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

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

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,offersthe opportunity to shed light on global scale variations of the geomagneticfield. This period of long constant polarity of the geomagnetic field mayconstrain Earth?s core and mantle evolution. Long periods of single polarity aswell as its opposite, periods with high reversal frequency, are still a matterof debate: up to now there are two contradicting theories, which try to relategeomagnetic field intensity and reversal rate. We aim to report the geomagneticfield strength during the CNS, because data are still scarce and of dissimilarquality. We investigated Cretaceous samples from two locationsfrom South America. Firstly, we report paleomagnetic results from the WesternCordillera of Colombia from 15 sites North of Cali. These volcanic rock samplesare close to age determinations, which have been made previously, giving an ageof 92.5 ± 1.1 Ma on average, occuring during the CNS. And secondly, we presentresults from an alkaline dike swarm in the Asunción Rift, Eastern Paraguay.Previous investigations suggest that these dikes extruded in a rather shortperiod of 126-127 Ma, during normal and reversed polarity field configuration,right before the onset of the CNS.Rock magnetic measurements indicate alow-coercivity mineral, e.g., (titanium)magnetite, as main magnetic component,with a range of grain sizes from multi- to single domain, or mixtures ofdifferent grain sizes in a sample. The Colombian samples yield an averageVirtual Geomagnetic Pole (VGP), whose latitude compares well to the one forSouth America of Besse and Courtillot (2002) with a similar age. Thedetermination of the Virtual Dipole moments (VDMs) with the Thellier-Coe methoddid not provide successful results, probably due to minerals in the range ofmultidomain grain size. Therefore, we applied the multispecimen protocol afterDekkers&Böhnel (2006) on ten specimens. Six successful determinations yieldan average VDM of (17.3 ± 5.1) x 10-21Am2.This rather low value is in agreement with other data from the same timeperiod. Directional investigations of the Eastern Paraguayan dike swarm showhighly clustered promising results with six out of 22 sites having an α95 ≤ 10.0°. Most of thesesites exhibit a reversed polarity of the ancient geomagnetic field. However,one intermediate polarity site has a very reliable directional results. Thisand the occurrence of normal polarity sites suggest that the dikes may have notappeared at the same time but rather during the transition from normal toreversed polarity right before the CNS at around 126-127 Ma. Specimens fromnine sites are promising for paleointensity determination due to their linearvector diagrams showing one component, the occurrence of magnetite as magneticcarrier, and their magnetic stability. Ongoing work is focusing oninvestigation of directions from specimens from a second field campaign locatedclose to the first one. Furthermore, paleointensity determinations are planned,after choosing the most successful specimens.