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: 2018
• 资助国家: 加拿大
• 起止时间: 2018-01-01 至 2019-12-31
• 项目状态: 已结题
• 来源: https://www.nserc-crsng.gc.ca/ase-oro/Details-Detailles_eng.asp?id=654775
• 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知名的标志，也是我研究的重点。韦格纳将盘古大陆描述为南部冈瓦纳大陆与北部劳俄罗斯石炭纪大陆碰撞的回应，形成了阿巴拉契亚山脉（在美洲）-瓦里斯坎（在欧洲）造山带。 人们如何解释阿巴拉契亚-瓦里斯造山带取决于这两个山脉特征的重要地图视图弯曲的性质。 这些地图视图弯曲是“主要”古地理特征（海角和凹陷），还是次要“山岳”——以前是线性造山带，后来绕垂直旋转轴弯曲？大多数模型假设这些弯曲是主要构造，并暗示最终碰撞之前有一个增生期，在此期间，“近冈瓦纳”大陆带（阿瓦隆尼亚、梅古马、南葡萄牙区）从冈瓦纳北缘裂开，打开了莱克洋与劳俄罗斯南缘碰撞并吸积。此外，主要模型表明： 1 - 近冈瓦纳地体仅限于以南部的瑞克缝合带和北部的劳俄罗斯本土为界的地带； 2 - 多条缝合线是劳俄罗斯南缘的特征，在那里它们分隔了近冈瓦纳地体，而一条单一的瑞克缝合线界定了完整的冈瓦纳被动边缘的北缘； 3 - 冈瓦纳大陆被动边缘进入俯冲到劳俄罗斯下方的俯冲带导致地壳增厚和造山作用； 4 - 盘古大陆的碰撞后变形可以用板内、大陆内的构造环境来解释。对替代模型的支持来自我们最近对伊比利亚瓦里坎造山带的研究。 古地磁、结构和地层研究表明，造山带的特征是形成于 305 Ma 至 295 Ma 之间的两个耦合的次生山斜，并定义了大陆规模的 S 褶皱。 山岳的回弹性恢复产生了一条 2300 公里长的先前线性 NNE 走向的造山带，并表明需要以超过 10 厘米/年的平移速度缩短 1100 公里才能产生山岳。在我们的新造山带模型中，口斜屈曲解释了造山带的地图视图几何形状： 1 - 近冈瓦南地体（南葡萄牙带）位于瑞克缝合线以东； 2 - 冈瓦纳大陆被动边缘层序不是受单一的瑞克缝合线束缚，而是形成了一条大陆带，其东边以瑞克缝合线为界，西边则以伊比利亚西北部的蛇绿岩为代表的一条单独的缝合线所束缚； 3 - 瓦里西亚造山作用是冈瓦纳带状大陆向西俯冲到洋弧下方而不是劳俄罗斯下方的产物； 4 - 所谓的碰撞后盘古大陆内山斜形成，涉及以 >10 cm/a 的速度平移 1100 公里，需要俯冲作用，因此盘古大陆中心存在海洋岩石圈并深入到二叠纪。一项地质、古地磁、地质年代学和地球化学研究计划，重点沿着瓦里斯造山带的选定横断面，将测试盘古大陆构造的这些对比模型。预期成果包括加深对地球古地理的了解；更好地约束地球系统演化（包括灭绝事件）与超大陆循环之间的关系；以及增加外展机会，包括公共和学校演讲。