科研之友
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摘要
New paleomagnetic, rock-magnetic and paleointensity results obtained on samples from 23 basaltic lava flows belonging to four different flow sequences (Mashavera, Kvemo Orozmani, Zemo Karabulaki and Diliska) of Pleistocene and Pliocene age from the eastern Djavakheti Highland, in southern Georgia, are presented. Radiometric dating of these sequences yields ages between 1.8 and 2.18 Ma for Mashavera, 2.07 and 2.58 Ma for Zemo-Karabulakhi and 2.12 and 3.27 for Diliska. No radiometric ages are available for the Kvemo Orozmani sequence, which is considered to be coeval to the Mashavera sequence.
Rock-magnetic experiments including measurement of thermomagnetic, hysteresis and IRM-acquisition curves suggest low-Ti titanomagnetite as main carrier of remanence, although a lower Curie-temperature component was also observed in several cases. Reversible and non-reversible curves were recorded in thermomagnetic experiments.
Paleomagnetic analysis generally indicated the presence of a single component (mainly in the Mashavera sequence), but also two more or less superimposed components in some other cases. In 21 sites a characteristic component could be determined and all except one were characterised by normal-polarity directions. Flows from the Mashavera sequence had a rather steep inclination (73.1 degrees). Nevertheless, a mean paleomagnetic direction of all four sequences is obtained (D = 8.5 degrees, I = 60.8 degrees, N = 4, alpha(95) = 11.7 degrees, k = 62.7) which agrees with the Plio-Quaternary directions obtained in previous studies in Georgia. The paleomagnetic pole obtained (latitude phi = 82.1 degrees, longitude lambda = 118.2 degrees, A(95) = 8.0 degrees, k = 240.7) agrees with the pole values of both the 0 Ma and the 5 Ma windows of the synthetic Eurasian polar wander path from Besse and Courtillot (2002). In order to analyse the behaviour of secular variation, the scatter of paleosecular variation of virtual geomagnetic poles of both the Mashavera flow and all 18 studied flows of Pleistocene age was calculated. It could be observed that both data-sets seem to fit well the expected scatter at latitude 41 degrees N.
Paleointensity experiments were carried out with the Coe modification of the Thellier method. Twenty-five out of 84 samples (30%) provided reliable paleointensity results. These successful results were mainly obtained in the Mashavera sequence. Most flows yielded paleointensity results in the 30-45 mu T range, in accordance with expected Pliocene to present day intensities. Two flows, however, located near the top of the Mashavera sequence yield high paleointensity values around 60 mu T. Anomalous paleointensity results in the upper-lying Mashavera flows together with the steep inclinations observed in that sequence, could perhaps signal the near onset of the Olduvai-Matuyama reversal.
- 出版日期2011-8
