Maxime Bertauts, A. Vezinet, Emilie Janots, Magali Rossi, Isabelle Duhamel-Achin, Philippe Lach, P. Lanari
Metallogenic models of polyphase mountain belts critically rely on robust geochronology. We combine petrology with Rb−Sr and U−Th−Pb in situ geochronology, paired at thin-section scale, to date mineralization in deformed hydrothermal Pb−Zn−Ag deposits along an east-west transect in the Western Alps, France. The Pb−Zn−Ag veins occur in shear zones with kinematic structures consistent with the mylonitized host rocks. The ore consists mainly of galena in a quartz-phengite gangue. The paragenesis can be related to hydrothermal crystallization during periods of variable strain. Both isotope systems yield only Cenozoic ages (ca. 35 Ma and 15−20 Ma) without any pre-Alpine inheritance, clearly indicating orogenic mineralization. The metallogenic model proposed here includes significant fluid circulation along major tectonic contacts between basement and sedimentary cover during Alpine convergence.
多相山带的成矿模型主要依赖于可靠的地质年代学。我们将岩石学与 Rb-Sr 和 U-Th-Pb 原位地质年代学结合起来，以薄片尺度配对，确定了法国西阿尔卑斯山东西横断面上变形热液铅锌银矿床的成矿年代。铅锌银矿脉出现在剪切带中，其运动结构与岩化母岩一致。矿石主要由方铅矿和石英黝帘石煤矸石组成。其成因可能与变应变时期的热液结晶有关。两个同位素系统都只得出了新生代的年龄（约 35 Ma 和 15-20 Ma），没有任何阿尔卑斯山前的遗传，这清楚地表明了造山运动的成矿作用。这里提出的成矿模式包括在阿尔卑斯山汇聚过程中沿着基底和沉积覆盖层之间的主要构造接触产生的大量流体循环。
{"title":"Cenozoic Pb−Zn−Ag mineralization in the Western Alps","authors":"Maxime Bertauts, A. Vezinet, Emilie Janots, Magali Rossi, Isabelle Duhamel-Achin, Philippe Lach, P. Lanari","doi":"10.1130/g51818.1","DOIUrl":"https://doi.org/10.1130/g51818.1","url":null,"abstract":"Metallogenic models of polyphase mountain belts critically rely on robust geochronology. We combine petrology with Rb−Sr and U−Th−Pb in situ geochronology, paired at thin-section scale, to date mineralization in deformed hydrothermal Pb−Zn−Ag deposits along an east-west transect in the Western Alps, France. The Pb−Zn−Ag veins occur in shear zones with kinematic structures consistent with the mylonitized host rocks. The ore consists mainly of galena in a quartz-phengite gangue. The paragenesis can be related to hydrothermal crystallization during periods of variable strain. Both isotope systems yield only Cenozoic ages (ca. 35 Ma and 15−20 Ma) without any pre-Alpine inheritance, clearly indicating orogenic mineralization. The metallogenic model proposed here includes significant fluid circulation along major tectonic contacts between basement and sedimentary cover during Alpine convergence.","PeriodicalId":12642,"journal":{"name":"Geology","volume":null,"pages":null},"PeriodicalIF":5.8,"publicationDate":"2024-02-16","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139961737","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Where, when, and why large-scale shear zones nucleate and propagate into the continental lithosphere are critical issues that challenge the research in tectonics. The East Variscan shear zone is one of the crustal-scale strike-slip faults that shaped the Variscan orogenic crust during late Carboniferous time. Field-based structural analysis and petrological observations demonstrate that suprasolidus high-strain deformation zones and metagranite occurrences are spatially correlated. Among the three dominant lithologies forming this orogenic middle crust (metapelite, metagraywacke, and metagranite), petrological observations and phase equilibrium modeling indicate that the latter is the first lithology that melts during collision-induced heating, in response to H2O-fluid-saturated melting. Our field data and modeling suggest that the water-fluxed melting of metagranite has a primary rheological control on the localization, instigation, and growth of crustal-scale shear zones in the middle crust. Thus, the distribution and geometry of metagranite at the crustal scale could be regarded as critical parameters influencing the rheological inheritance governing the tectonic evolution and localization of bulk strain in the continental lithosphere.
