Geochemical records of ancient periods of warm climate can be useful to help understand the looming effects of modern anthropogenic warming, including changes to biogeochemical nutrient cycles. Stable nitrogen isotope compositions of marine sediments archive the balance of processes in the global nitrogen cycle. However, the unusual isotopic signals of Mesozoic oceanic anoxic events (OAEs) remain enigmatic, thus hindering our understanding of nitrogen cycle processes and dynamics under conditions of ocean deoxygenation. Here, we present an ammonium "nutrient capacitor" model of the water-column nitrogen cycle to explain the anomalously negative isotopic compositions seen in Mesozoic OAE sediments. Our model applies isotopic inferences derived from high-resolution records of Lake Kivu sediments to show how periodic chemocline overturning of redox-stratified water columns during Mesozoic OAEs may have delivered ammonium to the photic zone in excess of primary producer requirements. Smoothed, stochastic sampling of the changing fluxes within the nitrogen cycle across these events can simulate OAE nitrogen isotope records.
{"title":"Ammonium “nutrient capacitor” model for δ15N signatures associated with marine anoxic events","authors":"Benjamin T. Uveges, Ann Pearson","doi":"10.1130/g51527.1","DOIUrl":"https://doi.org/10.1130/g51527.1","url":null,"abstract":"Geochemical records of ancient periods of warm climate can be useful to help understand the looming effects of modern anthropogenic warming, including changes to biogeochemical nutrient cycles. Stable nitrogen isotope compositions of marine sediments archive the balance of processes in the global nitrogen cycle. However, the unusual isotopic signals of Mesozoic oceanic anoxic events (OAEs) remain enigmatic, thus hindering our understanding of nitrogen cycle processes and dynamics under conditions of ocean deoxygenation. Here, we present an ammonium \"nutrient capacitor\" model of the water-column nitrogen cycle to explain the anomalously negative isotopic compositions seen in Mesozoic OAE sediments. Our model applies isotopic inferences derived from high-resolution records of Lake Kivu sediments to show how periodic chemocline overturning of redox-stratified water columns during Mesozoic OAEs may have delivered ammonium to the photic zone in excess of primary producer requirements. Smoothed, stochastic sampling of the changing fluxes within the nitrogen cycle across these events can simulate OAE nitrogen isotope records.","PeriodicalId":12642,"journal":{"name":"Geology","volume":"65 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2023-09-28","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"135385445","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}
Mantle metasomatism under ultrahigh-pressure (UHP) conditions is widely recognized in orogenic peridotites and pyroxenites from UHP terranes. However, the processes by which the deeply subducted continental crust reacts with the mantle remain obscure. To investigate the metasomatic regime under UHP conditions, we conducted layered reaction experiments between gneiss and peridotite at 5 GPa and 800−1100 °C, with free water added in some of the experiments. At temperatures below the gneiss solidus, the major metasomatic agent is aqueous fluid, with the main reaction products being orthopyroxene + phlogopite + K-richterite. At temperatures above the gneiss solidus, hydrous melt becomes the major metasomatic agent, and the major reaction products vary from orthopyroxene + phlogopite to orthopyroxene + garnet with increasing degree of melting. The transformation from phlogopite to garnet occurred between 1000 °C and 1100 °C for runs without water added and between 800 °C and 900 °C for runs with water added. Pyroxenites in UHP terranes are mainly characterized by the metasomatic growth of orthopyroxene and garnet at 4−6 GPa and 750−1000 °C. Only experiments with water added reproduced the metasomatic assemblage at similar pressure-temperature conditions, indicating that mantle metasomatism in UHP terranes is probably mainly induced by water-fluxed melting of the continental crust under UHP conditions.
