SUMMARY
We describe best-fitting angular velocities and MORVEL, a new closure-enforced set of angular velocities for the geologically current motions of 25 tectonic plates that collectively occupy 97 per cent of Earth's surface. Seafloor spreading rates and fault azimuths are used to determine the motions of 19 plates bordered by mid-ocean ridges, including all the major plates. Six smaller plates with little or no connection to the mid-ocean ridges are linked to MORVEL with GPS station velocities and azimuthal data. By design, almost no kinematic information is exchanged between the geologically determined and geodetically constrained subsets of the global circuit—MORVEL thus averages motion over geological intervals for all the major plates. Plate geometry changes relative to NUVEL-1A include the incorporation of Nubia, Lwandle and Somalia plates for the former Africa plate, Capricorn, Australia and Macquarie plates for the former Australia plate, and Sur and South America plates for the former South America plate. MORVEL also includes Amur, Philippine Sea, Sundaland and Yangtze plates, making it more useful than NUVEL-1A for studies of deformation in Asia and the western Pacific. Seafloor spreading rates are estimated over the past 0.78 Myr for intermediate and fast spreading centres and since 3.16 Ma for slow and ultraslow spreading centres. Rates are adjusted downward by 0.6–2.6 mm yr−1 to compensate for the several kilometre width of magnetic reversal zones. Nearly all the NUVEL-1A angular velocities differ significantly from the MORVEL angular velocities. The many new data, revised plate geometries, and correction for outward displacement thus significantly modify our knowledge of geologically current plate motions. MORVEL indicates significantly slower 0.78-Myr-average motion across the Nazca–Antarctic and Nazca–Pacific boundaries than does NUVEL-1A, consistent with a progressive slowdown in the eastward component of Nazca plate motion since 3.16 Ma. It also indicates that motions across the Caribbean–North America and Caribbean–South America plate boundaries are twice as fast as given by NUVEL-1A. Summed, least-squares differences between angular velocities estimated from GPS and those for MORVEL, NUVEL-1 and NUVEL-1A are, respectively, 260 per cent larger for NUVEL-1 and 50 per cent larger for NUVEL-1A than for MORVEL, suggesting that MORVEL more accurately describes historically current plate motions. Significant differences between geological and GPS estimates of Nazca plate motion and Arabia–Eurasia and India–Eurasia motion are reduced but not eliminated when using MORVEL instead of NUVEL-1A, possibly indicating that changes have occurred in those plate motions since 3.16 Ma. The MORVEL and GPS estimates of Pacific–North America plate motion in western North America differ by only 2.6 ± 1.7 mm yr−1, ≈25 per cent smaller than for NUVEL-1A. The remaining difference for this plate pair, assuming there are no unrecognized systematic errors and no measurable change in Pacific–North America motion over the past 1–3 Myr, indicates deformation of one or more plates in the global circuit. Tests for closure of six three-plate circuits indicate that two, Pacific–Cocos–Nazca and Sur–Nubia–Antarctic, fail closure, with respective linear velocities of non-closure of 14 ± 5 and 3 ± 1 mm yr−1 (95 per cent confidence limits) at their triple junctions. We conclude that the rigid plate approximation continues to be tremendously useful, but—absent any unrecognized systematic errors—the plates deform measurably, possibly by thermal contraction and wide plate boundaries with deformation rates near or beneath the level of noise in plate kinematic data.

