Benjamin S. Murphy, J. Caine, P. Bedrosian, Jade Crosbie
Three-dimensional magnetotelluric (MT) imaging in central Colorado revealed a set of north-striking high-conductivity tracks at lower-crustal (50−20 km) depths, with conductive finger-like structures rising off these tracks into the middle crust (20−5 km depth). We interpret these features to represent saline aqueous fluids and partial melt that are products of active extensional tectonomagmatism. These conductors are distributed over a wider region than the narrow corridor along which Rio Grande rift structures are traditionally mapped at the surface, and they consequently demarcate regions of the lower crust where accommodation of bulk extensional strain has concentrated conductive phases. Our observations reveal limitations in existing models of Rio Grande rift activity and may reflect unrecognized spatiotemporal variations in rift system evolution globally.
{"title":"Geoelectric evidence for a wide spatial footprint of active extension in central Colorado","authors":"Benjamin S. Murphy, J. Caine, P. Bedrosian, Jade Crosbie","doi":"10.1130/g51517.1","DOIUrl":"https://doi.org/10.1130/g51517.1","url":null,"abstract":"Three-dimensional magnetotelluric (MT) imaging in central Colorado revealed a set of north-striking high-conductivity tracks at lower-crustal (50−20 km) depths, with conductive finger-like structures rising off these tracks into the middle crust (20−5 km depth). We interpret these features to represent saline aqueous fluids and partial melt that are products of active extensional tectonomagmatism. These conductors are distributed over a wider region than the narrow corridor along which Rio Grande rift structures are traditionally mapped at the surface, and they consequently demarcate regions of the lower crust where accommodation of bulk extensional strain has concentrated conductive phases. Our observations reveal limitations in existing models of Rio Grande rift activity and may reflect unrecognized spatiotemporal variations in rift system evolution globally.","PeriodicalId":503125,"journal":{"name":"Geology","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"2024-02-09","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139788674","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Coastal marsh loss is commonly attributed to changes in external forcings, such as an increase in sea-level rise rate or a reduction in sediment supply. Here we show that extensive marsh loss can be caused by internal mechanisms alone, and specifically by autogenic tidal choking. This occurs when the marsh fills in, increasing tidal dissipation by bed friction and eventually decreasing the tidal range in its landward section. The reduced tidal range decreases sediment import on the marsh platform and increases ponding, both of which lead to interior marsh loss even with modest sea-level rise rates. This process is predicted to occur in dissipative microtidal marshes, which are experiencing some of the fastest rates of marsh loss worldwide. Considering this mechanism is essential to understanding the relationship between marsh loss, sea-level rise, and sediment supply and to eventually predicting future marsh evolution.
{"title":"Tidal dissipation morphodynamic feedback triggers loss of microtidal marshes","authors":"S. Zapp, G. Mariotti","doi":"10.1130/g51798.1","DOIUrl":"https://doi.org/10.1130/g51798.1","url":null,"abstract":"Coastal marsh loss is commonly attributed to changes in external forcings, such as an increase in sea-level rise rate or a reduction in sediment supply. Here we show that extensive marsh loss can be caused by internal mechanisms alone, and specifically by autogenic tidal choking. This occurs when the marsh fills in, increasing tidal dissipation by bed friction and eventually decreasing the tidal range in its landward section. The reduced tidal range decreases sediment import on the marsh platform and increases ponding, both of which lead to interior marsh loss even with modest sea-level rise rates. This process is predicted to occur in dissipative microtidal marshes, which are experiencing some of the fastest rates of marsh loss worldwide. Considering this mechanism is essential to understanding the relationship between marsh loss, sea-level rise, and sediment supply and to eventually predicting future marsh evolution.","PeriodicalId":503125,"journal":{"name":"Geology","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"2024-02-09","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139849793","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
The Cascade arc (western North America) is the world’s youngest continental arc, and because the down-going Juan de Fuca plate is young, it is also the hottest end member among subduction zones worldwide. We present evidence that the arc initiated <5 m.y. after accretion of the Siletzia oceanic terrane terminated the earlier subduction system and caused the northern portion of the Farallon slab to break off. Cascade magmatism began ca. 46 Ma with a new trench outboard of Siletzia, a reconfiguration commonly attributed to a seaward jump of the subduction zone. However, the presence of young buoyant oceanic lithosphere that would have resisted being forced into the mantle and the very rapid reestablishment of arc magmatism are hard to reconcile with initiation of a new subduction zone by this process. We propose an alternative mechanism in which the arc was reestablished as the intact southern portion of Farallon slab migrated northward from California (United States), converting a transform margin to a convergent one. This model utilizes plate reconstructions, petrology, mantle tomography, and geochronology to explain how subduction was initiated in a setting where the slab was young and hot and why the earliest Cascade magmatism occurred toward the middle rather than an end of the arc.
