The Alpine cooling history of the western Dolomites Indenter, European Southern Alps

The NW to N directed indentation of the Adriatic microplate into the European lithospheric domain, initiated in the upper Eocene following the closure of the Piemont-Liguria and Valais oceanic basins, constitutes a key feature of the Neoalpine orogenesis. The separation of the eastern Southern Alps (Dolomites Indenter) along the Giudicarie fault system from the late Oligocene (Middle Miocene at the latest) on and its increased northward push contributes significantly to major tectonic processes in the Eastern Alps north of the Dolomites Indenter: updoming, piggy-back top-N thrusting, and eastward lateral escape of the Tauern Window.

The interior of the Dolomites Indenter undergoes deformation as well, as documented, e.g., by the prominent, dominantly SSE-vergent fold and thrust belt of the Dolomites, as well as the top-WSW directed thrusts of the Dinaric chain and associated flysch sedimentation. New and compiled Apatite (U-Th)/He (AHe) and Fission Track (AFT) data allow the tracing of the exhumation history.

AFT data from the western Dolomites Indenter tend to cluster within consistent Dinaric and Neoalpine distinguishable tectonic blocks. However, the data are quite scattered. AHe data primarily indicate exhumation during the post-15 Ma Valsugana phase, showing a tendency of getting younger towards the east. A subordinate number of AHe datapoints document Eocene to Oligocene cooling as well.

Regional age-elevation profiles of consistent fault-delimited blocks exhibit (i) moderate cooling during the Mesoalpine Penninic subduction, (ii) fast Dinaric exhumation (in the Plose area), and (iii) fast Valsugana phase exhumation starting at approximately 15 Ma; Notably, this exhumation pulse starts earlier (Chattian/Aquitanian) in the northernmost tectonic block at the Indenter tip.

Time-temperature path modelling confirms the Valsugana phase as the most significant period of tectonic exhumation within the western Dolomites Indenter. According to the modeling, prior to this phase, a significant number of samples remained within the AFT annealing zone for an extended period of time, at least from Ladinian times onwards. This is due to a wide dispersion of single grain ages and suggests, the data does not necessarily represent a tectonic pulse. Moreover, many samples from sedimentary rocks of the Permian and Lower Triassic periods show a complete reset of the AFT system during the Middle Triassic, well before the maximum burial indicated by the stratigraphic record. This high-temperature anomaly could be attributed to the extensive Ladinian volcanism in the study area.

Based on the new thermochronological data, it can be inferred that the Middle Miocene Valsugana phase is the most significant exhumation phase in the Dolomites Indenter. Additionally, this phase begins earlier in the north than in the south. It is essential to consider the complex thermal history of the Dolomites Indenter and the possible long residence time of samples within the partial annealing zone prior to the Neoalpine exhumation when interpreting new data.