Study challenges traditional views on plate tectonics in Earth's early history

Study challenges traditional views on plate tectonics in Earth's early history
by Simon Mansfield
Sydney, Australia (SPX) Jan 10, 2024

The established theory of plate tectonics, a cornerstone of modern geology developed in the 20th century, has been instrumental in explaining various geological phenomena and processes throughout the Phanerozoic era. Yet, when it comes to the interpretation of geological records from the continental interior and the Precambrian period, particularly the Archean, this theory has often faced challenges. A novel integrated study led by Prof. Yong-Fei Zheng at the University of Science and Technology of China is revisiting this narrative, offering a fresh perspective on Archean cratons and their formation.

Traditionally, the theory of plate tectonics has been perceived as somewhat limited in its application to Precambrian geology. This view stems from the conventional approach, which tends to separate continental interior tectonics from continental margin tectonics in terms of their structural and compositional development. Such a dichotomy has inadvertently created a misconception that plate tectonics may not be entirely relevant to understanding the early geological history, particularly in the Archean era.

However, Prof. Zheng's study challenges this notion by successfully applying a 21st-century interpretation of plate tectonics to major geological phenomena observed in Archean cratons. The research takes into account key characteristic features of the Archean Earth: notably high convective mantle temperatures estimated between 1500-1700C, the formation of unusually thick basaltic oceanic crust around 30-40 km, and an asthenosphere composition akin to the primitive mantle, rather than the depleted mantle seen today.

By transitioning the plate tectonics theory from a traditional kinematic model to a more holistic kinematic-dynamic model, Prof. Zheng's work systematically examines the vertical transport of matter and energy at plate margins. This approach has yielded significant insights, demonstrating that plate tectonics can indeed interpret common geological characteristics of Archean cratons such as lithological associations, structural patterns, and metamorphic evolution.

The study goes further by proposing alternative tectonic models for the early Earth. These models envision vertical movements within a stagnant lid tectonic regime, incorporating both bottom-up processes like mantle plumes and heat pipes, and top-down processes such as lithospheric foundering and subduction. Importantly, these vertical processes are not exclusive to the Archean period but have continued into the Phanerozoic era, driven by mantle poloidal convection at varying depths.

Furthermore, the research provides explanations for various Archean geological features. For instance, the formation of tonalite-trondhjemite-granodiorite (TTG) rocks is attributed to the partial melting of the thick basaltic oceanic crust at convergent plate margins. The study also explains the structural patterns of gneissic domes and greenstone keels as a result of the buoyancy-driven emplacement of TTG magmas interacting with the basaltic crust at fossil convergent margins. Additionally, komatiites in greenstone belts are identified as products of mantle plume activity in the ancient plate tectonics regime.

The absence of certain volcanic associations and specific rock types in Archean igneous associations, such as calc-alkaline andesites and Penrose-type ophiolites, is explained in the context of the unique geological conditions of the time. For example, the lack of blueschist and eclogite, as well as classic paired metamorphic belts, suggests that convergent plate margins in the Archean were over-thickened, likely through warm subduction or hard collision of the thick oceanic crust under moderate geothermal gradients.

Prof. Zheng's study not only dispels the illusion that the Archean continental crust did not originate from a regime of plate tectonics but also paves the way for a revolutionary understanding of the formation and evolution of continental crust since the Archean. It underscores the relationships between ancient and modern plate tectonics regimes, contributing significantly to our understanding of the origin and evolution of continental crust on early Earth, and shedding light on the geodynamic mechanisms that led from stagnant lid tectonics to mobile lid tectonics.

In essence, this groundbreaking research by Prof. Zheng and his team opens new doors in the realm of geological science, providing robust arguments and evidence that reinforce and expand our understanding of plate tectonics and its pivotal role in shaping our planet's ancient geological history.

Research Report:Plate tectonics in the Archean: Observations versus interpretations