Landslides in the Tandayapa Valley, northern Andes, Ecuador: implications for landform development in humid and tectonically active mountain ranges |
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Authors: | A P Dykes M R Welford |
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Institution: | (1) Present address: School of Earth Sciences & Geography, Kingston University, Penrhyn Road, Kingston-upon-Thames, Surrey, KT1 2EE, UK;(2) Department of Geology and Geography, Georgia Southern University, Herty Building 1100, P.O. Box 8149, Statesboro, GA 30460-8149, USA |
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Abstract: | Landslides are common throughout the Ecuadorian Andes, but their causal and controlling factors and their roles in landform
development have not yet been systematically investigated. This paper reports observations and hypotheses arising from a reconnaissance
study of the Tandayapa Valley in the Cordillera Occidental, approximately 30 km west of Quito. This study area is characterised
by high local relief (ca. 800 m) associated with dissected mountainous terrain, high annual rainfall (>2,000 mm), and secondary-succession
wet montane ‘cloud forest’. Regolith cover is extremely thin on the very steep (45 to >60°) upper main valley slopes, but
there are thick accumulations of tephra on the slope crests and ridgetops. These deposits show periods of soil development
separated by deposition events. Natural landslides in this environment comprise rare large deep-seated bedrock failures and
occasional shallow failures on the steep upper slopes where potentially unstable thicknesses of tephra have accumulated. Landslides
associated with construction of roads and forest trails are more common. Back-analysis of one road-cut landslide, using field
and laboratory data to characterise the in situ weathered material where possible, indicated that natural shallow failures
are unlikely in the absence of a surficial cover of tephra. By analogy with another mountainous tropical landscape, it is
suggested that long-term landform development can be explained in terms of channel incision, driven by uplift, producing steep
lower valley slopes that eventually exceed bedrock failure thresholds. Therefore, large deep-seated landslides appear to control
valley slope form development, whilst the shallow landslides contribute to general denudation. |
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Keywords: | Ecuador Cordillera Occidental Tandayapa Valley Shallow landslides deep-seated landslides Steady-state topography Volcanic deposits |
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