Geologically current plate motions

/pdf/phanerozoic-polar-wander-palaeogeography-and-dynamics-3fc5flnbf0.pdf

Phanerozoic polar wander, palaeogeography and dynamics

A number of tectonic events occurred contemporaneously in the Mediterranean region and the Middle East 30–25 Myr ago. These events are contemporaneous to or immediately followed a strong reduction of the northward absolute motion of Africa. Geological observations in the Neogene extensional basins of the Mediterranean region reveal that extension started synchronously from west to east 30–25 Myr ago. In the western Mediterranean it started in the Gulf of Lion, Valencia trough, and Alboran Sea as well as between the Maures massif and Corsica between 33 and 27 Ma ago. It then propagated eastward and southward to form to Liguro-Provencal basin and the Tyrrhenian Sea. In the eastern Mediterranean, extension started in the Aegean Sea before the deposition of marine sediments onto the collapsed Hellenides in the Aquitanian and before the cooling of high-temperature metamorphic core complexes between 20 and 25 Ma. Foundering of the inner zones of the Carpathians and extension in the Panonnian basin also started in the late Oligocene-early Miocene. The body of the Afro-Arabian plate first collided with Eurasia in the eastern Mediterranean region progressively from the Eocene to the Oligocene. Extensional tectonics was first recorded in the Gulf of Aden, Afar triple junction, and Red Sea region also in the Oligocene. A general magmatic surge occurred above all African hot spots, especially the Afar one. We explore the possibility that these drastic changes in the stress regime of the Mediterranean region and Middle East and the contemporaneous volcanic event were triggerred by the Africa/Arabia-Eurasia collision, which slowed down the motion of Africa. The present-day Mediterranean Sea was then locked between two collision zones, and the velocity of retreat of the African slab increased and became larger than the velocity of convergence leading to backarc extension. East of the Caucasus and northern Zagros collision zone the Afro-Arabian plate was still pulled by the slab pull force in the Zagros subduction zone, which created extensional stresses in the northeast corner of the Afro-Arabian plate. The Arabian plate was formed by propagation of a crack from the Carlsberg ridge westward toward the weak part of the African lithosphere above the Afar plume.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2000TC900018

Mediterranean extension and the Africa‐Eurasia collision

A model of three-dimensional shear wave velocity variations in the mantle reveals a tilted low velocity anomaly extending from the core-mantle boundary (CMB) region beneath the southeastern Atlantic Ocean into the upper mantle beneath eastern Africa. This anomaly suggests that Cenozoic flood basalt volcanism in the Afar region and active rifting beneath the East African Rift is linked to an extensive thermal anomaly at the CMB more than 45 degrees away. In contrast, a low velocity anomaly beneath Iceland is confined to the upper mantle.

https://science.sciencemag.org/content/sci/286/5446/1925.full.pdf

Complex Shear Wave Velocity Structure Imaged Beneath Africa and Iceland.

A simplified model of continental extension including lower crustal flow is developed. The model employs the thin sheet approximation in estimating lithospheric yield strength and gravitational buoyancy forces arising from lateral variations in crustal thickness and temperature. The effect of advection and diffusion of heat on the temperature structure and yield strength of the extending region is calculated. The viscosity of the lower crust, which controls its rate of flow, is estimated in a manner consistent with the yield strength calculation. The change in the force required to extend the lithosphere is calculated after a finite amount of extension. When this force increases, the zone of extension is assumed to widen; when it decreases, the extension remains localized. Material parameters affecting the model results are the density difference between the crust and mantle, the gradient of brittle yield stress with depth, the abundance of heat-producing elements within the crust, and the ductile strengths of the crust and mantle as a functions of temperature and strain rate. The model predicts three distinct modes of extension depending on the model crustal thickness, heat flow, and strain rate. The modes are (1) core complex mode (concentrated upper crustal extension with lower crustal thinning over a broad area and any mantle lithosphere extending in a local area), (2) wide rift mode (uniform crustal and mantle lithospheric thinning over a width greater than the lithospheric thickness), and (3) narrow rift mode (concentrated crustal and mantle lithospheric extension). Models with a dry olivine mantle rheology and a dry quartz or dry anorthosite crustal rheology are consistent with geologic observations of crustal thickness and heat flow for all three extensional modes. An analysis of the force changes due to extension, ignoring conduction of heat, is carried out to give greater insight into how variations in crust and mantle rheologic parameters affect model results. Crustal rheologies several orders of magnitude stronger or weaker than dry quartz give results inconsistent with observations. The model implies a temporal progression from core complex mode to wide rift mode to narrow rift mode. Data from the southern Basin and Range Province are consistent with this progression.

