{"title":"A More Informative Way to Name Plutonic Rocks","authors":"A. Glazner, J. Bartley, D. Coleman","doi":"10.1130/GSATG384A.1","DOIUrl":"https://doi.org/10.1130/GSATG384A.1","url":null,"abstract":"","PeriodicalId":35784,"journal":{"name":"GSA Today","volume":" ","pages":""},"PeriodicalIF":0.0,"publicationDate":"2019-02-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"43695547","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Documentation of eustatic variations for the Triassic is limited by the paucity of the preserved marine stratigraphic record, which is confined mostly to the low and middle paleolatitudes of the Tethys Ocean. A revised sea-level curve based on reevaluation of global stratigraphic data shows a clear trend of low seastands for an extended period that spans almost 80 m.y., from the latest Permian to the earliest Jurassic. In the Early and Middle Triassic, the long-term sea levels were similar to or 10–20 m higher than the present-day mean sea level (pdmsl). This trend was reversed in the late Ladinian, marked by a steady rise and culminating in peak sea levels of the Triassic (~50 m above pdmsl) in the late Carnian. The trend reverses again with a decline in the late Norian and the base level remaining close to the pdmsl, and then dipping further in the mid-Rhaetian to ~50 m below pdmsl into the latest Triassic and earliest Jurassic. Superimposed upon this long-term trend is the record of 22 widespread third-order sequence boundaries that have been identified, indicating sealevel falls of mostly minor (<25 m) to medium (25–75 m) amplitude. Only six of these falls are considered major, exceeding the amplitude of 75 m. The long interval of Triassic oceanic withdrawal is likely to have led to general scarcity of preserved marine record and large stratigraphic lacunae. Lacking evidence of continental ice sheets in the Triassic, glacio-eustasy as the driving mechanism for the third-order cyclicity can be ruled out. And even though transfer of water to and from land aquifers to the ocean as a potential cause is plausible for minor (a few tens of meters) sea-level falls, the process seems counter-intuitive for third-order events for much of the Triassic. Triassic paleoenvironmental scenarios demonstrate a close link between eustasy, climates, and biodiversity. INTRODUCTION The Triassic Period encompasses 50.5 m.y., spanning an interval from 251.9 to 201.4 Ma (Ogg et al., 2016). By this time, the megacontinent of Pangaea had already assembled, surrounded by the Panthalassa Ocean that covered >70% of Earth’s surface, and by the mid-Triassic the Pangaean landmass was almost evenly distributed in the two hemispheres around the paleo-equator (see Fig. S1 in the GSA Data Repository1). The interval from latest Permian through the earliest Jurassic, a time span of nearly 80 m.y., represents the longest spell of low seastands of the Phanerozoic. The Triassic is also bracketed by two major biotic extinctions near the Permian-Triassic (P-T) and Triassic-Jurassic boundaries, the one at P-T boundary being the most severe biotic turnover of the Phanerozoic (Raup and Sepkowski, 1982; Hallam and Wignall, 1997; McElwain et al., 1999). The Late Triassic experienced the beginning of the lithospheric swell, ushering the breakup of Pangaea and its eventual split into discrete continents in the later Mesozoic (see Fig. S1 [see footnote 1]). The definite signs of the beginning o
