The Approach to Steady-State Thermochronological Distribution Following Orogenic Development in the Southern Alps of New Zealand Geoffrey E

[American Journal of Science, Vol. 301, April/May, 2001,P.374–384] THE APPROACH TO STEADY-STATE THERMOCHRONOLOGICAL DISTRIBUTION FOLLOWING OROGENIC DEVELOPMENT IN THE SOUTHERN ALPS OF NEW ZEALAND GEOFFREY E. BATT* Department of Geology, Royal Holloway, University of London, Egham, Surrey TW20 0EX, United Kingdom. ABSTRACT. A diachronous sequence of isotopic ages along the Southern Alps of New Zealand illustrates details of the development of the modern tectonic regime of this orogen at about 5 Ma. Coupled with the rapid cooling rates experienced in the Southern Alps, which ensure negligible residence time at temperatures allowing partial radiogenic product retention during exhumation (and thus effectively instantaneous thermochronological closure), this record presents important general insight into the transient physical and thermal effects of changes in tectonic conditions. The tectono- thermal response of the Southern Alps to the change in dynamical conditions at 5 Ma is resolved into two evolutionary stages that are observed with progressive exhumation. The removal by erosion of material that had cooled below the relevant closure temperature prior to the change in dynamics at 5 Ma results in an initial decrease in age. This is followed by a sharp drop to younger ages as material subjected to thermal re-equilibration associated with the modern orogenic regime is exposed, culminating in the eventual exposure of time-invariant ages reflecting the new steady-state dynamics of the region. This two-step response is a direct result of the relationship between cooling and exhumation and illustrates the care needed in reconstructing physical histories from isotopic ages and cooling rates in tectonically active regions.

introduction Thermochronological age reflects the time since a sample became cool enough for the products of a given radioactive decay process to be retained on geological timescales (effective isotopic closure – Dodson, 1973) (Clark and Ja¨ger, 1969; Dodson, 1973, 1976; Ja¨ger, 1979). Due to the prevailing monotonic temperature increase with depth in the crust, this relationship is commonly used to equate isotopic ages with exhumation (fig. 1). Such thermochronological assessment is of particular interest in tectonically active and deeply eroded settings, offering insight into aspects of the physical and thermal development of such regions that may have been lost from the direct structural and stratigraphic record at the surface. Two principal assumptions underlie this method of interpreting cooling ages in terms of tectonic rates and processes. (1) Samples must be exhumed from below the depth at which the ambient crustal temperature is equal to the closure temperature of the chronometer in question (fig. 1). Although isotopic closure is not in itself dependent on pressure, the importance of the subsequent exhumation makes the depth at which closure occurs (closure depth) a fundamentally important quantity in the tectonic interpretation of cooling ages. (2) The isotopic ages observed should be at steady state values, such that they reflect the time taken for a sample to be exhumed from its closure depth (fig. 1). If this requirement is not fulfilled, ages reflect an average of the exhumation rates experienced between closure and exhumation and so provide no direct information about local dynamics at any particular point in time. Assumption 2 also requires that the region concerned be in a topographic and thermal steady state on the timescales over which chronometers are exhumed following

* Formerly at Department of Geology and Geophysics, Yale University, PO Box 208109, New Haven, Connecticut 06520-8109.

