Data Release for Surficial Geology and Quaternary Tectonics of the Madison Valley and Fault Zone, Madison, Gallatin, and Beaverhead Counties, 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. The fault zone consists primarily of west-dipping normal faults with east-dipping antithetic faults, which create the Madison Valley graben and several northeast-trending intra-basin faults. The Madison fault zone (MFZ) and associated sections discussed herein refer to the main west-dipping, range-bounding fault along the eastern side of the valley. Detailed geologic mapping (1:12,000 scale) of the entire fault zone and fault scarp profiling (total of 102 profiles) of the MFZ reveal greater late Quaternary paleoseismic activity towards the south, with at least three post-Pinedale paleo-events along the southern part of the fault zone. Early to mid Holocene fans have vertical surface offsets that average between 2.0 and 3.0 m, and define 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 indicate slip rates ranging from 0.18 to 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 to 5 ka. The northern section of the fault zone is defined by multiple normal faults, which detached the hanging walls of Laramide thrust faults within the Paleozoic and Mesozoic strata. This resulted in the partitioning of extension along multiple pre-existing structures and less displacement along individual normal fault strands. Structural controls on lateral propagation of individual paleoevents involve the position of lateral ramps along pre-existing Laramide contractional faults. This resulted 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. Inferred ENE-trending, intra-basin, normal faults within the southern half of the fault zone have no late Pleistocene displacement.