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.
Highlights
Interactions between tectonic and magmatic processes, as well as the lithology and preexisting structure of the host rock, control the geometry and connectivity of magma conduits and reservoirs
Seismic reflection data and field observations suggest that upper crustal plumbing systems, those emplaced within sedimentary basins, may be dominated by interconnected sills and inclined sheets (e.g., Figs. 6–15)
Given that the host rock–magma interactions facilitating mafic sill transgression occur at the lateral tips of intermediate to felsic sills (e.g., Pollard and Johnson, 1973; Gudmundsson, 2012), it is likely that magma rheology is an important control on whether a sill complex or laccolith develops (Johnson and Pollard, 1973; McLeod and Tait, 1999; Currier and Marsh, 2015); i.e., it appears easier for mafic magma, which typically has a higher temperature and lower viscosity than intermediate to felsic magma, to form inclined sheets at sill tips
Summary
Interactions between tectonic and magmatic processes, as well as the lithology and preexisting structure of the host rock, control the geometry and connectivity of magma conduits and reservoirs (i.e., a magma plumbing system). Studies applying seismic techniques to analyzing plumbing systems have shown that (1) seismically imaged intrusions describe a range of morphologies that can be categorized, broadly, into strata concordant, saucer shaped (i.e., a flat inner sill that passes laterally into an encompassing, or partially encompassing, inclined limb; Thomson and Hutton, 2004; Magee et al, 2014), and inclined sheet geometries (Fig. 3; e.g., Thomson and Hutton, 2004; Planke et al, 2005; Magee et al, 2014); (2) different host-rock deformation mechanisms accommodating intrusion can be interpreted (e.g., Jackson et al, 2013; Magee et al, 2013a); and (3) magma flow pathways can be mapped across entire intrusion networks (e.g., Thomson and Hutton, 2004; Schofield et al, 2012b, 2015; Magee et al, 2014) Despite these advances, seismic interpretation is an underutilized tool in igneous-based research and remains an unfamiliar technique to many Earth scientists in the volcanic and magmatic community. Onlap relationships are observed throughout the folded succession, between the top Cretaceous and top lower Eocene boundaries (Fig. 8A), suggesting that sill emplacement and fold growth occurred incrementally over 15 m.y. between ca. 65 and 54 Ma (Magee et al, 2014)
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