Sayani Khan, T. Majumder, S. Patranabis‐Deb, D. Saha
Occurrence of slump folds and associated faults generated by soft-sediment deformation from the Paleoproterozoic Vempalle Formation, southwestern Cuddapah basin, India, is being reported here for the first time. The slump horizon is preserved within a more or less undeformed shallow to deep-water carbonate unit in the Cuddapah basin exposed near Parnapalle village, Andhra Pradesh, India. The stratigraphic framework includes the Gulcheru Quartzite, lowermost unit of the Cuddapah basin succession, deposited in an alluvial to shallow marine shelf environment, and the overlying Vempalle Formation, representing a ramp-type stromatolitic carbonate platform. The synsedimentary sliding along a steepened ramp is evidenced by northeast-verging kink-like folds with wavelength up to 400 m and an overprinting set of thrusts with ramp-flat geometry, fault-cored folds or small break-thrusts showing top-to-southwest displacement, and smaller congruent folds. From the isolated occurrence within a generally undeformed succession, association of structures, and the stratigraphic context, we suggest soft-sediment deformation at the toe of a large (kilometer-scale) slump, with the prevalent bedding-parallel anisotropy exploited for common flexural slip and ramp-flat geometry. In addition, the bedding-parallel slickensides in the Gulcheru Quartzite immediately below the Vempalle Formation indicate a top-to-east-northeast, normal sense of slip representing extensional slip at the slump head. Northeast-verging large folds in the Parnapalle slump horizon possibly represent structures formed during the translation phase of slumping. As the large northeast propagating slump was halted, the backthrust-like structures and associated folds developed at the slump toe.
{"title":"Deformation Structures in a Large Slump Horizon, Paleoproterozoic Vempalle Formation, Cuddapah Basin, Southern India","authors":"Sayani Khan, T. Majumder, S. Patranabis‐Deb, D. Saha","doi":"10.1086/712290","DOIUrl":"https://doi.org/10.1086/712290","url":null,"abstract":"Occurrence of slump folds and associated faults generated by soft-sediment deformation from the Paleoproterozoic Vempalle Formation, southwestern Cuddapah basin, India, is being reported here for the first time. The slump horizon is preserved within a more or less undeformed shallow to deep-water carbonate unit in the Cuddapah basin exposed near Parnapalle village, Andhra Pradesh, India. The stratigraphic framework includes the Gulcheru Quartzite, lowermost unit of the Cuddapah basin succession, deposited in an alluvial to shallow marine shelf environment, and the overlying Vempalle Formation, representing a ramp-type stromatolitic carbonate platform. The synsedimentary sliding along a steepened ramp is evidenced by northeast-verging kink-like folds with wavelength up to 400 m and an overprinting set of thrusts with ramp-flat geometry, fault-cored folds or small break-thrusts showing top-to-southwest displacement, and smaller congruent folds. From the isolated occurrence within a generally undeformed succession, association of structures, and the stratigraphic context, we suggest soft-sediment deformation at the toe of a large (kilometer-scale) slump, with the prevalent bedding-parallel anisotropy exploited for common flexural slip and ramp-flat geometry. In addition, the bedding-parallel slickensides in the Gulcheru Quartzite immediately below the Vempalle Formation indicate a top-to-east-northeast, normal sense of slip representing extensional slip at the slump head. Northeast-verging large folds in the Parnapalle slump horizon possibly represent structures formed during the translation phase of slumping. As the large northeast propagating slump was halted, the backthrust-like structures and associated folds developed at the slump toe.","PeriodicalId":54826,"journal":{"name":"Journal of Geology","volume":null,"pages":null},"PeriodicalIF":1.8,"publicationDate":"2020-11-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1086/712290","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"46174955","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Fei Liu, D. Lian, Guangying Feng, Zhao-li Li, Xiaolu Niu, Jingsui Yang
Ophiolites in the southern belt (SB) occur as much larger peridotite massifs compared with those of the northern belt (NB), sporadically overlain by a thin layer of isotropic gabbro in the western part of Yarlung Zangbo suture zone (YZSZ) in Tibet, which in turn is tectonically thrust over a volcanic-sedimentary sequence. Geochemical data and radiolarian fauna of cherts and detrital zircon ages of litho-quartz sandstones in the sequence provide robust constraints to elucidate the stratigraphic and paleo-depositional environments in which these rocks formed. Eight cherts from Purang, Dongbo, Daba Qu, East Daba, and Labuzha massifs in the SB reveal Late Jurassic–Early Cretaceous radiolarians; they are coeval with minimum detrital zircon U-Pb ages of 132 and 149 Ma, respectively, from two litho-quartz sandstones in the northwestern part of Purang massif. Thirty chert samples from five massifs geochemically show that they have high SiO2 contents of 86.51–95.93 wt%, and high mean ratios of Al/(Al+Fe+Mn) ranging from 0.59 to 0.78, indicating a nonhydrothermal, biogenic, and terrigenous origin. Ce/Ce* ratios of cherts range from 0.93 to 1.52, combined with claystone interlayered with radiolarian chert sporadically overlying litho-quartz sandstone and quartzose sandstone, suggesting a continental slope setting. Given the structural and stratigraphic evidences of ophiolites associated with sedimentary strata and no arc-related magmatism in the SB, we propose that SB ophiolites and ophiolitic mélanges represent southward-thrust nappes from the NB.
