New Geochronology and Paleoseismic Data for the Cheraw fault at the Old Ranch Site, SE Colorado

The Cheraw fault is one of the few faults east of the Rocky Mountains with evidence of Quaternary surface rupture that is considered in regional seismic hazard models of the central and eastern United States. The prior record of large earthquakes on the Cheraw fault includes three events from ~22 to ~8 ka interpreted by Crone et al. (1997; USGS Map I-2591). A possible 100 kyr interseismic interval preceding those events was partly estimated from the inferred age of nearby offset Rocky Flats Alluvium. Two events were constrained by ¹⁴C and TL ages, but sediments that would limit the age of the oldest event and the length of the prior interseismic interval were undated.

To extend the paleoseismic record and constrain the ages of the oldest faulted sediments, we re-excavated the Crone et al. (1997) trench and opened two soil pits on adjacent undated alluvial surfaces with larger scarp heights. Colluvial wedges and truncations in the new trench exposures show direct evidence for three late Quaternary surface ruptures, similar to the original trench. Net vertical offset for individual events is ~0.5 to ~1 m. Differential tilting and near-fault thickening of older hangingwall units provides indirect evidence for additional events. A combined set of nearly 30 OSL, TL, and ¹⁴C ages will constrain the ages of faulting and depositional units in the trench and the soil pits, including deposits mapped as Rocky Flats Alluvium.

Our new results are consistent with prior interpretations that the late Quaternary surface ruptures on the Cheraw fault were associated with M6.5 to 6.8 earthquakes. However, recent mapping of the Cheraw fault (Ostenaa and Zellman, 2018; CGS MI-97) now shows a minimum scarp length of 80 km vs. 45 km as mapped previously. The increased fault length implies that additional late Quaternary partial rupture events of similar magnitude likely occurred on the northern portion of the Cheraw fault, possibly doubling the frequency of M6.5 to 6.8 earthquakes from this source in SE Colorado. New age data will define limits for the prior interseismic intervals and test other dating results from Ostenaa and Zellman (2018) that imply that nearby faulted deposits mapped as Rocky Flats Alluvium may be younger than 160 to 125 ka. This latter result has significant implications for correlation of geomorphic surfaces on the High Plains of eastern Colorado.