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Abstract. The respective tectonic effects of back arc spreading and continental collision in Asia are considered either as two independent processes or as closely interrelated. Extrusion tectonics assumes that the opening of the South China Sea and the left-lateral motion along the Red River fault are geometrically linked in a pull-apart manner. This model is not accepted by several workers because the structural link between the two processes is not clearly demonstrated. In the case of the Japan Sea,, we can show without ambiguity that back arc opening was controlled by large intracontinental strike-slip faults which can be easily understood as effects of the India-Asia collision far from the indenter. The Japan Sea opened in the early Miocene in a broad pull-apart zone between two major dextral strike-slip shear zones.

We have identified Late Jurassic to Early Cretaceous magnetic anomaly lineations (M0 to M35 of the Japanese and Hawaiian lineation sets) and fracture zones in the northwestern Pacific more comprehensively than previous investigators. We fixed 3500 positions of magnetic anomalies identified from magnetic data collected along cruise tracks as well as 151 positions of fracture zones from bathymetric and seismic profiles. The resultant isochron map revealed the evolution of the triple junction of the Pacific, Izanagi, and Farallon plates and the intimate relationship between the triple junction and the origin of the Shatsky Rise. The triple junction stagnated on a hotspot (the Shatsky hotspot) from chron M21 (149.5 Ma) to chron M4 (126 Ma). The Shatsky Rise is a trace of the hotspot on the Pacific plate. A sudden appearance of the Shatsky hotspot between chrons M21 and M20 caused a regional reorganization of the Pacific‐Izanagi‐Farallon plate boundaries.

S U M M A R YLate Jurassic to Early Cretaceous (Mesozoic) magnetic anomaly lineations (the Phoenix, Magellan, Mid-Pacific Mountains, Hawaiian and Japanese lineation sets) with fracture zones in the west-central Pacific Ocean were identified more comprehensively than in any previous studies. We fixed 2100 positions of identified magnetic anomalies based on magnetic data of 283 cruise tracks. Two remarkable fracture zones, the Phoenix and Central Pacific Fracture Zones, were mapped and newly named. Our newly identified lineations from MlON to MO around the Mid-Pacific Mountains, which belong to the Hawaiian lineation set, illustrated that the sea-floor south of t h e Mid-Pacific Mountains has the same age as that of the north (132-118 Ma). Our analysis of skewness parameters revealed that the older part of the Phoenix set (M17-M29) has skewness different from that of the younger part (Ml-M14), implying an effect of magnetic overprints by the Cretaceous volcanism. It was confirmed that the spreading rate of the Mesozoic Pacific spreading system was the fastest in the world in the Mesozoic. A drastic change in spreading rates occurred simultaneously at the period between chrons M21 and M20 (149.5-148.5 Ma) in all the Mesozoic Pacific spreading systems. The event appears to be synchronous with events in other oceans such as the Mesozoic Atlantic and Indian Oceans.

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Indian Ocean hydrothermal vents are believed to represent a novel biogeographic province, and are host to many novel genera and families of animals, potentially indigenous to Indian Ocean hydrothermal systems. In particular, since its discovery in 2001, much attention has been paid to a so-called ‘scaly-foot’ gastropod because of its unique iron-sulfide-coated dermal sclerites and the chemosynthetic symbioses in its various tissues. Despite increasing interest in the faunal assemblages at Indian Ocean hydrothermal vents, only two hydrothermal vent fields have been investigated in the Indian Ocean. Here we report two newly discovered hydrothermal vent fields, the Dodo and Solitaire fields, which are located in the Central Indian Ridge (CIR) segments 16 and 15, respectively. Chemosynthetic faunal communities at the Dodo field are emaciated in size and composition. In contrast, at the Solitaire field, we observed faunal communities that potentially contained almost all genera found at CIR hydrothermal environments to date, and even identified previously unreported taxa. Moreover, a new morphotype of ‘scaly-foot’ gastropod has been found at the Solitaire field. The newly discovered ‘scaly-foot’ gastropod has similar morphological and anatomical features to the previously reported type that inhabits the Kairei field, and both types of ‘scaly-foot’ gastropods genetically belong to the same species according to analyses of their COI gene and nuclear SSU rRNA gene sequences. However, the new morphotype completely lacks an iron-sulfide coating on the sclerites, which had been believed to be a novel feature restricted to ‘scaly-foot’ gastropods. Our new findings at the two newly discovered hydrothermal vent sites provide important insights into the biodiversity and biogeography of vent-endemic ecosystems in the Indian Ocean.

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