LATE QUATERNARY TECTONIC ACTIVITY ALONG THE MADISON RANGE FAULT ZONE, NORTHERN ARM OF THE YELLOWSTONE TECTONIC PARABOLA, SOUTHWEST MONTANA

The NNW-striking Madison Range fault zone is approximately 95 km long, lying at the confluence of the northeast Basin and Range province and the Yellowstone tectonic parabola. Based on geomorphic expression and structural discontinuities, the fault zone has been subdivided into five sections with individual sections having potential events of Mw 6.5–7.2. Detailed geologic mapping reveals greater late Pleistocene paleoseismic activity towards the south, with at least three post-Pinedale paleoevents along the southern half of the fault zone. Early to mid Holocene fans have vertical surface offsets between 1.0 and 2.5 m, defining the characteristic single-event surface offset. Pinedale lateral moraines have surface offsets as great as 12.0 m. Late Quaternary multiple-event fault scarps are not beveled, suggesting short recurrence intervals and potential temporal clustering. Long-term tectonic activity rates (>100 k.y.) indicate slip rates range from 0.18 to 0.6 mm/yr. Morpho-metric analyses clearly delineate probable segment boundaries. Based on a comparison of fault-scarp height versus maximum slope angle, the most recent event occurred between 1 and 5 ka. The northern half of the fault zone is characterized by pre-existing structural controls on multiple normal faults within the hanging wall of Laramide thrust faults. This results in the coseismic partitioning of displacement along multiple structures and decreasing the displacement on any single fault strand. Along the northern section of the fault zone, structural controls on lateral propagation of individual paleoevents involve the position of pre-existing Laramide-age lateral ramps. Along the southern half of the fault zone, displacement occurs within the larger basement-cored structures, where extension is accommodated by one principal normal fault. ENE-trending, intra-basin normal faults have created transitional relay ramps and display only pre-late Pleistocene displacement.