{"title":"Crucial role of water-present melting in metagranite: Implications for the instigation of crustal-scale shear zones","authors":"J. Vanardois, P. Trap, D. Marquer","doi":"10.1130/g51790.1","DOIUrl":"https://doi.org/10.1130/g51790.1","url":null,"abstract":"Where, when, and why large-scale shear zones nucleate and propagate into the continental lithosphere are critical issues that challenge the research in tectonics. The East Variscan shear zone is one of the crustal-scale strike-slip faults that shaped the Variscan orogenic crust during late Carboniferous time. Field-based structural analysis and petrological observations demonstrate that suprasolidus high-strain deformation zones and metagranite occurrences are spatially correlated. Among the three dominant lithologies forming this orogenic middle crust (metapelite, metagraywacke, and metagranite), petrological observations and phase equilibrium modeling indicate that the latter is the first lithology that melts during collision-induced heating, in response to H2O-fluid-saturated melting. Our field data and modeling suggest that the water-fluxed melting of metagranite has a primary rheological control on the localization, instigation, and growth of crustal-scale shear zones in the middle crust. Thus, the distribution and geometry of metagranite at the crustal scale could be regarded as critical parameters influencing the rheological inheritance governing the tectonic evolution and localization of bulk strain in the continental lithosphere.","PeriodicalId":12642,"journal":{"name":"Geology","volume":null,"pages":null},"PeriodicalIF":5.8,"publicationDate":"2024-01-12","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139532347","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
M. Mohr, M. Schmitz, N. Swanson‐Hysell, K.E. Karlstrom, F.A. Macdonald, M. E. Holland, Y. Zhang, N.S. Anderson
The Southwestern Laurentia large igneous province (SWLLIP) comprises voluminous, widespread ca 1.1 Ga magmatism in the southwestern United States and northern Mexico. The timing and tempo of SWLLIP magmatism and its relationship to other late Mesoproterozoic igneous provinces have been unclear due to difficulties in dating mafic rocks at high precision. New precise U-Pb zircon dates for comagmatic felsic segregations within mafic rocks reveal distinct magmatic episodes at ca. 1098 Ma (represented by massive sills in Death Valley, California, the Grand Canyon, and central Arizona) and ca. 1083 Ma (represented by the Cardenas Basalts in the Grand Canyon and a sill in the Dead Mountains, California). The ca. 1098 Ma magmatic pulse was short-lived, lasting 0.25 –0.24 +0.67 m.y., and voluminous and widespread, evidenced by the ≥100 m sills in Death Valley, the Grand Canyon, and central Arizona, consistent with decompression melting of an upwelling mantle plume. The ca. 1083 Ma magmatism may have been generated by a secondary plume pulse or post-plume lithosphere extension. The ca. 1098 Ma pulse of magmatism in southwestern Laurentia occurred ∼2 m.y. prior to an anomalous renewal of voluminous melt generation in the Midcontinent Rift of central Laurentia that is recorded by the ca. 1096 Ma Duluth Complex layered mafic intrusions. Rates of lateral plume spread predicted by mantle plume lubrication theory support a model where a plume derived from the deep mantle impinged near southwestern Laurentia, then spread to thinned Midcontinent Rift lithosphere over ∼2 m.y. to elevate mantle temperatures and generate melt. This geodynamic hypothesis reconciles the close temporal relationships between voluminous magmatism across Laurentia and provides an explanation for that anomalous renewal of high magmatic flux within the protracted magmatic history of the Midcontinent Rift.
西南劳伦西亚大型火成岩带（SWLLIP）包括美国西南部和墨西哥北部大量、广泛的约1.1 Ga岩浆活动。由于难以对岩浆岩进行高精度的年代测定，SWLLIP岩浆活动的时间和节奏及其与其他中新生代晚期火成岩带的关系一直不明确。新的锆石U-Pb精确年代测定显示，黑云母岩中的杂岩性长英岩分块在大约1098Ma（以块状长英岩为代表）时发生了不同的岩浆活动。1098 Ma（以加利福尼亚州死亡谷、大峡谷和亚利桑那州中部的块状岩屑为代表）和约 1083 Ma（以大峡谷的卡德纳斯玄武岩和加利福尼亚州死亡山脉的岩屑为代表）。约大约 1098 Ma 的岩浆脉冲持续时间短，仅为 0.25 -0.24 +0.67 m.y.，岩浆量大，范围广，死亡谷、大峡谷和亚利桑那州中部≥100 m 的岩屑就是证明，这与上涌地幔羽流的减压熔融一致。约约 1083 Ma 的岩浆活动可能是由次级羽流脉冲或羽流后岩石圈延伸产生的。约 1098 Ma 的岩浆活动脉冲劳伦提亚西南部约 1098 Ma 的岩浆活动发生在劳伦提亚中部中洲裂谷大量熔融活动异常更新之前 2 m.y.，该活动由约 1096 Ma 的 Duluth Complex 记录。1096 Ma Duluth Complex层状岩浆侵入体所记录的。地幔羽流润滑理论预测的横向羽流扩散速率支持这样一个模型，即来自深地幔的羽流在劳伦提亚西南部附近撞击，然后扩散到中洲裂谷减薄的岩石圈，经过∼2 m.y.，使地幔温度升高并产生熔体。这一地球动力学假说调和了劳伦提亚各地大量岩浆活动之间的密切时间关系，并为中洲裂谷漫长岩浆史中的高岩浆通量异常更新提供了解释。
