Earth and Planetary Science Letters 306 (2011) 193–204

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Earth and Planetary Science Letters

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Rapid formation and exhumation of the youngest Alpine eclogites: A thermalconundrum to Barrovian metamorphism

Andrew J. Smye a,⁎, Mike J. Bickle a, Tim J.B. Holland a, Randall R. Parrish b, Dan J. Condon b

a Department of Earth Sciences, University of Cambridge, Cambridge, CB2 3EQ, UKb NERC National Isotope Geoscience Laboratories, Kinglsey Dunhem Centre, Keyworth, Nottingham, NG12 5GG, UK

⁎ Corresponding author. Tel.: +44 1223 333400; fax:E-mail address: as859@cam.ac.uk (A.J. Smye).

0012-821X/$ – see front matter © 2011 Elsevier B.V. Adoi:10.1016/j.epsl.2011.03.037

a b s t r a c t

a r t i c l e i n f o

Article history:Received 15 December 2010Received in revised form 28 March 2011Accepted 29 March 2011Available online 30 April 2011

Editor: R.W. Carlson

Keywords:eclogiteBarrovian metamorphismU–Pb geochronologyconductive heat transferallaniteTauern Window

Eclogite facies metamorphic rocks provide critical information pertaining to the timing of continental collisionin zones of plate convergence. Despite being amongst Earth's best studied orogens, little is understood aboutthe rates of Alpine metamorphism within the Eastern Alps. We present LA–MC–ICPMS and ID–TIMS U–Pbages of metamorphic allanite from the Eclogite Zone, Tauern Window, which when coupled with rare earthelement analysis and thermobarometric modelling, demonstrate that the European continental margin wassubducted to between 8 and 13 kbar (30–45 km) by 34.2±3.6 Ma. These data define: (i.) an upper limit onthe timing of eclogite facies metamorphism at 26.2±1.8 kbar (70–80 km) and 553±12 °C, (ii.) plate velocity(1–6 cm·a−1) exhumation of the Eclogite Zone from mantle to mid-crustal depths, and (iii.) a maximumduration of 10 Ma (28–38 Ma) for juxtaposition of Alpine upper-plate and European basement units andsubsequent conductive heating thought to have driven regional Barrovian (re)crystallisation at ca. 30 Ma.One-dimensional thermal modelling of tectonically thickened crust shows that conductive heating is too slowto account for Tauern Barrovian conditions (550 °C at 9–13 kbar) within the maximum 10 Ma intervalbetween eclogite exhumation and the thermal peak (28–32 Ma). Given that the Tauern Window is a classiclocality for understanding rates of conductive thermal relaxation in tectonically thickened crust, this workraises questions of fundamental importance concerning the length scales of the mechanisms responsible forheat transfer within orogenic crust.

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© 2011 Elsevier B.V. All rights reserved.

1. Introduction

High-pressure (HP) metamorphic rocks are a ubiquitous feature ofPhanerozoic mountain belts. Their occurrence provides importantconstraints on both the transfer of material between the crust andmantle, and also the timing of continental collision within zones ofactive and past convergence (De Sigoyer et al., 2000; Leech et al., 2005;Parrish et al., 2006; Rubatto and Hermann, 2001). Whilst subductionprovides a plausible mechanism by which tracts of oceanic andcontinental lithosphere are conveyed to upper mantle depths, theprocesses responsible for exhuming such rocks back to Earth's surfaceare lesswell understood (e.g. Agard et al., 2009, and references therein).Within the typical orogenic cycle (ultra)high-pressure ((U)HP) meta-morphism occurs early, linked to subduction of intervening oceanicrealms, before the inception of continental collision which thickens thecrust anddrives subsequentBarrovianmetamorphism. This accounts forcommonly observed amphibolite–greenschist facies overprinting ofblueschist–eclogite facies metamorphic rocks (e.g. De Sigoyer et al.,1997, 2004; England and Richardson, 1977). High-precision radioiso-

tope geochronology of both (U)HP and Barrovian metamorphic eventsallows calculation of average exhumation rates of (U)HP bodies.Recently, this approach has shown that exhumation rates are of thesameorder ofmagnitude to plate velocities—cm·a−1 (e.g.Baldwin et al.,2004; Meffan-Main et al., 2004; Parrish et al., 2006; Rubatto andHermann, 2001). Such rates of exhumation question the dominance ofconductive heat transfer over timescales of tectono-metamorphicevolution in orogenesis. In mountain belts such as the Alps and theHimalayas, where exposed rocks record both Barrovian and (U)HPmetamorphism associated with plate convergence it should be possibleto assess both the duration of thrusting and subsequent conductiverelaxation heating. This is of fundamental importance to understandingthe rates andmechanisms responsible for driving tectono-metamorphicevolution in areas of orogenesis.

The Tauern Window (Fig. 1) exposes the largest coherent eclogitefacies unit within the Eastern Alps. Eclogites senso stricto and eclogitefacies metasediments pertaining to the European margin are exposedin a nappe which, following HP metamorphism, experienced regionalBarrovian metamorphism associated with emplacement of theAustroalpine nappes. Tauern Barrovian metamorphism provided theimpetus behind pioneering studies of the thermal budget oftectonically thickened crust (Bickle et al., 1975; England, 1978;England and Richardson, 1977; England and Thompson, 1984;

Fig. 1. Tectonic sketch map of the Alps, focusing on the Tauern Window and the Eclogite Zone in particular. Modified after (Glodny et al., 2005). Collectively, the Venediger covernappe, the Eclogite Zone, the Rote-Wand/Modereck nappe and the Glockner nappe are referred to as the Schieferhülle nappes.

194 A.J. Smye et al. / Earth and Planetary Science Letters 306 (2011) 193–204

Oxburgh and England, 1980; Oxburgh and Turcotte, 1974). However,despite extensive petrological study (Frank et al., 1987; Getty andSelverstone, 1994; Holland, 1979a,b; Miller, 1974) the age of eclogitefacies metamorphism and therefore the duration of conductiveheating are poorly constrained. Previous geochronological under-standing relies heavily on the 40Ar–39Ar system (Kurz et al., 2008;Zimmermann et al., 1994), which is highly susceptible to excess 40Arcontamination under HP conditions (e.g. Sherlock and Kelley, 2002;Warren et al., 2010). Further geochronological studies have beenhampered by isotopic disequilibrium (Van Breemen andHawkesworth,1980), a paucity of datable accessory phases (Miller et al., 2007) and thelack of a quantifiable link to P–T conditions (Glodny et al., 2005). As aresult, despite the Eastern Alps being one of Earth's best studiedorogens, the timescales of its evolution are currently uncertain. Thispaper presents results of a combined thermobarometric, geochrono-logical and thermal modelling study which show that eclogiteformation, exhumation and subsequent Barrovian metamorphism alloccurred in under 10 Ma—within an order of magnitude quicker thanpreviously thought. We use high-precision U(–Th)–Pb allanite geo-chronology to illustrate the potential that allanite has as a progradegeochronometer in high grade metamorphic terranes.

