The Earth's magnetic field prior to the Cretaceous Normal Superchron: New palaeomagnetic results from the Alto Paraguay Formation

• Main Authors: Sánchez Bettucci, Leda, Mena, Mabel
• Language: unknown
• Published: 2013
• Subjects: large igneous province; palaeomagnetic poles; palaeosecular variation; Paraná flood basalts; the Alto Paraguay Formation; Cretaceous; flood basalt; geomagnetic field; magnetic reversal; natural remanent magnetization; paleomagnetism; secular variation; virtual geomagnetic pole; Alto Paraguay; Paraguay; Geomagnetic Pole
• Online Access: https://bibliotecadigital.exactas.uba.ar/collection/paper/document/paper_00206814_v55_n6_p692_Goguitchaichvili; https://hdl.handle.net/20.500.12110/paper_00206814_v55_n6_p692_Goguitchaichvili

We report a detailed palaeomagnetic investigation of 28 lava flows (221 standard palaeomagnetic cores) collected from the Paraguayan part of the Paraná flood basalts (the Alto Paraguay Formation). The initial aims of our study were to (i) document variability of the Earth's magnetic field during the time interval sampled, (ii) obtain a new Early Cretaceous palaeomagnetic pole (PP) for stable South America, and (iii) estimate the extrusion rate of the Paraná magma. We precisely determined the palaeofield direction for 26 sites for which the characteristic remanent magnetizations exhibit small within-site dispersion and high directional stability. No palaeodirections were determined for two sites because of a very complex and erratic behaviour of the remanence during the palaeomagnetic treatments. Nine sites display normal polarity magnetization, whereas nine others are reversely magnetized and the remaining eight sites yield intermediate palaeodirections. The mean palaeomagnetic direction of normal polarity sites give I = -41.8°, D = 4.9°, k = 112, and α 95 = 4.9°, whereas reversely magnetized sites give I = 37.1°, D = 181.4°, k = 23, and α 95 = 11.1°. The reversal test as defined by McFadden and McElhinny (1990; Classification of the reversal test in paleomagnetism: Geophysical Journal International, v. 103, p. 725-729) is positive, corresponding to Type B with γ = 8.7° and γ 0 = 3.7°. This ensures that the palaeomagnetic treatment successfully removed the secondary natural remanent magnetization and that the sampling adequately averaged the palaeosecular variation (PSV). The mean PP position obtained from 18 sites is palaeolongitude (Plong) = 359.2° and palaeolatitude (Plat) = 86.2° S. We show by means of probability plots and formal testing procedures that a Fisher distribution with a concentration parameter K = 65 satisfactorily fits the distribution of virtual geomagnetic poles (VGPs). The PP obtained in this study agrees reasonably well with coeval pole positions, in particular with those obtained from the ...