{"title":"Mid-Ordovician subduction, obduction, and eduction in western Newfoundland; an eclogite problem","authors":"J. Dewey, John F. Casey","doi":"10.25131/SAJG.124.0029","DOIUrl":null,"url":null,"abstract":"\n The narrow, short-lived Taconic-Grampian Orogen occurs along the north-western margin of the Appalachian-Caledonian Belt from, at least, Alabama to Scotland, a result of the collision of a series of early Ordovician oceanic island arcs with the rifted margin of Laurentia. The present distribution of Taconian-Grampian ophiolites is unlikely to represent a single fore-arc from Alabama to Scotland colliding at the same time with the continental margin along its whole length; more likely is that there were several Ordovician arcs with separate ophiolites. The collision suture is at the thrust base of obducted fore-arc ophiolite complexes, and obduction distance was about two hundred kilometres. Footwalls to the ophiolites are, sequentially towards the continent, continental margin rift sediments and volcanics and overlying rise sediments, continental shelf slope carbonates, and sediments of foreland flexural basins. The regionally-flat obduction thrust complex between the ophiolite and the rifted Laurentian margin is the collision suture between arc and continent. A particular problem in drawing tectonic profiles across the Taconic-Grampian Zone is several orogen-parallel major strike-slip faults, both sinistral and dextral, of unknown displacements, which may juxtapose portions of different segments. In western Newfoundland, most of the Grenville basement beneath the Fleur-de-Lys metamorphic complex (Neoproterozoic to early Ordovician meta-sediments) was eclogitised during the Taconic Orogeny and separated by a massive shear zone from the overlying Fleur-de-Lys, which was metamorphosed at the same time but in the amphibolite facies. The shear zone continued either to a distal intracontinental “subduction zone” or to the main, sub-fore-arc, subduction zone beneath which the basement slipped down to depths of up to seventy kilometres at the same time as the ophiolite sheet and its previously-subcreted metamorphic sole were being obducted above. Subsequently, the eclogitised basement was returned to contact with the amphibolite-facies cover by extensional detachment eduction, possibly enhanced by subduction channel flow, which may have been caused by slab break-off and extension during subduction polarity flip. Although the basal ophiolite obduction thrust complex and the Fleur-de-Lys-basement subduction-eduction surfaces must have been initially gently-dipping to sub-horizontal, they were folded and broken by thrusts during late Taconian, late Ordovician Salinic-Mayoian, and Acadian shortening.","PeriodicalId":49494,"journal":{"name":"South African Journal of Geology","volume":" ","pages":""},"PeriodicalIF":1.2000,"publicationDate":"2021-04-29","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":"4","resultStr":null,"platform":"Semanticscholar","paperid":null,"PeriodicalName":"South African Journal of Geology","FirstCategoryId":"93","ListUrlMain":"https://doi.org/10.25131/SAJG.124.0029","RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":null,"EPubDate":"","PubModel":"","JCR":"Q2","JCRName":"GEOLOGY","Score":null,"Total":0}
引用次数: 4
Abstract
