EEm RESEARCH A new indicator of movement direction during orogenesis: measurement technique and application to the Alps T.H. Bell, A. Forde and J. Wang Deparfmenf of Earfh Sciences, Jumes Cook University, Tawnsvifle, Qld 4811, Australia ABSTRACT The orientation of axes of sigmoidal, staircase or spiral inclusion trails within porphyroblasts provides an indicator of the direction of movement during deformation that is synchronous with metamorphism. A simple technique is presented in detail to find this axis in 3D by radially sectioning a horizontal slab cut from an oriented sample. When viewed from one direction, the switch in asymmetry of the porphyroblast inclusion trails in these sections defines the trend of the axis. Further radial sectioning of a vertical slab cut parallel to this trend determines the plunge of this axis. of sigmoidal or spiral-shaped inclusion 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. Measurements of spiral and sigmoid axes in garnet porphyroblasts from the European Alps show that they reflect the movement of the African Plate relative to Europe better than linear indicators preserved within the same rocks. This technique is independent of the model adopted for the formation Terra Nova, 7, 500-508, 1995. INTRODUCTION Stretching and mineral aggregate elon- gation lineations are used as indicators of movement direction during deforma- tion. However, they do not provide an easily used record of the relative convergent motion between plates (eg. Fig. 1). This possibly results from rotation of these lineations through large angles due to the effects of reactivation (e.g. fig. 19, Bell, 1986; fig. 28, Bell and Johnson, 1992) or reuse (Davis, 1993; Davis and Forde, 1994) of the foliation in which they lie during a subsequent deformation. Sigmoidal (Williams, 19941, staircase (Jones, 1994) or spiral-shaped inclusion trails (Bell et al., 1992) develop in por- phyroblasts from concurrent deforma- tion and metamorphism and two models exist for their development. These models differ in whether (Pas- sehier et al., 1992) or not (Bell et al., 1992) rotation of the porphyroblast occurs with respect to geographical space during inclusion trail develop ment. Irrespective of the model used to un- derstand these structures, clearly, the orientation of the axis of apparent rota- tion of these trails should supply useful information about the movement direc- tion during concurrent deformation and metamorphism at the time of por- phyroblast growth. If a rotational mod- el is favoured, the axis should lie perpendicular to the direction of shear- ing but within the foliation from which the inclusion trails formed (Rosenfeld, 1970; Schoneveld, 1979). If a non-rota- tional model is favoured the axis should represent the intersection of the overprinting high-angle foliations pre- served as inclusion trails (Fig. 2) and lie perpendicular to the relative direc- tion of bulk compression, but not neces- sarily within the matrix foliation (Bell et al., 1992; this foliation intersection axis preserved within the porphyroblasts is called the FIA). An approach is described, developed through 5 years of conducting this type of research, for accurately and repro- ducably determining the orientation of this rotation axis or FIA. This approach is independent of the mechanism by which the axis formed and is demon- strated using results that we have ob- tained from the Alps. This technique is internally self checking and, con- sequently, can be applied successfully by anyone, including undergraduate students. THE TECHNIQUE 1 Oriented sampling Accurately oriented samples, from outcrops that have not been subject to creep or slumping, Ire essential and must be marked for reorienting with minimum error. Consequently, Coclar compasses made by Breithaupt Kassel (Germany) are used which have a solid flap that can be placed in a stable position on the sample (other similar style compasses or ones with flap lids can be used in similar manner). With thumb in the centre, the compass flap is held against the most planar face, the compass oriented and then the flat is outlined with a felt-tip pen. A dip mark is added to the flap outline to indicate which way the face dips and whether or not it is inverted (Fig. 3). The minimum sample size needed is approximately 10 x 10 x lOcm with 15 x 15 x 15an being best. The compass is not adjusted for magnetic north. Magnetic north is corrected for the sample site, after determining the rota- tion axis or FIA orientation in our laboratory. 500 01995 Blackwell Science Ltd