TESTING THE BULGE: USE OF NEOGENE BEDROCK PALEO-VALLEYS TO TEST MODELS OF THE YELLOWSTONE THERMAL DOME AND RELATED DEFORMATION

Anders et al, (1989) proposed that deformation from the migration of the Yellowstone hotspot has resulted in a parabolic distribution of outward propagating seismicity centered upon the axis of the Snake River Plain. Recently, Smith et al (2009) utilized geodetic measurements to seismically image the hotspot plume in order to constrain the geographic extent of the Yellowstone thermal deformation. The study delineated a ~400- km-wide and ~500-m-high topographic dome centered on the Yellowstone Plateau. Both hypotheses place southwest Montana within the region of Yellowstone deformation. South of Great Falls, MT, the upper Missouri River and its headwater tributaries, the Madison and Jefferson, flow through the region of deformation. The modern river valleys follow Miocene half grabens containing Cenozoic valley fill (Fields et al, 1985) and a mid-Tertiary unconformity (Barnosky, 2008). The northern limit of the field area at Holter Lake, MT, has experienced very little tectonic activity. Strath terraces exposing the unconformity are typically ~80 meters above the adjacent Holocene floodplain. Moving south towards the Madison and Jefferson River valleys, the unconformity begins to gain elevation more rapidly than the current valley profile and is disrupted by increasing numbers of normal faults. Using the Mid Tertiary unconformity as a datum surface, this study will create a structural contour map in order to measure the uplift of southwest Montana in the periphery of the Yellowstone Hotspot.