フィッション・トラック

ニュースレター

第28号

6-7

2015年

Fission-track dating applied for reconstruction of tectonic processes
in the Balkanides
Eleonora Balkanska*, Alexandre Kounov** and Ianko Gerdjikov** Department of Geology, Paleontology and Fossil Fuels, Soﬁa University St. Kliment Ohridski,
Bulgaria** Institute of Geology and Paleontology, Basel University, Switzerland

In the Balkanides region, on the territory of Central Stara Planina Mountain, Bulgaria,
several geological structures, since the Variscan orogeny are well preserved. They arerelated to Alpine (Early and Late) and Neotectonic tectonic stage and can bedistinguished on the basis of clear structural features in the field. One of the mostprominent of them is the Late Alpine Botev Vrah Thrust – thick-skinned compressional
structure that represents northward emplacement of pre-Permian crystalline basement
over various by age and lithology rocks.
Low-temperature fission-track (FT) thermochronology combined with detailed
structural studies have been applied in order to decipher the Alpine tectonic and thermal
evolution of Central Stara Planina Mountain. Seven samples for zircon and apatite FT
dating from both hanging wall and footwall of Botev Vrah Thrust were analyzed (Fig.
1). Three samples from the autochthon – Permian volcanics, Triassic sandstones andPaleozoic granitic rocks yield zircon FT ages of 121±6 Ma, 118±10 Ma and 90±7 Ma,
respectively. These ages are related to cooling and post-tectonic denudation after theEarly Alpine (Austrian) thrusting when the whole unit was at temperatures higher than ̴
250º C. Two detrital populations were statistically separated in the authochthonous
Paleocene sandstone sample that yield zircon FT ages of 61±5 Ma and 90±13 Ma. The
three samples from the allochthon of the Late Alpine thrust yield zircon FT ages
between 82±4 Ma and 54±3 Ma. These ages are younger than the Late Cretaceous
magmatic activity (at about 92–91 Ma) in the area where the hanging wall rocks of theBotev Vrah Thrust originated. This suggests that the area was at temperatures higher
than ̴ 250º C during this magmatic event. The tectonically lowermost sample ofPaleozoic granitic rocks yields the youngest zircon FT age of 54±3 Ma. It could berelated to the cooling induced by the exhumation during the early stages of Late Alpine
thrusting. This age is similar to the age of the younger zircon population from theauthochthonous Paleocene sandstones, as well as to the age of their deposition. Thus, it
is assumed that the basin was sourced by the advancing to the north allochthon of Botev
Vrah Thrust. The existence of a foreland basin is also confirmed by the analysis of6

continuous sections of the Upper Cretaceous-Paleocene deposits where data show that
deposition and deformation occurred at the same time.
The obtained apatite FT ages of both the hanging wall and footwall samples are
between 31±3 Ma and 23±3 Ma. They are all younger than the time of Botev Vrah
Thrust emplacement, suggesting that the whole area was at temperatures higher than120º C during the late Alpine thrusting. These ages are related to syn- to post orogenic
denudation and cooling. The ten measured confined tracks in the granitic sample fromthe hanging wall of the Miocene normal fault give mean track length value of 13.21±0.5µm that presumes rather slow, probably related to erosion, cooling at about 30 Ma. The
apatite FT age-altitude correlation show fast cooling at about 23 Ma for the samples
from the footwall. This is probably related to the Miocene faulting and exhumation.
The applied fission-track dating to the region of Central Balkanides, along withthe structural data obtained, demonstrate that Botev Vrah Thrust represents a brittle fault
structure formed in the upper crustal levels at temperatures 125–250º C. It is suggested
that initial thrusting occurred in Late Paleocene. Analysis of the tectonic and thermal
evolution of Balkanides from Early Alpine to Neotectonic stage has been performed.

Fig. 1. Geological map of the region studied with sample locations shown7