The narrow, short-lived Taconic-Grampian Orogen occurs along the north-western margin of the Appalachian-Caledonian Belt from, at least, Alabama to Scotland, a result of the collision of a series of early Ordovician oceanic island arcs with the rifted margin of Laurentia. The present distribution of Taconian-Grampian ophiolites is unlikely to represent a single fore-arc from Alabama to Scotland colliding at the same time with the continental margin along its whole length; more likely is that there were several Ordovician arcs with separate ophiolites. The collision suture is at the thrust base of obducted fore-arc ophiolite complexes, and obduction distance was about two hundred kilometres. Footwalls to the ophiolites are, sequentially towards the continent, continental margin rift sediments and volcanics and overlying rise sediments, continental shelf slope carbonates, and sediments of foreland flexural basins. The regionally-flat obduction thrust complex between the ophiolite and the rifted Laurentian margin is the collision suture between arc and continent. A particular problem in drawing tectonic profiles across the Taconic-Grampian Zone is several orogen-parallel major strike-slip faults, both sinistral and dextral, of unknown displacements, which may juxtapose portions of different segments. In western Newfoundland, most of the Grenville basement beneath the Fleur-de-Lys metamorphic complex (Neoproterozoic to early Ordovician meta-sediments) was eclogitised during the Taconic Orogeny and separated by a massive shear zone from the overlying Fleur-de-Lys, which was metamorphosed at the same time but in the amphibolite facies. The shear zone continued either to a distal intracontinental “subduction zone” or to the main, sub-fore-arc, subduction zone beneath which the basement slipped down to depths of up to seventy kilometres at the same time as the ophiolite sheet and its previously-subcreted metamorphic sole were being obducted above. Subsequently, the eclogitised basement was returned to contact with the amphibolite-facies cover by extensional detachment eduction, possibly enhanced by subduction channel flow, which may have been caused by slab break-off and extension during subduction polarity flip. Although the basal ophiolite obduction thrust complex and the Fleur-de-Lys-basement subduction-eduction surfaces must have been initially gently-dipping to sub-horizontal, they were folded and broken by thrusts during late Taconian, late Ordovician Salinic-Mayoian, and Acadian shortening.
狭窄、短命的Taconic Grampian造山带发生在阿巴拉契亚-喀里多尼亚带的西北边缘,至少从阿拉巴马州到苏格兰,这是一系列早奥陶世洋岛弧与劳伦蒂亚裂谷边缘碰撞的结果。Taconian Grampian蛇绿岩的目前分布不太可能代表从阿拉巴马州到苏格兰的单一前弧沿其整个长度同时与大陆边缘碰撞;更有可能的是,有几个奥陶纪弧带独立的蛇绿岩。碰撞缝合线位于逆冲弧前蛇绿岩杂岩的逆冲基底,逆冲距离约200公里。蛇绿岩的下盘依次朝向大陆、大陆边缘裂谷沉积物和火山岩以及上覆隆起沉积物、大陆架斜坡碳酸盐岩和前陆弯曲盆地的沉积物。蛇绿岩与断陷劳伦期边缘之间的区域性平坦的倒冲-逆冲杂岩是弧与大陆的碰撞缝合线。绘制Taconic Grampian带构造剖面的一个特殊问题是,几个造山带平行的主要走滑断层,包括左旋和右旋断层,具有未知的位移,这些断层可能将不同区段的部分并置。在纽芬兰西部,Fleur de Lys变质杂岩(新元古代至早奥陶世变质沉积物)下方的大部分Grenville基底在Taconic造山运动期间被榴辉岩化,并被与上覆Fleur de Lys的巨大剪切带分隔开,Fleurr de Lys在同一时间变质,但处于角闪岩相。剪切带继续延伸至远端陆内“俯冲带”或主要的亚弧前俯冲带,在该俯冲带之下,基底滑至70公里深处,同时蛇绿岩片及其先前的亚网状变质基底在上方被挤压。随后,榴辉岩基底通过伸展脱离-喷出作用与角闪岩相覆盖层重新接触,这可能是由于俯冲极性翻转过程中板块断裂和伸展引起的俯冲通道流而增强的。尽管基底蛇绿岩冲断-逆冲杂岩和Fleur de Lys基底俯冲-抛射表面最初一定是平缓地倾斜到亚水平的,但它们在晚塔科尼阶、晚奥陶世Salinic Mayoian和阿卡迪亚缩短期间被逆冲褶皱和破碎。
期刊介绍:
The South African Journal of Geology publishes scientific papers, notes, stratigraphic descriptions and discussions in the broadly defined fields of geoscience that are related directly or indirectly to the geology of Africa. Contributions relevant to former supercontinental entities such as Gondwana and Rodinia are also welcome as are topical studies on any geoscience-related discipline. Review papers are welcome as long as they represent original, new syntheses. Special issues are also encouraged but terms for these must be negotiated with the Editors.