Petrology and structure of Thor-Odin Gneiss Dome, Shuswap Metamorphic Complex, British Columbia
Authors: Reesor J E , Moore J M 
Published: August 2015

Thor-Odin gneiss dome is a part of a large domal complex in the central, eastern part of the Shuswap Metamorphic Complex of the southern Canadian Cordillera. The dome is characterized by a structural-strati graphic succession divided into four lithologically
distinct zones. These are: the Core Zone , comprising migmatitic and granitic gneisses in the central part of the gneiss dome; the Mantling Zone , well-differentiated metasedimentary rocks ranging from quartzite and marble through calc-silicate gneiss to peliticschist; the Fringe Zone, in part
overlapping, and in part surrounding the Mantling Zone and characterized by large amounts of granitic and pegmatitic rocks; and the Supracrustal Zone that lies outside the gneiss complex, and forms a cover to the gneisses. Each zone exhibits a characteristic structural style and it is concluded that
a series of three successive fold movements can be recognized in each. The earliest consists of interfolding, in a northerly direction, of the Core Zone -Mantling Zone interface in the area of the present core. Large scale east - west folds developed synchronously in the Mantling Zone to the south,
independently of the dome. The second stage of folding reflects penetrative movement within the Core Zone as a whole, resulting from diapiric up rise of the Core Zone with respect to the Mantling gneisses. In the Mantling Zone, earlier folds were further compressed, partly redeformed, and strata
immediately south of the core were much thinned. Structural evolution of the gneisses reached a culmination during this episode and many complex mesoscopic structural features were developed. The third structural episode is more sporadic in its development and is represented by warps, local
crenulation of mica schist, and open upright folds in directions d epende nt upon the orie ntation of e arlier fold e d strata. Metamorphism nearly everywhere in the gneiss complex has reached the sillimanite -almandine -orthoclase subfacies, though at the outer periphery of the Fringe Zone musovite
appears in the assemblage. Metamorphic grade reached a maximum coincident with the earliest episode of folding and remained hi h throughout the second phase of movement. After all movement had ceased cordierite overgrowths formed on sillimanite in the western part of the Core Zone and andalusite
formed on kyanite in the outer Fringe Zone. Thus heating is considered to have not only pre dated or coincided with the earliest deformation, but far outlasted it in some zones. The gneiss complex is separated, in part, from the supracrustal rocks through a cataclasite zone and here, in the brittle
stage of deformation, retrograde metamorphism is extensive and the regional metamorphic isograds are displaced. The stratigraphic succession in Thor-Odin gneiss dome is broadly similar to the latest Precambrian- Paleozoic succession of the Purcell Selkirk Mountains. The rock of the Mantling Zone did
not rest on an older gneissic terrain, later to be remobilized into a classic 'Mantled Gneiss' dome . Rather, the intense migmatization and pegmatization of the Core Zone took place synchronously with the deformation and metamorphism of the complex as a whole. The core reflects the presence of an
original succession of pelites and semipelites compositionally suitable for migmatizati on with resultant mobilization and intense internal deformation. The quartzite - calcsilicate- marble-schist succession, on the other hand, resisted penetrative migmatization and consequently provided a
contrasting 'cover' for the Core Zone.