Numerical modelling of dykes deflected into sills to form a magma chamber

Abstract

Most shallow magma chambers are thought to evolve from sills. For this to happen, several conditions must be met. (1) There must be a discontinuity, normally a contact, that deflects a dyke (or an inclined sheet) into a sill. (2) The initial sill must have a considerable thickness, normally (depending on dyke injection rates) not less than some tens of metres. (3) The resulting sill must receive magma (through dykes) frequently enough so as to stay liquid and expand into a chamber. (4) The resulting magma chamber must remain at least partially molten and receive multiple magma injections over a given period of time to build up a volcano on the surface above. In this paper we present numerical models based upon field data and geophysical data as to how sills are emplaced and may subsequently evolve into shallow magma chambers. We suggest that most sills form when dykes meet contacts, particularly weak ones, which are unfavourable to dyke propagation. A contact may halt (arrest) a dyke altogether or, alternatively, deflect the dyke into the contact. The three main mechanisms for dyke deflection into a contact are (1) the Cook–Gordon debonding or delamination, (2) rotation of the principal stresses, generating a stress barrier, and (3) an elastic mismatch across a contact between adjacent layers. Elastic mismatch means that the layers have contrasting Young's moduli and varying material toughness. Once a sill is initiated, the developing magma chamber may take various forms. Many shallow magma chambers, however, tend to maintain a straight sill-like or somewhat flat (oblate) ellipsoidal geometry during their lifetimes. For a sill to evolve into a magma chamber there must be elastic-plastic deformation of the overburden and, to some extent, of the underburden. So long as the sill stays liquid, subsequent dyke injections become arrested on meeting the sill. Some magma chambers develop from sill complexes. For the sill complex to remain partially molten it must receive a constant replenishment of magma, implying a high dyke-injection rate. Alternatively, an initial comparatively thick sill may absorb much of the magma of the dykes that meet it and evolve into a single shallow magma chamber.