A data set comprising 110 spreading rates, 78 transform fault azimuths, and 142 earthquake slip vectors has been inverted to yield a new instantaneous plate motion model, designated Relative Motion 2 (RM2). The model represents a considerable improvement over our previous estimate, RM1 [Minster et al., 1974]. The mean averaging interval for the spreading rate data has been reduced to less than 3 m.y. A detailed comparison of RM2 with angular velocity vectors which best fit the data along individual plate boundaries indicates that RM2 performs close to optimally in most regions, with several notable exceptions. The model systematically misfits data along the India-Antarctica and Pacific-India plate boundaries. We hypothesize that these discrepancies are manifestations of internal deformation within the Indian plate; the data are compatible with northwest-southeast compression across the Ninetyeast Ridge at a rate of about 1 cm/yr. RM2 also fails to satisfy the east-west trending transform fault azimuths observed in the French-American Mid-Ocean Undersea Study area, which is shown to be a consequence of closure constraints about the Azores triple junction. Slow movement between North and South America is required by the data set, although the angular velocity vector describing this motion remains poorly constrained. The existence of a Bering plate, postulated in our previous study, is not necessary if we accept the proposal of Engdahl and others that the Aleutian slip vector data are biased by slab effects. Absolute motion models are derived from several kinematical hypotheses and compared with the data from hot spot traces younger than 10 m.y. Although some of the models are inconsistent with the Wilson-Morgan hypothesis, the overall resolving power of the hot spot data is poor, and the directions of absolute motion for the several slower-moving plates are not usefully constrained.

Present-day plate motions

A data set comprising 110 spreading rates, 78 transform fault azimuths and 142 earthquake slip vectors was inverted to yield a new instantaneous plate motion model, designated RM2. The mean averaging interval for the relative motion data was reduced to less than 3 My. A detailed comparison of RM2 with angular velocity vectors which best fit the data along individual plate boundaries indicates that RM2 performs close to optimally in most regions, with several notable exceptions. On the other hand, a previous estimate (RM1) failed to satisfy an extensive set of new data collected in the South Atlantic Ocean. It is shown that RM1 incorrectly predicts the plate kinematics in the South Atlantic because the presently available data are inconsistent with the plate geometry assumed in deriving RM1. It is demonstrated that this inconsistency can be remedied by postulating the existence of internal deformation with the Indian plate, although alternate explanations are possible.

