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.

The state of stress in the lithosphere provides strong constraints on the forces acting on the plates. The directions of principal stresses in the plates as indicated by midplate earthquake mechanisms, in situ stress measurements, and stress‐sensitive geological features are used to test plate tectonic driving force models, under the premises that enough data exist in selected areas to define a regionally consistent stress field and that for most such areas the dominant forces producing such stresses are plate tectonic in origin. Force models include buoyancy forces at ridges, subduction zones, and continental convergence zones and variously parameterized viscous shear between the lithosphere and the asthenosphere. A linear finite element method, based on the wave front solution technique, is used to predict the intraplate stress for each force model. Several long‐wavelength patterns for the orientation of horizontal principal deviatoric stresses are observable in the stress data. Maximum compressive stresses trend E‐W to NE‐SW for much of stable North America and E‐W to NW‐SE for continental South America. In western Europe the maximum compressive stresses trend NW‐SE, while in Asia the trend is more nearly N‐S, especially near the Himalayan front. In the Indian plate the trend varies from nearly N‐S in continental India to more nearly E‐W in Australia. Horizontal stresses are variable in Africa but tend to indicate a NW‐SE trend for the maximum compressive stress in west Africa and an E‐W trend for the minimum compressive stress in east Africa. Oceanic lithosphere away from plate boundaries is generally in a state of deviatoric compression, although few focal mechanisms can be constrained to define the orientation of the principal stresses. Comparison of stress orientations predicted for a wide range of driving force models to these regional stress observations provides a powerful test of the models. Ridge pushing forces are required in all models that match the stress orientation field. The net pulling force of subducted lithosphere, if such a force acts approximately symmetrically about the plate boundary, is at most a few times larger than other forces acting on the plates. Resistive forces associated with trench thrust faults and motion of the slab with respect to the mantle must therefore nearly balance the large gravitational potential of the slab. The upper limit on the ratio of net slab to ridge forces may be increased by less than a factor of 2 if net slab forces are reduced for the fastest moving plates by assuming that the resistance to subduction increases with convergence rate. Forces acting to resist further continental convergence along the Alpine‐Himalayan belt are important for models of the intraplate stress field in Europe, Asia, and the Indian plate. Inclusion of continental convergence zone forces, however, does not affect the upper bound on the ratio of net slab to ridge forces. A variety of possible lithosphere‐asthenosphere interactions have been tested. Resistive viscous drag forces act...

Abstract. 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 SHm•,, (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.

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The first-order South American intraplate stress field was modeled through • finite element analysis to evaluate the relative contribution of plate boundary forces and intraplate stress sources. The finite element mesh consisted of 3100 nodes in a network of 5993 equal-•rea triangular elements which provided a spatial resolution of about 1 ø at the equator. An important aspect of our modeling is the inclusion of topographic forces due to the cooling oceanic lithosphere along the Mid-Atlantic Ridge (e.g., ridge push), the continental margins along the east coast of Brazil and Argentina, and the elevated continental crust (e.g., the Andean Cordillera). Predicted intraplate stresses for two representations of the western collisional boundary forces are evaluated: pinned collisional boundaries and applied collisional boundary forces. Constraint for the modeling was provided by information about the orientation of the maximum horizontal compressive stress, SHm•, provided by 217 stress indicators from the World Stress Map Project as well as by SHm•x magnitude estimates and torque information from previous investigations. Our modeling results demonstrate that the first-order features of the observed stress field can be explained with simple tectonic models which balance the torque acting on the plate either with a fixed western margin or drag forces applied along the base of the plate. The predicted intraplate stress field is characterized by a nearly uniform E-W $gm• orientation throughout most regions of the plate, with stress magnitudes generally less than 20 MPa averaged over a 100-km-thick lithosphere. Significant perturbation of this regional stress field occurs in the western part of the plate in response to forces associated with the high topography of the Andes. Although the magnitude of the collisional boundary forces acting along the western margin remains poorly constrained, we estimate a plausible upper bound on the force per unit length acting along the Peru-Chile Trench to be about 2.5 x 101• N m -1. While some of our models are consistent with a driving basal drag to balance the torques acting on the plate, the magnitude of the drag torque is small compared to the contribution from other sources of stress such as the ridge push force. the lithosphere (i.e., cooling oceanic lithosphere, associated with "ridge push," acting outward from the Mid-Atlantic Ridge; the Brazilian continental margin; and elevated topography of the Andes) and forces transmitted across collisional boundaries along the Peru-Chile Trench. The plate therefore provides an ideal location to investigate the relative contributions of nonslab tectonic sources within the lithosphere. The present study builds on the previous modeling efforts of Richardson et al. [1979], Meijer and Wortel [1992], and Stefanick and Jurdy [1992]. An important aspect of this study is the use of a finite element grid with a spatial resolution of about 1 ø, or more than double the resolution employed in the previous studies. This allows the evaluation of predicted...