Eo-Alpine metamorphism and the ‘mid-Miocene thermal event’ in the Western Carpathians (Slovakia): New evidence from multiple thermochronology

A combination of zircon (U–Th)/He (ZHe), apatite fission track (AFT) and apatite (U–Th)/He (AHe) dating methods is applied to constrain the metamorphic and exhumation history of the Tatric part of the Branisko Mountains in the Western Carpathians. ZHe ages from the basement samples prove the basement experienced a very low-grade to low-grade Eo-Alpine metamorphic overprint in mid-Cretaceous times. Miocene AFT and AHe ages found in the basement and in the Palaeogene sediments conclusively demonstrate that the Branisko Mts experienced a ‘mid-Miocene thermal event’. This thermal event had a regional character and was related to magmatic and/or burial heating that exposed the sediment and basement samples to ~ 120–130°C and ~ 100–190°C, respectively

Geochronological evidence for the Alpine tectono-thermal evolution of the Veporic Unit (Western Carpathians, Slovakia)

Tectono-thermal evolution of the Veporic Unit was revealed by multiple geochronological methods, including 87Rb/86Sr on muscovite and biotite, zircon and apatite fission-track, and apatite (U-Th)/He analysis. Based on the new data, the following Alpine tectono-thermal stages can be distinguished: The Eo-Alpine Cretaceous nappe stacking (~135-95 Ma) resulted in burial of the Veporic Unit beneath the northward overthrusting Gemeric Unit and overlying Jurassic Meliata accretionary wedge. During this process the Veporic Unit reached metamorphic peak of greenschist- to amphibolite facies accompanied by orogen-parallel flow in its lower and middle crust. The subsequent evolution of this crust is associated with two distinct exhumation mechanisms related to collision with the northerly Tatric-Fatric basement. The first mechanism (~90-80 Ma) is associated with internal subhorizontal shortening of the Veporic Unit reflected by large-scale upright folding and heterogeneous exhumation of the Veporic lower crust in the cores of crustal-scale antiforms. This led to juxtaposition of the higher and lower grade parts of basement, all cooled down to ~350 °C by ~80 Ma. The second mechanism is associated with the overthrusting of the Veporic Unit over the attenuated Fatric crust. This led to a passive en-block exhumation of the Veporic crust from ~350 °C to 60 °C between ~80 and 55 Ma followed by erosion (~55-35 Ma). The erosion processes resulted in formation of planation surface before the Late Eocene transgression. After erosion and planation, a new sedimentary cycle of the Central Carpathian Palaeogene Basin was deposited with the sedimentary strata thickness of ~1.5-2.0 km (~21-17 Ma). The early to middle Miocene is characterised by destruction tectonic disintegration and erosion of this basin (~20-13 Ma) and formation of the Neogene Vepor Stratovolcano (~13 Ma). The final shaping of the area has been linked to erosional processes of the volcanic structure since the Late Sarmatian with accelerated processes during the Plio-Quaternary

Tracing metamorphism, exhumation and topographic evolution in orogenic belts by multiple thermochronology: a case study from the Nizke Tatry Mts., Western Carpathians

A combination of four thermochronometers [zircon fission track (ZFT), zircon (U–Th)/He (ZHe), apatite fission track (AFT) and apatite (U–Th–[Sm])/He (AHe) dating methods] applied to a valley to ridge transect is used to resolve the issues of metamorphic, exhumation and topographic evolution of the Nizke Tatry Mts. in the Western Carpathians. The ZFT ages of 132.1 ± 8.3, 155.1 ± 12.9, 146.8 ± 8.6 and 144.9 ± 11.0 Ma show that Variscan crystalline basement of the Nizke Tatry Mts. was heated to temperatures >210 °C during the Mesozoic and experienced a low-grade Alpine metamorphic overprint. ZHe and AFT ages, clustering at ~55–40 and ~45–40 Ma, respectively, revealed a rapid Eocene cooling event, documenting erosional and/or tectonic exhumation related to the collapse of the Carpathian orogenic wedge. This is the first evidence that exhumation of crystalline cores in the Western Carpathians took place in the Eocene and not in the Cretaceous as traditionally believed. Bimodal AFT length distributions, Early Miocene AHe ages and thermal modelling results suggest that the samples were heated to temperatures of ~55–90 °C during Oligocene–Miocene times. This thermal event may be related either to the Oligocene/Miocene sedimentary burial, or Miocene magmatic activity and increased heat flow. This finding supports the concept of thermal instability of the Carpathian crystalline bodies during the post-Eocene period