Paleosecular variation in the last 6 Ma recorded by lava flows from the East Carpathians () C.G. Panaiotu1, M. Visan2, C. Necula1 1 Paleomagnetic Laboratory, University of Bucharest, Romania ( [email protected])

2 “Sabba S. Stefanescu” Institute of geodynamics, Bucharest, Romania

T ime (Ma) ime Polarity

Chron

C4An 9.0 Introduction. In the last 10 Ma a post collisional volcanism took place in the East Carpathians East Carpathians -1 2r C4r 8.5 r 1 forming a 160 km long chain. We present paleomagnetic results from the volcanic rocks erupted in 8.0 2nC4n

n 1 3r 7.5 the last 6 Ma covering around 70 km from the volcanic chain. The geographic distribution of magnetic 2r/n C3Br 1r/n C3Bn 7.0 C3Ar polarity support the currently accepted model of a progressive migration of the volcanic activity from 2n Senetea 6.5 Mures Izvorul Muresului r C3An 1 Rachitis n North to the South, in time steps of around 1 Ma or less according to the magnetic polarity data (Fig. 6.0 Sumuleu Sandominic Ostoros C3r 5.5 1). This research is part of larger study to obtain data about paleosecular variation (PSV) from lava -Fierastraie

Ivo-Cocoizas 4n 5.0 r 3 flows in this part of Europe and its time variability in the last 10 Ma. n r C3n Madaras 2 n 4.5 r Varghis 1 arnava Mare n T

C2Ar 4.0 3n 3.5 r 46.21°N n 2 r C2An Methods. For the paleosecular variation study we have selected the sites following the criteria from 3.0 1

Valea Mare Ciuc Basin 2.5 Luci-Lazu 2r Johnson et al. (2008): minimum 5 samples per site, dispersion parameter k > 50 and colatitude cutoff HN85 C2r Olt

r 1 2.0 Cormos Cucu C2n 45°, both AF and thermal demagnetization, full demagnetization of each specimen. We selected a Pârâul Mare 2r 1.5 Fierarul C1r total of 156 sites divided in 3 age intervals: 0-1.1Ma (43 sites), 1.5-4 Ma (48 sites) and 4-6 Ma (65 Pilişca n 1.0 Herculian 1 Ciumadu r

Bradul sites). For stastitical analysis we used PmagPy 2.171. 0.5 Balvanyos Bixad C1n Braşov Basin Baraolt HN3 Braşov Basin

Malnaş HN5

Olt Racosu de Jos alley Olt V

normal polarity Comana reversed polarity Fig. 1 Correlation of magnetic polarity zones with the Polarity Time Scale (Lourens et al., 2004) 25.39°E Fig. 1 based on K-Ar ages (Peskay et al., 2006) and geographical distribution of magnetic data.

Results : Directions 0-1.1 Ma 1.5-4 Ma 4-6 Ma 0-6 Ma

43 sites 65 sites 156 sites 48 sites Group mean directions support All groups have a positive reversal test, but the group 1.5-4 Ma has a marginal negative test. the absence of significant tectonic rotations post-emplacement

Results: Which data set is Fisherian: directions or VGPs? Both are ! Conclusions 1. PSV was characterized both by

dispersion of virtual geomagnetic poles (SB) or precision parameter k of VGP or directions (Lhuillier & Gilder, 2013). The VGPs dispersion of the 1.5-4 Ma group is larger both than the global data base and the other two groups in this study. K parameter for this group is significant Directions (all combined 0-6 Ma) VGPs (all combined 0-6 Ma) different than k parameter for others groups. 2. Numerical simulations from statistical paleosecular variation models and dynamo Paleosecular variation models indicate the need for several VGP dispersion from lavas and their 95% hundred paleomagnetic sites and more 40 confidence intervals. Normal and reversed than 2 Ma to get an accurate determination (°)

B 35 combined data from this study are represented of the VGPs dispersion (Tauxe et al, 2003; with squares; Data from Johnson et al. (2008) Tauxe et al., 2008; Lhuillier & Gilder, 2013). 30 are represented with full symbols for Brunhes- Only the total data set (0-6 Ma) is near this 25 1.5-4 Ma age normal polarity, open symbols for condition and its dispersion is close to the 0-1Ma Matuyama-age reversed polarity data and grey TAFI data and the predicted dispersions for 20 0-6 Ma TK03 symbols for 0-5 Ma combined data set: Model Tk03.

VGPs dispersion, S 4-6 Ma 15 G inverted triangles represent data only from 3. The results from this study might imply an

10 TAFI studies, circles are latitudinally binned increase of VGP dispersion between 1.5-4 Ma starting at around 45°N, but more data 42 44 46 48 50 52 54 global data. Predicted dispersions for statistical Latitude (°N) model G and TK03: triangles. are needed both at local and global level to determine if this reflects the behavior of the geomagnetic field, or an incomplete database. Acknowledgements: This is a contribution of the CNCS – UEFISCDI project number PN-II-ID-PCE-2012-4-0177.