DETERMINING THE RELATIVE STRENGTHS OF ROCKS FROM DEFORMATIONAL GEOMETRY AND IMPLICATIONS FOR THE RHEOLOGY OF THE LOWER CRUST

Questions about the rheology of the deep crust include the following. (1) What role does melt extraction play in driving strain? (2) What is the relative strength of mafic and felsic rocks in the deep crust? Narrowly spaced mafic dikes within a migmatitic host orthogneiss allow us to explore both of these questions. The Bustard Islands, Ontario, Canada, lie within the Central Gneiss belt of the Grenville Province. Upper amphibolite facies metamorphism and deformation resulted in partial melting of the host orthogneiss and development of a cuspate-lobate structure in the mafic dikes. The sharp cusps of the deformed dikes are coincident with well-developed leucosome-rich necks in the orthogneiss. Possible explanations for the geometry include: (a) relatively weak dikes, resulting in cuspate-lobate geometry as expected from fluid-dynamics experiments, (b) relatively strong dikes, perhaps consistent with partial melting of the surrounding orthogneiss but inconsistent with the cuspate-lobate geometry, and (c) melt extraction resulting in volume loss and contraction of the host migmatite and associated development of the cuspate-lobate geometry independent of their relative strengths.

We explore these questions through microstructural analysis and 3D numerical modeling. Microstructural analysis focuses on the distribution of melt contemporaneous with deformation. The numerical modeling centers on identifying the sensitivity of developing the observed deformed geometry to the rheological structure of the different units. Preliminary modeling results suggest that the cuspate-lobate structure may form irrespective of the relative strength between the host rock and dikes. Preservation of these unusual geometries in combination with our preliminary analysis and modeling suggests that volume changes associated with melt extraction across a range of spatial and temporal scales may play an important role in development of deformational features found in the lower crust.