{"title":"Initiation of the Cascade arc","authors":"J. Tepper, Kenneth P. Clark","doi":"10.1130/g51888.1","DOIUrl":"https://doi.org/10.1130/g51888.1","url":null,"abstract":"The Cascade arc (western North America) is the world’s youngest continental arc, and because the down-going Juan de Fuca plate is young, it is also the hottest end member among subduction zones worldwide. We present evidence that the arc initiated <5 m.y. after accretion of the Siletzia oceanic terrane terminated the earlier subduction system and caused the northern portion of the Farallon slab to break off. Cascade magmatism began ca. 46 Ma with a new trench outboard of Siletzia, a reconfiguration commonly attributed to a seaward jump of the subduction zone. However, the presence of young buoyant oceanic lithosphere that would have resisted being forced into the mantle and the very rapid reestablishment of arc magmatism are hard to reconcile with initiation of a new subduction zone by this process. We propose an alternative mechanism in which the arc was reestablished as the intact southern portion of Farallon slab migrated northward from California (United States), converting a transform margin to a convergent one. This model utilizes plate reconstructions, petrology, mantle tomography, and geochronology to explain how subduction was initiated in a setting where the slab was young and hot and why the earliest Cascade magmatism occurred toward the middle rather than an end of the arc.","PeriodicalId":503125,"journal":{"name":"Geology","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"2024-02-08","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139853225","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
The termination of Neoproterozoic “Snowball Earth” glaciations is marked globally by laterally extensive neritic cap carbonates directly overlying glacial diamictites. The formation of these unique deposits on deglaciation calls for anomalously high calcium carbonate saturation. A popular mechanism to account for the source of requisite ocean alkalinity is the shallow-ridge hypothesis, in which initial spreading ridges surrounding fragments of Rodinia, assumed to be dominated by volcanic margins, were formed at sea level. The shallow ridges are inferred to have promoted widespread deposition and alteration of glassy hyaloclastite—a source of alkalinity. We test this hypothesis by quantifying the prevalence of shallow ridges along Pangea’s passive continental margins, and by assessing Neoproterozoic reconstructions of continents. We find that the most frequently occurring depth range for incipient mid-ocean ridges is 2.1 ± 0.4 km. Ridges with initial elevations of approximately sea level are rare and have anomalous crustal thicknesses >14 km that only occur proximal to large igneous provinces (LIPs). Hyaloclastite is uncommon on mid-ocean ridges as it is generally restricted to water depths of <200 m for tholeiitic basalts, instead forming mostly on intraplate seamounts. Additionally, ocean drilling recently found hyaloclastite to be insignificant along the outer Vøring Plateau (offshore Norway)—an exemplar of a volcanic margin. Reconstructions of Rodinia and associated LIPs demonstrate that volcanic margins potentially hosting minor hyaloclastites were scarce during the late Neoproterozoic. We conclude that the shallow-ridge hypothesis fails to explain the formation of cap carbonates and suggest that other mechanisms such as enhanced continental weathering may be largely responsible.
