[1] Previously, we estimated the angular velocity of the Nubian plate relative to the Somalian plate from an updated set of spreading rates and transform fault azimuths. We found that the Nubia-Somalia plate boundary intersects the Southwest Indian Ridge (SWIR) between %26°E and %32°E if both the Nubian and Somalian plates are rigid and if the boundary between them is narrow. These prior results are not completely satisfactory mainly for two reasons: (1) The four-plate circuit Somalia-Antarctica-Nubia-Arabia does not close. (2) The largest (M w 6.8) recorded ''African'' earthquakes that are near, but not along, the SWIR occur near 48°E, well east of the intersection with the Nubia-Somalia boundary. Here we investigate these problems through detailed analysis of plate motion data, especially those along the SWIR east of the Andrew Bain Transform Fault Complex. We find an improved fit to the data and improved plate circuit closure if a region of the African lithosphere is interpreted as a new component plate. This new plate lies between the Nubian and Somalian plates along the SWIR and is separated from the latter by a diffuse boundary that includes the locations of the largest off-ridge earthquakes. Following C. J. H. Hartnady, we call this new plate ''Lwandle.'' Use of this new plate geometry shifts the Nubia-Somalia pole of rotation northeastward to just south of South Africa and thus alters estimates of current India-Eurasia plate motion.

S U M M A R YA new analysis of geologically current plate motion across the Southwest Indian ridge (SWIR) and of the current location of the Nubia-Antarctica-Somalia triple junction is presented. Spreading rates averaged over the past 3.2 Myr are estimated from 103 well-distributed, nearly ridge-perpendicular profiles that cross the SWIR. All available bathymetric data are evaluated to estimate the azimuths and uncertainties of transform faults; six are estimated from multibeam data and 12 from precision depth recorder (PDR) data. If both the Nubian and Somalian component plates are internally rigid near the SWIR and if the Nubia-Somalia boundary is narrow where it intersects the SWIR, that intersection lies between ≈26 • E and ≈32 • E. Thus, the boundary is either along the spreading ridge segment just west of the Andrew Bain transform fault complex (ABTFC) or along some of the transform fault complex itself. These limits are narrower than and contained within limits of ≈24 • E to ≈33 • E previously found by Lemaux et al. from an analysis of the locations of magnetic anomaly 5. The data are consistent with a narrow boundary, but also consistent with a diffuse boundary as wide as ≈700 km. The new Nubia-Somalia pole of rotation lies ≈10 • north of the Bouvet triple junction, which places it far to the southwest of southern Africa. The new angular velocity determined only from data along the SWIR indicates displacement rates of Somalia relative to Nubia of 3.6 ± 0.5 mm yr −1 (95 per cent confidence limits) towards 176 • (S04 • E) between Somalia and Nubia near the SWIR, and of 8.3 ± 1.9 mm yr −1 (95 per cent confidence limits) towards 121 • (S59 • E) near Afar. The new Nubia-Somalia angular velocity differs significantly from the Nubia-Somalia angular velocity estimated from Gulf of Aden and Red sea data. This significant difference has three main alternative explanations: (i) that the plate motion data have substantial unmodelled systematic errors, (ii) that the Nubian component plate is not a single rigid plate, or (iii) that the Somalian component plate is not a single rigid plate. We tentatively prefer the third explanation given the geographical distribution of earthquakes within the African composite plate relative to the inferred location of the Nubia-Somalia boundary along the SWIR.

S U M M A R YA method previously used for estimating the uncertainty in rotation of a plate relative to the hotspots is shown to be insufficient because of the neglect of the uncertainty in one of the three degrees of freedom of a rotation. Two new methods for estimating best-fitting rotations and associated uncertainties for the rotation of a plate relative to the hotspots are presented. Both methods require a priori estimates of the uncertainties in the locations of individual hotspots and their ancient tracks for a particular age. The use of a priori uncertainties permits the hypothesis of hotspot fixity to be tested. The first method, the two-hotspot method, leads to simple geometrical interpretations of best-fitting rotations and of the eigenvectors and eigenvalues of the appropriate covariance matrix. This simplicity comes at the cost of using only two hotspots and their associated tracks with assumed equal circular uncertainties. The second method, the N-hotspot method, allows use of an arbitrary number of hotspots (≥2), with unequal elliptical uncertainties. This generality and power comes at the cost of losing the simple geometrical interpretation of the first method. Elliptical uncertainties are generally more appropriate because hotspot tracks are less well known along-strike than across-strike because of gaps and uncertainties in ages along the tracks.When both methods are applied to the estimate of the Pacific-hotspot rotation since the age of the Hawaiian-Emperor elbow, using the tracks of both the Hawaiian and Louisville hotspots, they produce similar rotations and similar uncertainties. The N-hotspot method is further applied to estimate Pacific-hotspot motion at six additional ages from 10.9 Ma to 67.7 Ma, corresponding to the ages of key magnetic anomalies used in global plate reconstructions. The goodness of the resultant fit shows that the assigned uncertainties and assumption of hotspot fixity are mutually consistent. The pre-elbow (i.e. pre-47.9 Ma or Emperor) pole of rotation is shown to differ significantly from the post-elbow (i.e. post-47.9 Ma or Hawaiian) pole of rotation. Moreover, rotation over the past 47.9 Ma is shown to have occurred in at least three statistically distinct stages, from 47.9 to 20.1 Ma, from 20.1 Ma to ≈6 Ma, and from ≈6 Ma to the present. The change in pole position at 20.1 Ma is accompanied by a highly significant 50 per cent increase in the rate of rotation of the Pacific plate relative to the hotspots.These encouraging results suggest that the new methods can be used to improve and to quantify plate motions relative to the hotspots for a wide variety of applications.

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 ϳ12% (ϳ5 mm yr ؊1 ) slower than, and ϳ20؇ 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. 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.

SUMMARY It has long been challenging to identify magnetic anomalies due to seafloor spreading in the equatorial Pacific. Here we show that Project Magnet vector aeromagnetic profiles from the equatorial Pacific record magnetic anomalies due to seafloor spreading much more clearly than do shipboard total intensity profiles. The anomalies are reliably recorded at wavelengths between ≈20 and ≈150 km in the vertical and east components, which have high coherence, differ in phase by ≈90°, and resemble synthetic magnetic anomaly profiles. From an analysis of a single near‐equatorial vector aeromagnetic profile we infer that the magnetic lineations strike ≈8°–10° counter‐clockwise of north and that magnetic anomaly 7 is located ≈400 km further east than previously estimated. The newly estimated location of anomaly 6 is consistent with a tentative estimate by Wilson from a low‐amplitude shipboard magnetic profile. Because the skewness of profiles over the seafloor formed near the paleoequator changes rapidly with paleolatitude and paleostrike, a skewness analysis of these data may provide useful bounds on the location of Pacific Plate paleomagnetic poles, and indicate that this seafloor has had little, if any, northward motion relative to the spin axis since it formed.