CAN RAPID EXHUMATION SUPPRESS FOUNDERING OF HIGH-DENSITY CRUSTAL ROOTS?

The Coast Mountains of British Columbia, expose the largest Phanerozoic batholith complex exposed on earth. The plutons were formed during subduction, collision, and translation of crustal slivers along the North American plate margin. Geochemical, geophysical, and structural studies have inferred that the Coast Orogen culminated in the formation of a large orogenic plateau that may have undergone foundering of mafic material, producing a differentiated continental crust.

The ability of a crustal root to founder into the mantle is closely related to the contrast in density between the crustal root and the mantle, the viscosity of the mantle, and the shape and size of the root. Using natural rock compositions, the density of crustal material has been calculated using forward thermodynamic models derived with the Theriak-Domino software. If the crust was greater than ~50 km thick, most crustal compositions investigated here would be negatively buoyant and have the potential to founder. Mafic rocks and garnet rich pelites have the greatest potential to founder, owing to their large garnet content at elevated pressure and temperature. The largest uncertainty on foundering rate is the viscosity of the mantle. At viscosity <10¹⁹Pa*sec, foundering is rapid, and the root will likely be removed. At viscosity >10²² Pa*sec foundering is slow compared to plate tectonic rates.

These calculations assume constant density for the crustal root, thus petrologic processes that effect density should be considered. The metamorphic core of the Coast Mountains are known to have decompressed as much as ~800 MPa in the time period between 65 and 50 million years ago, at exhumation rates of 2 mm/year, comparable to foundering rates at a range of estimated density and viscosity. Decompression results in the replacement of high-density phases like garnet and pyroxene by low-density phases such as cordierite and plagioclase. Thus, if the rate of foundering is near or slower than the exhumation rate, it is possible that parts of the crustal root will decrease in density and be unable to sink into the mantle. Thus, a critical geodynamic question for the Coast Mountains is if the present average crustal thickness of ~30 km beneath the metamorphic core is the result of root foundering, or thinning and density loss due to crustal extension.