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Although subparallel swarms of dikes are thought to be the primary feeders to voluminous volcanic eruptions, increasing recognition of volumetrically significant sill complexes suggests that they too play an important role in magma ascent through the shallow crust. However, the extent to which sills and interconnected, sill-fed dikes actually transport magma to the earth's surface in many large igneous provinces (LIP) is presently unclear. By analyzing field relationships and dimensions of intrusions of the Ferrar LIP in South Victoria Land, Antarctica, we show that sill-fed dikes were the likely feeders for voluminous flood basalt eruptions. These intrusions are small but numerous, with cumulative dimensions equivalent to a feeder network 308,000 km long and 1.8 m wide. Due to the tremendous aerial extent of this intrusive network, each individual dike-feeder segment would only be required to actively feed magma for 2 to 3 days on average to erupt the 70,000 km 3 of flood lavas represented by the Kirkpatrick basalts. The Ferrar intrusions form a broadly-distributed array of small, moderately dipping dikes (<2 km long, 1.8 m wide, 56° mean dip) exhibiting almost any orientation. This sill-fed dike network contrasts with dike swarms conventionally depicted to feed flood basalt provinces, and has the appearance of a variably “cracked lid” atop a sill complex. The cracked lid model may apply to a range of shallow feeder systems (<4 km depth) intruding sedimentary basins, where the effects of far-field tectonic stresses are negligible and sill intrusions exert the dominant control on dike orientations. We conclude that sill inflation, and resulting deformation of surrounding host rock, plays a critical role in the ascent of magma in shallow volcanic systems that span the full spectrum of eruptive volumes. [ABSTRACT FROM AUTHOR]

SUMMARY We analyse 10 representative intrusions from two sets of inclined diabase (Ferrar Dolerite) sheets exposed at Allan Hills (South Victoria Land, Antarctica), using petrographic and rock magnetic methods to determine microfabrics and infer magma flow directions. At least one diabase sample was collected at the margins of each intrusion. Magnetite and pyrrhotite contribute to magnetic fabrics of the samples. Thirty-six magnetic fabric directions, inferred from the mutual arrangement of either the magnetic lineation, or the magnetic foliation plane and local macroscopic flow indicators (e.g. horn-shaped apophyses and kinks) at the tips and margins of each intrusion reveal composite (i.e. both lateral and vertical) flow paths recorded along each intrusive segment. Petrographic textures and multiple flow directions inferred at sheet-segment tips reveal that 'passive' injection of magma via hydrofracturing produced the local shallow large igneous province plumbing as a sill-dominated intrusive complex very close to, or intersecting the palaeosurface. This contrasts with 'classic' arrays of either vertically or laterally injected blade-like dykes. [ABSTRACT FROM AUTHOR]

Field observations and structural data from intrusive complexes at Allan Hills and Terra Cotta Mountain, South Victoria Land, Antarctica, demonstrate that interconnected sills and inclined sheets transported magma through the shallow subsurface. These sills and sheets represent the upper-crustal (top 4 km) plumbing system of the 183 Ma Ferrar large igneous province. The sheets are short in length (<1500 m), are moderately inclined (47° and 51° means), and show meter-scale variations in attitude; in places, they intruded bedding planes, resulting in stepped sheet-sill geometries. Sheet geometries and their relationship to the surrounding country rock are consistent with peripheral sheet intrusion under local magmatic stresses arising from roof-lift during sill injection. The sheet intrusions thus reflect the intrusive process itself rather than a far-field tectonic stress regime. The sills and sheets, together with local dolerite masses, formed the intrusive network that supplied magma to the Mawson Formation pyroclastic rocks in various parts of South Victoria Land and, by inference, the Kirkpatrick flood basalt lavas. The predominance of inclined sheets rather than steeply dipping dikes indicates a magmatic environment that is unlike the Jurassic rift arm inferred by previous authors. This could be explained using any of the following three scenarios. (1) The axis of the rift, and hence any rift-hosted dikes, lies beyond the current exposure area. (2) The regionally extensive Ferrar sills may have provided rheologically weak horizons that limited mechanical coupling of the basement rocks and overlying Beacon Supergroup, locally detaching the upper 4 km of the crust from possible synmagmatic basement extension below. (3) The Ferrar large igneous province was emplaced in a neutral tectonic setting. In this scenario, broad-scale distribution of magma through the province was controlled by preexisting structure in the basement, and local intrusion geometries reflect the physical interaction of intruding magma with bedding anisotropy of the Beacon Supergroup. [ABSTRACT FROM AUTHOR]

The article discusses the structure of upper crustal magma plumbing systems that controls the distribution of volcanism and influences tectonic processes. It discusses the models involving the vertical transfer of magma via dykes, extending from a melt source to overlying reservoirs and eruption sites. The article mentions that plumbing systems need not be dominated by dykes and magma can be transported within widespread sill-complexes, promoting the development of volcanoes.