Geology Reference
In-Depth Information
Fig. 2.29
A fragment of
rotation model
are coded through plate identifiers. In order to
determine which of the two plates in a conju-
gate pair must be considered as the reference
plate, we shall conform to the principle that high-
degree nodes always appear at higher levels in
the hierarchical structure. Regarding the defini-
tion of the stage boundaries, it is necessary to
distinguish between the large first-order plates,
in a context of global tectonics, and the case
of small tectonic elements associated with intra-
plate deformation or collisional settings. A key
observation is that the changes of relative mo-
tion between large plates during the Mesozoic
and the Cenozoic, hence presumably also dur-
ing earlier time intervals, seem to have occurred
synchronously on a global scale, thereby the
major stage boundaries coincide. As an example,
the classic plate motions model of Müller et al.
(
1997
) is based on 15 synchronous stages from
the early Jurassic to the present. This implies that
the motions of the major tectonic plates cannot be
determined
exclusively
by processes occurring in
the mantle, including the subduction of slabs, and
independently from each other. Therefore, at any
time Earth's tectonic plates must be considered
as a system of interacting bodies. Conversely,
stage boundaries associated with changes of stage
poles between small plates and other tectonic el-
ements must be established on a geological basis
and are not necessarily synchronous with major
events of reorganization of the plate boundaries.
In the next section, we shall discuss some impor-
tant details of the procedures followed in plate
kinematics for the construction of plate motions
models.
2.8
Plate Tectonic
Reconstructions
Usually plate motions models include a recon-
struction of the initial configuration, preceding
the development of plate boundaries. Figure
2.30
illustrates an example of fit of Pangaea, the large
supercontinent that existed before the opening of
the Atlantic ocean.
In the previous sections, we have learnt that
there are two kinds of initial fits: pre-rift fits,
which show the configuration of the continen-
tal masses preceding the development of plate
boundaries, and post-rift fits, which match the
stretched continental margins and show the con-
figuration at the onset of sea floor spreading.
In both cases, the match between the conjugate
COBs is performed through a geometrical fitting
procedure. The algorithm used by Bullard et al.
(
1965
) in their reconstructions of Pangaea was
the first rigorous method for fitting continental
margins. Here we shall discuss an improved ver-
sion of this algorithm, which was proposed by
Schettino and Turco (
2009
). Let us assume that
the COBs to be fitted are represented by two
series of unit vectors, respectively (
p
1
,
p
2
, :::,
p
N
)
and (
q
1
,
q
2
, :::,
q
M
), which have been preliminar-
ily rotated to a geographic reference frame where
a test Euler pole
e
, with coordinates (œ
e
,¥
e
), has
been moved to the North Pole (Fig.
2.31
). A
transformation of the standard geographic coor-
dinates to this new reference frame is obtained
by rotating each position vector
p
i
and
q
j
about
an Equatorial pole placed at (0
ı
,¥
e
C
90
ı
)by
