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1. Examining a Channel Flow Hypothesis Along the Nashoba- Avalon Terrane Boundary, Eastern Massachusetts.

   Arolkar, N; Simoneau, V; Kuiper, YD; Link, F; Long, MD (December 2024, American Geophysical Union)

   The Acadian orogeny resulted from the accretion of the southeastern New England Avalon Terrane (AT) to the Nashoba Terrane (NT) - the trailing edge of Ganderia - to its northwest, in eastern Massachusetts. Ganderia and the AT are mostly Gondwana-derived. Previously, rocks of the NT were interpreted to have been extruded to the southeast over the AT as part of a channel flow zone. Only the top and center of this zone are exposed in the NT. Bedrock and structural mapping were carried out in the AT adjacent to the NT to locate the bottom of the channel flow zone. The main rock types are migmatitic biotite gneiss and mafic rock, quartzite, and igneous rocks, exposed in 10s of m to km scale blocks and lenses. Some of these rocks have been sheared and show evidence of mylonitization. Furthermore, they occur near, and in two areas are crosscut by, igneous plutons of unknown age. The foliations of migmatitic rocks, quartzites, and mylonites predominately dip NW, but the orientations of the mylonites vary, especially away from the terrane boundary. Lineations plunge NE and SW in migmatites, NE in quartzites, and NW in mylonites. Migmatitic rocks show abundant isoclinal folds. Predominantly NW to SW dipping normal faults with various slickenline orientations were observed in all rock types. The migmatitic biotite gneiss and its structures resemble those of the NT. However, U-Pb zircon data yielded a detrital zircon signature typical for Avalonia, with predominantly Mesoproterozoic and minor Paleoproterozoic and Tonian populations. Furthermore, zircon overgrowths are ~585 Ma, which suggests that the high-grade metamorphism and partial melting were Ediacaran and did not result from the Acadian orogeny and channel flow at that time. Based on the (1) blocky/lensoid outcrop pattern of rock types, (2) varied orientations of structures, and (3) abundance of faults, the area may represent a brittle fault zone that cut off the interpreted channel flow zone of the Nashoba terrane. Our structural analysis is complemented by and provides context for high-resolution seismic imaging of the crust enabled by the ongoing GENESIS deployment of broadband seismometers across the NT. Preliminary results from GENESIS suggest a transition in crustal structure across the boundary between the NT and AT, consistent with geological observations. 

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2. Deformation along the Nashoba-Avalon terrane boundary in Eastern Massachusetts

   Arolkar, Nikhil; Kuiper, Yvette D; Link, Fred; Long, Maureen D; Simoneau, Victoria (March 2025, Geological Society of America)

   At the onset of the Acadian orogeny (~421–400 Ma), the New England Avalon terrane (AT) accreted to the southeastern margin of the Nashoba terrane (NT), the trailing edge of Ganderia. In the northwest-dipping NT, widespread evidence for partial melting, the presence of high-grade rocks between lower-grade rocks of adjacent domains, and a progression from northwest-side-down folds at high structural levels towards symmetric folds at lower levels, suggests that NT terrane rocks may have undergone channel flow and ductile extrusion over the AT to the southeast. northwest-side up folds, expected towards the bottom of the extrusion zone, have not been found in the NT. Structural mapping was undertaken in a ~26 by ~11 km area in the AT adjacent to the NT to test whether the bottom of the zone may instead lie in the AT. This involved the documentation and analysis of structures in separate northeast and southwest structural domains. In the northeast domain, foliations dip northwest, while lineations plunge northwest and southeast. However, consistent northwest-side-up folds or shear zones were not recorded. Furthermore, new U-Pb zircon ages of a migmatitic paragneiss and a migmatitic mafic rock are ~585 Ma and ~591 Ma, respectively, indicating that high-grade metamorphism and partial melting occurred in the Ediacaran before the Acadian orogeny. In the southwest domain, foliated host rocks with various orientations are highly faulted and intruded by undeformed plutonic rocks. Zircons from a granitic sample that crosscuts mylonitic fabric yielded a ~355 Ma age of crystallization, indicating that mylonitization may either be related to the Acadian orogeny or occurred during the Ediacaran. Thus, evidence indicative of ductile extrusion during the Acadian orogeny was not found in the two structural domains. However, (1) the juxtaposition of NT rocks deformed by the Acadian orogeny alongside AT rocks unaffected by the event, and (2) the presence of fault rocks along the terrane boundary and within the southwest structural domain of the AT, suggest that the bottom of the ductile extrusion zone was cut off by a fault. Seismic data obtained through the GENESIS array of broadband seismometers (~5 km spacing) across the NT show a west-dipping transition in crustal structure across the NT-AT boundary. This may represent the base of the channel flow zone and/or a fault zone. 

