Date of Completion
Jean Crespi, Julie Fosdick, Tim Byrne
Field of Study
Master of Science
Thrust sheet emplacement is typically accompanied by internal deformation that can vary depending on a range of factors. In the Taconic slate belt in the northern part of the Giddings Brook thrust sheet of the Taconic allochthon, map-view curves and an along-strike change in structural level expose variations in internal deformation of a low-grade metamorphic thrust sheet. We characterized internal deformation during slaty cleavage formation by determining the orientations of cleavage and stretching lineation and by quantifying the strain using syntectonic fibers formed on subspherical core objects. The slate belt is everywhere characterized by top-to-west-northwest noncoaxial flow. The southern and northern domains have a north-northeast-striking, shallowly to moderately dipping slaty cleavage with a downdip stretching lineation. The strain magnitude and degree of fiber asymmetry, however, are lower in the structurally higher southern domain. The central domain has a north-northwest-striking, moderately dipping slaty cleavage with a moderately raking stretching lineation. A component of sinistral shear sets it apart from the other two domains, with the strain magnitude and degree of fiber asymmetry otherwise similar to the southern domain. We interpret the southern and northern domains as being dominated by dip-slip thrusting with strain increasing toward the base of the thrust sheet and the central domain as a region of inclined transpression. Two-dimensional kinematic models of thrust sheet deformation show that the southern and northern domains can be described by simple shear (with small amounts of thrust sheet thinning in the southern domain) in contrast to unmetamorphosed to weakly metamorphosed thrust sheets that exhibit thrust sheet thickening and higher-grade metamorphic thrust sheets that exhibit thrust sheet thinning. In addition, the dip-slip character of the southern and northern domains combined with the lack of evidence for orogen-scale kinematic partitioning implies nearly orthogonal plate convergence during the Taconic orogeny in the northeastern United States.
Robinson, Jessica, "Strain Variation in Low-grade Metamorphic Thrust Sheets: An Example from the Northern Taconic Allochthon of Vermont and New York, USA" (2018). Master's Theses. 1261.