Ch 11. PreCambrian tectonics

Archean crustal architecture

Let's return to the Pilbara. Eastern Pilbara is the "type locality" as it were for the dome-and-keel, greenstone-TTG terranes found in Archean cratons. It should be noted that this type of crustal architecture is not known from rocks younger than about 2.3 Ga, at least over large areas. In the Western Pilbara, the crustal architecture is distinctively different, as seen in this geologic map from Smithies et al. 2007. The Eastern Pilbara is about 3.5-3.25 Gyo, and the Western is about 3.15. The authors interpret this transition to record the emergence of modern subduction between 3.25-3.15 Ga.





Our own Superior Province has some distinctive similarities to the Pilbara model, but also some significant differences. (Map from Bedard et al. 2013, their redrawing)





Herzberg (2013) summarized the results of some numerical models investigating viscous flow patterns in a hotter lithosphere-asthenosphere system, the outcome of which is that perhaps mafic lower crustal layers flow/delaminate downward, rather than subducting in today's sense. A key difference would be the metamorphic/hydration history of the mafic "drip" vs. "slab"





Depending on your opinion of mantle temperature structure in the Archean, the P/T path of the descending mafic material might be different between these two models. This figure from Collins et al. (2004) shows some P/T paths of 1.8 Ga (Proterozoic) eclogites, and I have very gracefully added some geothermal gradients in rainbow colors. the steep CCW, isothermal exhumation paths show that peak metamorphic conditions for these rocks are consistent with geothermal gradients of something like 12-17C/km to > 50 km depth, staying below 800C. This temperature condition is characteristic of subduction. However, the mechanism of bringing these rocks back to the surface is very poorly understood in today's tectonics, making it difficult to compare to the past.





Bedard et al. (2013) proposed a model trying to avoid any semblance of subduction, but explaining horizontal compression preserved in crustal structures





Bedard et al.'s model seems to harken back to Wegener's original proposal - an "ice breaker" continent with no reasonable driving force:





Meanwhile, other workers find subduction-like attributes in the Archean system, such as the metamorphic P-T paths by Moyen et al. (2006) in the Barberton greenstone belt, South Africa. Note the stereonets!!




Moyen et al studied the metamorphic history of the sediments in a greenstone belt (3.5-3.6 G) in South Africa's Kaapvaal craton. The shapes of the P/T paths are indicative of rapid increase in depth, followed by fast isothermal decompression, then warming during slower exhumation.

De Wit (1997 and all his papers before and since then) argues that even the oldest Archean terranes can be easily explained using the same mechanisms as modern plate tectonics, if one allows for long term changes in the temperature/composition of the mantle as it evolves with recycling. One must just allow the relative importance of different processes (e.g. crust production vs. subduction; composition of subducting materials) to vary in time. He points out that the fundamental general shape of Archean cratons could be formed by more than one process:




GENERAL WARNING: The number of editorial opinion papers on Archean subduction meets or exceeds the number of significant contributions of new data.

What about OBSERVATIONS addressing the question of Archean Subduction?

Rocks associated with Phanerozoic subduction: Characteristics of the earth which result from subduction: