Seismic anisotropy beneath eastern North America, as expressed in shear wave splitting observations, has been attributed to plate motion‐parallel shear in the asthenosphere, resulting in fast axes ...aligned with the plate motion. However, deviations of fast axes from plate motion directions are observed near major tectonic boundaries of the Appalachians, indicating contributions from lithospheric anisotropy associated with past tectonic processes. In this study, we conduct anisotropic receiver function (RF) analysis using data from a dense seismic array traversing the New England Appalachians in Connecticut to examine anisotropic layers in the crust and upper mantle and correlate them with past tectonic processes as well as present‐day mantle flow. We use the harmonic decomposition method to separate directionally‐dependent variations of RFs and focus on features with the same harmonic signals observed across multiple stations. Within the crust, there are multiple features that may be correlated with stratification in the Hartford Basin, faults in the Taconic thrust belt, shear zones formed during Salinic/Acadian terrane accretion events, and orogen‐parallel crustal flow in the Acadian orogenic plateau. We apply a Bayesian inversion method to obtain quantitative constraints on the direction and strength of intra‐crustal anisotropy beneath the Hartford Basin. In the upper mantle, we identify a fossil shear zone possibly formed during oblique subduction of Rheic Ocean lithosphere. We also find evidence for a plate motion‐parallel flow zone in the asthenosphere that is likely disturbed by mantle upwelling near the southern margin of the Northern Appalachian Anomaly in the eastern part of the study area.
Plain Language Summary
Seismic waves with different propagation and oscillation directions can exhibit different velocities when going through a medium with some directional properties; this phenomenon is called seismic anisotropy. Seismic anisotropy observed beneath eastern North America is often attributed to present‐day flow in the upper mantle. The mantle flow causes shear waves oscillating in the direction of flow (e.g., in the direction of North America plate motion) to travel faster than those that travel in other directions. However, this pattern does not hold true for some regions in the Appalachian Mountains, suggesting that past tectonic events can result in long‐lived, “frozen‐in” anisotropy in the lithosphere, which modifies the predicted anisotropic behavior beneath these regions. In this study, we investigate sources of seismic anisotropy beneath southern New England using a method based on directionally dependent variations of P‐wave to S‐wave conversions at interfaces with contrasts in anisotropy. This method can separate signals caused by different anisotropic features and constrain the depth distribution of anisotropy. We identify various anisotropic layers in the crust and upper mantle and correlate them with Appalachian mountain‐building events and complex present‐day flow in the upper mantle.
Key Points
Anisotropic receiver function analysis reveals multiple anisotropic layers in the crust and upper mantle beneath southern New England
Anisotropic geometries at <10 km depth resolved by Bayesian inversion imply contributions from sediment layering and Paleozoic deformation
Array‐wide anisotropic interfaces in the upper mantle show plate motion parallel flow distorted by localized mantle upwelling
A detailed comparison is made between the mid-Paleozoic Norumbega fault system (NFS) in Maine, USA, and the San Andreas fault system (SAFS) of coastal California, USA, and their tectonic settings. ...The SAFS formed following subduction of an oceanic ridge-transform system, and the NFS is interpreted as having formed the same way. The parallel evaluation of mid-crustal processes associated with the NFS and surface processes associated with the SAFS gives a unique perspective on modern and inactive crustal-scale strike-slip fault systems.
The SAFS is separated from the Cascadia subduction zone by the Mendocino triple junction. Similarly, the mid-Paleozoic NFS is interpreted as having been separated from a convergent zone to the southwest by the interpreted Norumbega triple junction. The SAFS cuts a subduction complex, the Franciscan Complex, that formed when the margin was still convergent. No equivalent subduction complex associated with the NFS has been recognized, perhaps because it was eroded away or displaced, or because it never formed. The SAFS experienced periods of transpression and transtension, whereas the NFS shows transpressive structures only. Slab window magmatic rocks exist along the SAFS. Devonian to earliest Carboniferous plutonic rocks along the NFS may in part be slab window magmatic rocks, although the effects of the Acadian and prior orogenies in the Appalachians complicate their identification. Magmatic rocks along the SAFS are offset due to the high rate of displacement (20–40 mm/yr). Magmatic rocks along the NFS show little offset, consistent with the low rate of displacement (~3 mm/yr) in the model. Transverse structures are associated with both the SAFS and NFS.
Other Mendocino-type triple junctions may have existed in the past, but may be difficult to recognize. Where a past crustal-scale intracontinental strike-slip fault system terminates on an along-strike contemporaneous convergent system, such a fault-fault-trench triple junction may be considered.