2. Tauern Window eclogites

The TauernWindow exposes Penninic nappes of the Alpine orogen'slower plate, which are observed in a geometry reflecting exhumation,post-subduction associated with closure of the Valaisan oceanic realm(Fig. 1). Eclogites and associated HPmetasediments outcrop within theEclogite Zone (south-central TauernWindow)— a tectonic sliver 15 kmin length, intercalated between a crystalline basement complex — theVenediger Nappe, and an incomplete ophiolite suite — the GlocknerNappe. The Eclogite Zone comprises abundant metre to hundred metrescale eclogite boudins and pods hosted by a metasedimentary matrix,which is dominated by calc-schists, metapelites and quartzites. Bothmafic and sedimentary protoliths have experienced eclogite-faciesmetamorphism. The combination of mafic igneous and sedimentarylithologies supports interpretation that the successionwas deposited on

the southern passivemargin of the European continent prior to Tertiarysubduction and collision (Miller, 1974; Ratschbacher et al., 2004).

2.1. Tectono-metamorphic evolution

The Eclogite Zone behaved as a coherent unit within the Alpineorogenic episode and preserves petrographical signatures of threediscrete metamorphic events. Relicts of its prograde history are limitedto mica, chloritoid and epidote inclusions in garnets. Pseudomorphsafter lawsonite, within garnet poikiloblasts, have been inferred byseveral authors (Dachs, 1986;Droop, 1985;Spear, 1986). TheHPevent istypified by garnet–omphacite–kyanite–phengite assemblages in maficeclogites (Holland, 1979b) and garnet–chloritoid–kyanite–phengiteparageneses in metapelitic lithologies (Smye et al., 2010). Numerousthermobarometric calculations using peak eclogite-facies mineralassemblages yield a range in P–T conditions from 20 to 26 kbar and550–600 °C (Franz and Spear, 1983; Holland, 1979b; Hoschek, 2001;Smye et al., 2010; Stöckhert et al., 1997). During exhumation theEclogite Zone is thought to have been tectonically inserted between theunderlying Venediger nappe and beneath the overlying Glockner nappe(BehrmannandRatschbacher, 1989)under blueschist-facies conditions,indicated by partial replacement of eclogite-facies amphibole andpyroxene by glaucophane (Holland and Richardson, 1979). Regionallypenetrative greenschist–amphibolite facies metamorphism (ca. 7 kbarand 520–570 °C, Zimmermann et al., 1994) overprinted previousmetamorphic assemblages during conductive heating associated withisotherm relaxation after emplacement of the Austroalpine nappes(Bickle et al., 1975). Numerous radiometric dating studies have shownthat this event occurred ca. 27–32 Ma. Glodny et al. (2005) performedRb–Sr analyses on cogenetic white mica, titanite, amphibole andplagioclase from a structurally discordant vein assemblage in agreenschist retrogression aureole within mafic eclogite. They obtainedan isochron age of 31±0.5 Ma (MSWD=0.63). This age is supportedbythe data of Gleißner et al. (2007), who carried out Rb–Sr isotopicanalyses of a pseudomorphic assemblage after blueschist facies law-sonite in the Glockner nappe. A multimineral internal isochroninvolving white mica, apatite, epidote, clinozoisite, calcite and albiteyields an age of 29.82±0.54 (MSWD=2.2). The age is interpreted to

195A.J. Smye et al. / Earth and Planetary Science Letters 306 (2011) 193–204

represent syntectonic growth of the greenschist facies matrix assem-blage. Similarly, Inger and Cliff (1997) present white mica, epidoteand whole rock Rb–Sr isotopic data from eleven different localitiesfrom within the Eclogite Zone and Glockner nappe. All of the ages liebetween 27 and 31 Ma and are interpreted to record greenschistfacies metamorphism concomitant with penetrative deformation,influx of fluids and resetting of Sr isotopes. Phengite 40Ar– 39Ar agesobtained from Eclogite Zone lithologies range between 32 and 42 Ma(Kurz et al., 2008; Ratschbacher et al., 2004; Zimmermann et al.,1994) and are variably interpreted to represent cooling from highpressure metamorphism.

Both theVenediger andGlockner nappes appear tohaveexperienceda common metamorphic history with the Eclogite Zone subsequent tothe blueschist event (Holland and Ray, 1985). Our geochronology isbased on a single sample collected from the internal portions of theEclogite Zone (see Supplemental text S.1).

2.2. Sample petrography

Sample TH-680 is a peraluminous garnet–chloritoid–kyanitebearing metapelite collected from the central portion of the EclogiteZone. Smye et al. (2010) have shown that this assemblage is restrictedto a narrow, temperature controlled, stability field within the eclogitefacies. A psuedosection combined with isopleth calculations usingTHERMOCALC version 3.33 (Holland and Powell, 1998) constrain

5

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ssur

e (k

bar)

Temperat

coeqtz

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ctdchlep

gepcarjd

gctdepcarjd

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(+ g)(+ jd/- pa)

(- chl)

70.3113.761.293.645.553.330.240.090.185

SiO2Al2O3CaOMgOFeOK2ONa2OMnOO

Molar %

clin

ozoi

site

gro

wth

MnNCKFMASHO+ SiO2 + mu + H2O

(+

(+ jd)

gctdchlmaep

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(+ g)

(+ pa)

(+ car)

(+ ma)(- ctd)

Fig. 2. P–T pseudosection for sample TH-680. Italicised labels at the end of equilibria denotewhfield of clinozoisite growth represents the regionwhichbest correlateswith calculated ferric confrom(Holland andPowell, 1998)). Similarly, theellipse representing initiation of garnetgrowth i(rim), and Ca/(Ca+Fe2++Mg+Mn)=0.21(core)–0.18(rim). The P–Tpath is constrainedby awhich bracket kyanite growth and also the appearance of retrograde margarite.

considerable portions of the sample's prograde and retrograde P–Tpath given the presence of two generations of chloritoid which bracketkyanite growth (Fig. 2). Peak conditions determined using garnet rim,the core of matrix chloritoid blasts and matrix phengite compositionscoexisting with kyanite, quartz and H2O, are 26.2±1.8 kbar at 553±12 °C (2σ; calculations performed using the average P–T mode ofTHERMOCALC version 3.33), on the verge of UHP conditions andconsistentwith the pseudosection calculations. Sample TH-680 escapedpervasive overprinting during the Barrovianmetamorphic event. Zonedepidotes with allanitic cores and clinozoisite rims are present aselongated lozenges (150–900 μm length; 50–200 μmwidth) which arewrapped by the rock's crenulated phengite fabric (S0+S1), whichadditionally wraps around garnet poikiloblasts. Proximal to fold hinges,allanite grains are aligned parallel to the limbs of F2 generation micro-folds. Euhedral allanite grains are also present as rare inclusions withinboth core and rimportions of garnet poikiloblasts (Fig. 3a)—indicative ofpre-garnet growth. The textural setting of allanite in TH-680 is notablysimilar to that described by Hoschek et al. (2010) from a garnet–chloritoid–kyanite bearing micaschist collected nearby TH-680, withinthe Eclogite Zone.

