A closer look at deep crustal reflections

Since acquisition of the first crustal‐scale seismic reflection sections, Earth scientists have been intrigued by images of the inaccessibly deep parts of the crust. The pattern of discontinuous subhorizontal reflections at 20–40 km depths has evoked hypotheses as diverse as depositional layering, mafic sills/intrusive layering, shear zones, and fluidfilled fractures. It is clearly important to develop interpretational criteria to relate geological evolution to seismic images. A valid approach, practiced in seismic programs worldwide, is to study regions where the geological history is well understood. For example, the lower crust of the Basin and Range Province has a history of repeated recent mafic underplating, detectable seismically in high refraction velocities and widespread subhorizontal reflections. Such images observed in other extended regions may be interpreted in a similar fashion.