TSK 11 Göttingen 2006 Gröger et al.
 The timing of polyphase
 Miocene tectonics in North-
 ern Romania Vortrag
 Heike R. Gröger1 Matthias Tischler1
 Bernhard Fügenschuh2 Stefan M.
 Schmid1
 This study addresses the polyphase
 Miocene tectonic evolution in the
 Maramures area (northern Romania,
 Fig. 1) by combining field obser-
 vations, stratigraphic arguments and
 fission-track analysis (Tischler et al.
 in press). Fission-track analysis has
 been carried out on basement sam-
 ples from the Rodna horst, situated
 in the East Carpathians (Bucovinvian
 nappes). This area was affected by Cre-
 taceous medium- to low-grade metamor-
 phism, followed by post-collisional ex-
 humation and renewed moderate ther-
 mal overprint due to the deposition of
 Eocene to Early Miocene sediments.
 Based on paleostress analyses of meso-
 scale structures, three main tectonic
 phases can be disdinguished in the
 study area, all of which are post-
 date the earliest Miocene (Aquitanian,
 20.5Ma, Fig. 2). In late Early Miocene
 (Burdigalian) the Pienide nappes, non-
 metamorphic flysch series, were em-
 placed onto the Paleogene to Early
 Miocene sedimentary cover of the Bu-
 covinian nappes. Emplacement of the
 Pienides is consistently SE-directed and
 dominated by brittle thrusting while
 folding is of subordinate importance
 only. The curved map appearance of the
 thrust contact is due to lateral ramps
 and tear-off faults, accentuated by later
 1 Geologisch Paläontologisches Institut, Uni-
 versität Basel, Bernoullistrasse 32, 4056 Basel,
 Switzerland 2 Institut für Geologie und
 Paläontologie, Universität Innsbruck, Innrain
 52, Bruno Sander Haus, 6020 Innsbruck,
 Austria
 folding. This early Miocene thrust con-
 tact was sinistrally offset later on along
 the Bogdan-Voda fault (Fig. 1).
 Activity along the Bogdan-Voda fault
 started in Middle Miocene times as a
 sinistral transpressive fault related to
 the formation of open NW-SE trend-
 ing folds in the sedimentary units. In
 a next stage (late Middle Miocene to
 Late Miocene) kinematics along the co-
 evally active Bogdan-Voda fault and its
 eastern prolongation, the Dragos-Voda
 fault, formed one through going fault,
 characterised by sinistral transtension.
 Towards the east the left lateral offset
 along the main fault is continuously re-
 duced by coevally active SW-NE trend-
 ing normal faults (e.g. Greben fault,
 Fig. 1), thus terminating the Dragos-
 Voda fault in an extensional horsetail
 splay. The main activity along the
 Bogdan-Voda fault ends at about 10Ma,
 as is indicated by almost unaffected
 Neogene volcanics. In the western part
 of the study area, along the Bogdan-
 Voda fault, sedimentary and volcanic
 rocks provide timing constraints for the
 deformation. In the Eastern part of
 the study area, along the Dragos Voda
 fault, only basement units and Pale-
 ogene sediments crop out. Therefore
 the fault activity is indirectly dated
 and inferred from the cooling history of
 the syn-kinematically exhumed Rodna
 horst. A vertical profile covering about
 1000m of altitude difference has been
 sampled for apatite fission-track analy-
 sis. Burial by Eocene to Oligocene sed-
 iments led to full annealing of apatite
 in the Rodna Horst, and all samples
 yielded Middle to Late Miocene cool-
 ing ages (8.6–11.6Ma, see also Sanders
 1998, Fig. 3). The altitude vs. age re-
 lationship indicates enhanced exhuma-
 tion between 12–11Ma with exhuma-
 1
Gröger et al. TSK 11 Göttingen 2006
 Figure 1: Tectonic map of the study area.
 tion rates of at least 1mm−1, while a
 fast decrease in the rate of exhumation
 to around 0.1mma−1 is observed after
 10Ma (Fig. 3).
 Our fission-track data suggest enhanced
 exhumation of the Rodna horst be-
 tween 12–10Ma, i.e. during the strati-
 gaphically dated transtensional sinistral
 strike-slip phase.
 Literatur
 Sanders C (1998) Tectonics and erosion: com-
 petitive forces in a compressive orogen. A fis-
 sion track study of the Romanian Carpathi-
 ans. Ph.D. thesis, Free University of Ams-
 terdam, 204 p.
 Tischler M, Gröger HR, Fügenschuh B &
 Schmid SM (in press) Miocene tectonics in
 the Maramures area (Northern Romania)-
 implications for the Mid-Hungarian fault
 zone. Journal of international Earth Sci-
 ences.
 2
TSK 11 Göttingen 2006 Gröger et al.
 Figure 2: The three main Miocene tectonic
 phases observed in the study area.
 Figure 3: Apatite fission track data from
 the Rodna horst (encircled datum point
 from Sanders 1998) together with altitude
 vs. age diagram.
 3