2.3. Allanite

Allanite ((Ca, REE, Y)2 (Al, Fe)3(SiO4)3OH) can incorporate weightpercents of the LREE inventory and Th in crustal rocks (Hermann, 2002)

ure (°C)

Approxim

ate depth (km)

75

60

45

30

15

550 600 650

gctdchlep

gky

gctdky

gctdep

(- ma)

(- bi)

(- st)

g growth

av.P-T

gcarkyjd

(- car)(- ep) ky)

(+ jd)

gkyjd

gchlma

gkybima

gkychl

gkychlma

gmabist

(- ep) (+ st) (- chl)

ich phase is consumed or formed up-grade. The shaded transparencymarking the stabilitytents of clinozoisite rimportions (f(ep)=0.24–0.26— epidote activity–compositionmodels constrainedbygarnet isoplethvaluesof Fe2+/(Fe2++Ca+Mg+Mn)=0.66(core)–0.62forementioned isopleth calculations, the presence of two generations of chloritoid growth,

Fig. 3. Allanite chemistry: (a) High contrast BSE image of euhedral allanite in garnet poiklioblast. Clinozoisite rims to allanite cores are present in grains includedwithin garnet and also asmatrix porphyroblasts. The textural setting of allanite in TH-680 is very similar to that described byHoschek et al., 2010 of a similar lithology from the Eclogite Zone. (b) False colour X-raymap of Th concentration in allanite 1, TH-680. Euhedral core regions display complex structure relative to Th-poor clinozoisite rims. (c) Chondrite normalised LA–ICPMS REE patterns forallanite core regions and coexisting garnet. Allanite is enriched in LREEs and alsoHREEs relative to garnet. Note the absenceof a significant Eu anomaly in allanite, and also the presenceof apositive Ce anomaly. Garnet REE profiles showaprogressive depletion of HREEs during growth. (d) Total REE+Th versusAl (cationsper formula unit) plot after Petrik et al., 1995, showingthe chemical relationship between representative allanite and clinozoisite rim domains. (e)Modelled Y concentration profiles from core to rim, assuming Rayleigh fractionation betweengarnet andmetamorphicmatrix (Hollister, 1966) both before (red solid line; Dgarnet−matrix

Y =83.8; Cmatrix=30.62 ppm) and after (blue solid line; Dgarnet−matrixY =83.8; Cmatrix=5.96 ppm)

allanite growth. Measured profile constructed from LA–ICPMS Y data; 1σ errors are smaller than the width of an analysis point marker.

196 A.J. Smye et al. / Earth and Planetary Science Letters 306 (2011) 193–204

and forms a solution with epidote and clinozoisite by the coupledsubstitution of Ca2++Fe3+/Al3+ respectively for REE3++Fe2+.Allanite in TH-680 is zoned from core to rim by increasing clinozoisitecontent (Fig. 3b and d; Table S.D.1; Table S.D.2) which is commonlyascribed to prograde metamorphic growth (Janots et al., 2007, 2008,2009). Chondrite normalised REE patterns obtained via LA–ICPMS(Fig. 3c; Table S.D.2) show that the allanite is heavily enriched in theLREEs relative to HREEs (average La/Yb=294). Even so, the allaniteis enriched in HREEs relative to garnet (Fig. 3c). Allanite and garnetdominate the REE budget of sample TH-680 in which phosphateaccessory phases monazite, xenotime and apatite (common sinksfor REE elements in metapelitic rocks) are not observed. Yttrium isa trace element with a strong affinity for both allanite and garnet(i.e. Dallanite−matrix

Y and Dgarnet−matrixY NN1) and mineral Y profiles

are therefore very sensitive to the relative timing of their growth.Rayleigh fractionation modelling of garnet growth within TH-680(Supplemental text S.3; Table S.D.4) shows that the Y concentra-tions in garnet growing prior to allanite are calculated to be anorder of magnitude higher than measured values (∼2500 ppm versus∼500 ppm, Fig. 3e). Similarly, values of Dgarnet−matrix

Y calculated for TH-680 garnet cores against a TH-680 matrix composition depleted byallanite formation, are close to literature values unlike those estimatedwithmatrix compositions undepleted by allanite growth (see AppendixA.3). These calculations corroborate the textural evidence that theallanite grew prior to initiation of garnet growth. The allanite REE

spectra also lack a negative Eu anomaly, consistentwith growth outsidethe stability field of plagioclase (≤10 kbar).

Assuming that Σ[Si+Al+Ti+Fe+Mn+Mg]=6 c.p.f.u (Ercit,2002), structural formulae for allanite and clinozoisite were recalcu-lated in order to generate estimates for Fe3+ concentrations. Isoplethsfor constant ferric iron concentration, and hence constant epidote–clinozoisite ratio, were calculated between 5 and 25 kbar and 400–600 °C using the epidote activity–composition model of (Holland andPowell, 1998). Given the high concentration of REEs present in allanite,the two component epidote–clinozoisitemodelmay not be appropriate.However, lower REE concentrations of the rims surrounding allanitecores make such portions suitable for application of the epidote activitymodel. Rim epidote–clinozoisite ratios show a comparable range invalues to those calculated, between 8 and 13 kbar and 425–500 °C(Fig. 2). Further pseudosection calculations imply that garnet growthinitiated between 18 and 21 kbar and 550–580 °C, consistent withchemical and petrographic evidence showing that both allanite andclinozoisite fractions grew early along the prograde metamorphic path.

3. Geochronology of metamorphism

Current understandingof the timingof eclogite-faciesmetamorphismwithin the Tauern Window is based on Rb–Sr geochronology (Glodnyet al., 2005, 2008).However, the known susceptibility of Rb–Sr to isotopicresetting during deformation and retrogressive metamorphism (Thoni

0.77

0.79

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data-point error ellipses are 2

0.0

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34.6 ± 4.1 MaMSWD = 15

clinozoisite

allanite

LA-MC-ICP-MS

a

0.79

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data-point error ellipses are 2

ID-TIMS

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0.0

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Tera-Wasserburg concordia

b

clinozoisite

allanite

Fig. 4. Tera-Wasserberg concordia plots for (a) LA–MC–ICPMS and (b) ID–TIMS allanitedata. Colours of error ellipses correspond to individual grains ((a) red=X1 grain 1;green=X1 grain 7; (b) red=X1 grain 2; blue=X1 grain 4; green=X3 grain 3). InsetTera-Wasserberg plot illustrates the length of extrapolation required to generate anintercept age. All ages were calculated using Isoplot v. 3.00 (Ludwig, 2003).

