Geology Reference
In-Depth Information
Fig. 2.14
The Alpine-Himalayan collisional orogen.
This belt is composed by several plate boundaries linked
together (
blue lines
with labels). From the West, they are:
Alps, Dinarides (
DIN
), the North Anatolian Fault (
NAF
),
the Makran subduction zone (
MAK
), Sulaiman ranges
(
SUL
), and Himalayas (
HIM
).
Yellow dots
are earthquake
epicenters in the Harvard CMT catalog
the symbol
RRR
can be used to indicate a triple
junction where three spreading ridges meet,
TTR
would indicate a link between two trenches and a
ridge, and so on. At any given time, a plate tec-
tonic configuration can be represented by a graph
G
(
j
,
b
) (in the sense of Computer Science, e.g., see
three nodes and
b
edges, corresponding respec-
tively to triple junctions and plate boundaries. In
this representation, which is useful to investigate
the topological properties of the global system of
tectonic plates, a plate
P
is defined by the ordered
cyclic sequence
f
J
1
,
J
2
, :::,
J
n
g
of triple junctions
that tie its boundaries. It should be noted that
in this representation the exact geometry of the
plate boundaries is unessential, because what we
are describing is a system of relations between
tectonic plates, their interactions, not the specific
geologic details that implement them. If
J
is an
arbitrary node in
G
(
j
,
b
), then
J
is a vertex belong-
ing simultaneously to three adjacent cyclic se-
quences. Starting from this node, the sequence of
triple junctions defining one of the corresponding
tectonic plates is obtained applying the following
simple traversal algorithm:
Algorithm 2.1 (Plate Traversal Algorithm)
1. Select an arbitrary edge of the starting node
J
;
2. Move to the neighbor node through the se-
lected edge. If this is the starting node
J
,then
stop;
3. Now you have two alternative (left and right)
edges to leave the current node. Select the left
edge;
4. Jump to step #2.
In this algorithm, the choice of one of the three
plates associated with
J
is performed implicitly
at step #1 through the selection of a starting edge.
The algorithm can be used as a base for the design
of more sophisticated computational procedures
that investigate the structure of a global plate
tectonic configuration.
The theoretical definition of tectonic plates
discussed above allows to classify them accord-
ing to the number of triple junctions that tie
the corresponding sequence of plate boundaries.
We define the order
N
of a tectonic plate as
the number of triple junctions that are traversed
applying algorithm 2.1, as illustrated in Fig.
2.15
.
Figure
2.16
shows the present day plate tectonic
