Tertiary volcanic and sedimentary rocks were collected at 50 sites of both the central Ryukyu arc and the southern Ryukyu arc for a palcomagnetic and geochronological study, in an attempt to understand the opening of the Okinawa Trough back arc basin. Stable primary components of remanent magnetization were isolated through both thermal and alternating field demagnetization experiments from 12 sites of three geological units of the central Ryukyu arc and from 12 sites of two geological units of the southern Ryukyu are. The palcomagnetic results were compared with the Tertiary apparent pole wander path of Eurasia in order to investigate the tectonic movement of the Ryukyu arc with respect to the Asian continent. The comparison and the radiometric ages indicate that the southern Ryukyu arc rotated clockwise through 25 ø between 10 Ma and 6 Ma. In contrast, the central Ryukyu arc experienced little rotational motion since 17 Ma. We propose the following two-phase opening model for the Okinawa Trough opening. The first stage took place between 10 Ma and 6 Ma. The southern part of the trough opened by means of a "wedge" mode to cause the clockwise rotation of the southern Ryukyu are. In the central part the "parallel" opening occurred, and the Ryukyu arc drifted without any rotation. The second phase started recently in the whole Okinawa Trough. It is observed as the present activities in the trough. Geology of the Ryukyu Are An outline of the geology is described here following Kizaki [1986]. The Ryukyu arc is divided into north, central, and southem domains from both geological and geomorphological view points; the Tokara channel forms the boundary between the northern and the central Ryukyu arc, whereas the Miyako depression forms the boundary between the central and the southem Ryukyu arc (Figure 1). Figure 2 shows the schematic stratigraphy of the central and the southern Ryukyu arc. Weakly metamorphosed Mesozoic-Eocene sedimentary rocks are widely distributed in the northern and central Ryukyu domain. The sedimentary rocks of Okinawa-jima Island are named the Kayo Formation (Figure 3) and can be dated as Eocene by the presence of the foraminifera fossils (Nummulites sp. [Konishi et al., 1973]).

No abstract

The geomagnetic field intensity during Archaean times is evaluated from a palaeomagnetic and chronological study of a dolerite dyke intruded into the 3000 Ma Nuuk Gneisses at Nuuk (64.2"N, 51.7"W), west Greenland. Plagioclase from the dolerite dyke yields a mean K-Ar age of 2752 Ma. Palaeomagnetic directions after thermal demagnetization of the dyke and the gneiss reveal a positive baked-contact test, indicating that the high-temperature-component magnetization of the dyke is primary. Thellier experiments on 12 dyke specimens yield a palaeointensity value of 13.5 +_ 4.4 pT. The virtual dipole moment at ca. 2.8 Ga is 1.9 & 0.6 x 10" Am2, which is about onequarter of the present value. The present study and other available data imply that the Earth's magnetic field at 2.7 -2.8 G a was characterized by a weak dipole moment and that a fairly strong geomagnetic field similar to the present intensity followed the weak field after ca. 2.6 Ga.

No abstract

No abstract