197A.J. Smye et al. / Earth and Planetary Science Letters 306 (2011) 193–204

and Jagoutz, 1992)means that confirmation fromamore robust system isrequired. The U(–Th)–Pb system is the preferred choice given its highclosure temperature (≥600 °C), and the potential to demonstrably linkages with P–T conditions.

Typically allanite core regions contain 2–235 ppmU and have Th/Uratios between 1.7 and 21.6. Such low Th/U ratios focused geochro-nological study on the U–Pb system. We used U–Pb Isotope Dilution–Thermal Ionisation Mass Spectrometry (ID–TIMS) and U(–Th)–PbLaser Ablation–Multiple Collector–Inductively Coupled Plasma MassSpectrometry (LA–MC–ICPMS) methods to date laser-milled sepa-rates and in-situ allanite grains respectively. The U(–Th)–Pb data andanalytical methods are given in the Supplemental Data Set (Supple-mental text S.5; Table S.D.5; Table S.D.6). Preliminary laser ablationstudies were hampered by the absence of a suitable allanite standard.Analysis of Tertiary Alpine allanites previously used as standards(Gregory et al., 2007) reveals protracted crystallisation histories andsignificant spread in single grain U(–Th)–Pb contents. Against aprimary zircon mineral standard, analyses of both core and rimportions of zoned grains show that non-radiogenic Pb dominates thetotal Pb composition (Pb /Pbcb0.03). In particular, clinozoisite rimportions contain negligible radiogenic Pb (238U/206Pbb0.15) and thusprovide an estimate of the common Pb composition of the rock duringallanite growth. Analyses (n=14) of two allanite grains (allanite 1and 7) in thin section X1 provide a Tera-Wasserburg lower interceptage normalised to zircon standard 91500 (Wiedenbeck et al., 1995) of34.6±4.1 Ma (2σ; MSWD=15)and an initial 207Pb/206Pb composi-tion of 0.8211±0.0015 (95% conf.; Fig. 4a).

Given the lack of a matrix-matched primary standard, this age is notrobust enough for geological interpretation. In order to circumvent this,three allanite grains previously used for LA–MC–ICPMS were separatedfor U–Pb ID–TIMS analyses. Individual grains were extracted frompolished thin sections via laser milling of matrix material and weresubsequently broken into 50 μm diameter shards. ID–TIMS measure-ments corroborate observations on U–Pb systematics made from LA–MC–ICPMS measurements. Despite the limited spread in 238U/206Pbcompositional space (0.5–5), analyses of eight allanite–clinozoisiteshards define a Tera-Wasserburg regression with a lower concordiaintercept age of 34.2±3.6 Ma (2σ; MSWD=3.8) and an initial207Pb/206Pb composition of 0.8330±0.0014 (95% conf.; Fig. 4b),which is slightly more radiogenic than the 34 Ma average uppercrustal lead composition predicted by Stacey and Kramers (1975)(0.8379). Due to low allanite Th/U values, 207Pb/206Pb and 238U/206Pbratios corrected for ingrowth of 206Pb from the incorporation of excess230Th (Schärer, 1984), yield an age difference within the uncorrected2σ age uncertainty. Performing both ID–TIMS and LA–MC–ICPMSanalyses on the same allanites is a powerful geochronological approach,combining the spatial resolution of laser ablation and the high precisionof ID–TIMS. Although the ID–TIMS Tera-Wasserburg age is within errorof the LA–MC–ICPMS age, common Pb compositions are subtly different(Δ(207Pb/206Pb)≈0.0119). Identification of a suitable allanite standardis required to constrain matrix dependent isotopic fractionation forU(–Th)–Pb LA–ICPMS allanite geochronology.

4. Tectonic implications

We interpret the 34.2±3.6 Ma ID–TIMS age as the time at whichallanitic cores to clinozoisites grew between ∼8 and 13 kbar at 400–500 °C (see Figs. 2 and 5) prior to garnet growth during ongoingsubduction. The new geochronological data, combined with previ-ously reported data, permit calculation of exhumation rates and alsotimescales for the process of eclogite formation in the Eastern Alps.Taking the angle of subduction as 45° and a descending slab velocity of5 cm·a−1, the earliest peak pressure conditions (∼26 kbar) would beattained in the subductedmargin is ∼2 Ma after allanite growth i.e. byca. 33 Ma. However, given the 2σ uncertainty on the ID–TIMS age,peak pressure conditions are constrained to the interval 29–37 Ma.

This corroborates the 31.5±0.7 Ma Rb–Sr age of Glodny et al. (2005),obtained from a range of eclogitic mineral assemblages. The EclogiteZone was subjected to regionally pervasive Barrovian metamorphismbetween 27 and 32 Ma (Rb–Sr: Inger and Cliff (1997); Glodny et al.(2005)), which places a limit on the time taken to both form andexhume the eclogite-facies nappe to mid-crustal levels. Therefore, theEclogite Zone must have experienced ∼60 km of exhumation at ratesbetween ∼1 and 6 cm·a−1. It is extraordinary, then, that the EclogiteZone is understood to have been subjected to local recrystallisation inthe blueschist facies, prior to Barrovian overprinting as evidenced bythe growth of glaucophane at the expense of peak eclogitic omphacite.The P–T conditions of blueschist facies metamorphism are constrainedbetween 7 and 11 kbar and 400–450 °C (Eremin, 1994; Holland andRay, 1985). Given that the Eclogite Zone represents transitional crustlocated on the southernmargin of the European continent (Kurz et al.,1998, and references therein), the 29–37 Ma age of eclogite formationcritically provides a maximum estimate for the timing of continentalcollision in the Eastern Alps. This means that a maximum of 10 Ma

0

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kysill

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Az0 = 5.0 µW.m-3

qm = 27 mW.m2

κ = 31.54 km2.Ma-1 Inception of erosion at 30 Ma

Fig. 5. Often cited model of conductive relaxation heating (Bickle et al., 1975; Englandand Thompson, 1984), which assumes a 30 Ma hiatus prior to the initiation of erosion.Dots indicate 5 Ma increments; grey box delimits Tauern Barrovian metamorphicconditions.

198 A.J. Smye et al. / Earth and Planetary Science Letters 306 (2011) 193–204

separated emplacement of the Austroalpine nappes and attainment ofpeak Barrovian conditions at 27–32 Ma.

