|Dietl, C. and Koyi, H. A. 2002. Emplacement of nested diapirs: Results of centrifuge modelling. In: Schellart, W. P. and Passchier, C. 2002. Analogue modelling of large-scale tectonic processes. Journal of the Virtual Explorer, 7, 79 - 86.|
Emplacement of nested diapirs: Results of centrifuge modelling
Discussion and conclusions
Both intrusion mechanisms proposed for CEPs, diapirism and dyking, have their strong and weak points. Dykes are efficient as transport conduits for the construction of sheeted plutonic complexes and it is even possible to construct dyke-fed magma chambers with a concentrical internal fabric. However, it is not yet clear how magma ascent can be related to brittle and at the tip of the crack - elastic process like fracturing (Weijermars 1997), while the emplacement is driven purely by ductile (i.e. plastic) processes (i.e. magma chamber expansion in combination with a ductily deforming aureole). Diapirism, on the other hand, is a viable ascent and emplacement mechanism in the lower crust. It has been frequently debated, whether granitoids can rise as diapirs above the brittle-ductile boundary in the continental crust (e.g. Vigneresse and Clemens 2000).
The subsequent intrusions of nested diapirs into the same region or even magma chamber can extend the life time of an igneous system, because the consecutive intrusion of numerous melt bodies supplies additional heat and buoyant material, i.e. magma. Moreover, nesting of diapirs suggests that the first intrusive bodies created pre-heated pathways for later batches of typically more felsic magma, allowing the latter to rise faster and with less heat loss. Consequently, nesting makes it possible for diapirs to ascend farther than modeled hot-Stokes diapirs (Miller and Paterson 1999), which according to Marsh (1982) cannot rise farther than twice there diameter. The present experiment shows how nesting of diapirs can work in a ductile regime. It also points to the fact that CEPs are not necessarily the result of diapirism sensu strictu.
The first intrusion in the experiment was clearly diapiric and evolved all the diapiric features known from former labarotory experiments (e.g. Ramberg 1981). Mushroom shape with the very thin stems is typical for diapirs developing when viscosity ratio between overburden and buoyant layer (m = moverburden/mbuoyant) is Å 1 (Jackson and Talbot 1989). In the current experiment, this rate is 11.77. Dragging upward of the overburden material and the formation of rim synclines are also typical diapiric features (Figs. 5c-d).
The second buoyant layer (RG2) used the stems of the pre-existing diapirs as mechanically weak and conduit-like pathways. Use of these weak zones facilitated and accelerated the rise of RG2 compared to RG1. It took the first generation diapirs 930 to reach its level of neutral buoyancy and to spread beneath the PDMS layer, whereas the second intrusion needed at maximum 610.
Following the stems of the RG1 diapirs gives the ascending RG2 intrusions probably a dyke-like appearance with a large lateral extent relative to thickness, which is according to Spera (1980) the main characteristics of dykes. Moreover, according to the definitions for diapirism and dyking used in this study, the use of a zone of weakness as a pathway is typical for dyking. Consequently, the second intrusion was not unambiguously diapiric, but shows some geometric features characteristic of dykes. However, dyking is a mechanism whereby a magma fractures the crust by using the elastic properties of rocks (Lister and Kerr 1991). In the experiment described above only viscous materials were applied, which do not behave elastically. To solve this dilemma and to describe the ascent mechanism of the RG2 layer we introduce the term ductile dyking as the buoyancy-driven rise of viscous body with a large aspect ratio through a viscous medium along weak pathways and by ductile processes. In fact, also in nature, as the host rock will be deformed ductilely already during the first diapiric intrusion, ductile dykes rather than elastical fracturing of the host rock is more likely. Ductile dyking may leave some geochemical signature, which can be identified in the field and which might help distinguish it from dyking under brittle conditions. In case the earlier intrusions still contain melt, those can be mingling structures, back-veining of the older intrusive material and the incorporation of xenocrysts from the pre-intruded magma. In addition, since ductile dyking is associated with a relatively heated and hence dutile host rock it is less likely to find chilled margins in ductile dykes.
When reaching the bulbs of the pre-existing diapirs (i.e. the level of neutral buoyancy) the second stage intrusions spread and inflated the RG1 diapirs. The final shape of the nested diapirs is tabular as proposed by McCaffrey and Petford (1997) for most plutons. However, they are not the result of pure dyking + ballooning as suggested by these authors, but the outcome of combined diapirism, ductile dyking and ballooning. Transferred to nature the results of the experiment imply, that heavily deformed ductile aureoles form already during an early diapiric stage in the emplacement history of a CEP. Most internal fabrics of CEPs are created when subsequent dyke-like magma pulses enter the magma chamber, inflate it and overprint the fabrics which developed during the first intrusive stage.
In nature, nesting of multiple magma bodies extends the life time of igneous systems, because their consecutive intrusion adds additional heat to the system which may enable CEPs to pass the ductile-brittle transition at about 10 km depth. That is shown by the Ardara pluton, Cannibal Creek pluton and Tuolumne intrusive suite, respectively, which all were emplaced above the ductile-brittle transition (Paterson and Vernon, 1995).
In conclusion, model results show that CEPs can form by a combination of diapirism and subsequent ductile dyking of buoyant material through the stem of pre-existing diapirs. Multiple diapirs can form only when the overburden units deform ductiley during the different stages of diapirism. Multiple injections of magma into the magma chamber lead to ballooning and to a kinematic reactivation of the igneous system. The consecutive intrusion of several magma batches (first by diapirism, followed by ductile dyking) extends the longevity of CEPs through the additional heat input of the individual subsequent intrusion and allows them to rise beyond the brittle-ductile boundary within the Earths crust.
Special thanks go to A. Skelton, R. O. Greiling and E. Stein for discussion and comments. A. Skelton also helped to improve the English. Hemin A. Koyi is funded by the Swedish Research Council (Vetenskapsrådet).
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