The nature of seismic reflections from deep crustal fault zones

Abstract

Deep seismic reflection profiles have recorded reflections from ductile shear zones within the crystalline basement. Perhaps the best example is the Consortium for Continental Reflection Profiling (COCORP) Wind River line in Wyoming, which shows the Wind River thrust fault to be a strong reflector from the surface to depths of about 30 km. To identify the physical properties responsible for the seismic reflections from fault zones, we report measurements of compressional wave velocity and velocity anisotropy in mylonites recovered from exhumed ductile shear zones. These rocks are characterized by extensive ductile deformation of plastic minerals, brittle deformation of the more rigid minerals, grain size reduction, and development of a strong fabric. Average velocity and density were not found to change significantly and systematically between the mylonites and the adjacent undeformed rock. Seismic anisotropy of 7% or greater is present in several mylonites dependent upon their composition and fabric. Using our data on mylonite properties and models for the crustal structure near the Wind River line, we compute true amplitude synthetic reflection seismograms and compare them to true amplitude plots of the COCORP Wind River reflection data. When anisotropy and the finely laminated structure of the fault zone are considered, our modeling indicates fairly strong reflections from mylonite zones in most cases. We also show that elevated pore pressure in shear zones may produce strong reflections. However, a permeability of the order of 10−16 darcy is required to maintain sufficient pore pressure to produce a velocity anomaly in a fault zone which has long been inactive.