5. Thermal modelling

5.1. Background

The thermal diffusivity of continental lithologies determines thatconductive heating driven by relaxation of isotherms in regions ofover-thickened crust operates on timescales of the order of tens ofmillions of years (England and Thompson, 1984). However, high-precision isotope geochronology has recently shown that in someregional metamorphic terranes, peak Barrovian conditions areattained within much shorter durations (Ague and Baxter, 2007;Baxter et al., 2002; Oliver et al., 2000). Disparity between predictedand observed timescales of heating necessitates additional mecha-nisms to conductive heating. These include: elevatedmantle heat flow(Oxburgh and Turcotte, 1974; Stüwe and Sandiford, 1995), increasedradiogenic heat production (Huerta et al., 1999; Jamieson et al., 1998),shear heating (Grujic et al., 1996; Searle et al., 2008; Whittingtonet al., 2009), magmatism (Lyubetskaya and Ague, 2010), advectivetransport of heat by fluids (Bickle and McKenzie, 1987) and advectionof heat by tectonics (Burg and Gerya, 2005).

The Tauern Window is a classic locality for modelling the thermalevolution of overthrust terrains (Bickle et al., 1975; Cornelius et al.,1939; England, 1978; England and Richardson, 1977; England andThompson, 1984; Oxburgh and England, 1980; Oxburgh and Turcotte,1974; Richardson and England, 1979). Tauern Barrovian metamor-phism is understood to have been driven by conductive heatingfollowing northward emplacement of allochthonous Austroalpine

units during Alpine orogenesis (Oxburgh, 1968). Previous conductiveheating models call for a period of ca. 30 Ma post-emplacement of theAustroalpine nappes for the European basement to attain peakconditions of 520–550 °C at 7–8 kbar (see Fig. 5) — thus invokingages of East Alpine eclogite formation N60 Ma. This is incompatiblewith our geochronological data, which show that a maximum of10 Ma passed between eclogite formation and the peak of Barrovianmetamorphism. Here we use a one-dimensional thermal model (e.g.(Bickle et al., 1975; Oxburgh and Turcotte, 1974)) to constrainthermo-tectonic scenarios which satisfy rapid (b10 Ma) attainment ofTauern Barrovian conditions.

5.2. The model

The thermal development of the Tauern Window is modelled bya one-dimensionalfinite-difference approximation to the heat equationfollowing the approaches of Oxburgh and Turcotte, (1974), Bickleet al., (1975) and England, (1978) in which the thermal profilefollowing overthrusting is calculated for different values of totalheat generation (radiogenic heat production+mantle heat flow).Additionally, we consider the effect of thrust sheet thickness on theevolving thermal profile. Numerical details of the model are presentedin the (Supplemental text S.6).

The initial thermal profile is divided into two units representingthe allochthonous Austroalpine complex and lower plate Penninicbasement and cover nappes, including the Eclogite Zone. Diffusivity(κ) is kept constant throughout the profile at 31.54 km2·Ma− 1

(∼1 mm2·s−1) as temperature dependent feedbacks (Fig. 3 Bickleet al., 1975; Whittington et al., 2009) are only significant over theinvestigated length- and timescales in the presence of concentratedheat sources such as magmatism and shear heating, which are notapplicable to the Tauern. The initial temperature profile of the overthrustsheet is insignificant to the thermal evolution of the system (Bickle et al.,1975) and therefore, we use identical equilibrium geotherm relations(see Appendix A5) for both hanging- and footwall units. The surfacetemperature is 20 °C for all time steps and we assume a basement-top initial temperature of 20 °C, justified by its 1000 m thick coversequence (Kurz et al., 1998). The lower boundary condition is definedby a constant temperature gradient at 150 km depth. Constraints onthe thickness of the Austroalpine complex are limited due to internalimbrication and the absence of palaeo-horizontal markers. However,an estimate of the sheet's minimum thickness at the time of over-thrusting is obtained from peak pressures experienced within theZentralgneiss of theVenediger nappe, duringBarrovianmetamorphism:6–14 kbar equating to 20–45 km (this study). These thicknesses areslightly greater than assumed by previous studies (15–30 km — Bickleet al. (1975); England (1978); England and Thompson (1984)),reflecting advances in thermodynamic data used in calculatingmetamorphic pressures. Radiogenic elements in the Pennine basementare strongly concentrated within the Zentralgneiss—a Variscan calc-alkaline granitoid body. Measurements by Hawkesworth (1974b) areconsistent with an exponential distribution of radiogenic matter(Lachenbruch, 1968, 1970) with a characteristic drop-off length (D) of8 km and a surface radiogenic heat production (Az0) of 5.02 μW·m−3.Assuming such a distribution, Bickle et al. (1975) calculate an estimate ofthe total radiogenic heat production (Az0·D) possible from the basementof 50.16 mW·m−2 (1.19 HFU). Given the lack of constraints,mantle heatflow (qm) is allowed to vary between 20 and 100 mW·m−2 (0.47–2.39 HFU), encompassing values characteristic of stable continental crustto rifting margins (Sclater et al., 1980, 1981). It is important to note thatthe contribution of heat from the mantle is the single most importantparameter affecting the thermal evolution of the system (England,1978). Previous models invoke an erosional hiatus between emplace-ment of the Austroalpine at ca. 65 Ma and inception of erosionwithin theOligocene (Fig. 6; Bickle et al. (1975); England (1978); Oxburgh andTurcotte (1974)). However, our newgeochronological constraints on the

199A.J. Smye et al. / Earth and Planetary Science Letters 306 (2011) 193–204

timing of thrusting suggest that erosion was active immediately post-thrusting. Therefore, we use a constant erosion rate of 0.9 mm·a−1

(0.9 km·Ma−1). Shear heating is not considered because the plateinterface zonewithin the TauernWindow is dominated by low-strengthcarbonate lithologies. Shear heating in these rocks will be negligibleabove 300 °C as shown by the calculations of Bickle et al. (1975), whocalculated shear stress at the base of a 20 km thick overthrust sheetas function of temperature for Yule marble (Heard, 1963) assumingductile failure in a 4 km shear zone. Similarly, the effect of syn-metamorphic magmatism or fluid advection are not taken intoaccount as there is no Tertiary igneous activity known from withinthe TauernWindow; neither are extensive fluid vein networks of themagnitude required by England (1978) observed.