{"title":"Radiolarian Biochronology, Detrital Zircon Geochronological and Geochemical Constraints on Provenance and Depositional Environment of Cherts in the Southern Belt of the Western Yarlung Zangbo Suture Zone, Tibet","authors":"Fei Liu, D. Lian, Guangying Feng, Zhao-li Li, Xiaolu Niu, Jingsui Yang","doi":"10.1086/712185","DOIUrl":"https://doi.org/10.1086/712185","url":null,"abstract":"Ophiolites in the southern belt (SB) occur as much larger peridotite massifs compared with those of the northern belt (NB), sporadically overlain by a thin layer of isotropic gabbro in the western part of Yarlung Zangbo suture zone (YZSZ) in Tibet, which in turn is tectonically thrust over a volcanic-sedimentary sequence. Geochemical data and radiolarian fauna of cherts and detrital zircon ages of litho-quartz sandstones in the sequence provide robust constraints to elucidate the stratigraphic and paleo-depositional environments in which these rocks formed. Eight cherts from Purang, Dongbo, Daba Qu, East Daba, and Labuzha massifs in the SB reveal Late Jurassic–Early Cretaceous radiolarians; they are coeval with minimum detrital zircon U-Pb ages of 132 and 149 Ma, respectively, from two litho-quartz sandstones in the northwestern part of Purang massif. Thirty chert samples from five massifs geochemically show that they have high SiO2 contents of 86.51–95.93 wt%, and high mean ratios of Al/(Al+Fe+Mn) ranging from 0.59 to 0.78, indicating a nonhydrothermal, biogenic, and terrigenous origin. Ce/Ce* ratios of cherts range from 0.93 to 1.52, combined with claystone interlayered with radiolarian chert sporadically overlying litho-quartz sandstone and quartzose sandstone, suggesting a continental slope setting. Given the structural and stratigraphic evidences of ophiolites associated with sedimentary strata and no arc-related magmatism in the SB, we propose that SB ophiolites and ophiolitic mélanges represent southward-thrust nappes from the NB.","PeriodicalId":54826,"journal":{"name":"Journal of Geology","volume":null,"pages":null},"PeriodicalIF":1.8,"publicationDate":"2020-11-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1086/712185","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"46924763","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Granulite facies metamorphism of the lower crust has decreased in scale since the Late Archean, but many of its definitive features have persisted: (1) Punctuated, sometimes relatively short-lived, episodes of high-grade metamorphism. These are recorded, in favorably simple cases, by discrete growth rims on zircons. (2) A consistent age gap of a few to several tens of millions of years between juvenile magmatism (crustal accretion) and high-temperature metamorphism. The secondary thermal pulse is an event distinct from primary crustal accretion. (3) Involvement of mineralizing pore fluids of lowered H2O activity, that is, with high CO2 and saline concentrations. Very high oxidation states of some granulites implicate sulfur as an important fluid component. (4) Transcurrent faulting as a conspicuous feature of synmetamorphic deformation. This gives rise to characteristic transposed foliation and lineation. (5) Emplacement of coeval postorogenic K-rich granites at midcrust levels. These features can be rationalized by concepts of modern plate tectonics. High-angle plate collision is succeeded by orogen-parallel transport. This change of plate motion necessarily detaches the underthrust portion of the lithosphere, liberating asthenospheric melts and/or fluids in a postorogenic resurgence. A generation of volatile-rich mafic magmas invades the continental margin; high CO2 and halogen contents cause outgassing and freezing of the magmas at depth. Liberated volatiles effect granulite facies metamorphism by leaching H2O and lithophile elements, importantly K, and transporting these components and heat upward. Extensive melting of the lower crust is inhibited by the low H2O activity of saline-carbonic pore fluids at high pressure. Melting of orthogneiss and supracrustal rocks occurs at midcrust levels by increase of H2O activity as pressure on alkali chloride solutions falls below 0.6–0.5 GPa. The foregoing hypothesis is an alternative to the classical view that granite results from fluid-absent partial melting of, and extraction from, the lower crust, leaving granulites.
