Peak metamorphic temperature and thermal history of the Southern Alps (New Zealand)

Keywords: Southern Alps, Alpine Fault, RSCM thermometry, Alpine Schist, exhumation

The Southern Alps of New Zealand result from late Cenozoic convergence between the IndoAustralian and Pacific plates, and are one of the most active mountain belts in the world. Metamorphic rocks carrying a polymetamorphic legacy, ranging from lowgreenschist to high-grade amphibolites, are exhumed in the hanging wall of the Alpine Fault. On a regional scale, the metamorphic grade has previously been described in terms of metamorphic zones and mineral isograds; application of quantitative petrology being severely limited owing to unfavourable quartzofeldspathic lithologies. This study quantifies peak metamorphic temperatures (T) in a 300 x 20 km area, based on samples forming 13 transects along-strike from Haast in the south to Hokitika in the north, using thermometry based on Raman spectroscopy of carbonaceous material (RSCM). Peak metamorphic T decreases across each transect from ≥ 640°C locally in the direct vicinity of the Alpine Fault to less than 330°C at the drainage divide 15-20 km southeast of the fault. Thermal field gradients exhibit a degree of similarity from southernmost to northernmost transects, are greater in low-grade semischist than high-grade schist, are affected by folding or discontinuous juxtaposition of metamorphic zones, and contain limited information on crustal-scale geothermal gradients. Temperatures derived by RSCM thermometry are slightly (≤ 50°C) higher than those derived by traditional quantitative petrology using garnet-biotite thermometry and THERMOCALC modeling. The age of RSCM T appears to be mostly pre-Cenozoic over most of the area except in central Southern Alps (Franz Josef-Fox area), where the amphibolite facies schists have T of likely Cenozoic age. The RSCM T data place some constraints on the mode of exhumation along the Alpine Fault and have implications for models of Southern Alps tectonics.

Introduction

The kinematics and thermal structure of orogenic wedges result from the coupling between crustal and surface processes at convergent plate boundaries. Being one of the most active mountain belts in terms of both tectonic and surface processes, the Southern Alps of New Zealand offers a unique tectonophysical laboratory to investigate these interactions. The rocks of this mountain belt were formed by Paleozoic and Mesozoic subduction-accretion processes at the paleo-Pacific margin of Gondwana, split from Gondwana and were thinned during the Late Cretaceous, then rent by dextral strike-slip displacement as the Alpine Fault plate-boundary developed during the Neogene.

The Southern Alps, which comprise much of the South Island (Figure 1), began forming during the late Cenozoic as the IndoAustralian-Pacific plate motion became increasingly convergent in the Pliocene-Pleistocene. These mountains form against the Alpine Fault -a transpressive section of the Pacific and IndoAustralian plate boundary (see Cox and Sutherland, 2007 for review). The Pacific Plate presently appears to delaminate (e.g. [START_REF] Molnar | Continuous Deformation Versus Faulting through the Continental Lithosphere of New Zealand[END_REF] or subduct (e.g. Beaumont et al. 1994) within the orogen, actively exhuming a belt of mid-upper crustal material obliquely on the Alpine Fault, and accreting lower crustal material into a thickened crustal root (e.g. [START_REF] Gerbault | Three-dimensional lateral crustal thickening in continental oblique collision: an example from the Southern Alps, New Zealand[END_REF]. The plate boundary is widely cited as a type-example of deep geological processes and continent-continent collision (e.g. [START_REF] Okaya | Continent-continent collision at the Pacific/Indo-Australian plate boundary: background, motivation, and principal results, in A Continental Plate Boundary: Tectonics at South Island, New Zealand[END_REF].

Over the past twenty years, there has been considerable scientific effort trying to understand the architecture of the IndoAustralian-Pacific plate convergence in the South Island (e.g. [START_REF] Okaya | Continent-continent collision at the Pacific/Indo-Australian plate boundary: background, motivation, and principal results, in A Continental Plate Boundary: Tectonics at South Island, New Zealand[END_REF]. Evidence has been gathered on the depth of the crustal root, nature of lithosphere, and geometry of faults (see [START_REF] Okaya | Continent-continent collision at the Pacific/Indo-Australian plate boundary: background, motivation, and principal results, in A Continental Plate Boundary: Tectonics at South Island, New Zealand[END_REF]. This effort has been complemented by thermochronologic work to decipher the timing and thermal structure associated with mountain building and exhumation (e.g. ;Tippett and Kamp, 1993a,b;[START_REF] Batt | Thermochronological analysis of the dynamics of the Southern Alps, New Zealand[END_REF][START_REF] Herman | Low-temperature thermochronology and thermokinematic modeling of deformation, exhumation and development of topography in the central Southern Alps, New Zealand[END_REF]. Other studies have noted perturbations of the geotherm, producing high thermal gradients and hot spring activity (e.g. [START_REF] Allis | The thermal regime beneath the Southern Alps. In: The origin of the Southern Alps[END_REF][START_REF] Koons | Some thermal and mechanical consequences of rapid uplift: an example from the Southern Alps, New Zealand[END_REF][START_REF] Allis | New insights to temperature and pressure beneath the central Southern Alps, New Zealand[END_REF][START_REF] Sutherland | Drilling reveals fluid control on architecture and rupture of the Alpine Fault[END_REF][START_REF] Cox | Changes in hot spring temperature and hydrogeology of the Alpine Fault hanging wall, New Zealand, induced by distal South Island earthquakes[END_REF]. However, while the general metamorphic structure of the Southern Alps is qualitatively well established, there are very few quantitative constraints on the thermal state and thermal history of the crust. An understanding of the thermal history of the orogen is needed to constrain the information lowtemperature thermochronometers provide about erosion rates and the stability of landforms, as well as the rheology of rocks, behavior of faults at seismogenic depth [START_REF] Toy | Thermal regime in the central Alpine Fault zone, New Zealand : constraints from microstructures, biotite chemistry and fluid inclusion data[END_REF], and ultimately seismic hazard [START_REF] Sutherland | Do great earthquakes occur on the Alpine Fault in central South Island, New Zealand?[END_REF]. The lack of thermal state information is largely attributable to the bulk rock compositions (mainly metamorphosed quartzofeldspathic greywacke) that are chemically unfavourable for precise metamorphic petrology, and complicated further by the polymetamorphic and polydeformational history of the rocks and potential overprinting effects of fluid flow [START_REF] Koons | Fluid flow during active oblique convergence: a Southern Alps model from mechanical and geochemical observations[END_REF][START_REF] Vry | Zoned (Cretaceous and Cenozoic) garnet and the timing of high grade metamorphism, Southern Alps, New Zealand[END_REF][START_REF] Menzies | Incursion of meteoric waters into the ductile regime in an active orogen[END_REF].

In this study we introduce thermometry based on Raman spectroscopy of carbonaceous material (RSCM) [START_REF] Beyssac | Raman spectra of carbonaceous material from metasediments: a new geothermometer[END_REF]) that allows the quantitative estimate of peak metamorphic temperature (T) independently from the extent of retrogression and presence of diagnostic mineral assemblages. Owing to widespread presence of carbonaceous material in the local Alpine Schist and greywacke, this technique has enabled the generation of a large dataset covering most of the Alpine Fault hanging wall, both along strike and perpendicular to the fault. We present a dataset of 142 new temperature estimates covering a 300 x 20 km area (Table 1). We have also revisited traditional garnet-biotite thermometry results for some of the same samples used for RSCM thermometry, or collected from nearby locations. We provide those results for comparison, along with a few insights gained through comparison of the observed mineral assemblages with their stability fields in P-T pseudosections calculated using THERMOCALC. We then discuss the age of these temperatures by reviewing existing geochronologic constraints to separate the Mesozoic legacy from the late Cenozoic thermal overprint and the extent to which this varies along the plate boundary. Finally, we highlight some constraints these RSCM temperature distributions place on the style and nature of Southern Alps tectonics. Pacific Plate motion relative to the IndoAustralian Plate is 39.7 ± 0.7 mm/a at 245 ± 1° in the central South Island (MORVEL model of DeMets et al., 2010). The vector is 12° anticlockwise of the Alpine Fault, which strikes 053° and is inferred to dip ~40-60° SE [START_REF] Norris | The Alpine Fault, New Zealand, in A Continental Plate Boundary: Tectonics at South Island, New Zealand[END_REF][START_REF] Stern | Geophysical exploration and dynamics of the Alpine Fault zone, in A Continental Plate Boundary: Tectonics at South Island, New Zealand[END_REF], extending downward to depths of 25-30 km based on the presence of amphibolite facies schist exhumed in its hanging wall [START_REF] Grapes | Uplift and exhumation of Alpine Schist in the Franz Josef -Fox Glacier area of the Southern Alps, New Zealand[END_REF]. The generally accepted crustal model depicts the Alpine Fault shallowing eastward into a lower crustal décollement that delaminates the Pacific Plate (Figure 2, e.g., [START_REF] Wellman | An uplift map for the South Island of New Zealand and a model for uplift of the Southern Alps, in The Origin of the Southern Alps[END_REF][START_REF] Norris | The obliquely convergent plate boundary in the South Island of New Zealand: implications for ancient collision zones, New Zealand[END_REF][START_REF] Okaya | Continent-continent collision at the Pacific/Indo-Australian plate boundary: background, motivation, and principal results, in A Continental Plate Boundary: Tectonics at South Island, New Zealand[END_REF], although there is no conclusive evidence for such a detachment. Thermochronological modeling indicates uplift/cooling must be a two stage process first initiating on a gently rising trajectory beneath the dry pro-side of the mountains then occurring more-rapidly up the Alpine Fault ramp [START_REF] Herman | Low-temperature thermochronology and thermokinematic modeling of deformation, exhumation and development of topography in the central Southern Alps, New Zealand[END_REF]. While the maximum metamorphic grade of exhumed rocks has been used to infer the approximate depth of the Alpine Fault and Pacific Plate delamination, it is predicated on an assumed geothermal gradient and the assumption that previously-stable metamorphic assemblages were exhumed in the late Cenozoic. Although low-temperature thermochronologic ages are clearly the result of Neogene-Quaternary cooling and exhumation, many of the rocks reached peak metamorphic temperatures during the Mesozoic, so much care is needed when using metamorphic assemblages to constrain the present crustal structure.

Geological setting

General tectonics of the Southern Alps

Tectonostratigraphy and structural framework

Rocks of the Pacific Plate, southeast of the Alpine Fault, belong to the Eastern Province and mostly to the Torlesse Composite Terrane/Supergroup [START_REF] Mortimer | High-level stratigraphic scheme for New Zealand rocks[END_REF]. They are dominated by compositionally monotonous greywacke sandstone and argillite sequences of the Rakaia Terrane that were deposited in an accretionary prism on the margin of Gondwana during the Permian-Triassic [START_REF] Mortimer | New Zealand's geological foundations[END_REF].

