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: $ 2.19万
• 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: 2016
• 资助国家: 加拿大
• 起止时间: 2016-01-01 至 2017-12-31
• 项目状态: 已结题
• 来源: https://www.nserc-crsng.gc.ca/ase-oro/Details-Detailles_eng.asp?id=612285
• 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.

盘古大陆是地球科学中最具辨识度的标志，也是我研究的重点。韦格纳将盘古大陆描述为石炭纪南部的冈瓦纳大陆与北部的劳鲁西亚大陆碰撞的结果，形成了阿巴拉契亚（美洲）-瓦里斯坎（欧洲）造山带。如何解释阿巴拉契亚-瓦里斯坎造山带取决于两个山带特征的重要地图视图弯曲的性质。这些地图上的弯曲是“主要的”古地理特征（海峡和凹地），还是次生的“造山带”——以前是线性的造山带，后来被垂直的旋转轴扭曲了？大多数模型假设弯曲是初级构造，并暗示最后的碰撞之前是一个增生期，在此期间，从冈瓦纳北缘裂出的“近冈瓦纳”大陆带（阿瓦洛尼亚；Meguma；南葡萄牙区）打开了里海，与劳鲁西亚南缘碰撞并增生。此外，初步模式表明：1 -近冈瓦南地体南部受莱西缝合线束缚，北部受劳氏土族束缚；2 - Laurussia的南缘有多条缝合线，分隔了近冈瓦纳地体，而完整冈瓦纳被动地缘的北缘有一条单一的Rheic缝合线；冈瓦纳被动边缘进入月桂下倾的俯冲带导致地壳增厚和造山作用；4 -碰撞后盘古大陆的变形可以用板块内、大陆内的构造环境来解释。我们最近对伊比利亚Variscan造山带的研究支持了另一种模型。古地磁、构造和地层学研究表明，该造山带在305 ~ 295 Ma之间形成了两条耦合的次级造山斜，形成了一个大陆尺度的s褶皱。造山带的原始恢复产生了一个2300公里长的原线状北东向造山带，并表明以超过10厘米/年的平移速率，需要1100公里的缩短才能形成造山带。在我们的新造山带模型中，造山带的斜向屈曲解释了造山带的地图几何形状：1 -冈瓦南周边地体（南葡萄牙带）位于Rheic缝合线以东；冈瓦纳被动边缘序列不是由单一的Rheic缝合线束缚，而是形成了一条大陆带，东部由Rheic缝合线束缚，西部由一条以伊比利亚西北部蛇绿岩为代表的独立缝合线束缚；3 -瓦利斯坎造山作用是冈瓦南带大陆向西俯冲的产物，在大洋弧形之下，而不是在劳氏之下；4 -所谓的碰撞后盘古大陆内的造山口，以每年100 - 10厘米的速度进行了1100公里的平移，这需要俯冲作用，因此在盘古大陆的中心存在海洋岩石圈，直到二叠纪。一个由地质、古地磁、地质年代学和地球化学研究组成的项目集中在瓦里斯坎造山带的选定样带上，将测试这些盘古大陆构造的对比模型。预期的结果包括提高对地球古地理的理解；更好地约束地球系统演化（包括灭绝事件）与超大陆旋回之间的关系；并增加外联机会，包括公开和学校演讲。