Peak temperature of Tauern Barrovian metamorphism is wellconstrained by conventional thermodynamic methods to between500 and 570∘C (Bickle, 1973; Dachs, 1990; Dachs and Proyer, 2001;Hoschek, 2001) within the south-central Tauern. Variations intemperature estimates from the Schieferhülle nappes (Fig. 1) aregenerally within uncertainty of each other. Pressure estimates are lesswell constrained due to fewer relevant equilibria and uncertainties inactivity–composition relations. We performed average P–T calcula-tions (THERMOCALC version 3.33, Holland and Powell, 1998) on ametatonalite (TH-519—see Supplemental text S.1 and Table S.D.7)collected from the upper-most levels of the Zentralgneiss (Fig. 1)within the Großvenediger region. Equilibria generated from end-members of garnet, plagioclase, K-feldspar, biotite, muscovite andepidote yield an average pressure of 13.1±2.1 kbar at 601±38 °C(σ

fit=1.66). Although loosely constrained, the estimate is corrobo-

rated by pressures calculated from overprinted calc-schists of theEclogite Zone, 2–3 km structurally above the Zentralgneiss, (Hoschek,2001) between 9 and 10.2 kbar.

and

ky

sill

0 Ma

5 Ma

10 Ma

15 Ma

20 Ma

Temperature (oC)

Dep

th (

km)

60

50

40

30

20

10

0

0 100 200 300 400 500 600

25 Ma

2

4

6

8

10

12

14

16

18

Pressure (kbar)

30 Ma

35 Ma

Fig. 6. Numerically calculated Depth–T–t curves for variable thickness of overthrustsheet. Circles represent 5 Ma increments after thrusting. Mantle heat flow constant at27 mW·m−2 (0.95 HFU), crustal heat production is 5.0 μW·m−3 (11.95 HFU) anderosion rate is 0.9 mm·a−1. Grey box constrains metamorphic conditions at the peak ofAlpine Barrovian metamorphism.

Consequently, we consider the range of values for overthrust sheetthickness, radiogenic heat production and mantle heat flow in whichconditions of 9–13 kbar and 550 °C are attained in the upper 6 km ofthe lower-plate within 10 Ma of thrust sheet emplacement.

5.3. Model solutions

5.3.1. Overthrust sheet thicknessWe calculate Depth–T–t curves for the basement-top for a range of

overthrust sheet thicknesses between 20 and 60 km and a constantmantle heat flow of 27 mW·m−2 (0.64 HFU). Results are displayed inFig. 6. The P–T evolution of the upper-basement is characterised by aninitial phase of rapid heating (≤2Ma), driven by equilibration oftemperatures across the sawtooth-shaped temperature depression ofthe plate interface. Subsequent heating is exponentially slower untilthe basement is exhumed via erosion through colder isotherms. Thebasement top only attains Tauern temperatures within 10 Ma ofoverthrusting if the thrust sheet is greater than ∼55 km thick. Such ascenario would require peak basement pressures of 14–16 kbar. Peaktemperatures for overthrust sheets of 55 and 60 km thickness of548 °C at 11.2 kbar and 579 °C at 11 kbar respectively, are reachedbetween 20 and 25 Ma post thrusting if rapid exhumation begins after5 Ma. Interestingly, emplacement of a 30 km thick thrust sheet — thepreferred value of Bickle et al. (1975), heats the upper-basement to apeak temperature of 339 °C in 13 Ma — 200 °C short of Tauernconditions. For thrust sheets greater than 27 km thickness, erosionmust operate more rapidly than 0.9 mm·a−1.

5.3.2. Crustal heat production and mantle heat flowThe temperature attained by the basement-top following 10 Ma

conductive heating under a 30 km thick overthrust sheet is plottedagainst total heat contribution (A0·D) from the basement (Fig. 7).Erosion is not active and mantle heat flow varies between 0.2and 2.0 HFU (8.36–83.68 mW·m−2). Saliently, the calculations showthat for Tauern radiogenic heat production values 0.55–1.2 HFU(Hawkesworth, 1974a,b), Barrovian conditions will only be attainedwithin 10 Ma if mantle heat flow is N62.76–37.66 mW·m−2 (1.5–0.9 HFU) respectively, at the time of emplacement. Such values ofheat flow into the lithosphere correspond to conductive mantletemperature gradients of 25.10–15.06 °C·km−1, implying lithospheric

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

200 300 400 500 600 700 800

T after 10 Ma. (oC)

A0.

D (

H.F

.U)

Not applicable

Not applicable

1.8

2.0

1.6

1.4

1.2

1.0

0.8

0.6

H.F.U

0.4

0.2

Fig. 7. Temperature attained after 10 Ma conductive heating of the basement afteremplacement of a 30 km thick thrust sheet plotted against total heat generation withinthe basement complex. The plot is contoured for solutions of constant mantle heat flow(HFU); the shaded band delineates peak Tauern metamorphic temperatures; stippledregion corresponds to the probable range in A0·D for the Zentralgneiss, of 0.55–1.2 HFU(Hawkesworth, 1974a,b). For simplicity these solutions omit the effect of erosion.Thermal properties of the thrust sheet are identical to those of the basement.

200 A.J. Smye et al. / Earth and Planetary Science Letters 306 (2011) 193–204

thinning and asthenospheric upwelling (Sclater et al., 1980). However,helium isotopic ratios within East Alpine groundwaters show a virtualabsence of mantle-derived 3He ((3He/4He)/(3He/4He)airb0.2, Martyet al., 1992) and there is a notable lack of extensive Alpine magmatismwithin the Eastern Alps. Both observations are inconsistent withelevated mantle heat flows. In the absence of erosion, these resultsprovide lower-bound estimates as erosionwill shorten the time intervalover which conductive heating is operative. Erosion would displace theconstant mantle heat flow curves of Fig. 8 towards higher temperatures.

5.4. Alternative solutions

5.4.1. Tectonic emplacementAssuming that the Austroalpine nappes were emplaced ca. 65 Ma,

we now consider the scenario in which the Peripheral Schieferhüllenappes (Eclogite Zone, Rote-Wand and Glockner nappes) weretectonically inserted into a heated crustal pile ca. 35 Ma. This end-member tectonic scenario accounts for the lack of petrological evidencethat the basement experienced blueschist-facies metamorphism priorto Barrovian conditions, in addition to accounting for the Barrovianmetamorphism by conductive heating alone. Fig. 8 shows the thermalevolution of a crustal pile in which a 4 km thick slab is inserted at plateinterface depths, into a geotherm produced after 30 Ma of conductiveheating beneath a 30 km thick thrust sheet. Initial temperature of theslab is taken as a constant 400 °C, corresponding to temperatures of theblueschist-facies event (Holland and Ray, 1985). All other thermalparameters are as in Fig. 5 except that erosion is not considered due to

0

10

20

30

40

50

60

70

80

90

1000 200 400 600 800

Temperature oC

Dep

th k

m

emplacement

0.4 Ma 4 Ma

400 500 600300

Temperature (oC)

5

10

15

20

25

Pre

ssur

e (k

bar)

emplacement 30 Mapost-thrusting

tectonic emplacement

Fig. 8. One-dimensional conductive relaxation profile of a 4 km thick slab, collectivelyrepresenting the Glockner nappe and the Eclogite Zone, inserted into a crustal pile witha geotherm representing 30 Ma post-thrusting (doubling of thickness) conductiveheating. The model represents the Eclogite Zone's insertion into a blueschist faciesnappe stack and subsequent heating and exhumation to the upper-greenschist facies,∼550 °C (highlighted on P–T path of insert). Time steps are 0.4 Ma duration.

the short timescales investigated. Given that the thermal time constant(τ=a2/(π2κ)) of a 4 km slab under such conditions is 0.16 Ma, the slabattains Barrovian conditions rapidly, within ca. 1.2 Ma. However, inorder to reconcile tectonic emplacementwith eclogitization between28and38 Marequires subduction tobe active for 30 Mapost-emplacementof the Austroapline nappes at ca. 65 Ma.

