Nappe tectonics at the end of the Archaean

The beginning of modern-style plate tectonics is still debated in the absence of definite evidence. Because Earth’s mantle generates heat through radioactive decay and still contains heat left over from planetary accretion and core formation it must always have maintained some kind of heat transfer through some kind of circulatory motion involving the mantle and lithosphere. That must always too have involved partial melting and chemical differentiation that created materials whose density was lower than that of the mantle; e.g. continental crust. Since continental materials date back to more than 4 billion years ago and some may have been generated earlier in the Hadean, only to be lrgely resorbed, a generalised circulation and chemical differentiation have been Earth’s main characteristics from the start. One view is that early circulation was a form of vertical tectonics without subduction via a sort of ‘dripping’ or delamination of particularly dense crustal materials back into the mantle. A sophisticated model of how the hotter early Earth worked in this way has been called ‘lid tectonics’, from which plate tectonics evolved as the Earth cooled and developed a thicker, more rigid lithosphere. Such an outer layer would be capable of self-generating the slab pull that largely drives lateral motions of lithospheric plates. That process occurs once a slab of oceanic lithosphere becomes cool and dense enough to be subducted (see: How does subduction start?; August 2018).

The most convincing evidence for early plate tectonics would therefore be tangible signs of both subduction and large horizontal movements of lithospheric plates: common enough in the Neoproterozoic and Phanerozoic records, but not glaringly obvious in the earlier Archaean Eon. These unequivocal hallmarks have now emerged from studies of Archaean rocks in the Precambrian basement that underpins northern China and North Korea. The North China Craton has two main Archaean components: an Eastern Block of gneisses dated between 3.8 and 3.0 Ga and a Western Block of younger (2.6 to 2.5 Ga) gneisses, metavolcanics and metasediments. They are separated by a zone of high deformation. A key area for understanding the nature of the deformed Central Orogenic Belt is the Zanhuan Complex near the city of Kingtai (Zhong, YL. et al. 2021. Alpine-style nappes thrust over ancient North China continental margin demonstrate large Archean horizontal plate motions. Nature  Communications, v. 12, article6172, DOI: 10.1038/s41467-021-26474-7).

Schematic cross sections through the Zanhuan Complex of northern China, showing early and final development of the Central Orogenic Belt in the North China Block . (Credit: Zhong, YL. et al.;Figs 10b and c)

This small, complex area reveals that the older Eastern Block is unconformably overlain by Neoarchaean sediments, above which has been thrust a stacked series of nappes similar in size and form to those of the much younger Alpine orogenic belt of southern Europe. Though highly complex, the rocks involved having been folded and stretched by ductile processes, they are still recognisable as having originally been at the surface. Metavolcanics in the nappes can be assigned from their geochemistry to a late-Archaean fore-arc, through comparison with that of modern igneous rocks formed at such a setting in the Western Pacific. Thrust over the nappe complex is a jumble or mélange of highly deformed metasediments containing blocks of metabasalts and occasional ultramafic igneous rocks that geochemically resemble oceanic crust formed at a mid-ocean ridge. Some of them contain high-pressure minerals formed at depth in the mantle, indicating that they had once been subducted. The whole complex is cut by undeformed dykes of granitic composition dated at 2.5 Ga, confirming that the older rocks and the structures within them are Archaean in age. Thrust over the melange and tectonically underlying nappe complex are less-deformed volcanic rocks and granitic intrusions that closely resemble what is generally found in modern island arcs.

Orogenic belts bear witness to enormous crustal shortening caused by horizontal compressive forces. Assuming the average rate of modern subduction (2 cm yr-1) the 178 Ma history of the Zanhuan Complex implies more than 3,500 km of lateral transport. 2.5 billion years ago, higher radioactive heat production in the mantle would have made tectonic overturning considerably faster  The unconformity at the base of the complex suggests that it was driven over the equivalent of a modern passive, continental margin. So the complex provides direct evidence of horizontal plate tectonics and associated subduction during the latter stages of the Archaean that ranks in scale with that of many Phanerozoic orogenic belts, such as that of the European Alps. The Zanhuan Complex is a result of arc accretion that played a major role in many later orogens. The North China craton itself is reminiscent of continent-continent collision, as required in the formation of supercontinents.

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