Magma-driven accommodation structures formed during sill emplacement at shallow crustal depths : the Maiden Creek sill, Henry Mountains, Utah.

Abstract

In areas of exceptional exposure, upper-crustal intrusions and their immediate \nwall rocks commonly preserve direct evidence of the emplacement, \nmagma flow pathways, and strains associated with the intrusion process. \nSuch excellent exposure is displayed by the Paleogene Maiden Creek intrusion— \na small satellite body related to the Mount Hillers intrusive complex, \nHenry Mountains, Utah. An intermediate plagioclase-hornblende porphyritic \nmagma was intruded into the Entrada Sandstone Formation at an estimated \ndepth of ~3 km. The southern part of the intrusion is overlain by the newly \nidentified Maiden Creek shear zone (MCSZ): a subhorizontal, top-to-the-WNW \ndetachment formed at the contact with the overlying sandstone country rocks. \nFrom observations of both syn-emplacement deformation and the exposed \nintrusion geometries, it is proposed that the southern Maiden Creek intrusion \ncomprises westerly-derived, inclined sill sheets. Host-rock sandstones were \nsandwiched (~E–W constriction) between these intrusive bodies beneath the \nMCSZ. It is proposed that the MCSZ is a syn-emplacement magma-driven \naccommodation structure, with a shear sense antithetic to the magma flow \ndirection, which played a critical role in accommodating the westerly-derived \nsill intrusion. Our results show that inelastic syn-emplacement deformation \nstructures, such as the MCSZ, are very important in the accommodation of \nmagma in the subsurface. Such small structures are unlikely to be imaged by \nseismic-reflection surveys, highlighting the importance of detailed field studies \nin our understanding of intrusion geometry and emplacement mechanisms.