Mitchell McMillan

Wind erosion

Hyperarid regions, such as orogenic plateaus, are mainly eroded by wind, rather than rivers or glaciers.

The Salina del Fraile depression is a large (~300 sq. km) depression about 1 km deep, with a conspicuous rhomboidal shape. In some respects, it looks like a classic pull-apart basin created by strike-slip and normal faulting (i.e., tectonic extension) but we didn’t observe any such faults in the field. Instead, the major structures are clearly compressional, not extensional, and the floor of the depression is made up of older rocks. A series of anticlines (mapped as black arrows) seem to define the overall shape of the depression. This suggests that, instead of being lowered by faulting, the depression formed when rocks exhumed by the anticlines were eroded.

Map of wind-related landforms and their associated wind directions in the Salina del Fraile. Inset shows the wind directions from NASA climate reanalysis data over a 10 year period; the orientation of the strongest winds agrees very well with the wind directions inferred from various landforms. Strange landforms such as smoothed mesas and elongated ridges are also apparent.

Folds that exhume erodible strata in their cores, leading to them being hollowed out by erosion, are familiar phenomena. I mapped one such landform near Santa Fe, New Mexico as an undergrad student. There are also well-known ones in the Negev desert. The problem here? There’s no evidence of rivers or glaciers in this hyperarid region, especially ones that would be powerful enough to create such a large depression. My supervisor Lindsay Schoenbohm and I developed an alternate hypothesis: that the depression was carved by wind erosion. Once we made this inference, other oddities about the landform came into clearer focus, including the appearance of conspicuously smoothed topography, including large mesas and ridges, and gravel megaripples, large ripples created by strong winds that are made of gravel instead of sand. It seems that several million years of “normal” wind erosion (0.1 mm/yr) is enough to excavate a 1 km deep depression and to keep topographic slopes smooth, linear, and relatively free of debris. While we struggle to find similar landforms created by wind erosion on Earth, we did find that Mars has some smoothed topographic features, including large mesas and buttes that somewhat resemble the ones on the Puna. To read more, check out our paper published in the Journal of Geophysical Research: Earth Surface.

Although this project was originally a tangent from my lithospheric drip research, it actually has important implications there as well. The fact that we can now rule out a major episode of left-lateral transtension on a N-S trending fault (the original pull-apart basin hypothesis) means that the kinematic history of deformation is less complicated than previously thought. Multiple phases of ~E-W compressional deformation are evident from folded strata and unconformities exposed in the walls of the Salina del Fraile, and a later period ~NW-SE extension is evident from the pervasive normal faults and horst-and-graben structures in the SE portion of the map. Such a sequence of deformation may (or may not) be produced by a lithospheric drip.

Continental orogenesis
Investigating the tectonic, geodynamic, and climatic processes that produce large mountain belts
tectonic geomorphology, geodynamics, thermochronology
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Lithospheric foundering
A deep-seated geodynamic process that may affect the evolution of mountain belts.
geodynamics, orogenesis, numerical modelling
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Thermodynamics of lower crustal buoyancy.
thermodynamics, fluid-mediated reactions, crustal metamorphism
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Streambank erosion (M.Sc. thesis)
Applying models to predict streambank erosion rate in the Gulf Coastal plain.
fluvial geomorphology, numerical modelling, environmental geology
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