Disturbed metamorphic gradient indicates major plate tectonic thrust faulting 2.5 billion years ago.

What is it about?

Understanding the Archean mountain building is essential for exploring early Earth's tectonics, but limited knowledge about the structure and heat distribution in these zones hinders progress. Here we report a sharp inverted metamorphic gradient across an Archean suture zone in the North China Craton. The upper plate of the suture experienced temperatures of 650–700 ºC and pressures of 7–11 kbar, while the lower plate had temperatures of 500–600 ºC and pressures of 4–6 kbar, with slight increases near the front of the orogen. Notably, we show that the conditions drop sharply from 11 kbar/700 ºC to 4 kbar/500 ºC over just 1 km, representing a depth difference of almost 20 km across the main thrust fault. The pressure-temperature (P-T) paths of the upper plate show decompression, while those from the lower plate show increasing temperature. We suggest that this inverted gradient formed when thrust sheets were emplaced during an Archean arc-continent collision, with horizontal transport of hundreds of kilometers about 2.5 billion years ago. Peclet number analysis shows that this gradient formed at a thrusting rate of about 30 mm/year, similar to modern orogenic belts. This discovery enhances our understanding of Archean tectonics and reveals similarities in convergence rates and thermal structures between Archean and modern mountain-building processes.

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Why is it important?

This work presents the first well-documented inverted metamorphic gradient in an Archean (>2.5 billion years old) mountain belt. Younger inverted metamorphic gradients, such as those in the Alps and Himalaya, are associated with major plate tectonic thrust faults formed during plate collisions, and thrusting of a hot thrust sheet over a cold basement. This case shows that plate tectonic processes, with similar rates and scales to the present day, were operating 2.5 billion years ago.