{"title":"Young and Old Granulites: A Volatile Connection","authors":"R. C. Newton","doi":"10.1086/711026","DOIUrl":"https://doi.org/10.1086/711026","url":null,"abstract":"Granulite facies metamorphism of the lower crust has decreased in scale since the Late Archean, but many of its definitive features have persisted: (1) Punctuated, sometimes relatively short-lived, episodes of high-grade metamorphism. These are recorded, in favorably simple cases, by discrete growth rims on zircons. (2) A consistent age gap of a few to several tens of millions of years between juvenile magmatism (crustal accretion) and high-temperature metamorphism. The secondary thermal pulse is an event distinct from primary crustal accretion. (3) Involvement of mineralizing pore fluids of lowered H2O activity, that is, with high CO2 and saline concentrations. Very high oxidation states of some granulites implicate sulfur as an important fluid component. (4) Transcurrent faulting as a conspicuous feature of synmetamorphic deformation. This gives rise to characteristic transposed foliation and lineation. (5) Emplacement of coeval postorogenic K-rich granites at midcrust levels. These features can be rationalized by concepts of modern plate tectonics. High-angle plate collision is succeeded by orogen-parallel transport. This change of plate motion necessarily detaches the underthrust portion of the lithosphere, liberating asthenospheric melts and/or fluids in a postorogenic resurgence. A generation of volatile-rich mafic magmas invades the continental margin; high CO2 and halogen contents cause outgassing and freezing of the magmas at depth. Liberated volatiles effect granulite facies metamorphism by leaching H2O and lithophile elements, importantly K, and transporting these components and heat upward. Extensive melting of the lower crust is inhibited by the low H2O activity of saline-carbonic pore fluids at high pressure. Melting of orthogneiss and supracrustal rocks occurs at midcrust levels by increase of H2O activity as pressure on alkali chloride solutions falls below 0.6–0.5 GPa. The foregoing hypothesis is an alternative to the classical view that granite results from fluid-absent partial melting of, and extraction from, the lower crust, leaving granulites.","PeriodicalId":54826,"journal":{"name":"Journal of Geology","volume":null,"pages":null},"PeriodicalIF":1.8,"publicationDate":"2020-09-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1086/711026","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"45773286","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
J. V. L. Filho, R. Fuck, A. Ruiz, E. Dantas, J. Rodrigues, J. Scandolara
The Rio Apa Block, a crustal fragment of 20,000 km2 in the southern portion of the Amazonian Craton, is a Paleoproterozoic terrain, making up the basement of the Neoproterozoic Paraguay Belt. The block is not often considered in the evolution models of the Columbia supercontinent. Different hypotheses of the evolution of the Rio Apa Block consider this segment as part of the Amazonian Craton, and its correlation with other Paleoproterozoic accretionary systems remains controversial. We present new U-Pb and Sm-Nd data that suggest that the Rio Apa Block constitutes a continental arc system built around 1.8–1.7 Ga that can be a correlation element in supercontinent models. Recorded orogenic accretionary events and continental crust reworking divide the block into three main sectors. The western and central sectors comprise parts of the 1.88–1.71 Ga Amoguijá arc. The eastern sector consists of Orosirian high-K calc-alkaline and postcollisional A-type granites. The western segment comprises Orosirian banded orthogneisses of the Porto Murtinho Complex, intruded by initial phase granitoids of the Amoguijá arc. The central sector is characterized by weakly to moderately deformed granites of the Amoguijá Suite, Serra da Bocaina Formation metavolcanic rocks, Serra da Alegria gabbro-anorthosite suite, and Morro do Triunfo gabbro, overlain by the Amolar Group sedimentary rocks. The eastern segment comprises the backarc basin generated Alto Tererê Group, intruded by late to postorogenic granites of the Rio Apa Complex, including A-type granites. Mafic dikes and sill swarms related to a late extensional event intrude the preceding rocks. Two important metamorphic-deformational events are recorded in the opening of the Rb-Sr system in biotite around 1670 Ma, followed by thrust deformation and metamorphism probably related to the ca. 1300 Ma Rondonian-San Ignácio event, as indicated by Ar-Ar muscovite and U-Pb monazite ages.
