The purpose of this paper is to clarify the dynamic role of lithospheric density heterogeneities, in particular with respect to mountain building and other processes of intraplate deformation. Density anomalies within or just beneath the lithosphere constitute major sources for tectonic stress fields: the product of their magnitude by their depth is shown to characterize their ability to induce deformation. This rule of the density moment directly yields the lithospheric thickening or thinning rate when applied to structures of large lateral extent. For anomalies of lateral extent that is small in comparison with their depth, the deformation is vertically inhomogeneous and has been computed with the help of simple physical models of a stratified viscous Newtonian lithosphere. The analytical treatment is based on Fourier transform. Continent‐continent collision thickens not only the crust but the entire lithosphere. The cold root underlying a mountain chain induces strong regional compressive stresses able to sustain the mountain bulding process without further help from forces transmitted from far away. Thus the continental lithosphere is in a somewhat metastable mechanical state. Adiabatic, i.e. rapid, thickening (or thinning) tends to grow further once initiated. Tectonic phases of strong compression correspond to the climax of such instabilities. The response of models with cold lithospheric roots of various intensities has been computed both in two and three dimensions. They yield velocity distributions and stress fields. Instructive comparisons are made with earthquake focal mechanisms and in situ stress measurements in the Alpine and Appalachian regions. In the presence of lateral variations of the mechanical properties of the lithosphere, the tectonic style is not only function of the local topography and of the nature of its compensation. Deformations in neighbouring provinces are coupled as shown by 3‐dimensional models. For example, thickening sustained by a cold lithospheric root may generate extension in peripheral zones of weakness. These last results illustrate the point that the computation of regional tectonic stresses requires the knowledge of the density anomalies within the lithosphere on the one hand, and of geometrical constraints related to lateral mechanical heterogeneities on the other.

Megathrust earthquakes of magnitude close to 9 are followed by large-scale (thousands of km) and long-lasting (decades), significant crustal and mantle deformation. This deformation can be observed at the surface and quantified with GPS measurements. Here we report on deformation observed during the 5 yr time span after the 2010 M w 8.8 Maule Megathrust Earthquake (2010 February 27) over the whole South American continent. With the first 2 yr of those data, we use finite element modelling (FEM) to relate this deformation to slip on the plate interface and relaxation in the mantle, using a realistic layered Earth model and Burgers rheologies. Slip alone on the interface, even up to large depths, is unable to provide a satisfactory fit simultaneously to horizontal and vertical displacements. The horizontal deformation pattern requires relaxation both in the asthenosphere and in a low-viscosity channel along the deepest part of the plate interface and no additional low-viscosity wedge is required by the data. The vertical velocity pattern (intense and quick uplift over the Cordillera) is well fitted only when the channel extends deeper than 100 km. Additionally, viscoelastic relaxation alone cannot explain the characteristics and amplitude of displacements over the first 200 km from the trench and aseismic slip on the fault plane is needed. This aseismic slip on the interface generates stresses, which induce additional relaxation in the mantle. In the final model, all three components (relaxation due to the coseismic slip, aseismic slip on the fault plane and relaxation due to aseismic slip) are taken into account. Our best-fit model uses slip at shallow depths on the subduction interface decreasing as function of time and includes (i) an asthenosphere extending down to 200 km, with a steady-state Maxwell viscosity of 4.75 × 10 18 Pa s; and (ii) a low-viscosity channel along the plate interface extending from depths of 55-135 km with viscosities below 10 18 Pa s.

Abstract.We address the question of how convective processes control the thicknesses of oceanic and continental lithospheres. The numerical convection model involves a Newtonian rheology which depends on temperature and pressure. A repeated plate tectonic cycle is modeled by imposing a time-dependent surface velocity. One part of the surface, representing a continent, never subducts. The asymptotic equilibrium thickness of the lithosphere varies with the viscosity at the base of the lithosphere, but is not directly sensitive to the pressure dependence of the viscosity law and to the plate velocity. For small activation volumes, and average upper mantle viscosities deduced from postglacial rebound, the equilibrium plate thickness is more than 400 km (regimes I and 2). The equilibrium thickness of the oceanic lithosphere (around 100 km) implies that the viscosity in the asthenosphere is less than 7x10 •s Pa s. Only models with strongly pressure-dependent viscosity laws (activation volumes greater than 9x10 -6 ma/mol) are able to reconcile this value with the average upper mantle viscosity (5 x 10 •'ø Pa s). For these models, there are two lithospheric thicknesses such that the heat supplied by convection at the base of the lithosphere equals the surface conductive heat flow (regime 3). They could be that of an aged oceanic lithosphere and that of a shield lithospheric root. They indeed appear as points of prefered thickness in our numerical models. However, convection triggered by the lateral density jumps at the boundaries between the root and the thinner lithosphere slowly destabilizes the thick lithosphere. A plausible degree of chemical buoyancy in a depleted lithospheric root does not prevent convective erosion. In our simulations, long-term stability of a cratonic lithospheric root is best achieved when its material is both buoyant and more viscous than the surrounding mantle. Extensive devolatilization of the refractory rocks forming the root is invoked to explain this viscosity increase.

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On 16 September 2015, the M w 8.3 Illapel, Chile, earthquake broke a large area of the Coquimbo region of north-central Chile. This area was well surveyed by more than 15 high-rate Global Positioning System (GPS) instruments, installed starting in 2004, and by the new national seismological network deployed in Chile. Previous studies had shown that the Coquimbo region near Illapel was coupled to about 60%. After the M w 8.8 Maule megathrust earthquake of 27 February 2010, we observed a large-scale postseismic deformation, which resulted in a strain rate increase of about 15% in the region of Illapel. This observation agrees with our modeling of viscous relaxation after the Maule earthquake. The area where upper-plate GPS velocity increased coincides very well with the slip distribution of the Illapel earthquake inverted from GPS measurements of coseismic displacement. The mainshock started with a small-amplitude nucleation phase that lasted 20 s. Backprojection of seismograms recorded in North America confirms the extent of the rupture, determined from local observations, and indicates a strong directivity from deeper to shallower rupture areas. The coseismic displacement shows an elliptical slip distribution of about 200 km × 100 km with a localized zone where the rupture is deeper near 31.3°S. This distribution is consistent with the uplift observed in some GPS sites and inferred from field observations of bleached coralline algae in the Illapel coastal area. Most of aftershocks relocated in this study were interplate events, although some of the events deeper than 50 km occurred inside the Nazca plate and had tension (slab-pull) mechanisms. The majority of the aftershocks were located outside the 5 m contour line of the inferred slip distribution of the mainshock.