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_Journal of Structural Geology 33 (2011) 32-37_ Contents lists available at ScienceDirect Journal of Structural Geology journal homepage: www.elsevier.com/locate/jsg Sheets within diapirs - Results of a centrifuge experiment C. Dietl *, Hemin Koyib Institut für Geowissenschaften, Goethe-Universität, Altenheimer Straße 1, 60438 Frankfurt, Germany Geocentrum, Uppsala Universitet, Villavägen 16, Uppsala, Sweden Article info Article history: Received 21 February 2010 Received in revised form 9 September 2010 Accepted 30 October 2010 Available online 10 November 2010 Keywords: Centrifuge Diapir Balloon-on-string Finger-shaped Composite Abstract We carried out a centrifuge experiment to model the diapiric rise of a stratified PDMS layer from three perturbations through a non-Newtonian, ductile overburden. The experiment conducted at 700 g resulted in three composite diapirs fed by different PDMS layers. The three resulting diapirs represent two different stages of diapirism. One of the diapirs (diapir 1), which reached its level of neutral buoyancy and extruded at the surface of the model, was tabular in profile and copied by an internal intrusive body. The other two diapirs (diapirs 2 and 3) were still in the ascending stage when centrifuging was stopped and thus did not extrude at the surface. They displayed a typical balloon-on-string geometry, which develops at a high viscosity contrast between a highly viscous overburden and a less viscous buoyant material. The internal geometry of these last two diapirs, fed by the lower impure PDMS, however, did not copy the shape of their precursors. Instead, they had a finger-like shape. The finger geometry of the internal part of the diapirs might be the result of the higher viscosity of the impure lower PDMS intruding a less viscous clean PDMS. Compared to nature, diapir 1 represents a fully developed concentrically expanded pluton or nested diapir, while diapirs 2 and 3 resemble composite plutons which host magma batches of dyke-like geometry. Based on the results of our experiment we suggest that truly concentrically expanded plutons develop from the latter. © 2010 Elsevier Ltd. All rights reserved. 1. Introduction Numerous plutons contain sheet-like (mac) dikes and swarms of (mac) enclaves (Fig. 1a and b); the latter are commonly regarded as disintegrated dikes (Sparks and Marshall, 1986; Frost and Mahood, 1987). Examples include the Chila Pluton in Ethiopia (Tadesse-Alemu, 1998; Jungmann, 2009; Fig. 2a) and the Cannibal Creek Pluton in Australia (Paterson and Vernon, 1995; Fig. 2b). Most of these dikes are not dikes sensu strictu, because they do not crack solid rock which reacts elastically on the high magma pressure at the tip of the dike (Lister and Kerr, 1991). Rather they propagate “ductilely” through a mainly viscously behaving magma. It is this propagation process which finally leads to disintegration of these contiguous bodies and to the formation of enclave swarms. Nevertheless, they are dikes in a geometric sense, i.e. they have a high length width ratio (Spera, 1980), and are probably driven e as dikes s.s. e by a combination of buoyancy and the magma pressure within the dike (Petford et al., 2000). Other plutons consist of several magma batches which are nested into each other in a concentric manner. Two of the most intriguing examples of these composite plutons are the Joshua Flat-Beer Creek Pluton in California (Dietl and Longo, 2007; Fig. 3a) and the Ardara Pluton in Ireland (e.g. Paterson and Vernon, 1995; Sigesmund and Becker, 2000; Fig. 3b). Both features may be related to each other: the dikes could be the feeders of the magma batches which form the composite plutons. However, this possible genetic relation is neither proven nor known yet how they are related to each other. We carried out a centrifuge model which provides some interesting answers to these questions. 2. The experiment The model consisted of a 34 mm thick non-Newtonian, ductile overburden with a density ρ of 1.55 g cm³ and a viscosity μ of 2 × 10⁶ Pa s at a strain rate of 2.5 × 10⁻³ s⁻¹ and a buoyant, Newtonian polydimethylsiloxane (PDMS) layer of 6 mm thickness that was placed beneath the overburden. Both, the overburden and the buoyant layer were stratified in order to visualize the deformation structures within them. Stratification in the overburden was passive. The passive layers in the overburden were colored brown (7 strata, each 4 mm thick) and dark-grey (6 strata, each 1 mm thick), respectively. However, the buoyant layer consisted of two mechanically active layers; an upper clean PDMS layer (4 mm thick, ρ ~ 0.964 g cm³, μ ~ 2.4 × 10⁴ Pa s) which was stained purple and a lower impure light-grey layer (2 mm thick, ρ ~ 1.17 g cm³, μ ~ 5 × 10⁵ Pa s) which consisted of sand-contaminated PDMS. The model was topped by two thin plasticine strata, each 1 mm thick; the lower one was light-grey, the upper one white with a strain grid on its surface. To initiate three diapirs in the model, three perturbations of purple PDMS were initially put on top of the buoyant PDMS layer (Fig. 4). The ductile overburden simulated lower-crustal, siliciclastic rocks (ρ ~ 2.7 g cm³, μ ~ 10²² Pa s), the pure PDMS represented a felsic and highly viscous magma or partially molten silicic rock with ρ ~ 1.7 g cm³ and μ ~ 10²⁰ Pa s and the impure PDMS stands for a mac magma with ρ ~ 2 g cm³ and μ ~ 10²¹ Pa s. Viscosities of the “model magmas” are, of course, too high and the density too low with respect to a natural system. However, the observations from the model that we describe below and our interpretations are probably also valid for less viscous natural systems which were not possible to model with the currently available analogue materials. The setup of the model is displayed in Fig. 4, the material properties and scaling factors are listed in Table 1. The experiment was centrifuged for 6 min and 30 s at 700 g and stopped when the first PDMS diapir reached the model surface. The model was photographed in top view and sectioned for further examination. During centrifuging, three diapirs were initiated from the three perturbations. One of them (diapir 1) had pierced through the overburden and both the plasticine strata and spread on top of the model (Fig. 5a) to form an asymmetric, tabular extrusion (Fig. 5b). Around diapir 1, a fully developed rim syncline had formed. Even the two plasticine layers on top of the model were incorporated into the rim syncline and were deformed into recumbent, isoclinal folds. Where deformation is strongest - due to the extrusion of the PDMS - the upper, overturned limb of these folds is pulled off and thrusts have formed along which the tabular diapir head moved sideways during its extrusion (Fig. 5b). Diapir 1 is a composite structure and consists of both buoyant strata (i.e., the clean and the impure PDMS materials). The grey, impure PDMS copies the external geometry of the diapir which is formed by the purple PDMS (Fig. 5b) and which is almost elliptical in map view and tabular in profile. Hereby, diapir 1 resembles geometrically a typical composite pluton. Diapirs 2 and 3 had reached the upper half of the overburden when centrifuging was stopped. Nevertheless, they caused doming of their overburden and brittle fracturing of the plasticine strata. Both these diapirs have “balloon-on-string” geometry (Jackson and Talbot, 1989) with relatively thin stem in relation to the diapir head (Fig. 5c and d). Around these two diapirs also rim synclines had developed (Fig. 5c and d). The internal structures of diapirs 2 and 3 differ strongly from that observed in diapir 1. The grey PDMS body... 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