{"title":"Mantle metasomatism induced by water-fluxed melting of subducted continental crust at ultrahigh pressures","authors":"Mingdi Gao, Stephen F. Foley, Haijin Xu, Yu Wang","doi":"10.1130/g51547.1","DOIUrl":"https://doi.org/10.1130/g51547.1","url":null,"abstract":"Mantle metasomatism under ultrahigh-pressure (UHP) conditions is widely recognized in orogenic peridotites and pyroxenites from UHP terranes. However, the processes by which the deeply subducted continental crust reacts with the mantle remain obscure. To investigate the metasomatic regime under UHP conditions, we conducted layered reaction experiments between gneiss and peridotite at 5 GPa and 800−1100 °C, with free water added in some of the experiments. At temperatures below the gneiss solidus, the major metasomatic agent is aqueous fluid, with the main reaction products being orthopyroxene + phlogopite + K-richterite. At temperatures above the gneiss solidus, hydrous melt becomes the major metasomatic agent, and the major reaction products vary from orthopyroxene + phlogopite to orthopyroxene + garnet with increasing degree of melting. The transformation from phlogopite to garnet occurred between 1000 °C and 1100 °C for runs without water added and between 800 °C and 900 °C for runs with water added. Pyroxenites in UHP terranes are mainly characterized by the metasomatic growth of orthopyroxene and garnet at 4−6 GPa and 750−1000 °C. Only experiments with water added reproduced the metasomatic assemblage at similar pressure-temperature conditions, indicating that mantle metasomatism in UHP terranes is probably mainly induced by water-fluxed melting of the continental crust under UHP conditions.","PeriodicalId":12642,"journal":{"name":"Geology","volume":"34 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2023-09-27","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"135535230","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}
Rebecca G. Englert, Age J. Vellinga, Matthieu J.B. Cartigny, Michael A. Clare, Joris T. Eggenhuisen, Stephen M. Hubbard
Submarine channels parallel river channels in their ability to transport sediment. However in contrast to rivers, sediment transport and bed-form development in submarine channels are less well understood. Many steep (>1°), sandy submarine channels are dominated by upstream-migrating bed forms. The flow conditions required to form these upstream-migrating bed forms remain debated because the interactions between turbidity currents and active bed forms are difficult to measure directly. Consequently, we used a depth-resolved numerical model to test the role of flow parameters that are hypothesized to control the formation of upstream-migrating bed forms in submarine channels. While our modeling results confirmed the importance of previously identified flow parameters (e.g., densiometric Froude number), we found that basal sediment concentration in turbidity currents is the strongest predictor of upstream-migrating bed-form formation. Our model shows how locally steep gradients enable high sediment concentrations (average >5 vol%) in the basal parts of flows, which allow the development of cyclic step instabilities and their associated bed forms. This new insight explains the previously puzzling observation that upstream-migrating bed forms are abundant in proximal, steep, sandy reaches of submarine channels, while their occurrence becomes more intermittent downslope.
{"title":"Controls on upstream-migrating bed forms in sandy submarine channels","authors":"Rebecca G. Englert, Age J. Vellinga, Matthieu J.B. Cartigny, Michael A. Clare, Joris T. Eggenhuisen, Stephen M. Hubbard","doi":"10.1130/g51385.1","DOIUrl":"https://doi.org/10.1130/g51385.1","url":null,"abstract":"Submarine channels parallel river channels in their ability to transport sediment. However in contrast to rivers, sediment transport and bed-form development in submarine channels are less well understood. Many steep (>1°), sandy submarine channels are dominated by upstream-migrating bed forms. The flow conditions required to form these upstream-migrating bed forms remain debated because the interactions between turbidity currents and active bed forms are difficult to measure directly. Consequently, we used a depth-resolved numerical model to test the role of flow parameters that are hypothesized to control the formation of upstream-migrating bed forms in submarine channels. While our modeling results confirmed the importance of previously identified flow parameters (e.g., densiometric Froude number), we found that basal sediment concentration in turbidity currents is the strongest predictor of upstream-migrating bed-form formation. Our model shows how locally steep gradients enable high sediment concentrations (average >5 vol%) in the basal parts of flows, which allow the development of cyclic step instabilities and their associated bed forms. This new insight explains the previously puzzling observation that upstream-migrating bed forms are abundant in proximal, steep, sandy reaches of submarine channels, while their occurrence becomes more intermittent downslope.","PeriodicalId":12642,"journal":{"name":"Geology","volume":"3 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2023-09-27","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"135535536","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}
Warren W. Wood, Ward E. Sanford, John A. Cherry, Warren T. Wood
Groundwater solute mass flux discharged from the continents to the oceans is between 56% and 63% of particulate sediment transport mass flux. Herein we utilized newly developed continental geospatial groundwater concentration estimates that were multiplied by groundwater volumetric recharge flux to provide a continental-scale discharge mass flux to the oceans of 7.3 Pg DS/yr (petagrams dissolved solutes per year). This mass flux was evaluated from six continental ecosystems: direct ocean discharge (0.28 Pg DS/yr), endorheic basins (0.59 Pg DS/yr), cold-wet exorheic basins (0.55 Pg DS/yr), cold-dry exorheic basins (1.1 Pg DS/yr), warm-dry exorheic basins (0.82 Pg DS/yr), and warm-wet exorheic basins (4.0 Pg DS/yr), thus providing insight into the role of rainfall and temperature on continental weathering and denudation. A new, robust molar silicate/carbonate ratio of 0.42 was calculated for weathering of continental rocks, which is important in the Urey model of climate change. We estimate that rock weathering accounts for ∼50% of the total solute mass flux discharged from the continents, the remainder being from externally derived marine aerosols and organic-derived bicarbonate.
