Craig Magee

Sill morphology and comparison of brittle and non-brittle emplacement mechanisms

Magmatic sheet intrusions contribute significantly to the upper crustal magma transport network. The emplacement mechanism of the magmatic sheets controls the final geometry of the intrusions and the characteristics of host rock deformation. Previous observations have highlighted the preponderance of brittle structures, associated with shallow-level sheet intrusions. However, recent studies have suggested that non-brittle host rock behaviour also occurs, particularly related to the formation of magma fingers during shallow-level sill intrusion. Here, we examine both brittle and non-brittle intrusion mechanisms and expand upon them with field observations from a series of widespread and variable magmatic systems. Non-brittle emplacement appears primarily associated with viscous flow of the host rock during intrusion and is therefore intimately linked to the contemporaneous host rock rheology as well as magma dynamics. Purely brittle and non-brittle emplacement processes are found to be end members with many intrusions containing evidence of both behaviours. Deriving the host rock characteristics is therefore important for discerning potential diagnostic intrusion indicators and intrusion geometries both within the field and in modelling. Incorporation of variable host material behaviours in numerical and analogue modelling, tuned using direct field observations, may consequently further our understanding of the controls on shallow-level intrusion.

Lateral magma flow in mafic sill complexes

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.

The influence of normal fault geometry on igneous sill emplacement and morphology

Magma flow within the subsurface is heavily influenced by the pre-existing structure of the upper crust. During continental rifting, normal faults modify the geometry of igneous networks by providing preferential pathways for the intrusion of magma. However, the way in which magma intrudes into fault planes is poorly understood. Here, we quantitatively document the relationship between fault architecture and intrusion distribution and geometry using three-dimensional seismic reflection data from the Exmouth Sub-basin, offshore northwest Australia. Inclined segments of saucer-shaped sills intrude several faults along convex-into-the-hangingwall fault-plane corrugations. We suggest that stress field perturbations associated with the fault-plane corrugations provide suitable conditions for fault reactivation as magma conduits. Pre-existing faults also modify sill geometries through the offset of stratigraphic horizons that may be preferentially intruded, potentially resulting in the formation of a new sill or the development of minor intrusive steps. This work emphasizes the importance of the pre-existing structural template in controlling the growth and final geometry of intrusive networks.

Lithological controls on igneous intrusion-induced ground deformation

Ground deformation commonly precedes volcanic eruptions, although its relationship to underlying intrusion networks is complex. We use 3D seismic reflection data to examine the link between a saucer-shaped sill and an overlying forced fold formed at the contemporaneous palaeosurface. Our results highlight a disparity in size between the sill and fold, which we attribute to accommodation of magma by overburden uplift and fluid expulsion from the host rock. Sill transgression occurred in response to plastic deformation of the host rock and did not produce seismically resolvable uplift. Inversion models of ground deformation should therefore acknowledge host rock behaviour during intrusion. Supplementary material: Uninterpreted seismic section and sill thickness/fold amplitude data are available at www.geolsoc.org.uk/SUP18663.

Controls on the expression of igneous intrusions in seismic reflection data

The architecture of subsurface magma plumbing systems influences a variety of igneous processes, including the physiochemical evolution of magma and extrusion sites. Seismic reflection data provides a unique opportunity to image and analyze these subvolcanic systems in three dimensions and has arguably revolutionized our understanding of magma emplacement. In particular, the observation of (1) interconnected sills, (2) transgressive sill limbs, and (3) magma flow indicators in seismic data suggest that sill complexes can facilitate significant lateral (tens to hundreds of kilometers) and vertical (

Structure and dynamics of surface uplift induced by incremental sill emplacement

Shallow-level sill emplacement can uplift Earth’s surface via forced folding, providing insight into the location and size of potential volcanic eruptions. Linking the structure and dynamics of ground deformation to sill intrusion is thus critical in volcanic hazard assessment. This is challenging, however, because (1) active intrusions cannot be directly observed, meaning that we rely on transient host-rock deformation patterns to model their structure; and (2) where ancient sill-fold structure can be observed, magmatism and deformation has long since ceased. To address this problem, we combine structural and dynamic analyses of the Alu dome, Ethiopia, a 3.5-km-long, 346-m-high, elliptical dome of outward-dipping, tilted lava flows cross-cut by a series of normal faults. Vents distributed around Alu feed lava flows of different ages that radiate out from or deflect around its periphery. These observations, coupled with the absence of bounding faults or a central vent, imply that Alu is not a horst or a volcano, as previously thought, but is instead a forced fold. Interferometric synthetic aperture radar data captured a dynamic growth phase of Alu during a nearby eruption in A.D. 2008, with periods of uplift and subsidence previously attributed to intrusion of a tabular sill at 1 km depth. To localize volcanism beyond its periphery, we contend that Alu is the first forced fold to be recognized to be developing above an incrementally emplaced saucer-shaped sill, as opposed to a tabular sill or laccolith.

