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

Wilson, P.I.R and McCaffrey, K.J.W. and Holdsworth, R.E.H. (2019) 'Magma-driven accommodation structures formed during sill emplacement at shallow crustal depths: the Maiden Creek sill, Henry Mountains, Utah.', Geosphere., 15 (4). pp. 1368-1392.

Abstract

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

Item Type:Article
Full text:Publisher-imposed embargo
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Available under License - Creative Commons Attribution Non-commercial.
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Status:Peer-reviewed
Publisher Web site:https://doi.org/10.1130/GES02067.1
Publisher statement:This paper is published under the terms of the CC‑BY-NC license. © 2019 The Author(s).
Date accepted:19 April 2019
Date deposited:21 May 2019
Date of first online publication:24 June 2019
Date first made open access:26 May 2021

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