STRAIN DISTRIBUTION IN LEGUMES: A BEHAVIOURAL ANALOGUE FOR SCALY FABRIC

“Scaly fabric” is a network of anastomosing shear surfaces commonly observed in clay-rich fault zones, such as subduction thrust faults. Although these fabrics are widely observed, little work has been done to understand the strength and strain distribution in these fabrics during shear deformation. We studied the behavior of scaly fabrics using lentils as an analog materials to observe the distribution of strain and the development of slip on anastomosing surfaces. The apparatus consists of a box in which one half of the box descends at a controlled rate. We ran the shear experiments with different lentil shapes and starting fabrics (aligned parallel to (vertical) displacement or aligned perpendicular to displacement). Slip rate was approximately 0.4 - 5.0 mm/s and total displacement in the experiments was 5 cm. We used time-lapse photographs and movies, analyzed using a PIV (Particle Image Velocimetry) tool. We observed the size and shape of the deforming zone at intervals of 2 seconds (correlating to 0.7-10.0 mm of slip). We then used these records to describe the distribution and degree of localization through time during these experiments.

The width and geometry of the deformation region are established within the first less than 1 mm of slip and do not significantly change thereafter. The region includes transient slip surfaces which can accommodate the total motion of the system over 1 or more “faults” for short periods of time, but which are geometrically unstable and discontinuous. The “fault” geometry is in a constant state of re-organization. Transient shear surfaces are enabled by the temporary alignment and interlocking of lentils such that lentil surfaces line up across several lentils, enabling smooth slip. These alignments are re-arranged either by gradual shifting of localized shear outward across parallel interfaces (delocalization) or by a distinctive dilation associated with grain rotation and ended with a collapse event. Continued shear realigns the lentils, collapsing any dilation and restoring the pre-shearing porosity.

Our results show that in an anastomosing scaly fabric, shear can migrate through the system without leaving any geometric record, and can take highly tortuous paths through the particle network.