Abstract
Paleogeographic reconstructions largely rely on paleomagnetic data, mostly in the form of paleomagnetic poles. Compilations of poles are used to determine so called apparent polar wander paths (APWPs), which capture the motion through time of a particular location with respect to an absolute reference frame such as the Earth’s spin axis. Paleomagnetic datasets from sedimentary rocks are particularly relevant, because of their spatial distribution and temporal continuity. Several criteria have been proposed through the years to assess the reliability of paleomagnetic datasets. Among these, the latitudinal-dependent elongation of a given paleomagnetic directions distribution, predicted by a widely accepted paleosecular variations model, has been applied so far only to investigate inclination flattening commonly observed in sedimentary rocks. We show in this work that this concept can be generalized to detect “contamination” of paleomagnetic data derived from tectonic strain, which is not always detected by field observation only. After generating different sets of simulated geomagnetic directions at different latitudes, we monitored the variations in the shape of the distributions after applying deformation tensors that replicate the effect of increasing tectonic strain. We show that, in most cases, the “deformation” of the dataset can be detected by elongation vs. inclination ratios not conforming to the values predicted by the paleosecular variations model. Recently acquired paleomagnetic directions and anisotropy of magnetic susceptibility (AMS; a parameter very sensitive to tectonic strain) data from New Caledonia verifies the results of these simulations and highlights the importance of measuring AMS when using sedimentary paleomagnetic data for paleogeographic reconstruction. We suggest to include always AMS measurement and analysis of the distribution shape to assess sedimentary paleomagnetic data used for paleogeographic reconstructions.
Highlights
IntroductionPlate tectonics is a unifying theory that provides a solid background for understanding fundamental processes occurring on Earth
Paleomagnetism and Paleogeographic ReconstructionPlate tectonics is a unifying theory that provides a solid background for understanding fundamental processes occurring on Earth
Finite strain in sedimentary rocks induced by tectonism can deviate primary paleomagnetic directions
Summary
Plate tectonics is a unifying theory that provides a solid background for understanding fundamental processes occurring on Earth. In this model, the outer shell of the Earth consists of moving lithospheric plates and their past movement can be traced using geological data (Torsvik et al, 2008). The relative and absolute motion of the tectonic plates is reconstructed using combinations of ocean floor magnetic anomalies, hot-spot tracks, and paleomagnetic data. Can be converted into a virtual geomagnetic pole (VGP), which is the point on the Earth’s surface where the imaginary pole that would results in the measured D and I is located. At the base of this approach lies the assumption that the geomagnetic field averaged over a few thousand years can be approximated by the one generated by a geocentric axial dipole (GAD), with the characteristic that the paleomagnetic pole, obtained by averaging the available VGPs, and the geographic poles coincide (e.g., Butler, 1992; Tauxe, 2010)
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