Quantitative structural analyses and numerical modelling of ...

5 - 12 LEXA ET AL.: COLLISION IN WEST CARPATHIANSFigure 8. Finite strain pattern developed in weak zone after 7 Myr of shortening. (a) Distribution ofstrain intensity expressed in D value and orientation pattern of XY planes and X axes of finite strainellipsoid. (b) Distribution of angles between instantaneous and finite XY planes expressing the degree ofpossible simple shear reactivation of existing finite anisotropy. (c) Distribution of finite strain symmetryexpressed in K value. (d) Distribution of finite topography in meters after 7 Myr of shortening in front ofthe indenter.of material). The vertical strain rate and velocity are relatedto the horizontal velocity field by the incompressibilityequation. Ježek et al. [2002] have described the thin sheetmodel as sensitive to the angle of collision and maybeproducing a zone dominated by lateral simple shear close tothe indenter and a zone of dominant pure shear farther awayfrom the indenting boundary. In addition, we show thatthese general features can strongly interfere with finitedimension of the modeled area and imposed boundaryconditions.4.2.1. Definition of Domain Geometryand Boundary Conditions[36] As the Vepor promontories were considered to havebeen kinematically fixed throughout the whole deformationalhistory, the zero velocity (Dirichlet) boundary conditionwas applied to their boundaries (Figure 7a), so that theirgeometry corresponds to the Vepor basement recent shape.The northern boundary of our model connecting eastern andwestern promontories is characterized by a zero velocitygradient (Neumann) boundary condition allowing free outflowof material to the north. This is in agreement with theabsence of a deformation gradient in this area. Similarboundary conditions are applied to the rest of examineddomain boundaries since the extension of all geologicalunits in these particular areas is unknown, being covered byTertiary sediments. The southern edge of the model representsthe actively moving rigid body of oval shape followingprescribed trajectory with constant velocity of 1 cm/yr(Figure 7a). This velocity of plate movement is deliberatelychosen to demonstrate the principal tendencies in finitestrain patterns.4.2.2. Results of Numerical Modeling[37] The main results of our modeling are presented in aseries of three time steps equal to 1, 3 and 7 Myr. Wepresent maps of strain intensities, orientations of XY planesof finite strain and orientations of X strain axes (Figures 7b,7c, and 8a). Already after 1 Myr we can observe thedevelopment of arcuate pattern of XY trajectories aroundthe rigid indenter (Figure 7b). Another feature is thedecrease in strain intensity from the south (D =0.5)tothenorth, where the strain intensity is negligible. The strainintensity increases in the western part of the model close tothe western promontory. The strain symmetry is of planestrain type in the deformed area irrespective to the strainintensity. The X axis of finite strain is vertical in the entiredomain indicating predominant pure shear deformationregime.[38] After 3 Myr we can observe several domains withcontrasting strain parameters (Figures 7c and 7d). The centraldomain in front of the indenter shows exponential decrease instrain intensity from indenter margin (D = 1) to the north(D = 0.1). The strain gradient is poorly defined toward easternpromontory and the XY planes tend to be parallel with thepromontory margin. In the west, the strain intensity is highestacross the whole width of the shortened domain and thetrajectories of XY planes are fully parallel with the westernVepor promontory margin. The strain symmetry shows dominantflattening, K close to zero, along wide zones parallel to96