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

Contrasting early fold orientations from two tectonic blocks in the northern Teton Range support preliminary evidence that the blocks 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 in the northwest Teton Range suggest that metamorphism and deformation there was a result of a 2685-2671 Ma Himalayan-type orogeny. These gneisses are dominated by migmatites, with common kyanite-bearing pelite and garnet amphibolite. In contrast, in the eastern Teton Range, migmatites are rare or absent, rare pelites contain sillimanite without kyanite, and all but extremely iron-rich amphibolites lack garnet. εNd values from the gneisses exposed in the northwest are negative and εNd values from the gneisses exposed in the east are positive, indicating that gneiss in the two blocks formed from contrasting protoliths. Thermobarometric evidence suggests a doubling of crustal thickness in the northwest terrane (12 Kbars, 950° C peak metamorphism), whereas differences in rock type indicate 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 during or after the 2685-2671 Ma orogeny.

A comprehensive set of structural data suggests that the northwestern Teton Range has a structural history distinct from that of the northeast. 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 cut by the Mount Owen Quartz Monzonite (2550 Ma - Pb/U zircon). Lineations in rocks exposed west of a boundary defined by a string of ultramafic bodies outline a fold with an average hinge orientation of ~44°/257°; lineations east of this boundary are folded with the average hinge orientation of ~35°/126°. These two macroscopic fold orientations (even with superposed gently plunging fold trending 026° removed), taken with cross-section interpretations suggest that the two blocks experienced distinct early structural histories, supporting the hypothesis of juxtaposition of two terrains along the boundary during or after the 2685-2671 Ma Himalayan-type orogeny.