{"title":"Submarine volcanism along shallow ridges did not drive Cryogenian cap carbonate formation","authors":"A. Dutkiewicz, R. D. Müller","doi":"10.1130/g51884.1","DOIUrl":"https://doi.org/10.1130/g51884.1","url":null,"abstract":"The termination of Neoproterozoic “Snowball Earth” glaciations is marked globally by laterally extensive neritic cap carbonates directly overlying glacial diamictites. The formation of these unique deposits on deglaciation calls for anomalously high calcium carbonate saturation. A popular mechanism to account for the source of requisite ocean alkalinity is the shallow-ridge hypothesis, in which initial spreading ridges surrounding fragments of Rodinia, assumed to be dominated by volcanic margins, were formed at sea level. The shallow ridges are inferred to have promoted widespread deposition and alteration of glassy hyaloclastite—a source of alkalinity. We test this hypothesis by quantifying the prevalence of shallow ridges along Pangea’s passive continental margins, and by assessing Neoproterozoic reconstructions of continents. We find that the most frequently occurring depth range for incipient mid-ocean ridges is 2.1 ± 0.4 km. Ridges with initial elevations of approximately sea level are rare and have anomalous crustal thicknesses >14 km that only occur proximal to large igneous provinces (LIPs). Hyaloclastite is uncommon on mid-ocean ridges as it is generally restricted to water depths of <200 m for tholeiitic basalts, instead forming mostly on intraplate seamounts. Additionally, ocean drilling recently found hyaloclastite to be insignificant along the outer Vøring Plateau (offshore Norway)—an exemplar of a volcanic margin. Reconstructions of Rodinia and associated LIPs demonstrate that volcanic margins potentially hosting minor hyaloclastites were scarce during the late Neoproterozoic. We conclude that the shallow-ridge hypothesis fails to explain the formation of cap carbonates and suggest that other mechanisms such as enhanced continental weathering may be largely responsible.","PeriodicalId":503125,"journal":{"name":"Geology","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"2024-02-08","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139854113","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Zaili Tao, Jiyuan Yin, Christopher J. Spencer, Min Sun, W. Xiao, Andrew C. Kerr, Tao Wang, Pengpeng Huangfu, Yunchuan Zeng, Wen Chen
Subduction polarity reversal usually involves the break off or tearing of the downgoing plate (DP) along the continent-ocean transition zone, in order to initiate subduction of the overriding plate (OP) with opposite polarity. We propose that subduction polarity reversal can also be caused by DP-OP coupling and can account for the early Paleozoic geological relationships in the Western Kunlun orogenic belt in the northwestern Tibetan Plateau. Our synthesis of elemental and isotopic data reveals transient (∼2 m.y.) changes in the sources of early Paleozoic arc magmatism in the southern Kunlun terrane. The early-stage (ca. 530−487 Ma) magmatic rocks display relatively high εNd(t) (+0.3 to +8.7), εHf(t) (−3.6 to +16.0), and intra-oceanic arc-like features. In contrast, the late-stage (485−430 Ma) magmatic rocks have predominantly negative εNd(t) (−4.5 to +0.3), εHf(t) (−8.8 to +0.9), and higher incompatible trace elements (e.g., Th), similar to the sub-continental lithospheric mantle beneath the Tarim craton. This abrupt temporal-spatial variation of arc magmatism, together with the detrital zircon evidence, indicate that subduction polarity reversal of the Proto-Tethys Ocean occurred in a period of ∼10 m.y., consistent with the time interval reflected by ophiolite age. This rapid polarity reversal corresponds with the absence of ultrahigh-pressure (UHP) metamorphic and post-collisional magmatic rocks, features normally characteristic of slab break-off or tearing. Numerical modeling shows that this polarity reversal was caused by plate coupling during arc-continent collision. This coupling modified the normal succession of arc-continent collision events, preventing slab break-off or tearing-induced buoyant rock rebound and asthenosphere upwelling. Our model successfully explains early Paleozoic orogenesis in the Western Kunlun orogenic belt and may be applied elsewhere where post-collisional magmatic and UHP rocks are absent.