374 Geoffrey E. Batt 375

Fig. 1. Relationship of isotopic ages to cooling and exhumation in eroding regions. (A) Material is being exhumed from point A at a constant rate. Note that the geothermal gradient in this example is assumed to be linear with depth. The material effects discussed do not depend on this assumption, and it is made for clarity of illustration only. (B) Due to the geotherm, cooling is dependent on exhumation and thus on time. The gray box corresponds to the interval of partial retention for the relevant chronometer. (C) Once cooled to below the minimum temperature of the closure interval, the relevant radiogenic daughter product is completely retained, and thus isotopic age increases linearly with time. Projection of this linear zone of behavior back to an intercept with the age axis yields an effective “closure temperature” for the system, defined by Dodson (1973, p. 260) as “the temperature of the system at the time given by its apparent age.” The age observed at point B is a reflection of the time taken to exhume the material from the depth at which closure occurred (closure depth). As long as steady state conditions are maintained, this exposure age will not change, as material is continually being eroded off and replaced from below. closure, as change in either of these characteristics can alter the relevant closure depth, bringing about an accompanying transience in cooling ages at the surface. Thermochronological analysis of the Southern Alps of New Zealand presents important insight into how the physical and thermal characters of the crust evolve following disturbance during the return to steady state conditions. Systematic varia- tions in K-Ar and fission track ages along the orogen illustrate the dynamic response of the orogen to its initial development.A5to20kmwide zone adjacent to the Alpine Fault (fig. 2) along much of the length of the orogen displays patterns of cooling age variation indicative of the modern dynamics of convergence between the Australian and Pacific plates (Adams and Gabites, 1985; Kamp, Green, and White 1989; Tippett and Kamp, 1993; Batt and others, 1999, 2000). Recent assessment of the thermochro- nological record within this zone (Tippett and Kamp, 1993; Batt and others, 1999, 2000; Batt and Braun, 1999) indicates that the orogen is still adjusting dynamically to the development of the present tectonic regime at about 5 Ma. This work re-examines the thermochronological trends observed in the Southern Alps (Tippett and Kamp, 1993; Batt and others, 2000) and in particular builds on the 376 Geoffrey E. Batt—The approach to steady-state thermochronological distribution work of Batt and others (2000) in providing a new interpretation of those ages from the south of the orogen that, although displaying spatial links to the modern tectonic regime of the region, actually pre-date its initiation at 5 Ma. This enhanced understand- ing comes from interpreting the ages in the context of a progression toward steady state conditions following a recent major change in regional kinematics. This study serves to illustrate how these dynamical processes operate in orogenic systems and suggests potential indicators of such transience to consider when attempting to constrain tectonic evolution through thermochronology.

dynamic interpretation of isotopic ages The products of radioactive decay tend to diffuse out of a crystal lattice at elevated temperature (Sardarov, 1957; Hurley and others, 1962; Dodson, 1973), and thus an isotopic age is fundamentally a thermal datum. Such thermally activated diffusion is described by the Arrhenius relationship;

ϭ ͑ϪE/RT͒ D D0e where D is the diffusion coefficient, D0 is D at infinite temperature, E is the activation energy of the system, R is the gas constant, and T is temperature. Because of the exponential dependence of such diffusion on temperature, the partial retention interval (the range of temperatures over which a sample goes from open system behavior to effectively complete retention of a radionuclide) for many chronometers is small, relative to the temperature at which this isotopic “closure” occurs. The thermal basis of an isotopic age can thus often be approximated through the simplified concept of an effective closure temperature (Dodson, 1973) – the apparent threshold tempera- ture of this marked change in daughter product mobility (fig. 1C). Fission tracks are similarly unstable at elevated temperatures, with the radiogenic lattice damage that forms the basis of etchable and resolvable ‘tracks’ in crystals progressively healing over time. An analogous simplification is again commonly made of reducing this complex annealing behavior to a concept of ‘closure’ of fission track dating systems as rocks cool through the relevant “partial retention zone” (PRZ). The effective closure temperatures of different chronometers can be assessed through a combination of laboratory experiments extrapolated to geological time- scales (Fleischer, Price, and Walker, 1965; Krishnaswami and others, 1974; Harrison, Duncan, and McDougall, 1985; Lovera, Richter, and Harrison, 1989; Lister and Baldwin, 1996) and empirical geological studies relating well constrained thermal histories to observed sample age characteristics (Harrison, 1981; Hurford, 1986; Tagami , Carter, and Hurford, 1996). This temperature information can then be used to interpret the physical evolution of a region, based on an assumption that the age of a sample reflects the time taken to exhume that sample after it cooled through the relevant closure temperature (fig. 1).

