Triple Junction Analysis

Determine of the motion of the three plate boundaries at the triple junction, relative to the plates.  Determination of the triple junction motion first requires computing the motion of each pair of plates to determine the type of boundary.

• AB is the velocity vector of the boundary between the two plates
• ab is the line on which a point must be to stay on the boundary between those two plates
• Trench:
• Velocity line AB is parallel to the trench orientation on the map, or perhaps slightly oblique.
• ab goes through the point with the upper plate, because the trench is fixed to the upper plate.  The triple junction can only move along this boundary.
• Subduction (plate motion direction) does not have to be perpendicular to the trench
• Transform
• Velocity line AB is parallel to the transform.
• ab goes along line AB showing velocities of the two plates--the plates must have velocities parallel to the transform. In a few cases of "leaky transform faults", the motion is slightly oblique to the boundary. The triple junction can only move along the boundary, since crust is being neither added or destroyed at the boundary.
• Ridge
• Velocity line ab perpendicular to the ridge
• If spreading is symmetrical and perpendicular, then it's perpendicular bisector of AB.  This is the typical case, but there can be exceptions.  Both plates will be moving away from the triple junction.

A triple junction is stable only if the velocity lines for the three plate boundaries intersect in a single point. A stable triple junction will maintain it configuration over time, and it will almost certainly migrate with respect to the plates

If that point representing the triple junction's veclocity is not on the point representing the plate, the triple junction will be moving relative to that plate.  The triple junction cannot  be stationary with respect to all three plates, and in general will be migrate with respect to several of the plates.  A ridge-ridge-ridge triple junction will migrate with respect to all three plates.  The migration of the Mendocino triple junction north along the coast of North America between the Pacific and North American explains many of the salient features of the Cenozoic geologic history of California.

Examples:

Last revision 2/27/2019