{"title":"Subduction polarity reversal facilitated by plate coupling during arc-continent collision: Evidence from the Western Kunlun orogenic belt, northwest Tibetan Plateau","authors":"Zaili Tao, Jiyuan Yin, Christopher J. Spencer, Min Sun, W. Xiao, Andrew C. Kerr, Tao Wang, Pengpeng Huangfu, Yunchuan Zeng, Wen Chen","doi":"10.1130/g51847.1","DOIUrl":"https://doi.org/10.1130/g51847.1","url":null,"abstract":"Subduction polarity reversal usually involves the break off or tearing of the downgoing plate (DP) along the continent-ocean transition zone, in order to initiate subduction of the overriding plate (OP) with opposite polarity. We propose that subduction polarity reversal can also be caused by DP-OP coupling and can account for the early Paleozoic geological relationships in the Western Kunlun orogenic belt in the northwestern Tibetan Plateau. Our synthesis of elemental and isotopic data reveals transient (∼2 m.y.) changes in the sources of early Paleozoic arc magmatism in the southern Kunlun terrane. The early-stage (ca. 530−487 Ma) magmatic rocks display relatively high εNd(t) (+0.3 to +8.7), εHf(t) (−3.6 to +16.0), and intra-oceanic arc-like features. In contrast, the late-stage (485−430 Ma) magmatic rocks have predominantly negative εNd(t) (−4.5 to +0.3), εHf(t) (−8.8 to +0.9), and higher incompatible trace elements (e.g., Th), similar to the sub-continental lithospheric mantle beneath the Tarim craton. This abrupt temporal-spatial variation of arc magmatism, together with the detrital zircon evidence, indicate that subduction polarity reversal of the Proto-Tethys Ocean occurred in a period of ∼10 m.y., consistent with the time interval reflected by ophiolite age. This rapid polarity reversal corresponds with the absence of ultrahigh-pressure (UHP) metamorphic and post-collisional magmatic rocks, features normally characteristic of slab break-off or tearing. Numerical modeling shows that this polarity reversal was caused by plate coupling during arc-continent collision. This coupling modified the normal succession of arc-continent collision events, preventing slab break-off or tearing-induced buoyant rock rebound and asthenosphere upwelling. Our model successfully explains early Paleozoic orogenesis in the Western Kunlun orogenic belt and may be applied elsewhere where post-collisional magmatic and UHP rocks are absent.","PeriodicalId":503125,"journal":{"name":"Geology","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"2024-02-08","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139852192","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Zaili Tao, Jiyuan Yin, Christopher J. Spencer, Min Sun, W. Xiao, Andrew C. Kerr, Tao Wang, Pengpeng Huangfu, Yunchuan Zeng, Wen Chen
Subduction polarity reversal usually involves the break off or tearing of the downgoing plate (DP) along the continent-ocean transition zone, in order to initiate subduction of the overriding plate (OP) with opposite polarity. We propose that subduction polarity reversal can also be caused by DP-OP coupling and can account for the early Paleozoic geological relationships in the Western Kunlun orogenic belt in the northwestern Tibetan Plateau. Our synthesis of elemental and isotopic data reveals transient (∼2 m.y.) changes in the sources of early Paleozoic arc magmatism in the southern Kunlun terrane. The early-stage (ca. 530−487 Ma) magmatic rocks display relatively high εNd(t) (+0.3 to +8.7), εHf(t) (−3.6 to +16.0), and intra-oceanic arc-like features. In contrast, the late-stage (485−430 Ma) magmatic rocks have predominantly negative εNd(t) (−4.5 to +0.3), εHf(t) (−8.8 to +0.9), and higher incompatible trace elements (e.g., Th), similar to the sub-continental lithospheric mantle beneath the Tarim craton. This abrupt temporal-spatial variation of arc magmatism, together with the detrital zircon evidence, indicate that subduction polarity reversal of the Proto-Tethys Ocean occurred in a period of ∼10 m.y., consistent with the time interval reflected by ophiolite age. This rapid polarity reversal corresponds with the absence of ultrahigh-pressure (UHP) metamorphic and post-collisional magmatic rocks, features normally characteristic of slab break-off or tearing. Numerical modeling shows that this polarity reversal was caused by plate coupling during arc-continent collision. This coupling modified the normal succession of arc-continent collision events, preventing slab break-off or tearing-induced buoyant rock rebound and asthenosphere upwelling. Our model successfully explains early Paleozoic orogenesis in the Western Kunlun orogenic belt and may be applied elsewhere where post-collisional magmatic and UHP rocks are absent.
