Alpine Fault to less than 330°C at the main drainage divide, about 854
15-20 km southeast from the fault. The temperature decrease is relatively uniform in 855
the south, but distinct thermal field gradients are present across the central Southern 856
Alps. This is the first systematic and consistent dataset at the scale of the entire 857
Southern Alps with quantitative values for the peak metamorphic T experienced by 858
various textural and metamorphic zones. RSCM T increase with metamorphic and 859
textural grade, with reversals occurring only locally across folds and any apparent 860
steps where there are faults. Peak temperatures recorded by the RSCM method are 861
generally higher by ≤ 50°C than existing temperature estimates from petrology. 862
Biotite-in, garnet-in and K-feldspar-in first appearance ‘isograds’ occur at different 863
temperatures along the schist belt, which could reflect variable ages of peak 864
metamorphism, or potentially some truncation and juxtaposition of metamorphic 865
zones by faults and shear zones. RSCM T are mostly pre-Cenozoic except in the 866
Franz Josef - Fox area of the central Southern Alps, where these T are likely Cenozoic 867
in age. 868
The RSCM temperatures place limited constraints on thermal conditions 869
experienced within the orogen, with field temperature gradients potentially carrying 870
information on amounts of tilting and structural re-organisation of the Pacific Plate in 871
the Alpine Fault hangingwall, albeit disrupted by fault and shear zone juxtaposition. 872
Plots of RSCM T with respect to structural thickness (D) perpendicular to the Alpine 873
Fault, assuming a 45° dip, yield thermal field gradients that are consistently low, <20 874
°C/km, within the garnet-oligoclase and K-feldspar zones. It suggests these rocks 875
were neither fully rotated, nor structurally thinned, during exhumation. Given the 876
number and consistency of thermochronological data and geological observations 877
available, this dataset can constitute a basis to test thermokinematic and/or 878
thermomechanical models of mountain building processes in the Southern Alps. Such 879
models may have important implications in terms of thermal structure of the crust 880
27
before, during and after orogenic processes as well for our knowledge of crustal 881
rheology. 882
883
Acknowledgments 884
New samples from Westland National Park were collected under Department of 885 Conservation permit WC-22994-GEO, including material collected by Richard 886 Jongens, Mark Rattenbury and Lukas Nibourel. Older samples were sourced from 887 PETLAB collections at GNS Science, Victoria University of Wellington and 888
University of Otago. Rodney Grapes also provided access to archival samples and 889 analytical material. Holly Godfrey and Belinda Smith Lyttle provided technical 890 support. We also wish to thank our colleagues Tim Little, John Townend, and Rupert 891
Sutherland for discussions and helpful comments during the gestation of this work, 892 although not necessarily implying they agree with all of our interpretations and 893 conclusions. Olivier Beyssac acknowledges funding from ANR (GeoCARBONS 894 project), Sorbonne Universités (PERSU program) and CNRS-INSU. Simon Cox was 895
funded under GNS Science’s ‘Impacts of Global Plate Tectonics in and around New 896 Zealand Programme’ (PGST Contract C05X0203). Frederic Herman was funded by 897
