Fission-track constraints on the thermal and tectonic evolution of the Apuseni Mountains (Romania)

New zircon and apatite fission-track (FT) data, including apatite thermal modelling, are combined with an extensive literature survey and reconnaissance-type structural fieldwork in the Eastern Apuseni Mountains. This leads to a better understanding of the complex structural and thermal history of a key area at the boundary between two megatectonic units in the Balkan peninsula, namely the Tisza and Dacia Mega-Units. Following Late Jurassic obduction of the Transylvanian ophiolites onto a part of the Dacia Mega-Unit, that is, the Biharia nappe system, both units were buried to a minimum of 8km during late Early Cretaceous times when these units were underthrust below the Tisza Mega-Unit consisting of the present-day Codru and Bihor nappe systems. Tisza formed the upper plate during Early Cretaceous (‘Austrian') east-facing orogeny. Turonian to Campanian zircon FT cooling ages (95-71Ma) from the Bihor and Codru nappe systems and the Biharia and Baia de Arieş nappes (at present the structurally lowest part of the Dacia Mega-Unit) record exhumation that immediately followed a second Cretaceous-age (i.e. Turonian) orogenic event. Thrusting during this overprinting event was NW-facing and led to the overall geometry of the present-day nappe stack in the Apuseni Mountains. Zircon FT ages, combined with thermal modelling of the apatite FT data, show relatively rapid post-tectonic cooling induced by a third shortening pulse during the latest Cretaceous (‘Laramian' phase), followed by slower cooling across the 120°-60°C temperature interval during latest Cretaceous to earliest Paleogene times (75-60Ma). Cenozoic-age slow cooling (60-40Ma) was probably related to erosional denudation postdating ‘Laramian' large-scale updomin

Upper Cretaceous Gosau deposits of the Apuseni Mountains (Romania) - similarities and differences to the Eastern Alps

The Apuseni Mountains were formed during Late Cretaceous convergence between the Tisia and the Dacia microplates as part of the Alpine orogen. The mountain range comprises a sedimentary succession similar to the Gosau Group of the Eastern Alps. This work focuses on the sedimentological and geodynamic evolution of the Gosau basin of the Apuseni Mts. and attempts a direct comparison to the relatively well studied Gosau Group deposits of the Eastern Alps. By analyzing the Upper Cretaceous Gosau sediments and the surrounding geological units, we were able to add critical evidence for reconstructing the Late Mesozoic to Paleogene geodynamic evolution of the Apuseni Mountains. Nannoplankton investigations show that Gosau sedimentation started diachronously after Late Turonian times. The burial history indicates low subsidence rates during deposition of the terrestrial and shallow marine Lower Gosau Subgroup and increased subsidence rates during the period of deep marine Upper Gosau Subgroup sedimentation.The Gosau Group of the Apuseni Mountains was deposited in a forearc basin supplied with sedimentary material from an obducted forearc region and the crystalline hinterland, as reflected by heavy mineral and paleocurrent analysis. Zircon fission track age populations show no fluctuation of exhumation rates in the surrounding geological units, which served as source areas for the detrital material, whereas increased exhumation at the K/Pg boundary can be proven by thermal modeling on apatite fission track data. Synchronously to the Gosau sedimentation, deep marine turbidites were deposited in the deep-sea trench basin formed by the subduction of the Transylvanian Ocean. The similarities to the Gosau occurrences of the Eastern Alps lead to direct correlation with the Alpine paleogeographic evolution and to the assumption that a continuous ocean basin (South Penninic - Transylvanian Ocean Basin) was consumed until Late Cretaceous times