{"title":"Subduction polarity reversal facilitated by plate coupling during arc-continent collision: Evidence from the Western Kunlun orogenic belt, northwest Tibetan Plateau","authors":"Zaili Tao, Jiyuan Yin, Christopher J. Spencer, Min Sun, W. Xiao, Andrew C. Kerr, Tao Wang, Pengpeng Huangfu, Yunchuan Zeng, Wen Chen","doi":"10.1130/g51847.1","DOIUrl":"https://doi.org/10.1130/g51847.1","url":null,"abstract":"Subduction polarity reversal usually involves the break off or tearing of the downgoing plate (DP) along the continent-ocean transition zone, in order to initiate subduction of the overriding plate (OP) with opposite polarity. We propose that subduction polarity reversal can also be caused by DP-OP coupling and can account for the early Paleozoic geological relationships in the Western Kunlun orogenic belt in the northwestern Tibetan Plateau. Our synthesis of elemental and isotopic data reveals transient (∼2 m.y.) changes in the sources of early Paleozoic arc magmatism in the southern Kunlun terrane. The early-stage (ca. 530−487 Ma) magmatic rocks display relatively high εNd(t) (+0.3 to +8.7), εHf(t) (−3.6 to +16.0), and intra-oceanic arc-like features. In contrast, the late-stage (485−430 Ma) magmatic rocks have predominantly negative εNd(t) (−4.5 to +0.3), εHf(t) (−8.8 to +0.9), and higher incompatible trace elements (e.g., Th), similar to the sub-continental lithospheric mantle beneath the Tarim craton. This abrupt temporal-spatial variation of arc magmatism, together with the detrital zircon evidence, indicate that subduction polarity reversal of the Proto-Tethys Ocean occurred in a period of ∼10 m.y., consistent with the time interval reflected by ophiolite age. This rapid polarity reversal corresponds with the absence of ultrahigh-pressure (UHP) metamorphic and post-collisional magmatic rocks, features normally characteristic of slab break-off or tearing. Numerical modeling shows that this polarity reversal was caused by plate coupling during arc-continent collision. This coupling modified the normal succession of arc-continent collision events, preventing slab break-off or tearing-induced buoyant rock rebound and asthenosphere upwelling. Our model successfully explains early Paleozoic orogenesis in the Western Kunlun orogenic belt and may be applied elsewhere where post-collisional magmatic and UHP rocks are absent.","PeriodicalId":503125,"journal":{"name":"Geology","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"2024-02-08","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139792292","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
The termination of Neoproterozoic “Snowball Earth” glaciations is marked globally by laterally extensive neritic cap carbonates directly overlying glacial diamictites. The formation of these unique deposits on deglaciation calls for anomalously high calcium carbonate saturation. A popular mechanism to account for the source of requisite ocean alkalinity is the shallow-ridge hypothesis, in which initial spreading ridges surrounding fragments of Rodinia, assumed to be dominated by volcanic margins, were formed at sea level. The shallow ridges are inferred to have promoted widespread deposition and alteration of glassy hyaloclastite—a source of alkalinity. We test this hypothesis by quantifying the prevalence of shallow ridges along Pangea’s passive continental margins, and by assessing Neoproterozoic reconstructions of continents. We find that the most frequently occurring depth range for incipient mid-ocean ridges is 2.1 ± 0.4 km. Ridges with initial elevations of approximately sea level are rare and have anomalous crustal thicknesses >14 km that only occur proximal to large igneous provinces (LIPs). Hyaloclastite is uncommon on mid-ocean ridges as it is generally restricted to water depths of <200 m for tholeiitic basalts, instead forming mostly on intraplate seamounts. Additionally, ocean drilling recently found hyaloclastite to be insignificant along the outer Vøring Plateau (offshore Norway)—an exemplar of a volcanic margin. Reconstructions of Rodinia and associated LIPs demonstrate that volcanic margins potentially hosting minor hyaloclastites were scarce during the late Neoproterozoic. We conclude that the shallow-ridge hypothesis fails to explain the formation of cap carbonates and suggest that other mechanisms such as enhanced continental weathering may be largely responsible.
