Paleomagnetic Pole Positions and Geomagnetic Secular Variation from the Cretaceous Ponta Grossa Dike Swarm (Brazil)

摘要

A detailed paleomagnetic and rock magnetic investigation has been carried out on the Early Cretaceous Ponta Grossa dike swarm in southern Brazil. This formation seems an excellent target for paleomagnetic study. The dikes are widely distributed over a large area, easy to access, and they record faithfully the geomagnetic field at the time of the eruption. Most of them are fresh and have been dated by K-Ar and Ar-Ar. Thermomagnetic experiments (low-field versus temperature curves) suggest low-Ti titanonnagnetites as main remanence carriers, and their domain structure is characterised by a mixture of single-domain and multi-domain grains. Characteristic paleomagnetic directions are retrieved from 28 out of 29 sites (235 standard paleomagnetic cores). 17 sites show normal polarity, 10 sites show reverse polarity and one site shows an oblique direction, with negative inclination, separated 79 degrees of the mean normal directions. The reversal test is positive at the 95% confidence level which ensures that the secondary remanent magnetizations were successfully removed and the sampling adequately averaged the palaeosecular variation. The paleosecular variation parameters values obtained in this study correlate with those determined for the Cretaceous Normal Superchron, between 125 to 84 Ma. The new paleomagnetic pole position (222.0 degrees E, 88.1 degrees S, N = 24, K = 35.68, A(95) = 5.0 degrees) agrees well (within uncertainties) with the reference poles determined from Besse and Courtillot (2002), and disagree with those reported in previous studies. The new results should be considered for estimating the Cretaceous paleomagnetic poles for stable South America.