Dikes vs. diapirs in viscoelastic rock
Geophysical models have traditionally treated diapiric ascent as occurring in purely viscous host rock, and dike intrusion as occurring in purely elastic host rock. Such models are incapable of determining (1) what governs the transition between the two transport mechanisms, (2) the properties of diapirs that ascend via a combination of fracture and flow, and (3) the geologic regimes in which such hybrid behavior might occur. Observations that granitic plutons often intrude along crustal-scale faults suggest that transitional behavior may be important geologically. Here I explore a self-similar solution for a pressurized dike propagating in a linear viscoelastic medium. Dike growth is governed primarily by the ratio of two dimensionless parameters. The first, a measure of the elastic response of the host rock, is (po/G), wherepo is excess magma pressure at the dike entrance andG is elastic stiffness of the host rock. The second, a measure of the viscous response of the host rock, is(ηm/ηr)1/3, whereηm is magma viscosity andηr is host-rock viscosity. For expected values of (po/G) of 10−3 to 10−4, the host rock must be more than 11 to 14 orders of magnitude more viscous than the magma in order to behave as essentially elastic during dike intrusion. Dike thickness:length aspect ratios less than 10−1 require viscosity contrasts in excess of 6 to 8 orders of magnitude. These results indicate that rock will behave elastically during the intrusion of basalt and low-viscosity rhyolite dikes, but that viscous deformation can significantly exceed elastic deformation during intrusion of the most viscous rhyolite dikes into hot rock. Even in the latter case, however, the resulting intrusions appear dike-like. Thus while these results can explain the occurrence of rhyolite dikes with aspect ratios of 10−2 rather than the 10−3 to 10−4 expected for purely elastic host rock, the occurrence of granites in equidimensional plutons cannot be attributed solely to the viscosity contrast between liquid granites and hot rock.