{"title":"Submarine volcanism along shallow ridges did not drive Cryogenian cap carbonate formation","authors":"A. Dutkiewicz, R. D. Müller","doi":"10.1130/g51884.1","DOIUrl":"https://doi.org/10.1130/g51884.1","url":null,"abstract":"The termination of Neoproterozoic “Snowball Earth” glaciations is marked globally by laterally extensive neritic cap carbonates directly overlying glacial diamictites. The formation of these unique deposits on deglaciation calls for anomalously high calcium carbonate saturation. A popular mechanism to account for the source of requisite ocean alkalinity is the shallow-ridge hypothesis, in which initial spreading ridges surrounding fragments of Rodinia, assumed to be dominated by volcanic margins, were formed at sea level. The shallow ridges are inferred to have promoted widespread deposition and alteration of glassy hyaloclastite—a source of alkalinity. We test this hypothesis by quantifying the prevalence of shallow ridges along Pangea’s passive continental margins, and by assessing Neoproterozoic reconstructions of continents. We find that the most frequently occurring depth range for incipient mid-ocean ridges is 2.1 ± 0.4 km. Ridges with initial elevations of approximately sea level are rare and have anomalous crustal thicknesses >14 km that only occur proximal to large igneous provinces (LIPs). Hyaloclastite is uncommon on mid-ocean ridges as it is generally restricted to water depths of <200 m for tholeiitic basalts, instead forming mostly on intraplate seamounts. Additionally, ocean drilling recently found hyaloclastite to be insignificant along the outer Vøring Plateau (offshore Norway)—an exemplar of a volcanic margin. Reconstructions of Rodinia and associated LIPs demonstrate that volcanic margins potentially hosting minor hyaloclastites were scarce during the late Neoproterozoic. We conclude that the shallow-ridge hypothesis fails to explain the formation of cap carbonates and suggest that other mechanisms such as enhanced continental weathering may be largely responsible.","PeriodicalId":503125,"journal":{"name":"Geology","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"2024-02-08","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139794092","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
The Cascade arc (western North America) is the world’s youngest continental arc, and because the down-going Juan de Fuca plate is young, it is also the hottest end member among subduction zones worldwide. We present evidence that the arc initiated <5 m.y. after accretion of the Siletzia oceanic terrane terminated the earlier subduction system and caused the northern portion of the Farallon slab to break off. Cascade magmatism began ca. 46 Ma with a new trench outboard of Siletzia, a reconfiguration commonly attributed to a seaward jump of the subduction zone. However, the presence of young buoyant oceanic lithosphere that would have resisted being forced into the mantle and the very rapid reestablishment of arc magmatism are hard to reconcile with initiation of a new subduction zone by this process. We propose an alternative mechanism in which the arc was reestablished as the intact southern portion of Farallon slab migrated northward from California (United States), converting a transform margin to a convergent one. This model utilizes plate reconstructions, petrology, mantle tomography, and geochronology to explain how subduction was initiated in a setting where the slab was young and hot and why the earliest Cascade magmatism occurred toward the middle rather than an end of the arc.
