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

Whether or not there are extensional detachment faults in the Alboran basement can be tested directly because a part of the Alboran Basin is now emerged. These detachments, related to crustal thinning beneath the Alboran Basin, occurred from the Aquitanian to Tortonian. The resulting extensional geometries can be described in general terms. During the Serravalian a considerable southwest extension of the basin took place, accompanied by south-southeast extension in the northern Gibraltar Arc. Other detachments affected by Serravalian extension can be found. The spreading of the Alboran was nearly coeval with roughly westward migration of the Gibraltar mountain front.

Miocene extensional detachments in the outcropping basement of the northern Alboran Basin (Betics) and their tectonic implications

The Ocean Drilling Program (ODP) Leg 161 drilled in the Western Mediterranean Sea (May‐June, 1995) to investigate outstanding processes in the origin and structural evolution of the Alboran Basin. Studies at the Site 976 basement high, in the West Alboran Basin, demonstrate that the basin is floored with metamorphic rocks of continental origin (high-grade schist, migmatitic gneiss, marble, and calc-silicate rock, cross-cut by granite dikes) from the Alpujarride Complex of the Alboran Crustal Domain. Sediments encountered at the tops of the basement high are middle Miocene in age (about 11‐12 Ma, late Serravallian). The pressure-temperature (P-T) evolution of the high-grade schist shows heating during decompression, from pressures of up to 10.5 kbar at about 500 °C (corresponding to a burial depth of around 40 km) to final temperature conditions of 650o‐700°C at a pressure of 3‐4 kbar. The decompression was followed by cooling to T < 500o‐600 °C and P < 2‐3 kbar. Ar/Ar dating on coexisting muscovite and biotite shows that this cooling occurred from 426 °C at 20.0 ± 0.2 Ma to 330°C at 19.2 ± 0.7 Ma. Apatite fission-track shows that final cooling of the basement below 60 °C took place at 18.3 ± 1.0 Ma (Burdigalian). Site 976 reveals two periods of subsidence, in the middle Miocene (synrift subsidence, at 11‐10.7 Ma; rate of 3 km/m.y.) and since the latest Pliocene (postrift transtensive? subsidence, at 2.5‐0 Ma earliest rate of 0.5 km /m.y.), and uplift at 5‐2.5 Ma (ear ly Pliocene). Conspicuous lateral variations in rift-related subsidence phases in the basin suggest migration of the locus of exte nsion, and probably also changes in tectonic-transport direction during the Miocene rifting. Miocene crustal thinning in the Alb oran Domain is related to low-angle intracrustal detachments and shallow normal faulting. Since the latest Tortonian, relativel y uplifted and subsided areas coexist, but a generalized, continuous, and slower subsidence developed within the marine basin. Drilling results in the East and South Alboran basins (Sites 977, 978, and 979) define the timing of the later (post-Messinian) tectonic reorganization of the Alboran Basin into the present-day Alboran Sea basin. Ar/Ar dating of reworked but representative volcanic pebbles from tholeiitic, calc-alkaline, and shoshonitic series (Sites 977 and 978) constrains episodes of middle- tolate Miocene volcanism in the eastern Alboran region at 12.1 ± 0.2 Ma, 9.90 ± 0.4 Ma, 9.29 ± 0.02 Ma, and 6.1 ± 0.3 Ma. Incompatible elements in these rocks include both depleted (MORB-source) component and an ocean-island or plume-type component (“arc signature”). Calc-alkaline magmatic affinities and incompatible-element systematics were considered to support a Miocene oceanic subduction zone (from 6 Ma to at least 12 Ma) beneath the eastern Alboran region. However, these magmatic signatures prove to have also originated from variable crustal-contamination of MORB-type melts, produced by shallow melting during asthenosphere-mantle upwelling. Alternatively, modeled P-T paths in basement rocks suggest removal of mantle lithosphere below about 60 km depth (to explain further heating during decompression at shallow depth) and rapid exhumation during lithosphere stretching (4.5 km/m.y.), followed by final cooling with a minimum exhumation rate of 4 km/ m.y. It took ~9 m.y. for the exhumation of the basement rocks from 40 km depth to reach the surface at about 18 Ma, thus suggesting that extension in the Alboran Domain started at about 27 m.y. (late Oligocene), before the marine transgression occurred. We consider that strong evidence from rocks at the Site 976 basement high favors models that use the removal of mantle lithosphere as the driving force for extension generating the Alboran Basin. Further geological and geophysical data from the westernmost Mediterranean region are needed to corroborate our conclusions to define a fully satisfying model for the Neogene evolution of the lithosphere beneath the Alboran Basin.

