IN A recent review, Paterson & Fowler (1993) showed decreasing away from the pluton (as discussed by Gughow estimated volumes of displaced wall rock are lielmo 1993b). smaller than pluton volumes, and how the widths of Laboratory experiments indicate that rocks behave as strain aureoles around natural plutons are considerably power-law fluids with exponents n between 2.5 and 5 narrower than in models of diapirs where spheres rise (Hansen & Carter 1982, Kirby 1983, Wilks & Carter through Newtonian fluids (Cruden 1988, Schmeling et 1990). The power-law exponent n of a viscous fluid is a al. 1988). They then argued that mechanisms other than measure of the sensitivity of its viscosity-stress (or ductile flow of the wall rocks were necessary to account strain-role). For n = 1 (Newtonian fluid) the viscosity is for these observations. Although I accept that several constant for any strain rate. For n > 1 the fluid softens mechanisms may act simultaneously and change in imwith increased strain rate. The strain-rate dependency of portance as plutons are emplaced, I argue here that: (i) the viscosity of power-law fluids has important effects on the displaced volumes of wall rocks may be underestithe velocity and flow patterns of the ambient fluid mated because of difficulties in assessing the shape and around rising spheres (or diapirs)--and therefore the size of the three-dimensional flow cell in the wall rocks width of their strain aureoles (Crochet et al. 1984, caused by pluton emplacement; and more importantly Kawase & Moo-Young 1986, Weinberg & Podladchikov (ii) the narrow strain aureoles may result from diapiric in press). In the following discussion, the difficulties in rise of plutons through power-law rocks and/or result determining the volumes of rock displaced by diapir from the effects of thermal softening of the wall rocks, emplacement are first considered. The effect which Several field studies have measured the width of the power-law rocks might have on the width of the strain strain aureole caused by pluton emplacement (see Gugaureole is then discussed, and finally the possible effects lielmo 1993a, Paterson & Fowler 1993, and references of thermal softening of the wall rocks by the diapir are therein). The measured width depends on three main examined. factors: (i) The finite strain of the wall rocks, which depends on the velocity field imposed by the diapir. This in turn depends on the diapir's mechanism and depth of ROCK VOLUMES DISPLACED BY DIAPIRS emplacement, and on the rheology and physical boundaries of its wall rocks. The longer a rock volume is Paterson & Fowler (1993)studied horizontal shortensubmitted to a particular strain rate (velocity field), the ing in strain aureoles around plutons and then integrated larger is its strain. The velocity of flow decreases with shortening to three dimensions around plutons of differdistance from the contact of rising spheres, so that ent shapes. The amount of shortening they obtained for strains have to accumulate a long time to be distinguishseveral natural examples only accounted for about 30% able from any background strain. (ii) Late deformation of the volume of the plutons. They argued that this will mask the strain aureoles of pre-tectonic plutons, volume difference implies that other mechanisms such while interference with regional tectonic strains will as doming of the roof, stoping, assimilation and rigid affect the width of the aureoles of syn-tectonic plutons translation of wall rock must have operated at the same (Guglielmo 1993b). (iii) The measurable width of the time as viscous flow of the wall rock. aureole depends on distinguishing the strain due to the However, this discrepancy may largely be due to pluton from regional strains of any age (Paterson & difficulties in measuring small pluton-related strains Fowler 1993). This distinction becomes increasingly beyond the obvious strain aureole, and difficulties in more difficult as strains are spread over larger volumes, determining the shape of the flow cell around the pluton. The width of the strain aureole around a rising sphere depends on the power-law exponent n of the country *Current address: Research School of Earth Sciences, ANU, ACT rocks. It is widest in Newtonian fluids and narrows as n 6200, Canberra, Australia. increases. However, even in the case of rocks with high n
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Mar 1, 2000
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