Neoarchean Synmagmatic Crustal Extrusion in the Transpressional Yilgarn Orogen

Positive feedback mechanisms between magmatism and transpression allow efficient extraction, transfer, and delivery of synorogenic granitic magma. Although many Archean orogens are dominantly transpressional and record voluminous production of synorogenic granites, the role of crustal magmatism in Archean tectonics has been so far little explored. We detail here the structural evolution of the Ballard shear zone, a major synorogenic, inclined transpressional shear zone from the upper-crustal granite-greenstone system of the Neoarchean Yilgarn Orogen (Western Australia). We show that the bulk of the three-dimensional deformation was efficiently partitioned between greenstones and crystallizing granitic sheets, with the latter accommodating large amounts of orogen-parallel strain through hypersolidus viscous flow. Other large synorogenic Yilgarn shear zones show a similar structural evolution, allowing the formulation of a general tectonomagmatic model. The positive feedback between protracted magmatism and transpression promoted lower- to middle-crustal extraction and upward transfer of syntectonic magma. In upper-crustal sink regions, the rheological dichotomy between cold and strong greenstones and hot and weak sheets of crystallizing granite allowed efficient kinematic partitioning, resulting in the extrusion of the partially molten shear zone cores. This sequence of events occurred repeatedly throughout the orogeny, following magma pulses, with the bulk of the shearing along each shear zone waning with the cooling of each syntectonic pluton. Synorogenic magmatism played therefore a major role in promoting and localizing transpressional deformation, and ultimately in shaping the architecture of the whole Yilgarn orogen.