HR: 0800h
AN: V51C-0716    [Abstracts]
TI: Rigidity anisotropy as a control for the formation of sills and the growth of laccoliths and other plutons
AU: * Menand, T
EM: t.menand@bristol.ac.uk
AF: University of Bristol, Centre for Environmental and Geophysical Flows, Department of Earth Sciences, University of Bristol, Wills Memorial Building, Queen's Road, Bristol, BS8 1RJ, United Kingdom
AB: Despite a wealth of available field data, the mechanisms by which magmas stall in the crust and the physical processes leading to the formation of magma chambers and plutons remain unresolved. Field measurements of the geometry and dimensions of sills, laccoliths, plutons and batholiths suggest the existence of different growth processes related to the size of the intrusions considered. Many field and geochronological data also indicate that laccoliths, plutons and magma chambers develop and grow by amalgamation of numerous sills, and suggest a link between the time-scale associated with their growth and their size. In many cases sills appear to be the building blocks of larger laccoliths and plutons. Yet, the mechanics and dynamics of sill formation remain poorly understood. Different hypotheses for the emplacement of sills, proposed decades ago, have only recently been tested seriously. Analogue experiments involving the injection of fluid into a solid of gelatine reveal that under hydrostatic conditions the formation of sills requires the presence of layers of different rigidity, with sills forming only when their feeder dyke intersects an interface between an upper more rigid, stronger layer and a lower less rigid, weaker layer. That lithological discontinuities and rigidity contrasts can control sill formation provides a mechanism for the growth of laccoliths and plutons. Solidified sills provide favourable rigidity anisotropy for the emplacement of subsequent sills so that laccoliths and plutons can grow by over-accretion, under-accretion or even mid-accretion of successive sills. Supported by field data, this model predicts that laccoliths and plutons grow mainly by vertical expansion, representing the cumulative thickness of their internal sills, while maintaining a comparatively constant lateral extend. This model also predicts that the time-scale over which laccoliths and plutons form is essentially determined by the cumulative time between successive sill intrusions. The experiments also show that sill dynamics are controlled by viscous dissipation of the fluid along their length, which have consequences for their size and shape, enabling sills to propagate further and grow thicker than dykes of similar magmas.
DE: 8439 Physics and chemistry of magma bodies
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting