Seminar

Knowing how and when did plate tectonics began on Earth is essential for understanding the evolution of terrestrial planets. However, early tectonic systems and the timing/controlling factors of initiation of plate tectonics remain controversial. To investigate the viability of representative early Earth tectonic mode (i.e., plate tectonics and a variety of single-plate stagnant-lid modes), I compare new and established model predictions with well-preserved early Earth records hosted in Archean detrital zircons, rocks from the Eoarchean Isua supracrustal belt (SW Greenland) and rocks of the Paleoarchean East Pilbara Terrane (NW Australia). Detrital zircons from all >3.4 Ga ago global Archean strata have small age differences between crystallization and deposition, consistent with predictions of volcanic-dominated stagnant-lid modes and plate tectonics. The diversity and average values for such age differences largely increased over the ca 3.4–3.2 Ga ago period, such that <3.25 Ga ago records can only be explained by plate tectonics. The pre-3.5 Ga macro- and microstructures of Isua supracrustal rocks show spatially-randomly distributed two opposing shear senses as well as distributed deformation without evidence for 10-m scale discrete shear zones. These structures are inconsistent with proposed plate tectonic models, but are compatible with a “heat-pipe” style stagnant-lid model which predicts development of 0.1-m to km-scale a-type folds during a <3.66 Ga ago contraction event. This event would record either heat-pipe cooling or plate-breaking. Finally, minerology, geochemistry and phase equilibria results from ≥3.7 Ga ago ultramafic rocks of the Isua supracrustal belt show that these rocks are broadly similar to ultramafic cumulates of the East Pilbara Terrane, but are dissimilar to tectonically-emplaced mantle peridotites. Taken together, these findings support initiation of plate tectonics at ~3.66 Ga ago or at 3.25 Ga ago, or that both of these intervals represent episodic lid overturn events of an otherwise stagnant-lid regime.

Additional information: Mr. Jiawei ZUO, jwzuo@connect.hku.hk