The ∼2730Ma onset of the Neoarchean Yilgarn Orogeny

The timing of the onset of an orogeny is commonly constrained indirectly, because early orogenic structures are rarely exposed or are overprinted. Establishing the onset of an Archean orogeny is considerably more challenging, because of the more fragmented geological record and the general lack of consensus about Archean geodynamics. We combine existing tectonostratigraphic data with new structural and geophysical data sets to establish the onset of the Neoarchean Yilgarn Orogeny (Yilgarn Craton, Western Australia). We show that the surface of the similar to 2960-2750Ma deep-marine Yilgarn greenstone sequence was uplifted, eroded, and unconformably overlain by an similar to 2730Ma, syntectonic clastic sequence, deposited in shallow marine to subaerial conditions, and derived from the erosion of the underlying greenstones. This similar to 2730Ma regional unconformity predates the oldest known Yilgarn structures; therefore, its tectonic significance is so far unknown. At around the same time, at deeper crustal levels, the similar to 2728Ma Yarraquin pluton was being emplaced along an active, large-scale shear zone network. Our mesostructural and microstructural analysis shows that the bulk of the fabric in the granite and its country rocks developed during pluton emplacement and was largely assisted by magma-present shearing. Overall, these structures reflect an important event of synemplacement crustal shortening. The regional unconformity and the syndeformational emplacement of the Yarraquin pluton are both expressions of an similar to 2730Ma regional deformation event associated with significant crustal thickening, marking the onset of the Neoarchean Yilgarn Orogeny.