Motion of Nubia relative to Antarctica since 11 Ma: Implications
for Nubia-Somalia, Pacific–North America, and
India-Eurasia motion
Jean-Yves Royer Centre National de la Recherche Scientifique–Université de Bretagne Occidentale Domaines Océaniques,
Institut Universitaire Européen de la Mer, Place Copernic, 29280 Plouzané, France
Richard G. Gordon


Benjamin C. Horner-Johnson 
Department of Earth Science, Rice University, Houston, Texas 77005, USA
ABSTRACT
Recent estimates of the rotation between Nubia and Somalia have resulted in disparate
poles of rotation for the motion since 3.16 Ma (southwest of South Africa) compared with
that since 11.03 Ma (near the east tip of Brazil). Here we use magnetic anomaly profiles
unavailable in prior Nubia-Antarctica motion studies to significantly revise the estimate
of the rotation between Nubia and Antarctica since 11.03 Ma. We use this newly estimated
rotation to construct revised estimates of Nubia-Somalia, Pacific–North America, and
India-Eurasia motion. The new Nubia-Somalia rotation indicates substantial displacement
of Somalia relative to Nubia over the past 11.03 m.y.: 129 ! 62 km extensional, 90 ! 42
km right-lateral transtensional, and 52 ! 21 km right-lateral transtensional near the
northern extremity of the East African Rift, the northern Mozambique Basin, and the
Andrew Bain Fracture Zone complex, respectively. The substantial rotation between Nubia and Somalia implies that prior plate motion estimates based on a circuit through
Africa are biased by 60–85 km at 11.03 Ma and perhaps by much more for earlier reconstructions. Our results imply that India-Eurasia motion since 11.03 Ma has been !12%
(!5 mm yr"1) slower than, and !20# clockwise of, estimates that neglect Nubia-Somalia
motion. Our results further imply that Pacific–North America displacement since 11.03
Ma has been 5#–10# clockwise of prior estimates and require 58–75 km less extensional
displacement across the Basin and Range since 11.03 Ma than inferred before.
Keywords: Nubia-Somalia motion, Southwest Indian Ridge, Pacific–North America motion,
India-Eurasia motion.
INTRODUCTION
Although the relative motions between Africa and Antarctica have long been studied
(e.g., Le Pichon and Heirtzler, 1968; McKenzie and Sclater, 1971), it has only recently
been possible to resolve the difference between Nubia (West Africa)-Antarctica and Somalia (East Africa)-Antarctica motions.
DeMets et al. (1988) showed that the data then
available along the Southwest Indian Ridge
could not resolve distinct Nubia-Antarctica
and Somalia-Antarctica motion, although they
could not exclude it. Jestin et al. (1994) confirmed this result and, following DeMets et al.
(1990), speculated that the Nubia-Somalia
boundary intersected the Southwest Indian
Ridge. Jestin et al. (1994) further estimated
the angular velocity of Nubia relative to Somalia by combining data from the Red Sea,
Gulf of Aden, and Southwest Indian Ridge,
while assuming that the Nubia-Somalia
boundary intersects the Southwest Indian
Ridge near 30"E.
Using updated transform fault azimuths and
a greatly expanded set of 3-m.y.-averaged
spreading rates along the Southwest Indian
Ridge, Chu and Gordon (1999) were the first
to show that data along the Southwest Indian
Ridge required the existence of distinct Nubian and Somalian plates. Fitting magnetic
anomaly and fracture zone crossings also
shows that the flanks of the Southwest Indian
Ridge record the motion of two distinct plates,
Nubia and Somalia, relative to the Antarctic
plate (Lemaux et al., 2002). Surprisingly,
however, prior estimates of the rotation between Nubia and Somalia have resulted in disparate poles of rotation for the motion since
3.16 Ma compared with that since 11.03 Ma,
with the pole since 3.16 Ma being southwest
of South Africa (Horner-Johnson et al., 2005)
and the pole since 11.03 Ma being near the
east tip of Brazil (Lemaux et al., 2002). Here
we use new profiles collected along the western part of the Southwest Indian Ridge (Ligi
et al., 1999) to improve the estimate of NubiaAntarctica relative motion since the old end of
chron 5, denoted as chron 5o herein. More
specifically, we fit the part of the anomaly corresponding to chron 5n.2o, which is 11.03 Ma
according to the astronomically calibrated reversal time scale of Abdul Aziz et al. (2003).
Here we also explore the implication of this
revised reconstruction for the motion estimated between the Nubian and Somalian plates
across the East African Rift system, for the
motion estimated between the Pacific and
North American plates, and for the motion estimated between the Indian and Eurasian
plates.
