Flow and Fabric Development during the Diapiric Rise of Magma

Abstract

Scaled isothermal experiments of the internal circulation induced by viscous drag at a diapir/country rock interface are compared with published theoretical studies of flow induced finite deformation inside a rising sphere, and new numerical results are presented for the incremental strain pattern developed in spherical diapirs. The results predict a time-dependent deformation history within rising diapirs that produces finite strains characterized by strong oblate flattening strains at the margin and strong vertical prolate strains in the center with an intervening zone of low plane strain. Finite strains are discussed in terms of the different sub-fabrics expected to form in granitoids after considering the history of rheology and temperature distribution in rising magma. Strong rheological and temperature gradients are predicted for the margins of internally circulating diapirs that can lead to the formation of strong penetrative foliations at the margin, and magmatic or even isotropic fabrics in the interior. The experimental results account for many features normally attributed to ballooning of post-tectonic mesozonal granitoid plutons, as well as suggesting a mechanism for the dispersal, distortion, and mixing of entrained mafic marterial, and the formation of reversely zoned intrusions.