LATE QUATERNARY EARTHQUAKE HISTORY OF THE SANGRE DE CRISTO FAULT NEAR SAN LUIS, COLORADO

The ~300-km-long Sangre de Cristo fault bounds the western side of the Sangre de Cristo Range and the eastern margin of the San Luis Valley. Based on the presence of scarps on Holocene deposits, its length, and thousands of meters of net throw, it could be the most hazardous fault in Colorado, but little is known about its late Quaternary history, slip rate, and segmentation. On the central section of the fault at Rito Seco Creek near San Luis, Colorado, a 6.6-m-high scarp is present on fan-head alluvium, which may be as old as late-middle Pleistocene, and an ~2.7-m-high scarp is present on Holocene or latest Pleistocene (Pinedale?) terrace deposits. Previous work had inferred that the most recent faulting event (MRE) at this site occurred between 1.9 ka and 4.7 ka, but the dated deposits are difficult to relate to the fault. In September 2003, we excavated a trench across each scarp and exposed rocks of the Santa Fe Group overlain by sequences of fault-scarp colluvium and fluvial deposits that contain abundant eolian sand and silt.

Stratigraphy in the trench across the large scarp shows evidence for at least three faulting events, each having similar amounts of vertical displacement, that produced a cumulative total throw of at least 6.75 m. We measured 2.3 m of stratigraphic throw in the trench across the small scarp. Unfaulted channel-fill deposits contain carbon that yielded an age of 5593 +130/-259 cal. yr B.P.; thus, the MRE is somewhat older than previously thought. Pending luminescence age estimates from faulted eolian sand and silt will help date pre-Holocene earthquakes.

If the faulted older deposits correlate to the Bull Lake glaciation (~130 ka), then we can draw preliminary conclusions about the fault’s history at Rito Seco Creek. One Holocene surface-faulting earthquake has occurred at this site, whereas at least three such earthquakes have occurred in the past ~130 k.y. A minimum of 6.75 m of throw on ~130-ka deposits indicates an average late Quaternary slip rate of 0.05 mm/yr and suggests a recurrence interval on the order of 40-45 k.y. for this section of the fault. The occurrence of multiple late Quaternary events indicates that this section of the fault can potentially rupture, but the apparently long recurrence time and low slip rate suggest that it does not pose an extremely high hazard.