Bridging the gap between the foreland and hinterland II: Geochronology and tectonic setting of Ordovician magmatism and basin formation on the Laurentian margin of New England and Newfoundland

Abstract

Ordovician strata of the Mohawk Valley and Taconic allochthon of New York and the Humber margin of Newfoundland record multiple magmatic and basin-forming episodes associated with the Taconic orogeny. Here we present new U-Pb zircon geochronology and whole rock geochemistry and neodymium isotopes from Early Paleozoic volcanic ashes and siliciclastic units on the northern Appalachian margin of Laurentia. Volcanic ashes in the Table Point Formation of Newfoundland and the Indian River Formation of the Taconic allochthon in New York yield dates between 466.16 ؎ 0.12 and 464.20 ؎ 0.13 Ma. Red, bioturbated slate of the Indian River Formation record a shift to more juvenile neodymium isotope values suggesting sedimentary contributions from the Taconic arc-system by 466 Ma. Eight ashes within the Trenton Group in the Mohawk Valley were dated between 452.63 ؎ 0.06 and 450.68 ؎ 0.12 Ma. These ashes contain zircon with Late Ordovician magmatic rims and 1.4 to 1.0 Ga xenocrystic cores that were inherited from Grenville basement, suggesting that the parent magmas erupted through the Laurentian margin. The new geochronological and geochemical data are integrated with a subsidence model and data from the hinterland to refine the tectonic model of the Taconic orogeny. Closure of the Iapetus Ocean by 475 Ma via collision of the peri-Gondwanan Moretown terrane with hyperextended distal fragments of the Laurentian margin is not clearly manifested on the autochthon or the Taconic allochthon other than an increase in sediment accumulation. Pro-foreland basins formed during the Middle Ordovician when these terranes were obducted onto the Laurentian margin. 466 to 464 Ma ashes on the Laurentian margin coincide with a late pulse of magmatism in both the Notre Dame arc in Newfoundland and the Shelburne Falls arc of New England that is potentially related to break-off of an east-dipping slab. Following slab reversal, by 455 Ma, the Bronson Hill arc was established on the new composite Laurentian margin. Thus, we conclude that Late Ordovician strata in the Mohawk Valley and Taconic allochthon of New York and on the Humber margin of Newfoundland were deposited in retro-foreland basins.

Aseismic ridge subduction as a driver for the Ordovician Taconic orogeny and Utica foreland basin in New England and New York State

“…502 Ma above an eastdipping subduction zone on the east side of the Iapetus Ocean (present-day coordinates), away from the infl uence of Laurentia ( Fig. 1A; Macdonald et al, 2014Macdonald et al, , 2017Karabinos et al, 2017). As Iapetus closed, the arc migrated toward a microcontinent of Laurentian affi nity that lay on the west side of Iapetus.…”

“…The most recent Taconic tectonic models for western New England rely heavily on advances in detrital zircon data and radiometric dates (Macdonald et al, 2014(Macdonald et al, , 2017; for details of these and older models, see Appendix A). In these recent models, the Shelburne Falls arc initiated on the peri-Gondwanan Moretown terrane at ca.…”

“…Closure of the Iapetus Ocean at ca. 475 Ma led to the subsequent closure of the Taconic seaway, and early Taconic thrusts began forming (Macdonald et al, 2014(Macdonald et al, , 2017Karabinos et al, 2017) in response to crustal shortening or subduction or both. Following these events, breakoff of the eastwardly subducted oceanic slab at ca.…”

“…The northern Appalachian orogen of the United States and Canada has been a laboratory for the development and revision of plate-tectonic models, beginning with the seminal papers of Bird and Dewey (1970) and Dewey and Bird (1971), and continuing to the present (e.g., Macdonald et al, 2014Macdonald et al, , 2017van Staal et al, 2016;Karabinos et al, 2017).…”

Tropical weathering of the Taconic orogeny as a driver for Ordovician cooling: REPLY

“…In contrast, regional tectonics can cause subsidence and uplift on scales of several kilometers over millions of years. As discussed by Macdonald et al (2017), major Ordovician sequence boundaries in New England were driven by Taconic tectonism, and the Sauk-Tippecanoe sequence boundary itself may be associated with plate reorganization and subduction beneath Laurentia, similar to the modeled effects of dynamic subsidence in the Cretaceous interior seaway (Mitrovica et al, 1989). Inferring large-scale rapidly fluctuating global eustatic sea-level variation from facies in the Taconic allocthon, as done by Landing (2012), is problematic.Landing (2018) argues that the postulated high eustatic sea level would have eliminated the precipitation gradient between the tropics and subtropics and that a location of the Taconic orogenic belt within 10º of the equator, as opposed to 20º, would have no effect on its weatherability.…”