Earth interior II: Lithosphere

This presentation will discuss the advantages of using rotational invariants and curvature in addition to the components of the gradient tensor elements. The pros and cons are discussed by synthetic examples and using data from the GOCE satellite mission

Gradient data are nowadays available from an increasingly larger numbers of airborne surveys and from satellite missions like GOCE and Swarm. The reference frames for modelling and data acquisition are often not identical, e.g. data are given in a Cartesian or geographic coordinate system. Rotation of measured gradients to a model frame can be challenging, especially in high latitude regions.

For gravity gradients, the invariants overcome this challenge and have a depth sensitivity characteristic that represents the original gradient components well suited for lithospheric scale modelling. In forward modelling, typically done on a profile through a 3D geometry, the out of plane components are difficult to address and also here invariants are easier to use.

Magnetic gradients are even more complex as they depend on the orientation and strength of the induced and remanent magnetization. Because of the effect of remanence no simple reduction to the pole can be applied to the magnetic gradients. Invariants once again help to focus the magnetic anomalies and may be easier to interpret.

While the invariants are only to some degree make use of the directional information, the curvature components of the magnetic and gravity tensor can be used for structural interpretation and may offer an alternative for describing the bending of the lithosphere in reponse to topographic loads.

The crustal structure of Botswana represents one of the gaps that prevents fully understanding the tectonic evolution of the African continent. Although the area is rich in mineral resources, and especially diamond, the crust and upper mantle structure of the area are poorly investigated.

The continuous coverage and high accuracy of GOCE gravity gradients have opened a new era to explore the crust and upper mantle structures of previously unexplored areas on Earth. However, the inherent non-uniqueness of gravity inversion requires a priori information in order to produce reliable 3D subsurface models.

In this research,  a preliminary 3D shear-wave velocity model will be used as a priori information for GOCE gravity gradients inversion. The 3D shear-wave velocity model will be estimated using new data from the temporary seismological network NARS-Botswana. A joint inversion scheme of receiver function and ambient noise will be used to derive the 3D shear-wave velocity structure of the crust in southern and central part of Botswana. Then, the degree of agreement between the shear-wave velocity model and measured GOCE gradients will be evaluated by converting the shear-wave velocities into densities, calculating the forward signal and then compare it with the measured GOCE gradients. Finally, a joint inversion of GOCE gradients and shear wave velocities will be done to enhance our knowledge on subsurface crustal structures in Botswana.

The Swarm constellation of satellites was launched in November 2013 and has since then delivered high quality scalar and vector magnetic field measurements. A consortium of several research institutions was selected by the European Space Agency (ESA) to provide a number of scientific products which are available to the scientific community. Within this framework, specific tools were tailor-made to better extract the magnetic signal emanating from Earth’s the lithospheric. These tools rely on the scalar and vector gradients measured by the lower pair of Swarm satellites and rely on a regional modeling scheme that is more sensitive to small spatial scales and weak signals than the more standard spherical harmonic modeling. In this presentation, we report on various activities related to data analysis and processing. We assess the efficiency of the dedicated chain for modeling the lithospheric magnetic field using more than two years of measurements, and finally discuss refinements that are continuously implemented in order to further improve the robustness and the spatial resolution of the models describing the lithospheric magnetic field.

The study of the main subducting plates is usually performed by combining several different methodologies and observations, such as high accuracy mapping of seismicity and moment tensor catalogs, active-source seismic data and deep seismicity. In fact seismic derived data can for instance give a direct observation of the Benioff-Wadati zone (i.e. the boundary between the subducting oceanic lithosphere beneath the continental one). Moreover, high-resolution bathymetry and sediment thickness maps can help in defining the geometry of the subducting slab at the trenches. Since in correspondence of the subducting plate there is generally a large mass density discontinuity, due to the contrast between the light crust and the heavy mantle, gravity data can be used in order to study the geometry and some physical properties of subduction zones.

With the advent of the European Space Agency mission GOCE (Gravity field and steady-state Ocean Circulation Explorer) new important information can be added to the problem solution. The use of direct gravity observations has been up to now in fact quite limited due to the specific geometry of the problem: subducting plates usually extend in large areas thus requiring long and expensive airborne flight campaigns.

 In the present research ESA-GOCE along track gravity gradients have been used to improve our knowledge of the subduction zones and to estimate their shape parameters along with their density contrast. Practically the top of the subduction is considered known from seismic models (e.g. from SLAB1.0) and GOCE gravity gradients are inverted to recover the thickness and the density contrast between the subduction and the Earth mantle. The inversion method is based on a stochastic approach and the maximum probability solution is found by means of the simulated annealing algorithm.

 The algorithm has been firstly assessed on a closed-loop experiment to test its performance in detecting the parameters used to generate the simulated subduction signal; these parameters are estimated with a relative accuracy smaller than 10% even in presence of noise. After that, the Tonga subducting plate has been chosen as a natural laboratory to perform some numerical experiments; the resulting model seems to confirm the geometry already available in literature.

Gradients of magnetic field have higher spatial resolution than the fields themselves and are helpful in improving the resolution of downward continued satellite magnetic anomaly maps (Kotsiaros et al., 2015, Geophys. J. Int.; Sabaka et al., 2015, Geophys. J. Int.). Higher spatial resolution and fidelity of the magnetic field downward continued to the Earth’s surface translate into improvements in the interpretation of anomalies for recognition of geologic variability and tectonic processes (e.g., recognizing details of geologic provinces, anomalous seafloor spreading patterns, etc., that can help understand the evolution of the Earth).  Magnetic anomalies have sensitivity to thermal variations in the Earth’s lithosphere through the phenomenon of Curie temperature of ferromagnetic minerals. The response of the bottom of magnetization in the Earth’s crust/lithosphere is primarily observed in the long wavelength magnetic field up to at least 500 km (and sometimes longer). The Curie temperature depth can also be used to better map the thermal structure of the lithosphere because it is possible to theoretically include the Curie depth constraint in the derivation of the one dimensional geotherm (Ravat et al., 2015, in review). Despite having global set of observations from POGO, Magsat, Ørsted, CHAMP, and Swarm satellites (altitude > 400 km), preservation of intermediate wavelengths from about 100 to 375 km proves challenging (known as the “spectral gap”). Since the gradients of magnetic field have higher spatial resolution than the fields themselves, they are helpful in improving the coverage in the spectral gap. East-West and North-South (along orbit) gradients from Swarm magnetic field satellites provide an opportunity to examine the improvement in the anomaly coverage in the spectral gap and its effect on the interpretation, particularly the derived Curie depths and the thermal variation of the lithosphere. We examine the inaccuracies in anomalies and also their resulting interpretation using the U.S. aeromagnetic data where a full spectrum magnetic anomaly coverage is available (Ravat et al., 2009, USGS open files report OF09-1258) as a result of the availability of NURE data which were corrected with the comprehensive model of the magnetic field (CM4, Sabaka et al., 2004, Geophys. J. Int.). We specifically compare various levels and types of corrected data sets: uncorrected original North American Magnetic Anomaly Map compilation (ca. 2002), the original compilation corrected with satellite-altitude data sets, and Swarm constellation gradient corrected fields over the U.S.  Using this U.S. study as a test, we examine the possibility of improving the spectral coverage in many regions of the world where anomalies and their interpretations are still affected by the spectral gap.