RELATIONSHIPS BETWEEN DEFORMATION BAND DISTRIBUTION AND HOST ROCK CHARACTERISTICS, SAN ONOFRE, CALIFORNIA

Deformation bands are well exposed in the porous San Mateo sandstone adjacent to the Cristianitos normal fault at San Onofre State Beach, California. We investigated possible relationships between the distribution of these deformation bands and physical properties of the host rock. In general, deformation band density, a count of deformation bands within a 5-meter horizontal distance, decreases from the fault and fully dissipates within 500 meters. The average thickness of the deformation bands increases with distance from the fault. The decrease in deformation band density from the fault is not uniform – we investigated possible relationships between deformation band density and the composition and Young’s modulus values of the host rock, as well as any changes in Young’s modulus value with the formation of deformation bands. Using five samples of the host rock, we calculated the average grain size and percent feldspar composition. These samples suggest trends of decreasing feldspar and increasing grain size in greater distances from the fault. Using a Schmidt hammer in the field, we measured Young’s modulus values of the host rock and the deformation bands at 25 sites within 230 meters from the Cristianitos fault. The deformation band density appears insensitive to the Young’s modulus values of the host rock. The Young’s modulus values of the deformation bands at the majority of the sites are lower than that of the host rock, suggesting that the formation of the deformation bands may lower the elasticity of the rock unit.