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•Comparison between the mid-Paleozoic Norumbega fault system in Maine and San Andreas fault•Modern and ancient transform fault boundaries on land formed by oceanic ridge subduction•Formation and unique nature of fault-fault-trench triple junctions•Recognition of ancient fault-fault-trench triple junctions
The crust and upper mantle beneath the New England Appalachians exhibit a large offset of the Moho across the boundary between Laurentia and accreted terranes and several dipping discontinuities, ...which reflect Paleozoic or younger tectonic movements. We apply scattered wavefield migration to the SEISConn array deployed across northern Connecticut and obtain insights not previously available from receiver function studies. We resolve a doubled Moho at a previously imaged Moho offset, which may reflect westward thrusting of rifted Grenville crust. The migration image suggests laterally variable velocity contrasts across the Moho, perhaps reflecting mafic underplating during continental rifting. A west‐dipping feature in the lithospheric mantle is further constrained to have a slab‐like geometry, representing a relict slab subducted during an Appalachian orogenic event. Localized low seismic velocities in the upper mantle beneath the eastern portion of the array may indicate that the Northern Appalachian Anomaly extends relatively far to the south.
Plain Language Summary
Tectonic processes in the geologic past, such as the formation and breakup of supercontinents, modified the deep structures of the crust and upper mantle beneath eastern North America. In this study, we use a seismic imaging technique based on scattered wavefield back‐projection to investigate deep structures beneath southern New England. This imaging technique, which relies on seismic wave energy from distant earthquakes, is capable of resolving km‐scale structures when applied to data from closely spaced seismometers (∼10 km station spacing). We image an abrupt, step‐like change of the crustal thickness beneath southern New England; the details of this feature suggest a complicated tectonic history during the formation of the Appalachian Mountains. A west‐dipping interface in the upper mantle suggests the presence of a relict slab beneath southern New England, associated with a past subduction event. A region of low seismic velocity in the upper mantle beneath southeastern New England may reflect past impingement of a mantle plume or modern upwelling of asthenospheric mantle.
Key Points
We conducted 2‐D wavefield migration on a dense seismic array across northern Connecticut to investigate crust and upper mantle structures
We resolved a doubled Moho beneath the Laurentian margin as well as a relict slab and a low‐velocity anomaly in the upper mantle
Migration provides insights unattainable by receiver functions, including the Moho velocity contrast and the geometry of the relict slab
Structures associated with Ediacaran‐Ordovician alkaline magmatism and the timing of rare earth element (REE) mineralization in the Wet Mountains, CO, were analyzed using field, geophysical, and ...U‐Th‐Pb isotope methods to interpret their tectonic setting in the context of previously proposed rift models. The Wet Mountains are known for thorium and REE mineralization associated with failed rift‐related, Ediacaran‐Ordovician alkaline intrusions and veins. Structural field data indicate that alkaline dikes and mineralized veins are controlled by a system of northwest‐striking, high‐angle faults and tension fractures formed in a 040°‐directed extensional regime. Magnetic and surface expressions of Democrat Creek and McClure Mountain complexes show tectonic elongation toward ∼045°, consistent with NE‐directed extension. Magnetic data also suggest the existence of a fourth, previously unrecognized mafic‐ultramafic complex of inferred Cambrian age with a similar elongated orientation. Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA‐ICP‐MS) 208Pb/232Th analysis of low‐uranium zircon from carbonatite dikes and in situ 206Pb/238U LA‐ICP‐MS analysis of monazite in mineralized dikes yielded 465 ± 18 Ma and 489 ± 33 Ma ages, respectively. These ages are consistent with the expected age based on slightly older, cross‐cut syenite dikes and the hypothesized Ordovician end to failed rift‐related magmatism. The Ediacaran‐Ordovician age of alkaline magmatic rocks and the associated northeast‐directed extension direction are similar to those of the along‐strike, Ediacaran‐Cambrian Southern Oklahoma Aulacogen. Therefore, the failed rift system in the Wet Mountains is interpreted to be a northwestern continuation of the Southern Oklahoma Aulacogen with carbonatite magmatism and thorium/REE mineralization representing late intrusive phases.
