Tripple Junction:

The point where three Tectonic plate boundaries meet is known as tripple junction.
At the triple junction each of the three boundaries will be one of 3 types –
a ridge (R), trench (T) or transform fault (F) – and triple junctions can be described
according to the types of plate margin that meet at them (e.g. Transform-
Transform-Trench, Ridge-Ridge-Ridge, or abbreviated F-F-T, R-R-R).

Only a few types of triple junctions are stable over time ('stable' in this sense means
that the triple junction's geometrical structure will not change over geologic
time).Meeting of 4 plates is possible instantaneously.

The concept of tripple junction was published in 1969 by Dan McKenzie and W.

Jason Morgan.

Fig:Tripple Junction
Interpretation:

The kinematics of triple junctions on a flat Earth are essentially the same as those
on the surface of the sphere. Plate motions on a sphere are represented as relative
rotations around Euler Poles, and the relative motion along a plate boundary can be
determined using this rotation. However, since the space around a triple junction is
small (relative to the size of the sphere) and far enough from the pole of rotation,
the relative motion around the boundary can be considered to be steady.As a
consequence, triple junction analysis will typically be performed on a flat surface
with vector-defined motions.

Stability of Tripple Junction:

Triple junctions may be described and their stability assessed without use of the
geological details but simply by defining the properties of
the ridges, trenches and transform faults involved, making some simplifying
assumptions and applying simple velocity calculations. This assessment can
generalise to most actual triple junction settings provided the assumptions and
definitions broadly apply to the real Earth.

A stable junction is one at which the geometry of the junction is retained with time
as the plates involved move. This places restrictions on relative velocities and plate
boundary orientation. An unstable triple junction will change with time, either to
become another form of triple junction (RRF junctions easily evolve to FFR
junctions), will change geometry or are simply not feasible (as in the case of FFF
junctions).

By assuming that plates are rigid and that the Earth is spherical,  Euler’s theorem
of motion on a sphere can be used to reduce the stability assessment to determining
boundaries and relative motions of the interacting plates. The rigid assumption
holds very well in the case of oceanic crust, and the radius of the Earth at the
equator and poles only varies by a factor of roughly one part in 300 so the Earth
approximates very well to a sphere.

McKenzie and Morgan first analysed the stability of triple junctions using these

assumptions with the additional assumption that the Euler poles describing the
motions of the plates were such that they approximated to straight line motion on a
flat surface. This simplification applies when the Euler poles are distant from the
triple junction concerned. The definitions they used for R, T and F are as follows:
  R – structures that produce lithosphere symmetrically and perpendicular to
the relative velocity of the plates on either side (this does not always apply,
for example in the Gulf of Aden).

 T – structures that consume lithosphere from one side only. The relative
velocity vector can be oblique to the plate boundary.

 F – active faults parallel to the slip vector.

Stability criteria:

For a triple junction between the plates A, B and C to exist, the following condition
must be satisfied:

AvB + BvC + CvA = 0

where AvB is the relative motion of B with respect to A.

This condition can be represented in velocity space by constructing a velocity

triangle ABC where the lengths AB, BC and CA are proportional to the velocities
AvB, BvC and CvA respectively.

Further conditions must also be met for the triple junction to exist stably – the
plates must move in a way that leaves their individual geometries unchanged.
Alternatively the triple junction must move in such a way that it remains on all
three of the plate boundaries involved.

These criteria can be represented on the same velocity space diagrams in the
following way. The lines ab, bc and ca join points in velocity space which will
leave the geometry of AB, BC and CA unchanged. These lines are the same as
those that join points in velocity space at which an observer could move at the
given velocity and still remain on the plate boundary. When these are drawn onto
the diagram containing the velocity triangle these lines must be able to meet at a
single point, for the triple junction to exist stably.

These lines necessarily are parallel to the plate boundaries as to remain on the plate
boundaries the observer must either move along the plate boundary or remain
stationary on it.

For a ridge the line constructed must be the perpendicular bisector of the relative
motion vector as to remain in the middle of the ridge an observer would have to
move at half the relative speeds of the plates either side but could also move in a
perpendicular direction along the plate boundary.

For a transform fault the line must be parallel to the relative motion vector as all of
the motion is parallel to the boundary direction and so the line ab must lie along
AB for a transform fault separating the plates A and B.

For an observer to remain on a trench boundary they must walk along the strike of
the trench but remaining on the overriding plate. Therefore, the line constructed
will lie parallel to the plate boundary but passing through the point in velocity
space occupied by the overriding plate.

The point at which these lines meet, J, gives the overall motion of the triple
junction with respect to the Earth.

Using these criteria it can easily be shown why the FFF triple junction is not stable:
the only case in which three lines lying along the sides of a triangle can meet at a
point is the trivial case in which the triangle has sides lengths zero, corresponding
to zero relative motion between the plates. As faults are required to be active for
the purpose of this assessment, an FFF junction can never be stable.

Fig:Stability of Tripple junctions

Types:

McKenzie and Morgan determined that there were 16 types of triple junction
theoretically possible, though several of these are speculative and have not
necessarily been seen on Earth. These junctions were classified firstly by the types
of plate boundaries meeting – for example RRR, TTR, RRT, FFT etc. – and
secondly by the relative motion directions of the plates involved. Some
configurations such as RRR can only have one set of relative motions whereas TTT
junctions may be classified into TTT(a) and TTT(b). These differences in motion
direction affect the stability criteria.

McKenzie and Morgan claimed that of these 14 were stable with FFF and RRF
configurations unstable, however, York later showed that the RRF configuration
could be stable under certain conditions

Ridge Ridge Ridge Junction:

An RRR junction is always stable using these definitions and therefore very
common on Earth, though in a geological sense ridge spreading is usually
discontinued in one direction leaving failed rift zone. There are many examples of
these present both now and in the geological past such as the South Atlantic
opening with ridges spreading North and South to form the Mid-Atlantic Ridge,
and an associated aulacogen in the Niger Delta region of Africa. RRR junctions are
also common as rifting along three fractures at 120° is the best way to relieve
stresses from uplift at the surface of a sphere; on Earth, stresses similar to these are
believed to be caused by the mantle hotspots thought to initiate rifting in
continents.

The stability of RRR junctions is demonstrated below – as the perpendicular

bisectors of the sides of a triangle always meet at a single point, the lines ab, bc and
ca can always be made to meet regardless of relative velocities

Ridge-Trench-Fault junctions:

RTF junctions are less common, an unstable junction of this type (an RTF(a)) is
thought to have existed at roughly 12Ma at the mouth of the Gulf of California
where the East Pacific Rise currently meets the San Andreas Fault zone.The
Guadeloupe and Fallaron microplates were previously being subducted under the
North American Plate and the northern end of this boundary met the San Andreas
Fault. Material for this subduction was provided by a ridge equivalent to the
modern East Pacific Rise slightly displaced to the west of the trench. As the ridge
itself was subducted an RTF triple junction momentarily existed but subduction of
the ridge caused the subducted lithosphere to weaken and ‘tear’ from the point of
the triple junction. The loss of slab pull caused by the detachment of this
lithosphere ended the RTF junction giving the present day ridge – fault system. An
RTF(a) is stable if ab goes through the point in velocity space C, or if ac and bc are
colinear.

Trench-Trench-Trench junctions:

A TTT junction can be found in central Japan where the Eurasian plate overrides
the Philippine and Pacific plates, with the Philippine plate also overriding the
Pacific. Here the Japan Trench effectively branches to form the Ryukyu and Bonia
arcs.

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:

Gulf of California

Indian Ocean