Sediments were tectonically stacked and imbricated by accretion, and now locally include some metavolcanic-rich and Cretaceous sequences [START_REF] Cooper | Cretaceous sedimentation and metamorphism of the western Alpine Schist protoliths associated with the Pounamu Ultramafic Belt, Westland, New Zealand[END_REF]. Metavolcanic, chert and micaceous-rich schist sequences have been differentiated locally as the Aspiring lithologic association [START_REF] Craw | Lithologic variations in Otago Schist, Mt Aspiring area, northwest Otago, New Zealand[END_REF][START_REF] Nathan | Geology of the Greymouth area: scale 1:250,000[END_REF][START_REF] Cox | Geology of the Aoraki area[END_REF][START_REF] Rattenbury | Geology of the Haast area[END_REF] and may warrant separate terrane status [START_REF] Cooper | Cretaceous sedimentation and metamorphism of the western Alpine Schist protoliths associated with the Pounamu Ultramafic Belt, Westland, New Zealand[END_REF]. Regional low-grade metamorphism affected much of the Rakaia Terrane during the Jurassic [START_REF] Mortimer | Jurassic tectonic history of the Otago schist, New Zealand[END_REF][START_REF] Mortimer | Metamorphic discontinuities in orogenic belts : example of the garnet-biotite-albite zone in the Otago schist, New Zealand[END_REF], possibly involving several discrete metamorphic events [START_REF] Adams | K-Ar geochronology of Torlesse Supergroup metasedimentary rocks in Canterbury, New Zealand[END_REF][START_REF] Adams | Rb-Sr age and strontium isotope characterisation of the Torlesse Supergroup in Canterbury, New Zealand, and implications for the status of the Rakaia Terrane[END_REF].

Schist fabrics and most metamorphic mineral growth have occurred during Jurassic-Cretaceous metamorphism [START_REF] Cooper | Cretaceous sedimentation and metamorphism of the western Alpine Schist protoliths associated with the Pounamu Ultramafic Belt, Westland, New Zealand[END_REF][START_REF] Mortimer | Metamorphic discontinuities in orogenic belts : example of the garnet-biotite-albite zone in the Otago schist, New Zealand[END_REF][START_REF] Vry | Zoned (Cretaceous and Cenozoic) garnet and the timing of high grade metamorphism, Southern Alps, New Zealand[END_REF].

The 300 km-long belt of schist adjacent to the Alpine Fault contains multiple generations of metamorphic fabrics, folds, and syn-to post-metamorphic quartz veins [START_REF] Little | Variations in exhumation level and uplift-rate along the oblique-slip Alpine Fault, central Southern Alps, New Zealand[END_REF][START_REF] Cox | Geology of the Aoraki area[END_REF]. A near-continuous mid-upper crustal section is exposed southeastwards across the Southern Alps [START_REF] Grapes | Uplift and exhumation of Alpine Schist in the Franz Josef -Fox Glacier area of the Southern Alps, New Zealand[END_REF][START_REF] Grapes | Metamorphism and uplift of Alpine Schist in the Franz Josef-Fox Glacier area of the Southern Alps, New Zealand[END_REF][START_REF] Little | Variations in exhumation level and uplift-rate along the oblique-slip Alpine Fault, central Southern Alps, New Zealand[END_REF]. Mid-crustal mylonites and amphibolite facies schist adjacent to the fault contain evidence of a late Cenozoic ductile deformation overprint that constructively reinforced and reoriented the pre-existing Mesozoic metamorphic fabrics (Little et al. 2002a[START_REF] Little | Transpression models and ductile deformation of the lower crust of the Pacific Plate in the central Southern Alps, a perspective from structural geology, in A Continental Plate Boundary: Tectonics at South Island[END_REF]Norris andCooper 2003, 2007;[START_REF] Toy | Quartz textures in Alpine Fault mylonites: influence of pre-existing preferred orientations on fabric development during uplift[END_REF]. A steeply dipping array of late-stage shears that is present for 20 km in the Franz Josef -Fox area (central Southern Alps) represents an exhumed, fossil, brittleductile transition zone (BDTZ; Little et al., 2002b;[START_REF] Little | Transpression models and ductile deformation of the lower crust of the Pacific Plate in the central Southern Alps, a perspective from structural geology, in A Continental Plate Boundary: Tectonics at South Island[END_REF]. The zone separates schist from relatively undeformed greywacke and semischist sequences that were metamorphosed during the Mesozoic but suffered only brittle effects during the late Cenozoic IndoAustralian-Pacific plate transpression [START_REF] Cox | Structure and fluid migration in a late Cenozoic duplex system forming the Main Divide in the central Southern Alps, New Zealand[END_REF][START_REF] Cox | Potentially active faults in the rapidly eroding landscape adjacent to the Alpine Fault, central Southern Alps[END_REF].

Two regionally extensive foliations are developed in schist beside the Alpine Fault: An early foliation (S1 or S2) that is sub-parallel to remnant bedding and metamorphic isograds, and a steeply dipping crenulation foliation (S3) that is axial planar to folds of the S1/S2 fabric and bedding [START_REF] Grindley | Structure of the Alpine Schists of South Westland, Southern Alps, New Zealand[END_REF]Little et al. 2002a).

At regional scale, metamorphic mineral zones are slightly oblique to boundaries in the textural development of schistosity and cleavage [START_REF] Little | Variations in exhumation level and uplift-rate along the oblique-slip Alpine Fault, central Southern Alps, New Zealand[END_REF][START_REF] Cox | Geology of the Aoraki area[END_REF][START_REF] Rattenbury | Geology of the Haast area[END_REF]. The belt of schist varies in width along the orogen, being narrowest (8 km) in the Franz Josef -Fox area of the central Southern Alps, where tight to isoclinal folding has aligned the early schistosity with 035-045º strike weakly oblique to the 055º Alpine Fault [START_REF] Little | Variations in exhumation level and uplift-rate along the oblique-slip Alpine Fault, central Southern Alps, New Zealand[END_REF]. Elsewhere, the folding is more open, early schistosity strikes at a high-angle (nearly perpendicular) to the Alpine Fault, and the schist belt broadens. Mineral metamorphic grade changes are indicated by the presence or absence of key minerals or mineral assemblages but textural metamorphic zones, a semi-quantitative measure of the degree of cleavage, foliation and metamorphic segregation development, are considerably easier to apply and map in the field [START_REF] Bishop | Stratigraphic, structural, and metamorphic relationships in the Dansey Pass area, Otago, New Zealand[END_REF][START_REF] Turnbull | Textural zonations in the Haast Schista reappraisal, New Zealand[END_REF]. This classification is widely used throughout New Zealand, and offers a first-order representation of the increasing deformation gradient on a regional scale towards the Alpine Fault. From SE to NE, following increasing metamorphic grade, there are four main textural zones: uncleaved greywacke (TZ1), cleaved greywacke (TZ2a) to well cleaved semischists (TZ2b), foliated schists (TZ3) and well segregated schists (TZ4).

Metamorphism and geochronology in the Alpine Schist

The general metamorphic pattern is well mapped and qualitatively constrained in the Southern Alps by the presence or absence of key index minerals or mineral assemblages [START_REF] Nathan | Geology of the Greymouth area: scale 1:250,000[END_REF][START_REF] Cox | Geology of the Aoraki area[END_REF][START_REF] Rattenbury | Geology of the Haast area[END_REF].

There is a general SE-NW increasing metamorphic gradient, ranging from subgreenschist (prehnite-pumpellyite or pumpellyite-actinolite facies); to chlorite then biotite zones (greenschist facies); and garnet-oligoclase then K-feldspar zones (amphibolite facies). Metamorphic mineral zones have been mapped on the basis of the first appearance of minerals, but many of the 'isograd' boundaries between zones represent juxtaposition by brittle faults or ductile shear zones, such that the 'first appearance' of an index mineral cannot necessarily be assumed to represent a preserved mineral reaction surface [START_REF] Craw | Structural boundaries and biotite and garnet 'isograds' in the Otago and Alpine Schists, New Zealand[END_REF]. Blocks with distinct metamorphic and structural histories are commonly bound by faults or shear zones. Locally, distinct phases of biotite and garnet crystal growth can be distinguished. Fine-grained biotite and very fine-grained grossular-spessartine garnet are associated with rocks attaining garnet-biotite-albite zone of the greenschist facies during the Jurassic, and are distinguished by the biotite-1 and garnet-1 'isograds' [START_REF] White | Composition and zoning of garnet and plagioclase in Haast Schist, northwest Otago, New Zealand: implications for progressive regional metamorphism[END_REF][START_REF] Mortimer | Metamorphic discontinuities in orogenic belts : example of the garnet-biotite-albite zone in the Otago schist, New Zealand[END_REF][START_REF] Cox | Geology of the Aoraki area[END_REF][START_REF] Rattenbury | Geology of the Haast area[END_REF]. Whilst common in Otago, in most places in the Southern Alps these minerals were either (i) completely overgrown or consumed by growth of porphyroblastic 'biotite-2' and 'garnet-2' almandine as metamorphism reached amphibolite facies during the Cretaceous-Cenozoic, or (ii) have been retrogressively replaced by chlorite. As with textural zonation, the Alpine Schist metamorphic mineral zones are slightly oblique to the Alpine Fault and to the main SW-NE structural trend of the mountains.

There are few detailed and quantitative petrology studies on Alpine Schist. P-T estimates have been made using garnet-biotite thermometers and barometers that involve partitioning of Ca between garnet and plagioclase [START_REF] Cooper | Retrograde alteration of chromian kyanite in metachert and amphibolite whiteschist from the Southern Alps, New Zealand, with implications for uplift on the Alpine Fault[END_REF][START_REF] Grapes | Metamorphism and uplift of Alpine Schist in the Franz Josef-Fox Glacier area of the Southern Alps, New Zealand[END_REF][START_REF] Grapes | Uplift and exhumation of Alpine Schist in the Franz Josef -Fox Glacier area of the Southern Alps, New Zealand[END_REF]. Peak metamorphic conditions have been constrained to 600-700 °C and 9.2-10 kbar in the Franz Josef -Fox area [START_REF] Grapes | Metamorphism and uplift of Alpine Schist in the Franz Josef-Fox Glacier area of the Southern Alps, New Zealand[END_REF][START_REF] Grapes | Uplift and exhumation of Alpine Schist in the Franz Josef -Fox Glacier area of the Southern Alps, New Zealand[END_REF]. Such high temperatures were recently confirmed by [START_REF] Toy | Thermal regime in the central Alpine Fault zone, New Zealand : constraints from microstructures, biotite chemistry and fluid inclusion data[END_REF] applying Ti-in-Biotite thermometry in the mylonitic rocks from the Alpine Fault shear zone. Within a few kilometers of the Alpine Fault, peak P-T conditions decrease progressively to lower P-T conditions, e.g. 400-540 °C and 4-7 kbar towards the southeast [START_REF] Cooper | Retrograde alteration of chromian kyanite in metachert and amphibolite whiteschist from the Southern Alps, New Zealand, with implications for uplift on the Alpine Fault[END_REF][START_REF] Green | The Alpine Fault Zone of the Waitaha Valley area[END_REF][START_REF] Grapes | Uplift and exhumation of Alpine Schist in the Franz Josef -Fox Glacier area of the Southern Alps, New Zealand[END_REF][START_REF] Grapes | Metamorphism and uplift of Alpine Schist in the Franz Josef-Fox Glacier area of the Southern Alps, New Zealand[END_REF]. To the north, in the Hokitika region, [START_REF] Vry | Establishing the P-T path for Alpine Schist, Southern Alps near Hokitika, New Zealand[END_REF] established one of the rare P-T paths available for Alpine Schist, using pseudosection calculations. They obtained a prograde P-T path from ca. 380°C / 2.5 kbar to ca. 490°C / 8.5 kbar followed by a slight increase of temperature during decompression to reach ca. 500°C / 6.5 kbar.

