FORELAND ZONE FAULTS AND VEINS RELATED TO ALONG-STRIKE PROPAGATION OF SLAB BREAKOFF AT THE END OF THE TACONIC OROGENY

The breakoff of a subducted slab removes downward slab pull to switch the compressional stress in a collision zone to tensional stress, resulting in the collisional shortening structures being cut by strike-parallel normal faults. In the Taconic frontal zone we identify major normal faults truncating shortening structures. High-angle normal faults observed at Bald Mountain, in Troy and near Hudson in eastern New York suggests that a substantial part of the present western boundary of the Taconic Allochthon is marked by an east-side-down normal fault system, and this extensional event also includes the Mettawee Fault, which truncates the Champlain Thrust. This regional extension is constrained in the Albany area to have occurred immediately following the latest thrusting during the Caradocian, and before the latest Silurian; we propose it was caused by subducted slab breakoff. The previously documented reversal of Taconic subduction polarity provides a tectonic framework that requires slab breakoff. A conspicuous development of planar reverse-motion veins in the Taconic melange marks brittle deformation of the melange zones at the end of the Taconic shortening; we propose that these veins were caused by an enhanced "marginal" slab pull (from the dangling part of the slab), during northward propagation of slab breakoff. The strike-slip cross faults/veins occurring commonly at the margin of and in the foreland of the Taconic orogen in New York and Vermont can also be explained by accommodation of differential strains caused by migration of the marginal slab pull and the breakoff point. Propagation of the slab breakoff toward north during the cessation of Taconic convergence is supported by evidence of oblique collision of the Laurentian margin and progressively later occurrence of the last shortening events in the same direction.