Plain Language Summary
The Wet Mountains of south‐central Colorado contain elements and minerals important to modern electronics and green energy technology. These minerals formed along an ancient tectonic system where the Earth's crust pulled apart, which was associated with uprise of magma from greater depths. The orientation of the system was previously unknown. This study used field and geophysical data to analyze the faults and fractures that controlled the migration of magma and emplacement of associated mineralization in order to understand the direction in which the Earth's crust pulled apart. Results of this study reveal that mineralization occurred along northwest‐striking faults and fractures, and that large magma bodies were elongated to the northeast during emplacement. This information suggests that the crust pulled apart by northeast‐directed extension along a northwest‐trending system. Additionally, isotopic analyses of mineralized features indicate that mineralization occurred at the end of this tectonic event. The orientation and age of the system is similar to a comparable system in southern Oklahoma, suggesting that the two are related. Based on this information, it is possible that additional mineral deposits formed between southern Colorado and southern Oklahoma.
Key Points
Ediacaran‐Ordovician failed rift‐related magmatism in the Wet Mountains formed along a northwest‐trending extensional regime
The failed‐rift system is likely related to the northwest‐trending, Ediacaran‐Cambrian Southern Oklahoma Aulacogen
Rare earth element mineralization occurred near the end of failed rifting
Abstract
Orogenic gold deposits are comprised of complex quartz vein arrays that form as a result of fluid flow along transcrustal fault zones in active orogenic belts. Mineral precipitation in these ...deposits occurs under variable pressure conditions, but a mechanism explaining how the pressure regimes evolve through time has not previously been proposed. Here we show that extensional quartz veins at the Garrcon deposit in the Abitibi greenstone belt of Canada preserve petrographic characteristics suggesting that the three recognized paragenetic stages formed within different pressure regimes. The first stage involved the growth of interlocking quartz grains competing for space in fractures held open by hydrothermal fluids at supralithostatic pressures. Subsequent fluid flow at fluctuating pressure conditions caused recrystallization of the vein quartz and the precipitation of sulfide minerals through wall-rock sulfidation, with some of the sulfide minerals containing microscopic gold. These pressure fluctuations between supralithostatic to near-hydrostatic conditions resulted in the post-entrapment modification of the fluid inclusion inventory of the quartz. Late fluid flow occurred at near-hydrostatic conditions and resulted in the formation of fluid inclusions that have not been affected by post-entrapment modification as pressure conditions never returned to supralithostatic conditions. This late fluid flow is interpreted to have formed the texturally late, coarse native gold that occurs along quartz grain boundaries and in open spaces. The systematic evolution of the pressure regimes in orogenic gold deposits such as Garrcon can be explained by relative movement of fault-fracture meshes across the base of the upper crustal brittle-ductile transition zone. We conclude that early vein quartz in orogenic deposits is precipitated at near-lithostatic conditions whereas the paragenetically late gold is introduced at distinctly lower pressure.
Lu-Hf laser ablation – multi-collector – inductively coupled plasma – mass spectrometry (LA-MC-ICP-MS) analysis was conducted on ~1800 detrital zircon grains from successor basins of the Archean ...Abitibi and Pontiac subprovinces of Ontario and Quebec, Canada, and paired with previous U-Pb LA-MC-ICP-MS analyses of the same grains. Results are used to constrain the isotopic character of magmatic source domains of the zircon grains to establish the sedimentary provenance of the ~2690–2670 Ma successor basins, to provide constraints on terrane configurations and amalgamations at the time of basin formation, and to assess their significance for the record of crust-mantle growth in the region. The majority of results (95%) yield ɛHf values of + 1 to + 10 for ~2850–2675 Ma zircon, and clusters along compositions of the Archean depleted mantle (DM), which is based on projections of modern MORB compositions. Subordinate results, comprising ~2% of the data set, yielded values (ɛHf > +10) corresponding to extremely depleted mantle compositions, reflecting anomalously depleted sources in the ~2950–2670 Ma age range. The remaining 3% correspond to chondritic uniform reservoir (CHUR)-like to negative ɛHf values that reflect primitive sources and/or evolved magmas in zircon that crystallized in the ~3250–3050 Ma and ~2950–2670 Ma age ranges. While Neoarchean grains dominate the data set (~88%), approximately 12% are Mesoarchean. The Lu-Hf data collected on these zircon grains, when compared with published isotopic results, preserve signatures indicative of derivation from exotic crustal domains juxtaposed during ~2690–2670 Ma amalgamation of the southern Superior Province. Since depleted compositions are characteristic of Neoarchean and Mesoarchean zircon groups in the southern Superior Province, and sources include local and distal domains that were likely separated by many 100s of kilometers prior to amalgamation, it is inferred that a depleted upper mantle reservoir was not only well-established, but prevalent in the mantle below each of these areas during their construction. Based on the predominant Hf isotope signatures in the detrital zircon results and predicted isotopic trends produced by probable geodynamic mechanisms, crustal growth by direct differentiation from a depleted mantle reservoir is likely to have been moderated by subduction-accretion processes.
