Partitioning of strain and strain type during transpression:
Insights from analogue modelling
Sandra Piazolo
Department of Earth Sciences, 4 Brownlow Hill, Liverpool, L69 3GP
piazolo@liv.ac.uk
In recent years it has been established that transpressional and transtensional bulk deformation is commonly partitioned in zones of low and high strain as well as areas of different deformation type. If this is the case, the geometries of such zones of specific strain and deformation type may potentially be used to infer the overall general deformation type in terms of vorticity number Wn and strain rates. To gain a better understanding for these geometries and the influence of strain rates on strain localization, experiments were performed in general flow apparatus at different imposed strain rates and bulk deformation ranging from vorticities of 1 (simple shear), 0.95, 0.8 and 0.6.
Experiments show that even in a slightly non-Newtonian, viscoelastic material with a stress exponent of n=1.23, systematic patterns of strain partitioning develop rapidly. In general, two sets of dominant high strain zones develop, whereof one is generally more pronounced than the other. High strain zones are simple shear dominated, while low strain zones are pure shear dominated. No unequivocal correlation of high strain zone geometry and strain rate could be observed. Whereas the angle between two main high strain zone orientations changes in a systematic way. With increasing bulk pure shear component in the deformation geometry, the angle between two main high strain zone orientations increases (e.g. at Wn = 0.6, angle = approx. 40º). In addition, the main orientation is antithetic. This is in contrast to patterns developed at simple shear bulk deformation. Here, the angle between the main high strain zones is smaller (approx. 30º) and the orientation of the main high strain zones is synthetic.
The observed relationship between high strain zone angles, anti- and synthetic orientation and bulk deformation geometry may be useful in the analyses of shear zone geometries in the field. Results suggest that in rocks which exhibit power-law rheology vorticity analyses on the scale of partitioning may not necessarily represent the kinematic vorticity of bulk flow.