{"title":"\"Wrecking the rocks\": Continental weathering by groundwater","authors":"Warren W. Wood, Ward E. Sanford, John A. Cherry, Warren T. Wood","doi":"10.1130/g51571.1","DOIUrl":"https://doi.org/10.1130/g51571.1","url":null,"abstract":"Groundwater solute mass flux discharged from the continents to the oceans is between 56% and 63% of particulate sediment transport mass flux. Herein we utilized newly developed continental geospatial groundwater concentration estimates that were multiplied by groundwater volumetric recharge flux to provide a continental-scale discharge mass flux to the oceans of 7.3 Pg DS/yr (petagrams dissolved solutes per year). This mass flux was evaluated from six continental ecosystems: direct ocean discharge (0.28 Pg DS/yr), endorheic basins (0.59 Pg DS/yr), cold-wet exorheic basins (0.55 Pg DS/yr), cold-dry exorheic basins (1.1 Pg DS/yr), warm-dry exorheic basins (0.82 Pg DS/yr), and warm-wet exorheic basins (4.0 Pg DS/yr), thus providing insight into the role of rainfall and temperature on continental weathering and denudation. A new, robust molar silicate/carbonate ratio of 0.42 was calculated for weathering of continental rocks, which is important in the Urey model of climate change. We estimate that rock weathering accounts for ∼50% of the total solute mass flux discharged from the continents, the remainder being from externally derived marine aerosols and organic-derived bicarbonate.","PeriodicalId":12642,"journal":{"name":"Geology","volume":"18 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2023-09-27","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"135536463","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}
Youqiang Yu, Frederik Tilmann, Stephen S. Gao, Kelly H. Liu, Jiaji Xi
The initiation and evolution of marginal seas, especially those developing under a convergent setting, is one of the more enigmatic aspects of plate tectonics. Here, we report the presence of slab relics in the mid-mantle of the Woodlark rift in the southwestern Pacific based on a new map of the topography of the mantle discontinuities from a receiver function analysis and evidence from body-wave tomography. The widespread mantle transition-zone thickening rules out active mantle upwelling, and the revealed slab relics in both the upper and middle mantle may hydrate the upper mantle, which can be expected to further weaken the overlying lithosphere. Such a process can then promote initial continental rifting when this lithosphere is exposed to tensional stress like slab-pull stretching originating from the nearby active subduction.