{"title":"Initiation of the Cascade arc","authors":"J. Tepper, Kenneth P. Clark","doi":"10.1130/g51888.1","DOIUrl":"https://doi.org/10.1130/g51888.1","url":null,"abstract":"The Cascade arc (western North America) is the world’s youngest continental arc, and because the down-going Juan de Fuca plate is young, it is also the hottest end member among subduction zones worldwide. We present evidence that the arc initiated <5 m.y. after accretion of the Siletzia oceanic terrane terminated the earlier subduction system and caused the northern portion of the Farallon slab to break off. Cascade magmatism began ca. 46 Ma with a new trench outboard of Siletzia, a reconfiguration commonly attributed to a seaward jump of the subduction zone. However, the presence of young buoyant oceanic lithosphere that would have resisted being forced into the mantle and the very rapid reestablishment of arc magmatism are hard to reconcile with initiation of a new subduction zone by this process. We propose an alternative mechanism in which the arc was reestablished as the intact southern portion of Farallon slab migrated northward from California (United States), converting a transform margin to a convergent one. This model utilizes plate reconstructions, petrology, mantle tomography, and geochronology to explain how subduction was initiated in a setting where the slab was young and hot and why the earliest Cascade magmatism occurred toward the middle rather than an end of the arc.","PeriodicalId":503125,"journal":{"name":"Geology","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"2024-02-08","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139793567","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
A. Dutkiewicz, A. Merdith, Alan S. Collins, Ben Mather, Lauren Ilano, S. Zahirovic, R. D. Müller
The Sturtian “Snowball Earth” glaciation (ca. 717−661 Ma) is regarded as the most extreme interval of icehouse climate in Earth’s history. The exact trigger and sustention mechanisms for this long-lived global glaciation remain obscure. The most widely debated causes are silicate weathering of the ca. 718 Ma Franklin large igneous province (LIP) and changes in the length and degassing of continental arcs. A new generation of two independent Neoproterozoic full-plate tectonic models now allows us to quantify the role of tectonics in initiating and sustaining the Sturtian glaciation. We find that continental arc length remains relatively constant from 850 Ma until the end of the glaciation in both models and is unlikely to play a role. The two plate motion models diverge in their predictions of the timing and progression of Rodinia break-up, ocean-basin age, ocean-basement depth, sea-level evolution, and mid-ocean ridge (MOR) carbon outflux. One model predicts MOR outflux and ocean basin volume−driven sea level lower than during the Late Cenozoic glaciation, while the other predicts outgassing and sea level exceeding those of the Late Cretaceous hothouse climate. The second model would preclude a major glaciation, while the first model implies that the trigger for the Sturtian glaciation could have been a combination of an extremely low MOR outflux (∼9 Mt C/yr) and Franklin LIP weathering. Such minimal outflux could have maintained an icehouse state for 57 m.y. when silicate weathering was markedly reduced, with a gradual build-up of MOR CO2 in the atmosphere paired with terrestrial volcanism leading to its termination.
