Implications of rapid, dike-fed pluton growth for host-rock strain rates and emplacement mechanisms

Dikes must maintain a critical width and flow velocity in order to propagate several kilometers from source to sink without freezing; consequently, they must support a high volumetric magma flow rate. This in turn implies that plutons fed by dikes must fill rapidly. In an effort to predict host-rock strain rates required by dike-fed growth, we employ a three-dimensional geometric model consisting of concentric shells to track instantaneous strain rates in a homogeneously deforming aureole around spheroidal plutons with a range of aspect ratios. Using published values for magma and host-rock parameters appropriate for mid-crustal levels, we calculate, for example, a minimum instantaneous strain rate of approximately 10−10 s−1 in the deformation aureole of a 1 km radius spherical pluton. Aureole strain rates, which range upward to greater than 10−4 s−1, are primarily a function of position in the aureole, pluton size, and filling rate; pluton shape plays only a secondary role. Although preservation of evidence for high strain rates may be rare, and although multiple mechanisms probably operate simultaneously, we expect that deformation this rapid in the middle crust would be accommodated primarily by brittle mechanisms.