EVOLUTION OF A FRICTIONAL WEDGE USING VELOCITY AND DEFORMATION FIELDS FROM SAND EXPERIMENTS

We present simplified models of doubly-vergent sand wedges to illustrate basic concepts associated with wedge theory. An important objective is to show that the critical-taper concept is only one part of a larger wedge theory. The models were generated using loose sand (to avoid faulting which causes problems with scaling of material properties). The sand is deformed using servo-controlled stepper motors driving one or two "mylar" plates. The mylar is allowed to subduct or educt through slots in the base of the table, which allows the formation of double-vergent wedges in either advancing or retreating modes. The subduction and eduction points are equivalent to the S-point formulation in the Dalhousie numerical wedge models. The models are visualized in side and top views using high-resolution digital cameras and particle-tracking software. The resulting movies show in detail the evolution of various simple wedge scenarios based on the velocity and deformation fields operating within the wedge.

The talk will illustrate three points: 1) The S point concept predicts a set of limiting slip lines that bound a proside, a core, and a retroside within a generalized doubly-vergent wedge. The wedge core will be remain relatively undeformed for an advancing wedge, but will deform strongly for a retreating wedge. 2) A single wedge will typically have both critical and stable regions. Stable refers to a state where the integrated strength of the wedge is greater than the stress at the base of the wedge. Accretion and erosion favor a critical wedge, whereas sedimentation, a stable wedge. 3) Normal faulting is a common feature in the retroside of a convergent frictional wedge. There is no need for over-thickening or collapse to explain these structures.

Wedge behavior will change as thermally activated viscosity starts to dominate the interior of the wedge, but many of the concepts associated with frictional wedges are applicable to viscous wedges. There is often the notion that the wedge concept is somehow in conflict with the thin-sheet model or the channel-flow model. We will show that all of these concepts are inter-related. For example, the thin-sheet model behaves similar to a wedge with an infinitely weak basal surface. The channel flow model has a flow pattern similar to a viscous wedge with a strong gravity-driven return flow.