Geochronological and thermochronological datasets available for the Southern

Alps cover a range from low to high closure temperature systems. [START_REF] Batt | Thermochronological analysis of the dynamics of the Southern Alps, New Zealand[END_REF] generated a compilation of new and previously available fission track data from zircon and/or apatite (see also Tippett andKamp, 1993a,b, 1995), and K-Ar and 40 Ar-39 Ar data from muscovite, biotite and/or K-feldspar. This compilation was subsequently complemented in [START_REF] Herman | Low-temperature thermochronology and thermokinematic modeling of deformation, exhumation and development of topography in the central Southern Alps, New Zealand[END_REF]. These studies illustrated 2dimensional patterns of thermochronological ages with variation both along-strike and perpendicular to the Alpine Fault. Along strike, old apparent ages are almost systematically observed with all techniques close to Haast River in the southwest, representing a complex burial and exhumation history during the Cretaceous and Cenozoic, prior to the development of the modern tectonic regime. Ages decrease towards the northeast and are mostly late Cenozoic and 'reset' in Alpine Schist of the central Southern Alps. For instance, [START_REF] Chamberlain | Geochronologic constraints of the uplift and metamorphism along the Alpine Fault, South Island, New Zealand[END_REF] derived 40 Ar-39 Ar dates for hornblende in the Alpine Schist and obtained reset Neogene (3-6 Ma) ages in the Fox-Franz Josef area, as well as some disturbed 25 Ma ages with plateau ages suggestive of partial gas retention. Although two Cretaceous 40 Ar-39 Ar ages on hornblende were subsequently obtained by [START_REF] Little | Variations in exhumation level and uplift-rate along the oblique-slip Alpine Fault, central Southern Alps, New Zealand[END_REF] in Fox Valley and Waikukupa River, other hornblende ages and all other thermochronometers for schist between Copland and Whataroa Rivers record a Neogene exhumation history for the central Southern Alps. Further northeast, higher temperature thermochronometers record increasingly older ages, partially reset by the thermal regime of the present plate boundary. Perpendicular to the Alpine Fault, all thermochronometers show a zone of young ages adjacent to the fault, which increase southeastwards across the mountains through a series of exhumed partial annealing zones. The pattern of resetting is systematic, with annealing zones located further from the Alpine Fault, but also along strike away from the central Southern Alps, with decreasing closure temperature of the thermochronometer [START_REF] Batt | Thermochronological analysis of the dynamics of the Southern Alps, New Zealand[END_REF][START_REF] Herman | Low-temperature thermochronology and thermokinematic modeling of deformation, exhumation and development of topography in the central Southern Alps, New Zealand[END_REF][START_REF] Little | Variations in exhumation level and uplift-rate along the oblique-slip Alpine Fault, central Southern Alps, New Zealand[END_REF].

The long and complex burial and exhumation history of Alpine Schist makes it hard to decipher and distinguish the thermal history during late Cenozoic evolution of the plate boundary, and/or formation of the Southern Alps [START_REF] Mortimer | New Zealand's geological foundations[END_REF][START_REF] Cox | Regional geological framework of South Island, New Zealand, and its significance for understanding the active plate boundary[END_REF]. Schists in the Southern Alps are generally thought to be correlated with those in Otagoboth being part of the Haast Schist Group (see [START_REF] Mortimer | High-level stratigraphic scheme for New Zealand rocks[END_REF]. But while metamorphic assemblages in Otago were formed during Jurassic-Early Cretaceous and schist uplifted in the Early Cretaceous, some metamorphic growth in Alpine Schist occurred during the Late Cretaceous and a component of ductile-brittle deformation overprinted, but did not involve complete recrystallization of these fabrics during the Neogene (Little et al., 2002a,b). A few studies have suggested that locally in the central Southern Alps, peak metamorphic temperatures may have been reached in the Alpine Schist during the Neogene. For instance, on a large scale, by reconsidering available thermochonological data in light of geological and geophysical observations, [START_REF] Little | Variations in exhumation level and uplift-rate along the oblique-slip Alpine Fault, central Southern Alps, New Zealand[END_REF] proposed the local 'Alpine' exhumation of lower crustal rocks (T>500°C) in a narrow zone where 40 Ar-39 Ar ages on hornblende are totally reset. This narrow zone is located between Franz Josef -Fox and is not located right on the Alpine Fault but a few kilometers from it to the SE.

Combining detailed petrological and geochronological investigations on garnet porphyroblasts collected farther to the north, from schist near the Alpine Fault in the Hokitika region, [START_REF] Vry | Zoned (Cretaceous and Cenozoic) garnet and the timing of high grade metamorphism, Southern Alps, New Zealand[END_REF] were able to show that at least locally, peak metamorphic temperatures of ca. 600°C were reached during the late Cenozoic and recorded in the external growing zone of garnets. For this, they dated the different zones of the garnet porphyroblasts using Sm-Nd and Lu-Hf systems and calculated for each zone the P-T conditions of crystallization using garnet-biotite thermometry and garnet-plagioclase-muscovite-biotite barometry.

Methods

Sampling

Samples collected for this study have been contributed to the national New Zealand rock and mineral collection PETLAB (pet.gns.cri.nz). Samples already in the collection, obtained during many years of mapping and research investigations in the western Southern Alps (e.g. [START_REF] Grindley | Structure of the Alpine Schists of South Westland, Southern Alps, New Zealand[END_REF][START_REF] Cox | Geology of the Aoraki area[END_REF][START_REF] Rattenbury | Geology of the Haast area[END_REF], were also analysed. Schist and semischist samples have been systematically characterized for their lithology, main mineral assemblage, textural fabric development, and are georeferenced with standard coordinate systems. Any research action on the samples, such as geochronology, geochemistry or quantitative petrology, has been recorded, and substantial information has been returned to the PETLAB database where it is publically accessible. Supplementary material contains a kmz file of samples, sample descriptions and RSCM T results. Numbers prefixed by P, OU or VU refer to unique sample identifiers used by PETLAB.

Samples were collected from the field, or selected from PETLAB, to form thirteen transects across the Southern Alps named according to major rivers or places, to quantify variations both across and along the plate boundary. For each sample location, we calculated a three-dimensional structural distance D (in km) to the Alpine Fault plane by assuming the fault dips at 45° from its mapped surface trace and using the topographic altitude of the sample site, as depicted on Figure 2. Possible variation in D, associated with uncertainty in the subsurface geometry of the fault, was also assessed by varying the dip by ±15°, to 30° and 60° (see below).

RSCM thermometry

RSCM thermometry is based on the quantitative study of the degree of graphitization of carbonaceous material (CM) that is a reliable indicator of metamorphic temperature (T). Because of the irreversible character of graphitization, CM structure is not sensitive to the retrograde path during exhumation of rocks and records the maximum T reached during metamorphism [START_REF] Beyssac | Raman spectra of carbonaceous material from metasediments: a new geothermometer[END_REF]. Absolute T can be determined in the range 330-650°C with a precision of ± 50 °C due to uncertainties on petrologic data used for the calibration. Relative uncertainties on T are however much smaller, in the range 10-15 °C [START_REF] Beyssac | Thermal metamorphism in the lesser Himalaya of Nepal determined from Raman spectroscopy of carbonaceous material[END_REF].

Raman spectra were obtained using a Renishaw InVIA Reflex microspectrometer (ENS Paris). We used a 514 nm Spectra Physics argon laser in circular polarization. The laser was focused on the sample by a DMLM Leica microscope with a 100 × objective (NA=0.85), and the laser power at the sample surface was set around 1 mW. The Rayleigh diffusion was eliminated by edge filters, and to achieve nearly confocal configuration the entrance slit was closed down to 10-15 µm. The signal was finally dispersed using a 1800 gr/mm grating and analyzed by a Peltier cooled RENCAM CCD detector. Before each session, the spectrometer was calibrated with a silicon standard. Because Raman spectroscopy of CM can be affected by several analytical mismatches, we followed closely the analytical and fitting procedures described by [START_REF] Beyssac | Raman spectra of carbonaceous material from metasediments: a new geothermometer[END_REF][START_REF] Beyssac | On the characterization of disordered and heterogeneous carbonaceous materials using Raman spectroscopy[END_REF]. Measurements were done on polished thin sections cut perpendicularly to the main metamorphic rock fabrics (mostly S1 or S2) and CM was systematically analyzed below a transparent adjacent mineral, generally quartz. More than 15 spectra were recorded for each sample in the static or extended scanning mode (1000-2000 cm -1 ) with acquisition times varying from 30 (static) to 60 -150 (extended) seconds. Spectra were then processed using the software Peakfit using a fitting procedure with 3 bands with Voigt profiles [START_REF] Beyssac | On the characterization of disordered and heterogeneous carbonaceous materials using Raman spectroscopy[END_REF][START_REF] Beyssac | Application of Raman spectroscopy to the study of graphitic carbons in the Earth Sciences[END_REF].

Petrology

The principal aim of this paper was to investigate metamorphic temperatures using RSCM thermometry on a large scale. However we also felt it is important to place these data in the context of previously available petrology, albeit without providing a complete and parallel petrological assessment. Because published petrological data was at least one, and in most cases three, decades old we undertook some new analytical work and recalculations to update the available data.

Major element analyses of bulk-rock samples were carried out by X-ray fluorescence spectrometry, with Fe 2+ analyses by titration. Early results had been obtained at Victoria University [START_REF] Grapes | Analyses of Quartzofeldspathic Schist and Metacherts[END_REF]. More recent results were obtained at SpectraChem Analytical Ltd., Wellington, New Zealand, using methods described in [START_REF] Vry | Establishing the P-T path for Alpine Schist, Southern Alps near Hokitika, New Zealand[END_REF], with Fe 2+ analyses performed at the Albert-Ludwigs-Universität, Freiburg, Germany, following the techniques outlined in [START_REF] Heinrichs | Praktikum der Analytischen Geochemie[END_REF]. For these latter samples, the standard CRM2115 was also analysed, and the result was Fe 2+ = 8.54% compared to the stated value of Fe 2+ = 8.50%; the FeO determinations have a 1σ error of 0.08%. The powdered rock samples were prepared at Victoria University using a TEMA tungsten carbide mill. All analytical data are available from the PETLAB database (www.petlab.gns.cri.nz).

Electron probe microanalyses of rim compositions of garnet, plagioclase, and coexisting matrix biotite were obtained by R. Grapes, mainly in the early 1990's [START_REF] Grapes | Metamorphism and uplift of Alpine Schist in the Franz Josef-Fox Glacier area of the Southern Alps, New Zealand[END_REF][START_REF] Grapes | Uplift and exhumation of Alpine Schist in the Franz Josef -Fox Glacier area of the Southern Alps, New Zealand[END_REF] using a JEOL JXA-733 SuperProbe, with Moran Scientific software that incorporates modified ZAF and Bence-Albee matrix corrections. The operating conditions used were 15 kV accelerating voltage, 12 nA sample current, and a beam diameter of 1-3 µm. Some representative analyses are given in supplementary material. All analysis positions were located using backscattered electron imaging. Some representative data are given in supplementary material and are available from the PETLAB database (www.petlab.gns.cri.nz).