44. the origin and tectonic history of the alboran basin: insights from leg 161 results 1

Multibeam bathymetry and gravity data have been obtained along an ∼800-km-long section of the Mid-Atlantic Ridge from just south of the Hayes fracture zone at 33°N to the northern edge of the Azores Platform near 40°N. A three-dimensional analysis of these gravity and topography data, combined with results from earlier seismic refraction studies in this area, reveal two different scales of crustal heterogeneity. A systematic, along-axis, segment-scale (λ 8–9 km) near the middle of spreading segments and the thinnest crust (<3–4 km) near segment offsets. The magnitude of this along-axis variation in crustal thickness is proportional to the length of the spreading segment and the size of the adjacent ridge offset. There also is a distinct asymmetry in crustal structure across the rift valley near large segment offsets with gravity highs, inferred to be thinner crust, beneath the “inside corner” highs adjacent to these offsets. This segment-scale crustal heterogeneity is similar to that reported from the Kane-to-Atlantis section of the Mid-Atlantic Ridge and from parts of the intermediate-spreading southern Mid-Atlantic Ridge. It is superimposed on a second, longer wavelength variation in gravity and crustal thickness associated with the Azores hot spot. The most pronounced effect of the Azores hot spot on the Mid-Atlantic Ridge occurs between 38°N and 40°N where the ridge axis rapidly shoals by more than 1000 m, the crust thickens by over 2 km, and the rift valley largely disappears. The absence of a deep axial rift valley on the Azores Platform, and near the midpoints of some ridge segments along the Mid-Atlantic Ridge to the south, agrees well with the predictions of recent ridge crest thermal and rheological models that suggest a dependence of axial morphology on both crustal thickness and spreading rate. The transition from a rift valley to an axial high morphology at these spreading rates (∼11 mm/yr half rate) occurs at a crustal thickness of about 9±1km.

Gravity anomalies and crustal thickness variations along the Mid‐Atlantic Ridge between 33°N and 40°N

In order to handle the distortion of large-scale induced electric currents by local conductivity anomalies, the problem of electromagnetic induction in non-uniform thin sheets has been reformulated in terms of an integral equation over the anomalous domain. This formulation considers in the layered substratum in addition to toroidal currents also the poloidal current mode (vertical current loops), at the expense that two scalar functions have to be determined. Simple formulas for the required kernels are derived. The algorithm is applied to model the gross features of the northern Pyrenean induction anomaly. It is suggested that this pronounced anomaly results from a conductive channel between the Atlantic Ocean and the Mediterranean Sea.

/pdf/bimodal-electromagnetic-induction-in-non-uniform-thin-sheets-512s8h3l0h.pdf

Bimodal electromagnetic induction in non-uniform thin sheets with an application to the northern Pyrenean induction anomaly

The Azores triple junction evolution since 10 Ma from an aeromagnetic survey of the Mid-Atlantic Ridge

Magnetic observations made on la soufriere volcano (guadeloupe) during the 1976–1977 crisis

Summary Sixty stations measuring the time variations of the Earth's magnetic field were operated from 1972 to 1974 in the Northern Pyrenees. Magnetograms show a very large anomaly in the vertical and horizontal variation fields for the period range from a few minutes to a few hours. Owing to the particular measurement and reduction techniques used it was possible to show that, for the above period range, the anomalous field is the product of a function of space by a function of time. This anomaly corresponds to a current concentration the geometry of which is invariant with time. Most of the currents associated with the anomaly flow at a depth smaller than 15km. The orientation of the conductive structure is different from the orientation of superficial structures; this structure could correspond to old sedimentary basins inside the palaeozoic basement or (and) to a hidden accident in the basement as suggested by seismic evidence.

/pdf/a-geomagnetic-variation-anomaly-in-the-northern-pyrenees-5a9x17gx69.pdf

A Geomagnetic Variation Anomaly in the Northern Pyrenees

An aeromagnetic survey of the southwest of the Western Mediterranean: Description and tectonic implications

The lack of accurate and detailed magnetic information has, in the past, limited the development of well constrained models for the plate tectonic evolution of the Azores Triple Junction. An aeromagnetic survey, made possible by the existing airport facilities, has long been desired as it can provide high quality magnetic data, whose homogeneity and coherency is far better than those provided by classic marine surveys. The results presented in this paper concern only a part of the Aeromagnetic Survey conducted by the Portuguese Instituto Nacional de Meteorologia e Geofisica and already allow an improved definition of the basic tectonic boundaries at the central part of the Azores plateau.

J. C. Rossignol

Papers

The Azores triple junction evolution since 10 Ma from an aeromagnetic survey of the Mid-Atlantic Ridge

Magnetic observations made on la soufriere volcano (guadeloupe) during the 1976–1977 crisis

A Geomagnetic Variation Anomaly in the Northern Pyrenees

An aeromagnetic survey of the southwest of the Western Mediterranean: Description and tectonic implications

Tectonic framework of the Azores Triple Junction