12-2 Is surface geology along the western front of the New England Appalachians linked to the Moho overthrust?

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

Presenting Author:

Authors:

Abstract:

Reverse faults along the western front of the Berkshire and Green Mountain massifs place Laurentian basement and its Iapetan rift–drift cover sequences over younger Cambrian‒Ordovician rocks and crosscut thrust faults from the Ordovician Taconic Orogeny. Sampling transects across the reverse faults reveal field and (micro)structural evidence for polyphase deformation from the late Silurian–Mesozoic including reactivation and/or transposition of older foliations during shearing, crenulation cleavage development, and brittle fractures both parallel to and cross-cutting foliation planes. Samples collected along transects across structural domains yield multiple ⁴⁰Ar/³⁹Ar age populations and often complex age spectra with reproducible age gradients. Late Silurian‒Early Devonian argon ages once thought to represent slow cooling following the Taconic Orogeny instead document major phases of motion along the reverse faults beginning at ca. 420 Ma and continuing for at least 70 million years. Evidence for fault reactivation and younger faulting include the following: 1) the distribution of ages in fault zones along the western Appalachian front from southern Quebec to southern Massachusetts; 2) the preservation of similar ages in rocks from biotite to sillimanite grade Taconic metamorphic zones; and 3) the preservation of quartz microstructures in dated samples that require relatively rapid cooling post-deformation. The complexity of the geochronological data is consistent with polyphase deformation, including multiple age signals from single samples, and age variations across structural domains in a single outcrop. The map locations of the reverse faults roughly coincide with a projected surface trace of the Moho overthrust imaged by the New England Seismic Transect (NEST) and SEISConn passive seismic broadband arrays for which throw along the overthrust increases from north to south, a similar spatial trend recorded by increased complexity of argon ages and crustal structure associated with deformation ca. 360‒350 Ma. Data from the NEST seismic node arrays will help evaluate links between the surface geology and mantle structure.

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

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