EVIDENCE FOR THRUST SHEET THICKENING IN A REGION OF OUT-OF-SEQUENCE THRUST FAULTING IN THE TACONIC OROGENIC WEDGE

The Taconic allochthon (TA) is a stack of thrust sheets in NY and VT that developed during the Ordovician Taconic orogeny. The northern region of the Giddings Brook thrust sheet in the TA is composed of three structural domains: northern, central, and southern. These domains underwent regional folding, the development of slaty cleavage, and out-of-sequence thrust faulting. The central domain consists of two subdomains, an area of relatively high strain in the east and a larger area of relatively low strain in the west. From west to east in the central domain, out-of-sequence thrust faults become more prevalent and a crenulation cleavage locally develops. In order to expand our understanding of the high strain subdomain, additional orientation and stretch data are being collected and analyzed.

The strain data were analyzed by plotting the mean data for each site in the high strain subdomain on an R vs θ’ diagram. Previous work in the high strain subdomain indicates that the mean slaty cleavage is oriented at 010/43, the mean stretching lineation is oriented at 40/102, and the mean stretch along the X and Y axes is 2.01 and 1.11, respectively. Current work supports these results with the mean slaty cleavage for the new sites oriented at 007/50. Two strain combinations were investigated for the R vs θ’ diagrams: 1) simple shear and volume change and 2) simple shear and pure shear with no volume change. The R vs θ’ diagram for simple shear with volume change is internally inconsistent, with all but one site plotting in the volume gain field. However, on the R vs θ’ diagram for simple shear and pure shear with no volume change, the strain data plot in the thickening shear zone field, except for one site that plots on the simple shear curve. These data suggest that the high strain subdomain of the Giddings Brook thrust sheet underwent thickening in order to maintain the critical taper angle of the orogenic wedge. This interpretation is consistent with the prevalence of out-of-sequence thrust faults in the high strain subdomain.