Abstract

ABSTRACT

Basic, doleritic sills form a key component of subvolcanic complexes in the prospective parts of volcanic margins where they add considerably to the structural and thermal complexity of the subsurface geology. Whilst recent 3D seismic studies of sill complexes in volcanic margins have added considerable new insights into their development and significance, field studies at the sub- and trans-seismic scale are still required. The Theron Mountains in Antarctica offer perhaps the best exposures of a dolerite sill complex to be seen anywhere in the world and they create an unprecedented opportunity to understand the processes, on a variety of scales, that are important in individual sill emplacement. Large- and small-scale evidence indicates that forceful roof uplift is the fundamental emplacement mechanism. The sills contain an abundance of ‘bridge structures’, which are the remains of country rock that once lay between thin and closely separated precursor sills. Viewed on a variety of scales and inferred to be preserved at different stages in their development, these indicate that small proto-sills propagate ahead of the main sill body, exploiting thin weak horizons, such as coal and shale. The proto-sills expand laterally as they move downstream and come to overlap vertically along their lateral margins. Country rock bridges form between the sill overlaps and are rotated, bent and eventually broken as magma flow creates vertical inflation. Thus, a network of proto-sills exploiting a variety of weak horizontal horizons ahead of the main sill body unite, inflate and progressively allow the main sill body to move downstream. Although most thick sills are probably created by many thin proto-sills merging, some sills and some parts of sills are produced by the merging and inflation of single pairs of proto-sills and, in these examples, vertical inflation may run to more than 1000% of their original vertical separations.

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