identical in terms of shape and chronology. Subsequently,
by averaging nonoverlapping poles with 5–10 Myr windows, a master path was derived for Pangaea that could
be used in any continent in the Jurassic (Figure 1). The
resultant curve shows, when observed from South America, a W-E track between 215 and 197 Ma (Late TriassicEarliest Jurassic), followed by a SE-NW track between
197 and 185 Ma (Early Jurassic). The intersection of these
two tracks defines a cusp at $197 Ma. After 185 Ma until
170 Ma, a smaller track and consecutive standstill that
might have lasted until the Late Jurassic are observed.
The 197–185 Ma (Early Jurassic) track reveals a c. 50
shift in the pole positions and a minimum angular change
of poles of approximately 4
Myr
À1
.
Discussion
There are a few phenomena that could have caused such
shift in the pole positions: either the geomagnetic field
was not dipolar at that time or some geodynamic event
took place. With regard to the first, it is well accepted that
the Earth’s magnetic field behaved essentially as a geocentric dipole (e.g., McElhinny and Brock, 1975; Evans,
1976; Livermore et al., 1983, 1984). This means that the
c. 40
polar shift in the APW path cannot be attributed to
nondipole components. The cause of the polar shift is
rather related to some kind of geodynamic event, such as
lithospheric motion or true polar wander (TPW). In order
to appraise lithospheric motion, the South American plate
constitutes an illustrative case. Paleomagnetic data indicate that, during the Late Triassic and lowermost Jurassic,
the continent rotated 50
CW (Figure 1). Had such rotation resulted from sole lithospheric motion, South America would have been forced to override the trench as well
as the Pacific plate, thus causing major compression in
the overriding plate. Yet, the formation of the Andes comprised two main deformation stages, the first ($ 200 to
90 Ma) characterized by comprehensive extension in the
overriding plate, during which major rift systems like in
Peru, northern Chile (e.g., Cobbing et al., 1981; Åberg
et al., 1984; Mpodozis and Allmendinger, 1993), and
Argentina (e.g., Giambiagi et al., 2008) are formed. The
last stage on the other hand occurred in the
mid-Cretaceous and characterized a major compressional
regime (Somoza and Zaffarana, 2008).
Therefore, the polar shift might have been resulted from
the other geodynamical event which is TPW, defined as
the drift of the spin axis in relation with the solid Earth
over geological time. TPW is determined from hot spots,
which are assumed to form an array of fixed points in
the mantle-mantle reference frame. Defined as it is, TPW
makes all continents and mantle plumes rotate around an
Euler pole located at the Equator (Goldreich and Toomre,
1969; Jurdy and van der Voo, 1975). TPW is a global type
of event; therefore, it should be recognizable in all APW
paths from that time. The similarity in all four curves
(Figure 1) is hence the first hint of TPW. Despite that fixity
of hot spots is still hot matter of debate, many authors such
as Besse and Courtillot (2002) use this uniquely available
tool to determine TPW in the Jurassic.
To assess the occurrence of TPW, the most straightforward way is paleolatitudes plots. For this study, two South
American localities were analyzed (Figure 2). To do this,
continents were first rotated to South American presentday coordinates and rotated once again to position them
with respect to the mantle using Morgan’s (1983) grid of
hot spots. Two sets of paleolatitudes were thus calculated
for each locality, one derived from the master APW path
and the other from the array of hot spots (Figure 2). Had
no TPW existed, both paleolatitudinal sets should coincide. But data show a significant divergence of the curves
after 200 Ma, which could be interpreted as caused by
TPW. The curves show that whereas no evident latitudinal
change occurred from the hotspots reference frame, the
South American continent was actually moving northward. This is solid evidence of the occurrence of a clear
TPW event in the Early Jurassic.
In addition, the TPW path constructed after removing
plate motions in the APW path reveals that at $197 Ma,
poles displaced approximately 45
over a 12-Myr period
or a minimum angular change of 4
Myr
À1 (0.4 Myr
À1
).
This magnitude is consistent with the values predicted
by Prévot et al. (2000) of 1–5
Myr
À1
, or the 3
–10
Myr
À1 proposed by Sager and Koppers (2000) for the
Pacific plate. Subsequently by the end of the Early Jurassic, the path turned back on itself.
Paleomagnetism can provide paleolatitudes and orientations of the continents, but because the time-averaged
geomagnetic field is symmetric about the rotation axis,
the paleomagnetic method alone cannot assess
paleolongitudes. There are other methods to calculate
paleolongitudes such as hot spots. Thus, if the motion of,
e.g., South America, can be established in relation with
the grid of hot spots, all other plates can be put together
within this frame after applying the corresponding kinematic model. Therefore, by combining relative motions
of the plates, their motions relative to the hot spots, and
paleomagnetic data which indicate the ancient position
of the geomagnetic pole (paleopole) for a specific locality
and time, “absolute” paleoreconstructions can be
achieved.
Paleogeographic reconstructions thus obtained reveal
(Figure 3) that during the Late Triassic-Sinemurian
($210 to 200 Ma), Pangaea would have been located at
its southernmost position with continents rotated CCW
with respect to present-day orientation. Subsequently, the
supercontinent moved northward while rotated approximately 50
CW at higher speeds (c. 4
Myr
À1
). By
the Pliensbachian ($185 Ma), the supercontinent
achieved its northernmost latitudes. In the end of the Early
Jurassic, Pangaea moved to the south again until the Middle Jurassic when it positioned similar present-day
latitudes.
