Analogue and numerical modelling of shape fabrics: application to strain and flow determination in magmas

We summarise numerical and analogue models of shape fabrics, and discuss th eir applicability to the shape preferred orientation of crystals in magmas. Analyses of flow direction and finite strain recorded during the emplaceme nt of partially crystallised magmas often employ the analytical and numeric al solutions of the Jeffery's model, which describe the movement of noninte racting ellipsoidal particles immersed in a Newtonian fluid. Crystallising magmas, however, are considered as dynamic fluid systems in which particles nucleate and grow. Crystallisation during magma deformation leads to mecha nical interactions between crystals whose shape distribution is not necessa rily homogeneous and constant during emplacement deformation. Experiments c arried out in both monoparticle and multiparticle systems show that shape f abrics begin to develop early in the deformation history and evolve accordi ng to the theoretical models for low-strain regimes. At large strains and i ncreasing crystal content, the heterogeneous size distribution of natural c rystals and contact interactions tend to generate steady-state fabrics with a lineation closely parallel to the direction of the magmatic flow. This e ffect has been observed in all three-dimensional experiments with particles of similar size and for strain regimes of high vorticity. On the other han d, studies of feldspar megacryst sub-fabrics in porphyritic granites sugges t that these record a significant part of the strain history. Thus, the fab ric ellipsoid for megacrysts evolves closer to the strain ellipsoid than fo r smaller markers. This behaviour results from the fact that the matrix for ms of the melt and smaller crystals behave like a continuous medium relativ e to the megacrysts. Consequently, in the absence of these markers, and bec ause the fabric intensities of smaller particles such as biotite are stable and lower than predicted by the theory, finite strain remains indeterminat e. In that case, strain quantification and geometry of the flow requires th e addition of external constraints based on other structural approaches.