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Yuri Amelin - One of the best experts on this subject based on the ideXlab platform.
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reevaluation of the piermont frontenac Allochthon in the upper connecticut valley restoration of a coherent boundary mountains bronson hill stratigraphic sequence
Geological Society of America Bulletin, 2013Co-Authors: Douglas W Rankin, Robert D Tucker, Yuri AmelinAbstract:The regional extent and mode and time of emplacement of the Piermont-Frontenac Allochthon in the Boundary Mountains–Bronson Hill anticlinorium of the Upper Connecticut Valley, New Hampshire–Vermont, are controversial. Moench and coworkers beginning in the 1980s proposed that much of the autochthonous pre–Middle Ordovician section of the anticlinorium was a large Allochthon of Silurian to Early Devonian rocks correlated to those near Rangeley, Maine. This ∼200-km-long Allochthon was postulated to have been transported westward in the latest Silurian to Early Devonian as a soft-sediment gravity slide on a hypothesized Foster Hill fault. New mapping and U-Pb geochronology do not support this interpretation. The undisputed Rangeley sequence in the Bean Brook slice is different from the disputed sequence in the proposed larger Piermont-Frontenac Allochthon, and field evidence for the Foster Hill fault is lacking. At the type locality on Foster Hill, the postulated “fault” is a stratigraphic contact within the Ordovician Ammonoosuc Volcanics. The proposed Foster Hill fault would place the Piermont-Frontenac Allochthon over the inverted limb of the Cornish(?) nappe, which includes the Emsian Littleton Formation, thus limiting the alleged submarine slide to post-Emsian time. Mafic dikes of the 419 Ma Comerford Intrusive Complex intrude previously folded strata attributed to the larger Piermont-Frontenac Allochthon as well as the autochthonous Albee Formation and Ammonoosuc Volcanics. The Lost Nation pluton intruded and produced hornfels in previously deformed Albee strata. Zircons from an apophysis of the pluton in the hornfels have a thermal ionization mass spectrometry 207 Pb/ 206 Pb age of 444.1 ± 2.1 Ma. Tonalite near Bath, New Hampshire, has a zircon sensitive high-resolution ion microprobe 206 Pb/ 238 U age of 492.5 ± 7.8 Ma. The tonalite intrudes the Albee Formation, formerly interpreted as the Silurian Perry Mountain Formation of the proposed Allochthon. Collectively, these features indicate that the large Piermont-Frontenac Allochthon gravity slide of Silurian-Devonian strata, as previously proposed, cannot exist. Allochthonous rocks are restricted to a 25 km 2 klippe, the Bean Brook slice, emplaced by hard-rock thrusting in the post-Emsian Devonian. The Albee Formation, the oldest unit in the study area, is older than the Late Cambrian tonalite at Bath. The correlation and apparent continuity along strike to the northeast of the Albee Formation with the Dead River Formation suggest that the Albee Formation, like the Dead River Formation, is of Ganderian affinity and that the Bronson Hill magmatic arc in the Upper Connecticut Valley was built on Ganderian crust. The Dead River Formation is unconformably overlain by Middle and Upper Ordovician volcanic units; the unconformity is attributed to the pre-Arenig Penobscottian orogeny. Some of the pre-Silurian deformation in the Upper Connecticut Valley may be Penobscottian rather than Taconian. New stratigraphic units defined herein include the pelitic Scarritt Member of the Albee Formation, the Ordovician Washburn Brook Formation consisting of synsedimentary breccia and coticule, chert, and ironstone, and the Devonian–Silurian Sawyer Mountain Formation, probably correlative with the Frontenac Formation. The Partridge Formation is partially coeval with the Ammonoosuc Volcanics.
