LOWER CRUSTAL REFLECTIVITY MODELED BY RHEOLOGICAL CONTROLS ON MAFIC INTRUSIONS

Orogenic collapse, lithospheric extension, and associated basaltic mag matism have affected many Phanerozoic regions. This has led to the hyp othesis that the high lower crustal reflectivity of these areas origin ates from horizontal mafic dikes embedded in a more felsic matrix. Tec tonic extension, however, favors the intrusion of unreflective vertica l dikes. Stress distribution in an extending lithosphere varies over s everal orders of magnitude, which explains the simultaneous intrusion of vertical and horizontal dikes. Vertical dikes penetrate rheological ly strong zones and transform into horizontal dikes in weak zones wher e stress conditions are quasi-isostatic and a subhorizontal strain fab ric is likely to exist. The seismic response of a corresponding crusta l 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 prominen t end-member models on the origin of lower crustal reflections (i.e., ductile flow and mafic intrusions) is compatible with pertinent featur es associated with postorogenic lithospheric extension, such as high h eat flow, bimodal volcanism, anatectic granites, granulite-facies meta morphism, and uniform crustal thickness.