The Weird Tectonics of Play Sand and Silly Putty

Bruno C. Vendeville, Ph.D.

ABSTRACT
Physical modeling (a.k.a. "sandbox" modeling) has been used for almost two centuries with varying degrees of success. Since 1937, when King Hubbert adapted the theory of scaling and dimensional analysis from physics to geology, it has become possible to construct and deform models whose dynamic properties are much better constrained with respect to geological example and therefore allow for better testing of the physical parameters that govern geological processes. Along the way, the experimenter may uncover some unexpected ways by which the Earth's materials deform, ways that may depart from the conventional wisdom. This talk will illustrate two such cases, in which well-known geological features can be produced by unexpected processes or sets of boundary and geological conditions.

The first example deals with the onset of wrenching (i.e., strike-slip) movement within a tectonic plate. It is commonly assumed that intraplate wrenching occurs because of the presence of preexisting structural fabric or planes of weakness that are reactivated by strike-slip. Using physical models, I will show that a preexisting fabric is not required for strike-slip to occur. In a tectonic plate undergoing compression, the presence of weak areas, such as former extensional basins, is enough to trigger wrenching, depending on the basins' location. Experiments also show that the structural style of the wrench zone varies greatly (from a Dead Sea-style to a classic Riedel type), depending on slight changes in the imposed boundary conditions.

The second example deals with the origin of curved fold belts, such as the Jura and the Sierra Madre Oriental. Arcuate fold belts are traditionally attributed to the presence of an indenter in the hinterland, and/or of the curvature of the detachment pinch-out in the foreland. Experiments suggest that an unexpected other parameter is sufficient to promote the growth of arcuate folds, the curvature of the detachment on the hinterland side. This curvature in itself is enough to produce highly arcuate fold trains.

Bruno C. Vendeville received his Masterís degree from the University of Paris XI in 1984 and his Ph.D. from the University of Rennes, France, in 1987. He was a postdoctoral Fellow at the Center for Tectonophysics (Texas A&M University) from 1987 to 1988 before joining the Bureau of Economic Geology in 1989, where he is now a Senior Research Scientist.

Bruno specializes in salt tectonics, faulting analysis, and experimental modeling of tectonic processes, including salt tectonics, extensional tectonics, fold-and-thrust belts, magmatic intrusions, and gravity instabilities. He manages the experimental tectonics laboratory at the BEG.