KINEMATIC MODELING OF STRAIN VARIATION IN THE GIDDINGS BROOK THRUST SHEET OF THE TACONIC ALLOCHTHON AND IMPLICATIONS FOR THE DEVELOPMENT OF THE QUEBEC SALIENT

We evaluate various combinations of simultaneous simple shear, pure shear, and volume change in three dimensions to explain the strain patterns in the Giddings Brook thrust sheet, the largest thrust sheet in the Taconic allochthon. Our analysis is restricted to the slate belt where strain data are readily obtained from strain fringes around rigid core objects and where the relative homogeneity of the stratigraphic succession abets the modeling. Map-view curvature of the regional-scale fold axes and the very gentle south plunge of the thrust sheet define three structural domains: a northern domain, which is the structurally lowest domain, a central domain where the fold axes are anomalously oriented with respect to the overall trend of the Taconic allochthon, and a southern domain, which is the structurally highest domain. The kinematic modeling shows that the strain data are readily explained by constant volume + top-to-west-northwest thrusting simple shear ± left-lateral wrenching simple shear ± pure shear with the thrusting and wrenching components having a shared shear direction. The amount of thrusting shear strain and the ratio of wrenching to thrusting shear strain vary between the domains. The northern domain is described by the largest amount of thrusting shear strain, consistent with its structural position at the base of the Giddings Brook thrust sheet. The ratio of wrenching to thrusting shear strain is relatively high in the central domain compared to the northern domain while the southern domain shows no evidence for a component of wrenching simple shear. We interpret the anomalous orientation of the fold axes in the central domain as a product of the relatively high wrenching-to-thrusting shear strain ratio in this domain. More broadly, we interpret the evidence for distributed left-lateral wrenching simple shear in the northern and central domains as indicating that curvature of the Quebec salient was amplified during Taconian orogenesis. Thus, the Quebec salient evolved from a primary arc, with curvature inherited from the Iapetan rift architecture, to a progressive arc, with curvature forming in tandem with thrust sheet emplacement.