Polarities and amplitudes of intracrustal P-SV conversions (P waves converted to vertically polarized shear waves) in receiver functions from the Southeastern Suture of the Appalachian Margin Experiment array and USArray Transportable Array provide new constraints on the origin of seismic reflectivity delineating the Alleghanian detachment in the southern Appalachians(eastern United States). Forward modeling of receiver functions is consistent with a 3.5-km-thick, high shear-wave velocity (Vs = 3.9 km/s) section of deformed Paleozoic platform metasedimentary rocks beneath the Blue Ridge at 3–6.5 km depth. In the Inner Piedmont, conversions from the top and base of a low-Vs zone (3.1 km/s) at depths of 5–9 km are interpreted as a package of metasedimentary rocks or a shear zone characterized by radial anisotropy. The detachment continues to the southeast beneath the Carolina terrane, where high-amplitude negative conversions at 10–13 km depth are consistent with arc rocks (Vs = 4.0 km/s) overlying sheared rocks with lower Vs (3.2 km/s). Southeast-dipping conversions at 5–10 km depth mark the boundary between the Inner Piedmont and Carolina terrane. This study demonstrates that relatively high-frequency receiver functions (up to ∼3 Hz), though still lower in frequency than P-wave energy analyzed for reflection profiling (>20 Hz), can provide important links between surface geology and active-source experiments to better constrain models of crustal structure.