{"title":"High-precision U-Pb geochronology links magmatism in the Southwestern Laurentia large igneous province and Midcontinent Rift","authors":"M. Mohr, M. Schmitz, N. Swanson‐Hysell, K.E. Karlstrom, F.A. Macdonald, M. E. Holland, Y. Zhang, N.S. Anderson","doi":"10.1130/g51786.1","DOIUrl":"https://doi.org/10.1130/g51786.1","url":null,"abstract":"The Southwestern Laurentia large igneous province (SWLLIP) comprises voluminous, widespread ca 1.1 Ga magmatism in the southwestern United States and northern Mexico. The timing and tempo of SWLLIP magmatism and its relationship to other late Mesoproterozoic igneous provinces have been unclear due to difficulties in dating mafic rocks at high precision. New precise U-Pb zircon dates for comagmatic felsic segregations within mafic rocks reveal distinct magmatic episodes at ca. 1098 Ma (represented by massive sills in Death Valley, California, the Grand Canyon, and central Arizona) and ca. 1083 Ma (represented by the Cardenas Basalts in the Grand Canyon and a sill in the Dead Mountains, California). The ca. 1098 Ma magmatic pulse was short-lived, lasting 0.25 –0.24 +0.67 m.y., and voluminous and widespread, evidenced by the ≥100 m sills in Death Valley, the Grand Canyon, and central Arizona, consistent with decompression melting of an upwelling mantle plume. The ca. 1083 Ma magmatism may have been generated by a secondary plume pulse or post-plume lithosphere extension.\u0000 The ca. 1098 Ma pulse of magmatism in southwestern Laurentia occurred ∼2 m.y. prior to an anomalous renewal of voluminous melt generation in the Midcontinent Rift of central Laurentia that is recorded by the ca. 1096 Ma Duluth Complex layered mafic intrusions. Rates of lateral plume spread predicted by mantle plume lubrication theory support a model where a plume derived from the deep mantle impinged near southwestern Laurentia, then spread to thinned Midcontinent Rift lithosphere over ∼2 m.y. to elevate mantle temperatures and generate melt. This geodynamic hypothesis reconciles the close temporal relationships between voluminous magmatism across Laurentia and provides an explanation for that anomalous renewal of high magmatic flux within the protracted magmatic history of the Midcontinent Rift.","PeriodicalId":12642,"journal":{"name":"Geology","volume":null,"pages":null},"PeriodicalIF":5.8,"publicationDate":"2024-01-09","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139442155","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Matthias Sinnesael, Andrew R. Millard, Martin R. Smith
The first occurrence of trilobites at ca. 520 Ma is an iconic feature of the Cambrian Explosion. Developing a robust evolutionary view on early Cambrian life is generally hindered by large uncertainties in the ages of fossil finds and their global stratigraphic correlation. We developed an astrochronological interpretation for the Tiout section in Morocco that features some of the oldest trilobite fossils. Our novel approach to incorporating individual astronomical cycle durations in an integrated radioisotopic and astrochronological Bayesian age-depth model results in an age estimate of 519.62 Ma (519.70−519.54 Ma 95% highest posterior distribution) for the first occurrence of trilobites in West Gondwana. This level of precise age estimation is exceptional for biological events in deep time and demonstrates the power of our novel approach.
约 520 Ma 时首次出现三叶虫是寒武纪大爆发的标志性特征。520 Ma 是寒武纪大爆发的标志性特征。由于化石发现的年代及其全球地层相关性存在很大的不确定性，因此通常无法对寒武纪早期的生命进化形成有力的看法。我们对摩洛哥Tiout地段进行了天文年代学解释，该地段有一些最古老的三叶虫化石。我们采用新颖的方法，将单个天文周期的持续时间纳入一个综合的放射性同位素和天体年代学贝叶斯年龄深度模型，得出了西冈瓦纳首次出现三叶虫的年龄估计值为 519.62 Ma（519.70-519.54 Ma 95% 最高后验分布）。这种精确的年龄估计水平对于深时生物事件来说是绝无仅有的，同时也证明了我们新方法的威力。
{"title":"A Bayesian astrochronology for the Cambrian first occurrence of trilobites in West Gondwana (Morocco)","authors":"Matthias Sinnesael, Andrew R. Millard, Martin R. Smith","doi":"10.1130/g51718.1","DOIUrl":"https://doi.org/10.1130/g51718.1","url":null,"abstract":"The first occurrence of trilobites at ca. 520 Ma is an iconic feature of the Cambrian Explosion. Developing a robust evolutionary view on early Cambrian life is generally hindered by large uncertainties in the ages of fossil finds and their global stratigraphic correlation. We developed an astrochronological interpretation for the Tiout section in Morocco that features some of the oldest trilobite fossils. Our novel approach to incorporating individual astronomical cycle durations in an integrated radioisotopic and astrochronological Bayesian age-depth model results in an age estimate of 519.62 Ma (519.70−519.54 Ma 95% highest posterior distribution) for the first occurrence of trilobites in West Gondwana. This level of precise age estimation is exceptional for biological events in deep time and demonstrates the power of our novel approach.","PeriodicalId":12642,"journal":{"name":"Geology","volume":null,"pages":null},"PeriodicalIF":5.8,"publicationDate":"2024-01-08","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139445658","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Sajjad Akam, Pei-Chuan Chuang, Sergei Katsev, C. Wittkop, Michelle Chamberlain, Andrew W. Dale, Klaus Wallmann, Adam J. Heathcote, E. Swanner