{"title":"Insights into initial continental rifting of marginal seas from seismic evidence for slab relics in the mid-mantle of the Woodlark rift, southwestern Pacific","authors":"Youqiang Yu, Frederik Tilmann, Stephen S. Gao, Kelly H. Liu, Jiaji Xi","doi":"10.1130/g51528.1","DOIUrl":"https://doi.org/10.1130/g51528.1","url":null,"abstract":"The initiation and evolution of marginal seas, especially those developing under a convergent setting, is one of the more enigmatic aspects of plate tectonics. Here, we report the presence of slab relics in the mid-mantle of the Woodlark rift in the southwestern Pacific based on a new map of the topography of the mantle discontinuities from a receiver function analysis and evidence from body-wave tomography. The widespread mantle transition-zone thickening rules out active mantle upwelling, and the revealed slab relics in both the upper and middle mantle may hydrate the upper mantle, which can be expected to further weaken the overlying lithosphere. Such a process can then promote initial continental rifting when this lithosphere is exposed to tensional stress like slab-pull stretching originating from the nearby active subduction.","PeriodicalId":12642,"journal":{"name":"Geology","volume":"2 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2023-09-26","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"135718694","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}
Serpentinized oceanic peridotites might be an important reservoir delivering volatile elements including nitrogen (N) into the mantle via subduction. To determine N sources and estimate the budget of alteration-added secondary N in the oceanic mantle peridotite reservoir, we examined oceanic serpentinites from four Ocean Drilling Program (ODP) sites in the Pacific and Atlantic Oceans. Our results showed that, despite large variation in serpentinization condition (high temperatures up to >350 °C at Holes 895D, 1271B, and 920D; low temperatures <150 °C at Hole 1274A), serpentinites from all sites displayed ubiquitous and similar magnitude of N enrichment (3.2−18.6 ppm) from sediments/seawater sources (δ15N = −3.3‰ to +4.4‰), and these values were significantly elevated relative to the low N concentration (0.04−2.0 ppm) and δ15N value (−5‰ ± 2‰) of the depleted mantle. Based on these data, the serpentinized oceanic mantle is estimated to contribute 0.4 ± 0.2−14.7 ± 6.9 × 109 mol N annually to global subduction zones. Although this flux is smaller than that of subducting sediments (57 × 109 mol·yr−1), comparison between oceanic serpentinites and meta-serpentinites from subduction zones suggests that N can be effectively retained in serpentinites during prograde metamorphism. This implies that the serpentinized slab mantle could be a critical reservoir to deliver N enriched in 15N to the mantle (at least 70 km depth) and potentially to the deepest portions of the mantle sampled by deep-rooted mantle plumes.
{"title":"Oceanic serpentinites: A potentially critical reservoir for deep nitrogen recycling","authors":"Kan Li, Amber Jie Yu, Peter H. Barry, Long Li","doi":"10.1130/g51464.1","DOIUrl":"https://doi.org/10.1130/g51464.1","url":null,"abstract":"Serpentinized oceanic peridotites might be an important reservoir delivering volatile elements including nitrogen (N) into the mantle via subduction. To determine N sources and estimate the budget of alteration-added secondary N in the oceanic mantle peridotite reservoir, we examined oceanic serpentinites from four Ocean Drilling Program (ODP) sites in the Pacific and Atlantic Oceans. Our results showed that, despite large variation in serpentinization condition (high temperatures up to &gt;350 °C at Holes 895D, 1271B, and 920D; low temperatures &lt;150 °C at Hole 1274A), serpentinites from all sites displayed ubiquitous and similar magnitude of N enrichment (3.2−18.6 ppm) from sediments/seawater sources (δ15N = −3.3‰ to +4.4‰), and these values were significantly elevated relative to the low N concentration (0.04−2.0 ppm) and δ15N value (−5‰ ± 2‰) of the depleted mantle. Based on these data, the serpentinized oceanic mantle is estimated to contribute 0.4 ± 0.2−14.7 ± 6.9 × 109 mol N annually to global subduction zones. Although this flux is smaller than that of subducting sediments (57 × 109 mol·yr−1), comparison between oceanic serpentinites and meta-serpentinites from subduction zones suggests that N can be effectively retained in serpentinites during prograde metamorphism. This implies that the serpentinized slab mantle could be a critical reservoir to deliver N enriched in 15N to the mantle (at least 70 km depth) and potentially to the deepest portions of the mantle sampled by deep-rooted mantle plumes.","PeriodicalId":12642,"journal":{"name":"Geology","volume":"23 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2023-09-21","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"136130971","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}
Jack E. Stirling, Steven W. Denyszyn, Robert R. Loucks, Anthony I.S. Kemp, Johannes Hammerli, Marco L. Fiorentini, Jeffrey D. Vervoort
In modern plate tectonic regimes, continental crust is generated above subduction zones in magmatic island arcs. Models for continental growth—largely based on the modeling of geochemical processes that can transform mantle melts into the intermediate composition of bulk continental crust—have been hampered by a lack of definitive geochronology, which could clarify the temporal emplacement of igneous rocks at the base of island arc crust. The Kohistan Arc Complex (KAC) of Pakistan is a rare window into a nearly complete section of lower arc crust, revealing a sequence of igneous rocks representing discrete batches of magma underplated along the base of the arc crust. We present high-precision isotope dilution−thermal ionization mass spectrometry U-Pb geochronological data from zircon, and Lu-Hf and Sm-Nd isochron data from several mafic lower crustal cumulates within the KAC. These data establish a clear downward-younging age trend throughout this succession of cumulate complexes, demonstrating a total magmatic duration of ∼20 m.y., corresponding to a rate of lower crust formation of up to 200−260 km3 km−1 Ma−1. These results provide the first direct evidence of a sequential process of underplating, revealing a fundamental mechanism responsible for building the root of island arc crust.
