Contrasting styles of orogenesis and the amalgamation of Pangea

在过去的25亿年里，超大陆反复合并和扩散的循环对地球的演化产生了深远的影响。这些超级大陆中最晚的一块盘古大陆形成于约3亿年前的晚古生代(300 Ma)。尽管盘古大陆的存在是板块构造的基石，但导致其融合的机制却鲜为人知。如果将目前的理论地球动力学模型应用于早古生代古地理，它们不能以传统接受的形状重建盘古。作为推论，我们没有可行的模型来解释地球表面过程和内部过程之间的耦合，也没有可行的模型来理解长期全球变化的自然速度，而全球变化受到构造过程的深刻影响。这一建议研究了两个主要古生代大洋的演化：(1)“内陆”流变洋，其在晚古生代的闭合产生了盘古大陆内部大陆碰撞形成的山带(或造山带)(例如阿巴拉契亚-瓦里斯坎造山带)和(2)“外部”古太平洋，它通过沿盘古大陆外缘的俯冲和吸积形成山带(例如南陆造山带，TAO)。我、我的学生和同事将研究盘古大陆发展过程中的两个关键时间段：(I)480-400 Ma，当时瑞克洋内的俯冲带开始形成，最终形成盘古大陆；(Ii)360-290 Ma的盘古合并，我们将研究大陆碰撞期间地幔的演化和华力西造山带的形成。我们还将调查在这两个时间间隔内发生的两种根本不同类型的造山活动之间潜在的地球动力学联系。通过解决这些重大构造事件的机制和时间上的不确定性，我们将能够评估在盘古大陆组装期间在内陆发生的造山事件与沿其外围发生的造山事件之间的潜在地球动力学联系。我计划调查两个重要的区域，在那里Rheic海洋地质学的关键关系被很好地暴露出来：(I)伊比利亚，其最初的裂谷和最终闭合(缝合)的证据被保存得很好，以及(Ii)英国的康沃尔，它的地质位于一个主要的缝合带附近，是了解瓦里斯坎造山带构造演化的关键。沿着盘古大陆的外围，我将重点介绍墨西哥南部的古生代和早中生代杂岩，以及澳大利亚东部的TAO的塔斯马尼德褶皱带。墨西哥南部的地质学提供了一个罕见的古太平洋岩石圈俯冲的例子，该岩石圈叠加在碰撞缝合带上，并提供了研究从盘古大陆形成到盘古大陆解体开始期间不断变化的地幔和地壳源岩的机会。塔斯马尼德褶皱带在480-400 Ma之间有很好的裸露岩石，这将使我们能够评估Rheic海洋和古太平洋事件之间潜在的地球动力学联系。我和我的学生将研究这些关键区域的场关系以及构造、地球化学、同位素和岩石学演化，以推断导致盘古大陆组装和融合的构造事件的时间和机制。这项研究还将提供：(1)更好地了解主要古生代海洋演化之间的地球动力学联系；(2)研究导致古老超大陆反复聚集和扩散的过程，这些过程深刻地影响了地壳、大气、气候和生命的演化，以及过去2.5Ga全球气候变化的自然速度。

Repeated cycles of supercontinent amalgamation and dispersal over the past 2.5 billion years have had a profound effect on Earth’s evolution. The latest of these supercontinents, Pangea, formed in the Late Paleozoic, about 300 million years ago (300 Ma). Although the existence of Pangea is a cornerstone of plate tectonics, the mechanisms responsible for its amalgamation are poorly understood. If current theoretical geodynamic models are applied to an Early Paleozoic paleogeography, they fail to reconstruct Pangea in its conventionally accepted configuration. As a corollary, we have no viable model for the coupling between the Earth’s surface processes and its interior, or for understanding the natural pace of long-term global change, which is profoundly influenced by tectonic processes. This proposal examines the evolution of two major Paleozoic oceans: (i) the “interior” Rheic Ocean whose closure in the Late Paleozoic produced mountain belts (or orogens) formed by continental collisions in the interior of Pangea (e.g. the Appalachian-Variscan orogen) and (ii) the “exterior” Paleopacific Ocean, which produced mountain belts by subduction and accretion along the exterior margins of Pangea (e.g. the Terra Australis Orogen, TAO). I, my students and colleagues will investigate two critical time intervals in Pangea’s development: (i) the 480-400 Ma interval when subduction zones initiated within the Rheic Ocean that eventually gave rise to Pangea, and (ii) the 360-290 Ma Pangea amalgamation where we will investigate the evolution of the mantle during continental collision and the generation of the Variscan orogen. We also will investigate the potential geodynamic linkage between the two fundamentally distinct types of orogenic activities that occurred during these two time intervals. By resolving the uncertainties in the mechanisms and timing of these major tectonic events, we will be able to evaluate potential geodynamic linkages between orogenic events that took place within the interior and those along the periphery of Pangea during its assembly. I plan to investigate two key regions where critical relationships of Rheic Ocean geology are excellently exposed: (i) Iberia where evidence of its initial rifting and final closure (suturing) are well preserved, and (ii) Cornwall, U.K., whose geology lies adjacent to a major suture zone and is key to understanding tectonic evolution of the Variscan orogen. Along the periphery of Pangea, I will focus on Paleozoic and Early Mesozoic complexes in southern Mexico and the Tasmanide fold belt of the TAO in eastern Australia. The geology of southern Mexico provides a rare example of the subduction of Paleopacific lithosphere that was superimposed on a collisional suture zone and the opportunity to investigate the changing mantle and crustal source rocks from the time Pangea formed to the initiation of Pangea breakup. The Tasmanide fold belt has excellently exposed rocks in the 480-400 Ma interval that will allow us to assess potential geodynamic connections between events in the Rheic and Paleopacific oceans. I and my students will study the field relationships and the structural, geochemical, isotopic and petrologic evolution of these key regions in order to deduce the timing and mechanisms of tectonic events that led to the assembly and amalgamation of Pangea. This research also will provide (i) a greater understanding of the geodynamic linkages between the evolution of major Paleozoic oceans, and (ii) a gateway to examine processes responsible for the repeated assembly and dispersal of older supercontinents, which have profoundly influenced the evolution of the Earth’s crust, atmosphere, climate, and life as well as the natural pace of global climate change for the last 2.5 Ga.