the Swiss National Fund (grant PP00P2_138956). We thank Dave Craw and an 898 anonymous reviewer for very constructive help, and Jean-Philippe Avouac for his 899
editorial support. 900 901
28
902
References 903
904
Adams, C.J. (2003) K-Ar geochronology of Torlesse Supergroup metasedimentary 905 rocks in Canterbury, New Zealand. Journal of the Royal Society of New Zealand 906 33(1): 165-187. 907
Adams, C.J., Maas, R. (2004) Rb-Sr age and strontium isotope characterisation of the 908 Torlesse Supergroup in Canterbury, New Zealand, and implications for the status of 909
the Rakaia Terrane. New Zealand Journal of Geology and Geophysics 47(2): 201-217. 910
Allis, R.G., Henley, R.W., Carman, A.F. (1979) The thermal regime beneath the 911 Southern Alps. In: The origin of the Southern Alps. Bulletin of the Royal Society of 912
New Zealand, 18, (eds Walcott RI & Cresswell MM). Wellington, Royal Society of 913 New Zealand, 79-85. 914
Allis R.G., Shi, Y. (1995) New insights to temperature and pressure beneath the 915 central Southern Alps, New Zealand. New Zealand Journal of Geology and 916 Geophysics, 38(4), 585-592. 917
Andrews, P.B., Bishop, D.G., Bradshow, J.D., Warren, G. (1974) Geology of the Lord 918
Range, Central Southern Alps, New Zealand. New Zealand Journal of Geology and 919 Geophysics, 17(2), 271-299. 920
Batt, G.E., Braun, J., Kohn, B.P., McDougall, I. (2000) Thermochronological analysis 921 of the dynamics of the Southern Alps, New Zealand. Geological Society of America 922
Bulletin, 112, 250−266. 923
Beaumont, C., Kamp, P. J., Hamilton, J., Fullsack, P. (1996) The continental collision 924 zone, South Island, New Zealand: Comparison of geodynamical models and 925
observations. Journal of Geophysical Research: Solid Earth, 101(B2), 3333-3359. 926
carbonaceous material from metasediments: a new geothermometer. Journal of 928 Metamorphic Geology, 20, 859-871. 929
Beyssac O., Goffe, B., Petitet, J.P., Froigneux, E., Moreau, M., Rouzaud, J.N. (2003) 930
On the characterization of disordered and heterogeneous carbonaceous materials 931
using Raman spectroscopy. Spectrochimica Acta A, 59, 2267-2276. 932
Beyssac, O., Bollinger, L., Avouac, J.P., Goffe, B. (2004) Thermal metamorphism in 933 the lesser Himalaya of Nepal determined from Raman spectroscopy of carbonaceous 934 material. Earth and planetary Science Letters, 225, 233-241. 935
B. (2007) Late Cenozoic metamorphic evolution and exhumation of Taiwan. 937 Tectonics, 26, TC6001, doi:10.1029/2006TC002064. 938
Beyssac, O., Lazzeri, M. (2012) Application of Raman spectroscopy to the study of 939 graphitic carbons in the Earth Sciences. in: Applications of Raman Spectroscopy to 940
Earth Sciences and Cultural Heritage (J. Dubessy, M.-C. Caumon and F. Rull, 941 editors). EMU Notes in Mineralogy, 12, 415-454. European Mineralogical Union and 942 the Mineralogical Society of Great Britain & Ireland. 943
29
Bishop, D.G. (1974) Stratigraphic, structural, and metamorphic relationships in the 944 Dansey Pass area, Otago, New Zealand. New Zealand Journal of Geology and 945 Geophysics 17, 301-335. 946
Chamberlain, C.P., Zeitler, P.K., Cooper, A.F. (1995) Geochronologic constraints of 947 the uplift and metamorphism along the Alpine Fault, South Island, New Zealand. New 948
Zealand Journal of Geology and Geophysics, 38(4), 515-523. 949
Cooper, A.F. (1974) Multiphase deformation and its relationship to metamorphic 950 crystallisation at Haast River, south Westland, New Zealand. New Zealand Journal of 951 Geology and Geophysics, 17(4), 855-880. 952
