Igneous sills as a record of horizontal shortening: The San Rafael subvolcanic field, Utah

Abstract

Igneous sills can facilitate significant lateral magma transport in the crust, therefore it is important to constrain controls on their formation and propagation. Close spatial association between sills and dikes in layered (sedimentary) host rocks has led to a number of sill emplacement mechanisms that involve stress rotation related to layering; from horizontal extension and dike emplacement, to horizontal compression and sill emplacement. Here we use field observations in the San Rafael subvolcanic field (Utah, USA), on the Colorado Plateau, supported by mechanical modelling, to show that layering is not the dominant control in all cases of sill formation. We found no compelling evidence of large sills fed by dikes; all observed cases show that either dikes cut sills, or vice versa. Local sill contacts activate and follow host layer interfaces, but regionally, sills cut the stratigraphy at a low angle. The sills cut and are cut by reverse faults (1-3 m

Mechanical models to estimate the paleostress state from igneous intrusions

“…Fig. 4e; Gartner, 1986;Walker et al, 2017), and we have observed no dike-sill transitions for the major sills (e.g. Fig.…”

“…Fig. 4d; Walker et al, 2017). Richardson et al (2015) highlighted an example of a dike-to-sill transition for the Cedar Mountain sills (Fig.…”

“…Delaney and Gartner (1997) determined ages of dike emplacement between 4.6 and 3.7 Ma; the field relationships therefore constrain sill emplacement to a ∼ 1 Myr interval. Walker et al (2017) interpret the sills in the SRSVF as low-angle conjugate intrusions, and concluded that the sills record a phase of horizontal tectonic shortening, rather than relating to local deflections due to material layering. The sills range from < 10 cm to ∼ 30 m thick and are discordant to bedding, with dips of 1-25 • (Fig.…”

“…Notably, many field examples of sills and dikes exhibit near-parallel dilation vectors, regardless of the intrusion attitude (e.g. Hoek, 1991;Walker, 1993;Airoldi et al, 2011;Martinez-Poza et al, 2014;Muirhead et al, 2014;Eide et al, 2016;Walker et al, 2017). Intrusions that demonstrate shear offset of markers across their margins indicate that during emplacement the dilation vector was inclined from plane normal (Muirhead et al, 2014;Walker et al, 2017); this obliquity of opening can be characterised by the opening angle (Fig.…”

“…Gudmundsson, 2011a;Magee et al, 2016). Several studies have demonstrated that sills may also form during tectonic horizontal shortening, in which the minimum principal stress is vertical Walker et al, 2017;Stephens et al, 2017); hence sills can be used as paleostress indicators, provided they are accompanied by detailed kinematic characterisation.…”