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

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. The Macdonald et al. (2014) and older models involve eastward subduction and collapse of an ocean basin between ~502 Ma and variously ~475 Ma to ~450 Ma. This closure likely involved subduction of a spreading center. The sequence of the boninitic Shelburne Falls arc followed by the more easterly Bronson Hill arc with rhyolites (e.g., Ammonoosuc Volcanics) is consistent with 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, although westward subduction also could have occurred, 2) the alkalic volcanic ashes in the Utica Shale reflect the flat slab subduction and Bronson Hill arc, and 3) the ~453 Ma to ~450 Ma Utica basin and its syndepositional faulting would mark the filling and inversion of the Laurentian margin basin during collision. However, the expected gradual westward overstep of the Utica Shale as the Laurentian margin approached the trench is not observed; rather, black shales flooded the Laurentian carbonate bank synchronously across much of the basin.

Alternatively, the polarity flip to westward subduction at ~475 Ma invoked by recent models could have produced the Bronson Hill rhyolites during trench rollback (as occurred in Taiwan). In this model, the 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. The observed strike/oblique slip motion in the Utica basin may reflect escape tectonics during final convergence or general relative plate convergence vectors (e.g., Waldron et al. 2014).