The influence of plate strength on dynamic topography estimates

<p>A common method used to evaluate dynamic topography amplitudes begins with an estimate of Moho depth, usually from seismic data but sometimes - or also - from the inversion of gravity data. Then the principles of Airy isostasy are applied: surface topography is assumed to be in isostatic equilibrium, buoyantly supported by the displacement of high-density mantle material by the low-density crustal ‘root’ that compensates the surface topographic mass. Hence, the actual relief of the Moho yields an ‘isostatic topography’ which will depart from the actual, observed topography by a component that, in theory, must arise from convective support or subsidence. Notwithstanding the fact that the errors on the seismic Moho may be larger than the topography itself, there is another source of uncertainty, that of the flexural rigidity of the lithosphere. Airy isostasy is essentially an end-member of plate flexure models, one in which the flexural rigidity is zero. However there are very few places on Earth where the flexural rigidity, usually represented by its geometric analogue the effective elastic thickness (Te), is indeed zero. In most environments, the rigidity of the plate will act to resist flexure, with the implication that the ‘Airy isostatic topography’ and therefore the dynamic topography will be in error. Here several scenarios will be presented illustrating these issues, and paths for remediation recommended.</p>

GRAVITY GEOGRAPHY OF RESERVOIRS IN THE SVERDLOVSK REGION

The article presents the results of research into gravitational geography of reservoirs in the Sverdlovsk region used to clarify the rationality of the economic use of the regional natural capital and the environmental aspects of the spatial placement of productive forces. There was conducted a comprehensive study of the region's water system taking into account the status of its dams and reservoirs within one or several river basins. It has been established that of 7 key reservoirs, most do not satisfy the condition of compensatory pumping of the day surface aimed at improving the isostatic equilibrium of the surface. In practice, overloads are created even where the necessary conditions exist, which causes geoecological problems. It is shown that the reservoir at Reft complies the most with the condition of ideal compensation with the weight of water to establish the ideal isostatic equilibrium, but only in the zone of medium depths. For dam sites in all the reservoirs studied, this condition is not met, and the oretically the permissible load is exceeded multiple times.

Isostatic constraint for 2D nonlinear gravity inversion on rifted margins

We have developed a nonlinear gravity inversion for simultaneously estimating the basement and Moho geometries, as well as the depth of the reference Moho along a profile crossing a passive rifted margin. To obtain stable solutions, we impose smoothness on basement and Moho, force them to be close to previously estimated depths along the profile and also impose local isostatic equilibrium. Different from previous methods, we evaluate the information of local isostatic equilibrium by imposing smoothness on the lithostatic stress exerted at depth. Our method delimits regions that deviate and those that can be considered in local isostatic equilibrium by varying the weight of the isostatic constraint along the profile. It also allows controlling the degree of equilibrium along the profile, so that the interpreter can obtain a set of candidate models that fit the observed data and exhibit different degrees of isostatic equilibrium. Our method also differs from earlier studies because it attempts to use isostasy for exploring (but not necessarily reducing) the inherent ambiguity of gravity methods. Tests with synthetic data illustrate the effect of our isostatic constraint on the estimated basement and Moho reliefs, especially at regions with pronounced crustal thinning, which are typical of passive volcanic margins. Results obtained by inverting satellite data over the Pelotas Basin, a passive volcanic margin in southern Brazil, agree with previous interpretations obtained independently by combining gravity, magnetic, and seismic data available to the petroleum industry. These results indicate that combined with a priori information, simple isostatic assumptions can be very useful for interpreting gravity data on passive rifted margins.

ANALYSIS OF ISOSTATICALLY-BALANCED CORTICAL MODELS USING MODERN GLOBAL GEOPOTENTIAL MODELS

The knowledge of the Earth's gravity field and its temporal variations is the main goal of the dedicated gravity field missions CHAMP, GRACE and GOCE. Since then, several global geopotential models (GGMs) have been released. This paper uses geoid heights derived from global geopotential models to analyze the cortical features of the Tandilia structure which is assumed to be in isostatic equilibrium. The geoid heights are suitably filtered so that the structure becomes apparent as a residual geoid height. Assuming that the geological structure is in isostatic equilibrium, the residual geoid height can be assimilated and compared to the isostatic geoid height generated from an isostatically compensated crust. The residual geoid height was obtained from the EGM2008 and the EIGEN-6C4 global geopotential models, respectively. The isostatic geoid was computed using the cortical parameters from the global crustal models GEMMA and CRUST 1.0 and from local parameters determined in the area under study. The obtained results make it clear that the isostatic geoid height might become appropriate to validate crustal models if the structures analyzed show evidence of being in isostatic equilibrium.

Inversion of gravity data with isostatic constraints

We have developed a simple iterative gravity-inversion approach to map the basement and Moho surfaces of a rift basin simultaneously. Gravity anomalies in rift basins commonly consist of interfering broad, positive crustal-thinning anomalies and narrow, negative sedimentary-basin anomalies. In our model, we assumed that the Moho and basement surfaces are in Airy isostatic equilibrium. An initial plane-layered model was iterated to fit the gravity data. We applied the process to a model in which the inverted basement and Moho surfaces matched the model surfaces well and to a gravity profile across the Kosti Basin in Sudan.

Determination of Recent Movements of Lithosphere in Regions with Chemogenic Sediments on the Surface in the Dinarides Region

In the area of carbonate or karst Dinarides, besides the dominant carbonate rocks and subordinate clastics, in more places more significant mass of chemical sediments can be found, represented by gypsum and anhydrite. As chemical sediments are specifically lighter than overlying rocks, they have been, during the long geological history, trying to achieve isostatic equilibrium, or get out on the ground surface. In recent times, in areas with chemical sediments, a larger tectonic activity is noted than in the wider environment. This is certainly reflected in increased vertical and tangential displacements along major faults in relation to the surrounding area, which are built mainly of carbonate deposits. Determining the amount of movement of diapiric bodies, as absolute and relative, using the most modern surveying methods, precisely would assist the understanding of the tectonic movements and recent structural relations in a wider area