New geochemical data including Sm/Nd isotopic data show evidence for an early Paleozoic arc/back-arc complex in the Nashoba terrane of southeastern New England. The Nashoba terrane lies between rocks ...of Ganderian affinity to the northwest and Avalonian affinity to the southeast. It consists of early Paleozoic mafic to felsic metavolcanic and metasedimentary rocks that were intruded by intermediate to felsic plutons and metamorphosed to upper amphibolite facies conditions in the mid-Paleozoic. Major and trace element geochemical data indicate that the early Paleozoic igneous rocks contain a mix of arc, MORB, and alkaline signatures, and that the terrane formed as a primitive volcanic arc/back-arc complex built on thinned continental crust. Amphibolites have +4 to +7.5^sub εNd(500)^ values consistent with formation in a primitive volcanic arc with minimal crustal contamination. Intermediate and felsic gneisses have ^sub εNd(500)^ values between +1.2 and -0.75 indicating a mixture of juvenile arc magmas and an evolved (likely basement) source. Depleted mantle model ages of 1.2 to 1.6 Ga point to a Mesoproterozoic or older age for this source. Metasedimentary rocks yielded -6 to -8.3 εNd(500) values and 1.6 to 1.8 Ga model ages, indicating an isotopically evolved source (or sources) that included Paleoproterozoic or older material. The ^sub εNd(500)^ values and model ages of the intermediate and felsic and metasedimentary rocks indicate that the basement to the Nashoba terrane is Ganderian rather than Avalonian. The Nashoba terrane therefore represents a Ganderian arc/back-arc complex similar to the Cambrian Penobscot arc/back-arc seen in Maritime Canada and Newfoundland, and particularly in the Annidale and New River terranes of southern New Brunswick. This correlation has not previously been recognized in southeastern New England. The Ganderian affinity of the Nashoba terrane also extends Ganderia farther SE in New England than previously established and indicates that the Nashoba terrane did not originate as a separate oceanic arc/back-arc complex or microcontinent.
•The maximum depositional age of the rift-related Ghanzi Group is refined to 1085.5 ± 4.5 Ma.•Copper-bearing marine siliciclastic rocks of the Ghanzi Group were deposited after ∼1050 to ...1060 Ma.•Source regions contained both juvenile mantle and crustal reservoirs.•Compilation of magmatic rock U-Pb, Lu-Hf, and Sm-Nd data from the Kalahari Craton.•The southwestern margin of the Kalahari Craton was the primary source for the rift basin infill.
New igneous and detrital zircon laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS) U-Pb geochronology and Lu-Hf isotopic data are presented for the Mesoproterozoic Kgwebe Formation and the unconformably overlying Ghanzi Group in northwestern Botswana. The Makgabana Hills porphyritic rhyolite flow from the Ghanzi area yielded a U-Pb concordia age of 1085.5 ± 4.5 Ma and provides a new maximum depositional age for the unconformably overlying Ghanzi Group. Detrital zircon (n = 448) from the Ghanzi Group yielded a 207Pb/206Pb age distribution with a dominant (70 to 90%) Mesoproterozoic population (∼1450 to ∼1050 Ma), a smaller (5 to 20%) Paleoproterozoic (∼2200 to ∼1700 Ma) population, and a few (n = 4) older (∼3000 Ma to ∼2450 Ma) grains. A maximum depositional age constraint of ∼1060 to ∼1050 Ma was obtained for middle and upper Ghanzi Group based on the weighted-mean 207Pb/206Pb age of the youngest clusters of overlapping zircon ages for each sample.
Initial hafnium ratios (εHfi) and corresponding crustal residence model ages (TCDM) for the Paleoproterozoic zircon populations indicate either fractionation from a chondritic uniform reservoir (CHUR) or mixing between juvenile mantle and older crustal components. Mesoproterozoic zircon with εHfi values between −20 and +15 and TCDM model ages between 3000 and 1200 Ma suggest that the source terrane(s) contained magmatic rocks including both juvenile material and substantially reworked Paleoproterozoic and possibly Archean crust.
Comparison with a compilation of published U-Pb, Lu-Hf, and Sm-Nd data from the Kalahari Craton suggests that the predominant Mesoproterozoic zircon population was derived from the Namaqua Sector, Rehoboth Basement Inlier, Kwando Complex, and Choma-Kalomo Block; some zircon may have had distal sources in adjacent Rodinia landmasses. Both Archean cratonic components and juvenile ∼1200 to ∼1000 Ma magmatic rocks of the Natal Sector and the Maud and Mozambique belts on the eastern margin of the craton are unlikely sources for the detrital zircon based on isotopic composition. Sediment transported from the western margin of the Kalahari Craton entered the northwest Botswana rift and mixed with sediments from the Rehoboth Basement Inlier and Paleo- to Mesoproterozoic terranes that bound the northwest Botswana rift.
