Sea-floor spreading magnetic anomalies of Mesozoic age have been mapped for the entire northwest Pacific, from the mid-Mesozoic quiet zone to the Cretaceous quiet zone. There are two general lineation directions: NE-SW between Shatsky rise and Japan, and NW-SE to the east and southeast of Shatsky rise. These two patterns are joined in a magnetic bight southwest of Shatsky rise. The bight is assumed to have been produced by spreading from ridges of a former ridge-ridge-ridge (R-R-R) triple junction between the Pacific, Kula, and Farallon plates which existed at the beginning of the late Mesozoic geomagnetic reversal period (anomaly M-26, 155 m.y.B.P.). Evolution of the triple junction resulted in the formation of Shatsky rise during 142 to 116 m.y.B.P. and Hess rise during 116 to 95 m.y.B.P. Identification of the anomalies is based on a late Mesozoic reversal model constructed from four closely spaced, parallel profiles recorded across the complete late Mesozoic sequence of reversal anomalies near the Hawaiian ridge. Anomalies which are repeated in all four profiles are considered due to reversals, while those not repeated are shown to be associated with seamounts or other topographic features. It is concluded that at 155 m.y.B.P. (M-26) the Kula-Pacific, Pacific-Farallon, Pacific-Phoenix ridge system described a comparatively small triangle about the Pacific plate. Assuming that spreading existed at the same rates prior to the period of late Mesozoic reversals, and projecting it back in time, the system would have started essentially at a point in the vicinity of 15°N and 155°E at 185 to 190 ill.y.B.P. Spreading was faster on the Kula-Pacific and the Pacific-Phoenix ridges than on the Pacific-Farallon ridge during the late Mesozoic. Spreading direction of the Kula-Pacific and northern Pacific-Farallon ridges changed clockwise with time (40° from M-26 to Cenozoic anomaly 32), while spreading direction changed counterclockwise on the Pacific-Phoenix ridge. This evolution of the spreading system has persisted to the present time; thus, the Pacific plate has apparently formed from a small cell by the unfolding of the spreading ridges bordering it.

An evolutionary history of the Western Pacific is presented by reconstructing the spreading systems of the region since middle Mesozoic time on the basis of marine magnetic lineations, paleomagnetic data, DSDP results, and selected other data. Subduction along the Asian plate margin throughout this time, an E-W spreading ridge system which was offset by N-S transform faults of great length and extended from the Pacific into the Tethys Sea, and general northward movement of the plates surrounding Asia, with WNW movement of the Pacific plate after −45 m.y., are the major factors in the origin and development of the Western Pacific island arcs and marginal seas. As segments of the E-W ridge system were subducted along the Asian margin and plates south of these ridge segments began to subduct, new spreading ridges formed far to the south, rifting India from Antarctica at about −100 m.y. and Australia from Antarctica at about −52 m.y. The marginal seas have been initially formed by rifting of the Asian continental margin due to ridge subduction, entrapment of oceanic crust when former transform faults became subduction zones due to changes in plate motion, and rifting of plates along transform plate boundaries. Back-arc spreading is a result of an increased subduction rate and confined to late Oligocene to present time.

The Bonin Arc, from the Shikoku Basin to the trench axis, is broken into four crustal blocks, each separated by major right-lateral strike-slip faults. From north to south, they are termed the Sumisu Jima, the Sofu Gan and the Central Bonin Dislocations. These dislocations are delineated by 1. (1) changes in trends of en-echelon ridge and trough structures of the main arc. 2. (2) right-lateral offsets of the bathymetry and structure of the arc's outer ridge. 3. (3) right-lateral offsets in the free-air gravity contours. The most likely time of formation of the en-echelon structures and fracturing of the arc into the four crustal blocks is between 27 and 17 m.y. B.P. (Miocene). During this time, Japan was moving southeastward relative to Asia, and the Philippine Sea plate (including the Bonin Arc) was moving northward, subducting beneath southwest Japan. Also, convergence and subduction of the Pacific Plate was taking place along the east side of the Bonin Arc, which involved the collision of thick buoyant crustal blocks of the Marcus Necker Ridge near the south end of the Arc. These movements resulted in compressive forces acting on the Bonin Arc from the northwest and produced a clockwise torque within the arc. The response to these forces included development of NE-NNE structural trends within the arc and fracturing of the arc into four crustal blocks, offset from each other by NE-NNE right lateral displacements.