To provide insights into the P-T conditions for the stability fields of relevant mineral assemblages, P-T pseudosections were calculated in the 11 components [START_REF] Powell | An internally consistent dataset with uncertainties and correlations: 3, Applications to geobarometry, worked examples and a computer program[END_REF] and the internally consistent thermodynamic dataset 5.5 [START_REF] Holland | An internally consistent thermodynamic data set for phases of petrological interest[END_REF]. Mineral mixing models and nonideality parameters are based on [START_REF] Holland | An internally consistent thermodynamic data set for phases of petrological interest[END_REF] and [START_REF] Powell | Relating formulations of the thermodynamics of mineral solid solutions; activity modeling of pyroxenes, amphiboles, and micas[END_REF]. The activity-composition relationships used for garnet (CaMnFMAS), white mica and paragonite (NKFMASH), plagioclase (NCAS), biotite (KFMASHTO), epidote (CaFe 3+ ASH), and chlorite (MnFMASH) are as described in [START_REF] Vry | Establishing the P-T path for Alpine Schist, Southern Alps near Hokitika, New Zealand[END_REF]. The activity-composition model for magnetite (FTO) is from [START_REF] White | The effect of TiO2 and Fe2O3 on metapelitic assemblages at greenschist and amphibolite facies conditions: mineral equilibria calculations in the system K2O-FeO-MgO-Al2O3-SiO2-H2O-TiO2-Fe2O3[END_REF], and the ilmenite (MnFTO) is from [START_REF] White | An in-situ metatexite-diatexite transition in upper amphibolite facies rocks from Broken Hill, Australia[END_REF], but with non ideality described by W(ordered ilmenite, pyrophanite) = 2 kJ, W(disordered ilmenite, pyrophanite) = 2 kJ, and W(hematite, pyrophanite) = 25 kJ (R. Powell, personal communication, 2012). Albite, rutile, sphene, quartz, and H 2 O were treated as pure end-members.

system MnNCKFMASHTO (MnO, Na 2 O, CaO, K 2 O, FeO, MgO, Al 2 O 3 , SiO 2 , H 2 O, TiO 2 , O) using THERMOCALC v. 3.35

Results

Graphitic carbon in the Southern Alps

Graphitic carbon is widespread in rocks of the Southern Alps, generally dispersed in the mineral matrix and totally absent only at a few localities. The latter localities may correspond to unfavourable lithologies or lithologies affected by intense fluid circulation which may be responsible for bleaching of carbonaceous material in the rocks. In addition, distinctive hydrothermal graphite has been found in association with orogenic gold-quartz mineral deposits in Otago [START_REF] Pitcairn | Detecting hydrothermal graphite deposition during metamorphism and gold mineralisation[END_REF][START_REF] Henne | Synmetamorphic carbon mobility and graphite enrichment in metaturbidites as a precursor to orogenic gold mineralization, Otago Schist, New Zealand[END_REF][START_REF] Hu | Raman characterization of carbonaceous material in the Macraes orogenic gold deposit and metasedimentary host rocks, New Zealand[END_REF]. We found no such carbonaceous material in our samples or fieldwork in the central Southern Alps. Figure 3 depicts representative Raman spectra from the Southern Alps and demonstrates the general gradient of graphitization following increasing metamorphism from SE to NW towards the Alpine Fault. In the greywacke east of the drainage divide (Main Divide), carbonaceous material exhibits Raman spectrum with several defect bands (e.g. D1, D2, D3 and D4). Such spectra are characteristic of very disordered graphitic carbon that were transformed under low-grade metamorphism at temperature below 330°C [START_REF] Beyssac | Raman spectra of carbonaceous material from metasediments: a new geothermometer[END_REF][START_REF] Lahfid | Evolution of the Raman spectrum of Carbonaceous Material in low-grade metasediments: an example from the Glarus Alps (Switzerland)[END_REF]. In this study, the peak temperature has been assumed to be less than 330°C in these samples (Figure 4). To the west, there is a progressive increase of the degree of graphitization through semischist and schist towards the Alpine Fault. Close to the Main Divide, semischist samples exhibit spectra with an intense main defect band as well as a strong D2 defect band as a shoulder on the G peak: such spectra correspond to disordered graphitic carbon in which the tridimensionnal aromatic skeleton remains poorly developed. Going towards the Alpine Fault, both D1 and D2 bands decrease progressively with increasing metamorphic grade and finally completely disappear in some of the highest grade schist samples near the Alpine Fault. This shows a progressive graphitization process that is completely achieved on the Alpine Fault. Importantly, detrital graphitic carbon has been found in samples at all metamorphic grades. It can be easily distinguished from in situ graphitizing organic matter based on: (i) morphological criteria -as it generally appears as isolated grains or flakes; and (ii) Raman spectraas it usually exhibits a high crystallinity except in very high grade samples where it is difficult to distinguish from organic matter from the simple observation of the Raman spectra. Detrital graphite spectra were not included in RSCM T determinations. Note that the presence of detrital graphite throughout the sequence has been observed in many other metamorphic belts because graphite is easily recycled during the erosion/weathering cycle (see [START_REF] Galy | Selective recycling of graphite during Himalayan erosion: a geological stabilisation of C in the crust[END_REF], and occurs widely in sedimentary rocks.

More specifically, we have also carried out detailed investigation of inclusions of graphitic carbon in the garnet porphyroblasts studied by [START_REF] Vry | Zoned (Cretaceous and Cenozoic) garnet and the timing of high grade metamorphism, Southern Alps, New Zealand[END_REF], sample MA2 (VU37559 in Table 1). Graphitic carbon provides a marker of the garnet zoning as it is present in some zones and absent from others, matching the chemical zonation of garnets. The Raman spectrum of all such inclusions is constant and representative of highly crystalline graphite. Calculating the temperature for such spectra yields ca. 575°C (Table 1) in good agreement with the maximum T obtained for the external rim of garnet (ca. 600°C) and late Cenozoic ages. We conclude that all graphitic carbon in this sample recorded the maximum T while garnet composition was only equilibrated in the rims and not in the core during increasing metamorphism.

RSCM temperatures in the Southern Alps

All RSCM T are listed in Table 1 with key parameters such as location, geological information, the number of spectra per sample, the mean R2 ratio parameter and T with associated uncertainties. For very disordered graphitic carbon that is found in least metamorphosed rocks, we assign T<330°C which is the lower bound of the calibration by [START_REF] Beyssac | Raman spectra of carbonaceous material from metasediments: a new geothermometer[END_REF]. RSCM T are plotted on a regional-scale map (Figure 4) and on three local maps depicting the main textural zones, metamorphic mineral zone 'isograd' boundaries, faults and folds along the northern (Figure 5, Wanganui to Taramakau), central (Figure 6, Copland to Whataroa) and southern (Figure 7, Haast to Karangarua) segments of the Alpine Fault hanging wall. The latter figures also include T profiles against the structural distance (D) to the Alpine Fault, together with some approximate thermal field gradients drawn manually through the data points. Curve fitting was deemed unwarranted for these transect gradients, due to the relatively small numbers of samples involved in each transect. In addition, some of the RSCM T data are depicted along four geological cross sections (Figure 8) representing variations in geology along the Southern Alps: Waitaha river -Rakaia valley, Whataroa river -Havelock river, Franz Josef -Godley valley, Karangarua -Mt Cook village. Last, all RSCM T were plotted in frequency histograms for the various metamorphic and textural zones (Figure 9). Based on the dataset, these figures and Table 1, we make the following general observations:

-Highest RSCM T measurements were from K-feldspar zone mylonites and schist beside the Alpine Fault, where rocks locally contain only perfect graphite and are inferred to have reached a minimum T of 640°C. Such very high T values are observed in the Haast and Moeraki transects in the south, but also in Copland, Waikukupa and Whataroa transects. There are two samples from garnet-oligoclase zone rocks that contain only highly crystalline graphitic material yielding high T, one from just south of Karangarua and the other at Wanganui river quarry.

-Lowest RSCM T measurements, where values have been assigned 330°C to represent the current lower bound of the [START_REF] Beyssac | Raman spectra of carbonaceous material from metasediments: a new geothermometer[END_REF] -Plotting all RSCM T data versus metamorphic or textural zones shows the general systematic T increase with increasing metamorphic grade and deformation (Figure 9). We note that some metamorphic (e.g. biotite or garnet-oligoclase) or textural (e.g. TZ3 or TZ4) zones are characterized by a relative clustering of the T data, whereas other zones (e.g. chlorite zone or textural zone 2B) exhibit a significantly wider range of RSCM T measurements. This in part reflects the presence of some relatively undeformed TZ2b rocks of the Aspiring lithologic association between Waitaha-Arahura rivers, that are unusual in that they have reached amphibolite facies metamorphism and yet retain remnants of original sedimentary structures [START_REF] Cox | Geology of the Aoraki area[END_REF][START_REF] Cooper | Cretaceous sedimentation and metamorphism of the western Alpine Schist protoliths associated with the Pounamu Ultramafic Belt, Westland, New Zealand[END_REF].

-The RSCM T profiles exhibit a degree of similarity from the southernmost to the northernmost transects. Plots of T as a function of D (Figures 5,6,7) nearly all show RSCM thermal field gradients through the sub-greenschist to greenschist facies (chlorite and biotite zone) rocks that are higher (>35°C/km) than field gradients through amphibolite facies (garnet and K-feldspar zone) (<20°C/km).

In detail, the RSCM T data along the four geological sections of Figure 8 yields some insight on the thermal evolution of the Alpine Schist. To the south, along the Karangarua -Mt Cook village section (Figure 8d) there is extensive exposure of TZ3 and TZ4 rocks, corresponding to biotite and garnet zones, which comprise a map thickness of more than 10 km. The rocks exhibit a fan-like structure, marked on a broad scale by the opposite vergence of the Alpine Fault, which dips towards the SE, and faults in the southeast, including the Main Divide Fault Zone (Cox and Findlay, 1985), which dip NW. This fan shape structure is mimicked by the main S3 crenulation cleavage and schistosity, which dips towards the SE close to the Alpine Fault, is nearly vertical in the garnet zone and then progressively changes to dip towards the NW in the garnet and biotite zones. The enveloping surface of folded, early (S1/2) fabric and lithological variation dips gently southwest. Along this geological section and nearby profiles (e.g. Copland or Karangarua), the field gradients appear systematic with progressive increase in RSCM T towards the Alpine Fault (Figures 6,7). Yet in detail metamorphic sequences in pumpellyite-actinolite to biotite zone rocks (TZ2a-3) near the Main Divide are locally inverted by juxtaposition on NW-dipping faults [START_REF] Cox | Structure and fluid migration in a late Cenozoic duplex system forming the Main Divide in the central Southern Alps, New Zealand[END_REF][START_REF] Craw | Structural boundaries and biotite and garnet 'isograds' in the Otago and Alpine Schists, New Zealand[END_REF] a brittle juxtaposition which results in steep thermal field gradients. Along the Moeraki profile there are also kilometer-scale late-or post-metamorphic antiform and synform structures plunging 10-20° SW that fold the S2 surface [START_REF] Rattenbury | Geology of the Haast area[END_REF]. Here strong T reversals are present in RSCM measurements of garnet zone rocks (Figure 7). Other smaller temperature reversals occur locally in data from the Otoko and Haast profiles, which can also be attributed to late-or post-metamorphic folds (Figure 7; [START_REF] Cooper | Multiphase deformation and its relationship to metamorphic crystallisation at Haast River, south Westland, New Zealand[END_REF][START_REF] Rattenbury | Geology of the Haast area[END_REF]. Where such folds are prominent, resulting thermal field gradients are low. The Otoko profile also crosses an area of garnet-biotite-albite zone samples within the greenschist facies, that contain very fine-grained (<1 mm) grossular-spessartine garnets that are thought to be a remanent of Mesozoic metamorphism exposed more widely in Otago (Figure 1; [START_REF] Mortimer | Metamorphic discontinuities in orogenic belts : example of the garnet-biotite-albite zone in the Otago schist, New Zealand[END_REF][START_REF] Rattenbury | Geology of the Haast area[END_REF]. There do not appear to be any distinct steps or obvious thermal effects in the dataset associated with this zone, which is distinguished by the 'garnet-1 isograd' in Figure 7.

