# The importance of past rifting in large igneous province development

## TL;DR

The study shows that thinned lithosphere from past failed rifting does not always lead to future magmatism or rifting activity.

## Contribution

It challenges the assumption that lithospheric thin zones are always weak spots for magmatism and rifting.

## Key findings

- The Turkana Depression lacks thick lower crustal intrusions seen in the Ethiopian Plateau.
- The mantle lithosphere below the Depression retained a cool, fast-wavespeed structure.
- Failed rifting in the Anza Rift likely made the lithosphere refractory without later rejuvenation.

## Abstract

Lithospheric thin zones, such as recently failed rifts, are generally assumed to be weak spots where magmatism and deformation can concentrate during rifting and large igneous province development1–3. Yet, the Turkana Depression in East Africa, the site of the failed 66-million-year-old Anza Rift, did not experience the widespread flood magmatism seen on the adjacent Ethiopian Plateau, despite being a lithospheric thin spot when the region encountered hot plume material around 45 million years ago4. Here we jointly invert surface-wave and receiver function data to constrain crustal and upper-mantle seismic structure below the Depression to evaluate lithospheric thermo-mechanical modification. Evidence for thick lower crustal intrusions, ubiquitous below the uplifted Ethiopian Plateau5,6, is comparatively lacking below the Depression’s failed Anza Rift system, which ongoing East African rifting is circumnavigating, not exploiting. The mantle lithosphere below the Depression has also retained its cool, fast-wavespeed ‘lid’ character, contrasting the Ethiopian Plateau. Volatile depletion during failed Anza rifting probably rendered the thinned lithosphere refractory without later rejuvenation. Subsequent rifting and magmatism thus initiated away from the still-thin Anza Rift, in regions where fertile lithosphere enabled melting and the sufficient lowering of plate yield strength. Areas of thinned lithosphere are thus not necessarily persistent weak zones where significant extension and magmatic provinces will develop.

Seismic data from the Turkana Depression in East Africa show that areas of thinned lithosphere are not necessarily persistent weak zones where extension and magmatic provinces will develop.