Types and kinematic stability of triple junctions

Abstract

A triple junction is kinematically stable if the orientation of each plate boundary remains constant relative to other boundaries in the triple junction during a finite time-interval. Instantaneous relative-velocity vectors have been used elsewhere to indicate the relative motion of plates during a finite time-interval — a technique that is not generally valid where finite relative plate motion is not circular. In the typical case in which all three plates have non-zero velocities around different plate-specific poles of rotation, the direction and magnitude of relative plate velocity varies systematically with time. Five plate-boundary types are considered in evaluating triple-junction stability: ridges (R), right-lateral and left-lateral transform faults (F R and F L) and trenches in which the overriding plate is clockwise (T C) or anticlockwise (T A) from the trench, judged by a rotation around the triple junction. One hundred twenty five triplet combinations of boundary types are possible. Some combinations display symmetry with other combinations. Grouping all similar configurations, 25 types of triple junctions can be distinguished, of which 19 types may be stable when finite relative motion is circular. Vectorial descriptors of the geometry of ridges, transform faults and trenches permit the listing of general conditions for the kinematic stability of triple junctions, under conditions of both circular and non-circular relative motion. The TJ1 model for the evolution of RRR triple junctions provides estimates of the variation in the geometry of triple junctions in which spreading is orthogonal and near-symmetric. RRR triple junctions at which spreading is orthogonal and symmetric are always stable when the finite relative motion of all corresponding plates is circular; however, TJ1 indicates that RRR triple junctions are not generally stable when relative motion is non-circular. In general, triple junctions are not kinematically stable, but evolve with changes in the finite relative motion of plates.