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Reevaluation of the Piermont-Frontenac Allochthon in the Upper Connecticut Valley: Restoration of a coherent Boundary Mountains-Bronson Hill stratigraphic sequence
Geological Society of America Bulletin, 2012Co-Authors: Douglas W Rankin, Robert D Tucker, Yuri AmelinAbstract:The regional extent and mode and time of emplacement of the Piermont-Frontenac Allochthon in the Boundary Mountains–Bronson Hill anticlinorium of the Upper Connecticut Valley, New Hampshire–Vermont, are controversial. Moench and coworkers beginning in the 1980s proposed that much of the autochthonous pre–Middle Ordovician section of the anticlinorium was a large Allochthon of Silurian to Early Devonian rocks correlated to those near Rangeley, Maine. This ∼200-km-long Allochthon was postulated to have been transported westward in the latest Silurian to Early Devonian as a soft-sediment gravity slide on a hypothesized Foster Hill fault. New mapping and U-Pb geochronology do not support this interpretation. The undisputed Rangeley sequence in the Bean Brook slice is different from the disputed sequence in the proposed larger Piermont-Frontenac Allochthon, and field evidence for the Foster Hill fault is lacking. At the type locality on Foster Hill, the postulated “fault” is a stratigraphic contact within the Ordovician Ammonoosuc Volcanics. The proposed Foster Hill fault would place the Piermont-Frontenac Allochthon over the inverted limb of the Cornish(?) nappe, which includes the Emsian Littleton Formation, thus limiting the alleged submarine slide to post-Emsian time. Mafic dikes of the 419 Ma Comerford Intrusive Complex intrude previously folded strata attributed to the larger Piermont-Frontenac Allochthon as well as the autochthonous Albee Formation and Ammonoosuc Volcanics. The Lost Nation pluton intruded and produced hornfels in previously deformed Albee strata. Zircons from an apophysis of the pluton in the hornfels have a thermal ionization mass spectrometry 207 Pb/ 206 Pb age of 444.1 ± 2.1 Ma. Tonalite near Bath, New Hampshire, has a zircon sensitive high-resolution ion microprobe 206 Pb/ 238 U age of 492.5 ± 7.8 Ma. The tonalite intrudes the Albee Formation, formerly interpreted as the Silurian Perry Mountain Formation of the proposed Allochthon. Collectively, these features indicate that the large Piermont-Frontenac Allochthon gravity slide of Silurian-Devonian strata, as previously proposed, cannot exist. Allochthonous rocks are restricted to a 25 km 2 klippe, the Bean Brook slice, emplaced by hard-rock thrusting in the post-Emsian Devonian. The Albee Formation, the oldest unit in the study area, is older than the Late Cambrian tonalite at Bath. The correlation and apparent continuity along strike to the northeast of the Albee Formation with the Dead River Formation suggest that the Albee Formation, like the Dead River Formation, is of Ganderian affinity and that the Bronson Hill magmatic arc in the Upper Connecticut Valley was built on Ganderian crust. The Dead River Formation is unconformably overlain by Middle and Upper Ordovician volcanic units; the unconformity is attributed to the pre-Arenig Penobscottian orogeny. Some of the pre-Silurian deformation in the Upper Connecticut Valley may be Penobscottian rather than Taconian. New stratigraphic units defined herein include the pelitic Scarritt Member of the Albee Formation, the Ordovician Washburn Brook Formation consisting of synsedimentary breccia and coticule, chert, and ironstone, and the Devonian–Silurian Sawyer Mountain Formation, probably correlative with the Frontenac Formation. The Partridge Formation is partially coeval with the Ammonoosuc Volcanics.
Douglas W Rankin - One of the best experts on this subject based on the ideXlab platform.