Cooper, A.F. (1980) Retrograde alteration of chromian kyanite in metachert and 953
amphibolite whiteschist from the Southern Alps, New Zealand, with implications for 954 uplift on the Alpine Fault. Contributions to Mineralogy and Petrology, 75, 153-164. 955
Cooper, A.F., Norris, R.J. (2011) Inverted metamorphic sequences in Alpine fault 956 mylonites produced by oblique shear within a plate boundary fault zone, New 957 Zealand. Geology, 39(11), 1023-1026. 958
Cooper, A.F., Ireland, T.R. (2013) Cretaceous sedimentation and metamorphism of 959
the western Alpine Schist protoliths associated with the Pounamu Ultramafic Belt, 960 Westland, New Zealand. New Zealand Journal of Geology & Geophysics, 56(4), 188-961 199. 962
Cox, S.C., Findlay R.H. (1995) The Main Divide fault zone and its role in formation 963
of the Southern Alps, New Zealand. New Zealand Journal of Geology and 964 Geophysics, 38, 489−499. 965
Cox, S.C., Barrell D. J. A. (2007) Geology of the Aoraki area. Institute of Geological 966
and Nuclear Sciences 1:250,000 geological map 15. Lower Hutt, New Zealand: 967 Institute of Geological and Nuclear Sciences. 71 pages + 1 folded map. 968
Cox, S.C., Sutherland R. (2007) Regional geological framework of South Island, New 969 Zealand, and its significance for understanding the active plate boundary, in A 970 Continental Plate Boundary: Tectonics at South Island, New Zealand edited by D. 971
Okaya, T. Stern, F. Davey. AGU Geophysical Monograph Series, 175, 19-46. 972
Cox, S.C., Stirling, M.W., Herman, F., Gerstenberger, M., Ristau, J. (2012) 973
Potentially active faults in the rapidly eroding landscape adjacent to the Alpine Fault, 974 central Southern Alps, New Zealand. Tectonics, 31, TC2011. 975
Cox, S.C., Menzies, C.D., Sutherland, R., Denys, P.H., Chamberlain, C., Teagle, 976 D.A.H. (2015) Changes in hot spring temperature and hydrogeology of the Alpine 977 Fault hanging wall, New Zealand, induced by distal South Island earthquakes. 978 Geofluids (15): 216-239. DOI: 10.1111/gfl.12093. 979
Cox, S.C., Craw D., Chamberlain C.P. (1997) Structure and fluid migration in a late 980
Cenozoic duplex system forming the Main Divide in the central Southern Alps, New 981 Zealand. New Zealand Journal of Geology and Geophysics, 40, 359−374. 982
Craw, D. (1984) Lithologic variations in Otago Schist, Mt Aspiring area, northwest 983
Otago, New Zealand. New Zealand Journal of Geology and Geophysics, 27(2): 151-984 166. 985
Craw, D. (1998) Structural boundaries and biotite and garnet ‘isograds’ in the Otago 986
and Alpine Schists, New Zealand. Journal of Metamorphic Geology, 16, 395-402. 987
30
Cross, A.J., Kidder, S., Prior, D.J. (2015) Using microstructures and TitaniQ 988 thermobarometry of quartz sheared around garnet pophyroclasts to evaluate 989 microstructural evolution and constrain an Alpine Fault Zone geotherm. Journal of 990 Structural Geology, 75, 17-31. 991
DeMets, C., Gordon, R. G., Argus, D. F. (2010) Geologically current plate motions. 992
Ferry, J.M., Spear, F.S. (1978) Experimental calibration of partitioning of Fe and Mg 995 between biotite and garnet. Contributions to Mineralogy and Petrology, 66: 113-117. 996
Galy, V., Beyssac, O., France-Lanord, C., Eglinton, T. (2008) Selective recycling of 997
graphite during Himalayan erosion: a geological stabilisation of C in the crust. 998 Science, 322, 943-945. 999