里约热内卢阿帕地块是位于亚马逊克拉通南部的一块面积为20,000 km2的地壳碎片，是一个古元古代的地形，构成了新元古代巴拉圭带的基底。该块体在哥伦比亚超大陆的演化模式中不常被考虑。关于里约热内卢Apa地块演化的不同假说认为该板块是亚马逊河克拉通的一部分，其与其他古元古代增生系统的对比仍然存在争议。我们提出了新的U-Pb和Sm-Nd数据，表明里约热内卢Apa地块构成了一个建立在1.8-1.7 Ga的大陆弧系统，可以作为超大陆模式的一个相关元素。有记录的造山增生事件和大陆地壳改造将地块划分为三个主要板块。西部和中部地区包括1.88-1.71 Ga amoguij弧的一部分。东段由奥陶系高钾钙碱性花岗岩和后碰撞a型花岗岩组成。西段为Murtinho港杂岩的Orosirian带状正长岩，被amoguij弧的初始期花岗岩侵入。中部地区发育Amolar群沉积岩覆盖的amoguij套、Serra da Bocaina组变质火山岩、Serra da Alegria辉长岩-斜长岩套、Morro do Triunfo辉长岩等弱-中度变形花岗岩。东段为弧后盆地生成的Alto Tererê群，被里约热内卢Apa杂岩的晚-后造山花岗岩侵入，包括a型花岗岩。与晚期伸展事件有关的基性岩脉和岩床群侵入了前面的岩石。1670 Ma前后，黑云母Rb-Sr体系的开启记录了两次重要的变质-变形事件，随后的逆冲变形和变质作用可能与约1300 Ma Rondonian-San Ignácio事件有关，由Ar-Ar白云母和U-Pb独辉石年龄表明。
{"title":"Rio Apa Block: A Juvenile Crustal Fragment in the Southwest Amazonian Craton and Its Implications for Columbia Supercontinent Reconstitution","authors":"J. V. L. Filho, R. Fuck, A. Ruiz, E. Dantas, J. Rodrigues, J. Scandolara","doi":"10.1086/710999","DOIUrl":"https://doi.org/10.1086/710999","url":null,"abstract":"The Rio Apa Block, a crustal fragment of 20,000 km2 in the southern portion of the Amazonian Craton, is a Paleoproterozoic terrain, making up the basement of the Neoproterozoic Paraguay Belt. The block is not often considered in the evolution models of the Columbia supercontinent. Different hypotheses of the evolution of the Rio Apa Block consider this segment as part of the Amazonian Craton, and its correlation with other Paleoproterozoic accretionary systems remains controversial. We present new U-Pb and Sm-Nd data that suggest that the Rio Apa Block constitutes a continental arc system built around 1.8–1.7 Ga that can be a correlation element in supercontinent models. Recorded orogenic accretionary events and continental crust reworking divide the block into three main sectors. The western and central sectors comprise parts of the 1.88–1.71 Ga Amoguijá arc. The eastern sector consists of Orosirian high-K calc-alkaline and postcollisional A-type granites. The western segment comprises Orosirian banded orthogneisses of the Porto Murtinho Complex, intruded by initial phase granitoids of the Amoguijá arc. The central sector is characterized by weakly to moderately deformed granites of the Amoguijá Suite, Serra da Bocaina Formation metavolcanic rocks, Serra da Alegria gabbro-anorthosite suite, and Morro do Triunfo gabbro, overlain by the Amolar Group sedimentary rocks. The eastern segment comprises the backarc basin generated Alto Tererê Group, intruded by late to postorogenic granites of the Rio Apa Complex, including A-type granites. Mafic dikes and sill swarms related to a late extensional event intrude the preceding rocks. Two important metamorphic-deformational events are recorded in the opening of the Rb-Sr system in biotite around 1670 Ma, followed by thrust deformation and metamorphism probably related to the ca. 1300 Ma Rondonian-San Ignácio event, as indicated by Ar-Ar muscovite and U-Pb monazite ages.","PeriodicalId":54826,"journal":{"name":"Journal of Geology","volume":null,"pages":null},"PeriodicalIF":1.8,"publicationDate":"2020-09-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1086/710999","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"45399636","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Geochronologic data combined with a geographic information system-based spatial analysis of plutons in the Bohemian Massif, Variscan belt, allow us to describe the apparent magma fluxes and episodic temporal pattern of plutonism, different from the cyclic pattern of continental margin arcs. The former is interpreted as reflecting a secular thermotectonic evolution of the orogen and magma sources changing from mantle-dominated to intracrustal, paralleled by increasing significance of crustal recycling at the expense of crustal growth. The analysis also suggests that crustal thickening, commonly regarded as the main cause of plutonism in collisional orogens, may be of relatively lower significance than late-stage mantle delamination.