In the Franz Josef -Fox area, the high-grade TZ3 and TZ4 schist units metamorphosed to biotite zone and above, are much narrower (8 km, Figure 6), and have been juxtaposed against semischist sequences by the BDTZ with escalator-like component of dip-slip motion (Figures 6, 8c;Little et al., 2002b;[START_REF] Little | Transpression models and ductile deformation of the lower crust of the Pacific Plate in the central Southern Alps, a perspective from structural geology, in A Continental Plate Boundary: Tectonics at South Island[END_REF]. Here the distance between the Alpine Fault and the Main Divide is also the smallest, being the region where the late Quaternary dip-slip rates are the greatest on the Alpine Fault (>12 mm/year - [START_REF] Norris | Late Quaternary slip rates and slip partitioning on the Alpine Fault, New Zealand[END_REF], and uplift and exhumation rates of schist also appear to be the highest along the plate boundary [START_REF] Little | Variations in exhumation level and uplift-rate along the oblique-slip Alpine Fault, central Southern Alps, New Zealand[END_REF]. The high grade schist units contain an S3 crenulation cleavage dipping towards the SE, weakly oblique to the Alpine Fault, which steepens with distance from the fault (Little et al., 2002a). The geological section across the schist units contains upright SE plunging folds and a tight fan-like structure, with an abrupt truncation of the structural trend towards the NW at the BDTZ. Along this section, high RSCM T (graphitic material at >640°C) was observed in the Alpine Fault mylonite zone, but RSCM T are relatively constant in the range 525-570°C in garnet zone and biotite zone rocks structurally below the BDTZ (see Fox, Waikukupa, Franz profiles on Figure 6). Structurally above the BDTZ, where foliation and bedding have a predominantly NW dip, the thermal field gradients are much higher (>35°C/km, potentially reaching ~90°C/km). Field gradients are also steep through chlorite and biotite zone rocks of the Whataroa profile (Figure 6), which we interpret to be a combination of the effects of topography, flat lying S2 fabric, and fault juxtaposition of different metamorphic blocks (Figure 8b).

Features observed in the southern and central profiles are also present in the northern region (Figures 5,8a,b). The high-grade TZ3 and TZ4 schist units widen to 14 km in both map view and geological section, and there are a number of mapped kilometer-scale late-or post-metamorphic antiform and synform structures which appear to fold isotect and isograd boundaries between Wanganui and Hokitika rivers.

Such folds are relatively tight with steep limbs in foliated schist sequences, but typically open in the semischists (Figure 8a). The result appears to be a relatively flatlying S2 enveloping surface, similar to the Whataroa-Havelock section (Figure 8b), plunging gently-moderately to the northeast, which produces temperature reversals in the Waitaha profile (Figure 8a). TZ2a and TZ2b semischist sequences (greenschist facies) are relatively flat-lying, with structural blocks juxtaposed by subhorizontal folding or faults [START_REF] Andrews | Geology of the Lord Range, Central Southern Alps, New Zealand[END_REF]. Thermal field gradients are high (>45°C/km) in the sub-greenschist to biotite zone rocks, and very low (<10°C/km) across schist sequences.

Petrological constraints

Petrological data were collated for samples from the Franz Josef-Fox area, including older studies [START_REF] Grapes | Metamorphism and uplift of Alpine Schist in the Franz Josef-Fox Glacier area of the Southern Alps, New Zealand[END_REF][START_REF] Grapes | Uplift and exhumation of Alpine Schist in the Franz Josef -Fox Glacier area of the Southern Alps, New Zealand[END_REF]. Traditional garnet-biotite geothermometry, and temperature estimates based on results of the P-T pseudosection calculations new for this study are presented in Table 2. An example of pseudosection calculation is given as supplementary material. These data are shown on Figure 10 along with P-T estimates extracted from Figure 4 in [START_REF] Grapes | Metamorphism and uplift of Alpine Schist in the Franz Josef-Fox Glacier area of the Southern Alps, New Zealand[END_REF], and shown in map view on Figure 11. Note that garnet-biotite temperatures in our study are generally higher than those from [START_REF] Grapes | Metamorphism and uplift of Alpine Schist in the Franz Josef-Fox Glacier area of the Southern Alps, New Zealand[END_REF], based on the same mineral analyses. In our study, we used the calibration by [START_REF] Hodges | Geothermometry, geobarometry and the Al 2 SiO 5 triple point at Mt. Moosilauke, New Hampshire[END_REF] based on rim analyses using pressure estimates based on results of garnet-biotitemuscovite-plagioclase barometry (Hoisch, 1990, Fe-endmember). [START_REF] Grapes | Metamorphism and uplift of Alpine Schist in the Franz Josef-Fox Glacier area of the Southern Alps, New Zealand[END_REF] used the calibration by [START_REF] Ferry | Experimental calibration of partitioning of Fe and Mg between biotite and garnet[END_REF], as modified for Ca content by [START_REF] Hodges | Geothermometry, geobarometry and the Al 2 SiO 5 triple point at Mt. Moosilauke, New Hampshire[END_REF] and [START_REF] Hoinkes | Effect of grossular-content in garnet on the partitioning of Fe and Mg between garnet and biotite: an empirical investigation on staurolite-zone samples from the Austroalpine Scheeberg complex[END_REF], and the garnet-biotitemuscovite-plagioclase geobarometer of [START_REF] Ghent | Geobarometry and geothermometry of plagioclasebiotite-garnet-muscovite assemblages[END_REF], with modification by [START_REF] Hodges | Error estimation and empirical geobarometry for politic systems[END_REF]. The P and T uncertainties (± 1 kbar and ± 50°C; Grapes and Watanabe, 1992), as estimated from standard deviations, apply for both studies.

Discussion

Comparison of RSCM data with petrological constraints

At some localities close to the Alpine Fault, in the high-temperature range, there is good agreement between RSCM thermometry and the peak T estimated by petrology.

Nearly pure crystalline graphite is present yielding RSCM T above 580°C close to the Alpine Fault, and in many places perfect graphite is found in the central and southern areas yielding RSCM T above 640°C, the higher limit of the calibration by [START_REF] Beyssac | Raman spectra of carbonaceous material from metasediments: a new geothermometer[END_REF]. This is in agreement with the T estimates by [START_REF] Vry | Zoned (Cretaceous and Cenozoic) garnet and the timing of high grade metamorphism, Southern Alps, New Zealand[END_REF] who showed that the maximum T recorded by a garnet porphyroblast in Hokitika area (Mac's Creek) is ca. 600°C using garnet-biotite thermometry. The lowest RSCM T occur from the Main Divide southeastwards, where most samples have been assigned <330°C based on the lower limit of the [START_REF] Beyssac | Raman spectra of carbonaceous material from metasediments: a new geothermometer[END_REF] calibration. There are no quantitative petrological estimates of T for quartzofeldspathic lithologies metamorphosed to such low temperatures. However the observed low temperatures are in agreement with: 1) the observed metamorphism of rocks at prehnitepumpellyite and pumpellyite-actinolite facies. For instance, prehnite occurring together with pumpellyite typically indicates temperatures in the range 250-300 °C and pressures below about 2 kbar (e.g. Willner et al., 2013); 2) the presence of partially annealed fission tracks in zircons, with zircon ages which suggest rocks had exceeded 240°C closure temperatures and have been uplifted from partial annealing zone or deeper [START_REF] Batt | Thermochronological analysis of the dynamics of the Southern Alps, New Zealand[END_REF][START_REF] Cox | The Main Divide fault zone and its role in formation of the Southern Alps, New Zealand[END_REF]Tippett and Kamp 1993a,b);

3) K-Ar ages that suggest the rocks have partial retention of gas and remained below temperatures of ca. 300°C [START_REF] Batt | Thermochronological analysis of the dynamics of the Southern Alps, New Zealand[END_REF].

All methods record the decrease in peak temperature away from the Alpine Fault towards the southeast (Figure 11), but RSCM T are generally higher than any related estimate from petrology and yield significantly lower apparent field metamorphic gradients through the high-grade schist. This is most-clearly illustrated on Figure 10 where all RSCM T data for the southern, central and northern profiles are shown together with petrological T estimates from [START_REF] Grapes | Metamorphism and uplift of Alpine Schist in the Franz Josef-Fox Glacier area of the Southern Alps, New Zealand[END_REF] and this study (see Table 2) from the Franz Josef -Fox area. The T estimates by [START_REF] Grapes | Metamorphism and uplift of Alpine Schist in the Franz Josef-Fox Glacier area of the Southern Alps, New Zealand[END_REF] are definitely lower by several tens of degrees compared to RSCM T (central Southern Alps profiles) except those in the direct vicinity of the Alpine Fault where they converge towards ca. 600°C. THERMOCALC T estimates are higher than those of [START_REF] Grapes | Metamorphism and uplift of Alpine Schist in the Franz Josef-Fox Glacier area of the Southern Alps, New Zealand[END_REF] and for the most-part are lower than RSCM T. A possible explanation of the difference between RSCM T and petrological estimates is due the irreversibility of graphitization [START_REF] Beyssac | Raman spectra of carbonaceous material from metasediments: a new geothermometer[END_REF]) such that RSCM records the peak T and is not sensitive to the retrograde/exhumation path of the rocks. By way of contrast, mineral assemblages can re-equilibrate their chemistry during retrogression, modifying and erasing the peak T signal, especially when fluid circulation and deformation are important. In the Alpine Schist, garnet compositions are not simple Fe-Mg end-member mixtures, and can vary considerably from rock to rock, and the biotite can be subject to re-equilibration and regrowth during uplift and cooling. Fluid flow has locally affected some of the rocks, and the effects and timing of this may pass unrecognized, and have not been quantified. The MnO contents in the bulk rock compositions vary, and the first appearance of garnet in P-T pseudosections is very sensitive to this, as well as the choices of activity-composition models. In any case, we note that temperature estimates based on results of P-T pseudosection calculations for rock samples that contain relatively high-grade mineral assemblages (containing ilmenite, oligoclase, ± garnet), are typically only slightly lower than, and within error of, the RSCM T obtained from nearby samples. However, for the samples that contain lower-grade mineral assemblages with sphene, there is a larger temperature difference, with higher T estimates being obtained from the RSCM data (see Figure 10).

