Quantitative structural analyses and numerical modelling of ...

SCHULMANN ET AL.: STRAIN DISTRIBUTION ETG 6 - 11Figure 8. Superposition of strain intensity (D) and strain symmetry (K ) in terms of angle ofconvergence (a) for different widths of wrench-dominated zone: (a) 50%, (b) 20%, (c) 10%, and (d) 5%.For 100%, see Figure 6. These diagrams may correspond to different depth levels through thetranspressional zone with increasing width of wrench-dominated zone with depth in agreement with themodel of Pinet and Cobbold [1992].the rock (vertical for a >20°) we can conclude that in theframework of our model the maximum initial width ofancient transpressional zones should not exceed doubletheir actual width. Transpressive zones with horizontallineation corresponding to highly oblique convergence(low a) would have been 1.4 times wider originally. Thereforethe maximum initial width of ancient transpressionalzones could vary from 20 to 100 km. This width isapparently less than those of recently active transpressionalsystems.[43] Provided the average range of plate velocities ofcontinental convergence did not change through geologicalhistory, from upper Proterozoic to Recent, we can estimatethe range of lifetimes of ancient transpressional zones. Asexamples of highly obliquely convergent transpressionalzones with large amounts of finite strain data, we canconsider the Central Bohemian Shear Zone (horizontallineation, K =0.8 1, vertical foliation, D =0.5 1.5[Rajlich et al., 1988]), and the transpressional zone developedin the eastern Variscan Culm Basin (horizontal lineation,K = 0.6–1.2, vertical foliation, D = 0.4 1.5[Rajlich, 1990]). These transpressional zones are associatedwith homogeneous ductile deformation over a width of upto 15 km. Because these transpressional zones are associatedwith low- to very low grade metamorphism, weconsider also an example of finite strain analysis from ahighly oblique transpressional zone developed at high-grademetamorphic conditions (kyanite-sillimanite zone) in theEastern Bohemian Massif which show horizontal lineationwith strain symmetry ranging from to oblate to plane strainand D from 0.6 to 1.2 [Schulmann, 1990]. As an example oftranspression with a supposed high angle of convergenceand with a large amount of data on finite strain, we considera transpressional zone in the Archean greenstone belt ofMinnesota (vertical lineation and steep foliation, K variesaround plane strain, D ranges from 1.5 to 2.5 [Schultz-Elaand Hudleston, 1991]), where the deformed zone reaches anapproximate width of 30 km. Assuming plate velocities in79