Abstract
Abstract. Maps of magnetic and gravity field anomalies provide information about physical properties of the Earth's crust and upper mantle, helpful in understanding geological conditions and tectonic structures. Depending on data availability, whether from the ground, airborne, or from satellites, potential field anomaly maps contain information on different ranges of spatial wavelengths, roughly corresponding to sources at different depths. Focussing on magnetic data, we compare amplitudes and characteristics of anomalies from maps based on various available data and as measured at geomagnetic repeat stations. Two cases are investigated: southern Africa, characterized by geologically old cratons and strong magnetic anomalies, and the smaller region of Germany with much younger crust and weaker anomalies. Estimating lithospheric magnetic anomaly values from the ground stations' time series (repeat station crustal biases) reveals magnetospheric field contributions causing time-varying offsets of several nT in the results. Similar influences might be one source of discrepancy when merging anomaly maps from different epochs. Moreover, we take advantage of recently developed satellite potential field models and compare magnetic and gravity gradient anomalies of ∼ 200 km resolution. Density and magnetization represent independent rock properties and thus provide complementary information on compositional and structural changes. Comparing short- and long-wavelength anomalies and the correlation of rather large-scale magnetic and gravity anomalies, and relating them to known lithospheric structures, we generally find a better agreement in the southern African region than the German region. This probably indicates stronger concordance between near-surface (down to at most a few km) and deeper (several kilometres down to Curie depth) structures in the former area, which can be seen to agree with a thicker lithosphere and a lower heat flux reported in the literature for the southern African region.
Highlights
Geopotential field anomalies are the spatial magnetic and gravity field variations depending on surface geology, tectonics, changes in composition, physical properties, and thickness of the crust and upper mantle
Short wavelength or small scale in the following refers to the highest available resolution scalar and vector anomalies as shown in the left two columns of Figs. 4 and 5, while long wavelength or large scale refers to dimensions of several 100 km as determined by spherical harmonic models of potential field anomaly data truncated at degree and order 200
An exception is a dominance of positive scalar anomalies in the Namibian region in the high-resolution scalar anomaly map which is somewhat opposed to what is seen in the map from the global model
Summary
Geopotential field anomalies are the spatial magnetic and gravity field variations depending on surface geology, tectonics, changes in composition, physical properties, and thickness of the crust and upper mantle. Repeat stations are well-defined locations where magnetic absolute vector observations are carried out for one to a few days once a year to every couple of years They are mainly used to map the core (main) magnetic field and its secular variation on a regional scale Based on new repeat station results with improved external field correction, recently produced scalar anomaly maps, and regional as well as global vector magnetic anomaly models, we investigate the agreement between robust localized estimates of magnetic scalar and vector anomalies and available maps. Taking advantage of recent new satellite geopotential field information, we complement our study by a combination of the largescale (∼ 200 km resolution) magnetic anomalies with gravity gradient information of a comparable scale to discuss their links to specific lithospheric structures like terrane boundaries and faults. We discuss implications for geological and tectonic interpretation of magnetic and gravity anomalies before concluding
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