TECTONIC GEOMORPHOLOGY AND STRUCTURAL EVOLUTION OF CENTENNIAL VALLEY, MT: EXPLORING THE LINKAGE BETWEEN SUBSURFACE STRUCTURE, SURFACE FAULTING, AND LANDSCAPE EVOLUTION IN THE WAKE OF YELLOWSTONE

We acquired a north-south oriented, 10-km long, high-resolution reflection seismic line of the Centennial Valley, Montana, to better understand its fault kinematics, origin as part of a proposed left-lateral shear zone associated with Yellowstone and Basin and Range extension, the linkage between basin geometry and surficial geology, and seismic hazards for this part of the SW Montana seismic belt. Oblique-slip, en-echelon normal faults of opposing polarity, some of which cut late Pleistocene deposits, bound the valley flanks, but little is known about their linkage and interaction in the subsurface of the basin or expression in the surface geology. Our data illuminate subsurface stratigraphy and faults down to ~1000 m, clearly showing that the basin has a half-graben geometry with a southern depocenter driven by the listric geometry of the north-dipping Centennial fault. Allostratigraphic packages display reflectors with clear growth geometry against these faults. We also note a small, positive flower structure that we interpret as evidence of transpression and recent basin inversion in a broader accommodation zone. In the northern part of the line we were able to illuminate several steep, south-dipping faults that appear to be the eastward extension of the Lima Dam Fault that has a well expressed surface scarp known to have ruptured in the Holocene a few kilometers west of the line. Geologic and geomorphic mapping, including stream sinuosity and long profile modeling are consistent with regional north-tilting and local folding above faults that have little to no other obvious surface expression. Preliminary forward modeling using Move software helps reconstruct the timing of fault activity, including the transition from older, purely extensional to current transpressive kinematics. These results are consistent with a major left lateral shear zone forming north of the Snake River Plain in the Pleistocene and contributing to the observed seismicity in this part of Montana.