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The polyphase thermal history of the Northern Calcareous Alps in the light of Conodont Colour Alteration Indices


1Montanuniversität Leoben, Institut für Geowissenschaften: Prospektion und Angewandte Sedimentologie, Peter-Tunner-Straße 5, A-8700 Leoben; 2Universität Wien, Institut für Paläontologie, Geozentrum Althanstraße, A-1090 Wien; 3Universität Wien, Institut für Geologie, Geozentrum Althanstraße, A-1090 Wien

Conodont colour alteration (CAI) studies reflect a polyphase thermal history and show two distinct units with a sharp CAI boundary.
The southern unit ("Ultra-Tirolic nappe") and parts of the Hallstatt mélange show strong alteration (CAI 5.5-6.0, partly CAI 7.0; e. g. Hochkönig, Grimming, Mandling unit). The highest metamor-phism (CAI >5.5) is transported and predates the Upper Jurassic gravitational tectonic emplacement of the Hallstatt Mélange resp. nappes and the metamorphic unit (= Hochkönig, southern part of Tennengebirge) onto the Tirolicum in late Middle early Upper Jurassic times (late Callovian to early Oxfordian). The high CAI values are related to tectonic burial in an accretionary wedge formed during the closure of parts of the Tethys Ocean.
The northern units (= Bavaric, Tirolic nappes) exhibit a relatively homogeneous distribution of no or low grade conodont alteration (CAI 1.0-2.0) increasing to the south thereby crossing the nappe boundaries between the Tirolicum and the Hallstatt Mélange. This thermal overprint is younger than Kimmeridgian and older than Berremian. Another thermal overprint is related to metamorphism of the crystalline basement in the Middle Cretaceous and affects parts of the southern rim of the Northern Calcareous Alps with a continuous south to north and bottom to top decrease in temperature and with medium CAI values in the south (CAI 3.0-4.0, locally CAI 5.0).

Those data corresponds with the polyphase diachronous meta-morphic history in the Austroalpine basement.
A first metamorphic cycle which produced high-pressure meta-morphism in the Hallstatt zone, yielded radiometric ages roughly between 160 and 130 Ma. This event affected the Greywacke Zone and its Paleozoic equivalents and parts of the Northern Calcareous Alps. The second cycle which includes high-pressure metamor-phism in the crystalline basement, embraces ages from roughly 110 to 80 Ma. It is found in the Austroalpine crystalline basement and overprinted Paleozoic terrains and the southern parts of the Northern Calcareous Alps.

We present examples from units with
1. strong alteration in the central and eastern part of the Northern Calcareous Alps (e.g., Hochkönig, Mürzalpen unit), partly with CAI inversions,
2. locations with metamorphic blocks and slides in Upper Jurassic carbonate clastic radiolaritc flysch basins (e.g. Berchtesgaden area, Hallstatt area),
3. areas with backthrusting and imbrication of the CAI-zones with tectonic shortening (e.g. southern part of the Dachstein block) and
4. areas with medium CAI values with south to north and bottom to top decrease in temperature (e.g. Schneealpen unit, Salzburg and Berchtesgaden area).

In some parts of the Northern Calcareous Alps the CAI-zones are transsected by Miocene lateral tectonic extrusion.

The mapping of CAI zones is an important tool for the recon-struction of the paleogeographic and tectonic configuration in Upper Jurassic and Cretaceous times and helps to understand the present block puzzle of the Northern Calcareous Alps. For example, unknown tectonic boundaries can be localized and "classical" stratigraphic successions can be demonstrated as tectonically imbricated. The emplacement of southerly slides with CAI values of CAI 1.0 (e.g. Rettenstein, Hüpfliger slides) predates the youngest metamorphic overprint of the southern rim of the Northern Calcareous Alps, and should related to the Miocene lateral tectonic extrusion.