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How the first-order structure of the magnetic field changed through geological time is one of the most long-living and hotly debated questions in geomagnetism. For the last 5 Ma, the geomagnetic field is found to be essentially dipolar with about 5 per cent quadrupole term and, possibly, still smaller octupole term. Proving this became possible after accumulation of a large set of globally distributed reliable palaeomagnetic data and their analysis in the coordinates tied to the rotation axis. For older times, the data are less reliable, scarce and unevenly distributed, while tectonic motions destroyed the global coordinates. In this paper, a method to determine the geomagnetic field structure for a single plate is discussed. It is suggested that the coordinate system for analysis can be derived from great circles, each being drawn through a sampling point and the corresponding palaeomagnetic pole (SP-circle). Such circles are to intersect at a true (i.e. pure dipole) pole that is largely purified from the contributions of non-dipole zonal terms and inclination shallowing. The scrutiny of late Permian (270–250 Ma) palaeomagnetic poles from Northern Eurasia revealed two distinct groups: one includes several poles from a limited area in South France (SF), and the other consists of tightly clustered nine poles from locations spread over a large part of Eurasia. Tentatively, the former group can be explained by either rotation of SF with respect to main Eurasia, or less likely, remagnetization in Early Triassic time; irrespectively, these poles cannot be used for calculation of the true dipole pole nor for computation of the apparent polar wander path of Eurasia. The poles from the main part of Eurasia were used to find the position of the true dipole pole of this plate and for analysis of the field. The minimum of residual dispersion of the observed data is found for the quadrupole and octupole terms of less than 10 per cent when each term is varied separately. When both terms are varied together, the best fit is observed for the quadrupole and octupole terms of −4 and 12 per cent, respectively. Our analysis shows that the position of the true dipole pole is robust, while the estimates of non-dipole terms are not. We argue that these estimates should be regarded as the upper limits on the non-dipole contributions to the Late Permian geomagnetic field; at the same time, we are pretty sure that octupole term of 20 per cent or more are not permitted by the data. [ABSTRACT FROM AUTHOR]

Resistive forces along convergent plate boundaries have a major impact on surface deformation, most visibly at collisional plate boundaries. Although quantification of these forces is key to understanding the evolution and present state of mountain belts, they remain highly uncertain due to the complexity of plate boundary structures and rheologies. In previous studies of the Eurasian Plate, we have analysed the balance of plate boundary forces, tractions resulting from lithosphere–mantle coupling, and intraplate variations in topography and density structure. This yielded a range of acceptable force distributions. In this study, we investigate to which extent the observed present-day stress field provides further constraints on the distribution of forces.We address the dynamics of the Eurasian Plate as a whole. This enables us to base our analysis on mechanical equilibrium of a tectonic plate and to evaluate all forces as part of an internally consistent set of forces driving and deforming Eurasia. We incorporate tractions from convective mantle flow modelling in a lithospheric model in which edge and lithospheric body forces are modelled explicitly and compute resulting stresses in a homogeneous elastic thin shell. Intraplate stress observations used are from the World Stress Map project. Eurasia's stress field turns out to be particularly sensitive to the distribution of collision forces on the plate's southern margin and, to a much lesser extent, to lithospheric density structure and tractions from mantle flow. Stress observations require collision forces on the India–Eurasia boundary of 7.0–10.5 TN m−1 and on the Arabia–Eurasia boundary of 1.3–2.7 TN m−1. Implication of mechanical equilibrium of the plate is that forces on the contacts with the African and Australian plates amount to 1.0–2.5 and 0–1.3 TN m−1, respectively. We use our results to assess the validity of the classical view that the mean elevation of an orogenic plateau can be taken as a measure of the magnitude of the compressive (in this case: collision-related) forces involved. For both the Tibetan and the Iranian plateaus, two plateaus with significantly different average elevations, we find that the horizontal force derived from the excess gravitational potential energy (collapse force) is in balance with the collision force. [ABSTRACT FROM AUTHOR]

We develop an approach that allows us to invert for the mantle velocity structure within a finely parametrized region as a perturbation with respect to a low-resolution, global tomographic model. We implement this technique to investigate the upper-mantle structure beneath Eurasia and present a new model of shear wave velocity, parametrized laterally using spherical splines with ∼2.9° spacing in Eurasia and ∼11.5° spacing elsewhere. The model is obtained from a combined data set of surface wave phase velocities, long-period waveforms and body-wave traveltimes. We identify many features as narrow as few hundred kilometres in diameter, such as subducting slabs in eastern Eurasia and slow-velocity anomalies beneath tectonically active regions. In contrast to regional studies in which these features have been identified, our model encompasses the structure of the entire Eurasian continent. Furthermore, including mantle- and body-wave waveforms helped us constrain structures at depths larger than 250 km, which are poorly resolved in earlier models. We find that up to +9 per cent faster-than-average anomalies within the uppermost ∼200 km of the mantle beneath cratons and some orogenic regions are separated by a sharp gradient zone from deeper, +1 to +2 per cent anomalies. We speculate that this gradient zone may represent a boundary separating the lithosphere from the continental root, which might be compositionally distinct from the overlying lithosphere and remain stable either due to its compositional buoyancy or due to higher viscosity compared with the suboceanic mantle. Our regional model of anisotropy is not significantly different from the global one. [ABSTRACT FROM AUTHOR]