Gerbault, M., Davey, F.J., Henrys, S.A. (2002) Three-dimensional lateral crustal 1000 thickening in continental oblique collision: an example from the Southern Alps, New 1001 Zealand, Geophysical Journal International, 150(3), 770-779. 1002
Ghent, E. D., Stout, M. Z. (1981) Geobarometry and geothermometry of plagioclase-1003
biotite-garnet-muscovite assemblages. Contributions to Mineralogy and Petrology, 76, 1004 92-97. 1005
Grapes, R., Watanabe, T. (1992) Metamorphism and uplift of Alpine Schist in the 1006
Franz Josef-Fox Glacier area of the Southern Alps, New Zealand. Journal of 1007
Metamorphic Geology, 10, 171-180. 1008
Grapes, R.H., Watanabe, T., Palmer, K. (1982) X.R.F. Analyses of Quartzofeldspathic 1009 Schist and Metacherts, Franz Josef–Fox Glacier Area. University of Wellington 1010
Publication. no. 25. 1011
Grapes, R. H. (1995) Uplift and exhumation of Alpine Schist in the Franz Josef - Fox 1012
Glacier area of the Southern Alps, New Zealand. New Zealand Journal of Geology 1013 and Geophysics 38, 525−533. 1014
Green, D.C. (1982) The Alpine Fault Zone of the Waitaha Valley area. BSc(Hons) 1015
thesis, University of Otago, Dunedin 1016
Grindley, G.W. (1963) Structure of the Alpine Schists of South Westland, Southern 1017
Alps, New Zealand. New Zealand Journal of Geology and Geophysics 6(5): 872-930. 1018
Heinrichs, H., Herrmann, A. G. (1990) Praktikum der Analytischen Geochemie, 1019
Springer-Verlag, Berlin-Heidelberg-New York, pp. 669. 1020
Henne A, Craw D 2012. Synmetamorphic carbon mobility and graphite enrichment in 1021 metaturbidites as a precursor to orogenic gold mineralization, Otago Schist, New 1022 Zealand. Mineralium Deposita 47:781-797. 1023
Herman F., Braun, J., Dunlap, W. J. (2007) Tectonomorphic scenarios in the Southern 1024
Alps of New Zealand. Journal of Geophysical Research: Solid Earth 112, B04201, 1025 doi:10.1029/2004JB003472. 1026
Herman, F., Cox, S.C., Kamp, P.J.J. (2009) Low-temperature thermochronology and 1027 thermokinematic modeling of deformation, exhumation and development of 1028 topography in the central Southern Alps, New Zealand. Tectonics, 28, TC5011, 1029 doi:10.1029/2008TC002367. 1030
31
Hodges, K.V., Crowley, P. D. (1985) Error estimation and empirical geobarometry for 1031 politic systems. American Mineralogist, 70, 702-709. 1032
Hodges, K.V., Spear, F.S. (1982) Geothermometry, geobarometry and the Al2SiO5 1033 triple point at Mt. Moosilauke, New Hampshire. American Mineralogist, 67, 1118–1034 1134. 1035
Hoisch, T.D. (1990) Empirical calibration of six geobarometers for the mineral 1036 assemblage quartz + muscovite + biotite + garnet + plagioclase. Contributions to 1037 Mineralogy and Petrology, 104, 225-234. 1038
Hoinkes, G. (1986) Effect of grossular-content in garnet on the partitioning of Fe and 1039 Mg between garnet and biotite: an empirical investigation on staurolite-zone samples 1040
from the Austroalpine Scheeberg complex. Contributions to Mineralogy and 1041 Petrology, 92, 393-399. 1042
Holcombe, R.J., Little, T.A. (2001) A sensitive vorticity gauge using rotated 1043 porphyroblasts, and its application to rocks adjacent to the Alpine Fault, New 1044 Zealand. Journal of Structural Geology, 23, 979-990. 1045
Holland, T.J.B., Powell, R. (1998) An internally consistent thermodynamic data set 1046
for phases of petrological interest. Journal of Metamorphic Geology, 16, 309–343. 1047
Hovius, N., Stark, C. P., Allen P. A. (1997) Sediment flux from a mountain belt 1048 derived from landslide mapping. Geology, 25, 231-234. 1049
