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The Taconic thrust belt in New England is the type locality of the Ordovician Taconic orogeny, the result of partial subduction of the rifted Laurentian margin beneath the Gondwanan-derived Moretown terrane (MT) and the Shelburne Falls arc. Evidence for Ordovician deformation and metamorphism is only preserved in rocks of the Laurentian margin; Taconic deformation and metamorphism in the MT and suture zone were overprinted by Devonian Acadian tectonism. New thermochronological data from the Taconic thrust belt indicate that many faults were active during the Silurian and Devonian, well after the Taconic orogeny. Crust under accreted terranes in New England is much thinner (~30 km) than below the Grenville belt along the Laurentian margin (~45 km), and Li et al. (2018) noted a particularly abrupt change in crustal thickness in southwestern New England near the suture between Laurentia and the MT. New seismic evidence indicates that the abrupt offset in Moho depth in CT and MA occurs east of an anisotropic region (~25 km wide and ~15 km thick) that lies between the shallow Moho of the MT and the deep Moho of Laurentia. The Taconic and Acadian orogens are narrower in southern New England than they are to the north, suggesting greater crustal shortening, and high-grade metamorphic rocks exposed in southern New England indicate greater erosion of overlying crust. Hillenbrand et al. (2021) proposed that an Acadian plateau existed in southern New England from 380 to 330 Ma and that plateau collapse after 330 Ma led to the abrupt Moho offset. We suggest that an indenter in southern New England focused the Acadian collision between Laurentia and Avalonia leading to greater crustal shortening and uplift than elsewhere the Appalachians. The east-dipping suture zone and Neoproterozoic normal faults cutting the leading edge of Laurentia were reactivated as west-directed thrust faults. Further, the diffuse fault zone that displaced the MT and the leading edge of the Laurentian margin penetrated the crust and displaced the Moho beneath the MT creating a double Moho near the suture. The anisotropic zone between the double Moho region is likely composed of crustal and mantle rocks bounded by faults. It is unclear how far east rifted Grenville crust extends under the MT; it is possible that the MT is no longer above its original lithospheric mantle.
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Paleozoic evolution of crustal thickness and elevation in the northern Appalachian orogen, USA
The Acadian and Neoacadian orogenies are widely recognized, yet poorly understood, tectono-thermal events in the New England Appalachian Mountains (USA). We quantified two phases of Paleozoic crustal thickening using geochemical proxies. Acadian (425–400 Ma) crustal thickening to 40 km progressed from southeast to northwest. Neoacadian (400–380 Ma) crustal thickening was widely distributed and varied by 30 km (40–70 km) from north to south. Doubly thickened crust and paleoelevations of 5 km or more support the presence of an orogenic plateau at ca. 380–330 Ma in southern New England. Neoacadian crustal thicknesses show a strong correlation with metamorphic isograds, where higher metamorphic grade corresponds to greater paleo-crustal thickness. We suggest that the present metamorphic field gradient was exposed through erosion and orogenic collapse influenced by thermal, isostatic, and gravitational properties related to Neoacadian crustal thickness. Geobarometry in southern New England underestimates crustal thickness and exhumation, suggesting the crust was thinned by tectonic as well as erosional processes.
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- PAR ID:
- 10252120
- Date Published:
- Journal Name:
- Geology
- ISSN:
- 0091-7613
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract Gneiss domes are an integral element of many orogenic belts and commonly provide tectonic windows into deep crustal levels. Gneiss domes in the New England segment of the Appalachian orogen have been classically associated with diapirism and fold interference, but alternative models involving ductile flow have been proposed. We evaluate these models in the Gneiss Dome belt of western New England with U‐Th‐Pb monazite, xenotime, zircon, and titanite petrochronology and major and trace element thermobarometry. These data constrain distinct pressure–temperature–time (P‐T‐t) paths for each unit in the gneiss dome belt tectono‐stratigraphy. The structurally lowest units, Laurentia‐derived migmatitic gneisses of the Waterbury dome, document two stages of metamorphism (455–435 and 400–370 Ma) with peak Acadian metamorphic conditions of ~1.0–1.2 GPa at 750–780°C at 391 ± 7 to 386 ± 4 Ma. The next structurally higher unit, the Gondwana‐derived Taine Mountain Formation, records Taconic (peak conditions: 0.6 GPa, 600°C at 441 ± 4 Ma) and Acadian (peak: 0.8–1.0 GPa, 650°C at 377 ± 4 Ma) metamorphism. The overlying Collinsville Formation yielded a 473 ± 5 Ma crystallization age and evidence for metamorphic conditions of 650°C at 436 ± 4 Ma and 1.2–1.0 GPa, 750–775°C at 397 ± 4 to 385 ± 6 Ma. The structurally higher Sweetheart Mountain Member of the Collinsville Formation yielded only Acadian zircon, monazite, and xenotime dates and evidence for high‐pressure granulite facies metamorphism (1.8 GPa, 815°C) at circa 380–375 Ma. Cover rocks of the dome‐mantling The Straits Schist records peak conditions of ~1 GPa, 700°C at 386 ± 6 to 380 ± 4 Ma. Garnet breakdown to monazite and/or xenotime occurred in all units at circa 375–360 and 345–330 Ma. Peak Acadian metamorphic pressures increase systematically from the structurally lowest to highest units (from 1.0 to 1.8 GPa). This inverted metamorphic sequence is incompatible with the diapiric and fold interference models, which predict the highest pressures at the structurally lowest levels. Based upon P‐T‐t and structural data, we prefer a model involving, first, circa 380 Ma thrust stacking followed by syn‐collisional orogen parallel extension, ductile flow, and rise of the domes between 380 and 365 Ma. Garnet breakdown at circa 345–330 Ma is interpreted to reflect further exhumation during collapse of the Acadian orogenic plateau. These results highlight the power of integrating petrologic constraints with paired geochemical and geochronologic data from multiple chronometers to test structural and tectonic models and show that syn‐convergent orogen parallel ductile flow dramatically modified earlier accretion‐related structures in New England. Further, the Gneiss Dome belt documents gneiss dome development in a syn‐collisional, thick crust setting, providing an ancient example of middle to lower crustal processes that may be occurring today in the modern Himalaya and Pamir Range.more » « less
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Abstract Along‐strike variation of the Laurentian rifted margin and the Appalachian orogen has long been recognized in the geologic record. We investigated the manifestation of this along‐strike variation at depth by generating scattered wavefield migration profiles from four dense seismic arrays deployed across the Appalachian orogen at different latitudes. All profiles exhibit a similar crustal thickness decrease of 15–20 km from the Mesoproterozoic Grenville Province to the Paleozoic Appalachian accreted terranes, but the Moho architecture differs dramatically along strike. The profiles beneath the central and southern Appalachians show a smoothly varying Moho geometry; in contrast, there is an abrupt Moho depth offset beneath the New England Appalachians. This contrast in Moho geometry may result from variations in the Laurentian rifted margin architecture, changes in Taconic orogeny subduction polarity, and greater crustal shortening during the Acadian‐Neoacadian orogeny in southern New England and the Alleghanian orogeny in the central and southern Appalachians. A first‐order along‐strike transition in the behavior of Appalachian orogenic processes is located between the central and New England Appalachians.more » « less
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1130/G48705.1
