Stress-controlled magma-plumbing system during rift evolution

Rift zones host highly dynamic melt pathways in which magma may ascend vertically as dikes or migrate laterally to form sills, generating complex trans-crustal magma plumbing systems. While diking is a well-recognized consequence of extension, the mechanisms that promote sill emplacement during rifting remain poorly constrained. Here, we conduct high-resolution geodynamic models using ASPECT coupled with the surface process modeling code FastScape, to track the evolving lithospheric stress state over millions of years and assess its control on melt migration.

Inspired by the Main Ethiopian Rift, the reference model comprises 40 km-thick crust of intermediate strength, featuring a 25 km-thick wet quartzite upper crust and a 15 km-thick wet anorthite lower crust, and is extended at a rate of 6 mm/yr. The model spans 12 Myr and accommodates 72 km of extension. Despite the overall extensional regime, the results reveal the development of localized compressional stress. This horizontal compression arises from flexure of the mechanically competent lithosphere during rifting and is concentrated at depths of 15–20 km and 5 km. In contrast, in weak crustal configurations, compression is confined to shallower levels (<3 km).

Analysis of the temporal evolution of the stress field shows that horizontal compression initially develops in off-rift regions at the top of competent layers during early rift stages of rift-flank uplift. As extension proceeds, compression also emerges below the rift axis while it persists in some places outside the rift. We represent melt pathways as streamlines aligned with the maximum principal deviatoric stress (σ₁). Assuming that magma migration through the crust follows the direction of σ₁, vertical melt ascent occurs when σ₁ is oriented vertically, corresponding to a regime dominated by extension. Rotation of σ₁ into a horizontal orientation due to compression promotes lateral magma migration and sill emplacement beneath or within zones of compression. For intermediate orientations of σ₁, melt ascent proceeds obliquely. This approach enables melt pathways to be visualized solely as a function of the stress field: clustered streamlines indicate focused magma transport, whereas dispersed streamlines reflect more diffuse migration.

Such stress-controlled magma deflection provides a mechanism for the formation of stacked sills and multi-tiered plumbing systems observed in nature. Prolonged magma storage enhances crustal assimilation, facilitating the generation of evolved magmas from primitive melts. These results demonstrate that the evolving lithospheric stress state plays a key role on magma transport during rifting and provides a geodynamic framework for understanding plumbing-system architecture in magma-dominated rift segments, such as the East African Rift System, the Taupō Volcanic Zone, and Iceland.