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3. Structural and partial melting history of the Ganderian Nashoba Formation in eastern Massachusetts: Evidence for channel flow and ductile extrusion?

   https://doi.org/10.1130/GES02914.1  

   Kuiper, Yvette D; Buchanan, J Wesley; Hepburn, J Christopher; Buchwaldt, Robert (April 2026, Geosphere)

   Detailed structural mapping, U-Pb chemical abrasion–isotope dilution–thermal ionization mass spectrometry (CA-ID-TIMS), and sensitive high-resolution ion microprobe–reverse geometry (SHRIMP-RG) analyses were carried out on the Nashoba Formation of the northwestern Nashoba terrane, eastern Massachusetts, to better constrain metamorphic conditions and structural timing in the terrane and provide an initial test for channel flow and ductile extrusion hypotheses. The Nashoba terrane is an early Paleozoic arc/back-arc complex on the trailing edge of Gondwana-derived Ganderia. High-grade metamorphism and partial melting occurred during the latest Silurian to Devonian due to accretion of the New England Avalon terrane. The partially migmatitic rocks of the NW-dipping Nashoba terrane are separated by shear/fault zones from lower-grade rocks of the Avalon terrane to the southeast and the Merrimack belt to the northwest. The Nashoba Formation is characterized by NW-dipping isoclinal folds, overprinted by NW-side-down asymmetric folds, shear zones, and faults. The isoclinal folds formed during a ca. 430–410 Ma period of initial partial melting and extensive plutonism in the Nashoba terrane. Crustal thickening occurred between ca. 419 Ma and ca. 367 Ma. High-grade metamorphic conditions in the Nashoba terrane prevailed between ca. 410 Ma and ca. 370 Ma, while plutonic rocks intruded mainly in the overlying Merrimack belt. Between ca. 370 Ma and ca. 360 Ma, NW-side-down folding and subsequent localized shearing occurred in the Nashoba Formation, while the terrane started cooling, based on new ages of fine-grained dikes and previous 40Ar/39Ar hornblende ages. These data are interpreted in a model of channel flow and ductile extrusion similar to the Himalaya–Tibetan Plateau system. 

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4. Crustal Deformation and Indications for Crustal Flow beneath Eastern Massachusetts from Coherent Harmonic Signals Along a Dense Seismic Profile (Invited)

   Link, F; Long, MD; Kuiper, YD; Arolkar, N (December 2024, American Geophysical Union)

   During the early Paleozoic the terranes of Ganderia and Avalonia both rifted from Gondwana. They accreted to North America in the middle Paleozoic. The late Silurian-Devonian Acadian orogeny, as a result of accretion of Avalonia, originated folding, high-grade metamorphism and northwest-dipping shear zones within the Nashoba-Putnam terrane, the trailing edge of Ganderia. In addition, partial melting produced plutonic rocks in and to the northwest of the Nashoba terrane. These characteristics have previously been interpreted as a result of channel flow and ductile extrusion towards the southeast. In this study, we apply geologically informed seismic imaging to test the hypothesis of the potential occurrence of crustal flow in the tectonic history of the Appalachian orogeny. Such crustal flow is suggested to produce significant seismic anisotropy due to the alignment of minerals within the weakened crust of the flow zone. This anisotropy would result in a characteristic set of effects to the seismic wavefield, such as the splitting of shear-waves, directionally dependent travel-times of seismic phases and directionally varying conversions at boundaries of anisotropic domains. Such effects yield a harmonic pattern that can be best observed in receiver function imaging. We systematically analyze the coherent harmonic patterns in receiver functions along a new dense (~5 km spacing) seismic profile, known as the GENESIS array, that complements existing stations across the Nashoba terrane in Eastern Massachusetts. We identify harmonic signals in the upper and mid-crust and within the lithospheric mantle, suggesting differing mid-crustal anisotropy between two lateral blocks, which correlate well with Avalonia and Ganderia. While we don’t directly identify the contact zone of the two terranes in our imaging, the changes of structural and anisotropic patterns may be consistent with a northwest-dipping suture zone, which is based on geologic observations. 

