45-6 Insights into Archean Tectonics and a Vaalbara Supercraton Through the Perspective of Spherule Layers

Session: Surface Processes Across the Solar System (Posters)

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This study uses petrographic imaging methods and considers a sequence of four impact events that resulted in distinct, correlated impact spherule layers in both the Pilbara Craton of Western Australia (WA) and the Kaapvaal Craton of South Africa (SA). These deposits range in age from 3.47-2.49 Ga. In the Pilbara the spherule layers considered are S1 (3.47 Ga), Jeerinah (2.63 Ga), Paraburdoo (2.57 Ga), and Dales Gorge (2.49 Ga). The equivalent Kaapvaal layers are the S1 (3.47 Ga), Monteville (2.63 Ga), Reivilo (2.57 Ga), and Kuruman (2.49 Ga). Representative samples from each deposit were imaged, and diameter measurements were taken for the constituent spherules, to evaluate temporal and spatial changes in the size distributions of these particles and ultimately to inform interpretations regarding the relative distance between the cratons and the history of tectonic spreading between them. It is found that the average difference in spherule diameter between correlated layers increased through time (3.47 Ga event: 26 µm, 2.63 Ga event: 66 µm, 2.57 Ga event: 123 µm, 2.49 Ga event: 207 µm) with significance. Size distribution graphs show a similar trend, in that the distributions themselves become increasingly different statistically. A two-sample t-test and Kolmogorov-Smirnov test were used to evaluate the significance of the difference in means and in overall distributions, respectively. Other potential influences on spherule diameter, such as petrologic type, depositional environment, bed thickness, and impactor size, are ruled out as controlling this difference in spherule diameter. These observed changes through time thus implicate a corresponding change in distance from the impact site, and using this as a proxy for relative distance suggests that the Pilbara and Kaapvaal Cratons were progressively spreading from 3.47-2.49 Ga. This work supports the idea that the Pilbara and Kaapvaal Cratons were oriented as a “Vaalbara” supercontinent during the Paleoarchean, but had spread into individual landmasses by the Neoarchean, and has implications for the state of plate tectonic activity during this phase of Earth’s history.

Geological Society of America Abstracts with Programs. Vol. 58, No. 2, 2026

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