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reevaluation of the piermont frontenac Allochthon in the upper connecticut valley restoration of a coherent boundary mountains bronson hill stratigraphic sequence
Geological Society of America Bulletin, 2013Co-Authors: Douglas W Rankin, Robert D Tucker, Yuri AmelinAbstract:The regional extent and mode and time of emplacement of the Piermont-Frontenac Allochthon in the Boundary Mountains–Bronson Hill anticlinorium of the Upper Connecticut Valley, New Hampshire–Vermont, are controversial. Moench and coworkers beginning in the 1980s proposed that much of the autochthonous pre–Middle Ordovician section of the anticlinorium was a large Allochthon of Silurian to Early Devonian rocks correlated to those near Rangeley, Maine. This ∼200-km-long Allochthon was postulated to have been transported westward in the latest Silurian to Early Devonian as a soft-sediment gravity slide on a hypothesized Foster Hill fault. New mapping and U-Pb geochronology do not support this interpretation. The undisputed Rangeley sequence in the Bean Brook slice is different from the disputed sequence in the proposed larger Piermont-Frontenac Allochthon, and field evidence for the Foster Hill fault is lacking. At the type locality on Foster Hill, the postulated “fault” is a stratigraphic contact within the Ordovician Ammonoosuc Volcanics. The proposed Foster Hill fault would place the Piermont-Frontenac Allochthon over the inverted limb of the Cornish(?) nappe, which includes the Emsian Littleton Formation, thus limiting the alleged submarine slide to post-Emsian time. Mafic dikes of the 419 Ma Comerford Intrusive Complex intrude previously folded strata attributed to the larger Piermont-Frontenac Allochthon as well as the autochthonous Albee Formation and Ammonoosuc Volcanics. The Lost Nation pluton intruded and produced hornfels in previously deformed Albee strata. Zircons from an apophysis of the pluton in the hornfels have a thermal ionization mass spectrometry 207 Pb/ 206 Pb age of 444.1 ± 2.1 Ma. Tonalite near Bath, New Hampshire, has a zircon sensitive high-resolution ion microprobe 206 Pb/ 238 U age of 492.5 ± 7.8 Ma. The tonalite intrudes the Albee Formation, formerly interpreted as the Silurian Perry Mountain Formation of the proposed Allochthon. Collectively, these features indicate that the large Piermont-Frontenac Allochthon gravity slide of Silurian-Devonian strata, as previously proposed, cannot exist. Allochthonous rocks are restricted to a 25 km 2 klippe, the Bean Brook slice, emplaced by hard-rock thrusting in the post-Emsian Devonian. The Albee Formation, the oldest unit in the study area, is older than the Late Cambrian tonalite at Bath. The correlation and apparent continuity along strike to the northeast of the Albee Formation with the Dead River Formation suggest that the Albee Formation, like the Dead River Formation, is of Ganderian affinity and that the Bronson Hill magmatic arc in the Upper Connecticut Valley was built on Ganderian crust. The Dead River Formation is unconformably overlain by Middle and Upper Ordovician volcanic units; the unconformity is attributed to the pre-Arenig Penobscottian orogeny. Some of the pre-Silurian deformation in the Upper Connecticut Valley may be Penobscottian rather than Taconian. New stratigraphic units defined herein include the pelitic Scarritt Member of the Albee Formation, the Ordovician Washburn Brook Formation consisting of synsedimentary breccia and coticule, chert, and ironstone, and the Devonian–Silurian Sawyer Mountain Formation, probably correlative with the Frontenac Formation. The Partridge Formation is partially coeval with the Ammonoosuc Volcanics.
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Reevaluation of the Piermont-Frontenac Allochthon in the Upper Connecticut Valley: Restoration of a coherent Boundary Mountains-Bronson Hill stratigraphic sequence
Geological Society of America Bulletin, 2012Co-Authors: Douglas W Rankin, Robert D Tucker, Yuri AmelinAbstract:The regional extent and mode and time of emplacement of the Piermont-Frontenac Allochthon in the Boundary Mountains–Bronson Hill anticlinorium of the Upper Connecticut Valley, New Hampshire–Vermont, are controversial. Moench and coworkers beginning in the 1980s proposed that much of the autochthonous pre–Middle Ordovician section of the anticlinorium was a large Allochthon of Silurian to Early Devonian rocks correlated to those near Rangeley, Maine. This ∼200-km-long Allochthon was postulated to have been transported westward in the latest Silurian to Early Devonian as a soft-sediment gravity slide on a hypothesized Foster Hill fault. New mapping and U-Pb geochronology do not support this interpretation. The undisputed Rangeley sequence in the Bean Brook slice is different from the disputed sequence in the proposed larger Piermont-Frontenac Allochthon, and field evidence for the Foster Hill fault is lacking. At the type locality on Foster Hill, the postulated “fault” is a stratigraphic contact within the Ordovician Ammonoosuc Volcanics. The proposed Foster Hill fault would place the Piermont-Frontenac