Hu S, Evans, K, Craw D, Rempel K, Bourdet J, Dick J, Grice K 2015. Raman 1050
characterization of carbonaceous material in the Macraes orogenic gold deposit and 1051 metasedimentary host rocks, New Zealand. Ore Geology Reviews, 70, 80-95. 1052
Koons, P.O. (1987) Some thermal and mechanical consequences of rapid uplift: an 1053
example from the Southern Alps, New Zealand. Earth and Planetary Science Letters, 1054 86, 307-319. 1055
Koons, P.O., Craw, D., Cox, S.C., Upton, P., Templeton, A.S., Chamberlain, C.P. 1056 (1998) Fluid flow during active oblique convergence: a Southern Alps model from 1057 mechanical and geochemical observations. Geology, 26(2), 159-162. 1058
Koons, P.O., Norris, R.J., Craw, D., Cooper, A.F. (2003), Influence of exhumation on 1059 the structural evolution of transpressional plate boundaries: An example from the 1060
Southern Alps, New Zealand. Geology, 31, 3-6. 1061
Lahfid, A., Beyssac, O., Deville, E., Negro, F., Chopin, C. Goffe, B. (2010) Evolution 1062
of the Raman spectrum of Carbonaceous Material in low-grade metasediments: an 1063 example from the Glarus Alps (Switzerland). Terra Nova, 22, 354-360. 1064
Lamb, S., Smith, E., Stern, T., Warren-Smith E. (in press) Continent scale strike-slip 1065 on a low-angle fault beneath New Zealand’s Southern Alps: implications for crustal 1066 thickening in oblique collision zones. Geochemistry, Geophysics, Geosystems, DOI 1067
10.1002/2015GC005990. 1068
Little, T.A. (2004) Transpressive ductile flow and oblique ramping of lower crust in a 1069 two-sided orogen: Insight from quartz grain-shape fabrics near the Alpine Fault, New 1070
Little, T.A., Holcombe, R.J., Ilg, B.R. (2002a) Ductile fabrics in the zone of active 1072 oblique convergence near the Alpine Fault, New Zealand: identifying the neotectonic 1073
overprint. Journal of Structural Geology, 24(1), 193−217. 1074
32
Little, T.A., Holcombe, R.J., Ilg, B.R. (2002b) Kinematics of oblique continental 1075 collision and ramping inferred from microstuctures and strain in middle crustal rocks, 1076 central Southern Alps, New Zealand. Journal of Structural Geology, 24(1), 219−239. 1077
Little, T.A., Cox, S.C. , Vry, J.K., Batt, G. (2005) Variations in exhumation level and 1078 uplift-rate along the oblique-slip Alpine Fault, central Southern Alps, New Zealand. 1079
Geological Society of America Bulletin, 117(5), 707−723. 1080
Little, T.A., Wightman, R. Holcombe, R.J., Hill, M. (2007) Transpression models and 1081 ductile deformation of the lower crust of the Pacific Plate in the central Southern 1082 Alps, a perspective from structural geology, in A Continental Plate Boundary: 1083 Tectonics at South Island, New Zealand, edited by D. Okaya, T. Stern, F. Davey. 1084
Menzies, C.D., Teagle, D.A.H., Craw, D., Cox, S.C., Boyce, A.J., Barrie, D. (2014) 1086
Incursion of meteoric waters into the ductile regime in an active orogen. Earth and 1087 Planetary Science Letters 399, 1-13. 1088
Molnar, P., Anderson H. J., Audoine E., Eberhart-Phillips D., Gledhill K. R., Klosko 1089 E. R., McEvilly T. V., Okaya D., Savage M. K., Stern T., and Wu F. T. (1999) 1090 Continuous Deformation Versus Faulting through the Continental Lithosphere of New 1091
Timm, C., Townsend, D.B., Tulloch, A.J., Turnbull, I.M., Turnbull, R.E. (2014) 1096 High-level stratigraphic scheme for New Zealand rocks. New Zealand Journal of 1097
Geology and Geophysics 57(4): 402-419. DOI: 10.1080/00288306.2014.946062 1098