由于缺乏保存的海洋地层记录，三叠纪海平面变化的文献受到限制，这些记录主要局限于特提斯洋的低纬度和中纬度地区。根据对全球地层数据的重新评估，修订后的海平面曲线显示，从二叠纪晚期到侏罗纪早期，在近80万年的长时间内，海平面呈明显的低海平面趋势。在三叠纪早期和中期，长期海平面与现今平均海平面（pdmsl）相似或高出10-20米。这一趋势在拉丁尼亚晚期发生了逆转，其特征是稳步上升，并在卡尼亚晚期达到三叠纪的海平面峰值（pdmsl以上约50米）。这一趋势再次逆转，在诺里阶晚期下降，基准面保持在pdmsl附近，然后在雷蒂阶中期进一步下降至pdmsl下方约50m，进入三叠纪晚期和侏罗纪早期。叠加在这一长期趋势之上的是已经确定的22个广泛的三阶层序边界的记录，这表明海平面下降大多很小（占地球表面的70%），到三叠纪中期，Pangaean陆地几乎均匀分布在古赤道周围的两个半球（见图GSA数据库1中的S1）。从最晚的二叠纪到最早的侏罗纪，时间跨度近80 m.y.，代表了显生宙最长的低潮期。三叠纪还被二叠纪-三叠纪（P-T）和三叠纪-侏罗纪边界附近的两次主要生物灭绝所包围，P-T边界的一次是显生宙最严重的生物更替（Raup和Sepkowski，1982；Hallam和Wignall，1997；McElwain等人，1999）。晚三叠纪经历了岩石圈膨胀的开始，盘古大陆解体，并最终在中生代晚期分裂为离散的大陆（见图S1[见脚注1]）。到三叠纪末，随着中大西洋大规模岩浆区的玄武岩喷出，Pangaean碎裂开始的确切迹象明显显现（例如，见Marzoli等人，19992004；Davies等人，2017）。在过去的二十年里，三叠纪剖面的大量新地层数据被发现，并且在时间尺度上有了显著的改进，这使得对三叠纪海平面变化的审查和修正变得及时。修订后的三叠纪海平面曲线的文件虽然主要来自欧洲西北部和中部（特提斯西部），但现在也包括特提斯其他地区向东的部分，如阿拉伯地台、巴基斯坦、印度、中国和澳大利亚。来自北纬度的文件包括斯维尔德鲁普盆地、斯瓦尔巴群岛和巴伦支海的部分。本文旨在完成对整个中生代的回顾，因为白垩纪和侏罗纪的海平面变化已经得到了重新评估（Haq，20142017）。有关三叠纪古环境条件（海洋和气候）的背景，请参阅GSA数据库（见脚注1）。三叠纪时间尺度更新Preto等人对三叠纪时间尺度的方法进步和修改进行了简要讨论。（2010），Ogg等人对三叠纪地层学进行了详细讨论。（2014）。自上一次更新三叠纪三阶海平面变化（Haq和Al-Qahtani，2005）以来，三叠纪时间地层学已经进行了几次改进。最新版本的时间尺度（Ogg et al.，2016）将三叠纪标准阶段（年龄）的边界修改了<1 m.y.到近6 m.y.。与早期版本一样，新的时间尺度主要基于生物地层学，由选定的辐射测量日期锚定，一些间隔通过天文和气旋地层学微调进行细化，以及其他借助磁地层学的研究。牙形石和菊石构成了三叠纪生物地层对比的主体。三叠纪使用这些化石群进行更广泛的相关性的特殊问题包括分类学标准化、标记物的稀有性、牙形石首次和最后出现的潜在历时性，以及菊石的地方性。由于Pangaean景观的大部分由陆地沉积物主导，区域相关性通常依赖于孢粉学、介形虫和四足动物，而这些与海洋记录的相关性并不广泛。Ogg等人（2016）根据数据类型，将Bilal U.Haq、美国华盛顿特区史密森学会和法国巴黎索邦大学的三叠世阶段边界的综合误差归为0.2至0.59 m.y.（关于时间尺度的进一步讨论，另见GSA数据库[见脚注1]）。
{"title":"Triassic Eustatic Variations Reexamined","authors":"B. Haq","doi":"10.1130/GSATG381A.1","DOIUrl":"https://doi.org/10.1130/GSATG381A.1","url":null,"abstract":"Documentation of eustatic variations for the Triassic is limited by the paucity of the preserved marine stratigraphic record, which is confined mostly to the low and middle paleolatitudes of the Tethys Ocean. A revised sea-level curve based on reevaluation of global stratigraphic data shows a clear trend of low seastands for an extended period that spans almost 80 m.y., from the latest Permian to the earliest Jurassic. In the Early and Middle Triassic, the long-term sea levels were similar to or 10–20 m higher than the present-day mean sea level (pdmsl). This trend was reversed in the late Ladinian, marked by a steady rise and culminating in peak sea levels of the Triassic (~50 m above pdmsl) in the late Carnian. The trend reverses again with a decline in the late Norian and the base level remaining close to the pdmsl, and then dipping further in the mid-Rhaetian to ~50 m below pdmsl into the latest Triassic and earliest Jurassic. Superimposed upon this long-term trend is the record of 22 widespread third-order sequence boundaries that have been identified, indicating sealevel falls of mostly minor (<25 m) to medium (25–75 m) amplitude. Only six of these falls are considered major, exceeding the amplitude of 75 m. The long interval of Triassic oceanic withdrawal is likely to have led to general scarcity of preserved marine record and large stratigraphic lacunae. Lacking evidence of continental ice sheets in the Triassic, glacio-eustasy as the driving mechanism for the third-order cyclicity can be ruled out. And even though transfer of water to and from land aquifers to the ocean as a potential cause is plausible for minor (a few tens of meters) sea-level falls, the process seems counter-intuitive for third-order events for much of the Triassic. Triassic paleoenvironmental scenarios demonstrate a close link between eustasy, climates, and biodiversity. INTRODUCTION The Triassic Period encompasses 50.5 m.y., spanning an interval from 251.9 to 201.4 Ma (Ogg et al., 2016). By this time, the megacontinent of Pangaea had already assembled, surrounded by the Panthalassa Ocean that covered >70% of Earth’s surface, and by the mid-Triassic the Pangaean landmass was almost evenly distributed in the two hemispheres around the paleo-equator (see Fig. S1 in the GSA Data Repository1). The interval from latest Permian through the earliest Jurassic, a time span of nearly 80 m.y., represents the longest spell of low seastands of the Phanerozoic. The Triassic is also bracketed by two major biotic extinctions near the Permian-Triassic (P-T) and Triassic-Jurassic boundaries, the one at P-T boundary being the most severe biotic turnover of the Phanerozoic (Raup and Sepkowski, 1982; Hallam and Wignall, 1997; McElwain et al., 1999). The Late Triassic experienced the beginning of the lithospheric swell, ushering the breakup of Pangaea and its eventual split into discrete continents in the later Mesozoic (see Fig. S1 [see footnote 1]). The definite signs of the beginning o","PeriodicalId":35784,"journal":{"name":"GSA Today","volume":" ","pages":""},"PeriodicalIF":0.0,"publicationDate":"2018-11-29","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"43495176","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
