Kinematic and geochronologic constraints on the structural development of the northern taconic allochthon in western New England, U.S.A.

Date of Completion

January 1998






The primary goal of this research is to constrain the kinematics and timing of cleavage formation in the northern Taconic Allochthon, one of the classic slate belts in the world. By studying slate belts, where strain markers and kinematic indicators are typically well preserved, cleavage formation is better understood in the context of tectonic processes in convergent orogenic belts.^ The first chapter focuses on the geometry, distribution, intensity, and kinematics of S2, a slaty cleavage, S3, a crenulation cleavage, and S4, a spaced phyllitic cleavage. Kinematic data indicate that S2 and S4 developed during noncoaxial flow. S2 varies little in orientation and strain magnitude, and is interpreted in the context of the cleavage trajectory model for thrust sheets. Whereas S2 is associated with emplacement of the Taconic Allochthon, orientation data suggest that S3 and S4 developed after emplacement of the Allochthon.^ The second chapter presents an analysis of albite porphyroblasts with sigmoidal inclusion trails in Taconic phyllites. Microstructural observations suggest the sigmoidal inclusion trails are trapped microfolds and do not record synkinematic rotation of the porphyroblasts. The porphyroblasts have, however, rotated with respect to the geographic reference frame on the basis of orientation data for S3 and truncation planes in the porphyroblasts.^ The final chapter addresses the use of high-spatial resolution $\sp{40}$Ar/$\sp{39}$Ar laser microprobe geochronology for directly dating cleavage formation. The age of cleavages in Taconic slates and phyllites was constrained by analyzing zones of tightly packed cleavage domains. Dates obtained from these domains are tightly clustered and interpreted as crystallization ages for cleavage-defining white micas. A ca. 370 Ma age for S3 is consistent with independent age constraints for the main phase of the Acadian orogeny. A ca. 345 Ma age for S4 can be interpreted as reflecting a late Acadian deformation event.^ The results of this research emphasize the importance of noncoaxial flow in the development of cleavages in slate belts and show that the regionally developed cleavages in the northern Taconic Allochthon did not develop during a single progressive deformation but during discrete tectonic events. ^