在现代板块构造制度中，大陆地壳是在岩浆岛弧的俯冲带之上产生的。大陆生长的模型——主要基于地球化学过程的建模，地球化学过程可以将地幔熔体转化为大块大陆地壳的中间成分——由于缺乏明确的地质年代学而受到阻碍，而地质年代学可以澄清岛弧地壳底部火成岩的时间位置。巴基斯坦的Kohistan弧杂岩(KAC)是一个难得的窗口，几乎可以看到下弧地壳的完整部分，揭示了一系列火成岩，代表了沿着弧地壳底部被包裹的岩浆的离散批次。我们提供了来自锆石的高精度同位素稀释-热电离质谱U-Pb年代学数据，以及来自KAC内几个基性下地壳堆积的Lu-Hf和Sm-Nd等时线数据。这些数据在这一系列的堆积复合体中建立了一个明确的向下年轻化的年龄趋势，表明岩浆活动的总持续时间为~ 20 m.y，对应于下地壳形成速率高达200 - 260 km3 km - 1 Ma - 1。这些结果提供了第一个直接的证据，证明了一个连续的底沉积过程，揭示了岛弧地壳根部形成的基本机制。
{"title":"Formation of lower arc crust by magmatic underplating revealed by high-precision geochronology","authors":"Jack E. Stirling, Steven W. Denyszyn, Robert R. Loucks, Anthony I.S. Kemp, Johannes Hammerli, Marco L. Fiorentini, Jeffrey D. Vervoort","doi":"10.1130/g51375.1","DOIUrl":"https://doi.org/10.1130/g51375.1","url":null,"abstract":"In modern plate tectonic regimes, continental crust is generated above subduction zones in magmatic island arcs. Models for continental growth—largely based on the modeling of geochemical processes that can transform mantle melts into the intermediate composition of bulk continental crust—have been hampered by a lack of definitive geochronology, which could clarify the temporal emplacement of igneous rocks at the base of island arc crust. The Kohistan Arc Complex (KAC) of Pakistan is a rare window into a nearly complete section of lower arc crust, revealing a sequence of igneous rocks representing discrete batches of magma underplated along the base of the arc crust. We present high-precision isotope dilution−thermal ionization mass spectrometry U-Pb geochronological data from zircon, and Lu-Hf and Sm-Nd isochron data from several mafic lower crustal cumulates within the KAC. These data establish a clear downward-younging age trend throughout this succession of cumulate complexes, demonstrating a total magmatic duration of ∼20 m.y., corresponding to a rate of lower crust formation of up to 200−260 km3 km−1 Ma−1. These results provide the first direct evidence of a sequential process of underplating, revealing a fundamental mechanism responsible for building the root of island arc crust.","PeriodicalId":12642,"journal":{"name":"Geology","volume":"34 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2023-09-21","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"136130685","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}
Tu Xue, Diandian Peng, Kelly H. Liu, Jonathan Obrist-Farner, Marek Locmelis, Stephen S. Gao, Lijun Liu
Fundamental to plate tectonics is the subduction of cold and mechanically strong oceanic plates. While the subducted plates are conventionally regarded to be impermeable to mantle flow and separate the mantle wedge and the subslab region, isolated openings have been proposed. By combining new shear wave splitting measurements with results from geodynamic modeling and recent seismic tomography and geochemical observations, we show that the upper ∼200 km of the Cocos slab in northern Central America is intensively fractured. The slab there is strong enough to produce typical arc volcanoes and Benioff Zone earthquakes but allows mantle flow to traverse from the subslab region to the mantle wedge. Upwelling of hot subslab mantle flow through the slab provides a viable explanation for the behind-the-volcanic-front volcanoes that are geochemically distinct from typical arc volcanoes, and for the puzzling high heat flow, high elevation, and low Bouguer gravity anomalies observed in northern Central America.
