Thermal and mechanical aspects of magma emplacement in giant dike swarms

We consider the thermal history and dynamics of magma emplacement in giant feeder dikes associated with continental flood basalts. For driving pressur e gradients inferred for giant dike swarms, thicknesses of <10 m would enab le dikes to transport magma laterally over the distances observed in the fi eld (up to thousands of kilometers) without suffering thermal lock-up. Usin g time-dependent numerical solutions for the thermal evolution of a dike ch annel under laminar and turbulent flow conditions in the presence of phase transitions, we investigate the possibility that the observed dike thicknes ses (of the order of 100 m) result from thermal erosion of the country rock s during dike emplacement. This implies that the observed range of dike wid ths in giant dike swarms may reflect variations in the source volume and no t the excess magma pressure. It is found that the total volume of intruded magma required to produce an order of magnitude increase in dike width via wall rock melting broadly agrees with the estimated volumes of individual f lows in continental flood basalts. The presence of chilled margins and appa rently low crustal contamination characteristics of some giant dikes may be consistent with turbulent magma flow and extensive melt back during dike e mplacement. In this case, measurements of the anisotropy of magnetic suscep tibility most likely indicate magma flow directions during the final stages of dike intrusion. Shear stresses generated at the dike wall when the dike starts to freeze strongly decrease with increasing dike width, which impli es that thicker dikes may have less tendency to produce consistent fabric a lignment. Our results suggest that if the dike was propagating downslope of f a plume-related topographic swell, the mechanism responsible for flow ter mination could possibly have been related to underpressurization and collap se (implosion) of the shallow magma plumbing system feeding the intrusion. Radial dikes that erupted at the periphery of the topographic uplift might have increased (rather than decreased) extensional stresses in the crust wi thin the topographic uplift upon their solidification.