The greenhouse gas methane (CH4) contributed to a warm climate that maintained liquid water and sustained Earth’s habitability in the Precambrian despite the faint young sun. The viability of methanogenesis (ME) in ferruginous environments, however, is debated, as iron reduction can potentially outcompete ME as a pathway of organic carbon remineralization (OCR). Here, we document that ME is a dominant OCR process in Brownie Lake, Minnesota (midwestern United States), which is a ferruginous (iron-rich, sulfate-poor) and meromictic (stratified with permanent anoxic bottom waters) system. We report ME accounting for ≥90% and >9% ± 7% of the anaerobic OCR in the water column and sediments, respectively, and an overall particulate organic carbon loading to CH4 conversion efficiency of ≥18% ± 7% in the anoxic zone of Brownie Lake. Our results, along with previous reports from ferruginous systems, suggest that even under low primary productivity in Precambrian oceans, the efficient conversion of organic carbon would have enabled marine CH4 to play a major role in early Earth’s biogeochemical evolution.
{"title":"Methane-carbon budget of a ferruginous meromictic lake and implications for marine methane dynamics on early Earth","authors":"Sajjad Akam, Pei-Chuan Chuang, Sergei Katsev, C. Wittkop, Michelle Chamberlain, Andrew W. Dale, Klaus Wallmann, Adam J. Heathcote, E. Swanner","doi":"10.1130/g51713.1","DOIUrl":"https://doi.org/10.1130/g51713.1","url":null,"abstract":"The greenhouse gas methane (CH4) contributed to a warm climate that maintained liquid water and sustained Earth’s habitability in the Precambrian despite the faint young sun. The viability of methanogenesis (ME) in ferruginous environments, however, is debated, as iron reduction can potentially outcompete ME as a pathway of organic carbon remineralization (OCR). Here, we document that ME is a dominant OCR process in Brownie Lake, Minnesota (midwestern United States), which is a ferruginous (iron-rich, sulfate-poor) and meromictic (stratified with permanent anoxic bottom waters) system. We report ME accounting for ≥90% and >9% ± 7% of the anaerobic OCR in the water column and sediments, respectively, and an overall particulate organic carbon loading to CH4 conversion efficiency of ≥18% ± 7% in the anoxic zone of Brownie Lake. Our results, along with previous reports from ferruginous systems, suggest that even under low primary productivity in Precambrian oceans, the efficient conversion of organic carbon would have enabled marine CH4 to play a major role in early Earth’s biogeochemical evolution.","PeriodicalId":12642,"journal":{"name":"Geology","volume":null,"pages":null},"PeriodicalIF":5.8,"publicationDate":"2024-01-05","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139383523","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
K. Konrad, Matthew G. Jackson, Bernhard Steinberger, A. Koppers, Andrea Marie Balbas, Valerie Finlayson, J. Konter, Allison Price
Age-progressive seamount tracks generated by lithospheric motion over a stationary mantle plume have long been used to reconstruct absolute plate motion (APM) models. However, the basis of these models requires the plumes to move significantly slower than the overriding lithosphere. When a plume interacts with a convergent or divergent plate boundary, it is often deflected within the strong local mantle flow fields associated with such regimes. Here, we examined the age progression and geometry of the Samoa hotspot track, focusing on lava flow samples dredged from the deep flanks of seamounts in order to best reconstruct when a given seamount was overlying the mantle plume (i.e., during the shield-building stage). The Samoan seamounts display an apparent local plate velocity of 7.8 cm/yr from 0 to 9 Ma, 11.1 cm/yr from 9 to 14 Ma, and 5.6 cm/yr from 14 to 24 Ma. Current fixed and mobile hotspot Pacific APM models cannot reproduce the geometry of the Samoa seamount track if a long-term fixed hotspot location, currently beneath the active Vailulu’u Seamount, is assumed. Rather, reconstruction of the eruptive locations of the Samoan seamounts using APM models indicates that the surface expression of the plume migrated ∼2° northward in the Pliocene. Large-scale mantle flow beneath the Pacific Ocean Basin cannot explain this plume migration. Instead, the best explanation is that toroidal flow fields—generated by westward migration of the Tonga Trench and associated slab rollback—have deflected the conduit northward over the past 2−3 m.y. These observations provide novel constraints on the ways in which plume-trench interactions can alter hotspot track geometries.
