Post-Emplacement Stability and Instability of Magma Reservoirs

Abstract:

One of the main controls on the intrusion of magma in continental crust is magma buoyancy, which drives ascent through denser rocks and lateral spreading near the contact with ambient rocks of lesser density. The formation of a sizable magma reservoir involves the deformation of host rocks over large characteristic times due to slow viscous relaxation. During and after emplacement, magmas cool and crystallize, inducing significant variations of density and buoyancy which may have a large impact on reservoir dimensions and shape. Different behaviours are predicted depending on the viscosity contrast between magma and host rock, melt buoyancy and total volume of magma. Laboratory experiments with viscous fluids and numerical calculations that account for brittle and ductile deformation of host rocks illustrate the large number of possibilities for reservoir evolution. The most striking result is obtained with a buoyancy reversal due to cooling and crystallization, such that spreading of the reservoir in the horizontal direction slowly segues into sagging accompanied by lateral shrinking. A large range of reservoir shapes can be produced, including funnel-like bodies and tabular bodies capped, or floored, by a few vertical structures. Many of those have natural equivalents. This work suggests that there may be large differences between fully solidified plutons and intrusions and active magma reservoirs.