Estimation of Secular Change in the Size of Continents for Understanding Early Crustal Development

Hikaru Sawada

doi:10.3389/feart.2020.541094

Abstract

The size of continents is an essential parameter to understand the growth of the continental crust and the evolution of the solid Earth because it is subject to tectonism and mantle convection and affects the preservation of the crust. This article reviews the secular change in the size of continents on the early Earth, focusing on terrigenous clastic rocks, especially quartzose sandstones occurring on relatively large continents. The earliest continental crust in the Hadean or early Archean was produced with a width of ∼200–500 km, similar to modern oceanic island arcs along subduction zones or oceanic islands in hot spot regions by mantle plume heating. Through the collision and amalgamation of such primitive continental crusts, continental blocks over 500 km in width and length evolved and appeared by ca. 3.5 Ga. Through further amalgamation, during ca. 3.3–2.5 Ga, the Archean continents emerged with widths and lengths greater than 1,000 km, which were still smaller than those of modern continents. Continents with widths and lengths of nearly 10,000 km have existed since ca. 2.4 Ga (early Proterozoic). Further analyses of the composition and formation mechanism of clastic rocks will help reveal more quantitative secular changes in the sizes of continents.

Highlights

The bimodal topography, land masses consisting of several major continents with islands of various sizes and an ocean with an average depth of nearly 4,000 m, is unique to the Earth among other planets of our Solar System
This paper reviewed the crustal development in the early Earth and the current understanding of the size of continents over time
The size of continents is emphasized as an essential parameter to determine the geotectonic evolution of the continental crust

Summary

INTRODUCTION

The bimodal topography, land masses consisting of several major continents with islands of various sizes and an ocean with an average depth of nearly 4,000 m, is unique to the Earth among other planets of our Solar System. Recent numerical simulations of mantle convection combined with crustal production processes suggested a slightly more complex model of “lid-plume tectonics,” wherein the intermittent sagging and dropping of the mafic crust into the mantle is assumed, known as sagduction (Sizova et al, 2010; Thébaut and Rey, 2013) They considered that Archean felsic continental crusts were generated through the melting of the mafic crust sagging into the mantle. In Proterozoic and Phanerozoic continents of class 3 or 4, several kilometers thick quartzose sandstone strata deposited on continental rift basins and passive continental margins over nearly 100 million years as the continents were large enough to exist stably for a long period of time Based on these considerations, here we focus on the quartzite sandstone for the estimation of the size of continents in conjunction with lithology of the sedimentary sequences. Deposition of these thick sedimentary units is considered to be due to stable large continents during the Proterozoic era

2.4–1.8 Ga Early Proterozoic Sandstone and the Size of Continents

SUMMARY