30 New advances in geological and geophysical research on the Appalachian orogen. (Posters)

We invite contributions that discuss new research on the Appalachian orogen. Submissions should include, but are not limited to, geological or geophysical studies or works comprising a combination of both methods.

Monday, 23 March 2026
1:30 PM - 5:30 PM
CCC, Ballroom C

Rapid Propagation of Subduction Initiation Throughout the Iapetus Ocean

Paper number: 30-0
Program: Technical Sessions
Time: 1:30 PM – 1:30 PM

Authors

Abstract

Mafic–ultramafic complexes decorate the Laurentian margin along >6,600 km of the Appalachian–Caledonian Orogen, from Alabama, USA, to Arctic Norway. Individual complexes host tholeiitic metabasalts, recording decompression melting in a forearc setting, in addition to boninites/low-Ti metabasalts and calc-alkaline metabasalts, recording nascent to mature arc magmatism. The mafic–ultramafic complexes have been interpreted as suprasubduction zone (SSZ) ophiolites that formed by seafloor spreading and embryonic arc processes during subduction initiation. Dating of the earliest lithologies in individual ophiolites presents an opportunity to track the kinematics of new convergent margin formation over thousands of km of the Iapetus Ocean.

High-precision U-Pb zircon dating was performed by chemical abrasion, isotope dilution thermal ionization mass spectrometry (CA-ID-TIMS) for eight ophiolites along 6,060 km of the Appalachian–Caledonian ophiolite belt. The new CA-ID-TIMS ages, with 2σ uncertainties of 170–480 ka, supersede legacy ages for the ophiolites obtained by laser ablation inductively-coupled-plasma mass spectrometry or multigrain, upper-intercept ID-TIMS approaches, none of which should be interpreted with 2σ uncertainties less than ~2% (~10 Ma). Seven of the eight ophiolites define a linear position–age trend (R2 = 0.997), younging monotonically to the SW, from 491.4 ± 0.2 Ma in central Norway to 467.1 ± 0.2 Ma in Alabama. The position–age trend tracks propagation of subduction initiation throughout the Iapetus Ocean, constraining the process to unidirectional (east to west) at ~25 cm y-1. This value is significantly greater than known plate velocities, suggesting subduction may be seeded/established in one part of an ocean basin and then propagate at rates that allow for rapid establishment of large-scale convergent plate margins.

The Coastal Complex of Newfoundland yielded an older (499.6 ± 0.3 Ma) age that corresponds, within uncertainty, to that of a handful of arc igneous units from Vermont, USA, to Scotland. The earlier-Cambrian arc magmatism records a distinct, short-lived episode of (quasi-)subduction that preceded the ocean-wide subduction initiation episode at 491–467 Ma and subsequent Appalachian–Caledonian collision (starting with the middle Ordovician–early Silurian Grampian–Taconic event). Convergence at c. 500 Ma was spatially localized and did not yield the SSZ ophiolites produced during the Cambro–Ordovician episode of convergence. It may represent ephemeral subduction associated with the closure of a small, non-continuous seaway within the Laurentian hyperextended margin.

Geological Society of America Abstracts with Programs. Vol. 58, No. 2

doi: 10.1130/abs/2026NE-411138

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