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Present‐day plate motions

To date, more than 7300 in situ stress orientations have been compiled as part of the World Stress Map project. Of these, over 4400 are considered reliable tectonic stress indicators, recording horizontal stress orientations to within <±25°. Remarkably good correlation is observed between stress orientations deduced from in situ stress measurements and geologic observations made in the upper 1–2 km, well bore breakouts extending to 4–5 km depth and earthquake focal mechanisms to depths of ∼20 km. Regionally uniform stress orientations and relative magnitudes permit definition of broad-scale regional stress patterns often extending 20–200 times the approximately 20–25 km thickness of the upper brittle lithosphere. The “first-order” midplate stress fields are believed to be largely the result of compressional forces applied at plate boundaries, primarily ridge push and continental collision. The orientation of the intraplate stress field is thus largely controlled by the geometry of the plate boundaries. There is no evidence of large lateral stress gradients (as evidenced by lateral variations in stress regime) which would be expected across large plates if simple resistive or driving basal drag tractions (parallel or antiparallel to absolute motion) controlled the intraplate stress field. Intraplate areas of active extension are generally associated with regions of high topography: western U.S. Cordillera, high Andes, Tibetan plateau, western Indian Ocean plateau. Buoyancy stresses related to crustal thickening and/or lithospheric thinning in these regions dominate the intraplate compressional stress field due to plate-driving forces. These buoyancy forces are just one of several categories of “second-order” stresses, or local perturbations, that can be identified once the first-order stress patterns are recognized. These second-order stress fields can often be associated with specific geologic or tectonic features, for example, lithospheric flexure, lateral strength contrasts, as well as the lateral density contrasts which give rise to buoyancy forces. These second-order stress patterns typically have wavelengths ranging from 5 to 10+ times the thickness of the brittle upper lithosphere. A two-dimensional analysis of the amount of rotation of regional horizontal stress orientations due to a superimposed local stress constrains the ratio of the magnitude of the horizontal regional stress differences to the local uniaxial stress. For a detectable rotation of 15°, the local horizontal uniaxial stress must be at least twice the magnitude of the regional horizontal stress differences. Examples of local rotations of SHmax orientations include a 75°–85° rotation on the northeastern Canadian continental shelf possibly related to margin-normal extension derived from sediment-loading flexural stresses, a 50°–60° rotation within the East African rift relative to western Africa due to extensional buoyancy forces caused by lithospheric thinning, and an approximately 90° rotation along the northern margin of the Paleozoic Amazonas rift in central Brazil. In this final example, this rotation is hypothesized as being due to deviatoric compression oriented normal to the rift axis resulting from local lithospheric support of a dense mass in the lower crust beneath the rift (“rift pillow”). Estimates of the magnitudes of first-order (plate boundary force-derived) regional stress differences computed from modeling the source of observed local stress rotations magnitudes can be compared with regional stress differences based on the frictional strength of the crust (i.e., “Byerlee's law”) assuming hydrostatic pore pressure. The examples given here are too few to provide a definitive evaluation of the direct applicability of Byerlee's law to the upper brittle part of the lithosphere, particularly in view of uncertainties such as pore pressure and relative magnitude of the intermediate principal stresses. Nonetheless, the observed rotations all indicate that the magnitude of the local horizontal uniaxial stresses must be 1–2.5+ times the magnitude of the regional first-order horizontal stress differences and suggest that careful evaluation of such local rotations may be a powerful technique for constraining the in situ magnitude stress differences in the upper, brittle part of the lithosphere.

/pdf/first-and-second-order-patterns-of-stress-in-the-lithosphere-595hs86ljh.pdf

First‐ and second‐order patterns of stress in the lithosphere: The World Stress Map Project

Trench-arc systems (subduction zones) can be classified into two types depending on whether or not actively opening back-arc basins are associated with them. This suggests that subduction of an oceanic plate is not a sufficient condition for back-arc opening, though it may be necessary one. Mechanisms that cause the distinction between the two types have been investigated. Earthquake studies suggest that there is a significant difference in the mode of plate motion at interplate boundaries between the two types of trench-arc systems. Extreme cases are Chile, where plate motion is seismic, and the Marianas arc, where it is aseismic. This difference seems to indicate that the stress state in the back-arc area differs between the two types: compression in the Chilean type and tension in the Marianas type. This difference in the stress state is also manifested in other tectonic features, such as topography, gravity, volcanic activity, and crustal movement. Two possible mechanisms for the difference between the two types are suggested: (1) The nature of the contact zone between upper and lower plates changes from tight coupling (Chile) to decoupling (the Marianas) through the evolutionary process of subduction. The decoupling results in an oceanward retreat of the trench and back-arc opening. (2) The downgoing slab is anchored to the mantle, so that the position of a trench is also fixed with respect to the mantle. Since the motion in the mantle is slow compared to that of surface plates, it is the motion of the landward plate which controls the opening and nonopening of back-arcs.