Allochthon over the inverted limb of the Cornish(?) nappe, which includes the Emsian Littleton Formation, thus limiting the alleged submarine slide to post-Emsian time. Mafic dikes of the 419 Ma Comerford Intrusive Complex intrude previously folded strata attributed to the larger Piermont-Frontenac Allochthon as well as the autochthonous Albee Formation and Ammonoosuc Volcanics. The Lost Nation pluton intruded and produced hornfels in previously deformed Albee strata. Zircons from an apophysis of the pluton in the hornfels have a thermal ionization mass spectrometry 207 Pb/ 206 Pb age of 444.1 ± 2.1 Ma. Tonalite near Bath, New Hampshire, has a zircon sensitive high-resolution ion microprobe 206 Pb/ 238 U age of 492.5 ± 7.8 Ma. The tonalite intrudes the Albee Formation, formerly interpreted as the Silurian Perry Mountain Formation of the proposed Allochthon. Collectively, these features indicate that the large Piermont-Frontenac Allochthon gravity slide of Silurian-Devonian strata, as previously proposed, cannot exist. Allochthonous rocks are restricted to a 25 km 2 klippe, the Bean Brook slice, emplaced by hard-rock thrusting in the post-Emsian Devonian. The Albee Formation, the oldest unit in the study area, is older than the Late Cambrian tonalite at Bath. The correlation and apparent continuity along strike to the northeast of the Albee Formation with the Dead River Formation suggest that the Albee Formation, like the Dead River Formation, is of Ganderian affinity and that the Bronson Hill magmatic arc in the Upper Connecticut Valley was built on Ganderian crust. The Dead River Formation is unconformably overlain by Middle and Upper Ordovician volcanic units; the unconformity is attributed to the pre-Arenig Penobscottian orogeny. Some of the pre-Silurian deformation in the Upper Connecticut Valley may be Penobscottian rather than Taconian. New stratigraphic units defined herein include the pelitic Scarritt Member of the Albee Formation, the Ordovician Washburn Brook Formation consisting of synsedimentary breccia and coticule, chert, and ironstone, and the Devonian–Silurian Sawyer Mountain Formation, probably correlative with the Frontenac Formation. The Partridge Formation is partially coeval with the Ammonoosuc Volcanics.
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The Piermont Allochthon revisited and redefined at its type locality: Discussion
Geological Society of America Bulletin, 2006Co-Authors: Douglas W Rankin, Robert D TuckerAbstract:[Timms (2004)][1] has made a significant contribution to understanding a controversial structural feature of central New England: the Piermont Allochthon (see [Hadley, 1942][2], [1950][3]; [Rumble, 1969][4]; [Moench, 1990][5], [1992][6], [1996][7], [1999][8]; [Billings, 1992][9]; [Rankin, 1996][10
Robert D Tucker - One of the best experts on this subject based on the ideXlab platform.
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reevaluation of the piermont frontenac Allochthon in the upper connecticut valley restoration of a coherent boundary mountains bronson hill stratigraphic sequence
Geological Society of America Bulletin, 2013Co-Authors: Douglas W Rankin, Robert D Tucker, Yuri AmelinAbstract:The regional extent and mode and time of emplacement of the Piermont-Frontenac Allochthon in the Boundary Mountains–Bronson Hill anticlinorium of the Upper Connecticut Valley, New Hampshire–Vermont, are controversial. Moench and coworkers beginning in the 1980s proposed that much of the autochthonous pre–Middle Ordovician section of the anticlinorium was a large Allochthon of Silurian to Early Devonian rocks correlated to those near Rangeley, Maine. This ∼200-km-long Allochthon was postulated to have been transported westward in the latest Silurian to Early Devonian as a soft-sediment gravity slide on a hypothesized Foster Hill fault. New mapping and U-Pb geochronology do not support this interpretation. The undisputed Rangeley sequence in the Bean Brook slice is different from the disputed sequence in the proposed larger Piermont-Frontenac Allochthon, and field evidence for the Foster Hill fault is lacking. At the type locality on Foster Hill, the postulated “fault” is a stratigraphic contact within the Ordovician Ammonoosuc Volcanics. The proposed Foster Hill fault would place the Piermont-Frontenac Allochthon over the inverted limb of the Cornish(?) nappe, which includes the Emsian Littleton Formation, thus limiting the alleged submarine slide to post-Emsian time. Mafic dikes of the 419 Ma Comerford Intrusive Complex intrude previously folded strata attributed to the larger Piermont-Frontenac Allochthon as well as the autochthonous Albee Formation and Ammonoosuc Volcanics. The Lost Nation pluton intruded and produced hornfels in previously deformed Albee strata. Zircons from an apophysis of the pluton in the hornfels have a thermal ionization mass spectrometry 207 Pb/ 206 Pb age of 444.1 ± 2.1 Ma. Tonalite near Bath, New Hampshire, has a zircon sensitive high-resolution ion microprobe 206 Pb/ 238 U age of 492.5 ± 7.8 Ma. The tonalite intrudes the Albee Formation, formerly interpreted as the Silurian Perry