Leslie Jean Sonder
Associate Professor of Earth Sciences
I use both field and theoretical techniques to investigate the fundamental processes driving large-scale continental deformation. It is important to understand these processes because over tens of millions of years they, in combination with surface erosional processes, are responsible for shaping the earth as we know it: the configuration of the continents, the locations and sizes of mountain belts, and the regions of significant earthquake and volcanic hazards.
Mountain belts occur when the forces exerted on plates exceed the intrinsic strength of plates (the ability of plates to resist deformation). Such forces can be due to plate interactions (boundary forces), density differences within plate interiors (buoyancy forces), or due to motions in the convecting mantle under the plates. Determining the relative importance of these forces is fundamental to understanding the dynamics of continental deformation. I am particularly interested in the western United States deformation (e.g. Rocky Mountains, Basin and Range), because this region is far from the plate boundary, so that boundary forces are unlikely to be responsible for creating the mountains in these areas. My colleagues and I have calculated the distribution of buoyancy forces in the western United States. We show that, given available constraints on lithospheric strength, buoyancy forces are sufficient to produce the wide variety of rates and styles of observed deformation (including its absence in some locations like the Colorado Plateau). Thus plate boundary forces, while probably present, are not required to drive active deformation in the western United States east of California. Current work is under way to understand the relationship of present and passt deformation in the Western United States to the available driving forces and lithospheric strength.
I am also interested in understanding strike-slip deformation. In some, but not all, large strike-slip faults, paleomagnetism has demonstrated that crustal blocks are rotated around vertical axes as much as 90-100 degrees. However, the controls on the distribution along and across strike are poorly understood. Such information is important in understanding whether crustal blocks are driven by forces on their sides, similar to ball bearings, or by shearing underneath of ductilely deforming parts of the lithosphere; it also helps to constrain the strength of the continental lithosphere. In the Las Vegas Valley Shear Zone, Nevada, and in Sardinia, Italy, I and my colleagues have found paleomagnetic evidence for rotations that systematically decrease away from the major faults. However, in other strike-slip fault zones, other workers have found that crustal blocks rotate uniformly, like sets of dominoes. Why are there such fundamental differences in behavior? Mathematical modelling using both continuum and distinct element approaches is helping us to understand the dynamics of such deformation, in particular the controls on the size of blocks and the physical mechanisms for crustal block rotations.
Verdeyen, M, B Dade and L Sonder, “A Comparison of Sediment Volumes Supplied to and Retained in Alluvial Fans of NE Owens Valley, California, USA,” Eos Transactions , American Geophysical Union, Fall Meeting Supplement 2005, abstract #H53D-0497, 86:52 (Dec 2005).
Townsend, D A and L J Sonder, Rheologic Control of Buoyancy-driven Extension in the Rio Grande Rift, Journal Geophysical Research , 106:B8 (2001) 16,515-16,523.
“‘Ductile Shear Zones as Counterflow Boundaries in Pseudoplastic Fluids,’ by C J Talbot: Discussion and Theory,” Journal of Structural Geology , 23:1 (2001) 149-153.
Sonder, L J and C H Jones, “Western United States Extension: How the West was Widened,” Annual Review of Earth and Planetary Sciences , 27 (1999) 417-462.
Sonder, L J and R A Pockalny, Anomalously Rotated Abyssal Hills Along Active Transforms: Distributed Deformation of Oceanic Lithosphere, Geology , 27 (1999) 1003-1006.
Jones, C. H., Sonder, L. J., and Unruh, J., (1998), "Lithospheric gravitational potential energy and past orogenesis: Implications for conditions of initial Basin and Range and Laramide deformation," Geology, 26, 639-642.