Since latitude affects climate, it is possible to correlate
paleolatitudes derived from the paleomagnetic method
with paleoclimatic data derived from fossils having a
PALEOMAGNETISM AND JURASSIC PALEOGEOGRAPHY
633
by averaging nonoverlapping poles with 5–10 Myr windows, a master path was derived for Pangaea that could
be used in any continent in the Jurassic (Figure 1). The
resultant curve shows, when observed from South America, a W-E track between 215 and 197 Ma (Late TriassicEarliest Jurassic), followed by a SE-NW track between
197 and 185 Ma (Early Jurassic). The intersection of these
two tracks defines a cusp at $197 Ma. After 185 Ma until
170 Ma, a smaller track and consecutive standstill that
might have lasted until the Late Jurassic are observed.
The 197–185 Ma (Early Jurassic) track reveals a c. 50
shift in the pole positions and a minimum angular change
of poles of approximately 4
Myr
À1
.
Discussion
There are a few phenomena that could have caused such
shift in the pole positions: either the geomagnetic field
was not dipolar at that time or some geodynamic event
took place. With regard to the first, it is well accepted that
the Earth’s magnetic field behaved essentially as a geocentric dipole (e.g., McElhinny and Brock, 1975; Evans,
1976; Livermore et al., 1983, 1984). This means that the
c. 40
polar shift in the APW path cannot be attributed to
nondipole components. The cause of the polar shift is
rather related to some kind of geodynamic event, such as
lithospheric motion or true polar wander (TPW). In order
to appraise lithospheric motion, the South American plate
constitutes an illustrative case. Paleomagnetic data indicate that, during the Late Triassic and lowermost Jurassic,
the continent rotated 50
CW (Figure 1). Had such rotation resulted from sole lithospheric motion, South America would have been forced to override the trench as well
as the Pacific plate, thus causing major compression in
the overriding plate. Yet, the formation of the Andes comprised two main deformation stages, the first ($ 200 to
90 Ma) characterized by comprehensive extension in the
overriding plate, during which major rift systems like in
Peru, northern Chile (e.g., Cobbing et al., 1981; Åberg
et al., 1984; Mpodozis and Allmendinger, 1993), and
Argentina (e.g., Giambiagi et al., 2008) are formed. The
last stage on the other hand occurred in the
mid-Cretaceous and characterized a major compressional
regime (Somoza and Zaffarana, 2008).
Therefore, the polar shift might have been resulted from
the other geodynamical event which is TPW, defined as
the drift of the spin axis in relation with the solid Earth
over geological time. TPW is determined from hot spots,
which are assumed to form an array of fixed points in
the mantle-mantle reference frame. Defined as it is, TPW
makes all continents and mantle plumes rotate around an
Euler pole located at the Equator (Goldreich and Toomre,
1969; Jurdy and van der Voo, 1975). TPW is a global type
of event; therefore, it should be recognizable in all APW
paths from that time. The similarity in all four curves
(Figure 1) is hence the first hint of TPW. Despite that fixity
of hot spots is still hot matter of debate, many authors such
as Besse and Courtillot (2002) use this uniquely available
tool to determine TPW in the Jurassic.
To assess the occurrence of TPW, the most straightforward way is paleolatitudes plots. For this study, two South
American localities were analyzed (Figure 2). To do this,
continents were first rotated to South American presentday coordinates and rotated once again to position them
with respect to the mantle using Morgan’s (1983) grid of
hot spots. Two sets of paleolatitudes were thus calculated
for each locality, one derived from the master APW path
and the other from the array of hot spots (Figure 2). Had
no TPW existed, both paleolatitudinal sets should coincide. But data show a significant divergence of the curves
after 200 Ma, which could be interpreted as caused by
TPW. The curves show that whereas no evident latitudinal
change occurred from the hotspots reference frame, the
South American continent was actually moving northward. This is solid evidence of the occurrence of a clear
TPW event in the Early Jurassic.
In addition, the TPW path constructed after removing
plate motions in the APW path reveals that at $197 Ma,
poles displaced approximately 45
over a 12-Myr period
or a minimum angular change of 4
Myr
À1 (0.4 Myr
À1
).
This magnitude is consistent with the values predicted
by Prévot et al. (2000) of 1–5
Myr
À1
, or the 3
–10
Myr
À1 proposed by Sager and Koppers (2000) for the
Pacific plate. Subsequently by the end of the Early Jurassic, the path turned back on itself.
Paleomagnetism can provide paleolatitudes and orientations of the continents, but because the time-averaged
geomagnetic field is symmetric about the rotation axis,
the paleomagnetic method alone cannot assess
paleolongitudes. There are other methods to calculate
paleolongitudes such as hot spots. Thus, if the motion of,
e.g., South America, can be established in relation with
the grid of hot spots, all other plates can be put together
within this frame after applying the corresponding kinematic model. Therefore, by combining relative motions
of the plates, their motions relative to the hot spots, and
paleomagnetic data which indicate the ancient position
of the geomagnetic pole (paleopole) for a specific locality
and time, “absolute” paleoreconstructions can be
achieved.
Paleogeographic reconstructions thus obtained reveal
(Figure 3) that during the Late Triassic-Sinemurian
($210 to 200 Ma), Pangaea would have been located at
its southernmost position with continents rotated CCW
with respect to present-day orientation. Subsequently, the
supercontinent moved northward while rotated approximately 50
CW at higher speeds (c. 4
Myr
À1
). By
the Pliensbachian ($185 Ma), the supercontinent
achieved its northernmost latitudes. In the end of the Early
Jurassic, Pangaea moved to the south again until the Middle Jurassic when it positioned similar present-day
latitudes.
Since latitude affects climate, it is possible to correlate
paleolatitudes derived from the paleomagnetic method
with paleoclimatic data derived from fossils having a
PALEOMAGNETISM AND JURASSIC PALEOGEOGRAPHY
633