Mountain Formation of the proposed Allochthon. Collectively, these features indicate that the large Piermont-Frontenac Allochthon gravity slide of Silurian-Devonian strata, as previously proposed, cannot exist. Allochthonous rocks are restricted to a 25 km 2 klippe, the Bean Brook slice, emplaced by hard-rock thrusting in the post-Emsian Devonian. The Albee Formation, the oldest unit in the study area, is older than the Late Cambrian tonalite at Bath. The correlation and apparent continuity along strike to the northeast of the Albee Formation with the Dead River Formation suggest that the Albee Formation, like the Dead River Formation, is of Ganderian affinity and that the Bronson Hill magmatic arc in the Upper Connecticut Valley was built on Ganderian crust. The Dead River Formation is unconformably overlain by Middle and Upper Ordovician volcanic units; the unconformity is attributed to the pre-Arenig Penobscottian orogeny. Some of the pre-Silurian deformation in the Upper Connecticut Valley may be Penobscottian rather than Taconian. New stratigraphic units defined herein include the pelitic Scarritt Member of the Albee Formation, the Ordovician Washburn Brook Formation consisting of synsedimentary breccia and coticule, chert, and ironstone, and the Devonian–Silurian Sawyer Mountain Formation, probably correlative with the Frontenac Formation. The Partridge Formation is partially coeval with the Ammonoosuc Volcanics.
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Reevaluation of the Piermont-Frontenac Allochthon in the Upper Connecticut Valley: Restoration of a coherent Boundary Mountains-Bronson Hill stratigraphic sequence
Geological Society of America Bulletin, 2012Co-Authors: Douglas W Rankin, Robert D Tucker, Yuri AmelinAbstract:The regional extent and mode and time of emplacement of the Piermont-Frontenac Allochthon in the Boundary Mountains–Bronson Hill anticlinorium of the Upper Connecticut Valley, New Hampshire–Vermont, are controversial. Moench and coworkers beginning in the 1980s proposed that much of the autochthonous pre–Middle Ordovician section of the anticlinorium was a large Allochthon of Silurian to Early Devonian rocks correlated to those near Rangeley, Maine. This ∼200-km-long Allochthon was postulated to have been transported westward in the latest Silurian to Early Devonian as a soft-sediment gravity slide on a hypothesized Foster Hill fault. New mapping and U-Pb geochronology do not support this interpretation. The undisputed Rangeley sequence in the Bean Brook slice is different from the disputed sequence in the proposed larger Piermont-Frontenac Allochthon, and field evidence for the Foster Hill fault is lacking. At the type locality on Foster Hill, the postulated “fault” is a stratigraphic contact within the Ordovician Ammonoosuc Volcanics. The proposed Foster Hill fault would place the Piermont-Frontenac Allochthon over the inverted limb of the Cornish(?) nappe, which includes the Emsian Littleton Formation, thus limiting the alleged submarine slide to post-Emsian time. Mafic dikes of the 419 Ma Comerford Intrusive Complex intrude previously folded strata attributed to the larger Piermont-Frontenac Allochthon as well as the autochthonous Albee Formation and Ammonoosuc Volcanics. The Lost Nation pluton intruded and produced hornfels in previously deformed Albee strata. Zircons from an apophysis of the pluton in the hornfels have a thermal ionization mass spectrometry 207 Pb/ 206 Pb age of 444.1 ± 2.1 Ma. Tonalite near Bath, New Hampshire, has a zircon sensitive high-resolution ion microprobe 206 Pb/ 238 U age of 492.5 ± 7.8 Ma. The tonalite intrudes the Albee Formation, formerly interpreted as the Silurian Perry Mountain Formation of the proposed Allochthon. Collectively, these features indicate that the large Piermont-Frontenac Allochthon gravity slide of Silurian-Devonian strata, as previously proposed, cannot exist. Allochthonous rocks are restricted to a 25 km 2 klippe, the Bean Brook slice, emplaced by hard-rock thrusting in the post-Emsian Devonian. The Albee Formation, the oldest unit in the study area, is older than the Late Cambrian tonalite at Bath. The correlation and apparent continuity along strike to the northeast of the Albee Formation with the Dead River Formation suggest that the Albee Formation, like the Dead River Formation, is of Ganderian affinity and that the Bronson Hill magmatic arc in the Upper Connecticut Valley was built on Ganderian crust. The Dead River Formation is unconformably overlain by Middle and Upper Ordovician volcanic units; the unconformity is attributed to the pre-Arenig Penobscottian orogeny. Some of the pre-Silurian deformation in the Upper Connecticut Valley may be Penobscottian rather than Taconian. New stratigraphic units defined herein include the pelitic Scarritt Member of the Albee Formation, the Ordovician Washburn Brook Formation consisting of synsedimentary breccia and coticule, chert, and ironstone, and the Devonian–Silurian Sawyer Mountain Formation, probably correlative with the Frontenac Formation. The Partridge Formation is partially coeval with the Ammonoosuc Volcanics.
