PALEOSEISMIC ACTIVITY ON THE SANGRE DE CRISTO FAULT NEAR SAN LUIS, COLORADO

The Sangre de Cristo fault extends about 300 km along the western side of the Sangre de Cristo Range and the eastern margin of the San Luis Valley in southern Colorado and northern New Mexico. Despite its youthful expression, little is known about its late Quaternary history, including slip rates and potential segmentation. Near San Luis, Colorado, the central section of the fault forms a 7- to 8-m-high scarp on fan-head alluvium, and a 2.5- to 3-m-high scarp on latest Pleistocene (Pinedale) terrace deposits. We excavated a trench across each of these scarps at Rio Seco Creek, about 5 km NE of San Luis and exposed discrete packages of fault-scarp colluvium. The fault-scarp colluvium contains large amounts of eolian sand and silt that accumulated at the base of the scarps.

The small scarp from most recent faulting event (PE1) vertically offsets gravelly alluvial and debris-flow deposits 2.3 m. The time of this event is bracketed by unfaulted alluvium dated by AMS C14 at 5,529 ± 195 cal. yr B.P. and by faulted deposits that are luminsencence dated at 9.3 ± 0.6 ka and 12.4 ± 1.8 ka; our preferred age for PE1 is 9.0 ± 2.0 ka. About 100 m to the north, a 5- to 6-m-deep trench across the large scarp exposed evidence for a total of four faulting events in deposits luminescence dated at 48.1 ± 4 ka. Additional luminescence age estimates constrain the older events: PE2, ca. 23.4 ± 2 ka; PE3, ca. 30.3 ± 2 ka, and PE4, ca. 45.0 ± 4.3 ka.

Our age and stratigraphic constraints indicated that four surface-faulting events occurred on the Sangre de Cristo fault at Rito Seco Creek in the past ca. 50 k.y. The older deposits (ca. 48 ka) are offset at least 6.8 m vertically, which indicates a minimum late Quaternary slip rate of 0.17 mm/yr and recurrence intervals of about 12 k.y. for the central section of the fault (4 events between 9 and 45 ka). This slip rate is essentially the same as that derived from vertical offset of the 4-Ma Servillita basalt across the fault zone 10 km south of San Luis. Thus, the Sangre de Cristo fault has the highest documented normal-fault slip rate in the Southern Rocky Mountains and Rio Grande rift and should be viewed as a relatively hazardous structure for critical facilities.