Effects of side wall friction in compressional analog experiments with sand
Abstract
Analog experiments are often proposed to reproduce 2D sections through geological structures. However, friction on the side walls of the experimental box is often overlooked. We repeat experiments using two side wall set ups to estimate the influence of this friction. There are only two choices for the side walls: A) they move with respect to the basal plane, or B) they remain fixed. Configuration A is called "fixed back wall", or "pull setup", and is typical of boxes using a conveyor belt. Configuration B is called "mobile back wall", or "push setup". Typically, in both configurations, thrust traces at the surface exhibit curvature close to the side walls due to friction. We demonstrate, by comparing outcomes using the configurations A and B, and using the same material for side and basal walls, that magnitude of the experimental bias (i.e. the difference between the outcomes using configurations A and B) depends on the ratio of the area of the body in contact with the side walls (SL) to the area in contact with the basal surface (SB). For SL/SB ≤ 0.1, i.e. very little contact with side walls, there is no identifiable bias. For SL/SB increasing to 0.25, curvatures increase, yielding rather different cross-sections at the side walls, but still similar cross-ections in the central part of the sand body. For SL/SB ≥ 0.3, thrust planes occupy different regions of the box for each configuration, although their curvature is decreasing. At SL/SB = 0.9, curvatures have almost disappeared but the location of the thrusts and the length of the décollement vary strongly depending on the side wall configuration. Thus, the absence of curvature of the thrust traces is not a proof of the 2D character of the experiment. Analysis of the total forces at both front and back walls during shortening shows that the thrust locations are controlled by the difference between basal and side wall shear forces. However, the exact theoretical solution to these experiments, and in particular a 3D criterion for the stability of a Coulomb wedge, remain to be developed. In this respect, results using configurations A and B offer a useful test of this needed theory.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2010
- Bibcode:
- 2010AGUFM.T53A2113S
- Keywords:
-
- 8102 TECTONOPHYSICS / Continental contractional orogenic belts and inversion tectonics;
- 8194 TECTONOPHYSICS / Instruments and techniques