Magmatic-tectonic interactions in the Natron rift revealed by seismic anisotropy

Oldoinyo Lengai volcano is located the Natron Basin, a young (~3 Ma) magmatic rift segment of the East African Rift System. In this setting, magma transport, lithospheric deformation, and stress distribution are closely coupled, yet their relative roles in controlling volcanic and tectonic processes remain poorly constrained. The coexistence of an unusual natrocarbonatitic magmatic system with nearby silicic and basaltic volcanism points to a complex and evolving magma plumbing architecture that may both respond to and modify the regional stress field. Seismic anisotropy provides a sensitive indicator of stress-aligned fabric, deformation, and melt distribution within the crust and uppermost mantle.

Here, we combine local shear-wave splitting measurements with an inversion of anisotropic receiver functions to investigate stress modification and lithospheric deformation beneath Oldoinyo Lengai and the Natron Rift. We use data from the dense SEISVOL seismic network, spanning the region from Lake Natron to the extinct Gelai shield volcano, the monogentetic cone field Naibor Soito and active Oldoinyo Lengai volcano. We use the eigenvalue minimization method to analyze shear wave splitting of over ~10 000 volcano tectonic earthquakes. This provides a unique data set of shallow crustal anisotropy at unprecedented resolution. Azimuthally varying receiver-function signals are decomposed using harmonic regression and inverted within a probabilistic Bayesian framework, allowing us to resolve complex anisotropic layering and quantify uncertainties.

Our results reveal distinct anisotropic domains within the upper and mid-crust. Across much of the study area, fast-axis orientations align parallel to the rift axis, consistent with regional extensional stress. In contrast, pronounced lateral and depth-dependent variations in fast-axis orientation are observed beneath Oldoinyo Lengai and above a previously imaged sill complex underneath Naibor Soito, indicating localized stress perturbations associated with magmatic processes. These patterns closely correspond to the tension axes derived from focal mechanism solutions and stress modeling. However, local shear-wave splitting provides a much better spatial resolution of stress orientations at the scale of individual earthquake–station pairs and may even be susceptible to temporal changes of the magmatic plumbing system. Together, the combined anisotropic observations provide new constraints on the interaction between rift-related deformation and magmatic plumbing in the Natron Basin highlighting how seismic anisotropy offers substantial advantages to study these processes at high spatial and temporal resolution.