New insights from paleodirectional and intensity results of cretaceous dikes from Eastern Paraguay

KAPPER, K L; SANCHEZ BETTUCCI, L; GOGICHAISHVILI, A; CALVO-RATHERT, M; IRURZUN, M A; CEJUDO-RUIZ, R; MORALES, J; CARRANCHO, A; SINITO, A M

Seattle

Congreso; The Geological Society of America - Cordilleran Section - 113th Annual Meeting ? 2017; 2017

Geological Society of America

The investigation of the geomagnetic field variation during the Cretaceous, which lasted from about 145.5 ? 65.5 Ma, offers the opportunity to shed light on the controversy between reversal rate and strength of the paleointensity. Some authors relate a low reversal frequency to a simultaneous low paleointensity, while others relate it to high paleointensity, depending on different processes in Earth?s interior. Two periods of major interest in this controversy are the Cretaceous Normal Superchron (CNS), a period of long constant normal polarity, and the period right at the onset of the CNS, which exhibits frequent polarity chances.We aim to report the geomagnetic field direction and strength at the beginning of the CNS, because data are still scarce from this period. We investigate samples from an alkaline dike swarm in the Asunción Rift, Eastern Paraguay. Previous age determinations suggest that these dikes extruded in a rather short period at 126-127 Ma, during normal and reversed polarity field configuration.Rock magnetic measurements indicate a lowcoercivity mineral, e.g., (Ti)magnetite, as main magnetic component, with a range of grain sizes from multi to single domain, or mixtures of different grain sizes in a sample. Directional investigations show highly clustered promising results with six out of 22 sites having an α95 ≤ 10.0°. Most of these sites exhibit a reversed polarity of the ancient geomagnetic field. However, one intermediate polarity site has a very reliable directional results. This and the occurrence of normalpolarity sites suggest that the dikes may have not appeared at the same time but rather during the transition from normal to reversed polarity at around 126-127Ma.Specimens from nine sites are promising for paleointensity determination due to their linear vector diagrams that show one component, the occurrence of magnetite as magnetic carrier, and their magnetic stability. Ongoing work is focused on study of directions of specimens from a second field campaign located close to the first set of samples. Furthermore, paleointensity determinations with the Thellier-Thellier method are planned, after choosing the most successful specimens.