Seismic reflection profiling carried out with a sledgehammer source has imaged Tertiary extensional structures over a depth range of 45--500 m within lower plate rocks of the Ruby Mountains-East Humboldt Range metamorphic core complex. The 400-m CMP profile straddles an exposed contact between tectonic slices of dolomitic marble and metaquartzite emplaced by low-angle ductile-brittle normal faulting. Subhorizontal reflections from layering within the tectonic slices give way at 160 ms (160--220 m depth) to reflections that dip 15--45¿ to the east, in contrast with dips indicated in a poorly imaged segment of a coincident regional seismic line but in agreement with dips of foliation mapped for nearby up-plunge exposures of a late Proterozoic-early Cambrian sequence of metaquartzites, marbles, schists, and granitic rocks that forms the bulk of the underlying shear zone. Differences with the regional profile are attributed to the higher frequencies (30--100 Hz) generated by the smaller hammer source and the enhanced lateral resolution provided by the straighter profile and much smaller shot-receiver offsets (46--157 m) contributing to the stack for each CMP. The results suggest that the near-surface, east-dipping component of the anastomozing shear zone extends at least 2 km farther east than previously interpreted. Rough estimates of interval velocities (1500--4500 m/s) inferred from stacking velocities are consistent with velocities of mylonitic rocks measured perpendicular to foliation at low confining pressures when the effects of macroscopic fractures and joints are taken into account. Peaks in amplitude spectra of stacked traces suggest long-wavelength components of layering resolved at scales from 5--8 m (depth: 50 m) to 15--25 m (depth: 500 m). ¿ American Geophysical Union 1995