Structural analysis of tectonic lozenges in anisotropic rocksfied analysis and experimental modelling

Resumen

Partitioning of ductile deformation in rocks produces low-strain domains wrapped around by shear zones. Such low-strain domains use to display a characteristic elongated, rhombus-like shape that usually stands out from the outcrop and that are named tectonic lozenges. The focus of this study is on the 2-D structure of tectonic lozenges developed during ductile shearing in rocks with a pre-existing mechanical anisotropy. They have been investigated through experimental modelling and mainly through field analysis in different locations and geological settings, among which Cap de Creus has proven to be the best area to investigate. Although lozenges exist at all scales (and they show self-similarity patterns), the studied structures are metric-sized lozenges. Thus, the fieldwork performed was detailed structural analysis up to the centimetric scale. Lozenges are the less deformed domains (wall rock) bounded by comparatively more deformed domains (shear zones) and displaying an ellipsoidal shape. Consequently, there is no important internal deformation within them besides some mylonitization close to their rims, in the transition to the bounding shear zones. Since lozenges are defined by shear zones, they are strongly associated to them. In this sense, shear zone initiation, propagation and interaction have been also investigated in their relation to lozenges. The main part of this study is based on field analysis. It is combined to analogue and numerical modelling which have shown to support the field observations. Both the field and experimental research have shown that anastomosing shear zone networks in anisotropic rocks are appropriate settings for lozenge development since these networks are the result of shear zones interconnection and intersection and show characteristic romboidal morfology. Apart from Cap de Creus (Eastern Pyrenees), other reference field zones are the Laghetti area in Ticino (Swiss Alps), the Cabalón area in Cabo Ortegal (Galicia) and the Anti-Atlas of Morocco. The BCN-Stage at UAB has been used for the physical analogue experiments, and the platform ELLE for the numerical tests. The presence of strong anisotropy reveals to control the shear zone nucleation pattern, besides enhancing the anastomosing character of the network and therefore the presence of lozenges. A genetic classification of lozenges has been established. The geometry of the structure is analyzed and some geometric parameters are proposed that classify the variety of lozenge shapes. A typology on the internal geometry of lozenges in anisotropic rocks depending on the kinematics of the bounding shear zones is described. Models for the formation of lozenges in anisotropic rocks are also proposed. The evolution of lozenges with progressive strain is also analyzed. This study improves the knowledge on shear zone-related lozenges and clarifies the role and implication of lozenges in deformation. The study reveals that the geometry (symmetry) of lozenges do not to have a straightforward relation to the vorticity of the deformation, but to depend on many other variables, such as the orientation of the rock anisotropy with regard to the kinematic axes and the interaction between shear zones. The study also evidences that there is no an univoque relation between symmetry of the deformation and symmetry of the structures arosen from such deformation.