LATE QUATERNARY TECTONIC ACTIVITY ALONG THE MADISON FAULT ZONE, SOUTHWEST MONTANA

The NNW-striking Madison fault zone is approximately 95 km in length, lying at the confluence of the northeast Basin and Range province and the Yellowstone tectonic parabola. Detailed mapping and fault scarp profiling reveal greater paleoseismic activity towards the south, with at least three post-Pinedale paleoevents along the southern portion of the fault zone. Early to mid Holocene fans have vertical surface offsets that average between 2.0-3.0 m, which defines the characteristic single-event surface offset. Pinedale lateral moraines have vertical surface offsets as great as 12.0 m. Late Pleistocene to Holocene multiple-event fault scarps show little evidence of beveling suggesting short recurrence intervals and potential late Pleistocene/Holocene temporal clustering. Long-term average tectonic activity rates quantified by amount of offset of known-age deposits and morpho-metric analyses, including Ms, Vf ratios and faceted spur reconnaissance analyses, indicate slip rates ranging from 0.18-0.6 mm/yr. Based on a comparison of fault scarp height versus maximum slope angle of known regression lines developed from other paleoseismic investigations, the most recent event ranges from 1-5 ka.

The northern section of the fault zone is defined by multiple normal faults detaching the hangingwall of thrust faults within the Paleozoic and Mesozoic sections. This results in the partitioning of extension along multiple pre-existing structures and less displacement along individual normal faults. Structural controls on lateral propagation and segmentation of individual paleoevents involve the position of lateral ramps along pre-existing Laramide contractional faults. This results in greater displacement within the larger basement-cored structures along the southern section where extension is accommodated by one inferred principal basement-involved normal fault. ENE-trending normal faults in the southern half of the fault zone show only pre-late Pleistocene displacement. Morpho-metric analyses coupled with structural relationships and seismicity derive a seismic hazard assessment consisting of five segments with maximum credible earthquakes of Mw 6.5-7.1 and short late Pleistocene to Holocene recurrence intervals (<10 kyr) for the southern two segments of the fault zone.