At some localities (see Table 2), the discrepancy between RSCM T and petrological T estimates is, nonetheless, somewhat surprising. RSCM thermometry has now been used in many various geological contexts and generally exhibits good to excellent concordance with conventional petrology, including garnet-biotite thermometry (e.g. see Plunder et al., 2012 and references therein). However, in most available studies, RSCM T was applied to simple thermal histories, i.e. monometamorphic history with one single thermal event, except the notable examples of some internal units of Taiwan [START_REF] Beyssac | Late Cenozoic metamorphic evolution and exhumation of Taiwan[END_REF]). But given the high quality and consistency of the RSCM spectral data in the Alpine Schist, and considering that minerals may not record the true peak metamorphic temperature, we consider that RSCM thermometry yields a first quantification on a large scale of peak metamorphic T. for southern, central and northern Alpine Schist. RSCM T data from all metamorphic zones has also been projected onto an along-strike section parallel to the Alpine Fault (Figure 12). Also shown are the approximate position of key metamorphic mineral 'isograds' (first appearance of biotite; garnet and K-feldspar), and the zone where young (<6 Ma) 40 Ar-39 Ar and K/Ar ages have been obtained in the central Southern Alps. It is useful compare the along-strike projection against Figure 10 that shows the same data plotted perpendicular to the Alpine Fault. From Figures 10 and12, it seems there may be a decrease in peak T along the Alpine Fault in the K-feldspar zone going towards the northeast, although this trend may in part be apparent due to the absence of samples analysed from the northernmost area. Both the garnet-oligoclase and biotite zones exhibit a relatively clustered pattern of peak T around 560°C and 520°C respectively all along strike. There is dispersion in both cases around the mean values for each zone especially in the central Southern Alps although this may also be due to a denser sampling bias in this area. The chlorite zone all along the Alpine Fault covers an extremely wide range of T from ca. 360°C reaching as high as ca. 550°C. This is a very wide T range for a classical chlorite greenschist facies zone, especially towards the high T. One possibility is that the lithology/bulk chemistry may not have allowed growth of index minerals (garnet and/or biotite, see [START_REF] Vry | Establishing the P-T path for Alpine Schist, Southern Alps near Hokitika, New Zealand[END_REF], or the rocks suffered complete retrogression obliterating some high-grade minerals, resulting in the possibility that samples could have been 'misclassified' as being apparently of lower metamorphic grade than the temperature they actually experienced.

RSCM T pattern in the Southern Alps

Figure 10 shows that higher RSCM T extends farther from the Alpine Fault along the southern and, to a lesser degree, the northern compared to the central profiles. This is due to the wider map extension of high-grade units (biotite and garnet oligoclase) in the south and north, whereas these units are far less thick and steeper in the central Southern Alps. Although the data are somewhat scattered, it seems that the RSCM T pattern along both the central and northern profiles has nearly constant high T within the first kilometers close to the Alpine Fault (ca. 6 km for the central and ca. 8 km for the north) followed by a steep decrease of T in the eastern biotite and chlorite zones. In the south, the T profile is more continuous and progressive, with no break in the slope, and projection toward Alpine Fault suggest rocks here may have reached temperatures ~700°C. This is consistent with widespread occurrences of pegmatite in the Mataketake Range and less commonly elsewhere between Haast and Moeraki rivers, which reflect partial melting of the schist that occurred during the Late Cretaceous [START_REF] Chamberlain | Geochronologic constraints of the uplift and metamorphism along the Alpine Fault, South Island, New Zealand[END_REF]. The calculation of D for these data assumes the Alpine Fault has a constant dip of 45° (Figure 2). Also shown in Figure 10 are the range of possible D values and field gradients that could occur if the fault dipped as shallow as 30° or as steeply as 60°.

Age of RSCM temperatures in the Southern Alps

RSCM thermometry records the peak metamorphic T undergone by carbonaceous material and the host rock during the burial history but it carries no age information by itself. In the case of complex poly-phased metamorphic histories like in the Alpine Schist with at least two major thermal events, either associated with tectonics on the margin of Gondwana during Jurassic -Cretaceous or with evolution of the present plate boundary during late Cenozoic, an important issue remains with regards to the age of the recorded RSCM T. This is a significant question if one wants to use such data to infer and constrain models of the thermal structure of the Southern Alps, or rheology of Alpine Fault rocks at depth (e.g. [START_REF] Toy | Thermal regime in the central Alpine Fault zone, New Zealand : constraints from microstructures, biotite chemistry and fluid inclusion data[END_REF], as the T measurements will be most directly relevant if they are late Cenozoic in age. Confirming the RSCM T measurements as late Cenozoic is made more difficult by the complete absence of Cenozoic-aged metasedimentary units in the Southern Alps that would only contain the expression of late Cenozoic metamorphism. Age of mineral assemblages and of peak P-T conditions recorded in the Alpine Schist have therefore long been discussed in the literature (e.g. [START_REF] Chamberlain | Geochronologic constraints of the uplift and metamorphism along the Alpine Fault, South Island, New Zealand[END_REF]Little et al. 2002aLittle et al. ,b, 2005;;[START_REF] Vry | Zoned (Cretaceous and Cenozoic) garnet and the timing of high grade metamorphism, Southern Alps, New Zealand[END_REF][START_REF] Vry | Establishing the P-T path for Alpine Schist, Southern Alps near Hokitika, New Zealand[END_REF], but there is no real consensus as to the exact locations where peak P-T conditions recorded by the rocks are late Cenozoic. There are a number of arguments however that supports this to be the case, at least locally, for Alpine Schist between Fox and Franz Josef, and potentially Copland -Whataroa rivers, as we discuss below.

On a large scale, the spatial distribution of RSCM T is strongly correlated with the geometry of the main contacts and tectono-stratigraphic structures that were acquired during formation of the Southern Alps. But the classical two-dimensional model of exhumation by [START_REF] Wellman | An uplift map for the South Island of New Zealand and a model for uplift of the Southern Alps, in The Origin of the Southern Alps[END_REF], exhuming deep old rocks up a ramp formed by the Alpine Fault, would yield the same results whether the peak T was late Cenozoic, or a product of older Jurassic-Cretaceous metamorphism with passive exhumation up the Alpine Fault ramp during the late Cenozoic. Instead, thermochronologic data at least record when the rocks have cooled below some threshold temperature (closure temperature), depending on the selected thermochronometer. Except for U-Pb geochronology on zircon yielding crystallization ages, the highest temperature thermochronometer so far applied has been 40 Ar-39 Ar ages on hornblende. Between Franz Josef and Fox, hornblende from mylonite beside the Alpine Fault has 40 Ar-39 Ar ages older than Neogene formation of the plate boundary, yet garnet-oligoclase zone Alpine Schist been reset to < 6 Ma when the Southern Alps were uplifted (Figures 4, 11;[START_REF] Chamberlain | Geochronologic constraints of the uplift and metamorphism along the Alpine Fault, South Island, New Zealand[END_REF][START_REF] Little | Variations in exhumation level and uplift-rate along the oblique-slip Alpine Fault, central Southern Alps, New Zealand[END_REF]. The reset ages are distributed through a zone where measured RSCM T range between 530-565°C (Figure 11). These RSCM temperatures are therefore equivalent to, or higher, than the commonly assumed value for the closure temperature of 40 Ar-39 Ar ages on hornblende which is ca. 500-550°C. At least, in this zone it seems reasonable to assume that RSCM T represents a peak T which was reached during the late Cenozoic. Further support is given by the close correspondence between (i) peak T of ca. 600°C recorded by the most external rim of garnet porphyroblasts dated to Cenozoic ages [START_REF] Vry | Zoned (Cretaceous and Cenozoic) garnet and the timing of high grade metamorphism, Southern Alps, New Zealand[END_REF]) and (ii) RSCM T of ca. 575°C obtained on all graphitic inclusions on the same garnet from the core to the rim.

To the south or north, however, there are no other places where a late Cenozoic age for thermochronometers or RSCM T can be easily inferred. There are also observations, as outlined above, showing that peak RSCM T values are Mesozoic: (i) 40 Ar-39 Ar ages on hornblende are Miocene or older to the south, (ii) RSCM T reversals occur on profiles where the regional S1/S2 foliation has been folded and cross-cut by the S3 crenulation cleavage, suggesting the RSCM T data record the peak metamorphism associated with this older fabric, and (iii) that projection of RSCM temperatures on the Haast and Otoko profiles suggest the schists may have reached maximum T ~700°C, where partial melting that produced pegmatites have been dated as late Cretaceous. This does not mean that all RSCM T are Mesozoic, as for instance the coverage for high T dating systems is restricted to the Franz-Josef Fox region. In the south there are also some local complications that are yet to be fully mapped and understood, such as inversion of the metamorphic sequence in the mylonite zone caused by distributed shear on the Alpine Fault [START_REF] Cooper | Inverted metamorphic sequences in Alpine fault mylonites produced by oblique shear within a plate boundary fault zone[END_REF].

Some tectonic implications

The belt of schist along the Southern Alps has a seemingly continuous westward increase in metamorphic grade toward the Alpine Fault. It has long been considered (e.g. [START_REF] Suggate | The Alpine Fault[END_REF] to be due to an increase in depth of exhumation, with some 20-30 km of exhumation adjacent to the fault. It is commonly assumed that the metamorphic and textural boundaries in the schist were once sub-horizontal, as observed in the Otago region of southeast New Zealand, and have been rotated during Neogene tectonics. The notion of rock uplift or exhumation and cooling in the Neogene has been corroborated by thermochronologic studies, which demonstrated young 'reset' ages adjacent to the Alpine Fault (e.g. [START_REF] Tippett | Quantitative relationships between uplift and relief parameters for the Southern Alps, New Zealand, as determined by fission track analysis[END_REF][START_REF] Batt | Thermochronological analysis of the dynamics of the Southern Alps, New Zealand[END_REF][START_REF] Little | Variations in exhumation level and uplift-rate along the oblique-slip Alpine Fault, central Southern Alps, New Zealand[END_REF][START_REF] Herman | Tectonomorphic scenarios in the Southern Alps of New Zealand[END_REF][START_REF] Herman | Low-temperature thermochronology and thermokinematic modeling of deformation, exhumation and development of topography in the central Southern Alps, New Zealand[END_REF]. Structural models of the Southern Alps now nearly all infer some form of deformation and rotation of upper crustal material within the Southern Alps, as the Pacific Plate is delaminated and translated Alpine Fault ramp (see [START_REF] Okaya | Continent-continent collision at the Pacific/Indo-Australian plate boundary: background, motivation, and principal results, in A Continental Plate Boundary: Tectonics at South Island, New Zealand[END_REF] for a review). Petrological and field observations provide evidence the Alpine Fault hanging wall has been tilted southeastward in the central Southern Alps (Figure 2). Cumulative vertical displacements on an array of fractures in the BDTZ effect a 22 ± 8º of bulk SE tilt (Wightman and Little, 2007;[START_REF] Little | Transpression models and ductile deformation of the lower crust of the Pacific Plate in the central Southern Alps, a perspective from structural geology, in A Continental Plate Boundary: Tectonics at South Island[END_REF]. Structurally below the BDTZ, garnet zone rocks also preserve microstructural evidence of distributed ductile shear strain (Ɣ = 0.6, down to the east) with sufficient magnitude that could account for ~32° SE tilt of the schist sequence [START_REF] Holcombe | A sensitive vorticity gauge using rotated porphyroblasts, and its application to rocks adjacent to the Alpine Fault, New Zealand[END_REF]Little et al., 2002a).