tectonic character of the southern alps The Southern Alps (fig. 2) result from convergence between continental crust of the Australian and Pacific plates along the South Island of New Zealand (DeMets and others, 1990; Norris, Koons, and Cooper, 1990; Pearson and others, 1995). Although uplift rates in the orogen reach approx 10 mm/a adjacent to the Alpine Fault (Wellman, 1979; Tippett and Kamp, 1993), correspondingly rapid erosion and re- moval of material into the Tasman Sea by the efficient network of rivers along the western front of the range maintain orogenic topography at a relatively low level, with present mean elevations reaching only about 1750 m at the Main Divide (Kamp and Tippett, 1993). Despite an estimated 90 Ϯ 20 km of Late Cenozoic convergence (Walcott, 1998), this efficient erosion has effectively pinned the orogen at the Alpine following orogenic development in the Southern Alps of New Zealand 377

Fig. 2. Major geological features of the Southern Alps and localities mentioned in the text. Geology simplified from Grapes and Watanabe (1992).

Fault (fig. 2) and prevented significant overthrusting by the Pacific Plate and orogenic growth. The rapid uplift and exhumation of the Southern Alps cause correspondingly rapid cooling. Material close to the Alpine Fault in central regions of the orogen has cooled from temperatures higher than 300°C within the past million years (Tippett and Kamp, 1993; Batt and others, 1999, 2000). This extremely rapid quenching effectively removes the ambiguity of ‘partial retention’ (fig. 1). Even for the low- temperature apatite fission track chronometer, cooling at these rates leads to negli- gible residence times within the PRZ, yielding effectively instantaneous closure at a single closure temperature, and allowing for relatively unambiguous interpretation of cooling ages. This modern tectonic regime of oblique convergence and rapid uplift and exhumation in the Southern Alps has evolved only over the past approx 5 Ma (Sutherland, 1995; Walcott, 1998; Chamberlain and others, 1999; Batt and others, 1999; 2000). Prior to this, as far back as the latest Oligocene to early Miocene (Cooper and others, 1987), the Alpine Fault existed as a predominantly strike-slip or mildly transtensional structure linking the Hikurangi and Fiordland subduction zones (Kamp, 1986; Sutherland, 1996; Walcott, 1998), with minimal associated uplift and topography (Nathan and others, 1986; Tippett and Kamp, 1993). 378 Geoffrey E. Batt—The approach to steady-state thermochronological distribution

age variation across the southern alps Cooling ages vary in systematic fashion across the Southern Alps and can be divided into two main groups or domains of consistent behavior (Adams and Gabites, 1985; Tippett and Kamp, 1993; Batt and others, 2000). In the interior of the orogen, remote from the Alpine Fault, ages are at early Cenozoic to Mesozoic levels (Tippett and Kamp, 1993). These older outboard ages are related to the Rangitata Orogeny and other episodes of exhumation and cooling pre-dating the initiation of the modern plate boundary through the South Island of New Zealand. As such, these ages are not directly related to the modern tectonic regime of the Southern Alps, and their interpretation is not discussed further here. Toward the inboard margin of the Southern Alps, ages decrease dramatically to Pliocene to Pleistocene levels (Tippett and Kamp, 1993; Batt and others, 2000). These markedly younger samples retain no connection to the older thermal history observed in outboard regions, so they record only cooling associated with the recent-ongoing phase of exhumation in the area, related to the Kaikoura Orogeny. For the muscovite and biotite K-Ar chronometers, these ‘Kaikoura’ cooling ages exhibit systematic variation along the orogen (fig. 3). Between the Whataroa River and Fox Glacier (fig. 2), the youngest K-Ar mica ages observed are typically approx 1 Ma for biotite and 1.5 to 2 Ma for muscovite (fig. 3) (Adams, 1981; Batt and others, 2000). Farther south than the Paringa River (fig. 2), K-Ar mica ages are commonly affected by excess argon contamination (Adams and Gabites, 1985; Batt and others, 2000). Assessing and correcting for the presence of such contamination, however, yields a consistent trend of K-Ar ages increasing progressively southward, from approx 5 Ma at Fox Glacier to as high as 15 Ma for muscovite at Haast Pass (figs. 2 and 3) (Batt and others, 2000). At the transition between these two domains of varying behavior at Fox Glacier, a strong local relationship is observed between age and altitude (Batt and others, 2000), with a sharp increase from the approx 1 Ma ages of the central Southern Alps to the minimum 5 Ma ages of the southern age domain (fig. 3) over 1 km of local relief. Zircon fission track ages linked to Kaikoura orogenic activity (Kamp, Green, and White, 1989; Tippett and Kamp, 1993; Batt and others, 2000) are similarly consistent and low (Ͻ1 Ma) through central regions of the Southern Alps (fig. 3). These zircon fission track ages show no significant variation at Fox Glacier where the marked age discontinuity described above for K-Ar mica ages is observed (fig. 3). A sharp increase in zircon fission track age is, however, observed toward the south of the orogen, with minimum ages climbing to 3.6 Ϯ 0.8 Ma at Haast (Tippett and Kamp, 1993) and 5.6 Ϯ 0.5 Ma in the Arawata Valley (Tippett and Kamp, 1993) (figs. 2 and 3). This increase occurs toward the periphery of the studied regions (fig. 3), and its possible implica- tions for a second behavioral domain (as discussed for K-Ar mica ages above) are thus ambiguous. discussion The sequence of K-Ar mica ages exposed along the western (Kaikoura domain) margin of the Southern Alps provides an important window into the early evolution of the orogen, representing a time-transgressive sequence through the early development of the modern tectonic regime of the South Island. From as old as 15 Ma at Haast Pass, these K-Ar mica ages systematically decrease northward to approx 5 Ma immediately south of Fox Glacier, before dropping abruptly to as low as 1 Ma in Fox Glacier Valley and remaining at these young levels through much of the central region of the Southern Alps. The abrupt age transition at Fox Glacier cannot reasonably be attributed to a discontinuity in either the rates of exhumation or the total depth of exhumation of the modern orogen, as zircon fission track ages (which have a lower closure temperature, and therefore ostensibly a greater sensitivity to local dynamics) following orogenic development in the Southern Alps of New Zealand 379

