Paper No. 277-12
Presentation Time: 4:00 PM
RICHARD H. JAHNS DISTINGUISHED LECTURE: THE MOUNTAINS ARE FALLING APART; A SPECTRUM OF MASS FAILURES FROM LANDSLIDES THROUGH DEEP-SEATED GRAVITATIONAL SPREADING (SACKUNG), TO "UNFOLDING" OF FOLDS
All engineering geologists can recognize obvious, young landslides from their sharp, distinct geomorphic elements (headscarp, lateral margins, toe thrust, hummocky topography). However, we often see isolated elements without the others and are unsure of their exact origin and engineering significance. 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, a result of 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 rubbilized. Instead, stratigraphy and structure within the blocks are 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, 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. 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-nonseismogenic structures, but these two types have different hazard significance for engineering geologists. Two recent conferences in Europe on “Slope Tectonics” explore this topic, which has not yet been publicized in the USA (1st in Lausanne, Feb. 2008, pub. as Geol. Soc. London., Spec. Pub. 351; 2nd in Vienna, Sept. 2011).