Absolute Plate Velocity Research Articles

Patterns of hot spot volcanism

The aim of this paper is to identify the characteristic features, in terms of geophysically measured parameters, of the hot spot sites over the Earth's surface. We use a pattern recognition approach which identifies associations as well as single parameters, and our statistical algorithm allows us to work at a given significance level, avoiding any overfit. We use the following parameters: absolute plate velocity of the site, minimum distance between neighboring hot spots, minimum distance from a ridge, minimum distance from a trench, geoid anomaly of harmonic degrees 2–10 and 11–36, area of the host plate, lithospheric thickness and stress state. The hot spots, generally defined as volcanoes not obviously related to plate boundaries, are found to occur in sites characterized by either one of two main significant patterns: (1) positive values of geoid anomalies with harmonic degree between 2 and 10, which is characteristic of most of the hot spots with a “track” of extinct volcanoes; (2) clustered volcanoes (distance between volcanoes less than ∼900 km) in slow moving plates (velocity <2.5 cm/yr). A third pattern, representative of isolated volcanoes (distance between volcanoes greater than ∼900 km) anomalously close to mid‐ocean ridges (distance from the ridge less than ∼600 km) can also be tentatively identified. This result implies the existence of two main types of hot spot volcanism, the first one of sublithospheric origin with dynamics which is not influenced by surface tectonics, and the second one mainly due to a favorably “soft” lithosphere. A third type would probably consist of “anomalous” and magmatically very productive parts of mid‐ocean ridges.

Global patterns of azimuthal anisotropy and deformations in the continental mantle

SUMMARY We present a summary of measurements of azimuthal anisotropy in the continental mantle based on the SKS technique and performed mostly with the active participation of the authors. The directions of polarization of the fast quasi-shear wave and the time delays between the quasi-shear waves are obtained at nearly 70 locations in all continents, except Antarctica. These data are interpreted in terms of lattice-preferred orientation of olivine which is caused by deformations in the mantle. The depth interval responsible for anisotropy is unknown but the data suggest that it may reach at least 300 km. The fast directions in SKS do not show clear correlation with the fast directions of the teleseismic P at the same seismograph stations. In the regions of present-day convergence the fast direction of anisotropy usually aligns with the plate boundary. This correlation implies that the direction of shortening is the same in the crust and the upper mantle. In the regions of rifting, the inferred direction of mantle flow usually aligns with the direction of extension in the crust. Outside the regions of recent tectonic activity we, most likely, observe a combined effect of frozen anisotropy in the subcrustal lithosphere and of recently formed anisotropy in the asthenosphere. On a global scale, in these regions there is a positive correlation between the absolute plate velocity directions and the fast directions of anisotropy. The correlation is especially strong in central and eastern parts of North America. A clear absence of any evidence of large-scale azimuthal anisotropy in the data of long-range refraction profiling for the upper 100 km of the mantle of that region implies that the effect in SKS is generated mainly at greater depths, in the asthenosphere. Orientation of olivine at these depths reflects recent and present-day flow in the mantle rather than processes of a distant geologic past.

Global upper mantle tomography of seismic velocities and anisotropies

A data set of 2600 paths for Rayleigh waves and 2170 paths for Love waves enabled us to retrieve three‐dimensional distributions of different seismic parameters. Shallow layer corrections have been carefully performed on phase velocity data before regionalization and inversion at depth. The different seismic parameters include the five parameters of a radially anisotropic medium and the eight azimuthal anisotropic parameters as defined by Montagner and Nataf. It is found that the lateral heterogeneities of velocities and anisotropies in the upper mantle are dominated down to 250–30 km by plate tectonics with slow velocities below ridges, high velocities below continents and a velocity increasing with the age of the seafloor. Anisotropy is present in this whole depth range and the directions of maximum velocities are in good agreement with absolute plate velocities. Below 300 km, there is a sharp decreasing of the amplitude of lateral heterogeneities of seismic velocities and anisotropies. Below 450 km, lateral heterogeneities display a degree 2 and to a less extent a degree 6 pattern. Therefore, between 250 km and 450 km, there is a transition region where vertical circulation of matter is possible as shown by subducted slabs and “plumes” of slow velocities but which probably separates two types of convection. The first one is closely related to plate tectonics and to the distribution of continents. The second one dominates below 450 km and is characterized by two downgoing and two upgoing flows.

