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1. Field work
The areas investigated between September 2013 and November 2016 covered the volcanic rocks from the Gurghiu Mountains and the Calimani Mountains. We have extended the field work in the Quaternary volcanic areas of the Persani Mountains and Harghita Mountains (Ciomadu and Balvanyos areas) to increase the number of sites in the age interval 0 - 1.1 Ma. The total number of sites in the age interval 0 -1.1 Ma has increased from 23 to 44. Geological map is simplified after various published sources.
2. Laboratory work: Gurghiu Mountains and the southern part of the Calimani Mountains
2.1 Rockmagnetism
We have measured for a specimen from each site the S ratio = - IRM (-300 mT)/IRM(2T), where IRM is the isothermal remanent magnetization measured at 2T and in a reversed magnetic field of 300 mT, and V factor = 100(k(700 A/m)-k(50A./m)/k(50A/m), where k is the magnetic susceptibility measured with MFK1-A kappa bridge at two different magnetic fields. The results show that the feromagnetic magnetic mineralogy is dominated by magnetite or Ti-poor titanomagnetite. Ti-rich titanomagnetites were identifed only in basalts from the Calimani Mountains and the Persani Mountains. Hematite is present in the samples from the Ciomadu area.
Temperature variation of magnetic susceptibility was measured for selected specimens between room temperature and 700C using a CL3 furnace and MFK1-A kappabridge. These measurement confirm the presence of magnetite or Ti-poor titanomagnetite and Ti-rich titanomagnetite in the basalts. Red curve - heating; blue curve - cooling.
The hysteresis properties at room temperature using a VSM model 3900 (Princeton Measurements) with a maximum applied field of 1 T were measured for a specimen from each site. We have determined the following parameters: saturation magnetization (Ms), saturation remanent magnetization (Mrs), coercive force (Bc) and coercivity of remanence (Bcr). The Day diagram constructed using these parameters shows that most of samples are in the PSD (pseudomodomain) area following the theoretical curves for various mixtures of SD (single domain) and MD (multidomain) grains (Dunlop, 2002).
FORC diagrams confirm that the magnetic granulometry is formed by a mixture of SD and MD grains. The FORC diagrams show the progressive evolution from a mixture dominated by SD grains to MD structure.
2.2 Structure of the natural remanent magnetization
The structure of the natural remanent magnetization was analized using AF and thermal demagnetization or a combination of AF demagnetization followed by thermal demagnetization. Sites with unstable magnetization or affected by lighting were omitted from further analysis. For the rest of the sites we have computed the mean direction and the virtual geomagnetic pole (VGP) using the Fisher statistics.
2.3 Statistical analysis of paleomagnetic data
We have selected the VGPs according to the following criteria (Johnson et al., 2008) :
1. n (number of samples at site level) ≥ 5; 2. k (precision parameter from Fisher statistics) ≥ 50; 3. a constant VGP latitude cut-off of 45°.
The VGPs from this study combined with previous results from the East Carpathians (Panaiotu et al., 2012; Panaiotu et al., 2013; Visan et al., 2016) were grouped in 5 groups according to their age: 1. 9 - 7 Ma (89 sites from the Calimani and Gurghiu Mts); 2. 7 - 6 Ma (44 sites from the Gurghiu Mts); 3. 6 - 4 Ma (76 sites from the Gurghiu and Northern Harghita Mts); 4. 4 - 1.5 Ma (48 sites from the Southern Harghita Mts.) 5. 1.1 - 0 Ma (31 sites from the Persani Mts. and Ciomadu - Balvanyos area). Sites from the age group 1.1 - 0 Ma were grouped from 44 sites to 31 sites to avoid multiple sampling of simultaneous lava flows. All five groups follow a Fisherian distribution and have a positive reversal test.
Geographic distribution of the selected five age groups. Paleomagnetic sites with normal polarity and reversed polarity are represented with blue dots and red dots, respectively. Age of each group is based on available radiometric dating (Pécskay et al., 2006 and references there in; Panaiotu et al., 2013). Geological map is simplified after various published sources.
3. Results
3.1 Constrains for the duration of volcanic activity
In the figure sites with normal polarity are represented with blue circles and sites with reversed polarity with red circles. Based on the geographic distribution of the magnetic polarity and radiometric ages (Pécskay et al., 2006 and references there in; Panaiotu et al., 2013)) we have correlated the main periods of volcanic activity with the global polarity time scale (Lourens et al., 2004). Overall, the results are consistent with the currently accepted model of a progressive migration of the volcanic activity from North to the South. Starting with 7 Ma, the migration of the main volcanic activity took place in time steps of around 1 Ma or less according to the magnetic polarity data.
3.2 Tectonic implications
Using paleomagnetic results from Late Miocene - Quaternary volcanisc rocks from the East Carpathians, Pannonian sediments fo the Transylvanian Basin (11 - 9 Ma, de Leeuw et al., 2013) and late Miocene volcanics from the Apuseni Mts (12 - 10 Ma, Rosu et al., 2004) we have estimated the post-Sarmatian vertical axes rotations. These rotations and their 95% confidence angles are plotted on the map of Transylvanian Basin (de Leeuw et al., 2013). The reference paleomagnetic poles were those for stable Europe at 10 Ma and 0 Ma (Torsvik et al., 2012).
Significant rotations are observed only for the Pannonian sediments (de Leeuw et al., 2013) and for the Late Miocene volcanic rocks from the Apuseni Mountains, but they are absent in the volcanic chain of the East Carpathians. These opposite post-Sarmatian rotations reflect the deformation of the area iduring the uplift of the Transilvanian Basin around 8 Ma (Matenco et al., 2015) and the begging of the inversion of the Pannonian Basin (Balazs et al., 2016).
3.3 Paleosecular variation and inclination anomaly
Dispersion of the VGPs from lava flows (Sb) and their 95% confidence limits in the latitudinal band 36°N – 48°N. Blue symbols represent the data from the East Carpathians (Vişan et al., 2016; this study). Red symbols represent data from the United States (Johnson et al., 2008; Mankinen, 2008; Dominguez and Van der Voo, 2014). Near symbols are shown the age intervals in Ma. The number of sites is represented with bold italics fonts.
The angular dispersions in the East Carpathians are not significantly different from each other, with the exception of the angular dispersion between 1.5 – 4 Ma, which is larger than that recorded between 4 - 6 Ma. We did not observed significant difference between the reversed and normal polarity dispersion parameters. Our angular dispersions do not show any increases systematically with increasing age as the data from southwestern USA (Mankinen, 2008). More data are needed at a global level to determine if the differences between the angular dispersions distributions in this latitudinal band reflect the behavior of the geomagnetic field or an artefact of inadequate number of sites.
Inclination anomaly from lavas versus latitude with 95% confidence intervals. Data (0-5 Ma) from Johnson et al. (2008) are plotted with black squares. Best-fit two-parameter zonal field model (red line) after Johnson et al. (2008). The measured inclination anomalies are around 0° for all ages in accord with the global compilation of Johnson et al. (2008) at this latitude.
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