长期以来，人们一直利用岩石圈在静止的地幔羽流上运动所产生的年龄递增海山轨迹来重建绝对板块运动（APM）模型。然而，这些模型的基础要求地幔羽流的运动速度明显慢于覆盖岩石圈的速度。当羽流与辐合或发散板块边界相互作用时，它通常会在与这些机制相关的强局部地幔流场中发生偏转。在这里，我们研究了萨摩亚热点轨道的年龄演化和几何形状，重点是从海山深侧挖出的熔岩流样本，以便最好地重建特定海山何时覆盖地幔羽流（即在盾牌形成阶段）。萨摩亚海山的表观板块速度在 0 至 9 Ma 为 7.8 厘米/年，9 至 14 Ma 为 11.1 厘米/年，14 至 24 Ma 为 5.6 厘米/年。如果假定一个长期固定的热点位置（目前在活跃的维鲁鲁海山之下），那么目前的太平洋固定和移动热点APM模型就无法再现萨摩亚海山轨道的几何形状。相反，利用 APM 模型重建萨摩亚海山的喷发位置表明，羽流的表面表达在上新世向北迁移了 2°。太平洋海盆下的大规模地幔流动无法解释这种羽流迁移。相反，最好的解释是汤加海沟西移和相关的板块回滚产生的环状流场在过去2-3 m.y.使导管向北偏转。
{"title":"Toroidal flow around the Tonga slab moved the Samoan plume during the Pliocene","authors":"K. Konrad, Matthew G. Jackson, Bernhard Steinberger, A. Koppers, Andrea Marie Balbas, Valerie Finlayson, J. Konter, Allison Price","doi":"10.1130/g51588.1","DOIUrl":"https://doi.org/10.1130/g51588.1","url":null,"abstract":"Age-progressive seamount tracks generated by lithospheric motion over a stationary mantle plume have long been used to reconstruct absolute plate motion (APM) models. However, the basis of these models requires the plumes to move significantly slower than the overriding lithosphere. When a plume interacts with a convergent or divergent plate boundary, it is often deflected within the strong local mantle flow fields associated with such regimes. Here, we examined the age progression and geometry of the Samoa hotspot track, focusing on lava flow samples dredged from the deep flanks of seamounts in order to best reconstruct when a given seamount was overlying the mantle plume (i.e., during the shield-building stage). The Samoan seamounts display an apparent local plate velocity of 7.8 cm/yr from 0 to 9 Ma, 11.1 cm/yr from 9 to 14 Ma, and 5.6 cm/yr from 14 to 24 Ma. Current fixed and mobile hotspot Pacific APM models cannot reproduce the geometry of the Samoa seamount track if a long-term fixed hotspot location, currently beneath the active Vailulu’u Seamount, is assumed. Rather, reconstruction of the eruptive locations of the Samoan seamounts using APM models indicates that the surface expression of the plume migrated ∼2° northward in the Pliocene. Large-scale mantle flow beneath the Pacific Ocean Basin cannot explain this plume migration. Instead, the best explanation is that toroidal flow fields—generated by westward migration of the Tonga Trench and associated slab rollback—have deflected the conduit northward over the past 2−3 m.y. These observations provide novel constraints on the ways in which plume-trench interactions can alter hotspot track geometries.","PeriodicalId":12642,"journal":{"name":"Geology","volume":null,"pages":null},"PeriodicalIF":5.8,"publicationDate":"2024-01-03","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139389517","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
A.N. Martin, E. Stüeken, J.A.-S. Michaud, C. Münker, S. Weyer, E.H.P. van Hees, M.M. Gehringer
The biological nitrogen (N) cycle on early Earth is enigmatic because of limited data from Archean (meta-)sediments and the potential alteration of primary biotic signatures. Here we further investigate unusual 15N enrichments reported in 2.7 Ga meta-sediments from the Abitibi greenstone belt, Canada, purportedly related to a 15N-enriched Archean atmosphere. Given that sediments from this region are contemporaneous with large-scale volcanogenic massive sulfide deposits, we utilize Cu and Zn contents to trace the effects of hydrothermal circulation on N isotope fractionation. We show that high δ15Nbulk values as high as +23‰ are associated with Cu-Zn mineralization, whereas unmineralized organic-rich shales exhibit much lower δ15Nbulk and δ15Nkerogen values. Moreover, we find a large offset between δ15Nbulk and δ15Nkerogen of as much as 17‰ and relate this to the addition of organic-bound N during the late-stage emplacement of organic-rich veins. We conclude that the previously reported high δ15N values are most parsimoniously explained by biotic and abiotic mechanisms rather than a 15N-enriched atmosphere. Crucially, both mechanisms require the presence of NH4+ in hydrothermal fluids, supporting the hypothesis that hydrothermal discharge was an important nutrient source for Neoarchean marine life.
