PROGRESSIVE CHANGES IN THE DIRECTION OF BULK SHORTENING AND ITS EFFECTS ON PORPHYROBLAST GROWTH: AN EXAMPLE FROM THE KANMANTOO OF SOUTH AUSTRALIA

Quantitative microstructural analysis of porphyroblasts within the Cambrian Kanmantoo Group reveals that a lengthy history of deformation and metamorphism affected the turbidites of the Fleurieu Arc of the Adelaide fold-thrust belt that contrasts greatly with its relatively simple regional appearance. Foliation Intersection/Inflection Axis, or FIA, trends measured within garnet, staurolite and andalusite porphyroblasts, preserve a history of bulk shortening that was directed SW-NE, SE-NW, and WNW-ESE. The large scale regional folds that follow along the length this orogen formed prior to porphyroblast growth. They were overprinted by the effects of SW-NE directed shortening near orthogonal to their length when porphyroblast first began to grow and this formed the well known Fleurieu and Nackara oroclinal arcs in the orogen. They were further deformed sub-perpendicular to their axial planes during two younger periods of orogenesis when more porphyroblasts grew and it was probably at this time that the transecting cleavage that cuts across the Nackara oroclines was developed. Shear sense indicators within porphyroblasts have essentially a spiral geometry in total which strongly suggests that they formed to the east of the orogen core in a regime that involved an eastward thrusting on the opposite side of the orogen to the west directed thrust recognized by Jenkins & Sandiford (1992). These rocks were likely heated by granitoid emplacement associated with subduction of the rocks to the east below the orogen. Multiple phases of both staurolite and andalusite growth occur in a sequence that suggests the metamorphic path closely followed the reaction boundary between these two phases.