Elastic scattering data for proton scattering from ' C are analyzed phenomenologically by use of local, complex, one-channel optical-model potentials and in an extended coupled-reaction-channel environment. Although the back-coupling amplitudes in the elastic channel from pickup-stripping paths through "C intermediate states are found to be quite sizable, the optical-model potential is able to simulate most of the efFects quite well for both scattering and reaction processes at energies greater than about 60 MeV. The parameters of the optical-model potential and the residual optical-model potential of the coupled-reactionchannel environment are found to vary smoothly with energy. Below 60 MeV, these parameters have quite difFerent trends and the ability of the one-channel optical-model potential to simulate the singular effects of strong couplings is much reduced.NUCLEAR REACTIONS ' C+p system, energies up to 185 MeV; optical-model phenomenological analysis; coupled-reaction-channel analysis; comparison with DWBA calculations.
In September of 1990, a seismic refraction and reflection survey was conducted in the Japan Basin, in the northeastern part of the Japan Sea, as a part of the Japan‐USSR joint expedition. Twenty‐six ocean bottom seismometers (OBSs) were deployed on two 200‐km long lines. Explosives and an airgun were fired as controlled seismic sources on the two mutually perpendicular lines. Airguns were also used as the source for the multi‐channel reflection profiles. The crustal structure deduced is that of a typical oceanic basin: the crustal thickness is about 8.5 km including 2 km of sediment. We obtained a more detailed crustal structure than that obtained previously. From the dense airgun shooting data, the crustal structure is well resolved to show layer 1A, layer 1B, layer 2A, 2B, 2C, layer 3, and the mantle. The crust basically consists of laterally homogeneous layers but the Moho deepens slightly westward.
The Coastal Range in eastern Taiwan was originated from an oblique collision between the Luzon volcanic arc and Asian continent since the late Neogene. In this collision terrane, two intra-arc basins, the Pliocene Chingpu and Pleistocene Chengkung basins, were developed on the eastern part of the Neogene Chimei and Chengkuangao volcanic islands, respectively, prior to their accretion to eastern Taiwan. The tectonic evolution of these Neogene volcanic islands and associated intraarc basins is reconstructed by stratigraphic and sedimentological analysis, igneous rock geochemistry, and comparison with observations in modem collision zone in the regions off southeastern Taiwan. In the Coastal Range, the intra-arc basin sequences are 1.5-10 km wide and 40 km long, comparable in size to their modem analogues in the active collision zone. The basin axis trends subparallel to the volcanic ridge. In both basins, deepwater flysch overlies shallow marine reef carbonates, which in turn rest on volcanic basement, indicating rapid arc collapse (minimum rate of 1 krrdm.y.) soon after the arc-continent collision. The arc collapse occurred earlier in the north (Chimei, between 5.1 and 3.5 Ma) and later in the south (Chengkuangao, between 2.9 and 1.8 Ma), in concert with a southward propagation of the oblique collision. Sedimentation ended about 2 Ma and 1 Ma in the Chingpu and Chengkung basins, respectively, coeval with rotation of the Neogene volcanic islands. This suggests that the rotation inverted the intra-arc basin into thrusting, uplifting, and final emergence. Thus the duration of sedimentation for the intra-arc basins spanned only about 0.8-3.1 m.y. On the basis of land geology, offshore observations, and a clay model experiment simulating oblique arc-continent collision, a model for the intra-arc basin evolution in eastern Taiwan is proposed. During the collision, strike-slip faults would have been developed in the eastern part of volcanic islands to induce transtension movements, thus forming pull-apart, intra-arc basins on the collapsed volcanic island. This mechanism is believed to be responsible for the for-1Institute of Geology, National Taiwan University, Taipei. 2Institute
The Kurile Basin in the Okhotsk Sea, northwestern Pacific, is a back‐arc basin located behind the Kurile Island Arc. It is underlain by oceanic crust and its origin has been attributed to back‐arc spreading. Two models for the opening of the Kurile Basin exist, for which the spreading axis is oriented northeast–southwest and northwest–southeast, respectively. New data are presented here on the morphostructure of the slope of the northern Kurile Basin and of the central Kurile Basin which support a strike of the spreading axis in the latter direction. Bathymetric as well as single‐channel and multichannel seismic reflection data demonstrate the existence of dominant northwest‐striking normal faults on the northern slope of the Kurile Basin. In the central Kurile Basin a basement rise striking north‐northwest–south‐southeast (here named the Sakura Rise) was mapped. The rise morphology has the distinct imprint of a rift structure with symmetrical volcanic edifices on the rise axis and faulted blocks that tilt in opposite directions on the flanks. These data suggest that the Kurile Basin opened in a northeast–southwest direction. In the generally accepted plate tectonic reconstructions, northwest–southeast spreading associated with dextral strike–slip along the north–south‐striking shear zone of Sakhalin and Hokkaido islands has been assumed. In the present model, spreading in the Kurile Basin was presumably connected with dextral displacement along a northeast‐striking shear zone on the southern segment of the Okhotsk Sea.
In 1996, an airgun-ocean bottom seismometer survey was carried out in the northern part of the central Japan Basin. The crustal thickness in the central part is about 9 km, including a sedimentary layer with thickness of 1.5 km, and increases eastward. The obtained crustal structure is slightly different from those of typical ocean basins. The thickness and velocity of less than 6.5 km/s in the upper part of the crust do not correspond to that of a typical oceanic crust and the clear linear geomagnetic anomaly around this survey line has been unconfirmed. Although, this crust could be interpreted to be either anomalous thick oceanic crust formed at spreading centers influenced by a mantle plume or thinned continental crust at ocean-continental boundaries in passive margins, we prefer the latter as a conclusion, that is, it may be formed by thinning of a continental crust rather than by the melt of mantle plumes during the opening of the Japan Sea. In addition, the difference of the crustal structures in the study area and the northeastern Japan Basin where the crust is typical oceanic, indicates that the process of crustal formation may differ in the northern part of the central Japan Basin from in the northeastern Japan Basin.
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