STRAIN DISTRIBUTION IN SLATE BELTS: AN EXAMPLE FROM THE TACONIC ALLOCHTHON, VERMONT AND NEW YORK

Studies of slate belts in the Appalachians and Taiwan indicate that slaty cleavage is superposed on folds during a progressive deformation that results in the development of flattened folds. Because the strain related to slaty cleavage formation does not reflect the detailed kinematics of folding, the orientation and magnitude of the cleavage-related strain ellipsoid can be used to understand broadscale processes that affect the distribution of strain in orogenic belts.

In the northern Taconic allochthon, the strain related to slaty cleavage formation can be determined from strain fringes around core objects composed of framboidal pyrite and subspherical carbonaceous material and from the microstructure of fractured graptolites. Analyses of strain fringes and graptolites show that strain is homogeneous over areas greater than 100 sq. km but changes along strike of the allochthon. A northern domain is characterized by approximately dip-slip motion within a zone of thrust-sense shear. A southern domain is characterized by oblique motion within a zone of left-lateral, thrust-sense shear. The kinematics of the southern domain is interpreted to be a result of movement across an oblique ramp, which developed in the slope-rise strata as the thrust sheet moved over a subvertical basement fracture zone. Theoretical models of transpression in dipping deformation zones can be used to understand the distribution of strain in the two domains and to estimate the orientation of the oblique ramp. These models indicate that strain magnitude increases as the angle between the strike of the deformation zone and the bulk shortening direction decreases and as the dip of the deformation zone decreases. In the Taconic allochthon, the strain magnitude is lower in the southern domain than in the northern domain, and this is interpreted to be a result of the deformation zone having a very low dip in the northern domain. The orientation of the cleavage-related strain ellipsoid in the southern domain is consistent with a moderately dipping oblique ramp. This implies that the mechanical stratigraphy of the Laurentian passive margin had a strong effect on the deformation such that propagation of the oblique ramp from basement into the overlying dominantly fine-grained cover sequence resulted in a geometry roughly analogous to that of a flower structure.