Session 11: Modern modelling trends in tectonics

Djordje Grujic1

Neil S. Mancktelow2

1 Geologisches Institut, Universität Freiburg, 79108 Freiburg i.Br., Germany

2 Geologisches Institut, ETH-Zentrum, CH-8092 Zürich, Switzerland

"Shear zones" are recognized in the field on all scales as elongate, approximately planar zones of high strain. Good field constraint on the strain geometry and kinematics are rarely available, and it is generally assumed that they correspond to those of simple shear. If the adjacent walls are rigid, then the length of the shear zone remains constant during deformation and the only possible solutions involve some combination of perfect simple shear parallel to the walls and width change perpendicular to the walls, due to variation in volume. This restriction is removed if the walls are deformable, as is certainly the case in lower crustal shear zones developed at high metamorphic grade. Such high-grade shear zones are commonly intimately associated with partial melting and migmatite development, with leucosome material concentrated in the zones themselves. This study presents the results of analogue model experiments considering the geometry of such high-grade shear zones, both on the outcrop scale, where melt is generally distributed in an irregular anticlustered pattern, and on the larger map scale, where partial melting is often restricted to major regional shear zones. All experiments were performed under pure-shear boundary conditions. Models were made of paraffin wax with melting range 46-48°C and the melt was represented by weak inclusions of vaseline. Two shapes of weak sites were used, namely round cylinders and elongated prisms. Experiments were performed to consider both random, anti-clustered distributions of weak inclusions and planar zones of concentrated weak material, as an analogue for regional "shear zones". All the weak sites, regardless of their shape and orientation, initiated shear zones. Elongate weak sites (axial ratio in the XZ plane 1:2) initiated shear zones dependent on their orientation with respect to the bulk strain axes, and both dextral and sinistral shear zones formed within the same model. Sites with their longer or shorter axis parallel to the principal shortening direction flattened homogeneously and initiated conjugate shear zones at their tips, similar to the geometry in experiments with initially cylindrical sites (i.e. circular in the XZ plane). In experiments with a high concentration of weak inclusions, irrespective of their shape and orientation, a new planar fabric containing the weak phase (i.e. by analogy - melt) resulted at higher strain. This new fabric is, however, not planar but is defined by anastomosing shear zones surrounding less deformed areas. The anastomosing strain pattern developed in these analogue models, which were deformed with boundary conditions of pure shear, is very similar to natural examples from southern Madagascar and demonstrates that the automatic assumption of regional simple shear kinematics may not always be justified, and can lead to incorrect interpretations.