DATA
The Nubia-Antarctica plate boundary is
bounded on the west by the Bouvet triple
junction (BTJ), where the Southwest Indian
Ridge meets the southern Mid-Atlantic Ridge
and the South American–Antarctic Ridge (Fig.
1); it is bounded on the east by the Andrew
Bain Fracture Zone (FZ) complex (Fig. 2),
composed of a series of large transform offsets
that appear to be within or near the NubiaSomalia-Antarctica triple junction (Chu and
Gordon, 1999; Horner-Johnson et al., 2005;
Lemaux et al., 2002). Between the BTJ and
Andrew Bain FZ complex, the Southwest In-
Figure 1. Locations of plates and plate
boundaries. Simplified location of boundary
assumed between Nubia and Somalia is
shown by dashed line. Rectangles show locations of Figures 2 and 4.
! 2006 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected].
Geology; June 2006; v. 34; no. 6; p. 501–504; doi: 10.1130/G22463.1; 5 figures.
501
Figure 2. Crossings of old edge of magnetic anomaly 5 flanking western Southwest Indian Ridge (solid line) are displayed on map showing
shaded relief of satellite-derived gravity from Sandwell and Smith (1997). Black-filled circles show locations of magnetic anomaly crossings.
White-filled circles show locations of crossings after reconstruction about our new best-fitting Nubia-Antarctica rotation (2.134# at 6.32#S,
30.51#W with covariance matrix: $xx % 5.263, $xy % 2.011, $xz % "6.344, $yy % 8.676, $yz % "2.134, and $zz % 9.226 in units of 10"6
steradians). Magnetic profiles containing many of these anomaly crossings are shown in figures of Horner-Johnson et al. (2005).
dian Ridge extends over 2000 km with only 5
ridge segments (Fig. 2). Due to the slow
spreading rates (#16 mm yr$1 full rate) prevailing since 40 Ma, the large transform offsets along the Southwest Indian Ridge imply
large age offsets across them, and FZ trends
may be overprinted by changes in the direction of plate motion.
New profiles collected across the westernmost segments of the Southwest Indian Ridge
(Ligi et al., 1999) help in reinterpreting celestially navigated profiles (e.g., R/V Conrad,
1971; R/V Chain, 1974; R/V RSA, 1975) used
by Lemaux et al. (2002). The new data lead
to a new interpretation of paleoridge segmentation along the westernmost portion of the
Southwest Indian Ridge, including identification of a fracture zone unrecognized by Lemaux et al. (2002). Moreover, systematic reidentification of anomaly 2A (Horner-Johnson
et al., 2005) and other anomalies up to anomaly 6 along the Southwest Indian Ridge also
lead to self-consistent mapping of magnetic
lineations, which include further minor changes in prior identifications of anomaly 5. We
selected 58 magnetic crossings along 6 conjugate segments (vs. 60 crossings along 9 conjugate segments; Lemaux et al., 2002). Magnetic crossings were assigned a 1% uncertainty
of 4 km.
NUBIA-ANTARCTICA MOTION
Inversion of the new set of crossings of
anomaly 5o leads to several improvements.
First, the favorable distribution and geometry
of the magnetic crossings do not require any
fracture zone crossings to constrain the fit,
thus avoiding potential systematic bias due to
the complex fracture pattern. Second, the new
Nubia-Antarctica rotation pole moves !30" to
502
the southeast, closer to the Southwest Indian
Ridge (Fig. 3). The rotation pole for chron 5o
is now close to the recently revised pole for
chron 2A (Horner-Johnson et al., 2005). Third,
when the new Nubia-Antarctica rotation is
used to rotate crossings east of the Andrew
Bain FZ, i.e., those crossings believed to record motion between Somalia and Antarctica,
the gap in crossings thus reconstructed is larger than found before (42 & 13 km found here
vs. 23 & 6 km found by Lemaux et al., 2002;
Fig. 4). The size of this gap indicates an average rate of !4 mm yr$1 in the ridgeperpendicular component of Nubia-Somalia
displacement near the Southwest Indian
Ridge. This displacement corresponds to
right-lateral shearing along roughly NNESSW-striking fracture zones.
That the data are well fit by a single
Somalia-Antarctic rotation everywhere east of
the Andrew Bain FZ complex is consistent
with the Somalia-Antarctica plate boundary
continuing at least as far west as the Andrew
Bain FZ complex. The lack of conjugate magnetic data between the Andrew Bain FZ complex and the Du Toit FZ, however, makes it
impossible to determine whether NubianAntarctic or Somalian-Antarctic plate motion
is recorded there, although we suspect the former. West of the Du Toit FZ, the data are well
fit as part of the Nubia-Antarctica plate boundary as indicated for the past 3.16 m.y.
