CHARACTERIZATION OF EXTENSIONAL TECTONICS IN NORTH BOULDER BASIN, MONTANA, THROUGH DETAILED ANALYSIS OF LONGITUDINAL STREAM PROFILES

Rivers are influenced by tectonic activity; as such, they effectively serve as a record of past tectonic activity and can be used as a proxy for base-level changes. This research was conducted in the southern 92 km² of the North Boulder drainage basin in southwestern Montana, which contains nine tributaries that flow westward over a north-south-striking normal fault scarp into the North Boulder River. To better understand the timing and magnitude of the extensional tectonics in the region, we conducted a detailed analysis of tributaries that drain into the North Boulder River. We analyzed each tributary by using normalized steepness indices and total river incision taken from relict stream profiles reconstructions based on the stream-power limited detachment model. Another goal of this study was to provide a temporal context for the events that shaped the stream profiles. To establish this chronology, cosmogenic ³⁶Cl was used on the major normal fault scarp over which the streams flow, comprised entirely of in-situ limestone so that exposure ages caused by normal faulting could be determined. Each of the tributary stream beds contain a wide variety of Precambrian to Tertiary carbonate and siliciclastic lithologies that are unique to each stream. However, several streams flow almost entirely over the homogenous limestone of the massively bedded Mississippian Mission Canyon Formation.

Tools used in this study were: post-processing-kinematic surveys, 10 meter DEMs obtained from the USGS, Arc-GIS watershed tools, Topo Toolbox 2 MATLAB scripts, USGS topographical maps, and the CRONUS cosmogenic age exposure calculator. The various data collected allowed us to quantitatively determine an extensional tectonic boundary and exposure rates of the normal fault scarp. When the results were placed in the context of the limited-detachment stream-power model it was evident that tectonic forcing superseded the influences of rock strength and drainage area that shaped stream profiles over at least the past 47 k.a.