Nucleation and evolution of ridge-ridge-ridge triple junction: New theory and magic formula

Ridge-ridge-ridge triple junctions are among the most remarkable features of global plate tectonics but their nucleation and evolution remains incompletely understood. Here, we employ 3D numerical models to study the processes of the nucleation and evolution of triple junctions induced by multi-directional lithospheric extension. The simulations show that two major classes of junctions develop: (i) transient quadruple and triple plate rifting junctions formed by the initial plate breakup that are gradually converted into (ii) stable triple oceanic spreading junctions formed by the accretion of new lithosphere. Quadruple junctions break into two diverging triple oceanic spreading junctions connected by a linear spreading center lengthening with time. This process gradually decreases the length of deforming boundaries between four diverging rigid plates and thus the integral mechanical resistance of these boundaries to the spreading. The geometry of triple oceanic spreading junctions varies from asymmetrical T-junctions to ideal 120° junctions. The structure of these junctions includes two crucial tectonic elements: oceanic ridges (spreading centers) and intra-plate ranges (triple junction traces) dividing crust accreted from different spreading centers. The geometrical steady state is achieved within several million years. We propose a new geometrical theory of a migrating steady state triple junction, which describes its structure as a function of the relative plate velocities in a moving triple junction reference frame. The relative plate velocities define the orientations and growth rates of respective intra-plate ranges. Orientations and lengthening rates of oceanic ridges are in turn given by the average relative velocity of two adjacent plates. The steady state triple junction migration velocity satisfies the condition of minimal cumulative ridge lengthening rate weighted by the square root of the spreading velocity, which is proportional to the energy dissipation per unit ridge length. Geometry of triple junction thus maximizes the rate of the mechanical energy dissipation decrease (or minimizes the rate of the mechanical energy dissipation increase) in the spreading system. Our analytical theory agrees well with the results of the numerical simulations and natural data (Gerya and Burov, 2018). Reference: Gerya, T. Burov, E. (2018) Nucleation and evolution of ridge-ridge-ridge triple junctions: Thermomechanical model and geometrical theory. Tectonophysics,