早期地球上的生物氮（N）循环是一个谜，因为来自Archean（元）沉积物的数据有限，而且原始生物特征可能被改变。在这里，我们进一步研究了加拿大阿比提比绿岩带 2.7 Ga 元沉积物中报告的异常 15N 富集，据称这与 15N 富集的 Archean 大气有关。鉴于该地区的沉积物与大规模火山成因块状硫化物矿床同时存在，我们利用铜和锌的含量来追踪热液循环对 N 同位素分馏的影响。我们发现，高达+23‰的δ15Nbulk值与铜锌矿化有关，而未矿化的富含有机质页岩的δ15Nbulk和δ15Nkerogen值则低得多。此外，我们还发现δ15Nbulk与δ15Nkerogen之间存在高达17‰的巨大偏移，这与富含有机质的矿脉在后期置换过程中增加了有机结合氮有关。我们的结论是，以前报道的高δ15N值最有可能由生物和非生物机制而不是富含15N的大气层来解释。最重要的是，这两种机制都需要热液中存在 NH4+，这支持了热液排放是新石器时代海洋生物重要营养源的假说。
{"title":"Mechanisms of nitrogen isotope fractionation at an ancient black smoker in the 2.7 Ga Abitibi greenstone belt, Canada","authors":"A.N. Martin, E. Stüeken, J.A.-S. Michaud, C. Münker, S. Weyer, E.H.P. van Hees, M.M. Gehringer","doi":"10.1130/g51689.1","DOIUrl":"https://doi.org/10.1130/g51689.1","url":null,"abstract":"The biological nitrogen (N) cycle on early Earth is enigmatic because of limited data from Archean (meta-)sediments and the potential alteration of primary biotic signatures. Here we further investigate unusual 15N enrichments reported in 2.7 Ga meta-sediments from the Abitibi greenstone belt, Canada, purportedly related to a 15N-enriched Archean atmosphere. Given that sediments from this region are contemporaneous with large-scale volcanogenic massive sulfide deposits, we utilize Cu and Zn contents to trace the effects of hydrothermal circulation on N isotope fractionation. We show that high δ15Nbulk values as high as +23‰ are associated with Cu-Zn mineralization, whereas unmineralized organic-rich shales exhibit much lower δ15Nbulk and δ15Nkerogen values. Moreover, we find a large offset between δ15Nbulk and δ15Nkerogen of as much as 17‰ and relate this to the addition of organic-bound N during the late-stage emplacement of organic-rich veins. We conclude that the previously reported high δ15N values are most parsimoniously explained by biotic and abiotic mechanisms rather than a 15N-enriched atmosphere. Crucially, both mechanisms require the presence of NH4+ in hydrothermal fluids, supporting the hypothesis that hydrothermal discharge was an important nutrient source for Neoarchean marine life.","PeriodicalId":12642,"journal":{"name":"Geology","volume":null,"pages":null},"PeriodicalIF":5.8,"publicationDate":"2024-01-03","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139389571","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Leonardo M. Pichel, Ritske S. Huismans, Robert Gawthorpe, Jan Inge Faleide
Salt tectonics on passive margins are driven by sediment loading and gliding with minimal influence from basement-involved tectonics and is associated with variable and complex salt structures, such as minibasins and diapirs. A major enigma in salt tectonics is the origin of load-driven diapir-flanked minibasins, synclinal depocenters formed by localized subsidence of synkinematic sediments into salt. How can less-dense clastic sediments sink into the denser salt, promoting diapirism at their flanks? We use two-dimensional numerical modeling of lithospheric extension including syn- and post-rift sedimentation to understand the evolution of salt-tectonic minibasins along rifted passive margins. Our results show that these minibasins are driven by deposition of dense early post-salt carbonates and then amplified during progradation of less-dense and compacting clastics. In contrast, basin-scale salt flow driven by clastic progradation alone, without deposition of early post-salt carbonates, does not produce minibasins as observed on salt-bearing passive margins.
