Coulomb failure of Earth's brittle crust controls growth, emplacement and shapes of igneous sills, saucer-shaped sills and laccoliths

T Schmiedel,O Galland,Ø.T Haug,G Dumazer,C Breitkreuz

doi:10.1016/j.epsl.2019.01.011

T Schmiedel, O Galland + Show 3 more

Open Access

https://doi.org/10.1016/j.epsl.2019.01.011

Copy DOI

Abstract

Tabular intrusions are common features in the Earth's brittle crust. They exhibit a broad variety of shapes, ranging from thin sheet intrusions (sills, saucer-shaped sills, cone sheets), to more massive intrusions (domed and punched laccoliths, stocks). Such a diversity of intrusion shapes reflects different emplacement mechanisms caused by contrasting host rock and magma rheologies. Most current models of tabular intrusion emplacement assume that the host rock behaves purely elastically, whereas numerous observations show that shear failure plays a major role. In this study, we investigate the effects of the host rock's Coulomb properties on magma emplacement by integrating (1) laboratory models using dry Coulomb granular model hosts of variable strength (cohesion) and (2) limit analysis numerical models. Our results show that both sheet and massive tabular intrusions initiate as a sill, which triggers shear failure of its overburden along an inclined shear damage zone at a critical sill radius, which depends on the emplacement depth and the overburden's cohesion. Two scenarios are then possible: (1) if the cohesion of the overburden is significant, opening of a planar fracture along the precursory weakened shear damage zones to accommodate magma flow, leads to the formation of inclined sheets, or (2) if the cohesion of the overburden is negligible, the sill inflates and lifts up the overburden, which is dissected by several faults that control the growth of a massive intrusion. Finally, we derive a theoretical scaling that predicts the thickness-to-radius aspect ratios of the laboratory sheet intrusions. This theoretical prediction shows how sheet intrusion morphologies are controlled by a mechanical equilibrium between the flowing viscous magma and Coulomb shear failure of the overburden. Our study suggests that the emplacement of sheet and massive tabular intrusions are parts of the same mechanical regime, in which the Coulomb behavior of the Earth's brittle crust plays an essential role.

Highlights

Subvolcanic processes in the shallow Earth’s crust govern the link between the plutonic and volcanic realms (Breitkreuz and Rocchi, 2018; Burchardt, 2018)
In the high cohesion experiments (100% SF, 50/50 glass beads (GB)/SF; Fig. 3; Table 1), the intrusions were typically saucer-shaped, with circular, flat, horizontal inner sill and outer inclined sheets (Fig. 3)
In experiments with intermediate cohesions (80/20 GB/SF, 90/10 GB/SF; Fig. 3, Table 1), the intrusions exhibit mostly inclined sheets, with a subhorizontal bottom intrusion of radius smaller than those of intrusions emplaced in the high-cohesion experiments (

Summary

Introduction

Subvolcanic processes in the shallow Earth’s crust (e.g., magma storage, magma transport) govern the link between the plutonic and volcanic realms (Breitkreuz and Rocchi, 2018; Burchardt, 2018). Tabular igneous intrusions represent fundamental elements of igneous plumbing systems that accommodate the bulk of magma transport and emplacement in the Earth’s brittle crust (Magee et al, 2017; Cruden et al, 2018; Galland et al, 2018). Many of these intrusions form in sedimentary basins, in which they may exert significant mechanical and thermal effects on hydrocarbon systems (e.g., Senger et al, 2017; Spacapan et al, 2018).

Methods

Results

Discussion

Conclusion