Characterization of Extensional Tectonics in North Boulder Basin, Montana, through Detailed Analysis of Longitudinal Stream Profile

This research was conducted in the southern ~92 km2 area of the North Boulder basin, which contains nine tributaries that flow westward over a north-south striking normal fault scarp into the North Boulder River. The goal of this study was to be able to quantify the spatial distribution and magnitude of base level changes that influenced the evolution of North Boulder River tributaries by using normalized steepness indices, and total river incision taken from relict profile reconstructions based off the Stream-Power Limited Detachment model. An additional goal was to place events that shaped the stream profiles into a temporal context by determining exposure ages along the major normal fault scarp over which the streams flow by testing for cosmogenic 36Cl isotopes. Each of the tributary stream beds contain a wide variation of lithologies unique to each stream. While several stream flow almost entirely over homogenous limestone bedrock of the massively bedded Mississippian Mission Canyon formation, other streams flow over numerous lithological changes that range in age from the pre-Cambrian to the Tertiary consisting of sandstones, carbonates, siltstones, and shales. The tool set used to determine normalized steepness indices and reconstruct relict profiles for total incision values consisted of 1) Post Processing Kinematic surveys, 2) 10 meter DEMs obtained from USGS, 3) watershed tools in Arc GIS, 4) TopoToolbox 2 MATLAB scripts, and 5) USGS topographical maps. The Prime Labs facility processed limestone samples extracted from a vertical section of the normal fault scarp and the results were inputted into CRONUS in order to determine exposure ages so that timing and magnitude of base level changes could be determined. Not only was a tectonic boundary able to be quantitatively determined, but results obtained from these methods indicate that when placed in context of the shear-stress/stream-power model, tectonic forcing superseded the influences of rock strength and drainage area that shaped the stream profiles over at least the past ~47 k.y..