boundaries meet, taking as an example the only one on earth at the present time, in central Japan. In this area, the Pacific,. Philippine. Sea, and Eurasian. (PAC ...
TECTONICS,VOL. 6, NO. 1, PAGES35-45, FEBRUARY 1987
CENTRAL
Xavier
JAPAN
TRIPLE
JUNCTION
REVISITED
Le Pichon and Philippe
D•partement Sup•rieure,
Abstract.
de G•ologie, Paris, France
We
dimensional
show
Huchon Ecole
that
kinematic
Normale
the
two-
discussion
of
McKenzie and Morgan's TTT-type triple junction is not sufficient, as it ignores the interaction of the sinking plates. We derive
a
dimensional
junctions
relation
for
existence
and show that
of
it
TTT does not change (is stable) or cVA//bc (Figure 1, inset).
the
three-
present
situation
TTT
triple
evolve
to
is satisfied
at
the unstable central Japan triple junction. We then use the geometry of the subducting Pacific plate beneath the Philippine Sea and Eurasian plates to show that the present situation did not hold before about 3 Ma. Before that time, the triple junction was probably located north of the present one and very close to the pole of relative motion. Finally, we discuss the geological implications of this
plate boundaries intersect in TTT (Figure 1). They showed that the geometrical configuration of the plate boundaries at
model.
INTRODUCTION
McKenzie and Morgan [1969]
first
discussed the evolution of triple junctions where three consuming plate boundaries meet, taking as an example the only one on earth at the present time, in central Japan. In this area, the Pacific, Philippine Sea, and Eurasian (PAC, PHS, and EUR hereafter, respectively) consuming
a
is
if ab//ac Thus, the
unstable
different
one
and should as
Paper
Union.
number 6TO258.
0278-7407/87/006T-0258510.00
been
this paper, we show that a kinematic discussion in the plane tangent to the earth in TTT is not sufficient, as it ignores the three-dimensional (3-D) interaction of the sinking plates. We use simple geometrical considerations to deduce
an
existence
condition
for
such
a
TTT. Before applying this condition to the geometry of the central Japan triple junction, we briefly discuss the kinematics of the Philippine Sea plate. Because the motion along the Sagami trough is nearly transform (boundary (a,c) in Figure 1), the triple junction is nearly a TTF-type triple junction. Then, we will show that although the existence condition is satisfied, the present situation did not
hold
before
about
3 Ma
and
previously, the triple junction probably located further north, very
to the PHS/EUR pole of relative Copyright 1987 by the American Geophysical
has
discussed by Fitch and Scholtz [1971], Matsubara and Seno [1980], and most notablyby Senoand Maruyama[1984]. In
THREE-DIMENSIoNAL
GEOMETRICAL
The two-dimensional
(2-D)
that,
was close
motion.
DISCUSSION
discussion
of
McKenzieand Morgan[1969] concernsthe
36
Le Pichon and Huchon: Central
135'
Japan Triple
Junction
145 ø
140 ø
45 o
EUR • M
PAC I
40ø
I
/ / !
•. ø
c)
/ ! / / ! EURASIAN / / I•,
--
// Kashima selmount
35 o
Triple junction
N • % \ \\ \ \ PHILIPPINE SEA \ • • PLATE
•,c•
•
-TT tb,c)
•5 e
Fig. 1. General geodynamic context of central Japan triple lines are isobaths (in kilometers) of the descending Pacific
and Hasegawa[1982] relative
triple
velocity
junction. Contour plate after Umino
and T. Maki (personal communication,1985). Arrows:
vectors
(scaled values:
see text).
Right inset:
TTT(a)-type
junction configuration of McKenzieandMorgan[1969]. (a,b): boundary
between plates A and B, and similarly for (a,c) and (b,c). A, Eurasia; B, Pacific; C, Philippine Sea; PHS/EU and PHS/HS, poles of rotation of PHS
with respectto EUCHuchon, 1986]and to hot spotscomputed fromHuchon E1986] and CoxandEngebretson [1985]. Left inset: velocity triangle at central Japan triple
junction
(34.5øN, 142øE). PAC/EURis computed from Minster and Jordan's
[1978] parameters. The position of PHSin the triangle is given for the
following parameters: H[Huchon, 1986], KS•arig, 19;]5!, MEMinster and
Jordan, 1979], R [Rankenet al., 1984] and [Seno, 197 Note that K and R producedivergenceat the triple junction (see discussionby Huchon,[1986]).
Le Pichon and Huchon: Central
Japan Triple
geometric stability of TTT triple junctions. A stable triple junction keeps the same geometric configuration through time, whereas an unstable one changes to another configuration. However, if trenches are involved in the triple junctions, some configurations are impossible because of geometric interference from the sinking plates. Thus, another condition should be met in order to avoid this space problem. We call it
the 3-D existence condition. If all three subductions have
the
same
polarity (case TTT(b) of McKenzie and Morgan), such a triple junction can exist because the sinking plates do not interfere. If both polarities exist (case TTT(a) of McKenzie and Morgan), interference between plates may arise unless
a
is met.