{"title":"Post-salt carbonates control salt-tectonic minibasin formation","authors":"Leonardo M. Pichel, Ritske S. Huismans, Robert Gawthorpe, Jan Inge Faleide","doi":"10.1130/g51717.1","DOIUrl":"https://doi.org/10.1130/g51717.1","url":null,"abstract":"Salt tectonics on passive margins are driven by sediment loading and gliding with minimal influence from basement-involved tectonics and is associated with variable and complex salt structures, such as minibasins and diapirs. A major enigma in salt tectonics is the origin of load-driven diapir-flanked minibasins, synclinal depocenters formed by localized subsidence of synkinematic sediments into salt. How can less-dense clastic sediments sink into the denser salt, promoting diapirism at their flanks? We use two-dimensional numerical modeling of lithospheric extension including syn- and post-rift sedimentation to understand the evolution of salt-tectonic minibasins along rifted passive margins. Our results show that these minibasins are driven by deposition of dense early post-salt carbonates and then amplified during progradation of less-dense and compacting clastics. In contrast, basin-scale salt flow driven by clastic progradation alone, without deposition of early post-salt carbonates, does not produce minibasins as observed on salt-bearing passive margins.","PeriodicalId":12642,"journal":{"name":"Geology","volume":null,"pages":null},"PeriodicalIF":5.8,"publicationDate":"2024-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"138839995","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Chloé Bouscary, Georgina E. King, Djordje Grujic, Jérôme Lavé, Rafael Almeida, György Hetényi, Frédéric Herman
The Himalayan Main Frontal Thrust (MFT) currently accommodates approximately half, i.e., 12–23 mm/yr, of the convergence between the Indian and Eurasian tectonic plates by uplift and deformation of the Sub-Himalayas. While deformation is well documented at modern and million-year time scales, almost no quantitative data are available that constrain Quaternary time scale deformation rates along and within this key tectonic unit. Filling this knowledge gap is crucial to better understanding tectonics and the seismic cycle in this densely populated Himalayan region. We quantify exhumation rates in the Sub-Himalayas using the recently established luminescence thermochronometry technique over time scales of 105 yr, which documents exhumation over the final few kilometers of Earth’s crust. The ultra-low closure temperature of luminescence thermochronometry enables us to resolve thermal histories from the Siwalik Group (Nepal) rocks, which have experienced maximum burial temperatures of ~120 °C. An extensive set of 33 samples was collected from western Nepal to eastern Bhutan, from which 22 yield exhumation rates of ~3–11 mm/yr over the past ~200 k.y. We converted these values to minimum cumulative thrust slip rates of ~6–22 mm/yr, assuming a thrust dip angle of 30°. Our luminescence thermochronometry results show that the Sub-Himalayan fold-and-thrust belt, particularly the MFT, accommodates at least 62% of Himalayan convergence since at least 200 ka. Our data also show activity of some intra-Siwalik thrusts throughout this period, implying that internal deformation of the orogenic wedge and strain partitioning may have occurred.
{"title":"Sustained deformation across the Sub-Himalayas since 200 ka","authors":"Chloé Bouscary, Georgina E. King, Djordje Grujic, Jérôme Lavé, Rafael Almeida, György Hetényi, Frédéric Herman","doi":"10.1130/g51656.1","DOIUrl":"https://doi.org/10.1130/g51656.1","url":null,"abstract":"The Himalayan Main Frontal Thrust (MFT) currently accommodates approximately half, i.e., 12–23 mm/yr, of the convergence between the Indian and Eurasian tectonic plates by uplift and deformation of the Sub-Himalayas. While deformation is well documented at modern and million-year time scales, almost no quantitative data are available that constrain Quaternary time scale deformation rates along and within this key tectonic unit. Filling this knowledge gap is crucial to better understanding tectonics and the seismic cycle in this densely populated Himalayan region. We quantify exhumation rates in the Sub-Himalayas using the recently established luminescence thermochronometry technique over time scales of 105 yr, which documents exhumation over the final few kilometers of Earth’s crust. The ultra-low closure temperature of luminescence thermochronometry enables us to resolve thermal histories from the Siwalik Group (Nepal) rocks, which have experienced maximum burial temperatures of ~120 °C. An extensive set of 33 samples was collected from western Nepal to eastern Bhutan, from which 22 yield exhumation rates of ~3–11 mm/yr over the past ~200 k.y. We converted these values to minimum cumulative thrust slip rates of ~6–22 mm/yr, assuming a thrust dip angle of 30°. Our luminescence thermochronometry results show that the Sub-Himalayan fold-and-thrust belt, particularly the MFT, accommodates at least 62% of Himalayan convergence since at least 200 ka. Our data also show activity of some intra-Siwalik thrusts throughout this period, implying that internal deformation of the orogenic wedge and strain partitioning may have occurred.","PeriodicalId":12642,"journal":{"name":"Geology","volume":null,"pages":null},"PeriodicalIF":5.8,"publicationDate":"2024-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"138886869","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Qi Zhao, Yi Yan, Satoshi Tonai, Yildirim Dilek, Zuofei Zhu
Constraining the timing of tectonic coupling between converging plates is crucial for understanding the transition from continental subduction to continental collision. In the case of the India-Asia collision, thrusting of an accretionary complex onto the Indian continental margin provides the most direct temporal constraint on the early stages of continental collision, as it represents the most immediate upper-crustal fault system corresponding to plate coupling. Here, we used structural analysis combined with K-Ar dating and hydrogen isotopes of authigenic illite and muscovite to unravel the time-progressive development of the Zhongba-Gyangze thrust (ZGT), which represents a tectonic boundary fault in southern Tibet. Our results suggest that the ZGT evolved from its initiation as a single fault zone infiltrated by metamorphic fluids with high δD values (–47‰ to –55‰) at ca. 80 Ma to multiple deformation localization zones starting around 51 Ma. This latter phase is represented by the development of different generations of authigenic 1 M/1Md illite and significant input of meteoric fluids with δD values ranging from –71‰ to –98‰ through multiple episodes of brittle fault reactivation. A Late Cretaceous tectono-thermal event related to the subduction of a Neotethyan oceanic ridge may have been responsible for the formation of 2M1 illite/muscovite at ca. 80 Ma. The oldest (ca. 51 Ma) 1 M/1Md illite age coincides with the first major pulse of shortening in the upper plate after the initial India-Asia contact. Given the synchronous deceleration of India-Asia convergence, the ca. 51 Ma deformation pulse across the Yarlung-Zangbo suture zone demarcates strong coupling (i.e., the onset of continental collision) between India and Asia at this time.