(Horner-Johnson et al., 2005), and poorly
fit if assumed to be part of the SomaliaAntarctica plate boundary.
IMPLICATIONS FOR NUBIA-SOMALIA
MOTION
The effect of the new Nubia-Antarctica rotation is even more striking when it is com-
bined with the chron 5o Somalia-Antarctica
rotation of Lemaux et al. (2002). The resulting
Nubia-Somalia rotation pole moves from near
the east tip of Brazil to !5" northeast of the
BTJ (Fig. 3). The uncertainty region of our
new 11.03 Ma pole includes, and differs insignificantly from, a new pole for the past 3.16
m.y. (Horner-Johnson et al., 2005). The new
3.16 Ma pole, in turn, is far southwest of Chu
and Gordon’s (1999) 3.16 Ma pole (Fig. 3).
Extensional displacements of 129 & 62 km,
90 & 42 km, and 52 & 21 km are predicted
for the past 11.03 m.y. near the northern extremity of the East African Rift, the northern
Mozambique Basin, and the Andrew Bain FZ
complex, respectively (Fig. 3). Extension becomes increasingly oblique, with a large rightlateral component of shearing, as the NubiaSomalia boundary approaches the Southwest
Indian Ridge. The corresponding opening
rates, 12 & 6 mm yr$1, 8 & 4 mm yr$1, and
5 & 2 mm yr$1, are comparable, respectively,
to the rates over the past 3.16 m.y. of 8 & 3
mm yr$1, 6 & 2 mm yr$1, and 4 & 1 mm yr$1
indicated by the angular velocity of HornerJohnson et al. (2005). Within uncertainties,
our predicted rates are also compatible with
velocities estimated from space geodesy (from
7 mm yr$1 in the Ethiopian Rift to 2 mm yr$1
closer to the Nubia-Somalia-Antarctica triple
junction; e.g., synthesis of Fernandes et al.,
2004).
IMPLICATIONS FOR COMPLEX
PLATE CIRCUITS
Africa-Antarctica motion is a critical link in
global plate motion circuits. For example,
Pacific–North America relative motions are
generally estimated by summing motions
across the central Mid-Atlantic Ridge, the
GEOLOGY, June 2006
Figure 4. Reconstructions of magnetic
crossings located east of Andrew Bain Fracture Zone (FZ) complex: crossings are well
fit if assumed to record Somalia-Antarctica
motion but very poorly fit if assumed to record Nubia-Antarctica motion. Symbols:
black-filled circles—unrotated crossings
of magnetic anomaly 5o; white-filled
triangles—crossings rotated by best-fitting
Somalian-Antarctic rotation for chron 5 (Lemaux et al., 2002); white-filled squares—
crossings rotated by best-fitting NubianAntarctic rotation since 11.03 Ma found here
(Fig. 2).
Figure 3. Top: Displacements (and 95% confidence ellipsoids) going forward in time of
selected points on Somalian (Som) plate relative to Nubian (Nub) plate. Longer arrows with
large arrowheads show displacement since 11.03 Ma determined in this paper; shorter arrows with small arrowheads are displacements since 3.16 Ma (Horner-Johnson et al., 2005).
Bottom: Poles of rotation. Pole uncertainty regions are projections of three-dimensional
95% confidence regions for rotations. Squares are Nubia-Antarctic (Ant) poles of rotation
and circles are Nubia-Somalia poles of rotation estimated for chrons 2A and 5o. Revised
estimate of Nubia-Somalia rotation since 11.03 Ma is 1.268# at 50.33#S, 6.43#E with covariance matrix: $xx % 5.264, $xy % 2.070, $xz % "6.400, $yy % 1.076, $yz % "2.152, and $zz %
9.788 in units of 10"6 steradians. Chron 2A poles from Chu and Gordon (1999; CG, triangle)
and from Jestin et al. (1994; stars: JHG-bf best-fitting, JHG-4p 4-plate solution) are shown
for comparison. Small rectangles labeled A, B, and C show locations of upper panels.
Southwest Indian Ridge, and the PacificAntarctic Rise. For example, Atwater and
Stock’s (1998) estimates of Pacific–North
America relative motion do not rely on actual
Nubia-Antarctica motions, but on SomaliaAntarctica motions (i.e., data east of the Andrew Bain FZ complex from Royer and
Chang, 1991). Wilson et al. (2005) estimated
Pacific–North America relative motion using
the Nubia-Antarctica rotations of Chu and
Gordon (1999) and Lemaux et al. (2002). Figure 5 illustrates the effect of using our new
11.03 Ma Nubia-Antarctica rotation on estimates of Pacific–North America motion. The
results of different Pacific–North American
estimates are shown, differing only in the
‘‘Africa’’-Antarctica rotation used. One result
GEOLOGY, June 2006
(labeled A in Fig. 5) uses the rotation of Royer
and Chang (1991), which is effectively a
Somalia-Antarctica rotation because of the
near absence of data west of the Andrew Bain
FZ. A second result (labeled B) uses the
Nubia-Antarctica rotation of Lemaux et al.
