Characterization of Stream Channel Evolution Due to Extensional Tectonics Along the Western Margin of North Boulder Basin (Bull Mountain), SW Montana with the Use of Geologic Mapping and Thermochronologic (U-Th/He) Dating.

Landscape response to active tectonics, such as fault motion or regional uplift, can be recorded in river profiles as changes in slope (i.e. knickpoints) or topography. North Boulder basin region (SW Montana), experienced two separate phases of extension, from 45 - 35 Ma and again beginning 14 Ma to the present, producing basin-and-range style fault-blocks. Focusing on the Bull Mountain region, located on the western margin of the North Boulder basin, data is collected to test the hypothesis that Bull Mountain is located on the hanging wall of a half-graben. Our objective is to elucidate the active tectonics of the study area within a regional context by utilizing river profile analysis and thermochronometric data. High-resolution (< 5cm) river profile data is obtained from five of the main tributaries of Bull Mountain. Comprehensive geologic mapping along the main tributaries and topographic highs of the region allowed for the identification and measurement of knickpoints, composition of detailed lithologic descriptions, and analysis of key structural features. The absence of knickpoints within the four tributaries mapped on east Bull Mountain are consistent with a lack of tectonic activity. In contrast, Dearborn Creek, on western Bull Mountain, is located along an active normal fault and presents several knickpoints. Geologic mapping confirms that the primary lithologies of the region belong to the Elkhorn Mountain Volcanics. At lower elevations, there are massive plutonic intrusions of Quartz Monzonite and Diorite, both constituents of the Boulder batholith. These lithologies contain minerals suited for low-temperature thermochronology (U-Th/He) to constrain the timing of tectonic activity (i.e. uplift and exhumation) and erosion rates in the region. High-resolution stream profiles and a 10m DEM are used to delineate watersheds and produce steepness and concavity maps of major tributaries to investigate changes in slope or topography. The effects of extensional tectonic events can reshape drainage patterns of streams and their distribution of water, which is an important commodity in SW Montana for ranchers and farmers. Thus, the ability to discern the probability of recurring tectonic events and the effects on the regional watersheds, could help facilitate solutions before these events take place.