The collision of a seamount or microcontinental block with an arc may produce complex strain distributions and changes in the tectono-magmatic setting. Slab rollback and associated upwelling of asthenosphere occurred ca. 440 Ma near the margin of East Gondwana in response to subduction zone lock-up following arc collision ca. 455 Ma. World-class porphyry copper gold deposits (e.g., Cadia) formed in the backarc during extension and magmatism associated with slab rollback. Upwelling of the asthenosphere also provided the heat engine for generation of hydrothermal fluids. The result was elevated fluid pressures and compressional brittle deformation associated with orogenic gold mineralization (e.g., Bendigo) inboard of the East Gondwana margin.

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