HR: 0800h
AN: H51A-1113    [Abstracts]
TI: Regolith Thickness Instability and the Formation of Tors in Arid, Granitic Environments
AU: * Strudley, M W
EM: mws10@duke.edu
AF: Division of Earth and Ocean Sciences, Duke University, Box 90227, Durham, NC 27707-0227
AU: Murray, A B
EM: abmurray@duke.edu
AF: Division of Earth and Ocean Sciences, Duke University, Box 90227, Durham, NC 27707-0227
AU: Haff, P K
EM: haff@duke.edu
AF: Division of Earth and Ocean Sciences, Duke University, Box 90227, Durham, NC 27707-0227
AB: We previously developed a model that incorporates a feedback between bedrock weathering and physical erosion (stream flow and diffusive processes) to explain the origin and maintenance of the laterally extensive (km's), nearly uniformly thin regolith blanket that characterizes pediments in arid regions such as the southwestern United States. Specifically, physical erosion controls pediment form by modifying regolith thickness, which, in turn, regulates bedrock weathering rates (Anderson, 2002). This relationship yields coupled bedrock and alluvial surfaces that lower together through time. Hydrogeochemical considerations and field observations in arid, granitic environments suggest that the relationship between weathering rates and regolith thickness exhibits a maximum for a finite thickness of cover. Our model offers an explanation for the isolated bedrock knobs (tors/inselbergs) that often punctuate otherwise smooth pediments. These features may arise as a consequence of stochasticity in rainfall and sediment transport conditions driven by fluctuations in climate, combined with the "peaked" nature of our weathering relationship. Climate fluctuations, which may manifest themselves as periods of higher effective moisture (decadal to millennial time scales) or changes in local base levels, may invoke a transition in which mantled surfaces lower at rates exceeding the bare-bedrock weathering rate. (Typically, boundary conditions and geometrical constraints restrict mantled surfaces to lower at rates below the bare-bedrock weathering rate in our model.) With a mantled surface lowering at such a rate, the form of the weathering-rate curve predicts that the pediment will be susceptible to an instability that will lead to bare-bedrock; regolith thickness that falls below a threshold value will approach a stable attractor at zero thickness. If this instability is triggered in a spatially heterogeneous pattern, perhaps as runoff from an intense rainfall event interacts with heterogeneities in regolith thickness and surface topography, a tor field will tend to develop. Incipient tors may then grow due to accelerated denudation on mantled surfaces compared to bare rock surfaces. As the tors grow in height, they will also tend to develop the steep sides and angular junctions with the surrounding pediment that characterize classic tor fields such as those found on the flanks of Cima Dome or in Joshua Tree National Park in the Mojave Desert of Southern California. Subsequent shifts in climate or local base level that cause the sediment surface to lower at a rate less than the bare-bedrock weathering rate will lead to a progressive decrease in tor height, ultimately leading to their disappearance. Tors in these environments thus represent possibly transient features related to fluctuations in climate or local transport conditions.
DE: 3220 Nonlinear dynamics
DE: 1815 Erosion and sedimentation
DE: 1824 Geomorphology (1625)
DE: 1886 Weathering (1625)
DE: 1625 Geomorphology and weathering (1824, 1886)
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting