Numerical modeling of eastern Tibetan-type margin: Influences of surface processes, lithospheric structure and crustal rheology

The eastern Tibetan margin is characterized by a steep topographic gradient and remarkably lateral variations in crustal/lithospheric structure and thermal state. GPS measurements show that the surface convergence rate in this area is strikingly low. How can such a mountain range grow without significant upper crustal shortening? In order to investigate the formation mechanism of the eastern Tibetan-type margins, we conducted 2D numerical simulations based on finite difference and marker-in-cell techniques. The numerical models were constrained with geological and geophysical observations in the eastern Tibetan margin. Several major parameters responsible for topography building, such as th…

Crustal structure of the Reykjanes Ridge near 62°N, on the basis of seismic refraction and gravity data

Explosion deep seismic sounding data sections of high quality had been obtained with RV Meteor in the Reykjanes Iceland Seismic Project (RRISP77 [Angenheister, G., Gebrande, H., Miller, H., Goldflam, P., Weigel, W., Jacoby, W.R., Palmason, G., Bjornsson, S., Einarsson, P., Pavlenkova, N.I., Zverev, S., Litvinenko, I.V., Loncarecic, B., Solomon, S., 1980. Reykjanes Ridge Iceland Seismic Experiment (RRISP 77). J. Geophys. 47, 228–238]) which close an information gap near 62°N. Preliminary results were presented by Weigel [Weigel, W., 1980. Aufbau des Reykjanes Ruckens nach refraktionsseismischen Messungen. In: Weigel, W. (Ed.), Reykjanes Rucken, Island, Norwegischer Kontinentalrand. Abschluss…

Numerical approach to problems of gravitational instability of geostructures with advected material boundaries

SUMMARY We present a numerical approach for solving 2-D mantle flow problems where the chemical composition changes abruptly across intermediate boundaries. The method combines a Galerkin-spline technique with a method of integration over regions bounded by advected interfaces to represent discontinuous variations of material parameters. It allows direct approximation of a natural free surface position, instead of a posteriori calculation of topography from the normal stress at the upper free-slip boundary. We formulate a model where a viscous incompressible fluid filling a square box is divided into layers (not necessarily horizontal) by advected boundaries, across which the density and vi…

A magmatic failed rift beneath the Gongola arm of the upper Benue trough, Nigeria?

Abstract A magnetic anomaly map of the Gongola arm of the upper Benue trough was compiled from existing half-degree sheets of the upper Benue trough. The map is dominated by an elongated NE–SW magnetic low anomaly that stretches over a 100 km distance and has an average width of about 20 km. A combination of Werner deconvolution and forward modeling techniques on selected profiles across the major NE–SW anomaly lead to the conclusion that the anomaly may be caused by predominantly mafic rocks at depths of between 6 and 10 km, well below the base of the sediments, which are generally not more than 4.5 km thick. The presence of large and continuous bodies of mafic rocks indicated by these res…

Crust–mantle transition and Moho model for Iceland and surroundings from seismic, topography, and gravity data

An increasing data set exists on the nature and thickness of the Iceland crust. This paper relates topography, i.e., elevation and bathymetry (TOP), Bouguer gravity anomalies (BA) and Moho depths to each other to assess the consequences of the “thick crust model” for Iceland in the context of the North Atlantic. Results of regression of TOP and BA vs. Moho depth are converted into “Airy densities” Δρ* (mantle crust density contrasts in the case of ideal Airy isostasy). For Iceland, Δρ* is very low (105±10 kg/m3); for the adjacent continental margins and relicts, it is high and intermediate for the Jan Mayen Ridge. The values are affected by lithosphere cooling and systematic variations of i…

Hotspot Iceland: An introduction

Cluster Algorithm Integrated with Modification of Gaussian Elimination to Solve a System of Linear Equations