Fig. 3. Variation in Kaikoura Orogeny-related zircon fission track and muscovite and biotite K-Ar ages along the Southern Alps, modified from Batt and others (2000). K-Ar ages from southern regions of the orogen heavily affected by excess argon contamination (Adams and Gabites, 1985; Batt and others, 2000) are omitted for clarity, with only the corrected regional trends suggested by Batt and others (2000) shown for these areas. Note that, where not explicitly illustrated by error bars, uncertainty on the ages shown is less than the size of the individual data symbols (Ϯ0.5 Ma). display no such transition in the area. Indeed, zircon fission track ages maintain young levels compatible with the rapid exhumation and cooling of the modern Southern Alps as far south as the Arawata River before displaying an enigmatic increase in age at the southern limits of the published dataset. Independent thermochronological (Batt and others, 1999), geophysical (Suther- land, 1996; Walcott, 1998), sedimentological (Sutherland, 1995), and geochemical (Chamberlain and others, 1999) arguments all indicate that the 5 Ma timing of the abrupt K-Ar age transition at Fox Glacier coincides with the initiation of the present tectonic regime of the Southern Alps. This pattern of ages may thus illuminate the physical and thermal changes undergone during this critical episode of tectonic 380 Geoffrey E. Batt—The approach to steady-state thermochronological distribution change from strike slip motion to markedly oblique convergence along the Alpine Fault. A major increase in exhumation rate, such as would be expected from the initiation of oblique convergence across the Alpine Fault, would influence isotopic ages at the surface in two ways. (1) In a purely physical response, samples would be exhumed from their closure depth more rapidly, and ages at the surface would decrease accordingly. (2) The increase in exhumation rate would produce a dynamic perturbation of the local geothermal structure (England and Thompson, 1984; Koons, 1987), with crustal isotherms within the exhumed layer migrating toward the surface (fig. 4), further reducing age. This style of dynamically perturbed thermal structure is well documented for the Southern Alps. Both direct borehole measurements (Shi, Allis, and Davey, 1996) and indirect geological and geophysical evidence (Craw, 1988; Holm, Norris, and Craw, 1989; Craw, Rattenbury, and Johnstone, 1994; Ingham, 1995) indicate anomalously high near-surface temperatures and geothermal gradients throughout the orogen. The response of an idealized thermochronometer to an increase in exhumation rate at a time ti is illustrated in figure 4. Closure is assumed to occur instantaneously for this chronometer at a single temperature. Although this is a major simplification of the thermal sensitivity of isotopic (and fission track) chronometers (Dodson, 1973; Lovera, Richter, and Harrison, 1989), this simplification is largely appropriate for the rapid cooling regime of the modern Southern Alps. At ti, a profile of ages exists in the upper crust, from an exhumation age at the surface (point G in fig. 4) reflecting the rate of exhumation in the former regime to an age of zero at the closure depth of the system (point C in fig. 4). This entire layer of crust has already cooled through the relevant closure temperature and is thus insulated from the subsequent exhumation-driven changes to the thermal structure of the region. Before the full effects of the change in exhumation rate are observed at the surface, this ‘isotopically closed’ layer must thus be stripped off by erosion. This erosion causes a reduction over time in the ages observed at the surface, as material is exposed that has spent progressively more of its recorded exhumation history in the new regime of rapid exhumation (fig. 4B and C), until the base of this pre-existing isotopically closed layer is reached. Material is next exposed which closes isotopically after ti (points B and A), and which thus experiences the thermal re-equilibration arising from the increase in exhumation rates (England and Thompson, 1984; Koons, 1987), with a consequent further reduction in exhuma- tion age (fig. 4B and C). Only after this second thermal equilibration stage does this thermochronometer display steady-state ages that accurately reflect the new character of the orogen (fig. 4C). This model is able to account for the observed distribution of thermochronologi- cal ages along the Southern Alps by a southward decrease in the total exhumation (and by inference, the exhumation rate) experienced since the initial development of the modern tectonic regime of the region at 5 Ma. The consistently young (Ͻ2 Ma) ages of the central Southern Alps in this interpretation mark the more deeply exhumed areas, which have developed steady state ages indicative of the modern orogenic character, while the varying southern domain K-Ar mica ages reflect transitional ages from areas of the crust for which the relevant ‘isotopically closed’ layer related to the previous orogenic regime has yet to be fully stripped away. If this interpretation is correct, the sharp transition in K-Ar mica ages noted at Fox Glacier should move progressively southward over time as the orogen continues to evolve. This model should be applicable for any thermochronometer. The suggested increase in Kaikoura Orogeny-related exhumation from south to north should result in ages decreasing progressively northward along the Southern Alps. For any chronom- eter where ages drop below 5 Ma (indicating sufficient exhumation in the present following orogenic development in the Southern Alps of New Zealand 381

