Host rock rheology controls on the emplacement of tabular intrusions: Implications for underplating of extending crust

The pooling, ponding, and horizontal intrusion of basaltic magma at various depths into the crystalline crust is paradoxical because the stress conditions favoring such intrusions do not favor the opening of vertical feeder conduits necessary for their formation. The most rigid zones of the crust and upper mantle tend to behave elastically and store stress when subjected to tectonic forces, while the more ductile zones flow under stress. These rheological differences within the crust and upper mantle allow variation in the magnitude of deviatoric stress, and such variation has a profound effect on tabular intrusions. A vertical dike intruding into extending crust increases the horizontal least principal stress of the host rock when it is emplaced, and that effect is magnified in rheologically ductile zones where the pre‐existing deviatoric stress has been partially relaxed. Subsequent dikes intruding into the ductile zone may encounter stress conditions that have been altered to the extent that the local least principal stress has become vertical, and horizontal intrusion initiates. Multiple geological and geophysical observations of horizontal intrusions in extending crust indicate that such principal stress interchanges occur commonly.