Granular Mechanisms for Faulting in Nature and the Laboratory
Abstract
The Black Mountain detachments in Death Valley, CA, are type localities for large displacement faults that deformed by granular mechanisms. The granular mechanisms likely exerted a first-order control on the strength and stability of the faults. The granular hypothesis arises from: (1) A lack of mineralogical and geochemical data evidence for large fluid fluxes and, by inference, elevated fluid pressure; (2) Microstructural data that show that the fault rocks accommodated strain mostly by interparticle rotation, translation, and grain- boundary abrasion; (3) Structural relationships between the low-angle detachments and high-angle faults (including earthquake scarps) that illustrate that the strength and sliding-stability of the detachments changed over time. To further test the granular hypothesis a shear apparatus was constructed containing polymer disks that are birefringent (photoelastic) under stress. As an aggregate of the disks accumulates strain, the stress is observed by this method to be localized in "force chains" that evolve with deformation. At present the experiment is set up in a strike-slip reference frame with sigma-1 imposed horizontally. A split in the center of the apparatus prevents localization to the shear zone boundaries. A mixture of elliptical and circular particles allows for tracking particle orientations as well as positions, and will ultimately allow the apparatus to test the influence of grain shape on mechanics. Results to date show that stick-slip events coincide with abrupt localization of shear strain to the center of the apparatus, with little change to the force chain geometry near the shear zone boundaries. To simulate the Black Mountain detachments, future runs will find the conditions of large shear strains accommodated by more distributed deformation, while tracking the changes in bulk friction. The approach will be fruitful in combining geologic and laboratory observations to better understand shallow crustal faulting.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2006
- Bibcode:
- 2006AGUFM.S43C..03H
- Keywords:
-
- 7209 Earthquake dynamics (1242);
- 8000 STRUCTURAL GEOLOGY;
- 8010 Fractures and faults;
- 8100 TECTONOPHYSICS;
- 8163 Rheology and friction of fault zones (8034)