Ridge-ridge-ridge Triple Junction Instability: Unveiling the Dynamics and Evolution with 3-D Numerical Simulations

The most important form of plate evolution occurs where three tectonic plates meet, a triple junction1 (TJ). Ridge-ridge-ridge (RRR) TJs are unique because they are stable at all ridge configurations and spreading velocities1. This idealised and simplified RRR TJ with three straight rifts is challenged here by showing that it does not apply if at least one rift is not straight (sigmoidal). Despite its great importance for plate tectonics, how and why a stable RRR TJ configuration becomes unstable is unclear. The state-of-the-art 3-D numerical simulations reported here using the Azores TJ (ATJ) as a prototype bring a new understanding of RRR TJ stability and evolution. The current knowledge and critical similarities in both the simulations and prototype in terms of timing, transient grabens distribution, rift and triple junction jump, diffuse plate boundary, depth of the main rift, and rift shoulder uplift indicate that the ATJ is unstable because of the sigmoidal shape of the Azores-Gibraltar fracture zone and how Nubia has moved relative to Eurasia in the last 4 Ma. Consequently, the fracture zone that turned into a first-order plate boundary became unstable, and the Azores block moved from Eurasia to Nubia.