Witness to Strain: Subdomain Boundary Length and the Apparent Subdomain Boundary Density in Large Strained Olivine Grains

12 Electron Backscatter Diffraction (EBSD) investigation of strain mainly uses 13 polycrystalline samples to study fabric development. We extend the use of EBSD for analysis of 14 large single mineral grains, by measuring the apparent surficial subdomain boundary density per 15 unit area, reported here as Unit Segment Length (USL). We apply this USL technique to 16 examine and quantify the plastic deformation recorded by naturally shocked olivine in the low to 17 moderately shocked ureilite meteorite Northwest Africa 2221 and the highly-shocked martian 18 dunitic cumulate meteorite Northwest Africa 2737, by assessing the types of subdomain 19 boundaries and the increase of subdomain misorientation with increasing shock metamorphism. 20 We further compare USL results for the shocked olivine in the meteorites with those for 21 terrestrial deformation of Hawaiian olivine. USL of olivine increases with shock level, and USL