Crust-mantle coupling during the assembly and amalgamation of Pangea

盘古大陆组装合并过程中的壳幔耦合

• 批准号: RGPIN-2015-06385
• 负责人: Murphy, JamesBrendan
• 金额: $ 2.4万
• 依托单位: St. Francis Xavier University
• 依托单位国家: 加拿大
• 项目类别: Discovery Grants Program - Individual
• 财政年份: 2019
• 资助国家: 加拿大
• 起止时间: 2019-01-01 至 2020-12-31
• 项目状态: 已结题
• 来源: https://www.nserc-crsng.gc.ca/ase-oro/Details-Detailles_eng.asp?id=684618
• 关键词: Crust; mantle; coupling; during; assembly

The repeated assembly and dispersal of supercontinents have profoundly influenced the Earth's evolution. Pangea, the latest of these supercontinents, formed about 300 million years ago (300 Ma). There is consensus that "top down" tectonic processes are probably responsible for supercontinent amalgamation. However, if top-down processes are applied to a 500 Ma paleogeography, they fail to yield Pangea in the correct configuration. As a corollary, we have no viable model for the coupling between processes in the Earth's crust and its mantle, or for understanding long-term global change. ******Geodynamic and geological models are largely aimed at explaining the crustal record and the role of the mantle lithosphere is not adequately constrained. The oceanic lithosphere is largely consumed by subduction (and so we must use proxies to deduce its evolution) and the continental lithospheric mantle (CLM) is rarely exposed. The main objective of this proposal is to use the preserved geological record to better understand the evolution of the mantle lithosphere during the assembly and amalgamation of Pangea.******To achieve these objectives, I will focus on mantle processes that accompanied two time intervals pivotal to the amalgamation of Pangea: (i) the early evolution of the Rheic Ocean (490-440 Ma), a time interval accompanied by global-scale changes in mantle dynamics, and (ii) the demise of the Rheic Ocean in the Late Paleozoic which resulted in the continental collisions that produced the Appalachian-Variscan orogenic belt in the interior of Pangea. ******The early evolution of the Rheic Ocean coincided with global proxies for ocean dynamics including the highest global sea level and the highest 87Sr/86Sr concentration in ocean water in the Phanerozoic. The hypothesis I want to test is whether the 490-440 Ma evolution of the Rheic Ocean coincided with a mantle superplume event that fundamentally affected Paleozoic plate motions, setting the stage for Pangea to form in the Late Palaeozoic. ******For Rheic Ocean closure, I will deduce the extent of crust-mantle coupling by determining the isotopic (Lu-Hf, Sm-Nd) signature of rocks over a range of time (before, during, after collisional orogeny) so that mantle replacement events (MREs), which produce more juvenile isotopic compositions in mantle-derived magmas, can be deduced. These analyses, combined with detailed field work by myself and my students will document (i) the CLM evolution and timing of MREs relative to stratigraphic, structural, metamorphic and magmatic responses in the crust, and (ii) the potentially changing relationships between the CLM and overlying crust before, during and after collisional orogenesis. The results of this research will provide greater insight into the role of the oceanic and continental lithospheric mantle in the formation of Pangea, and supercontinents in general.*** **

超级大陆的反复组装和分散深刻地影响了地球的演化。盘古大陆是这些超级大陆中最新的一个，形成于大约3亿年前（300 Ma）。有一个共识，“自上而下”的构造过程可能是负责超大陆合并。然而，如果将自上而下的过程应用于500 Ma的古地理，它们无法产生正确配置的盘古大陆。作为一个必然的结果，我们没有可行的模型来解释地壳和地幔过程之间的耦合，或者理解长期的全球变化。 ** 地球动力学和地质模型主要是为了解释地壳记录，地幔岩石圈的作用没有得到充分的限制。大洋岩石圈在很大程度上被俯冲作用所消耗（因此我们必须使用代用指标来推断其演化），而大陆岩石圈地幔（CLM）很少暴露。 这项建议的主要目的是利用保存下来的地质记录，更好地了解在盘古大陆的组装和合并过程中地幔岩石圈的演化。为了实现这些目标，我将重点关注地幔过程，这一过程伴随着两个时间间隔，这两个时间间隔对盘古大陆的合并至关重要：（一）里克洋的早期演化（490-440 Ma），一个伴随着全球尺度地幔动力学变化的时间间隔，以及（ii）晚古生代莱茵洋的消亡导致了大陆碰撞，产生了阿巴拉契亚山脉-泛大陆内部华力西造山带。 ** 里克洋的早期演化与全球海洋动力学的代用指标相吻合，包括全球最高海平面和中生代海水中最高的87 Sr/86 Sr浓度。我想验证的假设是，490-440 Ma的莱茵洋演化是否与地幔超级柱事件相吻合，该事件从根本上影响了古生代板块运动，为盘古大陆在晚古生代形成奠定了基础。 ** 对于Rheic Ocean closure，我将通过确定岩石在一段时间内（碰撞造山运动之前、期间、之后）的同位素（Lu-Hf、Sm-Nd）特征来推断壳幔耦合的程度，从而可以推断出在幔源岩浆中产生更多年轻同位素组成的地幔置换事件（MREs）。这些分析，结合详细的实地工作，由我和我的学生将文件（i）CLM的演变和时间的MRE相对于地层，结构，变质和岩浆反应在地壳中，和（ii）潜在的变化关系之间的CLM和上覆地壳之前，期间和之后碰撞造山。这项研究的结果将使人们更深入地了解海洋和大陆岩石圈地幔在盘古大陆和一般超大陆形成中的作用。* **