Evolution of the Western Newfoundland Appalachian Orogen and its Foreland Basin

• Author / Creator

  White, Shawna E.

• The northern Appalachian Orogen was built during multiple orogenic episodes resulting from Cambrian through Devonian accretion of microcontinents and/or crustal ribbons to the eastern margin of Laurentia. Orogenic loading formed a foreland basin on the Laurentian craton. Orogen-derived sediments were deposited into the basin on top of a Cambrian through Early Ordovician passive margin, formed prior to the closure of the Iapetus Ocean. This study combines geophysical interpretation, geologic mapping, and geochronology to study the geologic history of deformed Paleozoic rocks of Laurentian affinity and the Appalachian foreland basin in the western Newfoundland Appalachians. Interpreted seismic reflection profiles and offshore aeromagnetic maps enable recognition of structure of the foreland basin, largely hidden beneath the Gulf of St. Lawrence. Isochron maps of the Middle Ordovician Goose Tickle Group demonstrate eastward thickening, suggesting loading by the Newfoundland segment of the orogen. A fast subsidence rate of ~ 0.17 km/Myr implies deposition in a pro-arc basin, formed on the eastward subducting Laurentian plate. Major fault-scarp units within the Goose Tickle Group indicate subsidence was controlled by large faults, in addition to distributed flexure resulting from orogenic loading and slab pull. Tectonic models indicate that subduction reversed polarity from east to west-dipping by the Late Ordovician, implying the Upper Ordovician Long Point Group was deposited in a retro-arc basin, on the upper plate. However, a fast subsidence rate of 0.17 km/Myr is consistent with deposition in a pro-arc basin. Isochron maps also demonstrate southward thickening of the Long Point Group, suggesting the basin was generated by loading from the Québec segment of the orogen. These observations indicate subduction polarity reversal was delayed in the Québec embayment, placing the Upper Ordovician basin in a hybrid retro/pro-arc setting. The hybrid setting, resulting from the irregular margin shape, is analogous to the modern plate boundary configuration along the northern Australian Plate. The record of Silurian activity is largely removed in the foreland basin at the latest Silurian Clam Bank unconformity which likely records uplift and erosion associated with slab break-off of the west-dipping subducting slab during the trailing end of (~ 440 – 420 Ma) Salinian orogenesis. Early Devonian (~ 420 – 400 Ma) Acadian orogenesis resulted in deposition of the latest Silurian to Early Devonian Clam Bank-Red Island Road succession. New mapping, aeromagnetic, and seismic interpretation at Parsons Pond allow recognition of structure in poorly exposed areas. The Humber Arm Allochthon contains a series of stacked and folded duplexes, typical of thrust belts. To the east, the Parsons Pond thrust transported basement, shelf and foreland-basin units westward above the allochthon. The Long Range thrust, farther east, shows less offset. Stratigraphic relationships indicate that basement-involved thrusts originated as normal faults, active during Neoproterozoic rifting and Taconian flexure. Devonian (Acadian) continental collision inverted the Parsons Pond and Long Range thrusts generating basement-cored fault-propagation folds, structurally analogous to uplifts of the Laramide Orogen in western USA. Similar deep-seated inversion structures may extend through the northern Appalachians, explaining enigmatic map patterns in New England. U-Pb ages of detrital zircon within the foreland basin are consistent with derivation from Laurentian sources. The Goose Tickle Group has abundant Mesoproterozoic and Archean grains. Paleoproterozoic ages are predominantly 1.85 Ga, indicating derivation from units within the Humber Arm Allochthon, which contain abundant zircons at 1.85 Ga. An abundance of Mesoproterozoic grains and conspicuous lack of 1.85 Ga zircons in the Long Point Group indicates a major provenance shift, whereby sediments were not derived from the Humber Arm Allochthon. Probability density plots of continental margin units in the Québec/New England segment of the orogen demonstrate a similar strong Mesoproterozoic and weak Paleoproterozoic signature, suggesting derivation of the Long Point Group from the Québec segment of the orogen. Similar provenance shifts in other foreland basins, formed at promontory to embayment transitions, may indicate a similar delay in subduction polarity reversal at the embayment. The absence of 2.0 – 2.2 Ga and 550 – 650 Ma Gondwanan zircons, and abundant 1.0 Ga grains within the Clam Bank-Red Island Road succession, is consistent with underthrusting of Ganderia and Avalonia during Salinian and Acadian orogenesis. Only Mesoproterozoic zircons were found in the Early Devonian Red Island Road Formation, consistent with derivation from Mesoproterozoic Grenville massifs uplifted during Devonian Acadian inversion.

• Subjects / Keywords

  • Newfoundland
  • Eastern Laurentia
  • Geology
  • Structural Geology
  • Detrital Zircon
  • Appalachian Orogen
  • Foreland Basin

• Graduation date

  Spring 2018

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R3416TF1Z

• License

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