DISPLACEMENT AND GRAVITY-ANOMALY PRODUCED BY A SHALLOW VERTICAL DYKE IN A COHESIONLESS MEDIUM

A crack model is presented for modelling magma emplacement within a shallow vertical dyke in a half-space which responds elastically in compression but has vanishing tensile strength. Realistic initial stress profiles in the solid rock are considered, corresponding to lithostatic and sedimentary equilibria, while the magma is assumed to possess higher density than the host rock and to conform to the hydrostatic pressure gradient. Equilibrium crack width and extension are computed for several sets of model parameters and, from these, uplift and gravity anomaly at the ground surface. It is shown that, within the model's assumptions, narrow gravity anomalies and uplift can be generated, reaching 50 mu gal and 1 m, respectively, provided that the dyke top is very shallow and its vertical extension is large enough (similar to 3 km). Dyke injection is accompanied by increasing compression in the host rock at depth, but tensile contributions are generated at shallower depths, around the magma-filled upper portion of the crack, which may play an important role in driving the flow of fluids permeating the upper crust. If the dyke propagates to a shallow enough depth, its uppermost part may remain empty of magma.