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1. TESTING FOR CHANNEL FLOW ALONG THE NASHOBA-AVALON TERRANE BOUNDARY IN EASTERN MASSACHUSETTS

   https://doi.org/10.1130/abs/2025NE-407565  

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

   The southeastern New England Avalon Terrane (AT) accreted to the southeastern margin of the Nashoba Terrane (NT) at the onset of the Acadian orogeny (latest Silurian to Devonian). The NT represents the trailing edge of Ganderia. Rocks of the NT have previously been interpreted as having been extruded to the southeast over the AT as part of a channel flow zone (CFZ). Based on fold symmetries, it was inferred that only the top and center of this zone are located in the NT. Bedrock and structural mapping were carried out in the AT adjacent to the NT to test whether the bottom of the CFZ may be located in the AT. Data were collected from migmatitic biotite gneiss, mylonite, foliated quartzite, and gneiss. Structural data were divided into NE and SW domains. In the NE domain, foliations dip predominantly NW, and lineations plunge NE and SW. Migmatitic and gneissic rocks are absent in the SW domain, and orientations of mylonite zones and foliations in quartzite vary. Compared to the NE domain, rocks in the SW domain are strongly faulted and intruded by Ediacaran and late Silurian/Devonian granitic and gabbroic plutons. The presence of migmatite and consistency in structural orientations in the NE domain, and the general resemblance of structures to those in the NT make the NE domain a likely candidate to represent the bottom of the CFZ. U-Pb zircon data of the migmatitic biotite gneiss 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 high-grade metamorphism and partial melting occurred in the Ediacaran, i.e., not during the Acadian orogeny. Hence, the migmatitic biotite gneiss in the AT terrane does not represent the bottom of the CFZ. We believe that the Bloody Bluff Fault along the Nashoba-Avalon terrane boundary may have cut off the bottom of the CFZ. Our 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 NT and AT, consistent with geological observations. 

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2. 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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3. Pre-Mississippian Stratigraphic Architecture of the Porcupine Shear Zone, Yukon and Alaska, and Significance in the Evolution of Northern Laurentia

   https://doi.org/10.2113/2021/7866155  

   Faehnrich, Karol; McClelland, William C.; Colpron, Maurice; Nutt, Charlotte L.; Miller, Rebecca S.; Trembath, Matthew; Strauss, Justin V. (June 2021, Lithosphere)

   Abu-Alam, Tamer S. (Ed.)

   Abstract The origin and displacement history of terranes emplaced along the northern margin of North America remain contentious. One of these terranes is the North Slope subterrane of the Arctic Alaska-Chukotka microplate, which is separated from the northwestern margin of Laurentia (Yukon block) by the Porcupine Shear Zone of Alaska and Yukon. Here, we present new field observations, geological mapping, detrital zircon U-Pb geochronology, and sedimentary/igneous geochemistry to elucidate the stratigraphic architecture of deformed pre-Mississippian rocks exposed within the Porcupine Shear Zone, which we distinguish herein as the newly defined Ch’oodeenjìk succession. The oldest rocks in the Ch’oodeenjìk succession consist of siliciclastic strata of the Lahchah and Sunaghun formations (new names), which yield detrital zircon U-Pb age populations of ca. 1050-1250, 1350-1450, 1600-1650, and 2500-2800 Ma (n =800). This succession is overlain by chert-bearing dolostone and limestone of the Caribou Bar formation (new name) that contains vase-shaped microfossils and yields carbonate carbon (δ13Ccarb) and strontium (87Sr/86Sr) isotopic data that range from ca. -3‰ to +3‰ and 0.70636 to 0.70714, respectively. These data suggest that Lahchah, Sunaghun, and Caribou Bar formations are late Tonian in age. These Neoproterozoic rocks are intruded by Late Devonian (Frasnian-Famennian) felsic plutons and mafic dikes, one of which yielded a sensitive high-resolution ion microprobe-reverse geometry (SHRIMP-RG) U-Pb age of 380 ± 4 Ma. Neoproterozoic strata of the Ch’oodeenjìk succession are also unconformably overlain by Upper Devonian-Carboniferous (?) siliciclastic rocks of the Darcy Creek formation (new name), which yields detrital zircon populations of ca. 365–385, 420-470 and 625-835 Ma, in addition to Proterozoic age populations similar to the underlying Tonian strata. Together, these new stratigraphic, geochronological, geochemical, and micropaleontological data indicate that pre-Mississippian rocks exposed within the Porcupine Shear Zone most likely represent a peri-Laurentian crustal fragment that differs from the adjacent Yukon block and North Slope subterrane; thus, the Porcupine Shear Zone represents a fundamental tectonic boundary separating autochthonous Laurentia from various accreted peri-Laurentian crustal fragments. 