The data accumulation and their inhomogeneous distribution lead to the issue of large and sparse systems solving in various fields: industrials, emergency management, etc. Complex structure in the data error creates additional risk to obtain an adequate solution. To facilitate problem-solving, we describe the technique that is based on intellectual division of data with following application of cluster algorithm and the modification of Gaussian elimination to different portions of data. In this paper, we present results of developed technique that was applied to samples of synthetic and real data. We compare them with outcomes of other algorithms (intelligence and classical) by using of num…

Calculation and inversion of two-dimensional gravity in the vicinity of Lake Tuz, Turkey

Abstract An example of gravity inversion and interpretation is presented which demonstrates how a priori information can be used to derive reliable, though complex models. In this case, the geometry of the studied model profile has been constructed on the basis of seismic and geological data. The densities used in the forward calculations were obtained from laboratory measurements of drill cores, from density-velocity relationships, and from Nettleton’s method of fitting gravity and topography. In the seismic section 21 ‘formations’ are distinguished leading to a very complex gravity model. What is called ‘formations’ here, are two-dimensional bodies which are distinguished from each other …

Eastern Pontides and Black Sea: gravity inversion, crustal structure, isostasy and geodynamics

Abstract Lacking detailed seismic data on the crustal structure of Anatolia and the transition to the Black Sea, it is attempted to invert gravity for crust-mantle structure with constraints from the limited a priori information available, as average continental and oceanic crust, local topography, an isostatic model, published marine seismic data and a tentative “fix point” from recent seismological receiver functions near the Black Sea coast. An initial 2D-model for a north–south profile along about 40°E longitude is constructed and adjusted to fit the Bouguer anomaly taken from a published Turkish map and results from satellite radar altimetry. Isostasy, seismic data and gravity inversio…

Inversion of gravity anomalies over spreading oceanic ridges

Abstract Models of spreading ocean ridges are derived by Bayesian gravity inversion with geophysical and geodynamic a priori information. The aim is to investigate the influence of spreading rate, plate dynamics and tectonic framework on crust and upper mantle structure by comparing the Mid Atlantic Ridge (MAR), the Indian Ocean Ridge (IND) and the East Pacific Rise (PAC). They differ in mean spreading rate, dynamic settings, as attached slabs, and plume interaction. Topography or bathymetry, gravity, isostasy, seismology and geology, etc. are averaged along the ridges and guide the construction of initial 2D models, including features as mean plumes, i.e. averaged along the ridge. This is …

A study of hydrothermal convection in saturated porous media

Abstract Because of its relevance to many geological and technical problems, hydrothermal convection is investigated here mainly with the aid of numerical models by a systematic analysis of the properties of this type of convection for a range of super-critical Rayleigh numbers. Calculations were performed for two-dimensional models with constant properties in a region of aspect ratio 2. The principal results in the case of temperature fixed at the impermeable top and bottom are the following for the Nusselt number Nu, the cell aspect ratio a, and the boundary layer thickness δ: Nu ≈ 1.7 R0.5, a ≈ 1.3 R−0.4, δc ≈ 0.4 R−0.4 for 2.5 R = R f /R f ∗ and Rf and R f ∗ are the ambient and critical…

Hotspot distribution, gravity, mantle tomography: evidence for plumes

Abstract Thermal convection is the motor of Earth dynamics and therefore is the link between plate motions, hotspots, seismic velocity variations in the mantle, and anomalies of the gravity field. Small scale mantle anomalies, such as plumes, do, however, generally escape detection by tomographic methods. It is attempted to approach the problem of detection in a somewhat statistical manner. Correlations are sought between spherical harmonic expansions of the fields under study: the hotspot distribution, mantle velocity variations, gravity, heat flow. Using spherical harmonic representations of global fields implies integration and averaging over the whole globe. Thus, although relationships…

Method to find the Minimum 1D Linear Gradient Model for Seismic Tomography

The changes in the state of a geophysical medium before a strong earthquake can be found by studying of 3D seismic velocity images constructed for consecutive time windows. A preliminary step is to see changes with time in a minimum 1D model. In this paper we develop a method that finds the parameters of the minimum linear gradient model by applying a two-dimensional Taylor series of the observed data for the seismic ray and by performing least-square minimization for all seismic rays. This allows us to obtain the mean value of the discrete observed variable, close to zero value.

