C. Denis

Tidal friction and its consequences in palaeogeodesy, in the gravity field variations and in tectonics

Abstract Fossils and tidal deposits as well as the possibility to compute values of the lunar tidal torque for different geological epochs allow us to model the variations in time of the Earths figure, assuming that the latter remains, on a global scale, close to a hydrostatic equilibrium figure. On this basis we were able to infer the variations of the Earths most important kinetic parameters over much of the geological past. Thus, the geometrical oblateness of the outer surface has decreased from 0.005 to 0.003 over the last two and a half billion years. This slow but continuous change of the Earths curvature brought about by tidal friction must have led to continuous stress accumulation in the uppermost part of the lithosphere, where the temperature is below 400 °C and the rheological behaviour is likely to remain brittle over geological time scales. We investigate the inevitable tectonic consequences of this stress buildup, and try to find some evidence in present-day worldwide seismicity, with a negative result. An interesting result of our study, which may open a new field of gravimetric research, is embodied in the fact that tidal friction causes a secular increase of the Earths normal gravity component at the equator at a rate of about 2 ngals yr−1, and a concomitant decrease at the poles of about 0.5 ngals yr−1. This tiny secular signal may just lie within observational reach of superconducting gravimeters.

Complex Interpretation of the Earth Despinning History

The length of day (LOD) values deduced from fossils and tidal deposits strongly suggest that the Earth’s despinning rate was on average about five times smaller in the Proterozoic than in the Phanerozoic. Moreover, these data indicate that between 250 and 100 million years ago, i.e. during Mesozoic there was a non-linear variation superimposed on the overall linear trend of the Earth’s rotation rate as a function of time. In order to understand these observations within a geodynamical framework, we investigated the variations throughout geological time of the oceanic tides, of tectonic plate speeds, and of geomagnetic paleointensities. In agreement with other authors, we found that in the Mesozoic, the geomagnetic moment underwent a minimum, but no statistically significant change could be inferred for the Proterozoic and the Archean. On the other hand, during the Mesozoic, concomitantly with the geomagnetic paleointensity minimum, the average oceanic tidal torque and the average lithospheric plate speeds, were significantly smaller than before and after this epoch.

Variations of the Geomagnetic Dipole Magnitude over the Past 400 My

The international bank of the virtual dipole moment data supplemented by the values from more recent publications is used as the basis for an analysis of the behavior of the virtual dipole moment values over the last 400 My. The results obtained revealed a positive linear trend from 4.1 × 1022 to 5.7 × 1022 A m2 during the last 400 My. Against the background of the linear increase, fluctuations with a periodicity of about 40 My were observed. In the Phanerozoic time, minimums within the intervals of 340–370, 290–300, 240–270, 190–210, 165-140 (chrons M17-M43), 130-120 (chrons M2-M10), 100–110 (chron 34), 75–85 (chron C33 and the beginning of chron C34), 70-60 (chrons C31-C27), and 40-15 (chrons C18-C5AD) My B.P. are found. The distribution of the virtual dipole moment is strictly related to the distribution of the ancient geomagnetic field and may be taken into consideration when modeling the magnetization of the inversive magnetic layer of the ocean.

Evolution of calculations of the virtual dipole moment of the Earth for reconstructing the oceanic inversion magnetic layer’s parameters

The VDM (virtual dipole moment) is one of the most significant characteristics describing the behavior of the time evolution of the terrestrial magnetic field. However, we have revealed that the formulas with which VDM calculations are performed often do not coincide with each other in various literature sources. Hence, results are obtained from these calculations that cannot be identical. Their correctness is verified by comparing the dimension and obtained results with the known value of the VDM for our time.

Evolution of the virtual dipole moment through the Paleoarchean-Phanerozoic

In the context of the dipole model of the earth’s magnetic field, the data from the international bank of digital information on the distribution of the virtual dipole moment (VDM) values in time, combined with the data obtained by subsequent investigations (7082 values in total), served as the basis for reconstructing its behavior through the Paleoarchean-Phanerozoic. The VDM behavior is characterized by a positive linear trend in the interval of 4.1 × 1022 Am2 (3.5 Ga ago) to 5.5 × 1022 Am2 (now). This background linear growth of the field strength is complicated by irregularly distributed VDM variations ranging in the amplitude from 1.7 × 1022 to 3.7 × 1022 Am2 with the wavelength varying from 220 to 920 Ma. The average wavelength of such fluctuations is estimated to be 570 Ma, which is approximately equal by duration to the Wilson geological cycles. The interval of 0.84–1.3 Ga is first established to be characterized by the relatively calm VDM region of 4.7–4.9 × 1022 Am2. The first defined deep minimum described by 10 data points with the extremum of 2.3 × 1022 Am2 (2.15 Ga ago) corresponds to the terminal phase in the formation of the earth’s core geometry and initiation of the formation of the modern dipole field.