/pdf/back-arc-opening-and-the-mode-of-subduction-1x39xr13ac.pdf

Back-arc opening and the mode of subduction

We investigate the distribution of focal depths for earthquakes that do not appear to be associated with zones of recent subduction, using both new results from analyses of individual events recorded at teleseismic distances and published data for both microearthquakes and larger events. The deepest events in oceanic regions occur in old lithosphere (≥100 Ma), and excluding earthquakes in active mountain belts, the deepest crustal events occur in old cratons (tectonic age ≥800 Ma). Therefore, the temperature at the source region is likely to be an important factor determining whether deformation occurs seismically or not. From estimates of the temperatures at depths of the deepest events, we conclude that those limiting temperatures are about 250°–450°C and 600°–800°C for crustal and mantle materials, respectively. In several regions of recent continental convergence, in addition to shallow crustal seismicity, there is seismic activity in the uppermost mantle. The lower crust, however, is essentially aseismic. We infer that both the upper crustal and the mantle seismic regions correspond to zones of relatively high strength and that they are separated by a zone of lower strength in the lower crust where aseismic, ductile deformation predominates. This simple interpretation is qualitatively in agreement with extrapolated values of brittle and ductile strengths of geologic materials studied under appropriate pressure and temperature conditions in the laboratory. A low-strength zone in the lower crust might allow detachment of crystalline nappes from the underlying mantle (and lower crustal) lithosphere. The apparently greater strength of mantle materials than crustal materials at the same temperature implies that oceanic lithosphere is much stronger than continental lithosphere, and this difference may account for why plate tectonics works well in oceanic regions but not in continents.

Focal depths of intracontinental and intraplate earthquakes and their implications for the thermal and mechanical properties of the lithosphere

In the present paper, the basic set of global intraplate stress orientation data is plotted and tabulated. Although the global intraplate stress field is complicated, several large-scale patterns can be seen. Much of stable North America is characterized by an E-W to NE-SW trend for the maximum compressive stress. South American lithosphere beneath the Andes, and perhaps farther east in the stable interior, has horizontal compressive stresses trending E-W to NW-SE. Western Europe north of the Alps is characterized by a NW-SE trending maximum horizontal compression, while Asia has the maximum horizontal compressive stress trending more nearly N-S, especially near the Himalayan front.

Tectonic stress in the plates

Torque poles are calculated for a variety of possible forces acting on the plates, including ridge push, slab pull, and collisional resistance. These torque poles are then compared to the directions of absolute plate motions. There is a strong correlation between ridge torque poles and the azimuth of absolute plate motions for the North American, South American, Pacific, Cocos, and Eurasian plates. Simple slab pull torques correlate well with absolute motion azimuths for the Pacific, Nazca, and Cocos plates and moderately well with the absolute motion azimuth of the IndoAustralian plate. Collisional resistance torque poles correlate with the absolute motion azimuth of the Eurasian plate only. The correlations are presented as further evidence that the absolute reference frame for plate motion is determined by the surface plates themselves. Torque poles for various forces are also compared with several long-wavelength features of the global intraplate stress field that also tend to be aligned with absolute motion directions. In general, ridge torque directions agree well with the orientations of maximum horizontal stresses for stable North America, western Europe, and South America and provide an alternative explanation for the alignment in terms of ridge push forces rather than basal drag. Collisional resistance forces can also explain the alignment of stresses in western Europe. For the IndoAustralian plate, the torque pole for collisional resistance forces is consistent with the general pattern of stresses in at least the western half of the plate but is not a good predictor of the entire data set for the plate. Other processes, in addition to collisional resistance, must be important for the Indo-Australian plate. Ridge push forces may account for a significant portion of the long-wavelength features of the intraplate stress field, especially away from continental collisions. Such a conclusion is consistent with negative buoyancy of the slab being an important component of the driving mechanism. As previously suggested, slab forces may be largely balanced within the subducted slab itself and thus have limited effect on deformation of the surface plates.

