Ma In Miocene Research Articles

Low absolute paleointensity during Late Miocene Noma excursion of the Earth’s magnetic field

Paleointensity variation is investigated for a sequence of andesitic lavas which records the Noma geomagnetic excursion at 6.66 ± 0.45 Ma. Sixty-six samples are collected from 11 lava flows at the Kamegaoka Mountain (31° 21′N, 130° 13′E) on the Noma Peninsula, Kyushu Island, Japan. Rock magnetic experiments confirm the presence of Ti-poor titanomagnetite grains with pseudo-single-domain or a mixture of single and multi-domains as the carrier of the remanent magnetization. The application of the Tsunakawa-Shaw method yields reliable geomagnetic paleointensity data set of 29 samples from 9 lavas that cover the Noma excursion. The mean paleointensities of lava flows vary from 1.9 to 10.3 µT. Paleointensity variation reveals a ‘V -shape’ of high–low–high paleointensities during the excursion. The highest value of 10.3 ± 3.5 µT appears prior to the excursion, the paleointensity decreases down to 1.9 ± 0.3 µT as the excursion progresses, and the paleointensity finally regenerates to 5.1 ± 3.0 µT at the end of the excursion. The virtual dipole moment (VDM) during the Noma excursion (0.3–1.1 × 1022 Am2) is less than 16% of the mean VDM during the period between 4 and 8 Ma in Miocene, suggesting that this weak geomagnetic field intensity is ascribed to the nature of excursions rather than the Miocene geomagnetic field properties. The weakest VDM (0.3 × 1022 Am2) in the Noma excursion is detected at the pole position that is located at fairly high latitude (58.9°N) in the opposite hemisphere. The Noma excursion is characterized by a large shift in virtual geomagnetic poles of more than 140° from the geographic pole and by a weak VDM of 1.0 × 1022 Am2 or less.

Fission track dates of Mandi granite and adjacent tectonic units in Kulu–Beas valley, NW Himalaya, India

We present zircon fission track (FT) ages from the lower parts of the Higher Himalayan crystalline (HHC) ranging from 3.9 to 4.3 Ma and apatite FT age as 1.9 ± 0.4 Ma . Combined with previous radiometric dates, these ages provide the low temperature data for the area and enable us to draw temperature–time plot which shows significant increase in cooling rates from 25 ∘ C / Ma in Oligocene and 4 ∘ C / Ma in Miocene to about 64 ∘ C / Ma during Pliocene. For the Mandi granite, the zircon FT ages range from 3.9 to 4.9 Ma while apatite FT obtained is 2.8 ± 0.5 Ma . The rapid Pliocene exhumation is characterized by a rapid increase in cooling rates from ∼ 3.4 ∘ C / Ma in Miocene to ∼ 77 ∘ C / Ma after 5.0 Ma. Our results confirm the rapid Pliocene to Pleistocene cooling of rocks of Kulu–Beas valley similar to that of the neighboring units from the frontal parts of HHC.