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1. 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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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. Late Cretaceous arc flare up and sinistral intra-arc ductile deformation in the southern California batholith, 2022, Late Cretaceous arc flare up and sinistral intra-arc ductile deformation in the southern California batholith

   Schwartz, J.; Lackey, J.S.; Miranda, E.; Klepeis, K.; Mora-Klepeis, G.; Robles, F.; Bixler, J. (October 2022, Geological Society of America Abstracts with Programs)

   We present >90 new igneous and metamorphic zircon and titanite petrochronology ages from the eastern Transverse Ranges of the Southern California Batholith (SCB) to investigate magmatic and tectonic processes in the frontal arc during postulated initiation of Late Cretaceous shallow-slab subduction. Our data cover >4000 km2 in the eastern Transverse Ranges and include data from Mesozoic plutons in the Mt. Pinos, Alamo Mountain, San Gabriel Mountain blocks, and the Eastern Peninsular mylonite zone. Igneous zircon data reveal 4 discrete pulses of magmatism at 258-220 Ma, 160-142 Ma, 120-118 Ma, and 90-66 Ma. The latter pulse involved a widespread magmatic surge in the SCB and coincided with garnet-granulite to upper amphibolite-facies metamorphism and partial melting in the lower crust (Cucamonga terrane, eastern San Gabriel Mountains). In this region, metamorphic zircons in gneisses, migmatites and calc-silicates record high-temperature metamorphism from 91 to 74 Ma at 9–7 kbars and 800–730°C. The Late Cretaceous arc flare-up was temporally and spatially associated with the development of a regionally extensive oblique sinistral-reverse shear system that includes from north to south (present-day) the Tumamait shear zone (Mt. Pinos), the Alamo Mountain-Piru Creek shear zone, the Black Belt shear zone (Cucamonga terrane), and the Eastern Peninsular Ranges shear zone. Syn-kinematic, metamorphic titanite ages in the Tumamait shear zone range from 77–74 Ma at 720–700°C, titanites in the Black Belt mylonite zone give an age of 83 Ma, and those in the eastern Peninsular Ranges mylonite zone give ages of 89–86 Ma at 680–670°C. These data suggest a progressive northward younging of ductile shearing at amphibolite- to upper-amphibolite-facies conditions from 88 to 74 Ma, which overlaps with the timing of the Late Cretaceous arc flare-up event. Collectively, these data indicate that arc magmatism, high-temperature metamorphism, and intra-arc contraction were active in the SCB throughout the Late Cretaceous. These observations appear to contradict existing models for the termination of magmatism and refrigeration of the arc due to underthrusting of the conjugate Shatsky rise starting at ca. 88 Ma. We suggest that shallow-slab subduction likely postdates ca. 74 Ma when high-temperature metamorphism ceased in the SCB. 

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4. 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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5. The Cretaceous–Paleogene Ross Lake fault system: Relationships between magmatism and faulting during large-scale translation in the northern North American Cordillera of Washington and British Columbia

   https://doi.org/10.1130/2025.2565(08)  

   Miller, Robert B; Eddy, Michael P; Gordon, Stacia M; Dragovich, Joe D; Raviola, Frank P; Park, Yuem; Karmakar, Sourav; Bakshi, Sarnav; Hoinville, Alex; Miller, Rachel T (January 2026, Geological Society of America)

   ABSTRACT The ≥10-km-wide, long-lived Ross Lake fault system is part of a ≥500-km-long zone of Paleogene, NW-striking, high-angle faults in the northern North American Cordillera. This fault system forms the eastern boundary of the exhumed Late Cretaceous–Eocene North Cascades magmatic arc and is marked by metamorphic discontinuities and focused magmatism. The Ross Lake system consists of two major sets of faults. The eastern set of the Hozameen–North Creek fault forms the western boundary of the Jurassic–Cretaceous Methow sequence and in part separates the sequence from its likely metamorphic equivalents. Dextral slip along this segment of the fault system is only constrained to between ca. 87 Ma and 48 Ma. The northernmost strand of the western fault set, the Ross Lake fault (sensu stricto), is a vertical zone of horizontally lineated mylonite that separates upper-amphibolite-facies rocks of the Skagit Gneiss Complex from lower-grade metamorphic rocks to the east. Traced to the SE, the Ross Lake fault is interpreted to step westward ~5 km across a gently to moderately dipping zone containing rocks buried to >30 km depth. The fault continues along strike for ~80 km as the Gabriel Peak tectonic belt, which is a moderately to steeply NE-dipping mylonite zone that records dextral shear in the north and reverse shear farther south. Movement in this transpressional zone occurred from potentially as early as ca. 82 Ma to ca. 58–55 Ma. The tectonic belt is truncated in the south by the Foggy Dew fault zone, which contains higher-grade fault rocks than the on-strike Hozameen–North Creek fault. The Foggy Dew fault zone was active from at least 63–55 Ma and ended by 49 Ma. It records oblique dextral-normal slip (down-to-the-E) marking the regional transition in northern Washington and southern British Columbia from transpression to transtension, and extension farther to the east. The Foggy Dew fault zone is truncated to the SE by the ca. 49–48 Ma Cooper Mountain batholith. South of this intrusion, slip is potentially transferred across strike westward to the Eocene dextral Entiat fault within the North Cascades arc. Field relations and geochronologic data are interpreted to indicate that during the active duration of the fault system, numerous magmatic intrusions led to complex rheological transitions that influenced the location of shear and helped the Ross Lake system respond to changing tectonic regimes. Melt probably initially weakened the shear zones, and strain was subsequently focused along rheologically strong pluton margins after pluton crystallization. Offsets of contacts in the northern extent of the Ross Lake fault system suggest that cumulative dextral slip on the fault system was likely ≥125 km but <400 km. This dextral slip occurred at a time when paleomagnetic data indicate large northward translation of the Insular superterrane. The fault system disrupted and shuffled rocks within the eastern part of paleomagnetically defined Baja–British Columbia and is important for paleogeographic reconstructions of the North American Cordillera. 

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