Growth of plutons by floor subsidence: Implications for rates of emplacement, intrusion spacing and melt-extraction mechanisms

Geophysical and field-based studies indicate that granitic plutons occur as either tabular (disk) or wedge (funnel) shapes whose length (L) to thickne ss (T ratio is controlled by the empirical Fewer law, T = 0.6(+/-0.15)L0.6( +/-0.1). The dimensions of plutons are selfsimilar to other natural subside nce phenomena (calderas, ice cauldrons? sinkholes, ice pits) and it is prop osed that they grow in a similar fashion by withdrawal of material (melt) f rom an underlying source, which is then transferred to the growing pluton w ithin the crust. Experimental studies show that growth of subsidence struct ures occurs by vertical inflation much greater than horizontal elongation o f an initial depression with ; = width of the source region. If pluton grow th is modelled in the same way I the empirical power law relating T and L d efines limits for pluton growth that are imposed by the width, thickness an d degree of partial melting from a lower crustal source. Several growth mod es that predict testable internal structural patterns are identified for pl utons, depending on whether they are tabular or wedge-shaped, grow by conti nuous or pulsed magma delivery and whether magma is accreted from bottom to top, or vice versa. Rates of pluton growth are geologically fast (hundreds to hundreds of thousands of years) if magma supply is effectively continuo us, but can also take millions of years if the time between magma delivery events is much longer than magma injection events. Plutons formed by melt e xtraction from an area directly beneath require large degrees of partial me lting and or very thick sources. Lower degrees of partial melting and thinn er sources are permitted when melt extraction occurs over a larger region, which can lead to the formation of spaced plutons. Tabular pluton growth wi ll tend to favour widely spaced plutons, unless degrees of partial melting in the source are high. Wedge-shaped plutons can form much closer together and require lower degrees of partial melting. These results are in general agreement with current geophysical, petrological and experimental estimates of partial melting in the lower continental crust. (C) 2001 Elsevier Scien ce Ltd. All rights reserved.