Bending a geological icon: Oroclines, Pangea, and the supercontinent cycle

弯曲的地质标志：Oroclines、Pangea 和超大陆旋回

• 批准号: RGPIN-2014-06533
• 负责人: Johnston, Stephen
• 金额: $ 0.17万
• 依托单位: University of Alberta
• 依托单位国家: 加拿大
• 项目类别: Discovery Grants Program - Individual
• 财政年份: 2015
• 资助国家: 加拿大
• 起止时间: 2015-01-01 至 2016-12-31
• 项目状态: 已结题
• 来源: https://www.nserc-crsng.gc.ca/ase-oro/Details-Detailles_eng.asp?id=589072
• 关键词: Bending; geological; icon; Oroclines; Pangea

Pangea, the most recognizable of Earth Science icons, is the focus of my research. Wegener depicted Pangea as having formed in response to the Carboniferous continental collision of Gondwana in the south with Laurussia in the north giving rise to the Appalachian (in America) – Variscan (in Europe) Orogen. How one interprets the Appalachian - Variscan Orogen is dependent upon the nature of significant map-view bends that characterize both mountain belts. Are these map-view bends 'primary' paleogeographic features (promontories and recesses), or are they secondary 'oroclines' - formerly linear orogens that have been subsequently buckled about vertical axes of rotation? Most models assume the bends are primary structures, and imply that the final collision was preceded by an accretionary period during which 'peri-Gondwanan' continental ribbons (Avalonia; Meguma; South Portuguese Zone) that rifted from the north margin of Gondwana, opening the Rheic Ocean, collided with and accreted to the southern margin of Laurussia. Furthermore, Primary models imply that: 1 - peri-Gondwanan terranes are restricted to a belt bound by the Rheic suture to the south and the Laurussian autochthon to the north; 2 - multiple sutures characterize the southern margin of Laurussia, where they separate the peri-Gondwanan terranes, whereas a single Rheic suture bounds the north margin of the intact Gondwana passive margin; 3 - crustal thickening and orogenesis resulted from entry of the Gondwana passive margin into a subduction zone dipping beneath Laurussia; and 4 - post-collisional deformation of Pangea is explicable in terms of a within-plate, intra-continental tectonic setting. Support for an alternative model comes from our recent research of the Variscan Orogen in Iberia. Paleomagnetic, structural and stratigraphic studies demonstrate that the orogen is characterized by two coupled secondary oroclines that formed between 305 and 295 Ma, and which define a continental scale S-fold. Palinspastic restoration of the oroclines yields a 2300 km long previously linear NNE-striking orogen, and indicate that 1100 km of shortening at translation rates in excess of 10 cm/a was required to produce the oroclines. In our new model of the orogen, in which oroclinal buckling explains the map-view geometry of the orogen: 1 - peri-Gondwanan terranes (South Portuguese zone) lie east of the Rheic suture; 2 - Instead of being bound by a single Rheic suture, the Gondwana passive margin sequence forms a continental ribbon bound to the east by the Rheic suture, and to the west by a separate suture represented by ophiolite in NW Iberia; 3 – Variscan orogenesis is a product of subduction of the Gondwanan ribbon continent to the west beneath an oceanic arc, and not beneath Laurussia; and 4 - so-called post-collisional intra-Pangean orocline formation, which involved 1100 km of translation at rates of >10 cm/a, requires the involvement of subduction and hence the presence of oceanic lithosphere within the heart of Pangea well into the Permian. A program of geological, paleomagnetic, geochronological and geochemical studies focused along select transects across the Variscan orogen will test these contrasting models of Pangea construction. Expected outcomes include an improved understanding of Earth's paleogeography; better constraints on the relationship between Earth System evolution, including extinction events, and the supercontinent cycle; and an augmentation of outreach opportunities including public and school presentations.

盘古是地球科学中最知名的图标，也是我研究的重点。韦格纳将盘古大陆描述为南部的冈瓦那与北部的劳鲁西亚石炭纪大陆碰撞形成的，形成了(美洲的)阿巴拉契亚-(欧洲的)瓦里斯坎造山带。人们如何解释阿巴拉契亚-瓦里斯坎造山带取决于这两个山脉带显著的地图视图弯曲的性质。这些地图图上的弯曲是“主要的”古地理特征(海角和凹陷)，还是次要的“造山带”--以前的线形造山带后来绕着垂直的旋转轴弯曲？大多数模型假设弯曲是主要的构造，并暗示在最终碰撞之前有一个增积期，在此期间，从冈瓦那北缘裂开、打开Rheic海洋、与劳鲁西亚南缘碰撞并吸积到劳鲁西亚南缘的‘周冈山’大陆带(阿瓦洛尼亚、梅古马、南葡萄牙带)。此外，初步模型暗示：1-冈德瓦南周地体局限于南靠Rheic缝合线、北靠劳鲁士地台的带；2-洛鲁西亚南缘具有多条缝合线的特征，在那里它们分隔了Peri-Gondwanan地体，而单一的Rheic缝合线则限定了完整的冈瓦纳岛被动边缘的北缘；3-地壳增厚和造山作用是由于冈瓦纳岛被动边缘进入到洛鲁西亚之下的俯冲带而产生的；4-盘古大陆的碰撞后变形可以从板块内、大陆内的构造背景来解释。对另一种模式的支持来自我们最近对伊比利亚瓦里斯坎造山带的研究。古地磁、构造和地层学研究表明，该造山带具有形成于30 5~2 95 Ma之间的两个相互耦合的次级造山斜带的特征，其大陆尺度为S褶皱。造山斜的古相恢复产生了一条2300公里长的先前线性的NNE向造山带，并表明需要以超过10厘米/年的平移速率缩短1100公里才能产生造山斜。在我们的造山带新模型中，造山斜向屈曲解释了造山带的地图几何形状：1-周冈德瓦南地体(南葡萄牙带)位于Rheic缝合线以东；2-Gonwaana被动边缘序列形成一个大陆带，向东由Rheic缝合线连接，向西由以伊比利亚西北的蛇绿岩为代表的单独缝合线连接；3-Variscan造山作用是冈德瓦南大陆向西俯冲到洋弧之下的产物，而不是在劳西亚之下；和4-所谓的碰撞后盘古内部造山斜建造，涉及1100公里的平移，速度为10厘米/a，需要俯冲的参与，因此在盘古大陆心脏内存在大洋岩石圈进入二叠纪。一项地质、古地磁、地质年代学和地球化学研究计划将重点放在瓦里斯坎造山带上的选定断面上，以检验这些盘古大陆构造的对比模型。预期成果包括：更好地了解地球的古地理；更好地制约包括灭绝事件在内的地球系统演变与超大陆周期之间的关系；扩大宣传机会，包括公共和学校介绍。