{"title":"Magmatic Tempos in Large Hot Orogens in Comparison with Continental Margin Arcs","authors":"J. Trubač, J. Žák, L. Kondrová","doi":"10.1086/711346","DOIUrl":"https://doi.org/10.1086/711346","url":null,"abstract":"Geochronologic data combined with a geographic information system-based spatial analysis of plutons in the Bohemian Massif, Variscan belt, allow us to describe the apparent magma fluxes and episodic temporal pattern of plutonism, different from the cyclic pattern of continental margin arcs. The former is interpreted as reflecting a secular thermotectonic evolution of the orogen and magma sources changing from mantle-dominated to intracrustal, paralleled by increasing significance of crustal recycling at the expense of crustal growth. The analysis also suggests that crustal thickening, commonly regarded as the main cause of plutonism in collisional orogens, may be of relatively lower significance than late-stage mantle delamination.","PeriodicalId":54826,"journal":{"name":"Journal of Geology","volume":null,"pages":null},"PeriodicalIF":1.8,"publicationDate":"2020-09-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1086/711346","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"41956915","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
In Anatolia, the northern branch of Neotethys is represented by Izmir-Ankara-Erzincan (IAE) Ocean, whose fragments and associated continental entities are preserved in the IAE Suture. However, the traces of this northerly located ocean are not solely bound to the IAE Suture, but can also be found further to the south. The Mersin Mélange, situated in southern Turkey, is such a place that preserves the southerly transported remnants of the northern Neotethys. The Mersin Mélange essentially displays block-in-matrix features, with blocks/slices of different age and lithological characteristics embedded in a clastic matrix. Within the mélange, the Degirmenocagi region consists of a volcano-sedimentary sequence of middle Carnian age, which is characterized by lavas and tuffites interbedded with pelagic sedimentary lithologies. The lavas are chemically of basaltic/basaltic-andesitic composition, whereas the tuffites are of rhyolitic composition. Both lavas and tuffites display negative Nb anomalies, suggesting the involvement of subduction-related components. The high Zr/Nb and low Nb/Yb ratios (compared with normal mid-ocean ridge basalt [N-MORB]) imply a variably depleted mantle source, which has experienced previous melt extraction. The ratio-based melting systematics reflects melt generation largely dominated by spinel-facies melts. The overall geological and geochemical characteristics of the Degirmenocagi sequence suggest an oceanic backarc setting during the Carnian, which may have developed by the rifting of the Sorgun Arc. Although this new finding shows that the Triassic intraoceanic subduction is not uncommon in the Neotethys, it also strengthens a pre-Liassic opening of the northern domain (i.e., the IAE Ocean) and may further explain the longevity of the Middle–Late Triassic intraoceanic subduction in the northern Neotethys.
{"title":"Carnian (Upper Triassic) Lavas and Tuffites from the Mersin Mélange: Evidence for Intraoceanic Arc Rifting in the Northern Neotethys","authors":"Kaan Sayıt, Yavuz Bedi, U. K. Tekin, C. Okuyucu","doi":"10.1086/711000","DOIUrl":"https://doi.org/10.1086/711000","url":null,"abstract":"In Anatolia, the northern branch of Neotethys is represented by Izmir-Ankara-Erzincan (IAE) Ocean, whose fragments and associated continental entities are preserved in the IAE Suture. However, the traces of this northerly located ocean are not solely bound to the IAE Suture, but can also be found further to the south. The Mersin Mélange, situated in southern Turkey, is such a place that preserves the southerly transported remnants of the northern Neotethys. The Mersin Mélange essentially displays block-in-matrix features, with blocks/slices of different age and lithological characteristics embedded in a clastic matrix. Within the mélange, the Degirmenocagi region consists of a volcano-sedimentary sequence of middle Carnian age, which is characterized by lavas and tuffites interbedded with pelagic sedimentary lithologies. The lavas are chemically of basaltic/basaltic-andesitic composition, whereas the tuffites are of rhyolitic composition. Both lavas and tuffites display negative Nb anomalies, suggesting the involvement of subduction-related components. The high Zr/Nb and low Nb/Yb ratios (compared with normal mid-ocean ridge basalt [N-MORB]) imply a variably depleted mantle source, which has experienced previous melt extraction. The ratio-based melting systematics reflects melt generation largely dominated by spinel-facies melts. The overall geological and geochemical characteristics of the Degirmenocagi sequence suggest an oceanic backarc setting during the Carnian, which may have developed by the rifting of the Sorgun Arc. Although this new finding shows that the Triassic intraoceanic subduction is not uncommon in the Neotethys, it also strengthens a pre-Liassic opening of the northern domain (i.e., the IAE Ocean) and may further explain the longevity of the Middle–Late Triassic intraoceanic subduction in the northern Neotethys.","PeriodicalId":54826,"journal":{"name":"Journal