Quantitative structural analyses and numerical modelling of ...

LEXA ET AL.: COLLISION IN WEST CARPATHIANS 5 - 13the Vepor promontories, whereas in the central domain theincrease of K parameter up to 0.5 occurs close to the indentermargin (Figure 7d). Pure shear is dominating along the wholedeformed area except small part southeast of the indenter,where subhorizontal stretching starts to develop. In this area,we can examine angle f between XY planes of instantaneousand finite strain ellipsoid. Here, the angle f exceeds 25°, butwith progressive northward movement of the indenter, thearea of high angle f enlarges. According to Tikoff and Teyssire[1994] and Ježek et al. [2002], high angle f is critical for thedevelopment of discrete partitioning and subsequent slipalong highly inclined surfaces. In this concept, the instantaneousstretching tensor has similar significance as does astress tensor for the development of faulting and reactivationof preexisting anisotropies [Tommasi and Vauchez, 2001].Therefore the sufficiently high angle between finite stainanisotropy and instantaneous maximum stretching axis maygenerate high resolved shear stress on preexisting surfacesand reactivate these planes as strike-slip faults. In our model,the discrete partitioning starts to operate after 3 Myr leading tocontinuous change in the indenter movement direction. Weshow the pattern developed after 7 Myr, where the observedtendencies are fully developed (Figure 8b). The central part ofweak domain shows exponential strain intensity decreasefrom D = 2.5 to approximately D = 0.5 in the most remotearea in the north (Figure 8a). Our highest calculated strainintensities correspond to maximum shortening of 70%,which is in agreement with values estimated for stronglydeveloped slaty cleavage [Siddans, 1972]. Therefore wesuggest that the zones of highest modeled strain intensitiescorrespond to the domains in the field with most intenselydeveloped cleavage mapped as stages 4 and 5 of Bell andRubenach [1983]. Similarly, modeled low strain intensities ofD = 0.5 (25% shortening) may be compared to heterogeneouslydeveloped cleavage stages 1 and 2 of Bell andRubenach [1983] in the field. Modeled strain symmetriesreach values of K = 0.5, which roughly correspond tomeasured values of Németh et al. [1997]. The area betweenthe indenter and western promontory forms now a narrowchannel in which the strain intensity is significantly higherand strain symmetry more oblate. Moreover, the X axis offinite strain becomes horizontal and starts to be controlled bysimple shear deformation. At that time, following the prescribedtrajectory (Figure 7a), the indenter bulk translationvector becomes parallel to the southern margin of the westernVepor promontory (Figure 8a). This change in indentermovement strongly affects the style of deformation in theeastern part of deformable domain. In this area, an intensestrain develops up to D = 2, the foliation trajectories beingparallel to the eastern promontory margin. The strainsymmetry remains oblate with K parameter close to zero(Figure 8c).5. Discussion5.1. Validation of Numerical ModelWith Respect to Field Data[39] The numerical model of thin viscous sheet deformationgenerated by an indenter of oval shape simulates thedevelopment of deformation pattern characteristic of GCF.We note that strain intensities decrease exponentially fromthe margin of indenting block, which is in agreement withcleavage pattern observed in the field.[40] The development of discrete partitioning betweenwestern Vepor promontory and indenting block agrees wellwith the observed secondary cleavage associated withTGSZ. We suggest that the TGSZ accommodates thechange in bulk indenter translation from northward tonortheastward during the deformation. The transcurrentmovement along the western promontory is responsiblefor propagation of TGSZ into interior of the Gemer Unit.This leads to separation of GCF into northern domain withpreserved northward movement related structures, whilethe southern domain becomes frontally convergent withthe eastern promontory. This process is responsible for thedevelopment of EGT.[41] However, the presented model has serious limitations.We are not able to simulate the deformation of thoseparts of the viscous sheet, which were thrust over rigidpromontories. This particularly concerns extensional strippingof the Gemer Unit from northeastern part of thewestern Vepor promontory. The noncoaxial extensionaldeformation in this area is most likely related to the activityof TGSZ and corresponds to pulling the allochthonousGemer Unit associated with sinistral shearing along thismajor shear zone. The model is unable to demonstrate theeffects of strain localization associated with discrete partitioning.In fact, the TGSZ is passively translating southernpart of the Gemer Unit without significant internal deformation.Similarly, the development of EGT appears to be amore localized feature than is shown in our model, andleads also to passive thrusting of the Gemer Unit over theeastern Vepor promontory.[42] Despite of these limitations, the presented modelallows to predict the strain pattern in front of indenting platein an area with complex boundary conditions. Our model isintended to quantify the cleavage patterns developed due tothe movement of rigid blocks as suggested by Woodcock et al.[1988], Sintubin [1999], and others. The basis of our modelingis the assumption that the cleavage represents the XYplaneof finite strain ellipsoid [Cloos, 1947; Sorby, 1853; Wood,1974]. Our model works with deformation of originallyisotropic medium and does not take into account problemsof existing internal anisotropy [Cobbold et al., 1971]. However,the major advantage of our approach is the interconnectionof complex kinematic frame with finite strain pattern,which was so far possible only for extremely simple boundarycondition models, e.g., simple shear, transpression, etc. Inaddition, the model explains the polyphase cleavagepatterns in terms of the complex shape of promontories andchanges in movements of indenting blocks. Moreover, usingthe regional mapping of cleavage patterns, we are now able todistinguish actively moving blocks from stationary rigidpromontories.[43] We are aware that infinite numbers of boundaryconditions exist, which may generate different strain distributionand superposition of structures. Therefore we deliberatelyselected the set of boundary conditions, which satisfy97