(2002), which we now know has incorrect correlations at the western end of the Southwest
Indian Ridge. The third result (labeled C) uses
our newly estimated Nubia-Antarctica rotation. Although the first and second results do
not differ significantly, the first and third differ by a significant 85 & 47 km. We also compare results from Wilson et al.’s (2005) original model (labeled W) and from the same
model updated with our revised NubiaAntarctica link (labeled Wcorr). The point re-
constructed at 11.03 Ma is shifted by 60 km
to the SW. This result indicates that previously
published estimates of Pacific–North America
displacement contain a bias that is several
times the size of the previously published formal uncertainty (!&20 km). Our new reconstruction indicates substantially less overlap
between the Pacific plate and the North American plate than found before. Such overlap
was used by Atwater and Stock (1998) and
Wilson et al. (2005) to infer lower bounds on
amounts of extensional displacement in the
Basin and Range Province in the western
United States. Our results indicate that the true
lower bounds at 11.03 Ma are 58–75 km
smaller than found by them.
Similarly, India-Eurasia relative motion is
traditionally estimated by summing motions
across the northern Mid-Atlantic Ridge, the
central Mid-Atlantic Ridge, the Southwest Indian Ridge, and the Southeast Indian Ridge,
again neglecting the rotation between Nubia
and Somalia. Even if a circuit uses a shortcut
across the Carlsberg Ridge to bypass the deformation of the equatorial Indian Ocean, the
India-Eurasia motions generally neglect the
motions across the East African Rift (e.g.,
503
Figure 5. Implications of new Nubia-Antarctica rotation on global plate circuits. Left: Track
of Pacific plate (Pac) relative to North America (Nam). Tracks A, B, and C (circle symbols)
are based on same rotations for North America–Nubia link (Müller et al., 1999) and
Antarctica-Pacific link (Cande et al., 1995) (updated by J. Stock, 1998, personal commun.).
They differ only in rotation used for Nubia-Antarctica link: track A uses chron 5 rotation
from Royer and Chang (1991); track B uses chron 2A rotation from Chu and Gordon (1999)
and chron 5 rotation from Lemaux et al. (2002); track C uses chron 2A rotation from
Horner-Johnson et al. (2005) and our new chron 5o rotation. Track W (dashed line and
closed triangles) is built with supplemental rotation table of Wilson et al. (2005), which
describes whole plate circuit; in track Wcorr (open triangles), we modify only NubiaAntarctica link with rotations used in track C. Track C and Wcorr are remarkably similar;
small differences in rate result from slightly differing rotations for North America–Nubia
link. Right: Tracks of point from India-Eurasia (Ind-Eur) suture zone relative to Eurasian
plate. Each track combines same Eurasia-Nubia rotations (J.-Y. Royer, 2004, personal commun.) and same Somalia-India rotations (DeMets et al., 2005). Track A assumes no motion
across East African Rift, whereas track B includes chron 2A Nubia-Somalia rotation from
Horner-Johnson et al. (2005) and our new chron 5o rotation. Ellipses on both panels show
resulting combined 95% uncertainty region for reconstructed points.
NUVEL-1; DeMets et al., 1990). In Figure 5,
one reconstruction assumes no motion along
the East African Rift; the other reconstruction
includes our new estimate of Nubia-Somalia
relative motion. The latter indicates motions
of India relative to Eurasia since chron 5 that
are slower than (35 & 2 vs. 40 & 1 mm yr$1),
and !20" clockwise of, those in the former.
Therefore our revised reconstruction along
the western Southwest Indian Ridge leads to
substantial and important revisions in estimates of both Pacific–North America motion
and India-Eurasia motion. The next step (our
current investigation) is the determination of
Nubia-Antarctica reconstructions further back
in time.
ACKNOWLEDGMENTS
We thank J. Stock, C. Chase, and P. Huchon for
constructive reviews. This work was funded by the
Centre National de la Recherche Scientifique/
National Science Foundation (NSF) cooperation
program (Royer and Gordon). Work at Rice University was further supported by NSF grants OCE-
504
0242905 and OCE-0453219. Institut Universitaire
Européen de la Mer contribution 982.
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Manuscript received 24 November 2005
Revised manuscript received 23 January 2006
Manuscript accepted 3 February 2006
Printed in USA
GEOLOGY, June 2006