Early Earth tectonics : evidence from Archean detrital zircon age and rock records of the Isua supracrustal belt and the East Pilbara Terrane

Abstract

Knowing how and when 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. Proposed pre-plate tectonic models include multiple single-plate stagnant-lid settings that lose heat dominantly via volcanic advection (heat-pipe tectonics), crustal convection (partial convective overturn), and/or conduction (cold stagnant-lid). Proposed initiation events range in time from Hadean (>4.3 Ga) to Neoproterozoic (~0.8 Ga), and the duration from 106- to 109-year time scales. This thesis constrains plate tectonic initiation time and viable Archean tectonic models via comparing model predictions (both new and established) with information from well-preserved Archean rocks/minerals, i.e., detrital zircons, and rock records from the Eoarchean Isua supracrustal belt and the Paleoarchean East Pilbara Terrane. This approach allows improved model testing by substantially mitigating problems associated with sparse geological predictions, preservation bias, and interpretative non-uniqueness. Archean detrital zircon age patterns could reflect early Earth tectonics. New detrital zircon age predictions for Archean tectonic models are generated by numerical simulations and data-based inferences. Heat-pipe and partial convective overturn models predict absence of >500-million-year older (relative to depositional ages) detrital zircons, fitting the >3.25 Ga detrital zircon age record well. Plate tectonics predicts diverse detrital zircon age patterns relative to depositional ages, fitting the <3.25 Ga record. Explaining the >3.25 Ga record with plate tectonics is viable, but would require strong preservation bias. The simple explanation is that the Archean detrital zircon record reflects a >3.25 Ga dominance of heat-pipe and/or partial convective overturn tectonics and dominance of plate tectonics by ~3.25 Ga. To investigate the tectonic setting of the Isua supracrustal belt, model predictions are compared with its pre-3.5 Ga structures, including quartz fabrics. Results show two opposing shear indicators (indicating top-to-southeast or top-to-northwest shearing) that are spatially randomly distributed. Strain intensity measurements support distributed deformation without the presence of 10-m scale discrete shear zones. These structures are inconsistent with proposed plate tectonic models, but are compatible with a heat-pipe model for this terrane which predicts development of 0.1-m to km-scale a-type folds during a <3.66 contraction event. This event would record either heat-pipe cooling or plate-breaking. Therefore, structures of the Isua supracrustal belt imply a <3.66 Ga initiation of plate tectonics. Ultramafic rocks from the Isua supracrustal belt have been interpreted as mantle emplaced by subduction. This thesis examines whether cumulate origins without requiring plate tectonics are viable. Minerology, geochemistry and phase equilibria results show that these rocks are broadly similar to ultramafic cumulates of the East Pilbara Terrane, but are dissimilar to tectonically-emplaced mantle peridotites. A viable alternative is that Isua ultramafic rocks may be crustal cumulates, and as such do not require ≥3.7 Ga operation of plate tectonics. Taken together, the Archean detrital zircons and rocks from the Isua supracrustal belt and the East Pilbara Terrane support initiation of plate tectonics at ~3.66 Ga or at 3.25 Ga, or that both of these intervals represent episodic lid overturn events of an otherwise stagnant-lid regime.