Can we use lines of no finite elongation to predict the orientations of cataclastic deformation bands?
Cataclastic deformation bands typically form in high porosity sandstones adjacent to and ahead of through-going fault surfaces. As such, the orientations of cataclastic deformation bands may provide information about the geometry and magnitude of finite strains within the wall rocks surrounding larger faults. This information may in turn provide clues as to the geometry and magnitude of the elastic strain field at the time of faulting (cf. Watterson 1999).
Similarities between the results of trishear numerical models and claybox experiments of fault propagation folding led Allmendinger (1998) to propose that secondary fractures may develop parallel to lines of no finite elongation (LNFEs). We test this proposal by comparing the orientations of deformation bands preserved in the footwall of a seismically active thrust, exposed at Clam Beach, McKinleyville, CA with the orientations of LNFEs predicted by a suite of two-dimensional (plane strain) trishear models. Although simplistic, the assumption of plane strain deformation provides a reasonable starting point in the absence of firm constraints on three-dimensional fault geometries and displacement gradients.
Models incorporating trishear angles of 50 and 70, and the maximum potential thrust offset of 56m, predict negligible strains in the footwall of the McKinleyville thrust along the Clam Beach section. Models incorporating a trishear angle of 90 predict that deformation occurs within the footwall up to ~120m away from the trace of the thrust, with maximum strains (Rxz) of 1.2. By contrast, the orientations of cataclastic deformation bands imply bulk footwall strains (Rxz) of 3 to 5, assuming that deformation bands form parallel to LNFEs. This prediction contrasts with a previous study, which used Fry analysis of sand grains within and adjacent to the cataclastic deformation bands to calculate footwall strains (Rxz) of 1.2 and 1.7 (Cashman & Cashman 2000). It is therefore unlikely that the deformation bands seen at McKinleyville have formed parallel to LNFEs, at least those modelled assuming constant volume plane strain deformation. Ongoing research will investigate how volume changes and triaxial strains during deformation band formation may have influenced the orientations of LNFEs.
Allmendinger, R.W. 1998. Tectonics 17, 640-656.
Cashman, S. & Cashman, K. 2000. Geology 28, 111-114.
Watterson, J. 1999. Journal of Structural Geology 21, 939-948