Formation and evolution of vertically extensive magma reservoirs in the continental crust: insights from numerical models

Abstract We apply numerical models of melt and magma transport through continental and arc crust to explore controls on trans-crustal magmatism. Our results suggest that repeated intrusion of mantle-derived basaltic magma in the deep crust produces vertically extensive, mush-dominated magmatic systems comprising individual reservoirs connected by episodic magma transfer via dykes. Upwards percolating melt differentiates and accumulates in each reservoir to form magma that can evacuate via magma-driven fractures if critical thresholds of buoyancy and melt fraction are reached. Melt and mush can also be generated by partial melting and assimilation of fertile crust. Processes in each reservoir control the size, frequency and composition of magma batches that ascend and intrude at shallower depth. Most simulated magma evacuations are temporally, spatially and compositionally decoupled from new intrusions, triggered by buoyant melt accumulation by reactive percolative flow. Other evacuations are a mix of intruded and rejuvenated magma triggered by new intrusions increasing the melt fraction (thermal rejuvenation) or buoyancy (buoyancy rejuvenation). Focusing of magma, as it evacuates from deeper reservoirs to intrude shallower reservoirs with smaller area, facilitates long-lived, upper-crustal magmatism by increasing the areal magma flux into the upper crust. The lower- to mid-crust is the most favourable environment for differentiation, generating evolved magmas that supply shallower parts of the system. System architecture is highly dynamic: reservoir depths vary through time and different parts of the system are magmatically active at different times. We do not observe the formation of a continuous, trans-crustal mush reservoir in any tested scenarios.