Southern Hemisphere Origin of the Cretaceous Laytonville Limestone of California

New paleomagnetic, paleontologic, and stratigraphic data from outcrops of the Laytonville Limestone (101 to 88 million years old) support a Southern Hemisphere origin. A paleomagnetic megaconglomerate test is statistically significant and suggests magnetization at 14 degrees +/- 5 degrees south, predating Late Cretaceous to Eocene (70 to 50 million years ago) accretion. Rapid Kula plate movement or the existence and demise of a now vanished oceanic plate (or both) are required to accommodate the greater than 50 degrees of poleward displacement implied by the paleomagnetic data. This rapid motion brings into question the validity of a "speed limit" for absolute plate velocity based on present-day plate motions.

Late Mesozoic and Cenozoic palaeomagnetism of Australia?II. Implications for geomagnetism and true polar wander

Summary. New palaeomagnetic results from Australia indicate that throughout the Cenozoic era the continent lay further south than suggested by hot-spot data. Moreover, while hot spots give a uniform rate of drift during most of the Cenozoic, the drift rate obtained from apparent polar wander varies considerably. The discrepancies between the palaeomagnetic and hot-spot results are analysed by comparing the Australian data with those of Europe and the central Pacific. The analysis suggests that the discrepancies are due to: (1) departures of the Earth's magnetic field from the geocentric axial dipole model, and (2), either true polar wander or a non-axial inclined dipole component. It is found that since the mid-Tertiary the dominant non-dipole component has been a quadrupole, and that during this period both the quadrupolar field and the true polar displacement/non-axial dipole component decreased progressively. During the Quaternary, and also at the earliest Tertiary, the non-dipole components appear to have been moderate or small. The comparison of data sets demonstrates that considerable errors may be incurred when Cenozoic, and presumably earlier, poles from one geographic region are used to derive those of another, widely separated, region. The results also imply that absolute plate velocities estimated from palaeomagnetic data can contain substantial errors, and that hot-spot data may need significant adjustments for true polar wander to yield correct palaeolatitudes. Finally, the new early Tertiary pole for Australia is used in conjunction with updated early Tertiary poles from other lithospheric plates to reapply the McKenzie test for true polar wander. The results indicate a small true polar displacement since the beginning of the Tertiary. The amount and direction of the displacement, however, differ from those generally obtained from hot-spot data.

Global plate motions relative to the hot spots 64 to 56 Ma

We examined the early Tertiary global plate velocities with respect to the hot spots and compared these velocities both to those at present and to early Tertiary velocities inferred by assuming that no net torque is exerted on the lithosphere as a whole. In our reconstruction, the velocities of the Pacific and African plates were inferred directly from their hot spot tracks, and the velocities of other plates with respect to the hot spots were calculated from the motion of the Pacific or African plate and by relative motions estimated from seafloor spreading data. The relative motion between assumed‐separate North Pacific and South Pacific (Chatham Rise) plates was estimated by assuming the hot spots in the Atlantic Ocean basin have been fixed with respect to those in the Pacific Ocean basin, thereby avoiding the use of a poorly defined relative motion circuit. We found that the characteristics of early Tertiary plate motions with respect to the hot spots resemble those at present. The root‐mean‐square velocity of every major continental plate with respect to the hot spots during the early Tertiary exceeds its present velocity but (with the exception of the Indian plate) is less than the root‐mean‐square velocity of every early Tertiary oceanic plate. Equatorial lithosphere moved faster than polar lithosphere during the early Tertiary, but the difference is less than at present. Over the interval 64 to 56 Ma, the no net‐torque absolute velocities differ from the assumed fixed‐hot spot velocities by only 0.04°/Ma, which is insignificant. Thus two dissimilar approaches, fixed hot spots and no net torque of the lithosphere, when applied to identical early Tertiary relative plate velocities, yield similar absolute plate velocities. We attribute the differences between these two reference frames found in prior investigations to relative plate velocities different than we assume here.

Tectonic interpretation of the Trans-Mexicano Volcanic Belt

Although some workers interpret subduction along the Middle America Trench to be directly associated with volcanism in the Trans-Mexico Volcanic Belt, geographic distribution of intermediate-depth earthquakes, analysis of Sn propagation, and a limited number of focal-mechanism solutions suggest that the belt is largely independent of subduction. These data, in conjunction with the position of the volcanic belt with respect to known plate boundaries in the region, suggest that the belt represents the incompletely developed northern boundary of a developing micro-plate. It is a zone of transtension that probably results from slightly different rates of absolute plate velocities in the region.

