STRUCTURAL EVIDENCE FOR JUXTAPOSITION OF TWO ARCHEAN TERRANES IN THE TETON RANGE, WESTERN WYOMING

Contrasting early fold orientations in two domains in the northern Teton Range support preliminary evidence that the domains were juxtaposed along the string of ultramafic bodies that separate them. The presence of Archean gneisses recording high pressure granulite facies metamorphism and associated leucogranites a in the northwest Teton Range suggest that metamorphism and deformation here was a result of a 2685-2671 Ma Himalayan type orogeny. Gneiss exposed in the NW Teton Range is dominated by migmatites, with common kyanite bearing pelite and garnet amphibolite. In contrast, in the eastern Tetons, migmatites are rare or absent, rare pelites contain sillimanite without kyanite, and all but extremely iron-rich amphibolites lack garnet. eNd values from the gneisses exposed in the northwest are negative andeNd values from the gneisses exposed in the east are positive, indicating that the northwestern gneisses were formed from different protoliths compared to the rocks in the east. Thermobarometric evidence suggests a doubling of crustal thickness in the northwest (12 Kbars, 950° C peak metamorphism), but indicates much lower pressures and temperatures in the east. This evidence suggests that the rocks in the eastern Teton Range were accreted to those to the west after the 2685-2671 Ma high pressure event.

A comprehensive set of structural data for the region is used to test if the structural history of the area is consistent with this hypothesis. Foliations define open folds with average hinge orientations ~20°/026° across the entire range; these folds postdate development of foliation and isoclinal folding, but are crosscut by the Mount Owen Quartz Monzonite (2550 Ma). Lineations to the west of a preliminary boundary defined by a string of ultramafic bodies define folding with average hinge orientations ~44°/257° while the lineations to the east of this boundary are folded about the average hinge orientation ~35°/126°. These two macroscopic fold orientations (even with gentle 026° trending folding removed) indicate that these blocks experienced distinct early structural histories, supporting the hypothesis of juxtaposition of two terrains along the boundary.