{"title":"Ongoing fragmentation of the subducting Cocos slab, Central America","authors":"Tu Xue, Diandian Peng, Kelly H. Liu, Jonathan Obrist-Farner, Marek Locmelis, Stephen S. Gao, Lijun Liu","doi":"10.1130/g51403.1","DOIUrl":"https://doi.org/10.1130/g51403.1","url":null,"abstract":"Fundamental to plate tectonics is the subduction of cold and mechanically strong oceanic plates. While the subducted plates are conventionally regarded to be impermeable to mantle flow and separate the mantle wedge and the subslab region, isolated openings have been proposed. By combining new shear wave splitting measurements with results from geodynamic modeling and recent seismic tomography and geochemical observations, we show that the upper ∼200 km of the Cocos slab in northern Central America is intensively fractured. The slab there is strong enough to produce typical arc volcanoes and Benioff Zone earthquakes but allows mantle flow to traverse from the subslab region to the mantle wedge. Upwelling of hot subslab mantle flow through the slab provides a viable explanation for the behind-the-volcanic-front volcanoes that are geochemically distinct from typical arc volcanoes, and for the puzzling high heat flow, high elevation, and low Bouguer gravity anomalies observed in northern Central America.","PeriodicalId":12642,"journal":{"name":"Geology","volume":"16 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2023-09-21","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"136130829","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}
Quantification of the crustal thickening processes that construct orogenic plateaus is essential for interpreting their genesis. In the North American Cordillera, a 2.75−3.5-km-elevation, 200−250-km-wide plateau was constructed to the west of the Cretaceous−Paleogene Sevier fold-and-thrust belt (SFTB). The SFTB deformed a Mesoproterozoic to Mesozoic sedimentary package that thickened westward from a 2−3-km-thick platform section that was deposited above the ∼40-km-thick craton to a 15−25-km-thick continental margin section that was deposited above middle to lower crust that had been significantly thinned during Neoproterozoic rifting. Shortening in the SFTB translated this thick sedimentary package as much as 265 km eastward, which resulted in the relative westward underthrusting of an equivalent length of thick cratonic basement beneath the hinterland region. Measurement of components of thickening with respect to the initial and final crustal thickness above and below the basal thrust décollement demonstrates that thickening accommodated by underthrusting outweighed thickening in the overlying SFTB by a factor of 1.5−3 and was likely the dominant thickening mechanism that constructed the broad hinterland plateau. In eastern Nevada, the reconstructed western edge of the underthrusted craton underlies the western limit of 2.75−3.5 km paleoelevations, which supports this interpretation. This analysis provides an important case study for underthrusting as a first-order thickening process in fold-and-thrust systems that deform sedimentary packages with a high pre-orogenic taper.
