VISITING SPEAKER  - Friday 23rd Sept.

Recent  geologic mapping in mountainous areas of Alaska, California, Colorado, and Utah, supplemented by  LiDAR   DEMs , has revealed an abundance of young scarps,  graben , bulges, and other geomorphic anomalies. Some landforms are  sackungen formed by deep-seated gravitational spreading (called by Doug Morton of USGS “half-a-landslide”). Gravitational spreading results in detachment of large masses of bedrock from mountain flanks, causing lateral bulging and vertical collapse of the crest. Detached blocks do not display hummocky topography because they are not rubble, as are landslide deposits. Instead, stratigraphy  and structure within the blocks is relatively undisturbed, so geologic maps show these areas as  unfailed  bedrock, and do not hint that the block margins are sites of recent deformation with engineering significance. As local relief in mountains increases (often due to valley  glaciation  in the Pleistocene) and active  tectonism  increases, the types of different failure styles and number of failures increase. In southern Alaska the combination of high relief and seismic shaking has even led to the Quaternary “toppling-unfolding” of pre Quaternary folds, due to gravitational spreading and development of extensional flexural-slip faults. Lower-relief areas in Colorado show similar swarms  of young  scarps related to unfolding of Cretaceous folds by  evaporite  dissolution  in underlying  rocks. Taken together, the landforms now known represent a nearly continuous spectrum of mass failures, ranging in scale from small landslides that we would all be able to recognize, to incipient landslides, to large detached parts of mountains, to deep-seated gravitational spreading that looks almost tectonic in nature. In mountain areas that contain active faults, it is difficult to separate the tectonic  seismogenic  structures from the gravitational-non  seismogenic  structures, but these two types have different hazard significance for engineering  geologists.   This talk is based on my research into  sackungs  since 1994, and on trenching  evaporite  scarps since 2010. The talk draws on examples from Colorado, Europe, California, Utah and Alaska.