EVALUATION OF THE APPLICABILITY OF KINEMATIC MODELS OF TRANSPRESSION TO A REACTIVATED IAPETAN FRACTURE ZONE IN THE TACONIC OROGEN

The orogen-scale geometry of the northern Appalachians implies the presence of northwest-striking, left-stepping, Iapetan fracture zones between the New York promontory and Quebec embayment. A synthesis of strain data in the Taconic allochthon and geologic relations in the Green Mountain massif provides evidence for an Iapetan fracture zone east of Clarendon, Vermont. Because slaty cleavage formation in the Taconic allochthon is inferred to have occurred during transport of the strata locally across a lateral ramp created by the reactivation of the fracture zone, it is possible to apply kinematic models of transpression to improve understanding of the Taconic collision.

Five common assumptions of kinematic models of transpression are homogeneous deformation, constant-volume deformation, lateral confinement of the deforming rock, basal confinement of the deforming rock, and a parallel-sided deformation zone. The first three assumptions can be evaluated for the Taconic allochthon using data from markers that record the strain related to slaty cleavage formation. Strain fringes around subspherical core objects allow measurement of the maximum and intermediate principal stretches, and obtained stretch values differ by no more than thirty percent within a structural domain. This evidence for relatively homogeneous deformation is consistent with the dominance of fine-grained siliciclastic rocks in the stratigraphic column. Although large volume loss has been proposed for the Taconic allochthon on the basis of stretch values obtained from graptolite thecal-spacing data, these stretch values are inconsistent with the graptolite microstructure. The thecal-spacing data yield a prolate state of strain, yet the brittlely fractured graptolite periderm displays chocolate-tablet boudinage. The discrepancy is attributed to an episode of prelithification volumetric contraction, and new volume-change estimates indicate approximately constant-volume deformation. Lateral confinement of the deforming rock is supported by the calculation of a stretch value of nearly one for the direction parallel to the inferred lateral ramp. In sum, these data suggest that the relatively simple boundary conditions of kinematic models of transpression are reasonably appropriate for some deformation zones.