Although seemingly continuous at a regional-scale, in detail the pattern of metamorphic zones represents a complex disrupted metamorphic pile, which involves slices that are variably affected by folding, and transitions ('isograd' boundaries) that involve juxtaposition on faults and shear zones. The presence of the BDTZ in the Franz Josef-Fox area, where an escalator-like back shearing process has occurred, is an important observation and reference frame (Wightman and Little, 2007;[START_REF] Little | Transpression models and ductile deformation of the lower crust of the Pacific Plate in the central Southern Alps, a perspective from structural geology, in A Continental Plate Boundary: Tectonics at South Island[END_REF]). Structurally highest chlorite and biotite zone rocks near the Main Divide, remained above the brittle-ductile transition, so only record a brittle expression of the modern phase of oblique convergence (e.g. [START_REF] Cox | The Main Divide fault zone and its role in formation of the Southern Alps, New Zealand[END_REF][START_REF] Cox | Structure and fluid migration in a late Cenozoic duplex system forming the Main Divide in the central Southern Alps, New Zealand[END_REF].

Below the BDTZ, ductile deformation resulted in constructive reinforcement of preexisting fabrics rather than superposition of a new foliation, and the SE tilt of the rock sequence. Clearly, field metamorphic gradients measured across major boundaries such as the BDTZ will contain little or no information about crustal-scale geothermal gradients, or pressure-temperature relationships during metamorphism. Measured field gradients might also be expected to be very different either side of the BDTZ due to the difference in nature and style of deformation either side of the boundary.

We observed high (>35°C/km) RSCM thermal field gradients through the subgreenschist to greenschist facies (chlorite and biotite zone) rocks and low (<20°C/km) field gradients through amphibolite facies (garnet and K-feldspar zone) (Figures 5,6,7,10).

The flux of Pacific Plate rock through the deforming zone has been suggested to have two distinct domains/stages, with pure-shear style motion and thickening in the eastern outboard domain, then inclined out-of-plane non-coaxial simple shear in the inboard domain [START_REF] Little | Transpressive ductile flow and oblique ramping of lower crust in a two-sided orogen: Insight from quartz grain-shape fabrics near the Alpine Fault, New Zealand[END_REF][START_REF] Cox | Potentially active faults in the rapidly eroding landscape adjacent to the Alpine Fault, central Southern Alps[END_REF]. The metamorphic and textural transition in the Franz Josef-Fox area (central Southern Alps) is considerably narrower in map view relative to schist than further to the north and south along the Alpine Fault (Figure 2), potentially the result of differing geometry of the fault at depth [START_REF] Little | Variations in exhumation level and uplift-rate along the oblique-slip Alpine Fault, central Southern Alps, New Zealand[END_REF]. Schist and semi-schist sequences in the central Southern Alps appear to have been thinned relative to the north and south, but although condensed, the westward prograde metamorphic mineral zonation has remained in sequential order. There have been arguments for structural thinning [START_REF] Grapes | Metamorphism and uplift of Alpine Schist in the Franz Josef-Fox Glacier area of the Southern Alps, New Zealand[END_REF] and 'extrusion' of lower crustal material [START_REF] Walcott | Modes of oblique compression: Late Cenozoic tectonics of the South Island New Zealand[END_REF] associated with uplift of the Alpine Fault hanging wall. On the basis of geobarometry (garnetbiotite-muscovite-plagioclase) and geothermometry (garnet-biotite) of high-grade schists, [START_REF] Grapes | Metamorphism and uplift of Alpine Schist in the Franz Josef-Fox Glacier area of the Southern Alps, New Zealand[END_REF] argued the crustal section in the central Southern Alps has been thinned to one third its original thickness. However, because there are significant uncertainties (at least ±1 kbar) in each pressure estimate, the 0.33 thinning ratio carries high uncertainty. If tilting of high-grade Alpine Schist involved thinning or shortening perpendicular to the Alpine Fault, we might expect to see it represented in metamorphic peak T field gradients recorded by the RSCM T data. For example, if thinning has been as substantial as the 0.33 ratio suggested by [START_REF] Grapes | Metamorphism and uplift of Alpine Schist in the Franz Josef-Fox Glacier area of the Southern Alps, New Zealand[END_REF], the field gradients might be expected to greatly exceed 'normal' crustal geothermal gradients of ~20-40°C/km. Instead, the RSCM T field gradients observed for garnet-oligoclase and K-feldspar zone schist in this study were only ~20°C/km perpendicular to the Alpine Fault in the central Southern Alps, and around 10°C/km or lower to the north and to the south (Figures 5,6,7).

Local temperature reversals in the Otoko and Waitaha profiles clearly reflect upright antiform and synform structures folding the S2 foliation surface and lowering observed field gradients. Here the enveloping surface of the isograds must have a relatively shallow SW dip and is near horizontal when considered perpendicular to the Alpine Fault. Importantly, there is little evidence in the RSCM T data anywhere for substantial thinning of the high-grade (garnet and K-feldspar zone) Alpine Schist sequence perpendicular to the fault, whether the RSCM T data represent Mesozoic or late Cenozoic peak temperatures. A thinning ratio of 0.33 would increase the apparent geothermal gradient by a factor of 3, although the 0.9 thinning ratio proposed in a 3-D kinematic model [START_REF] Little | Transpressive ductile flow and oblique ramping of lower crust in a two-sided orogen: Insight from quartz grain-shape fabrics near the Alpine Fault, New Zealand[END_REF] only requires a 1.11 increase, and is potentially supported by RSCM-T. We suggest any Neogene deformation that occurred to the high-grade schist sequence below the brittle-ductile transition is unlikely to have involved more than 0.5 thinning relative to a fixed Alpine Fault reference frame dipping 45°. Deformation by inclined simple shear would meet such criteria. By way of contrast, chlorite and biotite zone semischist sequences show moderate to high (40 -90°C/km) RSCM T field gradients. Here the rocks have remained above the brittleductile transition and Neogene deformation was accommodated by oblique dip-slip faulting (backthrusts), imbricated duplex-like stacking and local reversal of metamorphic grade [START_REF] Cox | The Main Divide fault zone and its role in formation of the Southern Alps, New Zealand[END_REF][START_REF] Cox | Structure and fluid migration in a late Cenozoic duplex system forming the Main Divide in the central Southern Alps, New Zealand[END_REF][START_REF] Craw | Structural boundaries and biotite and garnet 'isograds' in the Otago and Alpine Schists, New Zealand[END_REF]. The RSCM T field gradients in semischist sequences appear boosted by juxtaposition of metamorphic zones and we believe they are unlikely to represent any true geothermal gradients in the crust.

Our observations are presented using a calculated structural distance (D) with regard to an assumed Alpine Fault reference frame, fixed at 45° dip. What was perhaps surprising was how low the resulting thermal field gradients were in the highgrade Alpine Schist, particularly that nowhere can they be considered to have exceeded 20°C/km. Had a 60° Alpine Fault dip been selected, it would have resulted in even lower calculated field gradients (to about 10°C/km, depending locally on topography) and at 30° dip the field gradients would still not have exceeded 30°C/km. Since equally low (5-10°C/km) thermal gradients have been independently predicted during evolution of the Alpine Fault mylonite zone [START_REF] Toy | Thermal regime in the central Alpine Fault zone, New Zealand : constraints from microstructures, biotite chemistry and fluid inclusion data[END_REF][START_REF] Cross | Using microstructures and TitaniQ thermobarometry of quartz sheared around garnet pophyroclasts to evaluate microstructural evolution and constrain an Alpine Fault Zone geotherm[END_REF], it encourages us to think the low RSCM T field gradients might actually reflect the thermal state of high-grade Alpine Schist prior to uplift and exhumation. If not real, then such low geothermal gradients near the Alpine Fault can alternatively be explained by vertical thickening [START_REF] Little | Transpressive ductile flow and oblique ramping of lower crust in a two-sided orogen: Insight from quartz grain-shape fabrics near the Alpine Fault, New Zealand[END_REF], a crustal drag structure [START_REF] Little | Variations in exhumation level and uplift-rate along the oblique-slip Alpine Fault, central Southern Alps, New Zealand[END_REF] or imbricated reversals associated with distributed oblique-slip [START_REF] Cooper | Inverted metamorphic sequences in Alpine fault mylonites produced by oblique shear within a plate boundary fault zone[END_REF]. Perhaps the simplest alternative explanation is that the RSCM T field gradients dominantly represent an oblique slice through the Mesozoic crustal pile, and that the enveloping surface of Mesozoic isograds is much shallower than the Alpine Fault. A corollary is that the degree of rotation of the hanging wall was limited and it has not been completely rotated into parallelism with the Alpine Fault (as suggested by Wellman fig. 4a,c 1979, or Walcott fig 15, 1998). There are various ways this could be achieved. One is that the dip-slip displacement distributed on backthrust structures almost matches that on the Alpine Fault, so that blocks bound by faults and shear zones are only weakly rotated as they are exhumed up the Alpine Fault ramp (Wellman figure 4b, 1979). A corollary is that faults in the Southern Alps hanging wall must be active and have relatively high cumulative slip rates, which is important for seismic hazard assessment (see [START_REF] Wallace | Balancing the plate motion budget in the South Island, New Zealand using GPS, geological and seismological data[END_REF][START_REF] Cox | Potentially active faults in the rapidly eroding landscape adjacent to the Alpine Fault, central Southern Alps[END_REF]). An alternative hypothesis, that is not easily addressed with RSCM T data and is beyond the scope of our study, is that the Alpine Fault could itself have been rotated, or evolved to a shallower dip during the Neogene and Quaternary [START_REF] Koons | Influence of exhumation on the structural evolution of transpressional plate boundaries: An example from the Southern Alps, New Zealand[END_REF].

Conclusions

In this study, we present a dataset of peak metamorphic temperatures experienced by Alpine Schist, semischist and greywacke now exhumed in the hangingwall of the Alpine Fault. Carbonaceous material has been analysed in 142 samples, from 13 lowto high-grade transects, in which peak metamorphic temperatures decrease from ca. 650-700°C near the Alpine Fault to less than 330°C at the main drainage divide, about 15-20 km southeast from the fault. The temperature decrease is relatively uniform in the south, but distinct thermal field gradients are present across the central Southern Alps. This is the first systematic and consistent dataset at the scale of the entire Southern Alps with quantitative values for the peak metamorphic T experienced by various textural and metamorphic zones. RSCM T increase with metamorphic and textural grade, with reversals occurring only locally across folds and any apparent steps where there are faults. Peak temperatures recorded by the RSCM method are generally higher by ≤ 50°C than existing temperature estimates from petrology.

