Quantitative structural analyses and numerical modelling of ...

ETG 6 - 8 SCHULMANN ET AL.: STRAIN DISTRIBUTIONFigure 5. Vertical elevation rate for transpression expressed in terms of angle of convergence (a) andtime parameter (k t ) for different base depths, RFD = 40, 70 km, and different original sample depths z 0 =30, 60 km. The curves show elevation achieved by these samples in a given time. For example, in Figure5a (RFD = 40 km, z 0 = 30 km) for convergence angle a =80° sample is elevated to depth 10 km after k t =1.2. For the case of R vd = 0.1 the time of elevation corresponds to 12 Myr.deformation, where the rest of lateral displacement and thewhole across strike shortening are accommodated, can bededuced using our hypothetical strain map (Figure 6). Toevaluate the influence of partitioning on strain parameters,recalculation of values R vd and a are made using theequations (B1) and (B2) (in Appendix B) or they can betaken from Figure 7. The effect of discrete partitioning onfinite strain parameters is expressed by a virtual increase ofconvergence angle and decrease of R vd (decrease of plateconvergence velocity or increase of weak zone width). Thusfor a transpression zone with an angle of convergence of45°, R vd equal 0.1, and with 50% of lateral displacementconsumed by discrete faults, we use values of a 0 = 63.43°0and R vd = 0.079 (Figure 7). For 10 Myr of convergencewithout discrete partitioning, finite strain parameters are D =1.64 and K = 0.58. When discrete partitioning accommodates50% of the lateral displacement, the finite strainparameters are D = 0.9 and K = 0.8.[35] Ductile partitioning splits the deformed domain intoa pure shear zone (PSZ) and a wrench-dominated zone(WDZ). We assume that the pure shear-across-strike shorteningand elevation are homogeneously distributed acrossthe whole system, while simple shear-lateral displacement isaccommodated only in the WDZ. We examine developmentof strain parameters for different widths of the WDZ,expressed as ratio p 2 of the width of WDZ and the widthof the whole transpressional zone. Such a partitioning ofpure shear and simple shear within transpressional zones isresponsible for decomposition of the velocity gradienttensor into two separate tensors according to equations(C1) and (C2) (in Appendix C).[36] While the evolution of strain parameters in the PSZmay be obvious, the evolution of strain parameters in theWDZ zones is less so. The results of calculations for WDZzones of different width are shown in Figure 8. Fifty percentof ductile partitioning (Figure 8a) exhibits a similar patternof strain parameters as a nonpartitioned system. Nevertheless,the domain with horizontal lineation and the domainof oblate symmetry are both slightly enlarged. This isrelated to the shift of the lineation switch, which occurs76