Modes of continental lithospheric extension

The geology of northern and central Africa is characterized by broad plateaux, narrower swells and volcanism occurring from ∼45 Myr ago to the present. The greatest magma volumes occur on the >1,000-km-wide Ethiopian and east African plateaux, which are transected by the Red Sea, Gulf of Aden and east African rift systems, active since the late Oligocene epoch. Evidence for one or more mantle plumes having impinged beneath the plateaux comes from the dynamic compensation inferred from gravity studies, the generally small degrees of extension observed and the geochemistry of voluminous eruptive products1,2,3,4. Here we present a model of a single large plume impinging beneath the Ethiopian plateau that takes into account lateral flow and ponding of plume material in pre-existing zones of lithospheric thinning5. We show that this single plume can explain the distribution and timing of magmatism and uplift throughout east Africa. The thin lithosphere beneath the Mesozoic–Palaeogene rifts and passive margins of Africa and Arabia guides the lateral flow of plume material west to the Cameroon volcanic line and south to the Comoros Islands. Our results demonstrate the strong control that the lithosphere exerts on the spatial distribution of plume-related melting and magmatism.

Cenozoic magmatism throughout east Africa resulting from impact of a single plume

Occurring mainly over a week in September 2005, a 60-km-long section of the Afar depression in Ethiopia was torn apart by the injection of over 2 cubic kilometres of molten rock into the plate: an 8-metre-wide gap appeared at the surface. Satellite radar imagery reveals that a series of fissures opened, the rift shoulders rose, and the ground surface dropped above the molten rock. A similarly large rift took place in Krafla in Iceland 25 years ago, but in a series of events over a ten year period. The Afar incident suggests that magma intrusion into a dyke, rather than faulting of the crust, may be responsible for the segmentation of continental rifts. Seafloor spreading centres show a regular along-axis segmentation thought to be produced by a segmented magma supply in the passively upwelling mantle1,2. On the other hand, continental rifts are segmented by large offset normal faults, and many lack magmatism. It is unclear how, when and where the ubiquitous segmented melt zones are emplaced during the continental rupture process. Between 14 September and 4 October 2005, 163 earthquakes (magnitudes greater than 3.9) and a volcanic eruption occurred within the ∼60-km-long Dabbahu magmatic segment of the Afar rift, a nascent seafloor spreading centre in stretched continental lithosphere3,4. Here we present a three-dimensional deformation field for the Dabbahu rifting episode derived from satellite radar data, which shows that the entire segment ruptured, making it the largest to have occurred on land in the era of satellite geodesy. Simple elastic modelling shows that the magmatic segment opened by up to 8 m, yet seismic rupture can account for only 8 per cent of the observed deformation. Magma was injected along a dyke between depths of 2 and 9 km, corresponding to a total intrusion volume of ∼2.5 km3. Much of the magma appears to have originated from shallow chambers beneath Dabbahu and Gabho volcanoes at the northern end of the segment, where an explosive fissural eruption occurred on 26 September 2005. Although comparable in magnitude to the ten year (1975–84) Krafla events in Iceland5, seismic data suggest that most of the Dabbahu dyke intrusion occurred in less than a week. Thus, magma intrusion via dyking, rather than segmented normal faulting, maintains and probably initiated the along-axis segmentation along this sector of the Nubia–Arabia plate boundary.

/pdf/magma-maintained-rift-segmentation-at-continental-rupture-in-c9husmxlum.pdf

Magma-maintained rift segmentation at continental rupture in the 2005 Afar dyking episode

The Western rift, the western branch of the East African rift system, is bordered by high-angle normal fault systems bounding one side of spoon-shaped basins. Depth-to-detachment estimates of 20-30 km, the rollover geometry of asymmetric basins, and seismicity throughout the depth range 0-30 km suggest that planar border faults along one side of rift basins penetrate the crust. Along the length of the rift, ∼100-km-long en echelon border-fault segments are linked by obliqueslip transfer faults, ramps, and monoclines within comparatively high-strain accommodation zones. These transfer faults accommodate significant along-axis variations in the elevation of basins and uplifted rift flanks but do not appear to extend outside the 40- to 70-km-wide rift basins across the uplifted flanks. Kinematic constraints from several basins indicate a regional east-west extension direction that is consistent with existing seismicity data. Estimates of crustal extension based on fault geometries interpreted from seismic reflection data and field observations within 13 basins are less than 15% (<10 km). Volcanism began at ∼12 Ma in the north and at ∼7 Ma in the south prior to or concurrent with the initial development of Western rift sedimentary basins. During Pleistocene time, asymmetric basins have narrowed through progressive hanging-wall collapse and with uplift of the rift flanks. These geometric and temporal patterns of development are similar to those of the more magmatically active Kenya rift, where volcanic activity began ∼11 m.y. prior to initial volcanism in the Western rift. Both the poor correlation between Miocene-Recent border and transfer faults and pre-Miocene shear zones, and the repetitive basin geometries indicate that the along-axis segmentation generally is not inherited. Western rift border-fault segments propagated to the north and south to link originally isolated basins, and this along-axis propagation contributes to the segmentation of the Western rift valley.