Biotite-in, garnet-in and K-feldspar-in first appearance 'isograds' occur at different temperatures along the schist belt, which could reflect variable ages of peak metamorphism, or potentially some truncation and juxtaposition of metamorphic zones by faults and shear zones. RSCM T are mostly pre-Cenozoic except in the Franz Josef -Fox area of the central Southern Alps, where these T are likely Cenozoic in age.

The RSCM temperatures place limited constraints on thermal conditions experienced within the orogen, with field temperature gradients potentially carrying information on amounts of tilting and structural re-organisation of the Pacific Plate in the Alpine Fault hangingwall, albeit disrupted by fault and shear zone juxtaposition.

Plots of RSCM T with respect to structural thickness (D) perpendicular to the Alpine Fault, assuming a 45° dip, yield thermal field gradients that are consistently low, <20 °C/km, within the garnet-oligoclase and K-feldspar zones. It suggests these rocks were neither fully rotated, nor structurally thinned, during exhumation. Given the number and consistency of thermochronological data and geological observations available, this dataset can constitute a basis to test thermokinematic and/or thermomechanical models of mountain building processes in the Southern Alps. Such models may have important implications in terms of thermal structure of the crust before, during and after orogenic processes as well for our knowledge of crustal rheology.

Willner, A.P., Massonne, H.-J., Barr, S.M., White, C.E. (2013) Very low-to lowgrade metamorphic processes related to the collisional assembly of Avalonia in SE Cape Breton Island (Nova Scotia, Canada). Journal of Petrology 54(9) 1849-1874. Wightman, R., Little, T.A. (2007) 1). Selected metamorphic mineral 'isograd' zone boundaries and fold axes are also shown. On the right, RSCM T versus distance D to the Alpine Fault (see Figure 2) for the main river profiles. Rotated New Zealand Transverse Mercator projection, with NZGD2000 grid. 1). Selected metamorphic mineral 'isograd' zone boundaries and fold axes are also shown. On the right, RSCM T versus distance D to the Alpine Fault (see Figure 2) are represented for the main river profiles. Rotated New Zealand Transverse Mercator projection, with NZGD2000 grid. On this figure, temperature data (central area) from garnet-biotite thermometry of [START_REF] Grapes | Metamorphism and uplift of Alpine Schist in the Franz Josef-Fox Glacier area of the Southern Alps, New Zealand[END_REF], or reanalysed and recalculated from their dataset, and maximum temperature from THERMOCALC modeling (this study) are also shown (see

Figure captions

Figure 1

 1 Figure 1 depicts simplified geological and topographic maps of the South Island.

  calibration were observed in the vicinity of the Main Divide or to the southeast. Here the rocks are TZ2a cleaved greywacke or textural zone TZ2b semischist, metamorphosed to either sub-greenschist or chlorite zone.

Figure 10

 10 Figure 10 is a compilation of all local-scale profiles presented on Figures 5, 6 and 7

  , Deformation of the Pacific Plate above the Alpine Fault ramp and its relationship to expulsion of metamorphic fluids: An array of backshears, in A Continental Plate Boundary: Tectonics at South Island, New Zealand edited by D. Okaya, T. Stern, F. Davey. AGU Geophysical Monograph Series, 175, 177-205.

Figure 1 -

 1 Figure 1 -Simplified geological (left) and topographic map of the South Island of New Zealand. The left map depicts the main textural zones in schists and location for the other figures. The right map shows the topography of the South Island and the main kinematic vectors for the Pacific Plate relative to the IndoAustralian Plate (DeMets et al., 2010), and late Quaternary slip on the Alpine Fault[START_REF] Norris | Late Quaternary slip rates and slip partitioning on the Alpine Fault, New Zealand[END_REF].

Figure 2 -

 2 Figure 2 -Simplified geological cross-section across the central Southern Alps (see Figure 1 for location). Bottom is a general cross-section modified after Cox et al.(2012). Top is a sketch illustrating the calculation of the distance D for sampling points to the Alpine Fault at depth (see also Figure8).

Figure 3 -

 3 Figure 3 -Representative Raman spectra for rocks from the Southern Alps. For each spectrum, the calculated RSCM T is given. Note that the spectral window represented was reduced for the figure. P numbers refer to samples from the New Zealand National Rock and Mineral Collection (PetLab).

Figure 4 -

 4 Figure 4 -Simplified geological (left) and metamorphic (right) map of the central Southern Alps (see Figure 1 for location). Left depicts the main textural zones for the Alpine Schist with the RSCM T data and main 'isograd' boundaries between metamorphic mineral zones. Right depicts RSCM T data with the main metamorphic zones based on index mineral assemblages and the chrontours (lines of equal age) for 40 Ar-39 Ar thermochronology on biotite, muscovite and hornblende (after Little et al., 2005). New Zealand Transverse Mercator projection, with NZGD2000 grid.

Figure 5 -

 5 Figure 5 -Simplified geological map and RSCM T data for the northern segment (Wanganui to Taramakau valleys) of the Alpine Fault hanging-wall (see Figure 1 for location). Sample locations are color coded by metamorphic zone, annotated with RSCM T in °C (as provided in Table1). Selected metamorphic mineral 'isograd' zone

Figure 6 -

 6 Figure 6 -Simplified geological map and RSCM T data for the central segment (Copland to Whataroa valleys) of the Alpine Fault hanging-wall (see Figure 1 for location, Figures 5 or 7 for a legend). Sample locations are color coded by metamorphic zone, annotated with RSCM T in °C (as provided in Table1). Selected metamorphic mineral 'isograd' zone boundaries, fold axes and the exhumed brittleductile transition zone (BDTZ) are also shown. On the right, RSCM T versus distance D to the Alpine Fault (see Figure2) are represented for the main river profiles. Rotated New Zealand Transverse Mercator projection, with NZGD2000 grid.

Figure 7 -

 7 Figure 7 -Simplified geological map and RSCM T data for the southern segment (Karangarua to Haast valleys) of the Alpine Fault hanging-wall (see Figure 1 for location). Sample locations are color coded by metamorphic zone, annotated with RSCM T in °C (as provided in Table1). Selected metamorphic mineral 'isograd' zone

Figure 8 -

 8 Figure 8 -Geological cross-sections and RSCM T data from north to douth: for the profiles Waitaha to Rakaia (AA'), Whataroa to Havelock (BB'), Franz Josef to Godley (CC') and Karangarua to Mount Cook village (DD') (see Figure 1 for location). On each section, RSCM T data are represented according to metamorphic zones (see color code in Figures 5-7) with T value annotations in °C. Main structural features such as faults or fold axes are also depicted. BB',CC', DD' are modified after Little et al. (2005).

Figure 9 -

 9 Figure 9 -Histograms of frequency distribution for all RSCM T data as a function of metamorphic zones (left) and textural zones (right).

Figure 10 -

 10 Figure 10 -Diagram with all RSCM T data for the three main segments along the Southern Alps investigated versus the distance D to the Alpine Fault, assuming the fault dips at 45° (see Figure 2) with bars representing ± 15° uncertainty for the dip.

Figure 12 -

 12 Figure 12 -Diagram with all RSCM T data represented for the different profiles along the strike of the Alpine Fault by metamorphic zones. Main mineral first appearance 'isograds', drawn by hand, do not necessarily represent preserved metamorphic mineral reaction surfaces (see text for discussion). Zones where complete resetting is observed for 40 Ar-39 Ar data on hornblende (yellow) and biotite (white) are indicated (after Little et al. 2005).

  Table 2 and supplementary material). Enlarged map of the Franz Josef-Fox area, central Southern Alps, providing a comparison between RSCM T data (circles, Table 1), coloured according to temperature, and equivalent petrologic results (Table 2) derived from garnet-biotite geothermometry (squares, recalculated for this study using Hodges & Spear 1982) and THERMOCALC (squares). The zone of reset 40 Ar-39 Ar hornblende ages (Hnbl <6 Ma) from Little et al. (2005) is also shown. New Zealand Transverse Mercator projection, with NZGD2000 grid.

	Figure 11 -

Table 1 (1/3)

 1

	Sample	E (NZTM) N (NZTM) z (m) D (km) TZ Metamorphic zone	N	R2	SDV	T	SDV SE

Acknowledgments

New samples from Westland National Park were collected under Department of Conservation permit WC-22994-GEO, including material collected by Richard Jongens, Mark Rattenbury and Lukas Nibourel. Older samples were sourced from PETLAB collections at GNS Science, Victoria University of Wellington and University of Otago. Rodney Grapes also provided access to archival samples and analytical material. Holly Godfrey and Belinda Smith Lyttle provided technical support. We also wish to thank our colleagues Tim Little, John Townend, and Rupert Sutherland for discussions and helpful comments during the gestation of this work, although not necessarily implying they agree with all of our interpretations and conclusions. Olivier Beyssac acknowledges funding from ANR (GeoCARBONS project), Sorbonne Universités (PERSU program) and CNRS-INSU. Simon Cox was funded under GNS Science's 'Impacts of Global Plate Tectonics in and around New Zealand Programme' (PGST Contract C05X0203). Frederic Herman was funded by the Swiss National Fund (grant PP00P2_138956). We thank Dave Craw and an anonymous reviewer for very constructive help, and Jean-Philippe Avouac for his editorial support.

Table captions

Table 1 -RSCM temperature data obtained along various profiles in the Southern Alps. For each sample, information provided are: sample name (PETLAB database, http://pet.gns.cri.nz), Easting and Northing (New Zealand Transverse Mercator using NZGD2000), altitude z (in meters), Distance D to the Alpine Fault (in kilometers), Textural Zone (TZ), Metamorphic Zone, N number of Raman spectra, R2 ratio and associated standard deviation SDV, Temperature T and associated standard error SE (Standard error is the standard deviation divided by √N). See text for further details.

Table 2 -Summary of petrologic data obtained in this study along Franz, Fox and Waikukupa profiles. For each sample, information provided: sample name (PETLAB database, http://pet.gns.cri.nz), Easting and Northing (New Zealand Transverse Mercator using NZGD2000), altitude z (in meters), Distance D to the Alpine Fault (in kilometers), P-T conditions from [START_REF] Grapes | Metamorphism and uplift of Alpine Schist in the Franz Josef-Fox Glacier area of the Southern Alps, New Zealand[END_REF], Garnet-biotite geothermometry [START_REF] Hodges | Geothermometry, geobarometry and the Al 2 SiO 5 triple point at Mt. Moosilauke, New Hampshire[END_REF] results (grt-bio T) based on rim analyses, using pressure estimates based on results of garnet-biotite-muscovite-plagioclase barometry (Hoisch, 1990, Fe-endmember) for the same or nearby samples, from [START_REF] Grapes | Metamorphism and uplift of Alpine Schist in the Franz Josef-Fox Glacier area of the Southern Alps, New Zealand[END_REF]. X (grs+sps) is (Ca +Mn)/(Fe + Mg + Ca + Mn) in garnet, values <0.2 are generally more suitable for garnet-biotite thermometry. T TC is maximum temperature estimate based on observed mineral assemblages and results of P-T pseudosection calculations (this study), for pressure estimates based on nearby samples and the calculated mineral assemblage stability field. See the PETLAB database for other analytical data.