Crustal Deformation Related to Emplacement of the Socorro Magma Bod

The Socorro magma body is the second-largest known crustal magma body on Earth. It covers ∼3400 square kilometers, has a thickness of ~130 m, and is situated at 19 km depth below the surface. The magma body is located within the Rio Grande rift zone, in the crust below the southern Albuquerque basin, the Socorro basin, and the accommodation zone connecting these two basins. Previous surface-leveling and InSAR studies suggest that the Socorro magma body is still active; causing surface uplift with a maximum rate of 2.5∼3.0 mm/year centered around San Acacia, New Mexico. The Socorro seismic anomaly is a seismically very active region of mainly upper crustal earthquakes above the magma body.;In this study, Socorro magma body emplacement processes are studied, constrained by the sill-related surface uplift and seismicity patterns. I use the general finite-element software package AbaqusRTM to build models of crustal deformation. The fundamental analysis method used for all experiments is a fully-coupled thermal-stress analysis, in which results from thermal analysis depend upon displacement results and vice versa. The 2-D elastic models are composed of two parts: the crustal block, and the sill which is embedded into the crustal block at 19 km depth. Surface uplift above the Socorro magma body is caused by thermal expansion of the crustal material and/or a direct effect from inflation of the sill (magma addition). The two effects, thermal and mechanical, are both also modeled separately to understand their individual contributions, and combined in one model. The models predict thermal evolution, crustal deformation, and associated stresses.;The modeling results show the following crustal evolution upon sill emplacement: (1) A thermal aureole develops around the sill, resulting in thermal expansion. The uplift pattern caused by thermal expansion during the sill emplacement phase is wider, and the uplift rate is larger, than the observed uplift pattern; (2) After cooling of the emplaced sill starts, surface uplift rates are at least an order of magnitude smaller than observed uplift rates, and are below InSAR detection limits; (3) Net uplift caused by sill inflation is an approximately linear function of magma overpressure; (4) The temperature evolution suggests that the sill solidifies on a time scale of a few hundred years after an injection event; (5) Sill inflation causes crustal stresses, with a zone of horizontal compression directly above the sill, and horizontal tension between ~13 km depth and Earth's surface; (6) The average surface uplift rate during one heating-cooling cycle of the sill emplacement due to the combined effect of thermal expansion and inflation is similar to the observed uplift pattern.;Based on these results, I develop a new model for Socorro magma body emplacement. Because a newly emplaced sill cools down rapidly to temperatures below those associated with the formation of a crystal mesh, the currently imaged fluid sill must be younger than several hundred years, or, if it is older, a significant magma injection event must have occurred in the last few hundred years to maintain the molten status of the sill. The ongoing seismicity above the sill is interpreted to result from injection events, which stress the crust so that slip along favorably oriented faults occurs. If this is true, magma injection occurs every 5-10 years or so, concurrent with seismic swarms. The current surface uplift above the sill results from sill inflation, and longer-term thermal expansion.