Hermann Zeyen

Integrated lithospheric modeling combining thermal, gravity, and local isostasy analysis: Application to the NE Spanish Geotransect

A two-dimensional algorithm to determine the steady state thermal structure of the lithosphere that integrates thermal, gravity, and local isostasy analyses is presented. Gravity analyses together with seismic data are used to constrain spatial variations in density and crustal structure, while absolute elevation is used to determine the lithospheric mantle thickness. The calculation is performed using a finite element technique that links the different physical equations. The program optionally calculates the temperature at any material boundary and, with given rheological parameters, the strength distribution and the total lithospheric strength in selected columns. We apply the algorithm to the Northeastern Spanish Geotransect which extends from the Pyrenees to the Balearic Promontory and along which a strong variation in crustal and lithospheric thickness is evident. We assess the use of two different inferred density models for the lithospheric mantle: The first assumes a linear decrease in density with increasing temperature using the asthenospheric density as a reference; the second model assumes a constant density for the whole lithospheric mantle. Although conceptually the two hypotheses differ substantially, the results obtained do not show significant differences. Lithospheric thicknesses of 120–130 km below the Pyrenees, 60–65 km in the Valencia Trough, and 65–75 km below the Balearic Promontory are deduced. In all cases the mean lithospheric mantle density has to be 40–60 kg m−3 higher than the asthenospheric density. The algorithm is shown to be a powerful tool in lithospheric thermal modeling especially in areas where surface heat flow is poorly constrained because of the temperature-density-elevation relationship.

FA2BOUG-A FORTRAN 90 code to compute Bouguer gravity anomalies from gridded free-air anomalies: Application to the Atlantic-Mediterranean transition zone

In this paper we present a computer program written in FORTRAN 90 specifically designed to determine the Bouguer anomaly from publicly available global gridded free-air anomaly and elevation database sets. FA2BOUG computes the complete Bouguer correction (i.e. Bullard A, B and C corrections) for both land and sea points in several spatial domains according to the distance between the topography and the calculation point. In each zone a different algorithm is used. In a distant zone we consider the harmonic spherical expansion of the potential of each right rectangular prism representing an elevation grid point. In an intermediate zone we compute the gravitational attraction produced by each prism using the analytic formula. Finally, an inner zone contribution is divided into two parts: a flat-topped prism with a height equal to the elevation of the calculation point, and four quadrants of a conic prism sloping continuously from each square of the inner zone to the calculation point. The program has been applied to the Atlantic-Mediterranean transition zone to obtain a complete Bouguer anomaly map of the area, integrating available onshore Bouguer anomaly with satellite-derived free-air anomaly data. Positive Bouguer anomalies are found in the Atlantic oceanic domain (240-300mGal), central and eastern Alboran Basin (40-160mGal) and SW Iberian Peninsula (>40mGal). Major negative Bouguer anomalies are located beneath the west Alboran Basin (<-40mGal), the Rif, the Rharb Basin and the Atlas Mountains (<-120mGal). An isostatic residual anomaly map of the study area has been computed and compared with the crustal and lithospheric structure inferred from previous work.

Thermodynamic properties of solutions in metastable systems under negative or positive pressures

Metastable systems are created when the interface between the atmosphere (in which Patm = 1 bar) and water forms a spherical meniscus either concave toward the air (water filling capillaries, wherein Pwater Patm). Soil water, undergoing negative pressure (“capillary potential”) remains bound to the solid matrix (instead of flowing downward) by the capillary meniscus, concave toward the undersaturated dry atmosphere. The positive counterpart of tensile water in soils is the pressurized water contained in fine droplets suspended in oversaturated humid air, as in clouds. All these systems are anisobaric domains the phases of which have different pressures. Geochemical consequences of such characteristics are assessed here by calculating the consequences of the positive or negative water potential on the equilibrium constants of reactions taking place in stretched or pressurized aqueous solutions. Thermodynamic properties of aqueous species are obtained by using the TH model, used explicitly for positive pressures but extrapolated to negative ones for soil solutions. It appears that soil water dissolves gases, offering an alternative explanation of the observed enrichment of atmospheric noble gases in groundwater and of carbonic gas in the unsaturated zone below the root zone. Water droplets obviously show the opposite behavior, that is, a decreasing dissolutive capability with decreasing droplet size (water pressure increases), inducing some climatic consequences. An application of this approach to the solid-solution equilibria is performed by comparing experimental solubility of amorphous silica in unsaturated media on the one hand, to theoretical calculations taking account of the negative water pressure on the other hand. This comparison outlines the potential complexity of anisobaric situations in nature and the necessity to develop a suitable approach for solid pressure.

Eocene exhumation of the Tuareg Shield (Sahara Desert, Africa)

The arch-and-basin geometry that characterizes North Africa was achieved at the end of Paleozoic times. It has been subsequently reactivated during the Mesozoic-Cenozoic with, in particular, the development of large topographic anomalies. Among these, the Tuareg Shield forms a topographic high in which the Pan-African basement reaches 2400 m above sea level (Hoggar core). While Cretaceous sedimentary remnants suggest a possible stage of subsidence during the Mesozoic, currently the area forms a swell, emphasized by Cenozoic volcanic episodes since 35 Ma. In this context, we present the first apatite (U-Th)/He thermochronological data acquired across this swell, with mean ages ranging from 78 ± 22 Ma to 13 ± 3 Ma. These results demonstrate the existence of a widespread Eocene exhumation of the shield before volcanic activity began, which reflects large-scale vertical processes. In the northeastern part of the swell, Cretaceous continental sedimentary remnants unconformably lying on the basement close to our samples evidence that they were near the surface at that time. This study shows that basement rocks have undergone subsequent heating at ∼60–80 °C, suggesting a burial of more than 1 km after the Early Cretaceous. This conclusion can be possibly extended over the whole Tuareg Shield.

