Environmental Engineering Reference
In-Depth Information
3.3 Reconstruction methods
Until recently, Earth scientists have had no way of calculating the longitudes of
continents before the Cretaceous, leaving palaeomagnetism, which cannot deter-
mine longitude, as the only quantitative means of positioning continents on
the globe before that time. However, by choosing a reference continent that
has moved the least longitudinally (i.e. Africa), longitudinal uncertainty can be
minimized. The analytical trick is to rotate all palaeomagnetic poles to Africa and
calculate a global apparent polar wander path (GAPWaP) in African coordinates,
which serves as the basis for subsequent global reconstructions. This method,
dubbed the
approximation for Africa (Burke and
Torsvik,
2004
), has also allowed us to con
'
zero-longitudinal motion
'
dently estimate true polar wander
(TPW) since Pangea formation at around 320 Ma (Steinberger & Torsvik,
2008
;
Torsvik
et al
.,
2012
).
The past decade has also seen the dawn of so-called hybrid reference frames,
which combine different frames for different time periods. The
first of these
(Torsvik
et al
.,
2008a
) was based on a mantle reference (moving hotspot) frame
for the past 100 Myr, and, before that, a reference frame derived from the African
GAPWaP, making the assumption that Africa has not moved in longitude.
The longitude offset for Africa at 100 Ma (5
) was applied to all the older
reconstructions. Here we use a new moving hotspot reference frame (Dubrovine
et al
.,
2012
) for the past 125 Myr and
-
-
before that
the updated African GAPWaP
of Torsvik
et al
.(
2012
) with TPW correction.
Choosing Africa as a reference plate that has remained quasi-stationary with
respect to longitude only works back to Pangea
is assembly. This is because most
relative plate circuits are reasonably well-known from that time, except for
blocks or continents not belonging to Pangea, such as North and South China.
The latter blocks did not join Eurasia before the early Cretaceous closure of
the Mongol
'
Okhotsk Ocean, and thus well after the main Pangea break-up.
Reconstruction of the
c
. 258 Ma Emeishan LIP in South China therefore requires
a different approach. Using the surface-to-CMB correlation that we have identi-
-
fied (i.e. LIPs from the edges of Tuzo or Jason) we can relocate South China
in longitude so that Emeishan falls above the western margin of Jason (Torsvik
et al
.,
2008b
).
nement of the plume generation zones
Tuzo and Jason (
Figure 3.2
) are prominent features in all shear-wave tomographic
models. LIPs have erupted above their margins, dubbed the plume generation
zones (PGZs) by Burke
et al
.(
2008
). The exact de
3.4 Seismic tomography and re
nition of the PGZs depends
