Quantitative structural analyses and numerical modelling of ...

ETG 6 - 10 SCHULMANN ET AL.: STRAIN DISTRIBUTIONFigure 7. Relation between amount of discrete partitioning (amount of lateral displacement0accommodated by fault) and change of angle of convergence a and R vd ratio in transpression zone.Knowing the angle of convergence and the amount of discrete partitioning, new angle of convergence a 00and new R vd values can be depicted and used for estimation of strain parameters in diagram Figure 6.Thick black lines indicate amount of discrete partitioning for different active transpressive zones.mulate strain during the entire exhumation path. A rock’scapacity to accumulate strain is determined by its thermal,microstructural and rheological development. The bestexamples are syntectonically emplaced granites in transpressionalshear zones [e.g., Melka et al., 1992; Parry et al.,1997; Brown and Solar, 1998]. There is only a very shorttime of granite intrusion when the magma has enoughcrystals to record the deformation. Further cooling of themagma is responsible for hardening of granites and freezingof the fabrics at certain depth levels.[39] Numerous regional studies of transpressional zonesshow that rock complexes have experienced ductile deformationfollowed by late folding and brittle fracturing in thesame tectonic regime. This means that the accumulation ofductile strain ends at some depth below the surface dependingupon the ambient thermal regime. It is also common thatsamples originally located deep in lithospheric transpressionalzones have had their mineralogy changed due toretrograde metamorphism during exhumation. This meansthat deformation registered in transpressional zones wasdeveloping only during a small time interval during theconvergent activity.[40] To be able to correlate the external and internalparameters using the transpressional model, we need tospecify the time span (or the duration of the verticalelevation path) in which the accumulated strain is attributed.This means that in Figure 6 the time axis together withelevation contours (ratios of R vd /z 0 ) needs to be rescaled sothat times corresponding to the finite strain parameters, Dand K, are higher.9. Discussion[41] Transpressional models in general assume that aweak and deformable zone bounded by the rigid walls ofadjacent lithosphere is progressively shortened in the courseof convergence. We can put forward a question as towhether the measured internal (microscopic) parameters ofancient transpressional zones (lineation, foliation, K and Dvalues) may be used to estimate the initial external (macroscopic)parameters (R vd , RFD, and a)?9.1. Ancient Zones[42] The strain symmetry parameter K is sensitive to theangle of convergence (a), whereas the strain intensityparameter D is well correlated with across-width shorteningof a transpressional zone. On the basis of Figure 6 and usingmeasured natural finite strains and orientation of lineation in78