June 2024 - Volume 20, Number 3

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Cratons to Continents

Carol D. Frost and Paul A. Mueller – Guest Editors

Overview

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2024 Topics

Overview

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2024 Topics

Thematic Articles

Archean Cratons Time Capsules of the Early Earth

Studies of Archean cratons, and the rocks and minerals they contain, help us understand the processes that occurred on the early Earth, our place in the Solar System, and how the planet we live on today came to be. The articles in this issue examine different aspects of early Earth evolution from multiple perspectives relying on both theory and observation. We hope they will encourage you to investigate further this most fascinating time in Earth history. Here we introduce the basic characteristics of cratons, the challenges of inferring Earth evolution from the sparse Archean rock record, the concept of cratonic clans, the development of supercratons, and, by the end of the Archean, continents, supercontinents, and plate tectonics.

Earth’s Earliest Crust

By Jonathan O’Neil, Hanika Rizo, Jesse Reimink, Marion Garçon, Richard W. Carlson

The scarcity of rocks preserved from the first billion years (Gy) of Earth’s history hinders our ability to study the nature of the earliest crust. Rare >4.0-Gy-old zircons confirm that felsic crust was present within 500 million years of Earth’s formation. Given that most of that ancient crust has been destroyed, geochemical and isotopic tracers applied to rocks from the oldest sections of continents can be used to provide insights into the nature of the predecessor crust. Evidence from Earth’s oldest rocks and minerals suggests multiple early mantle depletion episodes, possibly linked to the formation of an initial, dominantly mafic, crust. This early crust was the precursor to evolved rocks that now constitute considerable portions of Earth’s oldest surviving crust.

At the Dawn of Continents: Archean Tonalite-Trondhjemite- Granodiorite Suites

By Oscar Laurent, Martin Guitreau, Emilie Bruand, Jean-François Moyen

Archean rocks of the tonalite-trondhjemite-granodiorite (TTG) suite are dominant constituents of Earth’s earliest preserved silicic crust, while conversely rare in Phanerozoic continental crust. Their formation represents the first critical step towards the construction and preservation of continents. Formation of most TTG magmas involved partial melting of hydrous, probably silicified, mafic rocks at various depths (20–50 km, possibly up to 100 km). Many possible tectonic scenarios fit the petrological and geochemical constraints on TTG formation, whether compatible with a global plate tectonic- like regime or not. Refining such scenarios is a major challenge that requires systematically integrating the constraints on TTG formation—relying especially on accessory minerals as key petrogenetic tools—with the geological context on a regional scale.

Archean Geodynamics Underneath Weak, Flat, and Flooded Continents

By Patrice Rey, Nicolas Coltice, Nicolas Flament

Although a significant volume of crust was extracted from the mantle early in Earth’s history, the contribution of felsic rocks to the sedimentary record was minimal until ~3.0 Ga. On a hotter Earth, this conundrum dissipates if we consider that the felsic crust was buried under thick basaltic covers, continents were flooded by a near-global ocean, and the crust was too weak to sustain high mountains, making it largely unavailable to erosion. Gravitational forces destabilized basaltic covers within these weak, flat, and flooded continents, driving intra-crustal tectonics and forcing episodic subduction at the edges of continents. Through secular cooling, this dual-mode geodynamics progressively transitioned to plate tectonics.

Embracing Craton Complexity at Depth

By Catherine M. Cooper, Meghan S. Miller

Variations within individual cratons, as well as across different cratons, are readily apparent at the Earth’s surface, providing indirect insight into the processes governing the formation and evolution of the under- lying regions. However, our views at depth are more limited. As such, there is a risk of interpreting the cratonic lithosphere as a monolith. Recent modeling and advances in seismological imaging have enhanced our perspective of vertical variations within the cratonic lithosphere, which has helped build a general conceptual model. While lateral variations also are increasingly identified, their significance still presents unanswered questions. In this review, we summarize the current state of knowledge of cratonic lithospheric structure and demonstrate the importance of lateral heterogeneity in craton evolution and stability.