In the following,
certain
3-D that
existence
condition.
Note
independent conditions
of the of McKenzie
applies to stable configurations. An
obvious
of a TTT(a)
cVA
condition
is
as
to
unstable
triple
for
junction
the
existence
is that
vector
is to the west of boundary (a,b)
and
that consequently• > O (see Figure 2). If • = 8, the triple junction is stable. If 0 < • < 8, the triple junction migrates along boundary (a,c) . If • > 8, it migrates Let
plate
the
of
plate
A
C is
ZC - (y cos ¾ - x sin ¾) tan e + P Along vector L, ZB - ZC. Assuming the same trench (a,c),
depth
along
boundaries
(a,b)
and
x tan • - ( y cos ¾ - x sin ¾) tan • The
coordinates -
cos
of
¾ tan
vector
L are
•
y - tan • + sin ¾ tan • z -
tan
•
tan
•
cos
¾
To obtain
the corresponding
relative
motion
axis (a,c) computations,
.
direction
in the horizontal L by an angle
of
plane,
e about
the
After some numerical one obtains the coordinates
of the rotated vector LH (see Figure 2) x'
- sin • cos ¾ + tan • sin ¾ cos ¾ (cos
y'
- sin • sin ¾
z'
-0
•
-
1 )
+ tan a sin 2 ¾ (cos • - 1) + tan •
along boundary (b,c). us now consider
bottom
where P is the depth at the trench axis. The depth to the bottom of plate A above
one must rotate
condition
the
ZB - x tan s + P
x
this
2-D stability and Morgan and
as well
C. The depth to above plate B is
condition
we derive this
Junction
general
case
where • > O. The existence condition must be
discussed
in
the
instantaneous
domain
because the vector LH is in the horizontal
because, except for the case where • -8,
plane.
the geometric configuration changes through time. The existence condition is that the subduction vector, originating at material triple point O in plate C be such that it still lies above plate B. Consider Figure 2, where the origin O is taken at the triple junction and where
motion is defined by angle • (Figure 2) such that tan •> x'/y'. Thus
The critical
direction
of
relative
tan• >cos ¾/ [sin¾+tan,•5 (sin e + tan • sin ¾ (cos •-
which is direction
the
of
critical relative
condition motion
for
1))
the with
Oy coincides with consuming boundary (ab). The coordinate system is orthonormal and direct, with the z axis positive downward. Working near the triple junction, we neglect changes in dips of plates along the boundaries. We also neglect differences in depths of trenches. Let • be the dip of the sinking plate B and e the dip of the sinking plate C. Let ¾ be the angle (ac, Ox) (counted positive clockwise from Ox). From figure 2, where
configuration is not stable. If • > 8, the triple junction migrates along (a,c) and,
we also
as can be seen in
show isodepth
lines
of the
bottom
of plate A, cVA cannot be closer to the y axis than the vector L, which is the intersection between subduction planes limiting
plates
A and B, and plates
A and
geometrical
parameters •,
•,
and ¾.
Note that, if 8 = •, the configuration is
stable,
in
the
sense
of McKenzie
and
Morgan•1969], and the above inequality should
exist.
be met for
this
configuration
This case is discussed further
to
by
Hamano •1986]. In
the
general
case,
Figure
however,
3a,
point
the
T J1
subducts during an interval of time It l, t2] along line TJ1-P, whichis parallel to the motion vector in the subducted portion of plate C (line L of Figure 2). No point
]8
Le Pichon and Huchon: Central
Japan Triple
Junction
Before discussing the application of this condition to the actual centra! Japan triple junction, we briefly discuss the present kinematics.
Y
PRESENT
KINEMATICS
CENTRAL
JAPAN
Although
AROUND
TRIPLE
the
THE
JUNCTION
data
set
used
for
the
determination is essentially the same, the parameters of rotation of the PHS plate vary widely according to various authors.
(a,c)
SenoE1977] andMinster andJordanE1979] located
Y
the pole of rotation
far
away from
(b,c) Fig. 2. Reference frame used for the computations (see text). Here ,• and e are the dip angles of plates A and C, respective ly.
of the subducted (a,b) than line existence
condition
plate C is TJ1-P so is
valid
in
closer that the
TJ1
TJ2
to our
finite
time domain. Physically, however, the previous discussion is not entirely valid because it assumes that, during the migration of the triple junction along (a,c), plate B immediately develops a scissor-type fault along segment TJ1-TJ2, which is probably unrealistic.
If
O < • < •,
migrates that
in
our
the
the triple
along (b,c), existence
finite
time
domain.
is
3b shows
also
However,
are again some significant problems because the previous
1
junction
and Figure
condition
2
TJ2
valid
there
physical discussion
assumes the existence of a bend along TJ1-P in the sinking plate C, as portion TJ1-TJ2-P of the sinking plate C does not have the same dip as the remaining part of the sinking plate C. To summarize, we have demonstrated that the previous inequality is the general existence condition for a TTT(a)-type triple junction in three dimensions,
although the idealized geometric cases raise significant physical problems. This
TJ1
0
