A NEW INDICATOR OF MOVEMENT DIRECTION DURING OROGENESIS - MEASUREMENTTECHNIQUE AND APPLICATION TO THE ALPS

The orientation of axes of sigmoidal, staircase or spiral inclusion tr ails within porphyroblasts provides an indicator of the direction of m ovement during deformation that is synchronous with metamorphism. A si mple technique is presented in detail to find this axis in 3D by radia lly sectioning a horizontal slab cut from an oriented sample. When vie wed from one direction, the switch in asymmetry of the porphyroblast i nclusion trails in these sections defines the trend of the axis. Furth er radial sectioning of a vertical slab cut parallel to this trend det ermines the plunge of this axis. This technique is independent of the model adopted for the formation of sigmoidal or spiral-shaped inclusio n trails and can be used to evaluate the mechanism by which they form. It can also be used to evaluate theories of folding and orogenesis. M easurements of spiral and sigmoid axes in garnet porphyroblasts from t he European Alps show that they reflect the movement of the African Pl ate relative to Europe better than linear indicators preserved within the same rocks.