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Advent of Continents: A New Hypothesis

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ABSTRACT

The straightforward but unexpected relationship presented here relates crustal thickness to magma type in the Izu-Ogasawara (Bonin) and Aleutian oceanic arcs. Volcanoes along the southern segment of the Izu-Ogasawara arc and the western Aleutian arc (west of Adak) are underlain by thin crust (10–20 km). In contrast those along the northern segment of the Izu-Ogasawara arc and eastern Aleutian arc are underlain by crust ~35 km thick. Interestingly, andesite magmas dominate eruptive products from the former volcanoes and mostly basaltic lavas erupt from the latter. According to the hypothesis presented here, rising mantle diapirs stall near the base of the oceanic crust at depths controlled by the thickness of the overlying crust. Where the crust is thin, melting occurs at relatively low pressures in the mantle wedge producing andesitic magmas. Where the crust is thick, melting pressures are higher and only basaltic magmas tend to be produced. The implications of this hypothesis are: (1) the rate of continental crust accumulation, which is andesitic in composition, would have been greatest soon after subduction initiated on Earth, when most crust was thin; and (2) most andesite magmas erupted on continental crust could be recycled from “primary” andesite originally produced in oceanic arcs.

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Related in: MedlinePlus

(a) Bathymetric features of the Izu-Ogasawara (Bonin) arcs. Old seafloor (135–180 Ma) of the western Pacific Plate subducts beneath the active Izu-Ogasawara arcs at the Izu-Ogasawara Trench. The location of the present-day volcanic front is shown by arrows, and is divided into the northern segment, the Torishima area and the southern segment, along which the wide-angle seismic profile21 is shown in Fig. 1b. Copyright (2007) The Geological Society of America. Reproduced with permission of GSA Publications. (b) Seismic velocity image along the volcanic front of the Izu-Ogasawara arcs obtained by seismic refraction tomography21. The crust beneath the arc volcanoes in the northern segment of the Izu-Ogasawara arcs is 32–35 km thick, which is twice as thick as the 16–21 km crust underlying the volcanoes in the southern segment. The crust beneath Torishima volcano has intermediate thickness between the northern and southern segments. Copyright (2007) The Geological Society of America. Reproduced with permission of GSA Publications. (c) The water depth and crustal thickness along the seismic profile, where densely deployed (~5 km spacing) ocean-bottom seismographs (OBSs) and a large air-gun array (~197 L) were used. The profile is along the volcanic front, but is just off the summits of the volcanoes. (d) Depth of water between arc-front volcanoes versus crustal thickness along the Izu-Ogasawara arcs. The northern segment is in shallower water and has thicker crust than the southern segment. Torishima is located and plots between them. The rectangle shows the range exhibited by the eastern Aleutian arc, east of Adak80.
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f1: (a) Bathymetric features of the Izu-Ogasawara (Bonin) arcs. Old seafloor (135–180 Ma) of the western Pacific Plate subducts beneath the active Izu-Ogasawara arcs at the Izu-Ogasawara Trench. The location of the present-day volcanic front is shown by arrows, and is divided into the northern segment, the Torishima area and the southern segment, along which the wide-angle seismic profile21 is shown in Fig. 1b. Copyright (2007) The Geological Society of America. Reproduced with permission of GSA Publications. (b) Seismic velocity image along the volcanic front of the Izu-Ogasawara arcs obtained by seismic refraction tomography21. The crust beneath the arc volcanoes in the northern segment of the Izu-Ogasawara arcs is 32–35 km thick, which is twice as thick as the 16–21 km crust underlying the volcanoes in the southern segment. The crust beneath Torishima volcano has intermediate thickness between the northern and southern segments. Copyright (2007) The Geological Society of America. Reproduced with permission of GSA Publications. (c) The water depth and crustal thickness along the seismic profile, where densely deployed (~5 km spacing) ocean-bottom seismographs (OBSs) and a large air-gun array (~197 L) were used. The profile is along the volcanic front, but is just off the summits of the volcanoes. (d) Depth of water between arc-front volcanoes versus crustal thickness along the Izu-Ogasawara arcs. The northern segment is in shallower water and has thicker crust than the southern segment. Torishima is located and plots between them. The rectangle shows the range exhibited by the eastern Aleutian arc, east of Adak80.

