LINKING DEFORMATION AND METAMORPHISM AT MULTIPLE SCALES

Deformation and metamorphism must be linked on multiple scales from local (hand specimen) to terrane to regional/tectonic scale, and different insights results from each. Local links start with texture/fabric relationships. Metamorphic assemblages and reactions can typically be correlated with the progression of local fabrics although many questions remain about porphyroblast-matrix fabric relationships and also about the larger significance of fabric events. The fabrics are useful for organizing and classifying metamorphic minerals into assemblages and metamorphic phase relations are useful in classifying structural fabrics into more regional deformation events. Compositional mapping of major and accessory phases, and especially maps of larger thin section domains, are a powerful tool for linking metamorphism and deformation. Linkages at the terrane scale commonly involve metamorphic field gradients, thermal gradients associated with igneous rocks, and deformation gradients. Although partitioning relationships can complicate interpretations, the feedbacks between the three processes can be key to linking them. At the largest scale, dynamic models can now be used to explore the thermal and rheological implications of different types of orogenic events and crustal configurations and can predict P-T paths for comparison with field observations. Calculated phase diagrams and pseudosections are invaluable for linking metamorphism and deformation and for integrating the various scales of linkages. Finally, in-situ geochronology and petrological analysis of chronometer phases (i.e. monazite, xenotime, titanite, allanite, etc.) are a critical part of the integration of scales and allow the tectonic interpretation of metamorphism and deformation. Isobaric terranes in the southwestern USA and northern Saskatchewan, Canada provide examples of deformation-metamorphism interactions and of the use of in-situ geochronology to illuminate interrelationships between metamorphism and deformation.