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The Young Uplift and Thermal History of the Central Eastern Alps (Austria/Italy), Evidence from Apatite Fission Track Ages

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The Young Uplift and Thermal History of the Central Eastern Alps (Austria/Italy), Evidence from Apatite Fission Track Ages Jb. Geol. B.-A. ISSN 0016-7800 Band 128 S.197-216 Wien, Oktober 1985 The Young Uplift and Thermal History of the Central Eastern Alps (Austria/Italy), Evidence from Apatite Fission Track Ages By GÜNTER GRUNDMANN & GIULlO MORTEANI*) With 13 figures and 2 tables Ostalpen Austroalpin Penninikum Tauern/enster Tauernkristallisation Radiometrische Altersbestimmungen Apatit-Spa Itspu renaIter Urangehalt Hebungsraten Abkühlungsgeschichte Zusammenfassung Die kontinuierliche Zunahme sowohl der Apatit-Spaltspuren- Die nach dem Abklingen der alpidischen R~gionalmetamor- alter als auch der Glimmer-Abkühlungsalter entlang eines phose (Tauernkristallisation) fortschreitende Hebung und Ab- West-Ost Profils innerhalb des Tauernfensters spricht gegen kühlung des Tauernfensters und seines austroalpinen Rah- die Existenz eines unabhängigen dritten, sog. "Neoalpinen" mens (zentrale Ostalpen) wurde mit Spaltspurenaltersdatierun- Metamorphoseereignisses in Form eines Wärmedoms vor etwa gen an Apatiten untersucht. Die Apatit-Konzentrate wurden 20 Ma. aus Gneisen und Metabasiten separiert. Die Apatit-Spaltspu- renalter des Tauernfensters liegen zwischen 5,1 :tO,S und 1O,5:t 1,1 Ma (:t2s), die seines nördlichen und südlichen aus- Summary troalpinen Rahmens zwischen 7,0:tO,8 und 42:t8 Ma (:t2s). The uplift and cooling of the Penninic Tauern Window and of Eine Probe aus dem südalpinen "granito di Bressanone" (Bri- its Austroalpine frame (central Eastern Alps) has continued xener Granit, 840 m ü. NN) südlich der Insubrischen Linie hat from the end of the Alpine regional metamorphism until the ein Alter von 16,6:t2,2 Ma (:t2s). present day. This postmetamorphic uplift and cooling history Über angenähert vertikale Probenprofile wurden für ausge- was studied by the fission track dating method applied to apa- wählte Gebiete die folgenden Hebungsraten bestimmt: Gra- tites which were separated from the metamorphic crystalline natspitz 0,4 mm/a, GroBvenediger 0,4 mm/a, Olperer - Ziller- rocks. taler Alpen 0,5 mm/a; diese Werte gelten für den Zeitraum zwi- The apatite fission track ages from the Tauern Window and schen 10 und 5 Ma. Die Hebungsrate des zentralen Tauern- its northern and southern Austroalpine frame (Eastern Alps) fensters war etwas langsamer als die des westlichen. Dieser range from 5.1 :t0.5 to 42:t8 Ma (:t2s); the elevations of the Unterschied ist auch aus den entsprechenden K/ Ar- und Rb/ sampling points range from 500 to 3500 m above sealevel. Sr-Glimmerabkühlungsaltern erkennbar. One sample of the Southern Alps, the " granito di Bressano- Als Anomalie in der sonst relativ homogenen Spaltspurenal- ne" (Brixen Granite, 840 m above sea level) south of the Peri- tersverteilung innerhalb des Tauernfensters treten im Granat- adriatic Lineament (Insubric Line), gives an age of 16.6:t2.2 spitz gehäuft sehr niedrige Alter auf, obwohl die Hebungsrate Ma (:t2s). hier etwa den gleichen Betrag wie die der Umgebung hatte. Sample groups along subvertical profiles from selected Als Erklärung stehen vorerst folgende zwei Modelle zur Dis- areas give the following uplift rates: The Granatspitz area kussion: 0.4 mm/a, the Venediger massif 0.4 mm/a, the Zillertaler Alps 1. Es erfolgte im Zeitraum zwischen 5 Ma und jetzt eine iso- 0.5 mm/a; these uplift rates are valid for the time span be- liert blockartige, schnellere Hebung des Granatspitzmas- tween 10 Ma and 5 Ma before the present. The uplift rate of sivs gegenübelj seiner Umgebung. the central Tauern Window was somewhat slower than that in 2. Der Geothermische Gradient im Granatspitzmassiv war si- the western part of the window. These different uplift rates are gnifikant höher als in den umgebenden Metabasiten. Die zu a long lasting phenomenon; indeed, they can also be deduced vermutende höhere Wärmeproduktion in den Granitgneisen from the K/ Ar- and Rb/Sr-biotite cooling ages fQund in the li- kann durch deren relativ höhere U-, Th- und K-Gehalte ver- terature. The youngest apatite fission track ages are found in ursacht worden sein. the Granatspitz area. These ages are anomalous with respect Apatit-Spaltspurenalter von 22,9:t3,8 Ma bis 42:t8 Ma to the rather homogeneous cooling pattern found in the wider (:t2s) aus der austroalpinen Grauwackenzone sind wahr- surroundings, particularly as the early uplift rate of the Granat- scheinlich nicht Abkühlungsalter der mittelalpinen Tauernkri- spitz area is similar to that of the surroundings. Two expla- stallisation (Temperaturmaximum zwischen 50 und 35 Ma) nations are possible to account for the young ages of the Gra- sondern eher als Mischalter bzw. Abkühlungsalter des "Eoalpi- natspitz area: nen" Ereignisses (Temperaturmaximum zwischen 90 und 1. During the time span between 5 Ma and today the uplift of 80 Ma) zu interpretieren. the Granatspitz occurred at a higher rate than the sur- roundings. 