The deformation of the Egersund-Ogna anorthosite massif, south Norway: finite-element modelling of diapirism

This paper aims at testing the mechanical relevance of the petrological model of anorthosite massif diapiric emplacement. The Egersund-Ogna massif (S. Norway) is of particular interest because recent petrological and geochronological data constrain the initial geometry, emplacement conditions and timing (about 2 m.y.). The formation of this anorthosite massif is in agreement with the classical petrological model, in which accumulation of plagioclase takes place in a deep-seated magma chamber at the crust-mantle limit, from which masses of plagioclase separate and rise through the lower crust up to the final level of emplacement at mid-crustal depths. The Egersund-Ogna massif also displays a foliated inner margin, in which strain ellipsoids have been reconstructed by investigating at 51 sites the deformation of megacrysts of high-alumina orthopyroxene. Based on these petrological data, a model made up of one rigid layer (upper granitic crust) and three viscous layers (lower part of the granitic crust, noritic lower crust and anorthosite) has been built up. The upper crust behaviour is represented by an elastoplastic law and the viscous layers obey elastic-viscoplastic laws with Newtonian viscosity. An inverse density gradient is considered between the lower crust (d = 3.00) and the anorthosite (d = 2.75), the loading consisting only in gravity. The modelling is carried out under axisymmetrical conditions, using the LAGAMINE finite-element code coupled with an automatic re-meshing algorithm designed to deal with large strains in complex structures. The results show that, from a mechanical point of view, the diapirism model is a robust and consistent assumption for the emplacement of anorthosites, because realistic diapir and rim-syncline shapes are obtained. Moreover, the numerically obtained emplacement time (about 2.5 m.y.) is in agreement with the available geochronological data, and the computed strain field is coherent with field measurements, especially regarding the circumferential extension, which becomes the largest extension strain component in the expansion phase. (C) 1999 Elsevier Science B.V. All rights reserved.