Abstract

A high-resolution seismic profile over the Layered Series of the Bjerkreim-Sokndal layered intrusion provides a high-quality seismic image of one of the rock types frequently cited as a source of dense lower crustal reflectivity. The data interpretation is controlled by a detailed comparison with exposed geology directly up-plunge and has implications for reaching a better understanding of deep crustal reflectivity. Reflections are produced by boundaries between major (≥100 m) lithologic units of moderately differing average composition, and small-scale modal layering (centimeters to several meters) contributes in a minor way. The seismic image provides a standard useful for identifying layered mafic intrusions within the upper and middle crust and for understanding their internal structure. The Bjerkreim data also imply that areas of highly reflective lower crust where underplating is believed to have occurred can result from internal lithologic layering resulting from fractionation and repeated influxes of magma to a closed system. The data show that highly developed modal layering can appear seismically transparent due to destructive interference.

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