New Mexico Geological Society Annual Spring Meeting — Abstracts

The Socorro Magma Body (SMB) is an ~130 m-thick partially molten sill at ~19 km depth below the central Rio Grande Rift. It is oval in shape, ~60 km in E-W width and ~80 km in N-S length. Numerous previous geodetic studies reveal that emplacement of the SMB causes surface uplift of up to ~3 mm/yr in a circular dome-like pattern centered on the northern ~2/3 of the SMB. Study of the SMB will provide insight regarding the mechanisms of deformation and emplacement of mid-crustal magma sills and potential related geo-hazards. We utilize the software ABAQUS© to model the process of magma sill emplacement in a two-dimensional elastic crust. Here we consider only thermal expansion of rocks (thermal expansion coefficient: 1×10^-5K^-1) around the SMB as the cause for surface uplift, in order to isolate the effects of thermal expansion from other processes, such as inflation. Tests with two other thermal expansion coefficients (5×10^-6K^-1and 3×10^-5 K^-1) indicate that the uplift amplitude is proportional to the thermal expansion coefficient. In the model heat is transmitted by conduction (thermal conductivity: 2.6 Wm^-1K^-1) only. Our simulation consists of two stages: the “heating phase,” when elements representing the cross sectional area of the SMB are heated from ambient temperature (~510°C) to magmatic temperature (basaltic liquidus, ~1200°C) over 100 yr, and a “cooling phase” of 50,000 yr, while the sill conductively cools. During the “heating phase” the surface uplift can be up to 3.5 m with a nearly constant maximum surface uplift rate of ~30 mm/year, about an order of magnitude faster than current maximum uplift rates. The width of the uplifted surface area is ~twice the width of the sill itself (similar to previous models of inflation of penny-shaped cracks), whereas the diameter of the geodetically measured uplift is similar to the short, E-W dimension of the oval SMB. During the cooling phase the surface uplift rate decreases to values similar to those measured geodetically within a few years but surface uplift continues across the width of the uplifting domain during the entire 50,000 years.

The mismatch in both shape and dimension of the measured uplift relative to the SMB itself suggests that the heat source has a different plan-view shape than the seismically imaged SMB. If heat is supplied across the full E-W width of the SMB, then the times when the model surface uplift is smaller than or comparable to the seismically imaged width are late enough in cooling phase that the slow, outer edges of the uplift pattern fall below the limit of geodetic resolution. The width of the surface uplift shrinks to values similar to those geodetically measured within several years after heat input into the SMB ends. These results suggest that the SMB is in the cooling phase and is no longer actively inflating and increasing its heat content.