The geology of a part of acadia and the nature of the Acadian orogeny across Central and eastern Maine

Rd. Tucker et al., The geology of a part of acadia and the nature of the Acadian orogeny across Central and eastern Maine, AM J SCI, 301(3), 2001, pp. 205-260
Citations number
Categorie Soggetti
Earth Sciences
Journal title
ISSN journal
0002-9599 → ACNP
Year of publication
205 - 260
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The zone of Acadian collision between the Medial New England and Composite Avalon terranes is well preserved in Maine. A transect from northwest (Rome ) to southeast (Camden) crosses the eastern part of Medial New England comp rising the Central Maine basin, Liberty-Orrington thrust sheet, and Frederi cton trough, and the western part of Composite Avalon, including the Graham Lake, Clarry Hill, and Clam Cove thrust sheets. U-Pb geochronology of even ts before, during, and after the Acadian orogeny helps elucidate the nature and distribution of tectonostratigraphic belts in this zone and the timing of some Acadian events in the Northern Appalachians. The Central Maine basin consists of sedimentary and volcanic rocks of Middl e Ordovician (similar to 470 to similar to 460 Ma) age overlain with probab le conformity by latest Ordovician(?) through earliest Devonian marine rift and flysch sedimentary rocks; these are intruded by weakly to undeformed p lutonic rocks of Early and Middle Devonian age (similar to 399-378 Ma). The Fredericton trough consists of Early Silurian gray pelite and sandstone to earliest Late Silurian calcareous turbidite, deformed and variably metamor phosed prior to the emplacement of Late Silurian (similar to 422 Ma) and Ea rly to Late Devonian (similar to 418 to similar to 368 Ma) plutons. The Lib erty-Orrington thrust sheet consists of Cambrian(?)-Ordovician (> similar t o 474 to similar to 469 Ma and younger) elastic sedimentary and volcanic ro cks intruded by highly deformed Late Silurian (similar to 424 to similar to 422 Ma) and Devonian (similar to 418 to similar to 389 Ma) plutons, possib ly metamorphosed in Late Silurian time (prior to similar to 417 Ma), and me tamorphosed to amphibolite facies in Early to Middle Devonian time (similar to 400 to similar to 381 Ma). The Graham Lake thrust sheet contains possib le Precambrian rocks, Cambrian sedimentary rocks with a volcanic unit dated at similar to 503 Ma, and Ordovician rocks with possible Caradocian Old Wo rld fossils, metamorphosed and deformed in Silurian time and intruded by mi ldly to undeformed Late Silurian (similar to 421 Ma) and Late Devonian (sim ilar to 371 to similar to 368 Ma) plutons. The Clarry Hill thrust sheet con sists of poorly studied, highly metamorphosed Cambrian(?) rocks. The Clam C ove thrust sheet contains highly deformed Precambrian limestone, shale, san dstone, and conglomerate, metamorphosed to epidote amphibolite facies and i ntruded by a mildly deformed pluton dated at similar to 421 Ma. Metamorphism, deformation, and voluminous intrusive igneous activity of Sil urian age are;common to both the most southeastern parts of Medial New Engl and and the thrust sheets of Composite Avalon. In contrast to Medial New En gland, the thrust sheets of Composite Avalon show only modest effects of De vonian deformation and metamorphism. Regional stratigraphic relations, pale ontologic findings, and U-Pb geochronology suggest that the Graham Lake, Cl arry Hill, and Clam Cove thrust sheets are far-traveled allochthons that we re widely separated from Medial New England in the Silurian. One hundred nine (109) new U-Pb analyses of zircon, monazite, and sphene fr om 25 samples of metamorphosed, stratified, and intrusive igneous rocks are used to decipher the history of events along the transect. In our view, many of the structures within Medial New England were formed d uring the closing of the back-are ocean during the waning stages of the Tac onian orogeny. These features include the precursors to the upright folds f ound in Silurian sedimentary rocks, the west-facing Liberty-Orrington thrus t sheet, and the Silurian plutons of Medial New England. Acadian tectonic features, including flysch sedimentation, igneous activity , deformation with nappe emplacement, and metamorphism record the progressi ve loading of Medial New England by a stack of thrust nappes emplaced in la test Silurian to Middle Devonian time. The Acadian orogeny is a prolonged event, lasting from earliest Late Siluri an to the Late Devonian, whose evolution involved: (1) convergence between Medial New England and Composite Avalon along an east-dipping subduction zo ne from earliest Late Silurian to Early Devonian time; (2) collision and co ncurrent delamination of lithospheric mantle beneath Medial New England in Early Devonian time resulting in deformation, high-grade metamorphism, and intrusive igneous activity in the most eastern part of Medial New England a nd the western parts of Composite Avalon; (3) Early to Middle Devonian nort hwest-migrating penetrative deformations of the Acadian Main stage, includi ng northwest-directed thrusting and recumbent folding followed by tightenin g of folds possibly produced in the waning stages of the Taconian orogeny a nd forming folded wave trains of isoclinal folds (similar to 419 to similar to 404 Ma), (4) asymmetrical folds produced by east-west shortening (simil ar to 399 to similar to 380 Ma); and (5) final west westward emplacement of Composite Avalon thrust sheets onto Medial New England in Late Devonian ti me (similar to 380 to similar to 371 Ma). The present boundary between rock s of Medial New England and Composite Avalon at the surface is the Sennebec Pond fault, a high-angle fault that cuts the Graham Lake and Clarry Hill t hrusts, and is intruded by the Mt. Waldo pluton (similar to 371), one of se veral Late Devonian plutons (similar to 371 to similar to 367 Ma) that mark the end of the Acadian orogeny in coastal Maine. The thrust sheets of Comp osite Avalon carried rocks of peri-Gondwanan affinity and with possible Old World fauna many kilometers to the west over Medial New England, thus pote ntially hiding the original suture at depth under the Gulf of Maine.