Mortimer, N. (1993) Jurassic tectonic history of the Otago schist, New Zealand. 1099
Tectonics, 12(1): 237-244 1100
Mortimer, N. (2000) Metamorphic discontinuities in orogenic belts : example of the 1101 garnet-biotite-albite zone in the Otago schist, New Zealand. International Journal of 1102
Earth Sciences, 89(2): 295-306. 1103
Mortimer, N. (2004) New Zealand’s geological foundations. Gondwana Research, 1104
7(1), 262-272. 1105
Nathan, S., Rattenbury, M.S., Suggate, R.P. (compilers) (2002) Geology of the 1106
Greymouth area: scale 1:250,000. Institute of Geological and Nuclear Sciences 1107 1:250,000 geological map 12. Lower Hutt, New Zealand: Institute of Geological and 1108 Nuclear Sciences. 58 pages + 1 folded map. 1109
Norris, R.J., Cooper, A.F. (2001), Late Quaternary slip rates and slip partitioning on 1110 the Alpine Fault, New Zealand. Journal of Structural Geology, 23, 507−520. 1111
Norris, R.J., Cooper, A.F. (2003) Very high strains recorded in mylonites along the 1112 Alpine Fault, New Zealand: Implications for deep structure of plate boundary faults. 1113 Journal of Structural Geology, 25, 2141-2257. 1114
Norris, R.J, Cooper A.F. (2007) The Alpine Fault, New Zealand, in A Continental 1115 Plate Boundary: Tectonics at South Island, New Zealand edited by D. Okaya, T. 1116 Stern, F. Davey. AGU Geophysical Monograph Series, 175, 157-175. 1117
33
Norris, R.J., Koons, P.O., Cooper A.F. (1990) The obliquely convergent plate 1118 boundary in the South Island of New Zealand: implications for ancient collision 1119 zones, New Zealand. Journal of Structural Geology, 12, 715-725. 1120
Okaya, D., Stern, T., Davey, F., Henrys, S., Cox, S. C. (2007) Continent-continent 1121 collision at the Pacific/Indo-Australian plate boundary: background, motivation, and 1122
principal results, in A Continental Plate Boundary: Tectonics at South Island, New 1123 Zealand edited by D. Okaya, T. Stern, F. Davey. AGU Geophysical Monograph 1124 Series, 175, 1-18. 1125
Pitcairn, I K, Roberts, S, Teagle, D A H & Craw, D. 2005. Detecting hydrothermal 1126 graphite deposition during metamorphism and gold mineralisation. Journal of the 1127
Geological Society, London 162: 429-432. 1128
Plunder, A., Agard, P., Dubacq, B., Chopin, C., Bellanger, M. (2012) How continuous 1129
and precise is the record of P-T paths? Insights from combined thermobarometry and 1130 thermodynamic modelling into subduction dynamics (Schistes Lustrés, W. Alps). 1131 Journal of Metamorphic Geology, 30, 323–346. 1132
Powell, R., Holland, T.J.B. (1988) An internally consistent dataset with uncertainties 1133 and correlations: 3, Applications to geobarometry, worked examples and a computer 1134
program. Journal of Metamorphic Geology, 6, 173–204. 1135
Powell, R. & Holland, T.J.B. (1999) Relating formulations of the thermodynamics of 1136
mineral solid solutions; activity modeling of pyroxenes, amphiboles, and micas. 1137 American Mineralogist, 84(1–2), 1–14. 1138
Rattenbury, M.S., Jongens, R., Cox S.C. (compilers) (2010) Geology of the Haast 1139 area. Institute of Geological and Nuclear Sciences 1:250,000 geological map 14. 1140
Lower Hutt, New Zealand: Institute of Geological and Nuclear Sciences. 67 pages + 1 1141 folded map. 1142
Stern, T., Okaya, D., Kleffman, S., Scherwath, M., Henrys, S., Davey, F. (2007) 1143 Geophysical exploration and dynamics of the Alpine Fault zone, in A Continental 1144 Plate Boundary: Tectonics at South Island, New Zealand edited by D. Okaya, T. 1145
