THE DEVELOPMENT OF DELTA-CLASTS IN NONLINEAR VISCOUS MATERIALS - A NUMERICAL APPROACH

Winged porphyroclasts are often used as kinematic or strain indicators in shear zones. In particular, it has been suggested that the geometr y of delta-clasts in mylonites can give information about the theologi cal behaviour of the matrix around the clast (Passchier et al., 1993). We tested this hypothesis with a numerical finite element simulation of the flow field of a viscous matrix around a relatively rigid clast. Power-law viscous flow properties of the matrix with stress-exponents of 1 (linear viscous), 3 and 5 were tested, as well as three differen t sets of approximately simple shear boundary conditions, which are re levant to experimental and/or natural situations, The simulations show that the flow field is primarily determined by the boundary condition s. The stress-exponent is of secondary importance. In general, the dif ferent boundary conditions produce two types of flow fields about the rigid clast: an eye-shaped flow field and a bow-tie shaped flow field. The former is produced by boundary conditions that approximate simple shear applied at an infinite distance from the rigid clast, and the l atter is produced by boundary conditions which are equivalent to a she ar box or ring shear apparatus. Although both the shear-box and ring-s hear boundary conditions produce bow-tie flow patterns, the wing geome tries which result from the flow patterns for these two cases can be s ignificantly different. The geometry of the wings that develop on a de lta-clast is a direct function of the flow field around the clast. For linear and non-linear theologies, only the ring-shear boundary condit ions produce wings that show ''stair-stepping'' (i.e. wings that are p arallel but not in the same plane). In natural shear zones, delta-clas ts both with and without stair-stepping are observed. In comparing geo logic observations with experimental or numerical modelling, it is ess ential to understand which boundary conditions are appropriate.