Mentions: Izu-Ogasawara-Mariana (IOM) system of arcs (also known as the Izu-Bonin-Mariana (IBM) arc system) is a typical oceanic arc produced by subduction of the Pacific Plate beneath the Philippine Sea Plate. The Izu-Ogasawara arcs extend from ~35°N near Tokyo in the north to ~24°N, the northern end of the Mariana arc in the south (Fig. 1a). Volume-weighted histograms of rock types from the northern segment of the Izu-Ogasawara arcs (30.5°–35°N), north of Torishima, are bimodal10, with basalt and dacite-rhyolite predominating. Rhyolitic submarine calderas15 are located between basalt-dominant island volcanoes, and it is suggested that rhyolite volcanoes have no mantle roots beneath the crust16. Instead it is believed that the rhyolite magmas are produced by melting of the existing underlying Oligocene middle crust triggered by basalt dikes (reflector X17) that travel laterally from the neighboring basalt volcanoes16. Thus, rhyolitic caldera-forming volcanoes likely resulted from crustal melting, and are omitted from discussion in this paper. The spacing of basalt-dominant island volcanoes in the northern segment is greater (~100 km) than the spacing of mostly submarine volcanoes in the southern segment (~50 km) (Fig. 1b). Supplementary Table S1 provides the sources of the analytical data from Quaternary volcanoes in the Izu-Ogasawara arcs (locations shown in Fig. 1b) used in this study. The table also shows data from Oligocene volcanic rocks collected at Omachi seamount in the Ogasawara arc18 and Guam, Rota and Saipan in the Mariana forearc1819. Geochemical data from the entire IOM system of arcs from the Oligocene to the present, plus thin-section photos and videos taken from JAMSTEC submersibles, are available on the GANSEKI database20.


Advent of Continents: A New Hypothesis
(a) Bathymetric features of the Izu-Ogasawara (Bonin) arcs. Old seafloor (135–180 Ma) of the western Pacific Plate subducts beneath the active Izu-Ogasawara arcs at the Izu-Ogasawara Trench. The location of the present-day volcanic front is shown by arrows, and is divided into the northern segment, the Torishima area and the southern segment, along which the wide-angle seismic profile21 is shown in Fig. 1b. Copyright (2007) The Geological Society of America. Reproduced with permission of GSA Publications. (b) Seismic velocity image along the volcanic front of the Izu-Ogasawara arcs obtained by seismic refraction tomography21. The crust beneath the arc volcanoes in the northern segment of the Izu-Ogasawara arcs is 32–35 km thick, which is twice as thick as the 16–21 km crust underlying the volcanoes in the southern segment. The crust beneath Torishima volcano has intermediate thickness between the northern and southern segments. Copyright (2007) The Geological Society of America. Reproduced with permission of GSA Publications. (c) The water depth and crustal thickness along the seismic profile, where densely deployed (~5 km spacing) ocean-bottom seismographs (OBSs) and a large air-gun array (~197 L) were used. The profile is along the volcanic front, but is just off the summits of the volcanoes. (d) Depth of water between arc-front volcanoes versus crustal thickness along the Izu-Ogasawara arcs. The northern segment is in shallower water and has thicker crust than the southern segment. Torishima is located and plots between them. The rectangle shows the range exhibited by the eastern Aleutian arc, east of Adak80.
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Related In: Results  -  Collection

