Quantitative structural analyses and numerical modelling of ...

5 - 10 LEXA ET AL.: COLLISION IN WEST CARPATHIANSFigure 6. Schematic block diagrams showing tectonic evolution of the studied area. (a) Development ofthe Gemer Cleavage Fan due to indentation of sub-Gemer basement. (b) Development of the Trans-Gemer Shear Zone and Eastern Gemer Thrust Zone resulting from interaction between the Veporpromontories with sub-Gemer basement.foliation of the EGT system are dipping to the SW, bear anintense stretching lineation plunging to the SW and show atop-to-the-NE sense of shearing.4. Tectonic Model of Cretaceous Collision[30] The structural pattern and succession of deformationstructures described above allow modeling of the tectonicevolution of SW Carpathians and development of polyphasecleavage patterns. Our interpretation is based on mutualstrength relationships between individual units at the onsetof Cretaceous convergence. Cretaceous tectonics of theWest Carpathians is characterized by north vergent collisionof a southern continent with the northerly lying WestCarpathian domain (European plate). Fragments of thesouthern continental domain, now located in northern Hungary(Bükk mountains), are considered to be Neo-Proterozoicin age [Pantó et al., 1967]. Here, the absence ofVariscan overprint is manifested by continuous sedimentationfrom the Early to Late Paleozoic. We suggest that thissouthern continent behaved as a rigid indenter controllingthe deformation of all northerly foreland crustal units, and inour coordinate system was actively moving toward thenorth. The mechanical contrast between the Vepor basementpromontories and the Gemer slates results from their contrastinglithologies and pre-Cretaceous evolution. Theinterval of thermal relaxation of the Vepor quartzofeldspathiccrust between the last (Late Carboniferous) importantthermal perturbation and Cretaceous collisioncorresponds to about 180 Ma. This indicates, that thegeotherm of the Vepor crust was equilibrated at the onsetof Cretaceous orogeny [Cloetingh and Burov, 1996; Morganand Ramberg, 1987]. Therefore we suggest that the VariscanVepor basement, composed of gneisses and granites, representeda mechanically strong promontory of irregular shape.In contrast, the Gemer Unit is represented mainly by lowgradeslates composed of fine-grained hydrous mineralswith rheology controlled by diffusion type of deformationmechanisms as pressure solution and diffusive mass transfer[Knipe, 1979, 1989]. For the same geotherm, as comparedwith laterally adjacent quartzofeldspathic rocks, the strengthof Gemer slates was incomparably lower. Taking intoaccount these rheological assumptions, the Gemer Unitduring the Cretaceous event is considered to be the weakestdomain accommodating most of the viscous deformation.[31] In agreement with Woodcock et al. [1988] andSintubin [1999], the cleavage patterns in deformable weakrocks reflect the geometry and direction of movement ofrigid blocks. In order to model the development of superposedcleavage pattern described above, it is important todefine boundary conditions.4.1. Definition of Kinematic Frame[32] The asymmetry of the GCF can be interpreted as aresult of movement of rigid indenting block to the north andback thrusting of metasediments over its northern margin.The rigid basement does not crop out, but it can be traced indeep seismic lines 2T and G1 [Tomek, 1993; Vozár andŠantavý, 1999; Vozár et al., 1996]. The seismic profilingshows that the Gemer Unit is about 5 km thick sheet-likebody resting on a basement of unknown age. This majorlithological boundary is represented by series of stronghorizontal reflectors. The most spectacular structure inseismic line G1 [Vozár and Šantavý, 1999; Vozár et al.,1996] is a highly reflective south dipping zone along whichthe horizontal base of the Gemer Unit is displaced to thenorth. This zone is interpreted as the major Sub-Gemerthrust fault responsible for northward thrusting of rigidbasement over weak sediments resulting in the developmentof GCF (Figure 6).[33] Important question in our model is the displacementof Vepor basement rocks with respect to the Gemer Unit. Itis well known that the whole central and southern Carpathiandomain was actively moving to the north (in recentcoordinates) as documented by Cretaceous progressiveclosure of Mesozoic Fatric basinal domain north of theVepor basement [Plašienka, 1997]. In this kinematic frameall units are shifted to the north but only differential move-94