CONTRIBUTION OF PENETRATIVE STRAIN AND OUT-OF-SEQUENCE THRUST FAULTS TO THRUST SHEET THICKENING IN THE TACONIC OROGENIC WEDGE OF THE NORTHERN APPALACHIANS

The Taconic allochthon is composed of several thrust sheets in eastern New York and western Vermont that developed during the Ordovician Taconic orogeny. The westernmost thrust sheet is the Giddings Brook thrust sheet, which contains the central domain, the focus area of this study. The central domain is composed of relatively homogenous slate and prevalent out-of-sequence thrust faults. The purpose of this study is to understand how the out-of-sequence thrust faults and the penetrative strain during slaty cleavage development within the central domain contributed to thickening of the Taconic orogenic wedge.

The central domain contains two structurally distinct regions: the western area, which exhibits a complex three-dimensional deformation, and the eastern area, which exhibits two-dimensional deformation. We used two-dimensional kinematic models to understand the penetrative strain during slaty cleavage development within the eastern area. Modeling of the eastern area is consistent with the region having undergone simultaneous simple shear and pure shear with a mean k_x ≈ 0.7 (thrust sheet thickening). These results indicate that the eastern area thickened through out-of-sequence thrust faulting and penetrative strain during cleavage formation. In contrast, previous work has shown the easternmost part of the western area, which contains more out-of-sequence thrust faults, did not experience significant thickening or thinning from penetrative strain during slaty cleavage formation. Even though these areas in the central domain accommodated strain differently, the amount of horizontal shortening was likely similar in the two areas. Together, these areas are a localized region of thrust sheet thickening in the Giddings Brook thrust sheet. These results suggest both penetrative strain and out-of-sequence thrust faults contributed to maintaining the critical taper angle of the Taconic orogenic wedge.