TWINNING STRAINS FROM SYN-FAULTING CALCITE GOUGE: SMALL-OFFSET STRIKE-SLIP, NORMAL AND THRUST FAULTS

We have evaluated the stress-strain behavior of calcite precipitated and mechanically twinned in small-offset strike-slip (5 sites), normal (7 sites) and thrust faults (7 sites) of a variety of ages and from a variety of tectonic settings (n=3001 twinned grains) on 6 continents. For each strike-slip fault system the twinning shortening strain (e₁) is horizontal and at an angle of 0°-60° to the respective fault plane (dextral or sinistral) although in the majority of cases the shortening axis is parallel to fault strike. In each normal fault example, dip-slip kinematic striations dominate the faulted surface yet the orientation of the maximum principal compressive stress (s₁) and shortening strain axis (e₁) are not 45° to the fault plane as predicted but are sub-horizontal and either strike-parallel or strike-normal. Thrust faults preserve shortening strain axes (e₁) parallel to the dip-slip kinematic direction, within the fault plane (plane strain) and not at 45° to the principal plane. None of the fault stress-strain field results reported here support the Andersonian or Mohr-Coulomb criteria for stress-strain relations predicted along faults.