Lateral Magma Flow in Mafic Sill‐complexes

Abstract

The structure of upper crustal magma plumbing systems controls the distribution of volcanism and influences tectonic processes. However, delineating the structure and volume of plumbing systems is difficult because (1) active intrusion networks cannot be directly accessed; (2) field outcrops are commonly limited; and (3) geophysical data imaging the subsurface are restricted in areal extent and resolution. This has led to models involving the vertical transfer of magma via dikes, extending from a melt source to overlying reservoirs and eruption sites, being favored in the volcanic literature. However, while there is a wealth of evidence to support the occurrence of dike-dominated systems, we synthesize field-and seismic reflection-based observations and highlight that extensive lateral magma transport (as much as 4100 km) may occur within mafic sill complexes. Most of these mafic sill complexes occur in sedimentary basins (e.g., the Karoo Basin, South Africa), although some intrude crystalline continental crust (e.g., the Yilgarn craton, Australia), and consist of interconnected sills and inclined sheets. Sill complex emplacement is largely controlled by host-rock lithology and structure and the state of stress. We argue that plumbing systems need not be dominated by dikes and that magma can be transported within widespread sill complexes, promoting the development of volcanoes that do not overlie the melt source. However, the extent to which active volcanic systems and rifted margins are underlain by sill complexes remains poorly constrained, despite important implications for elucidating magmatic processes, melt volumes, and melt sources.

Unravelling intrusion-induced forced fold kinematics and ground deformation using 3D seismic reflection data

“…The occurrence of subtle onlap and truncation observed within folded strata deposited between these principal phases of magmatism implies that sill emplacement occurred intermittently over~31 Myr (Figure 3, 6, and 7), consistent with previous observations that sills and sill-complexes can assemble incrementally via the accumulation of relatively small-volume magma pulses intruded across protracted periods of time [e.g. Annen 2011;Magee et al 2014;Annen et al 2015;Magee et al 2016;Magee et al 2017a]. We cannot constrain the precise volumes and timing of individual sill emplacement events because: (i) we cannot seismically image presumably thin sills fed by discrete magma pulses; and (ii) we lack detailed biostratigraphic data to constrain the precise ages of the key onlap surfaces and strata deposited during periods of forced folding.…”

“…At shallowlevels in sedimentary basins, intrusions often develop sill-like geometries as magma is emplaced along mechanical contrasts between layered strata, weak sedimentary rocks, and/or the minimum principal stress axis rotates to vertical [e.g. Kavanagh et al 2006;Gudmundsson 2011;Schofield et al 2012;Magee et al 2016;Walker et al 2017]. As intrusion continues and the sill inflates, space can be generated by uplift of the overburden and free surface to form forced folds [e.g.…”

“…The trapping mechanism of later pulses will have principally been controlled by the relative timing of the different magma pulses and the thermal history of the intrusions and host rock [e.g. Annen 2011; Annen et al 2015;Magee et al 2016]. For example, if there is sufficient time for previous magma pulses to fully crystallise, their basal contact with the underlying sedimentary host rock will act as a rigidity barrier that can deflect and trap intruding magma along its surface [e.g.…”

Synthesis of global satellite observations of magmatic and volcanic deformation: implications for volcano monitoring & the lateral extent of magmatic domains

“…This suggests that locations of deformation will vary more in rift zones than in volcanic arcs, where stress conditions may favour persistent magma reservoirs. Seismic reflection and field observations suggest that felsic sill complexes have total lateral extents of < 20 km, while shallow complexes of mafic sills may have much greater lateral extents (e.g., Magee et al, 2016). This suggests that there are likely to be major differences in the eventual geometry and extent of mature magmatic domains in different tectonics settings and for different magma compositions.…”

3D seismic imaging of the shallow plumbing system beneath the Ben Nevis Monogenetic Volcanic Field: Faroe–Shetland Basin

“…This is a powerful tool in understanding how monogenetic 10 volcanoes are fed and how pre-existing crustal structures can act as the primary influence 11 on their spatial and temporal distribution. This study examines the structure and lithology of 12 host-rock as an influence on edifice alignment and provides insight into the structure of consisting of a network of sills, dykes and inclined sheets, has implications for the 84 geochemical and petrological signature of magma erupted from monogenetic volcanoes 85 (Magee et al 2016). In addition, this study, and studies like it, can provide significant 86 information for volcanic risk to urbanised areas and infrastructure that are present within 87 active monogenetic fields (e.g.…”