1Geoscience/geoscientists is defined as all subdisciplines that are recognized as Scientific Divisions of GSA (geoarchaeology, karst, planetary geology, etc.), and may be extrapolated to areas that cross over to other subdisciplines that may not be strictly recognized as a GSA Scientific Division. INTRODUCTION The future of geoscience rests on your shoulders. Geologists are passionate about their science and discuss their interests with vigor, firmly understanding why geoscience is as important to society as physiology, agriculture, or engineering. In many cases, non-geologists don’t see the clear importance and implication of the profession, outside of natural disasters and events that have immediate and apparent human effects. Countless geo-scientists1, including professionals, academics, and students, are already vocal self-advocates; however, in our currently digital world, where information can be instantly disseminated at the push of a button, it is time we took a collective effort as the Geological Society of America to actively emphasize the importance of science to the non-geologist, forming a movement to assertively advocate for our field. We invite you to contribute to this discussion by responding with succinct, measurable, and clear reasons on how what you do affects society. Our collective views could be used to guide non-geologists to advocate for geoscience just as non-physicians advocate for medical advances. GSA is as effective as its members, who make up 21Scientific Divisions, which have numerous, tangible impacts on society. As GSA continues focusing efforts on the advancement of the Society into the twenty-first century, we are taking a critical look at what the Society is doing, whom it is doing it for, and how it could be doing it better. PROGRESS IS A GOOD THING The world has changed since GSA was founded in 1888. Integrated circuits have allowed us to use personal computers, we use antibiotics to fight deadly infections, wireless communication provides global access, and we can instantly transmit high-resolution videos to our friends. Technology advances because of society and society advances because of technology. Yet, technological advancement would not be possible without the discovery, understanding, and properties of raw materials, a direct outcome of the unending commitment of the geoscience community. We are all driven by an insatiable human desire for a better understanding of our world and everything it contains— not strictly speaking of geoscience, but all science, technology, engineering, and mathematics fields, and the humanities. What we learn now is different than what we learned in 1888. What university students learn in their geology courses today is different than what the authors of this contribution learned, and what the authors of this contribution learned is quite different from each other. This is the nature of progress. This is a good thing. The tools we use to study geoscience are adapting, innov
{"title":"GSA Tomorrow: An Open Challenge to Promote the Future of Geoscience","authors":"K. M. Dorfler, A. Friedrich","doi":"10.1130/GSATG377GW.1","DOIUrl":"https://doi.org/10.1130/GSATG377GW.1","url":null,"abstract":"1Geoscience/geoscientists is defined as all subdisciplines that are recognized as Scientific Divisions of GSA (geoarchaeology, karst, planetary geology, etc.), and may be extrapolated to areas that cross over to other subdisciplines that may not be strictly recognized as a GSA Scientific Division. INTRODUCTION The future of geoscience rests on your shoulders. Geologists are passionate about their science and discuss their interests with vigor, firmly understanding why geoscience is as important to society