MESOZOIC NORMAL FAULTS IN THE TACONIC SLATE BELT AND THE EFFECT OF PREEXISTING STRUCTURAL ANISOTROPY ON FAULT DEVELOPMENT

Strata of the Taconic slate belt are cut by numerous post-cleavage, outcrop-scale faults. These faults, which typically have a component of normal slip, are likely Mesozoic in age because their location in the Taconic orogenic belt is inconsistent with the predictions of models of orogenic collapse.

Fault-slip data were collected at three localities that have different structural and lithologic characteristics in order to understand the effect of preexisting structural anisotropy on fault development. At all three localities, the faults do not reactivate cleavage, which dips ~30° to ~45° east and is in a non-optimal orientation for slip in an Andersonian stress regime for normal faulting. At the two localities where bedding and cleavage are parallel, however, the faults generally strike parallel to cleavage and dip in the same direction as, but more steeply than cleavage. This is the case even though the trend and rake of the striae vary. The overall fault geometry is simpler and the angle between the faults and cleavage is lower at the locality where bedding and cleavage are parallel and the strata are lithologically homogeneous. These observations are consistent with the results of rock deformation experiments that show slip along cleavage when the anisotropy lies at 15° to 45° to the maximum compressive stress and slip along surfaces at a low angle to cleavage when the anisotropy lies at 45° to 60° to the maximum compressive stress. At the third locality, bedding and cleavage are not parallel, and the strata are locally folded into asymmetric folds that have steeply dipping limbs. The hinge lines of the folds are parallel to the striae on a number of planar normal faults, and entire folded surfaces are reactivated, the long limb as a normal fault and the short limb as a reverse fault. Kinematic axes from reactivated folded surfaces are internally consistent but do not agree with those determined from other post-cleavage faults.

These results show that a preexisting structural anisotropy can affect fault geometry even if it is not in a favorable orientation for reactivation. Because faults can act as conduits for fluid flow, the uniform dip direction of the faults can affect fluid migration pathways in the upper crust. In addition, these results show that strain compatibility can be an important control on fault kinematics.