THE REFLECTIVITY OF MAGMATIC UNDERPLATING USING THE LAYERED MAFIC INTRUSION ANALOG

The reflection characteristics of layered mafic intrusions provide an analog for characterizing the reflectivity of magmatic underplates. One-dimensional seismic models of a variety of mafic intrusions with different parent magmas demonstrate that cumulate layering may typically be strongly reflective with reflection coefficients as high as 0.18. A seismic profile across the Bjerkreim intrusion in southern Norway shows that even a moderately reflective cumulate sequence with reflection coefficients on the order of 0.05 produces detectable reflections. Two-dimensional models demonstrate that the subhorizontal cumulate layering in typical mid- to upper-crustal intrusions is conducive to seismic imaging but more complicated geometries such. as concentric layering created during synmagmatic deformation complicate the seismic image. Reflection coefficients predicted for granulite-facies mineral assemblages are similar to those predicted for the igneous assemblages. Reflection coefficients predicted for cumulates intruded or re-equilibrated at eclogite-facies conditions may be either decreased or enhanced with respect to the igneous assemblages, depending on the abundance of plagioclase in the cumulate layers and the pressure. Results suggest that magmatic underplating should not be invoked as an explanation for non-reflective crust except in cases where eclogitization has occurred.