Fig. 4. Conceptual illustration of the effects of an increase in exhumation rate at a time ti on isotopic ages for an idealized thermochronometer, as discussed in the text. Although this figure is conceptual, it is constructed by distilling and emphasizing the results of numerical modelling published in part by Batt and Braun (1997, 1999). (A) Depth of selected points at time ti. Note that for typical orogenic rates, exhumation of material advects heat towards the surface more rapidly than thermal conduction can diffuse it away into the surrounding crust (England and Thompson, 1984; Koons, 1987; Batt and Braun, 1997). This results in geothermal perturbation, with isotherms being moved closer to the surface. The strength of this effect increases with exhumation rate, and thus the increase in rate invoked here would be expected to move the relevant closure isotherm closer to the surface than it had been in the former regime. (B) Relative isotopic ages of the selected points at ti (white circles) and on exposure at the surface (black circles). Note that points C through G have already cooled through the relevant closure temperature at ti. This material is referred to as the “isotopically closed” layer, and it does not experience the dynamic change in geothermal structure resulting from the change in exhumation rates at ti. (C) Age variation observed in panel (B) plotted against time (or equivalently, increasing exhumation). Note the two transitional intervals between the change in regional dynamics at ti and the re-establishment of steady state ages. tectonic regime to strip off the relevant layer of cooled material inherited from the former regime of low exhumation rates) an abrupt drop in age to levels consistent with the modern thermally perturbed structure of the orogen should then be observed. The position of this transition would be south of Fox Glacier for chronometers with closure temperatures significantly lower than those of argon in muscovite and biotite and north of Fox Glacier for systems of higher closure temperature, reflecting the relative levels of exhumation required for material to reach the surface after cooling through the relevant temperature. The presently available thermochronological database for the Southern Alps is insufficient to fully test these predictions for other chronometers. 382 Geoffrey E. Batt—The approach to steady-state thermochronological distribution