Detecting faults and stratigraphy in limestone with Ground-Penetrating Radar: A case study in Rustrel

Surface Ground-Penetrating Radar (GPR) data have been acquired along the floor as well as along the vertical walls of a tunnel inside a karstic limestone reservoir in Rustrel. Geological study previously demonstrated the existence of stratification planes with an average dip of 25° to the south and numerous subvertical fault planes. The mono-offset GPR profile analysis acquired along the vertical wall of the tunnel demonstrates the presence of dipping reflectors that can be followed as deep as 16 m from the acquisition surface with 250 MHz nominal antennas. The position of these reflectors coincides with observations of faults recorded in a report written during the tunnel excavations.

Non-linear stochastic inversion of gravity data via Quantum-1 behaved Particle Swarm Optimization (QPSO): Application to 2 Eurasia-Arabia collision zone (Zagros, Iran)

Potential field data such as geoid and gravity anomalies are globally available and offer valuable information about the Earths lithosphere especially in areas where seismic data coverage is sparse. For instance, non-linear inversion of Bouguer anomalies could be used to estimate the crustal structures including variations of the crustal density and of the depth of the crust-mantle boundary, i.e. Moho. However, due to non-linearity of this inverse problem, classical inversion methods would fail whenever there is no reliable initial model. Swarm intelligence algorithms, such as Particle Swarm Optimization (PSO), are a promising alternative to classical inversion methods because the quality of their solutions does not depend on the initial model, they do not use the derivatives of the objective function, hence allowing the use of L1 norm, and finally they are global search methods, meaning the problem could be non-convex. In this paper, Quantum-behaved Particle Swarm (QPSO), a probabilistic swarm intelligence-like algorithm, is used to solve the non-linear gravity inverse problem. The method is first successfully tested on a realistic synthetic crustal model with a linear vertical density gradient and lateral density and depth variations at the base of crust in the presence of white Gaussian noise. Then, it is applied to the EIGEN 6c4, a combined global gravity model, to estimate the depth to the base of the crust and the mean density contrast between the crust and the upper-mantle lithosphere in the Eurasia-Arabia continental collision zone along a 400 km profile crossing the Zagros Mountains (Iran). The results agree well with previously published works including both seismic and potential field studies. This article is protected by copyright. All rights reserved

Automatic 1D integrated geophysical modelling of lithospheric discontinuities: a case study from Carpathian-Pannonian Basin region

Abstract Using a very fast 1D method of integrated geophysical modelling, we calculated models of the Moho discontinuity and the lithosphere-asthenosphere boundary in the Carpathian-Pannonian Basin region and its surrounding tectonic units. This method is capable to constrain complicated lithospheric structures by using joint interpretation of different geophysical data sets (geoid and topography) at the same time. The Moho depth map shows significant crustal thickness variations. The thickest crust is found underneath the Carpathian arc and its immediate Foredeep. High values are found in the Eastern Carpathians and Vrancea area (44 km). The thickest crust modelled in the Southern Carpathians is 42 km. The Dinarides crust is characterized by thicknesses more than 40 km. In the East European Platform, crust has a thickness of about 34 km. In the Apuseni Mountains, the depth of the Moho is about 36 km. The Pannonian Basin and the Moesian Platform have thinner crust than the surrounding areas. Here the crustal thicknesses are less than 30 km on average. The thinnest crust can be found in the SE part of the Pannonian Basin near the contact with the Southern Carpathians where it is only 26 km. The thickest lithosphere is placed in the East European Platform, Eastern Carpathians and Southern Carpathians. The East European Platform lithosphere thickness is on average more than 120 km. A strip of thicker lithosphere follows the Eastern Carpathians and its Foredeep, where the values reach in average 160 km. A lithosphere thickness minimum can be observed at the southern border of the Southern Carpathians and in the SE part of the Pannonian Basin. Here, it is only 60 km. The extremely low values of lithospheric thickness in this area were not shown before. The Moesian Platform is characterized by an E-W trend of lithospheric thickness decrease. In the East, the thickness is about 110 km and in the west it is only 80 km. The Pannonian Basin lithospheric thickness ranges from 80 to 100 km.

Integrated Modeling of the Crust and Mantle Structure in the Zagros Fold and Thrust Belt and the Mesopotamian Foredeep

The Zagros mountains formed by collision between Arabian and Eurasian plates from Miocene times. The region is characterized by intermediate seismicity, a low mantle velocity, a deep foreland basin, and an irregularly folded sedimentary cover. Despite extensive acquisition of geophysical data major unknowns are related to i) the nature of the crustal deformation during collision and the resulting crustal structure; ii) the existence of a mantle root and the possible strain partitioning between crust and mantle lithosphere; and iii) the basement deflection caused by the building of the Zagros mountains and the associated deep geometry of the foreland basin. These items are addressed in two ways. An integrated approach, combining the use of gravity, geoid and absolute elevation allows us to infer the 3D regional crustal and lithospheric structure. The resolution of the gravity inverse problem for that lithospheric configuration allow us to separate the regional and local field components which, in turn, allows for a more detailed 2D lithospheric modelling along selected geotransects. These geotransects are constrained by existing seismic profiles, surface elevation, gravity and geoid data. The crustal and lithospheric structure is modelled by using a numerical code that simultaneously solves the geopotential, lithostatic, and heat transport equations.