Stern, F. Davey. AGU Geophysical Monograph Series, 175, 207-233. 1146
Suggate, R.P. (1963) The Alpine Fault. Transactions of the Royal Society of New 1147
Zealand (Geology), 2, 105-129. 1148
Sutherland, R., Eberhart-Phillips, D., Harris, R.A., Stern, T.A., Beavan, R.J., Ellis, 1149
S.M., Henrys, S.A., Cox, S.C., Norris, R.J., Berryman, K.R., Townend, J., Bannister, 1150 S.C., Pettinga, J., Leitner, B., Wallace, L.M., Little, T.A., Cooper, A.F., Yetton, M., 1151 Stirling M.W. (2007) Do great earthquakes occur on the Alpine Fault in central South 1152 Island, New Zealand?, in A Continental Plate Boundary: Tectonics at South Island, 1153 New Zealand edited by D. Okaya, T. Stern, F. Davey. AGU Geophysical Monograph 1154
Series, 175, 235-251. 1155
Sutherland, R., Toy, V.G., Townend, J., Cox, S.C., Eccles, J.D., Faulkner, D.R., Prior, 1156 D.J., Norris. R.J., Mariani, E., Boulton, C., Carpenter, B.M., Menzies, C.D., Little, 1157 T.A., Hastings, M., De Pascale, G.P., Langridge, R.M., Scott, H.R., Lindroos, Z.R., 1158
Fleming, B., Kopf, A.J. (2012) Drilling reveals fluid control on architecture and 1159 rupture of the Alpine Fault, New Zealand. Geology, 40(12), 1143-1146; doi: 1160
1110.1130/G33614.33611. 1161
34
Tippett, J.M., Kamp, P.J.J. (1993a) The role of faulting in rock uplift in the Southern 1162 Alps. New Zealand Journal of Geology and Geophysics 36(4), 497−504. 1163
Tippett, J.M., Kamp, P.J.J. (1993b) Fission track analysis of the late Cenozoic vertical 1164 kinematics of continental Pacific crust, South Island, New Zealand. Journal of 1165 Geophysical Research, 98(B9), 16119−16148. 1166
Tippett, M.J., Kamp, P.J.J. (1995) Quantitative relationships between uplift and relief 1167 parameters for the Southern Alps, New Zealand, as determined by fission track 1168 analysis. Earth Surface Processes and Landforms, 20, 153-176. 1169
Toy, V.G., Prior, D.J., Norris R.J. (2008) Quartz textures in Alpine Fault mylonites: 1170 influence of pre-existing preferred orientations on fabric development during uplift. 1171
Journal of Structural Geology, 30(1), 602-621. 1172
Toy, V.G., Craw, D., Cooper, A.F., Norris, R.J. (2010) Thermal regime in the central 1173
Alpine Fault zone, New Zealand : constraints from microstructures, biotite chemistry 1174 and fluid inclusion data. Tectonophysics, 485(1-4): 178-192; 1175 doi:10.1016/j.tecto.2009.12.013 1176
Turnbull, I.M., Mortimer, N., Craw, D. (2001) Textural zonations in the Haast Schist - 1177
a reappraisal, New Zealand. Journal of Geology and Geophysics, 44(1), 171-183. 1178
Vry, J.K., Baker, J., Maas, R., Little, T.A., Grapes, R., Dixon, M. (2004) Zoned 1179 (Cretaceous and Cenozoic) garnet and the timing of high grade metamorphism, 1180
Southern Alps, New Zealand. Journal of Metamorphic Geology 22(3), 137-157. 1181
Vry, J.K., Powell, R., Williams, J. (2008) Establishing the P-T path for Alpine Schist, 1182 Southern Alps near Hokitika, New Zealand. Journal of Metamorphic Geology, 26(1), 1183 81-97. 1184
Walcott, R.I. (1998) Modes of oblique compression: Late Cenozoic tectonics of the 1185 South Island New Zealand. Reviews in Geophysics 36(1): 1-26. 1186
Wallace, L.M., Beavan, R.J., McCaffrey, R., Berryman, K.R., Denys P. (2007) 1187 Balancing the plate motion budget in the South Island, New Zealand using GPS, 1188 geological and seismological data. Geophysical Journal International, 168(1), 332-1189
