Orogenic collapse, lithospheric extension, and associated basaltic
magmatism have affected many Phanerozoic regions. This has led to
the hypothesis that the high lower crustal reflectivity of these
areas originates from horizontal mafic dikes embedded in a more felsic
matrix. Tectonic extension, however, favors the intrusion of unreflective
vertical dikes. Stress distribution in an extending lithosphere varies
over several orders of magnitude, which explains the simultaneous
intrusion of vertical and horizontal dikes. Vertical dikes penetrate
rheologically strong zones and transform into horizontal dikes in
weak zones where stress conditions are quasi-isostatic and a subhorizontal
strain fabric is likely to exist. The seismic response of a corresponding
crustal model faithfully reproduces banded reflectivity patterns
typical of extended provinces. Because the loci of mechanical weakness
vary with temperature, composition, and fluid content, other common
reflectivity patterns can be explained similarly. This combination
of two prominent end-member models on the origin of lower crustal
reflections (i.e., ductile flow and mafic intrusions) is compatible
with pertinent features associated with postorogenic lithospheric
extension, such as high heat flow, bimodal volcanism, anatectic granites,
granulite-facies metamorphism, and uniform crustal thickness.