Shallow dolerite intrusion and phreatic eruption in the Allan Hills region, Antarctica

TLDR

Shallow dolerite sill intrusion into Permian‑Triassic strata in the Allan Hills generates steam pressures that fracture sediments, producing mudflows and breccias, a process analogous to similar volcanoes elsewhere. En echelon dolerite sills formed at shallow depths where sediment folding and bedding‑plane slip were possible, creating a preserved extension space that acted as a conduit for fluid‑filled mud and rock. Curved, domed sills aligned with compensation surfaces, a wedgeshaped clastic sill whose matrix matches mudflow breccias, and roof fracturing that produced fluid‑filled dykes suggest that phreatic eruptions could occur, as indicated by calcite and zeolite fissures cutting the sills.

Abstract

Abstract Shallow intrusion of dolerite sills into porous and permeable Permian-Triassic strata in the Allan Hills region resulted in steam pressures which exceeded that of the roof rocks causing fracturing and fluidisation of the sediments and the formation of volcanic mudflows and explosion breccias. Shallow dolerite intrusion is suggested by curved and domed sills which were intruded parallel to surfaces of equal vertical rock pressure and magmatic pressure (compensation surfaces) so that they became gently curved to mirror the overlying topography. En echelon sills formed at shallow depths where folding and bedding-plane slip in the sediments was possible. A wedgeshaped clastic sill associated with a clastic dyke extending from it have a matrix similar to that of the mudflow breccias. The clastic sill is considered to be a preserved extension space which formed in front of an intruding dolerite sill. Fracturing of the roof of the extension space resulted in a fluidised mixture of mud, steam, and rock fragments intruded upwards as a dyke. Phreatic eruption would occur if such dykes reached the surface. Calcite and zeolite fissures cutting dolerite sills at Battlements Nunatak and Coombs Hills were probably the avenues of phreatic eruption of material from below the sills. Comparable breccia, mud, and scoria volcanoes of the Navajo-Hopi area, Arizona, Fifeshire, Swabia, and the Stormberg volcanoes of the Karroo System arc also considered to be the result of shallow sill intrusion and consequent phreatic eruption.