Mid-crustal magma storage in a continental rift: insights from an actively forming sill complex

Abstract:

We present an interdisciplinary study of the Socorro Magma body, a sill complex in the Rio Grande rift of the western U.S. This sill complex consists of both solidified and liquid sills, and has been supplied with magma injections in recent and historical times. Seismic velocities suggest that the sill complex is located in the uppermost ductile lower crust. It is sub-horizontal and of variable thickness (probably tens to hundreds of meters). The upper crust above the sill is seismically active, and swarms are common but irregularly spaced in time and spatially. The surface above the Socorro magma body has been uplifting since the early 1900’s, and GPS measurements show ongoing surface uplift, with varying rates. River terrace arching suggests a possibly latest Pleistocene- or Holocene-aged uplift event.

We use these data to develop a conceptual model for magma storage in a mid-crustal sill complex. In our model, magma migration follows a dynamically permeable and porous shear zone. Overpressure of the sill causes seismic swarms, and the sill grows vertically by emplacement of magma above and possibly below existing sills, and laterally by formation of lobes. Numerical models of magma injection show that the surface above the sill lifts up continuously over thousands of years by thermal expansion. This uplift rate is too low to be measured geodetically, but corresponds to surface uplift inferred from railroad construction in historic times and arched river terraces. Stresses induced by sill overpressure are large enough to cause slip on rift faults. Magma injection occurs frequently (every few years to ~15 years, corresponding to the seismic swarms), and causes surface uplift that corresponds to GPS measurements. Our model suggests that ductile shear zones play an important role in crustal magma storage.