Relation of dyke intrusion to deformation fields

Martin Casey, Joe Cann and James Edwards

School of Earth Science, University of Leeds, Leeds, LS2 9JT, U.K.

m.casey@earth.leeds.ac.uk

The mechanics of dyke intrusion is a long studied but not yet fully understood subject. It is generally considered to concern magma pressure, magma viscosity, fracture toughness, density contrast of intruding magma and wall rocks and the remote tectonic normal stress perpendicular to the plane of the intrusion. An application of fracture mechanics concepts reinforces the importance of tectonic normal stress in the process and an intrusion patch model is proposed. Additionally the possibility and manner in which stresses released by dyke intrusion can be restored by displacement on the remote boundaries is seen to be important.

We distinguish three regimes of dyke intrusion, on the basis of restoration of released stress:

Published work on the Krafla intrusions of 1975-82 is used to derive a model of individual intrusion events. Tilt meter data from a point near the magma chamber is consistent with an episodic charge and discharge of the magma chamber supplying the intrusions. Charge is associated with increase of magma pressure, discharge with decrease. Intrusion is observed to occur along sectors of the rift zone of lengths varying from 4-18km. There is a tendency for the sectors furthest from the magma chamber to be intruded first and for these to be longer.

On the basis of these observations it is proposed that intrusion is governed by the supply of magma in terms of the amount available for intrusion and its pressure on the one hand and the magnitude of tectonic stress perpendicular to the rift zone on the other. It is proposed that the tectonic stress is very heterogeneous, with the heterogeneity being determined by the long-term history of dyke intrusion and normal faulting in the rift.