Page 5 - i1052-5173-26-12

different continents and centered on ca. 145 Ma (Kent and Irving,
2010). The mean location of these three poles plots as the apex or
cusp of a novel APW path when reconstructed to common coor-
dinates using standard relative plate reconstructions. The APW
path was constructed from published paleomagnetic poles from
well-dated magmatic units and only those sedimentary units
corrected for inclination flattening (Tauxe and Kent, 2004; Kent
and Tauxe, 2005). The poles range in age from 230 to 50 Ma
(Triassic to Eocene) from all the major continents that could be
closely linked by plate circuits (see Fig. 4 in Kent and Irving,
2010). The resulting global composite APW path shows the pres-
ence of a previously undetected yet major polar shift of ~30° sepa-
rating the 160 Ma (Oxfordian, Late Jurassic) mean paleopole from
the mean cusp paleopole at 145 Ma, close in age to the Jurassic-
Cretaceous boundary.
Whether the 1–2°/m.y. of Jurassic polar shift represents an
episode of fast whole mantle rotation (TPW) or only rotation
of the reconstructed continental lithosphere (APW), the main
continents in either case rotated in unison clockwise by up to
~30° about a Euler pole centered on the equator (Fig. 1; Kent et al., of 9° (Kent and Irving, 2010).
2015). As a result, Arabia drifted southward from straddling the
ITCZ at ca. 160 Ma to plunging at up to 1–2°/m.y. (up to ~20 cm/yr)
into the tropical arid belt of the southern hemisphere by 145 Ma.
The main continents then rotated in unison anticlockwise about
virtually the same Euler pole so that Arabia drifted northward
back across the ITCZ and into the boreal arid belt by 120 Ma.
Such a rapid polar shift and the backtrack at the 145 Ma cusp are
not precluded either by any speed limit on plates (Meert et al.,
1993) coupled with their negligible momentum (Forsyth and
Uyeda, 1975) nor by models of whole-mantle motions for TPW
(Tsai and Stevenson, 2007; Greff-Lefftz and Besse, 2014).
The critical 145 Ma apex paleopole of Kent and Irving (2010) is
based on an average of three individual entries from radiometri-
cally dated igneous units:
1. The Ithaca kimberlite paleopole from New York State in
North America, based on high-temperature magnetic
component directions bearing dual (normal and reverse)
polarity from seven sites sampled in dikes intruding flat-
lying Devonian platform carbonates, giving a positive baked
contact test and with a geometry that precludes significant
post-emplacement tectonic disturbances (Van Fossen and
Kent, 1993); the Ithaca kimberlites are now provided with a
U/Pb perovskite date of 146.4 ± 1.4 Ma (Kent et al., 2015).
160 Ma arid GoM
arid 145 Ma Ghawar ITCZ E at 160 Ma (light gray shading) and 145 Ma (dark gray
arid Neuqén
GoM
ITCZ
E and counterclockwise from 145 to 120 Ma is located
arid
Neuqén
2. The ca. 145 Ma combined Bumbeni-Swartruggens paleopole
from South Africa, comprised of the Bumbeni syenite
complex dated with Ar/Ar at 145.8 ± 1.3 Ma, which yielded
bipolar paleomagnetic results from four sites, and the
kimberlite fissures at Swartruggens dated with K-Ar and Ar/
Ar to a mean of 142 ± 4 Ma, which also yielded bipolar paleo-
magnetic results from two sites (Main and Male kimberlites;
Hargraves et al., 1997, and references therein).
3. The Hinlopenstretet paleopole from Svalbard, based on high-
temperature magnetic component directions from 17 sites
sampled in dikes dated with K-Ar to a mean age of 144 ± 5 Ma
(Halvorsen, 1989).
The Bumbeni-Swartruggens and Hinlopenstretet paleopoles,
when transferred into North American coordinates using rotation
parameters to account for seafloor spreading in the Atlantic, fall
at only 6° and 8°, respectively, from the cratonic Ithaca paleopole;
this congruence of results from three continents allows erecting a
mean paleopole at ca. 145 Ma with a 95% confidence circle (A95)
The position of the ca. 145 Ma apex paleopole is confirmed by
entirely independent data from magneto-biostratigraphically
dated sections from Adria—the African promontory. These data
have been successfully correlated to the marine magnetic anomaly
M-sequence and corrected for sedimentary inclination shallowing
(Muttoni et al., 2013). Two mean paleomagnetic poles for Adria-
Africa are of particular interest for our discussion: a mean paleo-
pole based on three stratigraphic sections straddling magnetic
polarity chron CM22 at 151 ± 1 Ma in the Early Tithonian, and a
mean paleopole at 145 ± 2 Ma in the mid-Tithonian–Berriasian
(across the Jurassic-Cretaceous boundary) based on six sections
altogether from polarity chron CM17 to CM21 (Muttoni et al.,
2013). These two paleopoles at ca. 151 and ca. 145 Ma lie at only
1° and 7°, respectively, from the ca. 145 Ma paleopole of Kent and
Irving (2010) rotated into African coordinates (Muttoni et al.,
2013). Finally, a mean kimberlite paleopole from Ontario, Canada,
dated with state-of-the-art U/Pb on perovskite to 156 ± 1.6 Ma,
neatly plots in the middle of the Jurassic polar shift and provides
additional independent support for it (Kent et al., 2015).
Acknowledging the limitations of our small subset of nonethe-
less well-dated and inclination flattening-free Late Jurassic paleo-
poles from stable regions of different plates (North America,
Europe, Africa), the overall coherent progression they depict once
restored to a common reference frame, confirmed by magneto-
120 Ma Figure 1. Paleocontinental reconstructions bracketing GSA TODAY | www.geosociety.org/gsatoday
145 Ma the Jurassic polar shift: in left panel, reconstructions
Ghawar shading); in right panel, reconstructions at 145 Ma
(light gray shading; same as in left panel) and 120 Ma
(dark gray shading). The Euler pole (E) for rotating the
continental assemblage clockwise from 160 to 145 Ma
on the equator in the vicinity of the Bight of Benin of
western Africa. The stars connected by arrows
indicate the sites discussed in the text: Ghawar in
Saudi Arabia, Neuqén in Argentina, and the Gulf of
Mexico (GoM). Green latitudinal belts represent the
Intertropical Convergence Zone (ITCZ) within ±5°
equator and the temperate humid belts, whereas the
arid belts encompass the tropics up to ±30° latitude
(modified from Kent et al., 2015).
5