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f1: (a) Bathymetric features of the Izu-Ogasawara (Bonin) arcs. Old seafloor (135–180 Ma) of the western Pacific Plate subducts beneath the active Izu-Ogasawara arcs at the Izu-Ogasawara Trench. The location of the present-day volcanic front is shown by arrows, and is divided into the northern segment, the Torishima area and the southern segment, along which the wide-angle seismic profile21 is shown in Fig. 1b. Copyright (2007) The Geological Society of America. Reproduced with permission of GSA Publications. (b) Seismic velocity image along the volcanic front of the Izu-Ogasawara arcs obtained by seismic refraction tomography21. The crust beneath the arc volcanoes in the northern segment of the Izu-Ogasawara arcs is 32–35 km thick, which is twice as thick as the 16–21 km crust underlying the volcanoes in the southern segment. The crust beneath Torishima volcano has intermediate thickness between the northern and southern segments. Copyright (2007) The Geological Society of America. Reproduced with permission of GSA Publications. (c) The water depth and crustal thickness along the seismic profile, where densely deployed (~5 km spacing) ocean-bottom seismographs (OBSs) and a large air-gun array (~197 L) were used. The profile is along the volcanic front, but is just off the summits of the volcanoes. (d) Depth of water between arc-front volcanoes versus crustal thickness along the Izu-Ogasawara arcs. The northern segment is in shallower water and has thicker crust than the southern segment. Torishima is located and plots between them. The rectangle shows the range exhibited by the eastern Aleutian arc, east of Adak80.
Mentions: Izu-Ogasawara-Mariana (IOM) system of arcs (also known as the Izu-Bonin-Mariana (IBM) arc system) is a typical oceanic arc produced by subduction of the Pacific Plate beneath the Philippine Sea Plate. The Izu-Ogasawara arcs extend from ~35°N near Tokyo in the north to ~24°N, the northern end of the Mariana arc in the south (Fig. 1a). Volume-weighted histograms of rock types from the northern segment of the Izu-Ogasawara arcs (30.5°–35°N), north of Torishima, are bimodal10, with basalt and dacite-rhyolite predominating. Rhyolitic submarine calderas15 are located between basalt-dominant island volcanoes, and it is suggested that rhyolite volcanoes have no mantle roots beneath the crust16. Instead it is believed that the rhyolite magmas are produced by melting of the existing underlying Oligocene middle crust triggered by basalt dikes (reflector X17) that travel laterally from the neighboring basalt volcanoes16. Thus, rhyolitic caldera-forming volcanoes likely resulted from crustal melting, and are omitted from discussion in this paper. The spacing of basalt-dominant island volcanoes in the northern segment is greater (~100 km) than the spacing of mostly submarine volcanoes in the southern segment (~50 km) (Fig. 1b). Supplementary Table S1 provides the sources of the analytical data from Quaternary volcanoes in the Izu-Ogasawara arcs (locations shown in Fig. 1b) used in this study. The table also shows data from Oligocene volcanic rocks collected at Omachi seamount in the Ogasawara arc18 and Guam, Rota and Saipan in the Mariana forearc1819. Geochemical data from the entire IOM system of arcs from the Oligocene to the present, plus thin-section photos and videos taken from JAMSTEC submersibles, are available on the GANSEKI database20.

View Article: PubMed Central - PubMed

ABSTRACT

The straightforward but unexpected relationship presented here relates crustal thickness to magma type in the Izu-Ogasawara (Bonin) and Aleutian oceanic arcs. Volcanoes along the southern segment of the Izu-Ogasawara arc and the western Aleutian arc (west of Adak) are underlain by thin crust (10–20 km). In contrast those along the northern segment of the Izu-Ogasawara arc and eastern Aleutian arc are underlain by crust ~35 km thick. Interestingly, andesite magmas dominate eruptive products from the former volcanoes and mostly basaltic lavas erupt from the latter. According to the hypothesis presented here, rising mantle diapirs stall near the base of the oceanic crust at depths controlled by the thickness of the overlying crust. Where the crust is thin, melting occurs at relatively low pressures in the mantle wedge producing andesitic magmas. Where the crust is thick, melting pressures are higher and only basaltic magmas tend to be produced. The implications of this hypothesis are: (1) the rate of continental crust accumulation, which is andesitic in composition, would have been greatest soon after subduction initiated on Earth, when most crust was thin; and (2) most andesite magmas erupted on continental crust could be recycled from “primary” andesite originally produced in oceanic arcs.

No MeSH data available.


Related in: MedlinePlus