of Geology","volume":null,"pages":null},"PeriodicalIF":1.8,"publicationDate":"2020-09-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1086/711000","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"43101002","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Alfred Anderson, professor of geology at the University of Chicago from 1968, passed away on January 15, 2020. I met Fred in October 1982 when I first arrived as a postdocworkingwith Fred Ziegler. I beganmy postdoc before I had completed thefinal version of my dissertation. One of the components that I needed to complete was the photographing of thin sections. FredAndersonvery generously gave me access to his microscope—as he recalled in his acceptance speech of the Bowen Award in 2001— thiswas his startup package of $6.5Kwhenhe joined the faculty in 1968. It is a lovely microscope with an attached camera, and he also provided the film that I used to complete my dissertation. This simple, generous act completely typified Fred. He was a quiet, generous, self-effacing person who contributed broadly across volcanology and petrology to graduate, undergraduate, and continuing education students, to the Department of the Geophysical Sciences, and to the college. Fred was initially coeditor with BobNewton, but he later served as editor in chief of the Journal of Geology from 1984 to 2008. This is no small task, and Fred worked tirelessly and guided the journal with a steady hand for those 24 years. Fred mentored many students and postdocs, including in no particular order Guil Gualda, Dork Sahagian, Christine Skirius, Fanqiong Lu, Brett Peppard, Joe Dufek, Paul Wallace, and Stan Williams, among many others. They have gone on to successful careers, which I am sure are due in no small measure to Fred’s generous mentoring and continued support. Fredwas a gentle giantwhose acute observations, whether of hourglass bubbles in volcanic glasses or of volatile budgets in magmas, led to deep insights into volcanic eruptions and emplacement of granites. Fred received the Norman L. Bowen Award, the top award in volcanology in theworld, from the American Geophysical Union in 2001. Everyone should read the citation and Fred’s acceptance comments because they beautifully convey the
阿尔弗雷德·安德森，1968年起任芝加哥大学地质学教授，于2020年1月15日去世。1982年10月，我第一次和弗雷德·齐格勒(Fred Ziegler)一起做博士后，当时我遇到了弗雷德。我在完成论文的定稿之前就开始了我的博士后研究。我需要完成的一个组成部分是拍摄薄片。弗雷德·安德森非常慷慨地让我使用了他的显微镜——正如他在2001年鲍恩奖的获奖感言中回忆的那样——这是他1968年加入该学院时的启动资金，当时他的启动资金为6.5万美元。这是一个可爱的显微镜，附带一个相机，他还提供了我用来完成我的论文的胶卷。这个简单、慷慨的举动完全是弗雷德的典型。他是一个安静、慷慨、谦逊的人，在火山学和岩石学领域为研究生、本科生和继续教育的学生、地球物理科学系和学院做出了广泛的贡献。弗雷德最初是鲍勃·牛顿的共同编辑，但他后来在1984年至2008年期间担任《地质学杂志》的主编。这不是一项简单的任务，在那24年里，弗雷德不知疲倦地工作，坚定地指导着这本杂志。弗雷德指导了许多学生和博士后，包括Guil Gualda, Dork Sahagian, Christine Skirius, Fanqiong Lu, Brett Peppard, Joe Dufek, Paul Wallace和Stan Williams等等。他们都走上了成功的职业生涯，我相信这在很大程度上要归功于弗雷德慷慨的指导和持续的支持。弗雷德是一个温和的巨人，他敏锐的观察，无论是火山玻璃中的沙漏气泡还是岩浆中不稳定的平衡，都使他对火山爆发和花岗岩的就位有了深刻的认识。2001年，弗雷德获得了美国地球物理学会颁发的世界火山学最高奖项——诺曼·l·鲍恩奖。每个人都应该阅读引用和弗雷德的接受意见，因为他们很好地传达了
{"title":"Memorial to Alfred T. Anderson","authors":"D. Rowley","doi":"10.1086/709833","DOIUrl":"https://doi.org/10.1086/709833","url":null,"abstract":"Alfred Anderson, professor of geology at the University of Chicago from 1968, passed away on January 15, 2020. I met Fred in October 1982 when I first arrived as a postdocworkingwith Fred Ziegler. I beganmy postdoc before I had completed thefinal version of my dissertation. One of the components that I needed to complete was the photographing of thin sections. FredAndersonvery generously gave me access to his microscope—as he recalled in his acceptance speech of the Bowen Award in 2001— thiswas his startup package of $6.5Kwhenhe joined the faculty in 1968. It is a lovely microscope with an attached camera, and he also provided the film that I used to complete my dissertation. This simple, generous act completely typified Fred. He was a quiet, generous, self-effacing person who contributed broadly across volcanology and petrology to graduate, undergraduate, and continuing education students, to the Department of the Geophysical Sciences, and to the college. Fred was initially coeditor with BobNewton, but he later served as editor in chief of the Journal of Geology from 1984 to 2008. This is no small task, and Fred worked tirelessly and guided the journal with a steady hand for those 24 years. Fred mentored many students and postdocs, including in no particular order Guil Gualda, Dork Sahagian, Christine Skirius, Fanqiong Lu, Brett Peppard, Joe Dufek, Paul Wallace, and Stan Williams, among many others. They have gone on to successful careers, which I am sure are due in no small measure to Fred’s generous mentoring and continued support. Fredwas a gentle giantwhose acute observations, whether of hourglass bubbles in volcanic glasses or of volatile budgets in magmas, led to deep insights into volcanic eruptions and emplacement of granites. Fred received the Norman L. Bowen Award, the top award in volcanology in theworld, from the American Geophysical Union in 2001. Everyone should read the citation and Fred’s acceptance comments because they beautifully convey the","PeriodicalId":54826,"journal":{"name":"Journal of Geology","volume":null,"pages":null},"PeriodicalIF":1.8,"publicationDate":"2020-07-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1086/709833","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"44986693","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Jennifer N. Gifford, B. Platt, L. D. Yarbrough, A. O’Reilly, Mohammed Al Harthy