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4. A detrital zircon test of large-scale terrane displacement along the Arctic margin of North America

   https://doi.org/10.1130/G48336.1  

   Gibson, Timothy M.; Faehnrich, Karol; Busch, James F.; McClelland, William C.; Schmitz, Mark D.; Strauss, Justin V. (January 2021, Geology)

   null (Ed.)

   Abstract Detrital zircon U-Pb geochronology is one of the most common methods used to constrain the provenance of ancient sedimentary systems. Yet, its efficacy for precisely constraining paleogeographic reconstructions is often complicated by geological, analytical, and statistical uncertainties. To test the utility of this technique for reconstructing complex, margin-parallel terrane displacements, we compiled new and previously published U-Pb detrital zircon data (n = 7924; 70 samples) from Neoproterozoic–Cambrian marine sandstone-bearing units across the Porcupine shear zone of northern Yukon and Alaska, which separates the North Slope subterrane of Arctic Alaska from northwestern Laurentia (Yukon block). Contrasting tectonic models for the North Slope subterrane indicate it originated either near its current position as an autochthonous continuation of the Yukon block or from a position adjacent to the northeastern Laurentian margin prior to >1000 km of Paleozoic–Mesozoic translation. Our statistical results demonstrate that zircon U-Pb age distributions from the North Slope subterrane are consistently distinct from the Yukon block, thereby supporting a model of continent-scale strike-slip displacement along the Arctic margin of North America. Further examination of this dataset highlights important pitfalls associated with common methodological approaches using small sample sizes and reveals challenges in relying solely on detrital zircon age spectra for testing models of terranes displaced along the same continental margin from which they originated. Nevertheless, large-n detrital zircon datasets interpreted within a robust geologic framework can be effective for evaluating translation across complex tectonic boundaries. 

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5. Age and provenance of the Upper Cretaceous to Paleocene Valdez Group of the Chugach Terrane from the Richardson Highway and Northern Prince William Sound, Alaska.

   https://doi.org/10.1130/abs/2019CD-329673  

   Gross Almonte, N. (April 2019, Proceedings of the Keck Geology Consortium)

   The age and provenance of the southern Alaskan Campanian to Paleocene Valdez Group of the Chugach terrane and its relationship with the younger outboard Paleocene to Eocene Orca Group of the Prince William terrane is poorly understood but an important component of the Cordilleran collage (Plafker et al., 1994). The Valdez and Orca Groups are both part of the Chugach-Prince William terrane (CPW), which is a thick accretionary complex that extends 2200 km along the southern Alaskan margin (Fig. 1; Cowan, 2003). The deep-water turbidites of these terranes are quartzofeldspathic and volcanic-lithic sandstones and basaltic rocks (Dumoulin, 1987; Plafker et al., 1994). The CPW is intruded by near-trench plutons of the Sanak-Baranof belt (Davidson and Garver, 2017) and are believed to be related to a slab window that formed during subduction of Kula-Farallon or Kula- Resurrection spreading ridges (Marshak and Karig, 1977; Delong et al., 1978; Moore et al., 1983; Kusky et al., 1997a; Bradley et al., 2003; Haeussler et al., 2003). There are two hypotheses for the formation of the CPW along the North American Cordilleran margin: 1) either the CPW terrane formed in situ by subduction of the Resurrection plate (Haeussler et al. 2003); or 2) the rocks formed in the Pacific Northwest or California and were transported at least 2000 km along coastwise strike-slip fault systems (Cowan, 2003; Garver and Davidson, 2015). This study is an investigation into the age and provenance of the Valdez Group and its relationship with the Orca Group in the central Chugach Mountains using detrital zircon U-Pb dates. New detrital zircon U-Pb dates and their grain-age distributions from the Valdez and Orca Group turbidites are combined with dates from Kochelek et al. (2011), Amato et al. (2013), and Davidson and Garver (2017) and then synthesized to understand the difference in age between the units and provenance. New and existing U-Pb dates indicate maximum depositional ages (MDA) of the Valdez Group are concentrated in three groups: 84-78 Ma, 74-65 Ma, and 62-60 Ma. The youngest group of MDAs are age-correlative with the Orca Group but were collected from rocks in areas mapped as Valdez Group, indicating that either Orca Group rocks occur in the Valdez Group or the youngest Valdez Group rocks are stratigraphically equivalent to those of the oldest Orca Group. If the latter, the Valdez Group is not Campanian to Maastrichtian in age as has been traditionally viewed (Plafker et al., 1994) but is Upper Cretaceous to Paleocene and in part correlative to the lowest part of the Orca Group. 

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