3.3 Triple Junctions
What Is a Triple Junction?
A triple junction is a point on Earth's surface where the boundaries of three tectonic plates meet. Since the surface of a sphere partitioned into plates must have such meeting points (by Euler's polyhedron formula), triple junctions are a fundamental geometric consequence of plate tectonics. The modern classification and stability analysis of triple junctions was established by McKenzie & Morgan (1969) in a landmark paper that remains the foundation of the field.
At a triple junction, three plate boundaries meet. Each boundary can be one of three types: Ridge (R), Trench (T), or Transform fault (F). The combination of these three boundaries defines the type of triple junction. Since each of the three boundaries can be R, T, or F, there are nominally 10 distinct combinations (accounting for symmetry), expanding to 16 when boundary polarity (subduction direction for trenches) is considered.
The Velocity Triangle
The fundamental kinematic constraint at any triple junction involves three plates (A, B, C) and their pairwise relative velocities. Since the velocity of A relative to C must equal the velocity of A relative to B plus B relative to C, we obtain:
Velocity Closure at a Triple Junction
\[ \mathbf{v}_{AB} + \mathbf{v}_{BC} + \mathbf{v}_{CA} = \mathbf{0} \]
This is the velocity triangle condition. Geometrically, the three relative velocity vectors form a closed triangle in velocity space. This constraint must be satisfied at the junction point at every instant.
The velocity triangle is plotted in a 2D velocity diagram where each vertex represents the velocity of one plate at the junction point. The sides of the triangle are the relative velocity vectors. The lengths give spreading/convergence rates, and the orientations must be consistent with the boundary types (ridges spread perpendicular, transforms slide parallel, trenches converge in the dip direction).
Stability of Triple Junctions
A triple junction is stable if it can exist as a single point for a finite duration of time while maintaining the geometry of the three boundaries. An unstable junction immediately evolves into a different configuration or splits into two separate junctions.
Stability Criterion
\[ \text{Stable if: junction point lies on all three boundaries for } \Delta t > 0 \]
Formally, the junction point must simultaneously satisfy the geometric constraints of all three boundaries. The junction velocity vJ must be such that the junction remains at the intersection of all three boundaries as they evolve. This is analyzed graphically using velocity space diagrams.
The graphical method works as follows: each boundary type constrains where the junction point can lie in velocity space. A ridge requires the junction to move along the ridge axis. A transform requires the junction to lie on the fault line. A trench allows the junction to lie anywhere on the trench line (since the overriding plate consumes the trench). Stability requires that all three constraint lines (or regions) intersect at a single point.
Classification of Triple Junction Types
McKenzie & Morgan identified 16 distinct triple junction types based on the three boundary types and trench polarity. Their stability depends on boundary geometry and relative velocities.
| Type | Stability | Natural Example | Notes |
|---|---|---|---|
| RRR | Always stable | Afar (Red Sea/Gulf of Aden/E. African Rift) | Most common; velocity triangle always closes |
| RRF | Conditionally | Galapagos triple junction | Stable for specific transform orientations |
| RRT | Conditionally | Japan (Pacific/Philippine/Eurasia) | Depends on trench orientation |
| RTF | Conditionally | Mendocino (San Andreas/Cascadia/Gorda R.) | Classic California example |
| RFF | Conditionally | — | Rare; requires specific geometry |
| TTT | Conditionally | Central Japan (Boso triple junction) | Trench orientations must be compatible |
| TTF | Conditionally | Chile triple junction | Subduction polarity matters |
| TFF | Conditionally | — | Highly constrained geometry |
| FFF | Generally unstable | — | Three transforms rarely intersect stably |
After McKenzie & Morgan (1969). Polarity variants (e.g., TTT with different subduction directions) bring the total to 16 distinct types.
RRR: The Always-Stable Junction
The ridge-ridge-ridge (RRR) junction is unique in being stable for any combination of spreading rates and ridge orientations. This is because each ridge axis can adjust its position freely — it migrates with the half-spreading rate — so the three ridges can always intersect at a single point.
RRR Velocity Triangle
\[ \mathbf{v}_{AB} \perp \text{Ridge}_{AB}, \quad \mathbf{v}_{BC} \perp \text{Ridge}_{BC}, \quad \mathbf{v}_{CA} \perp \text{Ridge}_{CA} \]
Each relative velocity vector is perpendicular to its corresponding ridge segment. The velocity triangle always closes regardless of ridge orientations, ensuring unconditional stability.
Worked Example: The Afar Triple Junction
The Afar triple junction in East Africa is the textbook example of an RRR junction. It marks the meeting point of three divergent boundaries:
Red Sea Rift
Separating the Arabian and African (Nubian) plates. Full spreading rate ~1.6 cm/yr in the southern Red Sea. Ridge axis trends NNW-SSE.
Gulf of Aden
Separating the Arabian and Somali plates. Full spreading rate ~2.0 cm/yr. The Sheba Ridge trends ENE-WSW and propagates westward into Afar.
East African Rift
Separating the Nubian and Somali plates. Extension rate ~6 mm/yr. The youngest and least developed arm, transitioning from continental rifting to seafloor spreading.
Velocity Analysis
Let N = Nubian plate, A = Arabian plate, S = Somali plate. The closure condition requires:
\[ \mathbf{v}_{NA} + \mathbf{v}_{AS} + \mathbf{v}_{SN} = \mathbf{0} \]
With vNA ≈ 1.6 cm/yr (NE), vAS ≈ 2.0 cm/yr (SSE), the closure condition predicts vSN ≈ 0.6 cm/yr (approximately E-W), consistent with GPS measurements of the East African Rift extension rate. This agreement validates the triple junction model and confirms that Afar is a near-perfect RRR junction.
The Mendocino Triple Junction (RTF)
The Mendocino triple junction off northern California is a classic RTF junction where the Pacific, North American, and Gorda (Juan de Fuca) plates meet. It involves:
- San Andreas Fault (F): right-lateral transform between Pacific and North American plates
- Cascadia Subduction Zone (T): the Gorda plate subducts beneath North America
- Gorda Ridge (R): the Pacific and Gorda plates spread apart
This junction migrates northwestward along the coast at about 5 cm/yr, leaving a “slab window” in its wake. The stability of this junction depends on the precise orientation of the San Andreas fault relative to the Pacific–North America relative motion vector, and it is marginally stable under present-day kinematics.