Fission-track constraints on the thermotectonic evolution of the Apuseni Mountains (Romania)

The Apuseni Mountains, located inside of the Carpathian arc and bounding the Transylvanian basin to the west, constitutes the largest outcropping part of the Tisza block. This crustal fragment consists of a stack of several nappe sequences formed in response to continental collision, which followed the closure of the Neotethys Ocean. The northwestern part of the Apuseni Mountains represents a coherent nappe sequence consisting of the Bihor and Codru nappe systems. The tectonically highest Biharia nappe system, previously considered as part of Tisza plate, is attributed to the Dacia Mega-Unit. The assignment of the Biharia nappes to Dacia is made on the basis of the fact that both structurally underlie the obducted remnants of the Neotethys (Transylvanian Ophiolitic unit) leading to the assumption that an oceanic suture needs to be placed between Biharia nappe system and the Tisza block.
   The main high-grade metamorphic event in the basement rocks, outcropping in the Apuseni Mountains, is of pre-Mesozoic age because the metamorphics are covered by Triassic or Jurassic non-metamorphic sediments. The first Alpine tectonic event was probably related to the obduction of the Eastern Vardar Ophiolitic unit (Transylvanides) onto parts of the Dacia Mega-Unit (Biharia) in the latest Jurassic. This was followed by late early-Cretaceous final closure of the Neotethys remnants and the collision between Tisza and Dacia blocks producing top-E nappe stacking. The final emplacement of the nappes in the Apuseni Mountains involving top-W to NW superposition of the Biharia, Codru and Bihor nappe systems did not occur before Turonian time as documented by the late Turonian “Gosau” unconformity. Subsequent compressional deformations in the area are reported for the end of the Cretaceous and the Eocene.
   The Jurassic volcanics of the Transylvanides and their sedimentary cover, as well as the underlying Baia de Aries nappe (the highest structural unit of the Biharia nappe system) exhibit late early-Cretaceous zircon fission-track (FT) ages (Aptian and Albian, 120-103 Ma). The more westerly and structurally lower units (Biharia nappe of the Biharia nappe system, Codru and Bihor nappe systems), however, exhibit Late Cretaceous (Turonian to Campanian, 95-71Ma) zircon FT ages. The late early-Cretaceous zircon FT ages from the Baia de Aries nappe, together with the Jurassic ophiolites and their sedimentary cover, suggest that these rocks must have been buried to a minimum of 8 km during this time. Such associated temperatures have probably been attained during underthrusting of these units below the Tisza megatectonic unit (thrusting being top-east). The ages obtained from the Bihor, Codru and Biharia nappes (Turonian to Campanian, 95-71Ma) correspond to the age of the late Cretaceous top-NW event that led to the present-day nappe stack in the Apuseni Mountains. The internal parts of the Baia de Aries nappe and the overlying Transylvanides were not reheated during this second event since they occupied the highest tectonic position.
   Zircon FT age distribution, combined with thermal modelling of the apatite FT data, show that rapid post-tectonic cooling of the area during the late Cretaceous was followed by relatively slow cooling during the early Paleogene.