Boris Ya. Karp

Oceanic crust in the Japan Basin of the Japan Sea by the 1990 Japan‐USSR Expedition

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.

Tectonics of short‐lived intra‐arc basins in the arc‐continent collision terrane of the Coastal Range, eastern Taiwan

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 intra-arc basins is reconstructed by stratigraphic and sedimentological analysis, igneous rock geochemistry, and comparison with observations in modern 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 modern 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 km/m.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 formation of the Pliocene Chingpu and Pleistocene Chengkung basins as well as the present-day offshore intra-arc basins found on the Lutao and Lanhsu volcanic islands. The two intra-arc basins on Lutao and Lanhsu are predicted to be short lived. As collision continues, these two basins, together with their underlying northern part of the Luzon arc, will be rotated, thrust, and uplifted in the next 1 m.y. and, finally, will become part of the southern extension of the Coastal Range.

P-wave velocity structure in the northern part of the central Japan Basin, Japan Sea with ocean bottom seismometers and airguns

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.