A chronology of post-glacial landslides suggests that slight increases in precipitation could trigger a disproportionate geomorphic response

Large, deep-seated landslides are common throughout the south-eastern San Juan Mountains of Colorado and New Mexico, but their timing and initiation are not well understood. Determining when the landslides occurred would aid in clarifying the mechanisms for initiating landslides in the region and would help us to understand post-glacial landscape evolution. We studied seven pre-historic landslides located within the Tertiary volcanic rocks of the San Juan Volcanic Field. The landslides range in area from similar to 0.8km(2) to similar to 11.3km(2) and most are located in areas that were previously mapped as having been ice-covered during the last glaciation. Landslide deposits were dated using a variety of methods including surface exposure dating (chlorine-36, Cl-36), radiocarbon dating of basal bog sediments and organic material buried in soils, and relative soil development. The resulting limiting ages range from approximately 14ka to 2ka and show that deep-seated landsliding has occurred throughout the post-glacial period. This broad range in ages is inconsistent with our initial hypothesis, which proposed that landslides were likely the result of debuttressing of glacial walls during glacial retreat. Furthermore, the timing of landslides does not seem to correlate with documented post-glacial climatic shifts. Therefore, we conclude that landsliding in the region was the result of wetter than normal periods lasting months to years acting on weak bedrock preconditioned to failure and prepared by glacial debuttressing. Our findings suggest that the study area is likely still susceptible to deep-seated landsliding and may become even more prone to large-scale slope failure if future climate change increases precipitation in the San Juan Mountains. Copyright (c) 2017 John Wiley & Sons, Ltd.