RIDGE SUBDUCTION INCORPORATED INTO TECTONIC MODELS OF THE TACONIC OROGENY IN SOUTHERN NEW ENGLAND AND NEW YORK STATE AND ITS BEARING ON THE ORDOVICIAN UTICA FORELAND BASIN

Subduction of seismic and aseismic oceanic ridges is common in contracting ocean basins, and closure of main ocean basins generally involves subduction of a seismic ridge (spreading center). Ridge subduction can result in flat slab subduction, consequent arc retreat away from the trench, and (with later slab steepening) formation of a younger, more alkalic arc with continental contamination/melting signatures.

Tectonic models of the Appalachian Orogen, specifically the Taconic Orogeny, have rarely incorporated ridge subduction. Recent models involve eastward subduction and collapse of an ocean basin between ~502 Ma and ~470 Ma followed by westward subduction at ~470 Ma that continued to ~445 Ma (e.g. Macdonald et al. 2014). These ocean basin closures likely involved subduction of a spreading center.

In these models, the ~453 Ma to ~450 Ma Utica Shale was deposited in a retroarc foreland basin that experienced both extension and later thrusting. In Argentina, thrust belts in retroarc foreland basins (e.g. the Agrio belt, 500 km from the trench) are related to relatively flat slab subduction coupled with variations in relative convergence rate. Also consistent with ridge subduction are the Utica and Deicke-Millbrig volcanic ashes that had sources with a significant crustal component, based on isotope ratios, inherited zircon cores, and hornblende ages (Samson et al., 1989). The Bronson Hill arc rhyolites (e.g. Ammonoosuc Volcanics of Deicke-Millbrig age) could be related to this melting phase. Strike/oblique slip motion in the Utica basin may reflect escape tectonics or general relative plate convergence vectors (e.g. Waldron et al. 2014).

Alternatively, the sequence of the boninitic Shelburne Falls arc followed by the more easterly Bronson Hill arc with rhyolites (e.g. Ammonoosuc Volcanics) could reflect eastward-dipping, flat-slab subduction under a microcontinent-floored arc. Implications are 1) westward subduction is not necessary in order to construct the younger Bronson Hill arc, and 2) the Utica basin and its syndepositional faulting marks the filling and inversion of the Laurentian margin during final collision. Problems include the apparently exotic Moretown, subsidence timing and the lack of the expected gradual westward overstep of the Utica Shale as the Laurentian margin approached the trench.