12-9 The Mohawk Valley in New York State was not in a Deep Trench During Late Taconic Convergence (455-450 Ma)

Session: New advances in geological and geophysical research on the Appalachian orogen. (II)

Presenting Author:

Robert Jacobi

Authors:

Jacobi, Robert D.¹, Mitchell, Charles E.², Thompson, Margaret D.³, Crowley, James L.⁴, Elias, Robert J.⁵, Leslie , Stephen A.⁶

(1) Department of Earth Sciences, University at Buffalo, 126 Cooke Hall, Buffalo, , (2) Department of Earth Sciences, University at Buffalo,126 Cooke Hall, Buffalo, , (3) Department of Geosciences, Wellesley College, 106 Central Street, Wellesley, , (4) Department of Geosciences, Boise State University, 1910 University Dr. MS 1535, Boise, , (5) Department of Earth Sciences, University of Manitoba, 240 Wallace Building, Winnipeg, , (6) Department of Geology and Environmental Science, James Madison University, Harrisonburg, ,

Abstract:

One of the longest transects across the Laurentian shelf and into the hinterland of Taconic thrusts is from the Mohawk Valley in NYS to central VT. In the Mohawk Valley, the ~452.8 Ma, westward overstepping (present coordinates), basal black shale maintains a nearly constant thickness for over 60 km, and perhaps as much as 100 km. The constant thickness indicates that the carbonate shelf was nearly flat when the black shale overstepped. This geometry is in conflict with previous eastward subduction models in which faults in the Mohawk Valley dropped the carbonate bank down into a deep trench as it was overstepped by the black shale trench fill. Surprisingly, the eastward subduction model nonetheless remains viable with this flat-shelf constraint because the more craton-ward portion of the presently subducting Australian shelf also has a very low dip of about 1^o.

 

In the Australian convergence, shelfal carbonates were covered by at least 2-4 Myr of trench fill before the shelfal sediments were underthrust beneath the accretionary prism. In the Taconic case, however, new evidence suggests that thrusts were emplaced in central Vermont and the Mohawk Valley < 1 Myr after basal black shale deposition. Specifically, in central Vermont three new CA-TIMS U-Pb zircon dates of volcanic ash (453.39 ± 0.19, 453.50 ± 0.15, and 453.45 ± 0.18 Ma), as well as new coral and conodont ages, establish that the black shale overstep there occurred only about 0.6 Myr earlier than in the Mohawk Valley, and occurred < 1 Myr before the Giddings Brook slice of the Taconic allochthon thrust over the VT site.

 

The short duration between black shale overstep and thrusting at both localities, coupled with the flat shelf, fast overstep and thrust transport rates, and other considerations (e.g., age of sediment facies transition in the thrusts) place critical constraints on tectonic models. For example, the classic eastward subduction model (in which the shelfal carbonates dove into a deep trench), is not appropriate for the 450-455 Ma time period in this region. Instead, possible scenarios include a special set of circumstances for 1) closure of a narrow seaway that involved limited eastward subduction with a shallow trench fill overtopping the outer wall, or 2) retroarc thrusting in the subduction polarity reversal model.

Geological Society of America Abstracts with Programs. Vol. 58, No. 2, 2026

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