The Significance of Magnetic Fabric in Layered Mafic-Ultramafic Intrusions

Anisotropy of magnetic susceptibility (AMS) has been recognised as a well-established fabric analysis tool for intrusive igneous rocks since the 1990s. The AMS technique provides directional information for magnetic foliation and magnetic lineation fabric components of the AMS ellipsoid, potentially coupled with a quantification of the overall fabric strength and geometry. The magnetic susceptibility (and therefore the AMS) of igneous rocks is often dominated by ferromagnetic mineral phases such as magnetite or low-Ti titanomagnetite, even where present in very minor amounts (e.g., ~ 0.1 vol.%). Fe-bearing silicates exhibit subordinate paramagnetic behaviour but are volumetrically much more important constituents of igneous rocks than Fe-Ti oxides, so may also contribute considerably to the AMS.

The stratigraphic record of prebreakup geodynamics: Evidence from the Barrow Delta, offshore Northwest Australia

The structural and stratigraphic evolution of rift basins and passive margins has been widely studied, with many analyses demonstrating that delta systems can provide important records of postrift geodynamic processes. However, the apparent lack of ancient synbreakup delta systems and the paucity of seismic imaging across continent-ocean boundaries mean that the transition from continental rifting to oceanic spreading remains poorly understood. The Early Cretaceous Barrow Group of the North Carnarvon Basin, offshore NW Australia, was a major deltaic system that formed during the latter stages of continental rifting and represents a rich sedimentary archive, documenting uplift, subsidence, and erosion of the margin. We use a regional database of 2-D and 3-D seismic and well data to constrain the internal architecture of the Barrow Group. Our results highlight three major depocenters: the Exmouth and Barrow subbasins and southern Exmouth Plateau. Overcompaction of pre-Cretaceous sedimentary rocks in the South Carnarvon Basin, and pervasive reworking of Permian and Triassic palynomorphs in the offshore Barrow Group, suggests that the onshore South Carnarvon Basin originally contained a thicker sedimentary succession, which was uplifted and eroded prior to breakup. Backstripping of sedimentary successions encountered in wells in the Exmouth Plateau depocenter indicates that anomalously rapid tectonic subsidence (≤0.24 mm yr−1) accommodated Barrow Group deposition, despite evidence for minimal, contemporaneous upper crustal extension. Our results suggest that classic models of uniform extension cannot account for the observations of uplift and subsidence in the North Carnarvon Basin and may indicate a period of depth-dependent extension or dynamic topography preceding breakup.

Decoding sill emplacement and forced fold growth in the Exmouth Sub-basin, offshore northwest Australia: Implications for hydrocarbon exploration

AbstractIgneous sills emplaced at shallow levels in sedimentary basins commonly uplift the overburden and free surface. Uplift produces dome-shaped forced folds that may host economic hydrocarbon accumulations. These intrusion-induced forced folds are typically assumed to develop instantaneously, whereby the oldest onlapping strata constrain the age of sill emplacement, and accommodate the entire volume of intruded magma. However, several studies demonstrate that forced folds may grow over geologic timescales, with additional space-making mechanisms (e.g., compaction) partly accommodating the magma volume. It is thus critical to understand when forced fold traps form and how they evolve in relation to the timing of source rock maturation and migration. We analyze two forced folds imaged in 2D seismic reflection data from offshore northwest Australia. Analyzing the seismic stratigraphy of the forced fold overburden allows us to recognize several distinct phases of fold growth. Subhorizontal reflections onl...

Three-dimensional magma flow dynamics within subvolcanic sheet intrusions

Sheet intrusions represent important magma conduits and reservoirs in subvolcanic systems. Constraining the emplacement mechanisms of such intrusions is crucial to understanding the physiochemical evolution of magma, volcano deformation patterns, and the location of future eruption sites. However, magma plumbing systems of active volcanoes cannot be directly accessed and we therefore rely on the analysis of ancient systems to inform the interpretation of indirect geophysical and geochemical volcano monitoring techniques. Numerous studies have demonstrated that anisotropy of magnetic susceptibility (AMS) is a powerful tool for constraining magma flow patterns within such ancient solidified sheet intrusions. We conducted a high-resolution AMS study of seven inclined sheets exposed along the Ardnamurchan peninsula in northwest Scotland, and examined how magma flow in sheet intrusions may vary along and perpendicular to the magma flow axis. The sheets form part of the Ardnamurchan Central Complex, which represents the deeply eroded roots of an ∼58-m.y.-old volcano. Our results suggest that the inclined sheets were emplaced via either updip magma flow or along-strike lateral magma transport. It is important that observed variations in magnetic fabric orientation, particularly magnetic foliations, within individual intrusions suggest that some sheets were internally compartmentalized, i.e., different along-strike portions of the inclined sheets exhibit subtle differences in their magma flow dynamics. This may have implications for the flow regime and magma mixing within intrusions.