More Like this

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. 

   more » « less
2. 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. 

   more » « less
3. 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. 

   more » « less
4. Unraveling fault reactivations and their tectonic significance using integrated structural data and 40Ar/39Ar geochronology, examples from NW Vermont and SW New Zealand

   Klepeis, Keith; Webb, Laura E.; Merson, Matthew Q.; and Kim, Jonathan (April 2018, Abstracts with programs - Geological Society of America)

   Many large fault zones record multiple reactivations that can be difficult to resolve and interpret in the field. Here, we use examples from Vermont and New Zealand to illustrate how structural data combined with 40Ar/39Ar geochronology can be used to reconstruct fault reactivation histories and interpret their possible origins. In SW New Zealand, the Spey-Mica Burn fault zone parallels a transpressive boundary between the Pacific and Australian plates. Integrated structural and 40Ar/39Ar data obtained from pseudotachylyte, mylonite, and other fault rocks allow us to distinguish successive phases of faulting (i.e., reactivations) from cases where different styles of brittle and ductile deformation occurred simultaneously (or nearly so) in the fault zone. Apparent age spectra from multiple minerals show age gradients that reveal four reactivations spanning ~20 Ma. The style and timing of these events correlate well to times of increased convergence rate and collisions between oceanic ridge segments and a nearby trench. Fault zones in NW Vermont also record different styles of reactivation. The Hinesburg Thrust (HT), which juxtaposes Late Proterozoic-Early Cambrian rift clastic rocks against Ordovician carbonate rocks of the Champlain Valley belt, includes a ~30 m thick zone of mylonite that is cut by a cataclastic fault and deformed by folds. 40Ar/39Ar data suggest the mylonite formed during the Ordovician Taconic orogeny and later was folded into a series of domes and basins during the Late Silurian-Devonian Acadian orogeny. Farther west, the Champlain thrust fault (CT) juxtaposes Cambrian dolostones against Ordovician calcareous shales. Superposed faults within the foot wall of the CT show a progressive change in movement direction from W-directed thrusting, to NW-directed thrusting, to N-S slip, and NE-SW slip. These changing slip directions appear to reflect wholly Taconic motion along a north-dipping lateral ramp between Burlington and Shelburne where the CT cuts up section to the south. Acadian reactivation of the CT appears restricted to late folding similar to the HT. These examples highlight the utility of combining structural data with 40Ar/39Ar geochronology to unravel slip histories in continental fault zones and to distinguish among the different styles and origins of fault reactivation. 

   more » « less
5. AEROMAGNETIC ANOMALIES OF THE APPALACHIANS

   Kuiper, Y D; Long, M D (March 2024, Abstracts Geological Society of America)

   We provide an overview of aeromagnetic characteristics of the Appalachians. Whether terrane or domain boundaries are clearly defined by aeromagnetic lineaments depends on differences in rock types, and on whether the boundary is intruded by later plutons or overlain by sedimentary rocks. Sedimentary rocks and plutonic rocks of the Meguma terrane in Nova Scotia form magnetic lows, resulting in clearly defined boundaries with higher anomalies in adjacent terranes. The Caledonia terrane, an Avalonian terrane in New Brunswick, is defined by a strong high. The boundary between peri-Laurentian arcs and Carolinia in the southeastern U.S. is well defined, where the arcs form a magnetic high and parts of Carolinia form a low. Other terrane and domain boundaries are less well defined by aeromagnetic imagery. The Goochland terrane of Virginia shows a sharp boundary with the magnetic highs of the peri-Laurentian arcs to the northwest, but a diffuse boundary with the slightly lower magnetic rocks of Carolinia to the southeast. Peri-Laurentian arcs of the northern Appalachians form a low, while in the southern Appalachians they have mixed signatures. The boundary between Avalonia and Ganderia in Newfoundland is cut by plutonic rocks, and difficult to trace based on aeromagnetic images. Late Devonian to Permian basins, including the Maritimes basin of southeastern Canada and Narragansett basin of southeastern New England, and Mesozoic basins, including the Hartford basin of southwestern New England, form magnetic lows within and across terranes. They obliterate underlying rocks and structures. Strong magnetic contrasts also occur within terranes or domains. An example is the magnetite-bearing part of the Nashoba Formation in the Nashoba terrane of eastern Massachusetts, which is part of the trailing edge of Ganderia. Plutonic rocks may form magnetic highs, such as the German Bank Pluton southwest of Nova Scotia, or lows, such as the South Mountain Batholith of Nova Scotia. The New York – Alabama lineament separates a magnetic high to the northwest from a low to the southeast. NE-trending faults, including the Norumbega Fault of eastern New England, form strong lineaments. Two sets of east- and ESE-trending lineaments throughout the Appalachians represent late structures. 

   more » « less