HR: 17:00h
AN: T22E-05    [PDF]
TI: Position of the Topographic Divide as a Measure of Tectonics, Steady-State, and Bulk Rock Strength
AU: * Ellis, M A
EM: mellis@memphis.edu
AF: CERI, Dept. Earth Sciences, The University of Memphis, Memphis, TN 38152 United States
AU: Densmore, A L
EM: densmore@erdw.ethz.ch
AF: Institute of Geology, Dept. Earth Sciences, ETH-Zentrum, Zurich, 8092 Switzerland
AB: The fractional position of the topographic divide relative to local base-levels defines the first-order shape of mountains over a wide range of length-scales, from spur-ridges to entire mountain ranges. We propose that the fractional position is a sensitive measure of the relative roles of tectonics, local base-levels, and bulk-strengths of rocks exposed on adjacent flanks of the range. In the absence of tectonics and in a homogenous lithology and climate, the fractional position of a steady-state divide is given by D = F/(F+R) where F and R are the reliefs of adjacent flanks of the range. Departures from the predicted value indicate that one or more of the stated conditions is invalid. In cases where the topography is being generated over an active thrust, for example, the divide position expresses the relative role of tectonics. Examples from the Siwalik Hills, along the southern margin of the Himalaya, suggest that base-level is the primary control over the first-order shape of the emerging ranges. This means that attributes of the topography cannot be simply linked to the state of active faulting, but must be made with some reference to the geological history of marginal base-level changes. In cases where tectonics and differences in bedrock bulk strength can safely be disregarded, departures from the value of D speak to the response time of landscapes that are adjusting to recent changes in base-level. Such changes may be induced by climate change or to significant base-level changes (e.g., eustatic) that have subsequently migrated to the local region. In the absence of all external forcings (tectonics and climate), departures from D measure directly the difference between the bulk-strength of rocks exposed on adjacent flanks of a range. This measure is at a length-scale previously unavailable and may be of use in landscape evolution models and in landslide hazard analyses. All of this is predicated on the assumption that mean slopes measured from the topographic divide to local base-levels are ideally equal, which is a straightforward result if slopes are generated via landslides and debris flows. We propose and show supportive evidence that this is also the case where fluvial erosion is primarily responsible in the generation of range slopes.
DE: 1815 Erosion and sedimentation
DE: 1824 Geomorphology (1625)
DE: 8107 Continental neotectonics
SC: Tectonophysics [T]
MN: 2003 Fall Meeting