Models for convergence, subduction and back-arc development

Abstract Largely because of the wide variety of observational constraints which must be satisfied, the search for a viable driving mechanism is perhaps the most perplexing problem related to plate tectonics. The mechanism must be compatible with the rigid behavior of lithospheric plates, and with a wide range of plate sizes, shapes and motions. It must be consistent with complex configurations of plate boundaries and equally complex boundary interactions, such as the destruction of ridges at subduction zones. The mechanism must produce steady-state relative and absolute plate motions which persist for tens of millions of years, but must also account for sudden dramatic changes. Finally, the plate driving mechanism must be consistent with the non-Newtonian properties of olivine and with the fabrics of upper mantle peridotites. Mounting evidence suggests that plate motions result from forces associated with plate boundaries and that the principal resisting force is drag at the base of the lithosphere, particularly beneath continents Several investigators have suggested that gravitational forces acting on thermally-induced, lateral density variations in the upper mantle are the principal driving forces for plate tectonics. If so, plate motions are ultimately controlled by the temperature distribution in the upper mantle, and plate tectonics represents a state of dynamic equilibrium in which plate motions are both the cause and the consequence of temperature and density variations in the mantle. This concept requires that average absolute plate velocities be predictable from the characteristics of individual plates, and that plates tend to move down horizontal temperature gradients. A simple linear relation which includes contributions from ridge push (RP) , slab pull (SP) , trench suction (TS) and continental drag ( CD ): ( cm / y ) = (2.6 ± 0.4) + (4.8 ± 1.8) RP + (14.3 ± 1.7) SP +(3.5 ± 2.5) TS −(5.1 ±0.7) CD predicts plate velocities with an rms error of 0.44 cm/y, and a correlation coefficient of 0.98. That plate velocities can be accurately predicted from their own boundary configurations and proportions of continental lithosphere is strong evidence that plate motions result from negative buoyancy forces associated with plate boundaries.

Continental drift and true polar wandering

Summary. Evidence in the form of 75 yr of ILS data is accumulating which suggests that true polar wander may be currently taking place. It seems likely that true wander of some magnitude must always accompany plate motions, but the extrapolated ILS rate is an order of magnitude larger than the rate of true polar wander deduced from palaeomagnetic data over the past 55 Myr. The conflict between palaeomagnetic and latitude data provides the motivation for investigating one possible excitation of polar wander, the mass redistribution which accompanies continental drift. The mass redistribution arises mainly because of the contrasting density structure of oceanic and continental regions. The change in the inertia tensor resulting from 106yr of plate motions is found to be negligibly small; even consideration of episodic plate movements, anelasticity, or a decoupled lithosphere cannot boost the effect to the ILS rate of polar wander. These conclusions are strengthened by the fact that any one of several absolute plate velocity models, based on extremely diverse assumptions, yields the same results. In contrast, preliminary findings regarding the effect of Pleistocene deglaciation activities on the inertia tensor reveal that such non-isostatic phenomena may have a large influence on polar wander.

Global plate tectonics and the secular motion of the Pole

Astronomical data compiled during the last 70 years by the international organizations (ILS/IPMS, BIH) providing the coordinates of the instantaneous pole, clearly shows a continuous drift of the “mean pole” (≡barycenter of the wobble cycle with respect to the Conventional International Origin (CIO). This study was undertaken to investigate the possibility of an actual secular motion of the barycenter (approximated by the earth’s maximum principal moment of inertia axis or axis of figure) due to differential mass displacements from lithospheric plate rotations. The method assumes the earth’s crust modeled as a mosaic of 1° × 1° blocks, each one moving independently with their corresponding absolute plate velocities. The differential contributions to the earth’s second-order tensor of inertia were computed, resulting in no significant displacement of the earth’s axis of figure. In view of the above, the possibleapparent displacement of the “mean pole” as a consequence of station drifting due to absolute plate motions was also analyzed, again without great success. As a further step the old speculation of the whole crust possibly sliding over the upper mantle is revived and the usefulness of the CIO is questioned.