The Chesterian (Mississippian) Hartselle Sandstone is a tar sand exposed in the Black Warrior Basin and southern Appalachian fold belt in northern Mississippi and Alabama. Previous studies disagree about the delivery direction and relative contributions of sediment from the cratonic interior, the Appalachians, and the Ouachitas. The goal of this research is to investigate lateral trends in sedimentary and geochemical properties to provide new details about the provenance of the Hartselle. Samples were collected along a west-to-east transect and analyzed using petrography, X-ray fluorescence (XRF) geochemistry, and U-Pb detrital zircon (DZ) geochronology. Point counting indicated a major cratonic interior source with a minor recycled orogen signal. Sillimanite in the easternmost sample narrows down the potential Appalachian sources to areas of high-grade metamorphism. Petrographic observations suggest both western and eastern sources. The XRF results showed high concentrations of Ti and Mo in the westernmost sample, suggestive of a nearby terrigenous source. All of the samples had similar U-Pb DZ age distributions except for the easternmost sample, which had statistically higher proportions of Paleozoic- and Archean-aged grains (Kolmogorov-Smirnov test: p < .02). For the first time, Eoarchean and Paleoarchean grains were found in the Hartselle, indicating a likely sediment source from the Minnesota River Valley gneisses of the Superior Craton prior to Grenvillian overprinting. Based on integration of all data, we conclude that a large fluvial system draining the continental interior provided a substantial volume of sediment from the northwest. At the same time, a more dispersed drainage basin associated with the Appalachian front contributed recycled orogenic material from the northeast.
{"title":"Integrating Petrography, X-Ray Fluorescence, and U-Pb Detrital Zircon Geochronology to Interpret Provenance of the Mississippian Hartselle Sandstone, USA","authors":"Jennifer N. Gifford, B. Platt, L. D. Yarbrough, A. O’Reilly, Mohammed Al Harthy","doi":"10.1086/709700","DOIUrl":"https://doi.org/10.1086/709700","url":null,"abstract":"The Chesterian (Mississippian) Hartselle Sandstone is a tar sand exposed in the Black Warrior Basin and southern Appalachian fold belt in northern Mississippi and Alabama. Previous studies disagree about the delivery direction and relative contributions of sediment from the cratonic interior, the Appalachians, and the Ouachitas. The goal of this research is to investigate lateral trends in sedimentary and geochemical properties to provide new details about the provenance of the Hartselle. Samples were collected along a west-to-east transect and analyzed using petrography, X-ray fluorescence (XRF) geochemistry, and U-Pb detrital zircon (DZ) geochronology. Point counting indicated a major cratonic interior source with a minor recycled orogen signal. Sillimanite in the easternmost sample narrows down the potential Appalachian sources to areas of high-grade metamorphism. Petrographic observations suggest both western and eastern sources. The XRF results showed high concentrations of Ti and Mo in the westernmost sample, suggestive of a nearby terrigenous source. All of the samples had similar U-Pb DZ age distributions except for the easternmost sample, which had statistically higher proportions of Paleozoic- and Archean-aged grains (Kolmogorov-Smirnov test: p < .02). For the first time, Eoarchean and Paleoarchean grains were found in the Hartselle, indicating a likely sediment source from the Minnesota River Valley gneisses of the Superior Craton prior to Grenvillian overprinting. Based on integration of all data, we conclude that a large fluvial system draining the continental interior provided a substantial volume of sediment from the northwest. At the same time, a more dispersed drainage basin associated with the Appalachian front contributed recycled orogenic material from the northeast.","PeriodicalId":54826,"journal":{"name":"Journal of Geology","volume":null,"pages":null},"PeriodicalIF":1.8,"publicationDate":"2020-06-24","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1086/709700","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"44903727","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Deschamps and Mottez (hereafter DM) argue that the Gauss-Matuyama terrestrial magnetic field reversal may have left a vanishing main dipole moment to the field for a time of order 10,000 years. They say this may have allowed an enhanced cosmic ray flux, boosting the effect we proposed in Melott and Thomas (2019). We point out that the bulk of the cosmic ray flux from a nearby supernova should be too energetic, up to a million times more energetic than the limits of deflection by the terrestrial magnetic field. In fact, only those highly energetic ones will directly reach the troposphere, relevant for cloud-to-ground lightning. From Cosmic Explosions to Terrestrial Fires?: A Discussion. F. Deschamps and F. Mottez. J. Geology 128, online ahead of print. (2020) From Cosmic Explosions to Terrestrial Fires?: A Reply A.L. Melott and B.C. Thomas. J. Geology 128, online ahead of print. (2020) From cosmic explosions to terrestrial fires? (A.L. Melott and B.C. Thomas) Journal of Geology, 127, 475-481 10.1086/703418 (2019) [arXiv:1903.01501]
