COEVAL DEVELOPMENT OF TACONIC RETRO-ARC ALLOCHTHONS, UTICA BASIN, ASH BEDS, AND THE BIMODAL OLIVERIAN PLUTONIC SUITE: EFFECTS OF ASEISMIC RIDGE SUBDUCTION?

Recent Taconic tectonic models involve eastward subduction and collapse of the Iapetan ocean basin between ~502 Ma and ~470 Ma followed by westward subduction at ~460 Ma that continued to ~440 Ma (e.g., Karabinos et al., 2017; Macdonald et al. 2014, 2017). In these models, the 453 Ma to 450 Ma Utica Shale was deposited in a retroarc foreland basin. Volcanic ashes in the Utica and the Deicke-Millbrig volcanic ashes in underlying Trenton-age carbonates have a significant crustal component (e.g., Samson et al., 1989). The Highlandcroft/Oliverian plutonic series in the Bronson Hill arc (and similarly-aged units farther south) could be a source for the Utica ashes, and the Bronson Hill arc rhyolites of the Ammonoosuc Volcanics could be related to a Deicke-Millbrig-like melting phase.

If the westward subduction model is correct for the time of Utica basin subsidence, final Taconic allochthon emplacement, volcanic ash layers, and Oliverian Plutonic Suite, then we suggest that aseismic ridge subduction and related effects should be considered as a causal mechanism for these coeval elements. Subduction of aseismic oceanic ridges is a common occurrence. Using the Andes, the Antilles, and the Middle America trench as models, we developed a set 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 broadly synchronous subsidence in a retro-foreland basin. This aseismic ridge subduction model can be incorporated in the west-dipping subduction phase of the Taconic Orogeny to explain the development of the retro-arc Taconic allochthons, thrust-loaded Utica basin, bimodal Oliverian Plutonic series and associated ashes in the Utica.