ASEISMIC RIDGE SUBDUCTION, FLAT SLAB SUBDUCTION, THE TACONIC OROGENY IN WESTERN NEW ENGLAND, AND THE UTICA FORELAND BASIN IN EASTERN NEW YORK STATE

Subduction of aseismic oceanic ridges is a common occurrence, and has been proposed for nearly the entire Andean continental margin during its contractional history. Using the Andes, the Antilles, and the Middle America trench as models, we have developed a set of six characteristic elements of aseismic ridge subduction that can be used to identify ridge subduction in ancient orogenic belts. Aseismic ridge subduction results in flat slab subduction with a volcanic gap accompanied by significant uplift; slab steepening after passage of the ridge results in asthenospheric upwelling, decompression melting, and lithospheric delamination. These processes can cause bimodal volcanism far from the trench and an arc with continental contamination and melting signatures even farther from the trench (< 800 km) with voluminous rhyo-dacitic ashes associated with super-caldera formation. The thinned crust, coupled with high convergence rates, results in intracontinental thrusting that loads the foreland, resulting in a subsiding retro-foreland basin. In Argentina, these thrust belts in retroarc foreland basins such as the Agrio belt can be 500 km or more from the trench.

Tectonic models for the Taconic Orogeny in western New England have never incorporated ridge subduction into the evolution of the Appalachians, until recently. Recent models involve eastward subduction and collapse of the Iapetan 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, 2017).

In these models, the ~453 Ma to ~450 Ma Utica Shale was deposited in a retroarc foreland basin. We suggest that subsidence of the Utica foreland basin resulted from thrusting related to fast relative convergence rate coupled with a weakened lithosphere after passage of an aseismc ridge. Consistent with the flat-slab subduction model are volcanic ashes in the Utica and the Deicke-Millbrig volcanic ashes in underlying Trenton-age carbonates that have a significant crustal component, based on isotope ratios, inherited zircon cores, and hornblende ages (Samson et al., 1989). The Bronson Hill arc rhyolites of the Ammonoosuc Volcanics could be related to the Deicke-Millbrig melting phase and the Highlandcroft/Oliverian plutonic series of the Ammonoosuc arc could be a source for the Utica ashes.