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 ma. trix. 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.

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