Construction of the Ukrainian Carpathian Wedge from low-temperature thermochronology and tectono-stratigraphic analysis

Abstract. The evolution of orogenic wedges can be determined through stratigraphic and thermochronological analysis. We used apatite fission-track (AFT) and apatite and zircon (U-Th-Sm)/He (AHe and ZHe) low-temperature thermochronology to assess the thermal evolution of the Ukrainian Carpathians, a prime example of an orogenic wedge forming in a retreating subduction zone setting. Whereas most of our AHe ages are reset by burial heating, eight out of ten of our AFT ages are partially reset, and all ZHe ages are non-reset. We inverse-modelled our thermochronology data to determine the time-temperature paths of six out of the 8 nappes composing the wedge. The models were integrated with burial diagrams derived from the stratigraphy of the individual nappes, which allowed us to distinguish sedimentary from tectonic burial. This analysis reveals that accretion of successive nappes and their subsequent exhumation mostly occurred sequentially, with an apparent exhumation rate increase towards the external nappes. Following a phase of tectonic burial, the nappes were generally exhumed when a new nappe was accreted, whereas, in one case, duplexing resulted in prolonged burial. An early orogenic wedge formed with the accretion of the innermost nappe at 34 Ma, leading to an increase in sediment supply to the remnant basin. Most of the other nappes were accreted between 28–18 Ma. Modelled exhumation of the outermost nappe started at 12 Ma, and was accompanied by out-of-sequence thrusting. The latter was linked to emplacement of the wedge onto the European platform and consequent slab detachment. The distribution of thermochronological ages across the wedge, showing non-reset ages in both the inner and outer part of the belt, suggests that the wedge was unable to reach dynamic equilibrium for a period long enough to fully reset all thermochronometers. Non-reset ZHe ages indicate that sediments in the inner part of the Carpathian embayment were mostly supplied by the Inner Carpathians, while sediments in the outer part of the basin were derived mostly from the Trans-European suture zone or the European margin. Our results suggest that during the accretionary phase, few sediments were recycled from the wedge to the foredeep. Most of the sediments exhumed from the Ukrainian Carpathian wedge were likely transported directly to the present pro- and retro- foreland basins.