Preservation and thermal alteration of organic matter in the Ortles and Quattervals nappes (upper Austroalpine, north-eastern Lombardy, Italy): Preliminary results and implications for regional geology

About 150 samples collected in the Triassic sedimentary successions of the Ortles and Quattervals Nappes (Central Austroalpine) have been processed in order to evaluate and compare the thermal history of the two tectonic units. The study of the organic matter preservation based on the optical comparison of the palynofacies components colour (TAI) demonstrated that the Quattervals Nappe (occupying a tectonically higher position) had been subjected to higher diagenetic-metamorphic temperatures (estimated above 300 degrees C) with respect to the Ortles Nappe (few tens of degrees lower), Heating is mainly ascribed to tectonic burial related to higher tectonic units eroded or tectonically displaced after the thermal peak and before the development of the present-day nappe edifice: it occurred before the emplacement of the present thrust nappes which was not followed by a new temperature rise able to overprint previously recorded temperatures. Heating and thrust development of the succession both occurred during the eo-alpine orogenetic stages (post-Turonian). The presence, in the study area, of a previously documented increasing metamorphic gradient from west to east fits with the hypothesis of a southeastern provenance of the higher tectonic unit (Quattervals Nappe) with respect to the lower one, as earlier suggested by different authors.