Tectonic development of the western branch of the East African rift system

Mechanical processes largely control the along-axis segmentation of continental rifts; however, asthenospheric processes strongly influence the along-axis segmentation of mid- ocean ridges. We examine the distribution of strain and magmatism in the transitional active Main Ethiopian rift. Magmatic construction, diking, and faulting during the past 1.6 m.y. have created ∼20-km-wide, ∼60-km-long magmatic segments with or without axial valleys. Magmatic segments are arranged en echelon within the ∼100-km-wide rift valley bounded by mid-Miocene border faults. Geodetic data show that magmatic segments accommodate >80% of the strain across the rift, indicating that border faults are no longer the locus of extension. Comparison with mid-ocean ridges suggests that magmatic segments, rather than detachment faults, mark the ocean-continent boundary in rifts with a ready magma supply. Magmatic margins, therefore, may contain detachments abandoned during continental breakup. The processes of localized dike intrusion with underplating would produce strips of mafic crust transitional to oceanic crust, but without coherent seafloor-spreading magnetic anomalies.

Continental breakup in magmatic provinces: An Ethiopian example

Both oceanic and continental rifts show regular along-axis segmentation, but the relationship, if any, between the two is poorly understood. The tectonically active East African Rift system encompasses systematic along-axis variations in extension and magmatism, making it possible to explore the links between strain, magmatism, and the length scales of faulting and depositional systems during rift development. We summarize Quaternary along-axis segmentation within the Afar Rift system, which is transitional to seafloor spreading, and compare it with segmentation in a “continental” rift sector, the Main Ethiopian Rift (MER) system to the south. We use high-resolution satellite imagery calibrated by field studies and digital topography data to delineate faults and magmatic centers in unmapped areas and to compare with existing geological reports. From south to north we see a significant and systematic decrease in the lengths of young basin-bounding normal faults (∼50 km long in MER to ≤ 15 km long in northern Afar), in the lengths and widths of rift basins (from 50 to 100 km long × 30 to 80 km wide in MER to ≤ 20 km long × ≤ 5 km wide in northern Afar), and in the relief of the uplifted rift flanks surrounding the basins (from ≥ 1000 m high in MER to ≤ 100 m high in northern Afar). Erosional escarpments along the older, outer rift margins of Afar reveal long, high border fault segments similar to those found in the less evolved MER, suggesting that the Afar Rift was originally bounded by much longer faults. In northern Afar, there is a drastic increase in the volume of Quaternary basalts, with segmentation now dominated by 50- to 80-km-long volcanic ridges, which are similar in size, morphology, and spacing to the second order, nontransform offset segmentation of slow-spreading mid-oceanic ridges. The along-axis changes in rift segmentation are concomitant with decreasing crustal thickness (i.e., cumulative strain), decreasing effective elastic thickness estimates, and increasing magma supply. These temporal and spatial patterns suggest that segmented continental rifts can evolve into segmented oceanic rifts dominated by mantle sourced magmatic processes.

Cynthia Ebinger

Papers

Cenozoic magmatism throughout east Africa resulting from impact of a single plume

Magma-maintained rift segmentation at continental rupture in the 2005 Afar dyking episode

Tectonic development of the western branch of the East African rift system

Continental breakup in magmatic provinces: An Ethiopian example

Variations in the along‐axis segmentation of the Afar Rift system