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The Piermont Allochthon revisited and redefined at its type locality: Discussion
Geological Society of America Bulletin, 2006Co-Authors: Douglas W Rankin, Robert D TuckerAbstract:[Timms (2004)][1] has made a significant contribution to understanding a controversial structural feature of central New England: the Piermont Allochthon (see [Hadley, 1942][2], [1950][3]; [Rumble, 1969][4]; [Moench, 1990][5], [1992][6], [1996][7], [1999][8]; [Billings, 1992][9]; [Rankin, 1996][10
Stanley C. Finney - One of the best experts on this subject based on the ideXlab platform.
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recognition of fine scale imbricate thrusts in lower paleozoic orogenic belts an example from the roberts mountains Allochthon nevada
Geology, 1999Co-Authors: Paula J. Noble, Stanley C. FinneyAbstract:Dating of lower Paleozoic cherts is now possible through advancements in radiolarian biostratigraphy, permitting greater refinement of structural and stratigraphic models than has previously been possible. A detailed study of the type section of the Ordovician Vinini Formation, Vinini Creek, Roberts Mountains Allochthon, Nevada, has yielded anomalously young Devonian ages. These ages were unanticipated and indicate that structural imbrication exists in a classic and previously well dated section of basinal strata. Previous ages of cherts were inferred from their stratigraphic position relative to graptolite-bearing shales. New mapping and biostratigraphy reveal considerable structural interleaving of Ordovician and Devonian strata within the Allochthon, and many of the cherts appear to have structural bases. The Roberts Mountains Allochthon, considered to be Allochthonous but not far traveled, displays a highly complex structural history, including out of sequence thrusting and/or overprinting by later contractional deformation having a similar transport direction. This deformation was previously unrecognized because of the lack of overlap sequences in critical areas and the paucity of internal age control. Because the Roberts Mountains Allochthon is not considered to be exotic, but rather has undergone only a modest amount of transport, this study shows that convergent margins with high degrees of structural imbrication do not require large amounts of transport. A biostratigraphic approach that examines all lithologies, coupled with detailed observations of field relations, is required for the construction of detailed and accurate structure sections in early Paleozoic orogenic belts.
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Recognition of fine-scale imbricate thrusts in lower Paleozoic orogenic belts—An example from the Roberts Mountains Allochthon, Nevada
Geology, 1999Co-Authors: Paula J. Noble, Stanley C. FinneyAbstract:Dating of lower Paleozoic cherts is now possible through advancements in radiolarian biostratigraphy, permitting greater refinement of structural and stratigraphic models than has previously been possible. A detailed study of the type section of the Ordovician Vinini Formation, Vinini Creek, Roberts Mountains Allochthon, Nevada, has yielded anomalously young Devonian ages. These ages were unanticipated and indicate that structural imbrication exists in a classic and previously well dated section of basinal strata. Previous ages of cherts were inferred from their stratigraphic position relative to graptolite-bearing shales. New mapping and biostratigraphy reveal considerable structural interleaving of Ordovician and Devonian strata within the Allochthon, and many of the cherts appear to have structural bases. The Roberts Mountains Allochthon, considered to be Allochthonous but not far traveled, displays a highly complex structural history, including out of sequence thrusting and/or overprinting by later contractional deformation having a similar transport direction. This deformation was previously unrecognized because of the lack of overlap sequences in critical areas and the paucity of internal age control. Because the Roberts Mountains Allochthon is not considered to be exotic, but rather has undergone only a modest amount of transport, this study shows that convergent margins with high degrees of structural imbrication do not require large amounts of transport. A biostratigraphic approach that examines all lithologies, coupled with detailed observations of field relations, is required for the construction of detailed and accurate structure sections in early Paleozoic orogenic belts.
