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Sediment Diapirism and Gravity Sliding of the 2 Ma Huckleberry Tuff Near the Teton Dam, Idaho: Small-Scale Structural Constraints

Mark Millard, Robert W. Clayton, and Clayton S. Painter. Dept. of Geology, Brigham Young University - Idaho, Rexburg, ID 83460, [email protected]

A 20 by 20 km sheet of the 2 Ma Huckleberry Tuff and underlying Pliocene alluvial gravel, basalt, and tuffaceous lacustrine sediments were involved in gravity sliding and flow shortly after deposition of the tuff. Large scale structures are similar to those observed in the Gulf of Mexico resulting from soft-sediment and salt flow, and include overturned anticlines >100 m in amplitude, strike slip faults with up to 1 km displacement, sedimentary diapirs, and an arcuate pull-apart valley 12 km long. Because displacement occurred during compaction but before devitrification, the tuff deformed plastically in its lower parts but contains brittle joints in its upper. The brittle joints were opened during shearing by as much as 1 m. We classify and analyze the orientations of plastic shear zones, plastically deformed joints, orientation of zones within the tuff, and joint sets to document the kinematics of deformation. The small-scale structures are consistent with the secondary deformation and gravity sliding originally proposed by Embree and Hoggan in 1999. Sliding was toward the southwest, resulting in NW-SE fold axes, SW-NE trending strike slip faults, and cooling joints that were plastically sheared toward the southwest. The opened joints played a role in the failure of the Teton Dam in 1976.