as physiology, agriculture, or engineering. In many cases, non-geologists don’t see the clear importance and implication of the profession, outside of natural disasters and events that have immediate and apparent human effects. Countless geo-scientists1, including professionals, academics, and students, are already vocal self-advocates; however, in our currently digital world, where information can be instantly disseminated at the push of a button, it is time we took a collective effort as the Geological Society of America to actively emphasize the importance of science to the non-geologist, forming a movement to assertively advocate for our field. We invite you to contribute to this discussion by responding with succinct, measurable, and clear reasons on how what you do affects society. Our collective views could be used to guide non-geologists to advocate for geoscience just as non-physicians advocate for medical advances. GSA is as effective as its members, who make up 21Scientific Divisions, which have numerous, tangible impacts on society. As GSA continues focusing efforts on the advancement of the Society into the twenty-first century, we are taking a critical look at what the Society is doing, whom it is doing it for, and how it could be doing it better. PROGRESS IS A GOOD THING The world has changed since GSA was founded in 1888. Integrated circuits have allowed us to use personal computers, we use antibiotics to fight deadly infections, wireless communication provides global access, and we can instantly transmit high-resolution videos to our friends. Technology advances because of society and society advances because of technology. Yet, technological advancement would not be possible without the discovery, understanding, and properties of raw materials, a direct outcome of the unending commitment of the geoscience community. We are all driven by an insatiable human desire for a better understanding of our world and everything it contains— not strictly speaking of geoscience, but all science, technology, engineering, and mathematics fields, and the humanities. What we learn now is different than what we learned in 1888. What university students learn in their geology courses today is different than what the authors of this contribution learned, and what the authors of this contribution learned is quite different from each other. This is the nature of progress. This is a good thing. The tools we use to study geoscience are adapting, innov","PeriodicalId":35784,"journal":{"name":"GSA Today","volume":"29 1","pages":"44-45"},"PeriodicalIF":0.0,"publicationDate":"2018-11-20","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"47512647","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Tracking biodiversity changes based on body fossils through geologic time became one of the main objectives of paleontology in the 1980s. Trace fossils represent an alternative record to evaluate secular changes in diversity. A quantitative ichnologic analysis, based on a comprehensive and global data set, has been undertaken in order to evaluate temporal trends in diversity of bioturbation and bioerosion structures. The results of this study indicate that the three main marine evolutionary radiations (Cambrian Explosion, Great Ordovician Biodiversification Event, and Mesozoic Marine Revolution) detected in the body-fossil record are also expressed in the trace-fossil record. Analysis of ichnodiversity trajectories in marine environments supports Sepkoski’s logistic model, which was originally based on analysis of marine body fossils. The tracefossil record of continental environments suggests variable rates of increases in ichnodiversity, with major radiations in the Ludlow–Early Devonian, Cisuralian, Early Jurassic, Late Cretaceous, and Eocene, and slower increases or plateaus in between these periods. Our study indicates that ichnologic information represents an independent line of evidence that yields valuable insights to evaluate paleobiologic megatrends.
