Summary
Large-scale gravitational spreading and movement along fractures of steep-sided ridges in the mountainous areas of the western United States are characterized by linear fissures, trenches, and uphill-facing scarps on tops and sides of ridges. Spreading appears to take place by movement along disconnected planes and/or by slow plastic deformation of a rock mass. In some places, valleyward squeezing out of weak shales overlain by rigid rocks causes extensional fracturing and outward movement of the rigid layers, as illustrated by extension of two laccoliths overlying Mancos Shale, one at Dolores Peak and another at Crested Butte in western Colorado. Gravitational forces acting on a ridge of more homogeneous material causes tensional spreading of the ridge parallel to its long axis, for example in fractured granitic rock north of Mt. Massive in central Colorado, where a survey course has been established to monitor the movement. Recognition and understanding of these large-scale gravitational features and the mechanism that causes them are pertinent to site selection and design of engineering structures in high mountains. If fractures extend to considerable depth and if movement is continuing, engineering structures in valleys or tunnels through the spreading ridges could be damaged.
Résumé
L’étalement à grande échelle causé par la force de gravitation et le mouvement le long de fractures affectant les arêtes à pente accentuée dans les régions montagneuses de l’ouest des Etats-Unis d’Amérique sont caractérisés par des fissures linéaires, des fossés et des escarpements façant l’amont, localisés sur les sommêts et les versants des crêtes. L’étalement semble prendre place par mouvement le long de plans disjoints, et/ou par déformation plastique lente d’une masse rocheuse. Dans quelques localités, l’extrusion vers l’aval d’argiles schisteuses meubles surmontées par des roches rigides cause des fractures d’extension et un mouvement des couches rigides vers l’extérieur, comme le montre l’extension de deux laccolites recouvrant l’argile schisteuse de Mancos (l’un à Dolores Peak et l’autre à Crested Butte, dans l’ouest du Colorado). Les forces de gravitation agissant sur une crête formée dans un matériau plus homogène causent l’étalement de la crête par tension le long de son axe longitudinal; par exemple dans la roche granitique fracturée au nord du Mt. Massive dans le Colorado central, òu des mesures d’arpentage sont faites régulièrement pour contrôler le mouvement. La reconnaissance et la compréhension de ces phénomènes à grande échelle dus à la force de gravitation et de leurs mécanismes sont utiles à la sélection des sites de construction et au calcul des ouvrages d’art en haute montagne. Si les fractures s’étendent à des profondeurs considérables et si le mouvement continue a présent, les ouvrages d’art situés dans les vallées ou les tunnels transversaux aux crêtes qui s’étalent peuvent être endommagés.
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Radbruch-Hall, D.H., Varnes, D.J. & Savage, W.Z. Gravitational spreading of steep-sided ridges (“sackung”) in Western United States. Bulletin of the International Association of Engineering Geology 13, 23–35 (1976). https://doi.org/10.1007/BF02634754
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DOI: https://doi.org/10.1007/BF02634754