VORTICITY AND KINEMATIC ANALYSIS OF THE CORDILLERA BLANCA SHEAR ZONE, PERU

Quantitative vorticity analyses applied to naturally deformed rocks are essential for studying kinematics in shear zones. An understanding of vorticity, or the contribution of pure vs. simple shear can permit for the modeling of shear zone development in a deformed region. It is often assumed that the kinematic plane and vorticity normal surface are parallel. This is not always the case. Determining the orientation of the vorticity normal surface is critical to ensure kinematics are accurately interpreted. A range of methods for vorticity analyses have been proposed over the years. Recent (5 Ma-present) deformation in the Cordillera Blanca Shear Zone (CBSZ) of the Peruvian Andes has exposed sections of the middle crust at the surface, allowing for direct observation of crustal levels otherwise inaccessible by researchers. Outcrops in the CBSZ allow for analyses of shear zone processes and microstructures that help shed light on the kinematics during the deformation of the CBSZ. Oblique grain shape (OGS) analysis and crystallographic vorticity axis (CVA) analysis were employed to determine the kinematic vorticity number and the orientation of the vorticity normal surface in the CBSZ. We propose a new method of OGS analysis utilizing electron backscatter diffraction data to determine vorticity values as well as the orientations of flow apophyses and instantaneous stretching axes (ISA’s). Calculated vorticity values and flow apophyses orientations were used to calculate the convergence vector between the Nazca and South America Plates. The use of vorticity analysis for modeling plate motion has yet to be used on such a young system where the angle of convergence is known.