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 and can be performed using a range of methods, which have been developed over the last two decades. 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. Recent (5 Ma-present) deformation in the Cordillera Blanca Shear Zone of the Peruvian Andes has exposed sections of the middle crust at the surface, allowing for observation and analysis of shear zone processes. Oblique grain-shape (OGS) analysis and crystallographic vorticity analysis were employed to determine the kinematic vorticity number and the orientation of the vorticity normal surface in the CBSZ. We also 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). 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.