CYLINDRICAL FOLDS AT A LOW ANGLE TO THE STRETCHING LINEATION IN THE TACONIC SLATE BELT

High-strain zones commonly contain folds with hinges oriented subparallel to the stretching lineation.  This is interpreted to result from rotation of the fold hinges and stretching lineation toward the fabric attractor as strain accumulates during non-coaxial flow.  In the Taconic slate belt, fold hinges are typically subhorizontal and lie at a high angle to the stretching lineation in the north where deformation is characterized by monoclinic strain symmetry and at a moderate angle to the stretching lineation in the south where deformation is characterized by triclinic strain symmetry.  Outcrop-scale folds within a relatively large exposure on the eastern side of the region characterized by triclinic strain symmetry, however, depart from this pattern: fold hinges plunge moderately to the southeast and lie at a low angle to the stretching lineation.

The exposure containing the unusual folds is in the footwall of an out-of-sequence thrust fault directly west of the apex of a concave-to-the-west curve in the fault trace.  The folds are cylindrical, display S-shaped asymmetry, and have open interlimb angles.  The intersection between bedding and slaty cleavage is parallel to the moderately plunging fold hinges.  Although the region of triclinic strain symmetry is characterized by a stretching lineation that rakes moderately from the south on the slaty cleavage and R_XZ for slaty cleavage formation of about 2, the exposure containing the unusual folds has an approximately down-dip stretching lineation and a higher but still relatively low R_XZ for slaty cleavage formation of about 6.  The cylindrical character of the folds, the open interlimb angles, and the R_XZ value suggest that the fold hinges have not undergone significant rotation.

The low angle between the fold hinges and stretching lineation may be the result of a local change in kinematics through time near the out-of-sequence thrust fault.  Early-stage non-coaxial flow with triclinic strain symmetry reoriented bedding such that it was obliquely oriented to late-stage non-coaxial flow with monoclinic strain symmetry.  The obliquity yielded outcrop-scale folds with hinges that initiated at moderate to low angles to the stretching lineation and underwent only modest rotation during slaty cleavage formation.