{"title":"From Cosmic Explosions to Terrestrial Fires? A Reply","authors":"A. Melott, B. Thomas","doi":"10.1086/709751","DOIUrl":"https://doi.org/10.1086/709751","url":null,"abstract":"Deschamps and Mottez (hereafter DM) argue that the Gauss-Matuyama terrestrial magnetic field reversal may have left a vanishing main dipole moment to the field for a time of order 10,000 years. They say this may have allowed an enhanced cosmic ray flux, boosting the effect we proposed in Melott and Thomas (2019). We point out that the bulk of the cosmic ray flux from a nearby supernova should be too energetic, up to a million times more energetic than the limits of deflection by the terrestrial magnetic field. In fact, only those highly energetic ones will directly reach the troposphere, relevant for cloud-to-ground lightning. From Cosmic Explosions to Terrestrial Fires?: A Discussion. F. Deschamps and F. Mottez. J. Geology 128, online ahead of print. (2020) From Cosmic Explosions to Terrestrial Fires?: A Reply A.L. Melott and B.C. Thomas. J. Geology 128, online ahead of print. (2020) From cosmic explosions to terrestrial fires? (A.L. Melott and B.C. Thomas) Journal of Geology, 127, 475-481 10.1086/703418 (2019) [arXiv:1903.01501]","PeriodicalId":54826,"journal":{"name":"Journal of Geology","volume":null,"pages":null},"PeriodicalIF":1.8,"publicationDate":"2020-06-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1086/709751","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"46053455","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Several observational studies have revealed the explosion of a supernova in the early Pleistocene at about 100 pc from the Solar System. The proof of this explosion is based on the signature associated with Fe deposits in Earth’s sediments and on the Moon’s regolith. The supernova remnant was the cause of additional input of galactic cosmic rays (GCRs) in the Solar System. Their propagation from the supernova remnant to Earth and the effects of the associated cascades of secondary particles triggered by their interactionwith Earth’s atmosphere were analyzed by Melott and Thomas (2019). According to their study and previous works cited in their article, high-energy cosmic rays (above 1 TeV) caused a 20-fold increase of irradiation by muons on Earth’s surface and on the ocean and an order of magnitude increase of the atmospheric ionization that could have lasted more than 1,000 years. The increase in irradiation could have contributed to a minor mass extinction in the Pliocene–Pleistocene transition, 2.6 My ago. Melott and Thomas (2019) analyzed the climatic consequences of the increased atmospheric ionization leading tomore frequent lightning and, therefore, to an increase in nitrate deposition and in wildfires. Increased wildfires, evidenced by an increase in soot and carbon deposits over the relevant period, would have contributed to the transition from forest to savanna in northeastern Africa, long argued to have been a factor in the evolution of hominin bipedalism.
{"title":"From Cosmic Explosions to Terrestrial Fires? A Discussion","authors":"F. Deschamps, F. Mottez","doi":"10.1086/709750","DOIUrl":"https://doi.org/10.1086/709750","url":null,"abstract":"Several observational studies have revealed the explosion of a supernova in the early Pleistocene at about 100 pc from the Solar System. The proof of this explosion is based on the signature associated with Fe deposits in Earth’s sediments and on the Moon’s regolith. The supernova remnant was the cause of additional input of galactic cosmic rays (GCRs) in the Solar System. Their propagation from the supernova remnant to Earth and the effects of the associated cascades of secondary particles triggered by their interactionwith Earth’s atmosphere were analyzed by Melott and Thomas (2019). According to their study and previous works cited in their article, high-energy cosmic rays (above 1 TeV) caused a 20-fold increase of irradiation by muons on Earth’s surface and on the ocean and an order of magnitude increase of the atmospheric ionization that could have lasted more than 1,000 years. The increase in irradiation could have contributed to a minor mass extinction in the Pliocene–Pleistocene transition, 2.6 My ago. Melott and Thomas (2019) analyzed the climatic consequences of the increased atmospheric ionization leading tomore frequent lightning and, therefore, to an increase in nitrate deposition and in wildfires. Increased wildfires, evidenced by an increase in soot and carbon deposits over the relevant period, would have contributed to the transition from forest to savanna in northeastern Africa, long argued to have been a factor in the evolution of hominin bipedalism.","PeriodicalId":54826,"journal":{"name":"Journal of Geology","volume":null,"pages":null},"PeriodicalIF":1.8,"publicationDate":"2020-06-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1086/709750","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"41347291","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
京公网安备 11010802042870号
京ICP备2023020795号-1
扫码关注我们
求助内容：
应助结果提醒方式：