Contrasting styles of orogenesis and the amalgamation of Pangea

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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.