S. Henry Williams - One of the best experts on this subject based on the ideXlab platform.
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Tectonics and foreland basin development at the leading edge of the Humber Arm Allochthon, western Newfoundland, Canadian Appalachians
American Journal of Science, 2020Co-Authors: Ryan A. Lacombe, John W. F. Waldron, S. Henry WilliamsAbstract:The thrust front of the Northern Appalachians involves Ediacaran rift-related structures of the Laurentian margin that were re-worked in Taconic (Ordovician), Salinic (mainly Silurian) and Acadian (mainly Devonian) deformation events. Much of the thrust front is concealed under the Gulf of St. Lawrence, but Port au Port Peninsula, in western Newfoundland provides a cross-section through the Laurentian passive margin, overlying foreland basins, and the Humber Arm Allochthon. A large area of foreland basin strata is present in areas formerly mapped as Humber Arm Allochthon. Abrupt thickness changes and an abundance of fault-scarp-derived limestone conglomerate indicate that deposition was influenced by block faulting and differential subsidence in a basin undergoing active flexural extension. A coarsening upward trend in the Goose Tickle Group represents a transition from a distal to a proximal sediment source as the Humber Arm Allochthon was emplaced westwards. Goose Tickle Group separates two packages of Allochthonous rocks. The West Bay Thrust Sheet, the lower package, represents the leading edge of the Taconic Allochthon. Its timing of emplacement is constrained by Darriwilian 3 graptolites both above and below the thrust sheet. Its present-day configuration results from a combination of processes including thrusting, extension by gravity spreading, gravity gliding, and subsequent erosion of material that was deposited in the Goose Tickle Group. The structurally higher Lourdes Thrust Sheet is an out-of-sequence structure associated with Acadian (Devonian) orogenesis. High-angle faults show a protracted history of movement that includes early Taconic flexural extension, Acadian inversion, and later Carboniferous or Mesozoic strike-slip motion. The leading edge of the Humber Arm Allochthon was influenced by both thin-skinned and thick-skinned tectonics throughout the development of the Appalachian orogen.
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Mélanges and disrupted rocks at the leading edge of the Humber Arm Allochthon, W. Newfoundland Appalachians: Deformation under high fluid pressure
Gondwana Research, 2019Co-Authors: Ryan A. Lacombe, John W. F. Waldron, S. Henry Williams, Nicholas B. HarrisAbstract:Abstract The Humber Arm Allochthon was structurally emplaced onto the Laurentian margin in western Newfoundland during Taconian (Ordovician) and Acadian (Devonian) deformation. On Port au Port Peninsula, disrupted Allochthonous rocks previously mapped as melange and scaly shale include three mappable, variably disrupted, stratigraphic units; in addition, mixed rocks constitute melange with much smaller area than previously mapped. At outcrop scale, a qualitative assessment of disruption distinguishes broken, but coherent stratigraphy from a more disrupted and mixed melange unit. Within coherent regions, three generations of folds are probably related to Taconian, Acadian and Carboniferous deformation events. More disrupted regions show an average of ~24% blocks to 76% matrix with block sizes 0.5–158 cm. A new sampling technique allowed recovery of oriented melange samples for thin-section. Multiple orientations of extensional fractures suggest approximately coaxial extension. Abundant carbonate and less common bitumen-filled veins suggest that high fluid pressure played a role in the emplacement of the Allochthon. High fluid pressure was probably also responsible for dewatering structures, sandstone dykes and partially brecciated carbonate beds. Map relationships, outcrop and thin-section scale observations lead to a reinterpreted structural history for western Newfoundland in which an early, Taconian, West Bay Thrust Sheet was rapidly emplaced onto the Laurentian margin. During emplacement, debris flows initially contributed igneous blocks to the Allochthon, but the majority of fragmentation took place in an environment of horizontal tectonic extension promoted by high fluid-pressure that encouraged brittle fracture. The West Bay thrust sheet was subsequently overridden by the out-of-sequence Lourdes Thrust. Parts of the Allochthon were probably re-imbricated in later events, but because of previous disruption, an organized imbricated thrust belt was not developed. At the tip of an advancing thrust wedge, a clear distinction between tectonic and surficial processes of melange formation may not be possible.
