Paleomagnetic and intensity investigation of Cretaceous igneous rocks: new contributions from Colombia and Paraguay

R. CEJUDO RUIZ; Á. CARRANCHO; A. M. SINITO; A. NIVIA GUEVARA; MANUEL CALVO-RATHERT; M.A. IRURZUN; JUAN MORALES; V. MEJIA; LISA KAPPER; L. SANCHEZ BETTUCCI; AVTO GOGUITCHAICHVILI; C. S. G. GOGORZA

Puerto Vallarta

Congreso; Reunión Anual de la Unión Geofísica Mexicana (UGM 2016).; 2016

Unión Geofísica Mexicana

The investigation of the Cretaceous Normal Superchron (CNS), which lasted from 120.6 83.0 Ma, offers the opportunity to shed light on global scale variations of the geomagnetic field. This period of long constant polarity of the geomagnetic field may constrain Earth?s core and mantle evolution. Long periods of single polarity as well as its opposite, periods with high reversal frequency, are still a matter of debate: up to now there are two contradicting theories, which try to relate geomagnetic field intensity and reversal rate. We aim to report the geomagnetic field direction and strength during the Cretaceous, because data are still scarce and of dissimilar quality.We investigated Cretaceous samples from two locations from South America. Firstly, we report paleomagnetic results from the Western Cordillera of Colombia from 15 sites North of Cali. These volcanic rock samples are close to age determinations, which have been made previously, giving an age of 92.5 ± 1.1 Ma on average, occuring during the CNS. And secondly, we present results from an alkaline dike swarm in the Asunción Rift, Eastern Paraguay. Previous investigations suggest that these dikes extruded in a rather short period of 126127 Ma, during normal and reversed polarity field configuration, right before the onset of the CNS.Rock magnetic measurements indicate a low coercivity mineral, e.g., (titanium)magnetite, as main magnetic component, with a range of grain sizes from multi to single domain, or mixtures of different grain sizes in asample. The Colombian samples yield an average Virtual Geomagnetic Pole (VGP), whose latitude compares well to the one for South America of Besse and Courtillot (2002) with a similar age. The determination of the Virtual Dipole moments (VDMs) with the Thellier Coe method did not provide successful results, probably due to minerals in the range of multidomain grain size. Therefore, we applied the multispecimen protocol after Dekkers & Böhnel (2006) on ten specimens. Six successful determinations yield an average VDM of (17.3 ± 5.1) x 1021 Am2. This rather low value is in agreement with other data from the same time period. Directional investigations of the Eastern Paraguayan dike swarm show highly clustered promising results with six out of 22 sites having anα95 ≤ 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 normal polarity sitessuggest that the dikes may have not appeared at the same time but rather during the transition from normal to reversed polarity right before the CNS at around 126127 Ma. Specimens from nine sites are promising for paleointensity determination due to their linear vector diagrams showing one component, the occurrence of magnetite as magnetic carrier, and their magnetic stability. Ongoing work is focusing on investigation of directions from specimens from a second field campaign located close to the first one. Furthermore,paleointensity determinations are planned, after choosing the most successful specimens.