HR: 14:10h
AN: V23C-03    [Abstracts]
TI: Constraining Magma Flow in Sills, Dykes and Laccoliths, Their Relationships to Contemporaneous Volcanism and Basin Structure and the Implications for Sill Emplacement Mechanisms
AU: * Thomson, K
EM: k.thomson@bham.ac.uk
AF: University of Birmingham, School of Geography, Earth and Environmental Science University of Birmingham Edgbaston Birmingham B15 2TT United Kingdom, Birmingham, B15 2TT United Kingdom AB: 3D seismic data demonstrate that sills intruded into sedimentary strata display a complex range of morphologies based on a simple template. A fully developed sill has a concave upwards form with either radial or bilateral symmetry and consists of a relatively flat inner saucer surrounded by a steep inclined sheet which connects it to a flat outer rim. An evolving sill may not develop all of these components resulting in immature forms including half-saucers, half-troughs and sills with only partially developed inclined sheets or outer rims. The periphery of the inner saucer is commonly associated with fracturing and dyking, suggesting a laccolith-like mode of emplacement. When intruded into faulted strata sills tend to maintain their concave upwards form with the development of inclined sheets preferentially exploiting faults. Seismic volume visualisation techniques have permitted the identification of magma flow units and flow directions within sills. The magma flow associated with the emplacement of sills is directed upwards and away from the magma feeder, which may be a dyke or another sill emplaced at a deeper stratigraphic level. This results in a complex network of steep magma tubes rising from discrete locations at the inner saucer periphery with each magma tube subsequently branching towards the sill margin. Although a laccolith-like mode of emplacement seems the most appropriate for sills intruding sedimentary strata the 3D seismic data reveals that there are several features that require modification to the classic model to be made. For example, the sill outer rim can be a series of discrete segments around the sill periphery that are emplaced at different stratigraphic levels. A sill can also feed magma to shallower intrusions (e.g. sills and laccoliths) as well as discrete volcanic centres and eruptive fissures. Similarly, a dyke can feed several sills emplaced at a variety of stratigraphic levels and contemporaneous volcanic eruptions. These observations suggest that neutral buoyancy/levels of compensation concepts may not be the principal control determining the level of sill emplacement in sedimentary strata. Instead, magma encountering a shale horizon at or below the level of neutral buoyancy, particularly if the shale is overpressured, could initiate intrusion as a result of volatilisation and fluidisation of the country rock. As the sill spreads laterally gradual thickening of the sill will result in fracturing at the inner saucer periphery. Faulting dominates for deeply emplaced sills but at shallower emplacement depths the fracturing of the hinges of forced folds may be more important. As these faulting or fracturing processes will not develop at the same rate around the entire margin of the inner saucer they will breach the sill's chilled margin at a number of discrete points leading to the development of a limited number of steep magma tubes. These may subsequently coalesce to form a fully developed inclined sheet surrounding the entire inner saucer or if the steep magma feeders only developed along part of inner saucer margin a partial inclined sheet. The inclined sheet will subsequently climb and branch until another horizon suitable for intrusion is encountered. At this new level the outer rim will develop and possibly a new fully developed concave upwards sill.
DE: 0935 Seismic methods (3025, 7294)
DE: 8005 Folds and folding
DE: 8035 Pluton emplacement
DE: 8414 Eruption mechanisms and flow emplacement
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005