Do cataclastic deformation bands form parallel to lines of no finite elongation (LNFE) or zero extension directions?

Abstract

Conjugate cataclastic deformation bands cut unconsolidated sand and gravel at McKinleyville, California, and dip shallowly towards the north-northeast and south-southwest. The acute dihedral angle between the two sets of deformation bands is 47° and is bisected by the sub-horizontal, north-northeast directed incremental and finite shortening directions. Trishear models of fault propagation folding above the McKinleyville fault predict two sets of LNFE (lines of no finite elongation) that plunge steeply and shallowly to the south and north. These predictions are inconsistent with deformation band orientations and suggest that deformation bands did not form parallel to these LNFE. During plane strain, zero extension directions with acute dihedral angles of 47° develop when the dilatancy rate (dV/dε1) is −4.3. Experimental dilatancy rates for Vosges sandstone (cohesion > 0) and unconsolidated Hostun sand suggest the deformation bands either developed parallel to zero extension directions or in accordance with the Mohr–Coulomb criterion, assuming initial porosities of 22% and 39%, respectively. An empirical relationship between dV/dε1, relative density and mean stress suggests that dilatancy rates for Vosges sandstone overestimate dV/dε1 at McKinleyville. Deformation bands at McKinleyville likely developed either in a Mohr–Coulomb orientation, or an intermediate orientation bounded by the Mohr–Coulomb (θC) and Roscoe (θR) angles.