{"title":"The other biodiversity record: Innovations in animal-substrate interactions through geologic time","authors":"L. Buatois, M. Mángano","doi":"10.1130/GSATG371A.1","DOIUrl":"https://doi.org/10.1130/GSATG371A.1","url":null,"abstract":"Tracking biodiversity changes based on body fossils through geologic time became one of the main objectives of paleontology in the 1980s. Trace fossils represent an alternative record to evaluate secular changes in diversity. A quantitative ichnologic analysis, based on a comprehensive and global data set, has been undertaken in order to evaluate temporal trends in diversity of bioturbation and bioerosion structures. The results of this study indicate that the three main marine evolutionary radiations (Cambrian Explosion, Great Ordovician Biodiversification Event, and Mesozoic Marine Revolution) detected in the body-fossil record are also expressed in the trace-fossil record. Analysis of ichnodiversity trajectories in marine environments supports Sepkoski’s logistic model, which was originally based on analysis of marine body fossils. The tracefossil record of continental environments suggests variable rates of increases in ichnodiversity, with major radiations in the Ludlow–Early Devonian, Cisuralian, Early Jurassic, Late Cretaceous, and Eocene, and slower increases or plateaus in between these periods. Our study indicates that ichnologic information represents an independent line of evidence that yields valuable insights to evaluate paleobiologic megatrends.","PeriodicalId":35784,"journal":{"name":"GSA Today","volume":" ","pages":""},"PeriodicalIF":0.0,"publicationDate":"2018-09-25","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"46609951","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
{"title":"When oil and water mix: Understanding the environmental impacts of shale development","authors":"D. Soeder, D. Kent","doi":"10.1130/GSATG361A.1","DOIUrl":"https://doi.org/10.1130/GSATG361A.1","url":null,"abstract":".","PeriodicalId":35784,"journal":{"name":"GSA Today","volume":"1 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2018-07-03","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"41586149","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Emily Geraghty Ward, Geology Program, Rocky Mountain College, 1511 Poly Drive, Billings, Montana 59102, USA; Diana Dalbotten, National Center for Earth-Surface Dynamics, St. Anthony Falls Laboratory, University of Minnesota, 2 Third Ave. SE, Minneapolis, Minnesota 55414, USA; Nievita Bueno Watts, Indian Natural Resources Science & Engineering Program, Humboldt State University, Walter Warren House #38, 1 Harpst Street, Arcata, California 95521, USA; Antony Berthelote, Hydrology, Salish Kootenai College, P.O. Box 70, Pablo, Montana 59855, USA.
{"title":"Using place-based, community-inspired research to broaden participation in the geosciences","authors":"E. G. Ward, D. Dalbotten, N. Watts, A. Berthelote","doi":"10.1130/GSATG366GW.1","DOIUrl":"https://doi.org/10.1130/GSATG366GW.1","url":null,"abstract":"Emily Geraghty Ward, Geology Program, Rocky Mountain College, 1511 Poly Drive, Billings, Montana 59102, USA; Diana Dalbotten, National Center for Earth-Surface Dynamics, St. Anthony Falls Laboratory, University of Minnesota, 2 Third Ave. SE, Minneapolis, Minnesota 55414, USA; Nievita Bueno Watts, Indian Natural Resources Science & Engineering Program, Humboldt State University, Walter Warren House #38, 1 Harpst Street, Arcata, California 95521, USA; Antony Berthelote, Hydrology, Salish Kootenai College, P.O. Box 70, Pablo, Montana 59855, USA.","PeriodicalId":35784,"journal":{"name":"GSA Today","volume":" ","pages":""},"PeriodicalIF":0.0,"publicationDate":"2018-06-12","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"44493425","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
G. Stock, A. Guerin, N. Avdievitch, B. Collins, M. Jaboyedoff
Greg M. Stock, National Park Service, Yosemite National Park, El Portal, California 95318, USA; Antoine Guerin, Risk Analysis Group, Institute of Earth Sciences, University of Lausanne, Lausanne, Switzerland; Nikita Avdievitch, National Park Service, Yosemite National Park, El Portal, California 95318, USA; Brian D. Collins, U.S. Geological Survey, Menlo Park, California 94025, USA; Michel Jaboyedoff, Risk Analysis Group, Institute of Earth Sciences, University of Lausanne, Lausanne, Switzerland.