确定会聚板块之间构造耦合的时间对于理解从大陆俯冲到大陆碰撞的过渡至关重要。就印度-亚洲碰撞而言，增生复合体向印度大陆边缘的推力为大陆碰撞的早期阶段提供了最直接的时间约束，因为它代表了与板块耦合相对应的最直接的上地壳断层系统。在这里，我们利用构造分析结合K-Ar年代测定法以及自生伊利石和麝香石的氢同位素，揭示了仲巴-江孜推力（ZGT）的时间演进发展过程，ZGT代表了西藏南部的构造边界断层。我们的研究结果表明，仲巴-江孜断层在大约80Ma时由一个被变质流体浸润的单一断层带演变为多个变形局部。80Ma到51Ma左右开始的多重变形局部带。在后一阶段，通过多次脆性断层再活化，不同世代的自生1M/1Md伊利石和大量δD值在-71‰至-98‰之间的流体输入得到发展。晚白垩世的构造热事件与新特提安洋脊的俯冲有关，可能是在大约 80 Ma 时形成 2M1 辉石/迷石棉的原因。80 Ma。最古老的（约 51 Ma）1M/1Md 伊利石年龄与最初的印度-亚洲接触后上板块缩短的第一个主要脉冲相吻合。考虑到印度-亚洲辐合的同步减速，横跨雅鲁藏布江的约 51 Ma 的变形脉冲与印度-亚洲辐合的同步减速相吻合。51Ma的变形脉冲横跨雅鲁藏布缝合带，标志着此时印度与亚洲之间的强耦合（即大陆碰撞的开始）。
{"title":"Timing of India-Asia collision and significant coupling between them around 51 Ma: Insights from the activation history of the Zhongba-Gyangze thrust in southern Tibet","authors":"Qi Zhao, Yi Yan, Satoshi Tonai, Yildirim Dilek, Zuofei Zhu","doi":"10.1130/g51615.1","DOIUrl":"https://doi.org/10.1130/g51615.1","url":null,"abstract":"Constraining the timing of tectonic coupling between converging plates is crucial for understanding the transition from continental subduction to continental collision. In the case of the India-Asia collision, thrusting of an accretionary complex onto the Indian continental margin provides the most direct temporal constraint on the early stages of continental collision, as it represents the most immediate upper-crustal fault system corresponding to plate coupling. Here, we used structural analysis combined with K-Ar dating and hydrogen isotopes of authigenic illite and muscovite to unravel the time-progressive development of the Zhongba-Gyangze thrust (ZGT), which represents a tectonic boundary fault in southern Tibet. Our results suggest that the ZGT evolved from its initiation as a single fault zone infiltrated by metamorphic fluids with high δD values (–47‰ to –55‰) at ca. 80 Ma to multiple deformation localization zones starting around 51 Ma. This latter phase is represented by the development of different generations of authigenic 1 M/1Md illite and significant input of meteoric fluids with δD values ranging from –71‰ to –98‰ through multiple episodes of brittle fault reactivation. A Late Cretaceous tectono-thermal event related to the subduction of a Neotethyan oceanic ridge may have been responsible for the formation of 2M1 illite/muscovite at ca. 80 Ma. The oldest (ca. 51 Ma) 1 M/1Md illite age coincides with the first major pulse of shortening in the upper plate after the initial India-Asia contact. Given the synchronous deceleration of India-Asia convergence, the ca. 51 Ma deformation pulse across the Yarlung-Zangbo suture zone demarcates strong coupling (i.e., the onset of continental collision) between India and Asia at this time.","PeriodicalId":12642,"journal":{"name":"Geology","volume":null,"pages":null},"PeriodicalIF":5.8,"publicationDate":"2024-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"138840006","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
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