The lower crust has long been recognized as a place where significant chemical evolution of magma occurs. Understanding the conditions that lead to long-term storage in or rapid transport through these lower crustal systems has remained a challenge. In this study, active-source seismic data are processed to image prominent reflectors in the lower crust and upper mantle near Mount St. Helens. Results show strong and weak near-Moho reflectivity located near previously inferred regions of magma storage and high-density bodies underlying volcanic centers, respectively. To explain these observations, we hypothesize that magmas intersecting the high-density lower crustal bodies experience enhanced vertical buoyancy forces/transport due to the increased density contrast between melt and host rock. The reduced reflectivity near high-density bodies is therefore a result of both decreased impedance contrasts between crust and mantle host rocks and diminished lower crust magma accumulation, which ultimately causes focusing of volcanism above these features.
ASJC Scopus subject areas
- Earth and Planetary Sciences(all)