Secular trend of the Earth's rotation pole: Consideration of motion of the latitude observatories

Secular polar motion has been recorded in ILS data over the past 75 years, an amount greater by a factor of ten than the ‘true polar wandering’ deduced from paleomagnetic data. In this work, the possibility that the secular trend is an observational artifact of the continental drift of the ILS stations is directly examined by consideration of several absolute plate velocity models earlier proposed by Minster et al. (1974), Kaula (1975), and Solomon, Sleep & Richardson (1975). The assumptions underlying those models are discussed; in general, the absolute velocity models are more likely to be valid when geologically short timescales are considered. The corrections to the ILS data due to the stations' motion fail by an order of magnitude to explain the ILS trend; even by taking into account possible plate hyperactivity and non-rigidity, the corrections could explain no more than 30 per cent of the trend. The corrections are small because the absolute plate velocities of North America and Eurasia are small and primarily east—west. Consequently, the rotation pole is undergoing significant motion of its own relative to the surface of the Earth. The Kimura z term found by the ILS observations provides an independent means of estimating the relative motion between Eurasia and North America. It also contains other geophysical information; the 7.5-yr periodicity discovered by Naito & Ishii (1974) may be widespread. Lastly, tectonically induced changes in the zenith direction, such as at Mizusawa, are probably too small to be detected, contrary to earlier proposals.

Implications of absolute plate motions and intraplate stress for mantle rheology

Recent tests of driving force models for plate tectonics include comparison of predicted absolute plate motions with those allowed by paleomagnetism and comparison of the directions of principal deviatoric stresses predicted within the lithosphere with those inferred from midplate earthquake mechanisms and in-situ measurements. These tests are extended in this paper to possible non-linear relationships between plate velocity and shear stress at the base of the plate, including the relationships expected if dislocation creep is the rate-limiting flow mechanism in the mantle and if viscous heating and a temperature-dependent viscosity control the shear stress. Neither absolute plate velocities nor intraplate stress can be used to distinguish linear viscosity in the asthenosphere from a rheology appropriate to dislocation creep. Absolute plate motions for effective rheologies controlled by shear heating at the base of the lithosphere are in conflict with paleomagnetic limits on net polar wander, thereby restricting viscous heating as a regulator of shear stress to at most the oldest oceanic and continental lithosphere.

Intraplate stress as an indicator of plate tectonic driving forces

To test driving force models for plate tectonics, the global intraplate stress fields predicted by various force systems are compared with the long-wavelength features of the observed stress field as determined by midplate earthquake mechanisms, in situ measurements, and stress-induced geologic structures. The calculated stresses are obtained by a finite difference solution to the equilibrium equations for thin elastic spherical shells in the membrane state of stress. Buoyancy forces at spreading centers and convergent plate boundaries and viscous drag at the base of the lithosphere are modeled as surface tractions applied to the shell. Drag is modeled as both a resistive and a driving force, and both symmetric and nonsymmetric forces at subduction zones are considered. The net driving push at spreading centers is found to be at least comparable in magnitude to other forces acting on the lithosphere and in particular is 0.7 to 1.5 times the net driving pull at convergence zones. Subducting lithosphere, which from seismic and thermal evidence has more potential energy available to drive plates than does a spreading center, thus converts relatively little of this energy to a net force acting on the surface plates. The drag coefficient at the base of the lithosphere may be greater by a factor of 3 to almost 10 beneath continents than beneath oceans without substantially affecting the fit between calculated and observed stress fields. Intraplate stresses calculated for models in which viscous drag at the base of the lithosphere acts in the direction of absolute plate velocity to drive plate motion are in much poorer agreement with observed stresses than are those calculated for models in which drag resists plate motions.

Some simple physical models for absolute plate motions

Although the relative angular velocities of the earth's plates are well known, the velocities relative to the underlying mantle and the nature of the driving forces are not. We calculate several solutions to the ‘absolute’ velocity field from the hypothesis that no net torque is exerted on the lithosphere as a whole and a series of assumptions about the forces driving plate motions. The force models include viscous drag at the base of all plates (plus arbitrary forces at ridges and trenches that exert torques of equal magnitude but opposite sign on adjacent plates), drag concentrated beneath continents, drag concentrated to oppose the horizontal translation of sinking slabs, gravitational pull by sinking slabs, and linear combinations of these. These models generally give absolute plate velocities that are very similar to those calculated on the premise that a set of ‘hot spots’ provides a fixed reference frame. This removes some of the rationale for attributing to these hot spots any significant contribution to the driving forces. The similarity of plate motions inferred from markedly different driving mechanisms precludes using the velocity fields to discriminate among proposed models. Many of the hot spots may be simply due to intraplate tensional stress associated with the forces acting on present plate boundaries. This suggests that calculation of intraplate stress can ultimately serve to choose among force models.