New Mexico Geological Society Annual Spring Meeting — Abstracts

An earlier work comparing the gross characteristics of P and S-phase reflections indicated that rapid lateral variations occur in the internal structure of the Socorro magma body southwest of Socorro. The present study analyzed the structure of the body at that location by breaking the propagation path effects into two parts: upper-crustal/near surface effects and mid-crustal/magma body interaction effects. The upper-crustal and near surface effects have been approximated by the initial portion of the direct P-phase due to the favorable geometry of source,
reflector, and receiver and the small magnitudes of the swarm events used in the analysis. Synthetic seismograms which included attenuation and interbed multiples were calculated and compared to the observed P-phase reflections. The modelling results indicate that the magma body is better approximated by a two-layer structure, a thin layer of full-melt underlain by a crystalline mush, than by a single layer model. The layer
thicknesses vary by small, but resolvable amounts over the ~ 500m distance covered by the reflection points

To determine an "average" structure for the magma body in this area, the stacked spectrum of the observed P-phase reflections was modelled. The best-fit model consists of a thin (~ 70m) layer of non-rigid, low-velocity material underlain by a second, thin (~ 60m) layer of slightly higher velocity material. This second layer may be an earlier, partially solidified intrusion or a crystalline mush at the base of a single magma
chamber. The resulting magma body is less than 150m in total thickness. A body this thin, even at wid-crustal depths, would need to be replenished often to remain in a non-rigid state.