Emplacement of nested diapirs: Results of centrifuge modelling

Concentrically expanded plutons (CEPs) are characterized by compositional zoning, mainly concentric magmatic fabric within the pluton and ductile fabric in the contact aureole concordant with the pluton/host rock contact. Two intrusion mechanisms have been proposed for CEPs: dyking + ballooning and diapirism. Here, we present results of a centrifuge model to study the kinematics and dynamics of CEPs. The model was centrifuged twice. For each stage one buoyant layer - each with the same physical parameters - was attached to the bottom of the model. After the first stage, diapirs of a blue stained buoyant layer intruded and deformed the overburden to spread as mushroom-shaped diapirs below a less-dense upper layer. After second centrifuging with a red buoyant layer attached, an intrusion of this buoyant material occurred along the stem of the pre-existing diapirs using them as a mechanically weak pathway. This latter intrusion was not diapiric, but the buoyant material rose as a "ductile dyke". Once reaching the bulb of the pre-existing diapirs, the intrusive material spread laterally resulting in extensive spreading and expansion of the overhang of the pre-existing diapirs. Model results show that CEPs can be the result of combined initial diapirism and subsequent "ductile dyking". Multiple diapirs can form only when the overburden units deform ductilely during the different stages of diapirism.