DIKE-SILL TRANSITIONS IN THE SAN RAFAEL VOLCANIC FIELD, UTAH: IMPLICATIONS FOR SILL EMPLACEMENT MECHANISMS

The San Rafael Volcanic Field is composed of numerous dikes, sills, and conduits near the San Rafael Swell in central Utah. Together, these features represent the underground plumbing system of a monogenetic basaltic volcanic field. Erosion since the field was active 4 m.y.a. has exposed the system at a depth of approximately 8 km. At this level, the transition from dike to sill is exposed in several localities. These exposures yield insights into sill emplacement mechanisms. Multiple horizontal sheets are intruded from a single dike that rolls over. In the rollover, thrust faults are exposed beneath the intruding sheet. Within the rollover zone, a network of thin closely-spaced sills was injected opposite of the propagating direction of the main sheets. In one locality, the dike appeared to continue a short distance above the level of the sill. These features suggest that sills were injected when dikes could not continue vertical propagation through the host Jurassic-Cretaceous Entrada Formation. Sills spread out laterally, exploiting weaknesses—both vertical and horizontal—in the host rock. The network of sills suggests that the magma was injected under high pressure. These features suggest that buoyancy, in addition to rigidity contrasts, may not play a role in sill emplacement beyond determining the highest extent the original dike reaches.