{"title":"Rapid 3-D analysis of rockfalls","authors":"G. Stock, A. Guerin, N. Avdievitch, B. Collins, M. Jaboyedoff","doi":"10.1130/GSATG374GW.1","DOIUrl":"https://doi.org/10.1130/GSATG374GW.1","url":null,"abstract":"Greg M. Stock, National Park Service, Yosemite National Park, El Portal, California 95318, USA; Antoine Guerin, Risk Analysis Group, Institute of Earth Sciences, University of Lausanne, Lausanne, Switzerland; Nikita Avdievitch, National Park Service, Yosemite National Park, El Portal, California 95318, USA; Brian D. Collins, U.S. Geological Survey, Menlo Park, California 94025, USA; Michel Jaboyedoff, Risk Analysis Group, Institute of Earth Sciences, University of Lausanne, Lausanne, Switzerland.","PeriodicalId":35784,"journal":{"name":"GSA Today","volume":" ","pages":""},"PeriodicalIF":0.0,"publicationDate":"2018-06-12","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"48203243","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
The sizes and numbers of tectonic plates are thought to record the importance of plate division, amalgamation, and destruction at divergent and convergent margins. Changes in slope apparent on log area versus log frequency plots have been interpreted as evidence for discrete populations of plate sizes, but the sizes of lithospheric plates are also closely approximated by a continuous density function in which diameters of individual plates are exponentially distributed; such size frequencies are dependent only on the total area and number of designated elements. This implies that the spatial locations of plate boundaries are controlled by a myriad of complicated and interrelated processes such that the geographic occurrence of any particular boundary is largely indeterminate and thus spatially independent of the proximity of other plate boundaries. Observed breaks in slope on linearized size versus frequency plots are merely coincidental and of themselves do not support an interpretation of discrete tectonic processes operating over distinct length scales. Although a purely random distribution of plate boundaries also implicates a similar chance distribution of plate sizes, some smaller plates are indeed clustered along convergent boundaries in the southwestern Pacific. Such association of plates of similar (small) sizes suggests that locations of plate boundaries are best described as reflecting nonhomogeneous Poisson processes wherein probabilities of reaching some plate boundary vary along any Earth-surface transect. Size frequencies of continents, calderas, and many other geologic entities where dimensions are expressed as areal extent exhibit similar size-frequency distributions, suggesting that lateral occurrences of their boundaries are also largely unpredictable, thus reflecting the inherently complicated nature of processes associated with their formation.
{"title":"Broken Sheets—On the Numbers and Areas of Tectonic Plates","authors":"B. Wilkinson, B. McElroy, Carl N. Dummond","doi":"10.1130/GSATG358A.1","DOIUrl":"https://doi.org/10.1130/GSATG358A.1","url":null,"abstract":"The sizes and numbers of tectonic plates are thought to record the importance of plate division, amalgamation, and destruction at divergent and convergent margins. Changes in slope apparent on log area versus log frequency plots have been interpreted as evidence for discrete populations of plate sizes, but the sizes of lithospheric plates are also closely approximated by a continuous density function in which diameters of individual plates are exponentially distributed; such size frequencies are dependent only on the total area and number of designated elements. This implies that the spatial locations of plate boundaries are controlled by a myriad of complicated and interrelated processes such that the geographic occurrence of any particular boundary is largely indeterminate and thus spatially independent of the proximity of other plate boundaries. Observed breaks in slope on linearized size versus frequency plots are merely coincidental and of themselves do not support an interpretation of discrete tectonic processes operating over distinct length scales. Although a purely random distribution of plate boundaries also implicates a similar chance distribution of plate sizes, some smaller plates are indeed clustered along convergent boundaries in the southwestern Pacific. Such association of plates of similar (small) sizes suggests that locations of plate boundaries are best described as reflecting nonhomogeneous Poisson processes wherein probabilities of reaching some plate boundary vary along any Earth-surface transect. Size frequencies of continents, calderas, and many other geologic entities where dimensions are expressed as areal extent exhibit similar size-frequency distributions, suggesting that lateral occurrences of their boundaries are also largely unpredictable, thus reflecting the inherently complicated nature of processes associated with their formation.","PeriodicalId":35784,"journal":{"name":"GSA Today","volume":"1 1","pages":"4-10"},"PeriodicalIF":0.0,"publicationDate":"2018-06-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"47609922","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
C. Stein, S. Stein, R. Elling, G. R. Keller, J. Kley
Two prominent Precambrian geologic features of central North America are the Midcontinent Rift (MCR) and Grenville Front (GF). The MCR, an extensive band of buried igneous and sedimentary rocks outcropping near Lake Superior, records a major rifting event at ca. 1.1 Ga that failed to split North America. In SE Canada, the GF is the continent-ward extent of deformation of the fold-and-thrust belt from the Grenville orogeny, the sequence of events from ca. 1.3–0.98 Ga culminating in the assembly of the supercontinent of Rodinia. In the central U.S., lineated gravity anomalies extending southward along the trend of the front in Canada have been interpreted as a buried Grenville Front. However, we use recent tectonic concepts and data analyses to argue that these anomalies delineate the eastern arm of the MCR extending from Michigan to Alabama, for multiple reasons: (1) These anomalies are similar to those along the remainder of the MCR and quite different from those across the front in Canada; (2) the Precambrian deformation observed on seismic reflection profiles across the presumed “front” appears quite different from that across the front in Canada, cannot confidently be assigned to the Grenville orogeny, and is recorded at least 100 km west of the “front”; and (3) during the Grenville orogeny, deformational events from Texas to Canada were not caused by the same plate interactions and were not necessarily synchronous. Hence the commonly inferred position of the “Grenville Front” in the central U.S. is part of the MCR, and should not be mapped as a separate entity.