352, doi:10.1111/j.1365-246X.2006.03183.x. 1190
Wellman, H.W. (1979), An uplift map for the South Island of New Zealand and a 1191
model for uplift of the Southern Alps, in The Origin of the Southern Alps, edited by 1192 R. I. Walcott, and M. M. Cresswell, Royal Society New Zealand Bulletin, 1, 13−20. 1193
White, R.W., Powell, R., Holland, T.J.B., Worley, B.A. (2000) The effect of TiO2 1194 and Fe2O3 on metapelitic assemblages at greenschist and amphibolite facies 1195 conditions: mineral equilibria calculations in the system K2O-FeO-MgO-Al2O3-1196 SiO2-H2O-TiO2-Fe2O3. Journal of Metamorphic Geology, 18, 497-511. 1197
White, R.W., Pomroy, N.E., Powell, R. (2005) An in-situ metatexite-diatexite 1198
transition in upper amphibolite facies rocks from Broken Hill, Australia. Journal of 1199 Metamorphic Geology, 23, 579-602. 1200
White, S. (1996) Composition and zoning of garnet and plagioclase in Haast Schist, 1201
northwest Otago, New Zealand: implications for progressive regional metamorphism. 1202 New Zealand Journal of Geology and Geophysics, 39(4), 515−531. 1203
35
Willner, A.P., Massonne, H.-J., Barr, S.M., White, C.E. (2013) Very low- to low-1204 grade metamorphic processes related to the collisional assembly of Avalonia in SE 1205 Cape Breton Island (Nova Scotia, Canada). Journal of Petrology 54(9) 1849-1874. 1206
Wightman, R., Little, T.A. (2007), Deformation of the Pacific Plate above the Alpine 1207 Fault ramp and its relationship to expulsion of metamorphic fluids: An array of 1208
backshears, in A Continental Plate Boundary: Tectonics at South Island, New Zealand 1209 edited by D. Okaya, T. Stern, F. Davey. AGU Geophysical Monograph Series, 175, 1210 177-205. 1211
1212
36
1213
Table captions 1214
1215
Table 1 – RSCM temperature data obtained along various profiles in the Southern 1216
Alps. For each sample, information provided are: sample name (PETLAB database, 1217
http://pet.gns.cri.nz), Easting and Northing (New Zealand Transverse Mercator using 1218
NZGD2000), altitude z (in meters), Distance D to the Alpine Fault (in kilometers), 1219
Textural Zone (TZ), Metamorphic Zone, N number of Raman spectra, R2 ratio and 1220
associated standard deviation SDV, Temperature T and associated standard error SE 1221
(Standard error is the standard deviation divided by √N). See text for further details. 1222
1223
Table 2 – Summary of petrologic data obtained in this study along Franz, Fox and 1224
Waikukupa profiles. For each sample, information provided: sample name (PETLAB 1225
database, http://pet.gns.cri.nz), Easting and Northing (New Zealand Transverse 1226
Mercator using NZGD2000), altitude z (in meters), Distance D to the Alpine Fault (in 1227
kilometers), P-T conditions from Grapes and Watanabe (1992), Garnet-biotite 1228
geothermometry (Hodges and Spear, 1982) results (grt-bio T) based on rim analyses, 1229
using pressure estimates based on results of garnet-biotite-muscovite-plagioclase 1230
barometry (Hoisch, 1990, Fe-endmember) for the same or nearby samples, from 1231
Grapes and Watanabe (1992). X (grs+sps) is (Ca +Mn)/(Fe + Mg + Ca + Mn) in 1232
garnet, values <0.2 are generally more suitable for garnet-biotite thermometry. T TC 1233
is maximum temperature estimate based on observed mineral assemblages and results 1234
of P-T pseudosection calculations (this study), for pressure estimates based on nearby 1235
samples and the calculated mineral assemblage stability field. See the PETLAB 1236