This book contains papers which are grouped into general studies and regional studies. The first section examines the dynamic processes as well as the systematic geological and geophysical relationships found in the region as a whole. The second section focuses on specific areas and features of the Western Pacific.

The Institute of Oceanographic Sciences' long‐range sidescan sonar ‘Gloria’ was operated over almost 20,000 km of ship track during the recent 56‐day cruise 110 of R.R.S. Discovery in the eastern Pacific Ocean. The cruise took place between April and June 1980 and ran between Balboa (Panama), the East Pacific Rise, and Callao (Peru). The main objectives were geophysical studies of fast‐ and medium‐spreading midocean ridges (including the Galapagos Triple Junction) and fracture zones, and the Peru Trench. This is the first time that this unique sonar has been used in the Pacific or on a fast‐spreading midocean ridge.

A geophysical survey of the East China Sea, Yellow Sea, and Ryukyu Island arc and trench was conducted during October-November 1968, by the U. S. Naval Oceanographic Office with participation by scientists from ECAFE member nations. More than 12,000 km of continuous seismic reflection profiling, magnetic, and bathymetric data were recorded. The distribution of sedimentary strata appears to be controlled by a series of NE-SW trending ridges that dam and separate large sediment-filled depressions. The sediments were derived mainly from the large section of Chinese mainland drained by the Yellow and Yangtze Rivers. The Fukien-Reinan massif (between southeastern Korea and the mouth of the Yangtze River) consists of Precambrian to Mesozoic rocks uplifted during the Mesozoic Era. It dams a thick sequence of Paleogene and Neogene sedimentary rocks in the Yellow Sea. A ridge of folded sedimentary and igneous rock near the edge of the continental shelf is probably the sea-floor expression of the Taiwan-Sinzi folded zone which began to be active during Paleogene time. Approximately 1 million cu km of Neogene sedimentary strata (at aining a thickness of probably at least 5 km) and some Paleogene sedimentary rocks are dammed by this ridge. An extensive unconformity separates Neogene from Paleogene strata beneath the Yellow Sea and the continental shelf. The Neogene unit is generally undeformed, whereas the Paleogene unit shows evidence of structural deformation followed by erosion. Beyond the continental shelf the Ryukyu Ridge, composed of volcanic rocks, folded Paleogene and faulted Neogene strata, and Paleozoic and Mesozoic igneous and metamorphic rocks, has dammed a belt of sedimentary strata of probable Neogene age in the Okinawa Trough. Sediment fill in the trough exceeds 1.2 km in thickness and contains many internal reflectors that may be turbidite sand layers. Abundant faults and complex folds beneath the si e slopes suggest fault origin of the Okinawa Trough. Another dam halfway down the eastern edge of the Ryukyu Ridge has trapped additional sediments to form a broad terrace. The Ryukyu Trench at the foot of the Ryukyu Ridge contains zero to about 600 m of sediment and terminates against Taiwan in a trench-transform fault junction. Sedimentary strata beneath the continental shelf and the Yellow Sea appear to have a high potential for oil and gas. Much of the continental shelf north of Taiwan has sedimentary thickness exceeding 2 km, most of which is believed to be of Neogene age, as are the oil-producing strata on Taiwan. Present information suggests that the strata are dominantly shale but with some interbedded sandstone zones that serve as acoustic reflectors. Numerous anticlines, faults, and unconformities were recorded. Our reconnaissance study indicates that detailed seismic surveys are warranted and that eventual test drilling will be required to establish the potential for oil and gas in the region.

Continuous seismic profiles recorded south of Shikoku, Japan, reveal the shallow structure of the Tosa Terrace, continental slope, Nankai Trough, and adjacent Shikoku Basin. Tosa Terrace contains at least 1.5 km of terrestrially derived horizontally bedded sediments trapped by an outer structural dam. Deposition has probably continued from early Neogene time. These sediments unconformably overlie a deformed sedimentary unit of probable Paleogene age that appears truncated at its upper exposed surface. The deformed sediments and igneous basement of the continental slope have been faulted to form benches on which more recent sediment has accumulated, and scarps which expose basement. An average of approximately 600 m of interbedded turbidite and pelagic sediments are present in the Shikoku Basin. