Characteristics of the Earth's Magnetic Field Prior to the Cretaceous Normal Superchron: New Paleomagnetic Results from Alto Paraguay Formation

We report a detailed paleomagnetic investigation from 28 lava flows (221 standard paleomagnetic cores) collected in the Paraguayan part of the Paraná Flood Basalts (Alto Paraguay Formation) in order to (i) document the variability of the Earth's magnetic field during the early Cretaceous, (ii) estimate the extrusion rate of Paraná magma and (iii) obtain a new Cretaceous paleomagnetic pole for stable South America. The paleofield direction is precisely determined for 26 sites for which, the remanent magnetization is characterized by a small within-site dispersion and a high directional stability. Nine sites give normal polarity magnetization and other 9 are reversely magnetized while 8 remaining sites show intermediate paleodirections. The mean paleomagnetic direction of normal polarity sites is I=-41.8, D=4.9, k=112, a95=4.9 while reversely magnetized sites give I=37.1, D=181.4, k=23, a95=11.1. These results point to almost antipodal mean directions, since the reversal test is positive. The mean paleomagnetic pole position obtained from 18 sites is Plong= 179.2E, Plat= 86.2S, R=17.74, k=64.56, A95=4.3. The positions of Virtual Geomagnetic Poles show a reasonably good fit with a Fisherian distribution when probability plots as well as formal testing procedures are used. The pole obtained in this study agrees reasonably well with coeval pole positions, in particular with those obtained from CPMP (Central Paraná), Los Adobes, Misiones and SAMC. However, some other similar age paleomagnetic poles show significant departure that may be attributed to local tectonic rotations or insufficient sampling to overcome the paleosecular variation. The paleosecular variation parameters are in agreement with the selected data reported for the Cretaceous Normal Superchron. In contrast, VGP angular dispersions found here are lower with respect to the Jurassic and Plio-Pleistocene data. The intermediate VGPs show a cluster in southern hemisphere of 6 VGPs located near the pacific cost of south America while two other VGPs are located in Indian Ocean and Australia matching with still debated two preferred longitudinal bands.