Three-dimensional passive flow and temperature structure beneath oceanic ridge–ridge–ridge triple junctions

Abstract

This study uses numerical modeling to investigate generalized characteristics of mantle flow and thermal structure in the vicinity of ridge–ridge–ridge (RRR) triple junctions. Oceanic triple junctions present a unique opportunity to study three-dimensional (3D) mantle dynamics in a tectonic setting considerably different than where only two plates diverge. In many prominent oceanic triple junctions, including Rodrigues, Azores, and Galapagos, the slowest-spreading ridge branch intersects the near-collinear faster-spreading branches quasi-orthogonally. This study focuses on triple junctions free of influence from nearby hotspots, similar to the geometry of the Rodrigues Triple Junction. A finite element model was used to calculate the steady-state 3D velocity flow field and temperature patterns resulting from advective and conductive heat transfer. For the slowest-spreading branch, model results predict a strong component of along-axis velocity directed away from the triple junction. Both upwelling velocity and temperature are calculated to increase along the slowest-spreading ridge toward the triple junction, approaching the upwelling rate and temperature of the fastest-spreading branch. Within 200 km of the triple junction, upwelling velocity is predicted to increase more than threefold along the slowest-spreading ridge. In contrast, the calculated upwelling velocity and temperature for the fastest-spreading branch are not significantly different from the case of a single spreading ridge. For triple junctions where the three ridges spread faster than the Rodrigues Triple Junction system, such as the Galapagos Triple Junction, the contrast in axial upwelling velocity and temperature between the slowest- and fastest-spreading ridges is predicted to be less significant. However, for triple junctions with overall slower spreading velocities, such as the Azores Triple Junction, this contrast is more pronounced.