The evolution of a thrust belt within a continental indenter: investigating the internal deformation of the Dolomites Indenter, eastern Southern Alps, in a combined low-temperature thermochronology, field and analogue modelling study

The Dolomites Indenter represents the eastern front segment of the Neogene to ongoing N(W)-directed continental indentation of Adria into Europe. Concomitant shortening is accommodated within a south-vergent thrust belt, situated between the Periadriatic Fault system and the South-Alpine front. A combination of low temperature thermochronological analyses, focussed mapping and analogue modelling is used for unravelling the Neoalpine history of the crustal and lithospheric scale tectonic processes during indentation.

In our analogue models, extensional platform-basin geometries, formed at passive continental margins, are subject to subsequent shortening and orogenesis. Parallel to oblique (10 to 20 degrees with respect to the basin axes) contraction has been applied leading to the inversion of the pre-orogenic basins. The experiments show that the simple presence of an inherited platform-basin configuration controls the overall style of compressional deformation, no matter of including frictional or viscous basal décollements, of varying the rheology of the basin fill, or of changing platform-basin thickness ratios. Orientations of thrust faults change laterally across inherited platform-basin transitions throughout all experiments. New fault slip data and shortening directions from fold axes along the western segment of the Belluno thrust of the Valsugana fault system support variations of thrust fault orientation and a lateral change in shortening direction (from SSW to SSE along strike) along one single fault. Based on our modelling results, we infer that this variability of shortening directions depends on inherited structures and do not necessarily reflect different deformation phases.

Our low-temperature thermochronological dataset (zircon and apatite (U-Th)/He, and apatite fission track analyses) focuses on the Dolomites Indenter and spans from the Periadriatic Fault System (Pustertal-Gailtal fault) in the north to the footwall of the Bassano thrust in the south. The results argue against an only in-sequence fault activity within the dominantly WSW – ENE striking thrust belt but indicate a more complex fault system development, including backstepping fault activity. This is locally supported by field observations of fault cross-cutting relationships (e.g., at the Moschesin fault near Agordo). Remarkable that west of the Transalp Corridor Mesozoic apatite fission track ages are preserved within fault delimited areas while further to east all apatite fission track data show Cenozoic ages indicating younger exhumation in the eastern part of the study area.