THE INTRUSION ‘SPACE PROBLEM’: A COMPARATIVE CASE STUDY OF ACCOMMODATION AND DEFORMATION STRUCTURES ASSOCIATED WITH THE EMPLACEMENT OF THE MAIDEN CREEK AND TRACHYTE MESA SATELLITE INTRUSIONS, HENRY MOUNTAINS, UTAH

Most studies of magmatic intrusions concentrate on geometry and internal architecture: only a few pay particular attention to emplacement-related deformation structures in the host rock that record how magma is accommodated within the crust, i.e. the ‘space problem’. This research aims to develop a greater understanding of how igneous intrusive bodies are emplaced and accommodated within the shallow crust, using classic exposures found in the Henry Mountains, Utah.

Two satellite intrusions to the Mt Hillers intrusive centre show highly contrasting geometries, host-rock deformation structures, accommodation structures and apparent emplacement mechanisms. Trachyte Mesa, the most distal satellite intrusion of Mt Hillers, has a relatively simple elongate (NW-SE) geometry, concordant with the Entrada Sandstone it intrudes. The intrusion is comprised of multiple, stacked intrusive sheets. Syn-emplacement deformation structures observed in the host rocks consist of a conjugate set of intrusion margin-parallel deformation bands and extensional brittle faults, the latter occurring at the tips of intrusive sheets. These structures, along with a post-emplacement set of intrusion margin parallel and perpendicular tensile joints, indicate extensional strain normal to the intrusion margin, consistent with a two-stage growth mechanism for the overall intrusion.

In comparison, Maiden Creek, more proximal to the intrusive centre, shows a more complex elliptical shape with several finger-like lobes. Detailed outcrop studies across two neighbouring lobes have identified a sub-horizontal shear zone that runs along the top contact of each intrusive lobe. This shear zone separates low-/moderately-deformed sandstones above from highly deformed sandstones below and between the two lobes, hence acting as a detachment zone. Strain within the highly deformed sandstones is dominated by compressional faults, fractures and fabrics that point to a ‘bull-dozing’ mechanism for lobe emplacement. Fabrics (stretched phenocrysts) within the igneous rock, seen on the upper surface of the intrusive lobes directly beneath this shear zone, show that the shear zone was contemporaneous with magma emplacement. The shear zone therefore appears to have played a critical role in accommodating magma emplacement.