A 3-D GEOLOGICAL MODEL OF THE SEVIER THRUST BELT IN THE TENDOY MOUNTAINS, SOUTHWESTERN MONTANA

We created a 3-D geological model of part of the Tendoy Mountains in southwestern Montana. The purpose of the model is to examine the geologic structures of the leading edge of the Sevier thrust belt at a location where Paleozoic units are thrust over Cretaceous – Paleocene conglomerates. We digitized fault traces and contacts from the Lima 30x60 quadrangle geologic map (Lonn et al., 2000) and used minimum tension gridding algorithms in EarthVision software by Dynamic Graphics, Inc., to project geologically realistic surfaces into the subsurface. Strata in the model area range from Mississippian to Paleocene. Because they are thin in critical areas, Quaternary sediments were not modeled. The model area includes stacked thrust faults that trend northwest, and the northwest-trending Red Rock normal fault at the mountain front. The leading thrust fault, the Tendoy thrust, places Mississippian units over the Cretaceous – Paleocene Beaverhead Group conglomerates. The Tendoy thrust varies from steep to shallow-dipping and is parallel to hanging wall strata. Other thrusts have local hanging wall and footwall ramps. Folds range from tight to open, are typically asymmetrical, and many are overturned. The process of 3-D modeling reveals the significance of the pre-Tertiary surface, which is interpreted as a low-angle detachment on the geologic map, and the lack of constraints on the thickness of the Beaverhead Group conglomerates. Subsurface thrust fault geometries are poorly constrained at depth, and require interpretive modeling.