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 mod el of anorthosite massif diapiric emplacement. The Egersund-Ogna massif (S. Norway) is of particular interest because recent petrological and geochron ological data constrain the initial geometry, emplacement conditions and ti ming (about 2 m.y.). The formation of this anorthosite massif is in agreeme nt with the classical petrological model, in which accumulation of plagiocl ase takes place in a deep-seated magma chamber at the crust-mantle limit, f rom which masses of plagioclase separate and rise through the lower crust u p to the final level of emplacement at mid-crustal depths. The Egersund-Ogn a massif also displays a foliated inner margin, in which strain ellipsoids have been reconstructed by investigating at 51 sites the deformation of meg acrysts of high-alumina orthopyroxene. Based on these petrological data, a model made up of one rigid layer (upper granitic crust) and three viscous l ayers (lower part of the granitic crust, noritic lower crust and anorthosit e) has been built up. The upper crust behaviour is represented by an elasto plastic law and the viscous layers obey elastic-viscoplastic laws with Newt onian viscosity. An inverse density gradient is considered between the lowe r crust (d = 3.00) and the anorthosite (d = 2.75), the loading consisting o nly in gravity. The modelling is carried out under axisymmetrical condition s, using the LAGAMINE finite-element code coupled with an automatic re-mesh ing algorithm designed to deal with large strains in complex structures. Th e results show that, from a mechanical point of view, the diapirism model i s a robust and consistent assumption for the emplacement of anorthosites, b ecause realistic diapir and rim-syncline shapes are obtained. Moreover, the numerically obtained emplacement time (about 2.5 m.y.) is in agreement wit h the available geochronological data, and the computed strain field is coh erent with field measurements, especially regarding the circumferential ext ension, which becomes the largest extension strain component in the expansi on phase. (C) 1999 Elsevier Science B.V. All rights reserved.