Ridge forces, absolute plate motions, and the intraplate stress field

Summary Absolute plate motions and intraplate stress both serve as tests of models for the forces acting on plate boundaries. Plate velocities relative to a presumedly fixed underlying mantle are calculated from the hypothesis that no net torque is exerted on the lithosphere. Intraplate stress is calculated by solving the equilibrium equations for thin elastic shells in the membrane state of stress. The absolute velocity fields predicted from a wide assortment of physical and geological models are all very similar. While the global pattern of absolute velocity is probably close to those predicted by such models, the absolute motions do not therefore provide a strong test of the driving mechanism. Comparison of predicted intraplate stress with the long-wavelength features of the global stress field, however, as determined by in situ measurements, earthquake mechanisms, and stressinduced geological structures, does prove to be a powerful test of possible driving forces. All absolute velocity models have several interesting properties. Lithosphere in the equatorial half of the Earth is moving significantly faster than lithosphere in the polar half. Some connection to the Earth's rotation is implied since this statement is demonstrably untrue for co-ordinate poles much different from the geographic pole. Subducted slabs are characterized by a slow horizontal translation perpendicular to strike that is independent of the plate convergence rate, confounding attempts to explain the dip angles of Benioff zones in terms of a uniform vertical sinking and a variable absolute velocity for the overthrust plate. Ridges must migrate at a wide range of velocities relative to their underlying source of new lithosphere; such rapid migration may be a necessary but is not a sufficient condition for ridge jumps. Force models considered in velocity and stress calculations include driving forces at spreading centres and subduction zones and various parameterizations of drag at the base of the lithosphere. From the rms absolute velocities of individual plates, there is a weak indication that pull by subducted lithosphere at trenches is an important driving force and that drag may be greater beneath continental than oceanic lithosphere. The predicted intraplate deviatoric stress cannot match the well-determined stress fields in North America and Europe unless the driving force exerted at ridges is at least comparable in magnitude to other forces in the system. The mid-plate stresses are very sensitive to the nature of drag at the base of the lithosphere and thus measured stresses may ultimately provide a sensitive test of absolute plate velocities.

On the Forces Driving Plate Tectonics: Inferences from Absolute Plate Velocities and Intraplate Stress

The relative contribution of topographic (e.g., ridge push, continental margins, and elevated continental crust) and plate boundary (e.g., subduction and collisional) forces to the intraplate stress field in the Indo-Australian plate (IAP) is evaluated through a finite element analysis. Two important aspects of the IAP intraplate stress field are highlighted in the present study: (1) if substantial focusing of the ridge push torque occurs along the collisional boundaries (i.e., Himalaya, New Guinea, and New Zealand), many of the first-order features of the observed stress field can be explained without appealing to either subduction or basal drag forces; and (2) it is possible to fit the observed SHmax, (maximum horizontal stress orientation) and stress regime information with a set of boundary conditions that results in low tectonic stress magnitudes (e.g., tens of megapascals, averaged over the thickness of the lithosphere) throughout the plate. This study therefore presents a plausible alternative to previous studies of the IAP intraplate stress field, which predicted very large tectonic stress magnitudes (hundreds of megapascals) in some parts of the plate. In addition, topographic forces due to continental margins and elevated continental material were found to play an important role in the predicted stress fields of continental India and Australia, and the inclusion of these forces in the modeling produced a significant improvement in the fit of the predicted intraplate stresses to the available observed stress information in these continental regions. A central focus of this study is the relative importance of the boundary conditions used to represent forces acting along the northern plate margin. We note that a wide range of boundary conditions can be configured to match the large portion of the observed intraplate stress field, and this nonuniqueness continues to make modeling the IAP stress field problematic. While our study is an important step forward in understanding the sources of the IAP intraplate stress field, a more complete understanding awaits a better understanding of the relative magnitude of the boundary forces acting along the northern plate margin.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/97JB02381

Randall M. Richardson

Papers

Tectonic stress in the plates

Ridge forces, absolute plate motions, and the intraplate stress field

On the Forces Driving Plate Tectonics: Inferences from Absolute Plate Velocities and Intraplate Stress

Topography, boundary forces, and the Indo‐Australian intraplate stress field

The origins of the intraplate stress field in continental Australia