斯图尔特 "雪球地球 "冰川期（约 717-661 Ma）被认为是地球历史上最极端的冰室气候时期。这次持续时间较长的全球冰川作用的确切诱因和滞留机制仍然模糊不清。争论最多的原因是约 718 Ma 富兰克林大火成岩的硅酸盐风化。718 Ma富兰克林大型火成岩带（LIP）的硅酸盐风化以及大陆弧长度和脱气的变化。现在，新一代两个独立的新近纪全板块构造模型使我们能够量化构造作用在启动和维持斯图尔特冰川作用中的作用。我们发现，在两个模型中，大陆弧长度从 850 Ma 到冰川结束都保持相对恒定，不太可能起作用。两个板块运动模式在预测罗迪尼亚断裂的时间和进程、大洋盆地年龄、大洋盆地深度、海平面演变和大洋中脊碳外流方面存在分歧。一种模式预测大洋中脊碳外流和大洋盆地体积驱动的海平面低于晚新生代冰川时期的海平面，而另一种模式预测排气和海平面超过晚白垩世温室气候时期的海平面。第二种模式排除了大冰川的可能性，而第一种模式则意味着斯图尔特冰川的触发因素可能是极低的MOR外流（∼9 Mt C/yr）和富兰克林LIP风化作用的结合。当硅酸盐风化作用明显减弱时，这种极低的外流可以维持57 m.y.的冰室状态，大气中的MOR CO2逐渐增加，再加上陆地火山活动，最终导致冰期的结束。
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A. Dutkiewicz, A. Merdith, Alan S. Collins, Ben Mather, Lauren Ilano, S. Zahirovic, R. D. Müller
The Sturtian “Snowball Earth” glaciation (ca. 717−661 Ma) is regarded as the most extreme interval of icehouse climate in Earth’s history. The exact trigger and sustention mechanisms for this long-lived global glaciation remain obscure. The most widely debated causes are silicate weathering of the ca. 718 Ma Franklin large igneous province (LIP) and changes in the length and degassing of continental arcs. A new generation of two independent Neoproterozoic full-plate tectonic models now allows us to quantify the role of tectonics in initiating and sustaining the Sturtian glaciation. We find that continental arc length remains relatively constant from 850 Ma until the end of the glaciation in both models and is unlikely to play a role. The two plate motion models diverge in their predictions of the timing and progression of Rodinia break-up, ocean-basin age, ocean-basement depth, sea-level evolution, and mid-ocean ridge (MOR) carbon outflux. One model predicts MOR outflux and ocean basin volume−driven sea level lower than during the Late Cenozoic glaciation, while the other predicts outgassing and sea level exceeding those of the Late Cretaceous hothouse climate. The second model would preclude a major glaciation, while the first model implies that the trigger for the Sturtian glaciation could have been a combination of an extremely low MOR outflux (∼9 Mt C/yr) and Franklin LIP weathering. Such minimal outflux could have maintained an icehouse state for 57 m.y. when silicate weathering was markedly reduced, with a gradual build-up of MOR CO2 in the atmosphere paired with terrestrial volcanism leading to its termination.
斯图尔特 "雪球地球 "冰川期（约 717-661 Ma）被认为是地球历史上最极端的冰室气候时期。这次持续时间较长的全球冰川作用的确切诱因和滞留机制仍然模糊不清。争论最多的原因是约 718 Ma 富兰克林大火成岩的硅酸盐风化。718 Ma富兰克林大型火成岩带（LIP）的硅酸盐风化以及大陆弧长度和脱气的变化。现在，新一代的两个独立的新近纪全板块构造模型使我们能够量化构造在引发和维持斯图尔特冰川作用中的作用。我们发现，在两个模型中，大陆弧长度从 850 Ma 到冰川结束都保持相对恒定，不太可能起作用。两个板块运动模式在预测罗迪尼亚断裂的时间和进程、大洋盆地年龄、大洋盆地深度、海平面演变和大洋中脊碳外流方面存在分歧。其中一个模式预测大洋中脊碳外流和洋盆体积驱动的海平面低于晚新生代冰川时期，而另一个模式则预测排气和海平面超过晚白垩世温室气候时期。第二种模式排除了大冰川的可能性，而第一种模式则意味着斯图尔特冰川的触发因素可能是极低的MOR外流（∼9 Mt C/yr）和富兰克林LIP风化作用的结合。当硅酸盐风化作用明显减弱时，这种极低的外流可以维持57 m.y.的冰室状态，大气中的MOR CO2逐渐增加，再加上陆地火山活动，最终导致冰期的结束。
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