2. The geothermal gradient in the Granatspitz gneiss body *) Authors' addresses: Dr. GÜNTERGRUNDMANN,Prof. Dr.-Ing. was significantly higher than in the surrounding metabas- GIULlO MORTEANI,Lehrstuhl für Angewandte Mineralogie und ites due to a higher heat production caused by higher U, Th Geochemie, Technische Universität München, Lichtenbergstra- and K contents in the granitic gneisses than in the meta- Be 4, 0-8046 Garching, BRO. basites. 197 Fission track ages of about 22.9:t3.8 and 42:t8 Ma (:t2s) In the Central Alps an increase of apatite fission track found in the Austroalpine Grauwackenzone can be considered ages with increasing topographic elevation was obser- as cooling ages of the Eoalpine event (temperature maximum between 80 and 90 Ma) rather than those of the regional midd- ved from which uplift rates between 0.2 and 1.1 mm/a le Alpine metamorphism (= "Tauernkristallisation" , temperatu- were calculated (WAGNER& REIMER,1972; WAGNERet re maximum between 35 and 50 Ma). aI., 1977). Evidence from the continuous change of apatite fission track Using the mean geothermal gradient of 30°C/km sug- ages and mica cooling ages along an EW profile within the gested by SASSIet al. (1974) for the Eastern Alps and Tauern Window argues against the existence of an indepen- dant third Alpine metamorphic event at around 20 Ma. by WAGNERet al. (1977) for the Central Alps and using uplift rates from 0.3 to 0.8 mm/a as calculated for the Tauern area by CLARK(1979) on the basis of heat flow data from the Tauern tunnel (Eastern Tauern Window), 1. Introduction and problem the closure temperature of fission tracks in apatite for the study area is about 100°C. This corresponds ac- The main intention of this paper is to analyse the cording to DODSON(1979), to a cooling rate between young uplift and thermal history of the Penninie rocks of 10°C/Ma and 30°C/Ma. the Tauern Window, based on a study of fission track The closure temperature of Rbi Sr and KlAr in biotite ages from apatites. The investigated area is shown in is about 300°C:t50°C (PURDY& JÄGER,1976; JAGER, Fig.1 on a geological sketch map of the Eastern Alps. 1979). This closure temperature is obviously closer to The fission track ages of apatites from a low to medium the peak temperature of the last metamorphic event grade regional metamorphic terrain like the Penninie than that of the fission tracks in apatite. Therefore, a unit in the Eastern Alps are cooling ages (WAGNER, combination of the apatite fission track ages with the 1968; WAGNER& REIMER,1972). They give the time Rb/Sr- and KI Ar-ages of biotite and white mica extends elapsed since the rocks were uplifted through a certain the information on the cooling history to times closer to isotherm corresponding to the closure temperature of the metamorphic peak. the fission tracks in apatite. The closure temperature is The cooling history of a geological unit is influenced dependent on the cooling rate (NAESER& FAUL,1969; by the heat production in the unit itself and by the heat DODSON,1979). The slow cooling model (8) according flow from below. Most of the heat produced in the rocks to WAGNER(1979) is typical for regional metamorphic is produced by the decay of natural unstable isotopes terrains like the Penninie Tauern Window in the Eastern of K, U and Th. No data exist on the heat production in Alps. the study area. München o Salzburg Graz Ljubljana 10\11 DINARIDES [[[Il'OstaIPin" cs:3"SChiChtstufenland" o Milano []]"SüdalPin" f:[[J;;M!!!!!~l ~gnnd~ff~srocks SS~ Verona ~O\.~ [TTTl"O' 'd ,,~"Außeralpine varis- ~ man en ~ zische Gebirge" o 100km a)~ a)Helvetic, Flysch, I I - :. bl Penninic rocks undiff. b) Penninic rocks of the Tauern Window Fig. 1: Geological sketch map of the Eastern Alps. The investigated area is within the frame. 198 ~E o N 1 J 0 o 0 o o LI) LI) LI) -.J 0 ::J CI> '_ I I !l..(f)I->N c: CD~ co I- (f) a> d -=c: .~£ c: o .~ .!!l tii Cii ~ Q c: 'lij.... a;.2i 2 co'O~ 0 ..... I- Q) = c: ..c ~ ~a> :::J .0 Eg. -1 00 ~ :C~ ~ Q.co 000 Ea> 0 • co+= -'-a>tii Z E~ tiiCl <t S.~ 'öCi ~E co a>oo c: 'ä.~ o <I- o a> , -= \ Q Ö \ 00 '0 \ co \ er Cil , o \ .!!l o '0 \~ c: '\ o co 00 \ E I- \ N CD c--<t ..c: \~ 0 CI> Ö ..c 0 X .!!l 'L: a> m ..c: ~. I- \ \ • C\l \ o Cl \ Li: \ \ ~\ g \ o is \ ~.\ ..!; I / 199 Most of the radiometric ages in the Penninic rocks of 2. Sampling and Analytical Techniques the Tauern Window and the Austroalpine units are Rb/ Sr or K/ Ar ages on biotite and white mica (BESANG et The sampled area covers the western and central aI., 1968; JÄGER et aI., 1969; RAITH et aI., 1978; SATtR, Tauern Window as well as its northern and southern 1975; BORSI et aI., 1973, 1978; DEL MORa, 1982; HAM- Austroalpine frame i.
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