1,721,059 research outputs found
Postglacial rebound in a non-Newtonian spherical Earth
No full textUsing finite-elements we study the postglacial rebound in an axisymmetric Earth with a non-Newtonian rheology. Our approach allows for the first time a self-consistent study of the effect of a power-law creep on the long-wavelength deformations, which cannot be performed using flat models. We show that, for a non-Newtonian upper mantle, the time-variations of the low-degree Stokes coefficients become largely insensitive to the viscosity of the lower mantle. This is in contrast with previous evidence based on Newtonian models, which emphasized the importance of the low degree signatures for constraining the lower mantle viscosity
Structural and rheological constraints on source depth and overpressure estimates at Campi Flegrei Caldera, Italy
No full textModelling unrest episodes at Campi Flegrei caldera employing inelastic constitutive laws
No full textPlate motion and dragging of the upper mantle: Lateral variations of lithospheric thickness and their implications for intraplate deformation
No full textThe impact of lateral variations in the thickness of the lithosphere on surface topography, horizontal intraplate deformation and stress accumulation is studied for plates that drift with respect to the highly viscous lower mantle and the transition zone. The lithosphere and upper mantle are described by a viscoelastic Maxwell rheology within the framework of a finite element scheme which allows the modeling of heterogeneous lithospheric structures in 2D vertical cross sections. The geophysical signatures are found to be extremely sensitive to lateral viscosity contrasts which interact with the upper mantle flow. This mechanism can contribute to a certain extent and in concert with the other driving forces of plate tectonics to the evolution of back‐arc basins, to the explanation of the largest angle of subduction in west‐dipping slabs and to the initiation of subduction of an oceanic lithosphere underneath a stable continental one. Copyright 1992 by the American Geophysical Union
Inversions by 3D finite element solutions: deformation of Mount Etna from 1993 to 1997
No full textInversions of geodetic data are usually performed by using analytical forward models.
Very few analytical solutions exist, and they are characterized by fixed source geometry
(e.g. planar dislocation, sphere, dike) and simplified medium properties (e.g.
homogeneous, flat and elastic halfspace). These assumptions can potentially bias the
estimation of realistic source parameters. We develop a general tool to perform inversions
of geodetic data taking into account lateral variations of mechanical properties
of the medium and effects due to the topographic relief. The forward models are realized
by Finite Element technique. The deformation source is a combination of simple
point source mechanisms, i.e. dipoles and double couples. The FE forward technique
is based on the equivalence, under specific conditions, of the element-source and the
deformation of a 3D ellipsoid dilating under a constant pressure. This procedure is
applied to study the inflation process on Mount Etna from 1993 to 1997, as evidenced
by data recorded by GPS stations, EDM measurements and analysis of InSAR images.
We build a matrix of displacement solutions at data points for each potential elementsource.
We consider forward models characterized by heterogeneous medium and topographic
free surface. A direct search is performed in the parameters space using the
neighbourhood algorithm followed by an appraisal of the sampled solutions. From the
inversions we retrieve a source located below the NW sector of Etna, at 6-7 km b.s.l.
Our results suggest that while the effect of topography can be negligible, elastic heterogeneities
in the medium can significantly alter the position of the inferred source.
Furthermore, since the data show a significant signal in the SE sector due to flank
instability, we also include in our study some simple sliding mechanisms
Dynamic models of subduction: Geophysical and geological evidence in the Tyrrhenian Sea
No full textPredictions based on a 2-D finite-element model for subduction underneath the Calabrian Arc in southern Italy are compared with a variety of geophysical and geological data, such as the present-day stress pattern within the slab, uplift from the elevation of marine terraces in Calabria and subsidence in the Tyrrhenian Marsili Basin from ODP Leg 107. We model the behaviour of the slab driven by slab pull, in agreement with the present tectonic style in this part of the Mediterranean as suggested by several investigators. The model accounts for the crustal, lithospheric and mantle structures in a vertical cross-section perpendicular to the Calabrian subduction zone. The shape of the slab is constrained on the basis of new tomographic images in the southern Tyrrhenian Sea, which were obtained from the regional seismic stations of the Istituto Nazionale di Geofisica, while the rheological properties of the mantle are taken from global dynamic models. Density contrasts between the subducted slab and the surrounding mantle, based on petrological models, drive the flow in our viscoelastic model; stress values, displacements and vertical velocities at the surface are sampled at different times after loading until dynamic equilibrium is reached. Our estimates are appropriate for a time window of 100 kyr; the validity of our comparison with the geological record is based on the assumption that the tectonic configuration in the past was not substantially different from that of the present day. Two families of models, with unlocked and locked subduction faults, are considered. The unlocked models allow for roll-back of the trench of about 20 mm yr-1, in agreement with some geological estimates; the same family of models predicts uplift of the Calabrian Arc of about 1 mm yr-1 and subsidence in the Marsili Basin of 1-2 mm yr-1, in agreement with geological surveys. The deviatoric stress obtained from the unlocked model is consistent with the continuous distribution of deep seismicity in the southern Tyrrhenian Sea, with minor concentration within the lithospheric wedge. Locked models fail to reproduce these geophysical and geological observations. Predictions derived from a detached slab model are not consistent with the continuous hypocentral distribution of deep seismicity in the southern Tyrrhenian Sea. Deformation at the surface and the stress patterns at depth for a detached slab differ substantially from those of a continuous plate: dynamic topography and horizontal motions are reduced, when compared with the continuous plate, with deviatoric stresses concentrated within the relict slab. Our results indicate that subduction is a major tectonic process in the southern Tyrrhenian Sea
The role of subduction on the horizontal motions in the Tyrrhenian Basin: A numerical model
No full textThe horizontal motions in the Tyrrhenian basin and surrounding chains, induced by subduction along the Calabrian Arc and Northern Apennines, are analyzed by means of numerical models based on finite element techniques. The driving mechanism, in 2‐dimensional vertical cross‐sections perpendicular to the trench, is the slab‐pull due to the negative buoyancy of a stratified viscoelastic plate that models the Ionic oceanic lithosphere or Adriatic plate sinking in the upper mantle. For the modern tectonic setting of the area, we test the sensitivity of extension in the back‐arc and of roll‐back of the overriding plate to variations in the viscosity structure and in the geometry of subduction. Our results are compared with extensional velocities inferred from geology and SLR‐VLBI data. Copyright 1994 by the American Geophysical Union
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