Mid- to Late-Quaternary Geomorphic and Paleoseismic Event History, Cheraw Fault, Southeastern Colorado

Despite its subdued expression and isolated location within the Great Plains of southeastern Colorado, the 80 km long Cheraw fault may be one of the most active faults in North America east of the Rocky Mountains. We present geomorphic analyses, geochronology, and paleoseismic trenching data to (1) document the rupture history of the similar to 45 km long southwestern section of the Cheraw fault over the past similar to 19 ka, and (2) evaluate slip rate changes for the entire fault over the past similar to 200 ka. Results from new trenches excavated at the Old Ranch site show evidence of four surface rupture events since similar to 19 ka, each with an average vertical displacement of 0.75 +/- 0.15 m. An additional event is likely only slightly older than similar to 19 ka. Evidence for relatively small displacements at and near the Old Ranch site indicates that most of these earthquakes were magnitude (M) 7 or less and likely did not rupture the full length of the Cheraw fault. Since similar to 19 ka, the average slip rate is similar to 0.16 +/- 0.3 mm/yr near the Old Ranch site with an average interevent time of 3-5 ka. New geochronologic data for mid- to late-Quaternary geomorphic surfaces cut by the Cheraw fault imply rapid incision by local Arkansas River tributaries from similar to 145 to similar to 100 ka. Maximum vertical offsets of 7-9 m for these surfaces indicate that from similar to 19 to > 200 ka the average slip rate was no greater than similar to 0.03 mm/yr. The accelerated slip rate since similar to 19 ka indicates a possible response to rapid erosional unloading and/or a limited late Cenozoic, < 40 ka, paleoseismic history for the Cheraw fault.