The partitioning of triclinic flow into domains of apparent monoclinic and apparent orthorhombic flow is described and discussed, using the Aiken River shear zone (ARSZ) as an example. The ARSZ is a ...1–1.5 km wide east–west trending, dextral, north-side-up, mylonite zone, within the northern part of the Superior Province in Manitoba. It displays a high along-strike stretch (∼10), which is most likely indicative of an escape-tectonic setting.
Although the central mylonite zone exhibits an apparent monoclinic fabric symmetry, the actual flow field was probably triclinic with a high simple shearing over pure shearing ratio, which resolves potential strain compatibility problems with neighbouring domains. The simple shearing-dominated zone is relatively narrow and has well-defined boundaries. An up to ∼20 km wide zone adjacent to the ARSZ shows an apparent orthorhombic fabric symmetry with shear zone boundary-parallel horizontal stretch and shear zone-orthogonal shortening. However, the actual flow may have been triclinic with a low simple shearing over pure shearing ratio. Either way, the pure shear component of the ARSZ is distributed over a much broader area than the simple shear component and has diffuse boundaries. This is consistent with simple shearing being a softening and pure shearing a hardening process.
► Partitioning of triclinic flow into domains of apparent monoclinic and apparent orthorhombic flow. ► Discussion of strain compatibility across shear zones with high shear zone boundary-parallel horizontal stretch. ► The pure shearing component of a shear zone is normally distributed over a much broader area than the simple shearing component. This is consistent with simple shearing being a softening and pure shearing a hardening process.
Southern New England exhibits diverse geologic features resulting from past tectonic events. These include Proterozoic and early Paleozoic Laurentian units in the west, several Gondwana‐derived ...terranes that accreted during the Paleozoic in the east, and the Mesozoic Hartford Basin in the central part of the region. The Seismic Experiment for Imaging Structure beneath Connecticut (SEISConn) project involved the deployment of a dense array of 15 broadband seismometers across northern Connecticut to investigate the architecture of lithospheric structures beneath this region and interpret how they were created and modified by past tectonic events in the context of surface geology. We carried out P‐to‐S receiver function analysis on SEISConn data, including both single‐station analysis and common conversion point (CCP) stacking. Our images show that the westernmost part of Connecticut has a much deeper Moho than central and eastern Connecticut. The lateral transition is a well‐defined, ∼15 km step‐like offset of the Moho over a ∼20 km horizontal distance. The Moho step appears near the surface boundary between the Laurentian margin and the Gondwana‐derived Moretown terrane. Possible models for its formation include Ordovician underthrusting of Laurentia and/or modification by younger tectonic events. Other prominent features include a strong positive velocity gradient (PVG) beneath the Hartford basin corresponding to the bottom of the sedimentary units, several west‐dipping PVGs in the crust and mantle lithosphere that may correspond to relict slabs or shear zones from past subduction episodes, and a negative velocity gradient (NVG) that may correspond to the base of the lithosphere.
Plain Language Summary
The eastern margin of North America has a complicated tectonic history. It has been shaped by past episodes of landmasses coming together to form a supercontinent, with later breakup of the supercontinent to form a new ocean basin. This supercontinent cycle involves fundamental plate tectonic processes including subduction, the accretion of geologic microcontinents, the formation of mountain ranges, and rifting during the breakup of continents. These processes have led to the complex geology of southern New England that is visible at the surface, and they have also likely modified the deep structures of the crust and upper mantle. In this study, we analyzed data from seismometers deployed across northern Connecticut to investigate underground interfaces separating layers with different properties. We measured seismic waves from distant earthquakes and looked for evidence of specific wave behavior at these interfaces. We found that the interface separating the crust and the mantle, known as the Moho, is not continuous beneath this region. The Moho is much deeper in the west of our study area than in the east, and the transition from thick to thin crust corresponds to a key geologic boundary. We also identified several other interfaces, providing information on past tectonic events.
Key Points
We conducted Ps receiver function analysis on a dense seismic array across northern Connecticut to investigate lithospheric structures
We observe a step‐like change in Moho depth near the boundary between Laurentia and the Gondwana‐derived Moretown terrane
Several seismic discontinuities within the lithosphere have implications for the complicated tectonic history of southern New England
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