The increase in zircon fission track age toward the south of the orogen (fig. 3) is consistent with this model, but data from farther south would be needed to confirm the significance and extent of this age increase. Available 40Ar-39Ar amphibole age constraint (Chamberlain, Zeitler, and Cooper, 1995) is similarly ambiguous, as again, amphibole ages are consistent with the outlined predictions but insufficient in both geographical coverage and data quality to test the veracity of the model.

conclusions The isotopic ages of rocks exposed by erosion are controlled by cooling and resultant isotopic closure, occurring at depth. The age of a sample on exposure at the surface reflects the time taken to exhume that sample from the depth at which it cooled through this closure temperature. It is this principle that leads to the use of isotopic ages to constrain the dynamics and evolution of tectonically active regions (Wagner and Reimer, 1972; Wagner, Reimer, and Ja¨ger, 1977; Zeitler and others, 1982; Fitzgerald and Gleadow, 1988). This importance of closure depth to exhumation age also leads to an inherent time lag between a change (be it an increase or decrease) in exhumation rates and the full effects of that change being observed in isotopic ages at the surface. Even for an instantaneous change in regional dynamics, a protracted interval of transience in exhumation ages will follow, as first the entire layer of crust that had already cooled through the relevant closure temperature prior to the change in exhumation rate is removed, and then the effects of re-equilibration of the geotherm (which is itself dynamic and affected by exhumation rates) are progressively exposed. This unavoidable re-equilibration process and the superficial similarity of the resulting age transition to that related to other, genetically distinct, processes (in particular, the exposure of an exhumed partial retention zone) illustrate the caution required in the use of geographic or temporal variations in isotopic age as indicators of discrete tectonic events or changes in regional dynamics. Diachroneiety in the thermochronological record exposed along the Southern Alps of New Zealand presents an ideal and possibly unique case in which to examine such a regional transition to thermochronological steady state following tectonic disturbance. This orogen experienced a profound change in tectonic regime approx 5 my ago, with the initiation of oblique convergence across the Alpine Fault causing an associated increase in uplift and exhumation rates east of the fault. Variable levels of exhumation along the orogen since 5 Ma have exposed a sequence of ages illustrating the re-equilibration of the physical and thermal regimes of the region to these changed dynamical conditions. This record is particularly clear for the Southern Alps due to the rapid exhumation and cooling associated with the current regime, which result in effectively instantaneous thermochronological closure. A characteristic and predictable sequence of thermochronological age variation is observed along the Southern Alps, as first the physical and then the thermal conse- quences of the change in tectonic regime at 5 Ma are exposed. During this transient tectono-thermal re-adjustment, isotopic ages do not reflect the dynamics of either the old or new tectonic regimes directly. The one valid direct tectonic indicator in this sequence for the Southern Alps is the age corresponding to the base of the ‘isotopically closed’ layer for K-Ar mica ages from the older regime, which is exposed immediately to the south of Fox Glacier (figs. 2 and 3). This feature records the inception of the present tectonic regime of the Southern Alps at 5 Ma, as these samples had cooled through the relevant closure temperature immediately prior to the tectonic change and hence had an age of zero at the time of this dynamical change, but have been recording time ever since. following orogenic development in the Southern Alps of New Zealand 383

acknowledgements The ideas presented in this paper were formulated in large part during my time at the Australian National University and have also benefited from discussions with many colleagues in the ensuing 3 yr. Jean Braun, Barry Kohn, Ian McDougall, and Mark Brandon are particularly thanked for their respective contributions. The clarity and focus of the paper were enhanced by reviews from C. Page Chamberlain and Dave Schneider. Chris Carson, and Mark Brandon are also thanked for their comments on an earlier version.

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