5.4.2. Syn-thrusting heatingOxburgh (1968) suggests that roughly 100 km of displacement has

occurred between the Penninic nappes of the southern TauernWindow and the overlying Austroalpine during Alpine convergence.If the maximum duration of thrusting is 10 Ma (38–28 Ma), thenthrust sheet emplacement must have occurred at rates greater than1 cm·a−1, akin to plate convergence rates. At high rates of thrustingthe upper-basement will experience rapid heating due to the constantreplenishment of heat by the allochthonous sheet — effectivelymaintaining a constant thermal gradient within the upper plate,analogous to the action of a hot iron. However, for this scenario to beplausible, the basement must be located proximal to the inception ofthrusting so as to avoid conductive cooling of the thrust sheet. Fig. 9shows the thermal evolution of a crustal pile undergoing synthrustingheating for a thrusting rate of 1 cm·a−1. It is clear that syn-thrustingheating would form inverted isotherms within the footwall duringearly stages of thrusting — as observed in the Himalaya. Withsubsequent conductive relaxation and radiogenic heat production,footwall isotherms are smoothed and attain similar values to the baseof the overthrust sheet within 10 Ma.

0

10

20

30

40

50

60

70

80

90

1000 100 400 500 600200 300 700 800 900

Temperature (°C)

Dep

th (

km)

0 Ma 2 4 6 8 10

continual thrusting

Thrusting rate (r) = 10 km.Ma-1

Lbase. < 39 kmDisplacement = 100 km

100

80

60

40

20

00 10 20 30 40 50 60

Thr

ustin

g ra

te -

r (

km.M

a-1)

Depth from plate interface - Lper (km)

Displacement = 100 kmκ = 31.54 km2.Ma-1

Lper = √(κ.τper)

Exposed basement

Fig. 9. The ‘Hot Iron’ scenario. One-dimensional conductive heating of a basementcomplex with qm=27 mW·m− 2, Az0=5.0 μW·m− 3 and D=8 km continually over-thrust by a 35 km thick thrust sheet; erosion is neglected; 100 km displacementoccurs at 10 km·Ma− 1 (1 cm·a− 1). Horizontal distance from inception of thrusting(Lbase) ≤39 km. Grey box reflects Tauern metamorphic conditions. Inset plot showsthe depth a thermal perturbation (i.e. a sawtooth geotherm of an overthrust sheet)would penetrate against rate of thrusting over 100 kmdisplacement. Graph calculatedfrom thermal time constant relations: Lper=

ffiffiffiffiffiffiffiffiffiffiffiffiffi

κ:τperp

where Lper=depth of penetra-tion, κ=diffusivity and τper is the time over which the temperature perturbationexists (i.e. duration of thrusting). Grey region delimits the range of basement depthsexposed within the Tauern Window.

201A.J. Smye et al. / Earth and Planetary Science Letters 306 (2011) 193–204

6. Discussion

Amaximum interval of 10 Ma separating thrust sheet emplacementand Barrovian metamorphism in the Eastern Alps raises profoundquestions over the length scales of heating responsible for regionalmetamorphism. The classic conductive relaxationmodel shows that thelength scale of conductive heating required to attain peak conditions inb10 Ma is incompatible with Barrovian pressure estimates and detritalsediment loads (see beneath). Elevated mantle heat flow explains therapid heating, but lacks supporting geochemical evidence. Tectonicinsertion of the Schieferhülle nappes along the plate interface accountsfor rapid (b2 Ma) heating from blueschist to Barrovian metamorphicconditions. However, this scenario requires amechanism to exhume theEclogite Zone from 70–80 km to∼30 kmdepth at plate tectonic rates of1–6 cm·a−1, between ca. 38 Ma and Barrovian metamorphism ca. 27–32 Ma. Similarly, concomitant thrusting andheating explain the b10 Matimescale of heating, but demand temperatures at the base of theoverthrust sheet close to those of peak metamorphism and also a pulsein detrital sediment supply to circum-East Alpine basins.

The Eclogite Zone is bounded by mylonite zones at its contactswith the Glockner and Venediger nappes. The basal contact ischaracterised by thrust-sense, top-to-the-NNE deformation, whilstits sense of movement relative to the hanging wall is less clear,showing both top-to-the-N and S-side-down pre-syn Barrovianshear indicators. Nevertheless, it is likely that these high strainzones accommodated between 50 and 70 km of displacement duringexhumation of the nappe back up the subduction channel, prior tojuxtaposing the Eclogite Zone between the basement and Glocknernappes as an extrusion wedge (Behrmann and Ratschbacher, 1989;Glodny et al., 2008). Rapid exhumation of the nappe could have beenfacilitated by a buoyancy contrast between the subducted crustalmaterial and surrounding mantle (Δρ=300 kg·m−3, England andHolland, 1979), following detachment from the slab (Smye et al.,2010). However, the presence of kilometre-scale stratigraphiccontinuity in both marbles and quartzite bands of the Eclogite Zonesuggests that the nappe was exhumed as a coherent unit (Behrmannand Ratschbacher, 1989). As fluid-assisted granular flow (Stöckhert,2002) or power–law deformation of aragonite is expected to havecontrolled the rheology, this raises considerable doubt over whetherthe transitional crust would be strong enough to support suchbuoyancy forces throughout 70–80 km exhumation (e.g. Renneret al., 2001). Alternatively, return flow of low viscosity materialwithin a confined subduction channel has been postulated to accountfor the exhumation of (U)HP rocks (Cloos, 1982; Gerya and Stöckhert,2006; Gerya et al., 2002; Warren et al., 2008). High viscosity of thesubducted margin provides an explanation for the coherent natureof the Eclogite Zone.

Despite plausible exhumation mechanics, insertion of the EclogiteZone as an extrusive wedge into a heated crustal pile requires thatcontinental collision occurred ∼30 Ma prior to eclogite formationbetween 38 and 28 Ma, effectively refrigerating subducted oceaniccrust for up to 30 Ma. This is implausible given that temperatures of theblueschist overprint (200–450 °C) are indicative of an active subductionregime between 38 and 28 Ma. Furthermore, shear sense indicatorsalong theplate interface are strongly thrust-sense, supporting late-stageemplacement of the Austroalpine nappes as a tectonic ‘lid’ (Bachmannet al., 2009; Bickle and Hawkesworth, 1978; Liu et al., 2001; Wallis,1988; Wallis and Behrmann, 1996). In light of these issues regardingthe mechanism of tectonic insertion we do not consider it a feasibleexplanation to the problem.