{"title":"Westward underthrusting of thick North American crust: The dominant thickening process that built the Cordilleran orogenic plateau","authors":"Sean P. Long","doi":"10.1130/g51339.1","DOIUrl":"https://doi.org/10.1130/g51339.1","url":null,"abstract":"Quantification of the crustal thickening processes that construct orogenic plateaus is essential for interpreting their genesis. In the North American Cordillera, a 2.75−3.5-km-elevation, 200−250-km-wide plateau was constructed to the west of the Cretaceous−Paleogene Sevier fold-and-thrust belt (SFTB). The SFTB deformed a Mesoproterozoic to Mesozoic sedimentary package that thickened westward from a 2−3-km-thick platform section that was deposited above the ∼40-km-thick craton to a 15−25-km-thick continental margin section that was deposited above middle to lower crust that had been significantly thinned during Neoproterozoic rifting. Shortening in the SFTB translated this thick sedimentary package as much as 265 km eastward, which resulted in the relative westward underthrusting of an equivalent length of thick cratonic basement beneath the hinterland region. Measurement of components of thickening with respect to the initial and final crustal thickness above and below the basal thrust décollement demonstrates that thickening accommodated by underthrusting outweighed thickening in the overlying SFTB by a factor of 1.5−3 and was likely the dominant thickening mechanism that constructed the broad hinterland plateau. In eastern Nevada, the reconstructed western edge of the underthrusted craton underlies the western limit of 2.75−3.5 km paleoelevations, which supports this interpretation. This analysis provides an important case study for underthrusting as a first-order thickening process in fold-and-thrust systems that deform sedimentary packages with a high pre-orogenic taper.","PeriodicalId":12642,"journal":{"name":"Geology","volume":"159 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2023-09-20","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"136313293","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}
Selina R. Cole, David F. Wright, Jeffrey R. Thompson
Seawater magnesium (Mg) and calcium (Ca) have undergone secular fluctuations throughout the Phanerozoic, controlling whether the dominant calcium carbonate precipitant is calcite or aragonite + high-Mg calcite. Although these oscillations in seawater Mg/Ca ratios have been implicated as an important control on Phanerozoic diversification of calcifying marine organisms, determining the degree to which Mg/Ca ratios affected different clades requires integration of experimental data with historical patterns of biodiversity from the fossil record. We explore short-term and long-term responses of echinoderms to shifting calcite-aragonite seas by combining experimental and deep-time biodiversity investigations. While experimental results support a strong relationship between Mg/Ca ratios and short-term echinoderm regeneration rates, patterns of Phanerozoic echinoderm diversification dynamics show no correspondence with Mg/Ca ratios or calcite-aragonite sea transitions. This decoupling between short- and long-term responses of echinoderms to seawater Mg/Ca ratios suggests echinoderms were relatively unaffected by seawater chemistry throughout their evolutionary history, possibly due to their ability to alter skeletal Mg fractionation and/or adapt to gradual shifts in seawater chemistry. Notably, our results indicate a strict uniformitarian extrapolation of experimental results over geological time scales may not be appropriate for many calcifying marine invertebrates. Instead, the effect of seawater Mg/Ca ratios should be evaluated for individual clades using both experimental and deep-time biodiversity data in a time series.
{"title":"Calcite-aragonite seas as a driver of echinoderm evolution? Experimental insight and deep-time decoupling","authors":"Selina R. Cole, David F. Wright, Jeffrey R. Thompson","doi":"10.1130/g51444.1","DOIUrl":"https://doi.org/10.1130/g51444.1","url":null,"abstract":"Seawater magnesium (Mg) and calcium (Ca) have undergone secular fluctuations throughout the Phanerozoic, controlling whether the dominant calcium carbonate precipitant is calcite or aragonite + high-Mg calcite. Although these oscillations in seawater Mg/Ca ratios have been implicated as an important control on Phanerozoic diversification of calcifying marine organisms, determining the degree to which Mg/Ca ratios affected different clades requires integration of experimental data with historical patterns of biodiversity from the fossil record. We explore short-term and long-term responses of echinoderms to shifting calcite-aragonite seas by combining experimental and deep-time biodiversity investigations. While experimental results support a strong relationship between Mg/Ca ratios and short-term echinoderm regeneration rates, patterns of Phanerozoic echinoderm diversification dynamics show no correspondence with Mg/Ca ratios or calcite-aragonite sea transitions. This decoupling between short- and long-term responses of echinoderms to seawater Mg/Ca ratios suggests echinoderms were relatively unaffected by seawater chemistry throughout their evolutionary history, possibly due to their ability to alter skeletal Mg fractionation and/or adapt to gradual shifts in seawater chemistry. Notably, our results indicate a strict uniformitarian extrapolation of experimental results over geological time scales may not be appropriate for many calcifying marine invertebrates. Instead, the effect of seawater Mg/Ca ratios should be evaluated for individual clades using both experimental and deep-time biodiversity data in a time series.","PeriodicalId":12642,"journal":{"name":"Geology","volume":"13 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2023-09-12","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"135878840","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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