The collapse of the Balkan Mountains and the formation of the eastern part of the South Balkans Graben System: genetic constraints derived from a field kinematic study.
This report is based on a field research in the Bulgarian Balkans. This research has been done on account of a bachelor thesis done by students from the Free University of Amsterdam, the Netherlands.
The main focus was at first to indentify the normal faults and how to link them to the prior thrust phases.
It is known that the extension is linked to the orogenic collapse of the Balkan Mountains which was caused by a roll-back driven process of the African plate. But there is little known about the exact timing and the effect of this extensional phase. The last aim was to describe an inherited underlying nappe structure and his influence on the extensional phase.
These aims are reached by a structural fieldwork were kinematic information such as folds and faults and bedding-, cleavage planes were measured. The data was used to derive stereoplots, cross-sections and a structural map.
The faults in the area can be divided in three groups:normal faults, reverse faults and strike slip faults. All these groups have their own characteristics but they also have things in common. Most of the faults are dipping to the South, and all groups can be divided in faults with a large amount of displacement and faults with a small amount of displacement.
After characterising the faults, there was looked at the differences between the basins and the differences between the structural time-units. It can be seen that there are significant differences in fault orientation and dip angle.
The first known deformation phase this area underwent occurred during the Upper Permian-Lower Triassic and caused the formation of the back-arcs, which caused in turn the formation of half grabens. The normal faults were reversed to thrusts during the Upper Triassic compression phase. After that came the Late Alpine orogenesis during the Cretaceous, this caused large thrust sheets with top to 15°, combined with the formation of tear faults. The end of the Cretaceous underwent an extension phase, causing the formation of back-arcs. During the Eocene large low-angle thrust sheets developed with a transport direction to the North, about 350°. These nappes took the older basement, like Paleozoic granite, to the surface. The basins formed during Middle-Late Miocene extension as result of low angle normal faults dipping to the South, with a variation between Southwest and Southeast. These faults in fact are reactivated low angle thrusts.
The main focus was at first to indentify the normal faults and how to link them to the prior thrust phases.
It is known that the extension is linked to the orogenic collapse of the Balkan Mountains which was caused by a roll-back driven process of the African plate. But there is little known about the exact timing and the effect of this extensional phase. The last aim was to describe an inherited underlying nappe structure and his influence on the extensional phase.
These aims are reached by a structural fieldwork were kinematic information such as folds and faults and bedding-, cleavage planes were measured. The data was used to derive stereoplots, cross-sections and a structural map.
The faults in the area can be divided in three groups:normal faults, reverse faults and strike slip faults. All these groups have their own characteristics but they also have things in common. Most of the faults are dipping to the South, and all groups can be divided in faults with a large amount of displacement and faults with a small amount of displacement.
After characterising the faults, there was looked at the differences between the basins and the differences between the structural time-units. It can be seen that there are significant differences in fault orientation and dip angle.
The first known deformation phase this area underwent occurred during the Upper Permian-Lower Triassic and caused the formation of the back-arcs, which caused in turn the formation of half grabens. The normal faults were reversed to thrusts during the Upper Triassic compression phase. After that came the Late Alpine orogenesis during the Cretaceous, this caused large thrust sheets with top to 15°, combined with the formation of tear faults. The end of the Cretaceous underwent an extension phase, causing the formation of back-arcs. During the Eocene large low-angle thrust sheets developed with a transport direction to the North, about 350°. These nappes took the older basement, like Paleozoic granite, to the surface. The basins formed during Middle-Late Miocene extension as result of low angle normal faults dipping to the South, with a variation between Southwest and Southeast. These faults in fact are reactivated low angle thrusts.
The location of the basins
One of the cross-sections made of the area, showing the reactivation of thrust faults into normal faults
Structural map based on the resuls of the fieldwork
The aims were to derive thekinematic directions of deformation and its timing for the research area, andat second to derive the mechanism responsible for the extensional collapse ofthe Balkans and the formation of the graben system.
The large deformation phases:
1) The Caledonic orogenesis at the Upper Cambrian. This ductile phasecaused the intrusion of granites and metamorphism.
2) Upper Permian – Lower Triassic extension phase. This phase caused thedevelopment of back-arcs. And on its turn, these back-arcs caused the formationof half graben structures.
3) The Upper Triassic has some sedimentation,erosion and small compression phases, during this compression phase the normalfaults which earlier formed the grabens, now became thrusts.
4) The Late Alpine orogenesis at the Cretaceous. This compression phasecaused large thrust sheets with top to the North, about 15°. Due toirregularities in the subsurface, this thrusting is combined with the formationof tear faults.
5) The Upper Cretaceous extension phase. During this phase back-arcs wereformed as result of subduction mollases (roll back of the slab). After theformation of back-arcs there was a phase of volcanism which intruded thestratigraphy.
6) Eocene compression phase. Formation of nappes at which the older rockslike Srednagorie came to the surface. These nappesare thrusting to the North, about 350°. During this phase the syntectonicEocene sediments are also deposited.
7) Middle-Late Miocene extension. This extension is caused by largeroll-back. During this phase thrusts, strike-slip and back thrusts arereactivated. The nappes are reactivated through normal faults which cause theformation of the basins. The normal faults are dipping to the South, with avariation between Southwest and Southeast.
The extension is caused bynormal faulting combined with tear faults and transfer faults. The normalfaults exist on the Northern side of the basins and are low angle faults whichare influenced by the underlying reactivated low angle nappes. It is possiblethat most of the time at the basis the normal fault follows the thrust and justbelow the surface they spilt.
The tear faults andtransfer faults are caused by irregularities in the subsurface. They are linkedwith the shape of the basins. The transfer faults connect the basins and causethe elongated shape of the basins.