Concomitant heating and thrusting are often overlooked in thermalmodelling studies. However, our calculations show that they providethe impetus for rapid heating of footwall units close to the inception ofthrusting (Lbase≤r.τ, where Lbase=distance over which rapid heatingoccurs; r=thrusting rate and τ=thermal time constant of the thrustsheet; Fig. 9). Our allanite geochronology constrains average thrusting

rates for the 100 kmof displacement between upper and lower platesto≤10 km·Ma−1—similar to rates calculated by Dewey et al. (1989)and Schmid et al. (2004) via plate tectonic and palinspastic restorationrespectively. Therefore a range of thrusting rates between 10 and100 km·Ma−1 is representative of Eocene–Oligocene convergencewithin the Eastern Alps. Synthrusting heating initially affects theupper-most levels of the basement, forming inverted isothermswhich are subsequently smoothed by conductive heating. Fig. 9shows that synthrusting heating beneath a 30 km thick thrust sheetdisplaced at rates between 10 and 100 km·Ma−1 could feasiblypenetrate the upper 20 km of basement—consistent with Barrovianpressure estimates between 9 and 13 kbar calculated from basementrocks.

Overthrusting of a 30 km thick allochthon across the Eastern Alps(1.8×104 km2 — Oxburgh, 1968) would provide roughly 1.45×1018 kgof sediment to circum-East Alpine basins in the interval 40 Ma–present day, assuming a 10 km present day average thickness of theAustroalpine nappes, a crustal density of 2700 kg·m− 3 and anaverage erosion rate of 1.1 mm·a− 1. Compared to England (1981)'sestimate of 1.1×1018 kg for the East Alpine detrital sediment load,this suggests inconsistency between the detrital sediment record andmetamorphic pressures, which could be accounted for by a hiddenEocene–present age sediment trap or an overestimate of the metamor-phic pressures i.e. a thinner allochthon. The flysch–molasse transitionwithin circum-Alpine basins occurred across the Eocene–Oligoceneboundary (Kuhlemann, 2000; Kuhlemann et al., 2006; Sinclair, 1997),which strongly supports contemporaneous thrust sheet emplacement.Plate velocity overthrusting of the Austroalpine nappes provides anelegant explanation for equally rapid exhumationof the Eclogite Zoneasthe same basal thrust system would be responsible for both processes(Fig. 10). Similarly, the scenario accounts for continued subductionpost-eclogite formation, explaining blueschist facies overprinting prior toBarrovian conditions.

Syntectonic growth of Barrovian assemblages would provide strongevidence in support of footwall heatingdue to rapid overthrustingof hotrock, as noted in the Lesser Himalaya (Caddick et al., 2007). However,the relationship between Barrovian mineral growth and fabric forma-tion in the Penninic units of the Eastern Alps is less clear. Kurz et al.,(1996) state that westward directed shearing initiated synchronous tothe Barrovianpeak,whilst Droop (1985) and Inger and Cliff (1997) statethat the metamorphic peak post-dated regional fabric formation.Furthermore, evidence for Barrovian conditions at the base of theAustroalpine overthrust sheet is lacking; temperatures were highenough to facilitate Ar diffusion in biotite, chloritisation and develop-ment of low-temperature quartz microstructures ca. 30 Ma. (Wallis,1988; Waters, 1976), i.e. ≥300–350 °C. Under these conditions,synthrusting heating would not account for the rapid timescales ofBarrovian metamorphism. However, due to the heavily reworked andanhydrous nature of the polymetamorphic basement to the Austroal-pine nappes (the Altkristallin; e.g. Siegesmund et al., 2007), it isuncertain whether pervasive development of Barrovian mineralogywould occur under higher grade conditions, particularly given the shorttimescales of heating.

In the absence of an advective component to heating, syn-thrustingheating provides a theoretical explanation for the rapid (b10 Ma)tectono-metamorphic evolution of the Eastern Alps, but lacks directlysupporting field evidence. Tectonic replenishment of heat to footwallunits may be a fundamental mechanism to other overthrust terranes,such as the Himalaya, where syn-kinematic Barrovian assemblages areobserved.

7. Conclusions

1. We present the first U–Pb geochronological evidence for the age ofeclogite-facies metamorphism within the Eclogite Zone, TauernWindow. Allanite–clinozoisite grew during the prograde portion

Fig. 10. Tectonic schematic of scenario in which concomitant syn-thrusting heating of the Penninic basement and plate velocity exhumation of the Eclogite Zone occur. This ispossible because the same basal thrust is responsible for the rates of both thrust sheet emplacement and exhumation. Isotherms are approximate and represent an early thermalarchitecture associated with oceanic subduction. Continental collision would raise isotherms to shallower crustal levels.

202 A.J. Smye et al. / Earth and Planetary Science Letters 306 (2011) 193–204

of the Eclogite Zone's P–T–t path, between 8 and 13 kbar at 34.2±3.6 Ma, prior to initiation of garnet growth. Combined withpreviously reported Barrovian metamorphism at 27–32 Ma, thisplaces a maximum 10 Ma period between ∼28 and 38 Ma duringwhich the peak of East Alpine eclogite-facies metamorphismoccurred. Eclogites facies rocks were exhumed at plate velocityrates, between 1 and 6 cm·a−1, over a vertical distance of∼50 km.

2. These new geochronological data drastically shorten the duration oftime available for conductive relaxationof isotherms following thrustsheet emplacement, from ∼30 Ma to ≤10 Ma. We show that thethermal evolution of the Tauern Window cannot be explained byconductive heating of overthickened crust on lithosphere withnormal continental thermal gradients. Either the thermal evolutiontook place with a significant thermal contribution from the mantle,for which there is little supporting evidence or heating occurredduring thrustingwith heat provided by anoverlying thrust sheet. It ispossible that syn-thrusting heating by rapid emplacement of a hotoverthrust sheet, overlooked in previous thermal models, couldplausibly have played an influential role in the tectono-metamorphicevolution of the Eastern Alps and in overthrust terrains in general.

Supplementarymaterials related to this article can be found onlineat doi:10.1016/j.epsl.2011.03.037.

Acknowledgements

A.S further acknowledges support of NERC studentship NE/F007647/1. Matt Horstwood (NIGL) is thanked for directing LA–ICP–MC–MSwork.We thank John Cottle and Vannessa Paschley (NIGL) forguidance and advice with LA–ICP–MC–MS work. Peter Nabelek(University of Missouri) is thanked for discussions regarding thermalmodelling. Clare Warren (Open University) provided helpful reviewsof an earlier version of the manuscript and assisted with fieldwork.Mark Caddick (ETHZ) is thanked for advice and discussion bothwithinthe field and in the lab. Max Wigly ran bulk rock ICPMS analyses.Constructive reviews by two anonymous reviewers served tohighlight additional areas of importance and clarify the manuscript.

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