{"title":"Is the “Grenville Front” in the central United States really the midcontinent rift?","authors":"C. Stein, S. Stein, R. Elling, G. R. Keller, J. Kley","doi":"10.1130/GSATG357A.1","DOIUrl":"https://doi.org/10.1130/GSATG357A.1","url":null,"abstract":"Two prominent Precambrian geologic features of central North America are the Midcontinent Rift (MCR) and Grenville Front (GF). The MCR, an extensive band of buried igneous and sedimentary rocks outcropping near Lake Superior, records a major rifting event at ca. 1.1 Ga that failed to split North America. In SE Canada, the GF is the continent-ward extent of deformation of the fold-and-thrust belt from the Grenville orogeny, the sequence of events from ca. 1.3–0.98 Ga culminating in the assembly of the supercontinent of Rodinia. In the central U.S., lineated gravity anomalies extending southward along the trend of the front in Canada have been interpreted as a buried Grenville Front. However, we use recent tectonic concepts and data analyses to argue that these anomalies delineate the eastern arm of the MCR extending from Michigan to Alabama, for multiple reasons: (1) These anomalies are similar to those along the remainder of the MCR and quite different from those across the front in Canada; (2) the Precambrian deformation observed on seismic reflection profiles across the presumed “front” appears quite different from that across the front in Canada, cannot confidently be assigned to the Grenville orogeny, and is recorded at least 100 km west of the “front”; and (3) during the Grenville orogeny, deformational events from Texas to Canada were not caused by the same plate interactions and were not necessarily synchronous. Hence the commonly inferred position of the “Grenville Front” in the central U.S. is part of the MCR, and should not be mapped as a separate entity.","PeriodicalId":35784,"journal":{"name":"GSA Today","volume":"28 1","pages":"4-10"},"PeriodicalIF":0.0,"publicationDate":"2018-05-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"45307726","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Frequency of mention in books can be used to trace the evolution of a discipline, from the first recorded use of the word or phrase to its current standing, as measured by the number of books that include the phrase. Ngram Viewer, the tool developed by a team at Google Books (Michel et al., 2011) places a database (“corpus”) of >500 billion words at the disposal of its users (http://books.google.com/ngrams). Here I describe how this tool can be used to examine patterns suggested by qualitative ideas about the intellectual development of the geosciences. An example of the Ngram Viewer output is given in Figure 1.
{"title":"Charting the geosciences with google ngram viewer","authors":"D. S. Brandt","doi":"10.1130/GSATG348GW.1","DOIUrl":"https://doi.org/10.1130/GSATG348GW.1","url":null,"abstract":"Frequency of mention in books can be used to trace the evolution of a discipline, from the first recorded use of the word or phrase to its current standing, as measured by the number of books that include the phrase. Ngram Viewer, the tool developed by a team at Google Books (Michel et al., 2011) places a database (“corpus”) of >500 billion words at the disposal of its users (http://books.google.com/ngrams). Here I describe how this tool can be used to examine patterns suggested by qualitative ideas about the intellectual development of the geosciences. An example of the Ngram Viewer output is given in Figure 1.","PeriodicalId":35784,"journal":{"name":"GSA Today","volume":"28 1","pages":"66-67"},"PeriodicalIF":0.0,"publicationDate":"2018-05-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"44174792","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
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