The tempo of subduction-related magmatic activity over geological time is episodic. Despite intense study and their importance in crustal addition, the fundamental driver of these episodes remains unclear. We demonstrate quantitatively a first order relationship between arc magmatic activity and subduction flux. The volume of oceanic lithosphere entering the mantle is the key parameter that regulates the proportion of H2O entering the sub-arc. New estimates of subduction zone H2O budgets over the last 150 million-years indicate a three- to five-fold increase in the proportion of H2O entering the sub-arc during the most recent global pulse of magmatism. Step changes in H2O flux enable proportionally greater partial melting in the sub-arc. Similar magmatic pulses in the ancient Earth could be related to variability in subduction flux associated with supercontinent cycles.

The production of H2O during metamorphism along active plate boundaries is inferred to contribute to low-frequency tectonic tremor seismicity. This study combines predictions from phase equilibria and mechanical modelling of coincident volume changes to investigate links of tremor with hydrofracturing and fluid migration under the actively forming Southern Alps, New Zealand. Our predicted location of metamorphic fluid production correlates with published geophysical images of inferred permeability enhancement, fluid accumulation and potential fluid flow. As the hanging-wall rocks are translated towards the surface by motion along the Alpine Fault, they can undergo metamorphic reactions that involve positive volume changes. Production of metamorphic fluids leads to hydrofracturing and the development of tremor hypocentres in regions along, and above deep reflectors of the Alpine Fault. The capacity of metamorphic rocks to generate or consume fluid along portions of the pressure–temperature path exerts a fundamental control on the distribution of stresses in the crust.