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5. U-Pb detrital zircon analysis of sedimentary rocks of the southeastern New England Avalon terrane in the US Appalachians: Evidence for a separate crustal block

   Kuiper, Yvette D; Murray, Daniel P; Crowley, James L (October 2021, Geological Society of America data repository)

   Kuiper, Yvette D; Murphy, J Brendan; Nance, R Damian; Strachan, Rob A; Thompson, Margaret D (Ed.)

   The Avalon terrane of southeastern New England is a composite terrane, in which various crustal blocks may have different origins and/or tectonic histories. The northern part (west and north of Boston, Massachusetts) correlates well with Avalonian terranes in Newfoundland, Nova Scotia and New Brunswick, Canada, based on rock types and ages, U–Pb detrital zircon signatures of metasedimentary rocks, and Sm–Nd isotope geochemistry data. In the south, fewer data exist, in part because of poorer rock exposure, and the origins and histories of the rocks are less well constrained. We conducted U–Pb laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) analysis on zircon from seven metasedimentary rock samples from multiple previously interpreted subterranes, in order to constrain their origins. Two samples of Neoproterozoic Plainfield Formation quartzite from the previously interpreted Hope Valley subterrane in the southwestern part of the southeastern New England Avalon terrane and two from the Neoproterozoic Blackstone Group quartzite from the adjacent Esmond-Dedham subterrane to the east have Tonian youngest detrital zircon age populations. One sample of Cambrian North Attleboro Formation quartzite of the Esmond-Dedham subterrane yielded an Ediacaran youngest detrital zircon age population. Detrital zircon populations of all five samples include abundant Mesoproterozoic zircon and smaller Paleoproterozoic and Archean populations, and are similar to those of the northern part of the southeastern New England Avalon terrane and the Avalonian terranes in Canada. These are interpreted as having a Baltican/Amazonian affinity based primarily on published U-Pb and Lu-Hf detrital zircon data. Based on U-Pb detrital zircon data, there is no significant difference between the Hope Valley and Esmond-Dedham subterranes. Detrital zircon of two samples of the Price Neck and Newport Neck formations of the Neoproterozoic Newport Group in southern Rhode Island is characterized by large ~647–643 and ~745–733 Ma age populations and minor zircon up to ~3.1 Ga. This signature is most consistent with a northwest African affinity. The Newport Group may thus represent a subterrane, terrane or other crustal block with a different origin and history than the southeastern New England Avalon terrane to the northwest. The boundary of this Newport Block may be restricted to the boundaries of the Newport Group, or it may extend as far north as Weymouth, MA, as far northwest as (but not including) the North Attleboro Formation quartzite and associated rocks in North Attleboro, MA, and as far west as Warwick, RI, where eastern exposures of the Blackstone Group quartzite exist. The Newport Block may have amalgamated with the Amazonian/Baltican part of the Avalon terrane prior to mid-Paleozoic amalgamation with Laurentia, or have arrived as a separate terrane after accretion of the Avalon terrane. Alternatively, it may have arrived during the formation of Pangea and been stranded after the breakup of Pangea, as has been proposed previously for rocks of the Georges Bank in offshore Massachusetts. If the latter is correct, then the boundary between the Newport Block and the southeastern New England Avalon terrane is the Pangean suture zone. 

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