Abstract
Field and seismic observations show that numerous sills exhibit lobate morphologies. Each lobe corresponds to a distinct igneous segment exhibiting a finger-like shape, the long axis of which is commonly interpreted as a magma-flow indicator. Robust understanding of the emplacement mechanisms of finger-shaped sills, and direct observations supporting finger orientation as magma-flow indicator are lacking. In this paper, we present the results of detailed structural mapping on an exceptional, easily accessible 1-km long outcrop in the Neuquén Basin, Argentina, that exhibits a sill, its contacts and the structures in the finely layered sedimentary host rock. We show that the sill is made of distinct segments that grew, inflated or coalesced. We also demonstrate that the fingers were emplaced according to the viscoelastic fingering or viscous indenter models, with no field evidence of tensile elastic fracture mechanism as commonly assumed in mechanical models of sill emplacement. We identified new structural criteria at the intrusion's contacts for inferring magma flow direction during the magma emplacement. Our small-scale structural observations carried out on a seismic-scale outcrop have the potential to considerably aid the structural interpretation of seismic data imaging igneous sills, i.e. to fill the standard gap between outcrop-scale field observations and seismic-scale geophysical data.
Highlights
Igneous sheet and tabular intrusions, such as dykes, sills and laccoliths, are fundamental magma conduits and reservoirs through the Earth's crust (e.g., Walker, 1975; Rubin, 1995; Petford et al, 2000; Cartwright and Hansen, 2006; Magee et al, 2016)
We present the results of detailed structural mapping of an exceptional, accessible 1-km long outcrop at Las Loicas, located in the Neuquén Basin, Argentina (Fig. 2), which exhibits a sill composed of a string of igneous fingers, their contacts and the structures in their finely layered sedimentary host rock
All structures in the host rock adjacent to the intrusions' edges reflect shortening. These observations are in agreement with viscoelastic fingering mechanisms produced in the laboratory experiments of Bertelsen et al (2018), i.e. the viscous magma indents its host rocks, causing it to deform by ductile flow or shear failure
Summary
Igneous sheet and tabular intrusions, such as dykes, sills and laccoliths, are fundamental magma conduits and reservoirs through the Earth's crust (e.g., Walker, 1975; Rubin, 1995; Petford et al, 2000; Cartwright and Hansen, 2006; Magee et al, 2016). We present the results of detailed structural mapping of an exceptional, accessible 1-km long outcrop at Las Loicas, located in the Neuquén Basin, Argentina (Fig. 2), which exhibits a sill composed of a string of igneous fingers, their contacts and the structures in their finely layered sedimentary host rock This outcrop (1) reveals how the host rock accommodates the emplacement of the fingers, and (2) provides direct structural evidence of magma flow direction through the studied fingers. The fingers and sill appear disconnected in outcrop, they are likely connected in the third dimension as one large andesitic sheet, as they exhibit very similar andesitic composition and magmatic texture They are emplaced in the organic-rich shales of the Mendoza Group, but whether the host rock is the Vaca Muerta Fm. or the Agrio Fm. is unknown. The emplacement depth of sill complexes in the shale of the Mendoza groups is estimated between 2000 and 2500 m depth (Witte et al, 2012; Rabbel et al, 2018; Spacapan et al, 2018)
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