The Significance and Application of Foliation Intersection/Inflection Axes (FIA) within Porphyroblasts: A review


An increasing number of structural and metamorphic studies through­out the past sixty years have shown the importance of using porphyroblast microstructures to gain further understanding of deformation and meta­morphic history and mechanisms. A recent very significant advance has been the development of techniques for the measurement of Foliation In­tersection/Inflection Axes (FIAs) preserved within porphyroblasts. FIAs provide a quantitative method of microstructural analysis that enables the direct linkage and integration of the deformation, metamorphic and tec­tonic history. The FIA technique determines the orientation of or the axis of curvature of a foliation effected by an overprinting event, or the inter­section lineation between successive foliations, preserved as inclusion trails within porphyroblasts. Because the axis is determined completely indepen­dent of the model of formation of these included foliations, it allows such models to be thoroughly tested. The method involves three-dimensional examination of microstructures within porphyroblasts in several vertically orientated thin sections that fan around the compass and which have been cut from spatially orientated samples. FIA measurements allow reconstruction, correlation and dating of regional and large-scale defor­mation and metamorphic histories. Such reconstructions of the tectonic evolution had been conducted in several orogenies around the world. New insights include (1) the intimate control of deformation partitioning on metamorphic reactions, (2) a possible technique for determining the direc­tion of relative plate motion within ancient orogens, and (3) the role of rota­tion versus non-rotation of porphyroblasts relative to geographical coordi­nates during deformation have also been raised.

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