A self-consistent 2-D model for the dip angle ofmantle downflow beneath an overriding continent

Abstract We demonstrate how the inclination of descending mantle flow is affected by thepresence of a continent overriding the downwelling zone. The mantle is modelled by a fluidhaving a temperature- and pressure-dependent viscosity which describes a low viscosityasthenosphere and a higher viscosity lower mantle. The fluid occupies a square x:y=10:1 boxheated from below. In the absence of overlying continents thermal convection sets in with colddownwellings being nearly vertical. A continent is placed on the convecting mantle and starts todrift. The continent is assumed to be a thick rigid plate. We consider models of two types. Model1 involves a free-floating continent. The continent is pu…

Continental collision and the dynamic and thermal evolution of the Variscan orogenic crustal root — numerical models

Abstract Orogeny is modelled numerically by treating continental collision within full convection solutions, in order to better understand some aspects of the Variscan structures and processes. Three different approaches are taken: (1) collision where one ‘continental plate’ is ‘pushed’ against another across a zone of weakness; (2) gravitational instability of a lithospheric mantle root leading to delamination, slab break-off and crustal root reduction; (3) melting in the lower part of a crustal orogenic root. The first approach demonstrates that thick (but in the models: cool) roots can accumulate, in which upper crustal rocks are carried to great depth and mantle material may be carried …

Tomography Application to Complex Seismic Data of the Tjornes Fracture Zone (Iceland)

The Tjornes Fracture Zone (TFZ) is an active seismic zone in Northeast Iceland. It plays a key role to understand the geodynamic movement and location of tectonic plates. However the seismic experiment can not be performed close to earthquake sources, because sources are mainly located in the Greenland Sea. The unusual geological structure of TFZ and the limited conditions of an experiment lead to significant deviations between real observations and the values that are calculated in accordance with theoretical models. Consequently, there is a loss of adequacy and stability of tomography systems. Outcomes of the method, which takes into account this problem, are analyzed in the present chapt…

Effects of Rayleigh number length thickness of continent on time of mantle flow reversal

Abstract Numerical experiments are carried out to study the effects of continents on the structure of thermal convection in the mantle. The mantle is modelled by a viscous fluid occupying a horizontally extended rectangular 2-D region of aspect ratio 10:1. Continents are treated as thick rigid heat-conducting plates placed in the mantle, with free-slip and with no-slip conditions. Continents restrict the heat release from the underlying mantle; the mantle material heats up and becomes lighter; as a result, a hot upwelling flow replaces downwelling. We calculate the characteristic time τ of this restructuring for various values of model parameters and obtain analytical approximations for τ a…

Successes and failures in geodynamics

Abstract The evolution of Earth models is reviewed and the open questions and problems are highlighted. Generally, evolution of science was not linear, but proceeded in “steps” of paradigms; where old ones remained within useful limits. “Geodynamic hypotheses”, while embedded into the general concepts of space and time, were often mutually exclusive and competing until the 1900s. Wegener's concept of continental drift was the first successful globally unifying view, but it was discarded by most Earth scientists. The “real” change of paradigms did not come before mid-century through geophysical observations in paleomagnetism, seismology, bathymetry, seafloor geology and dating, leading to th…

Gravity Anomalies Over The Gongola Arm, Upper Benue Trough, Nigeria

A regional gravity survey of the Gongola Arm of the Benue trough was carried out with the aim of determining structures of interest. The results of the gravity interpretation showed that the area of study is characterized by negative Bouguer anomalies that trend in the NE-SW direction and range in value from -75 to -15 mGal with an average of -42 mGal. A first order polynomial was used to approximate the regional anomalies in the area. The residual gravity anomaly map shows anomalies that range in value from -32 to +20 mGal. 2.5-D modeling of the residual gravity anomalies suggests that the area has a horst and graben structure, with the grabens being as large as 60 km in width and filled w…