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 sh allow vertical dyke in a half-space which responds elastically in comp ression but has vanishing tensile strength. Realistic initial stress p rofiles in the solid rock are considered, corresponding to lithostatic and sedimentary equilibria, while the magma is assumed to possess hig her density than the host rock and to conform to the hydrostatic press ure gradient. Equilibrium crack width and extension are computed for s everal sets of model parameters and, from these, uplift and gravity an omaly at the ground surface. It is shown that, within the model's assu mptions, narrow gravity anomalies and uplift can be generated, reachin g 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 ro ck at depth, but tensile contributions are generated at shallower dept hs, 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.