Plate Motions - Quantitative Plate Tectonics - page 48

Fig. 2.17 Evolution of RRR , FFF ,and TTT triple junc-
tions. In all panels, a reference frame with origin in O
is fixed to A. Top left :An RRR junction. Arrows are full
spreading velocity vectors at the triple junction J .The
new location of J after a small time interval t can be
anywhere in the central dashed triangle . The ridges link
to this new location of J by propagation, development of
new transforms, or oblique spreading. Dark green lines
are points having the same sea floor age (isochrons). To p
right : An unstable FFF triple junction, which collapses
into a new triple junction microplate. In this example,
three new RRF junctions are created that substitute the

original junction J. Dashed lines represent initial locations
of plate boundaries. Bottom : The two different kinds of
TTT junctions. Dotted lines represent subducted points
of lower plate margins. The left panel shows a stable
situation where A is always upper plate, B is both upper
and lower plate, and C is always lower plate. The junction
J migrates rightward along the A C boundary. The right

panel illustrates a much more complicated configuration,
in which A , B ,and C are all upper and lower plates at the
same time. In this instance, the triple junction is not stable,
and new strike-slip boundaries develop ( blue lines )

Fig. 2.18 Development of
strike-slip faults along
flipping convergent
boundaries

away from the ridge, after a time interval t ,
by a distance v t /2 (Fig. 2.6 ). If we link all the
displaced points of age t from one side of a ridge,
and combine these segments with points placed
along the fracture zones, we obtain a line that
represents the geometry of the mid-ocean ridge
at a certain time t in the past. Such a line is called
an isochron . In Chap. 5 , we shall learn the process
of construction of isochron maps , which describe
the pattern of sea-floor spreading through the
geological time. For the moment, it is sufficient
to note that in the reference frame of a plate A ,
an isochron of the conjugate plate B moves at
full velocity v , just like trenches and strike-slip
boundaries, as illustrated in Fig. 2.17 .

2.6 Tectonic Elements

Computer modelling of plate kinematics has the
primary objective of reconstructing sequences of
past plate configurations. It requires, at a first
step, the specification of the tectonic elements
that will be included in the reconstructions. On
a present day tectonic map, these are defined
as rigid crustal blocks , bounded by paleo-faults,
which have had an independent kinematic history
in the geologic past (Ross and Scotese 1988 ).
This quite general definition applies equally well
to different scales of modelling (global, regional,
or local) and to different structural features, such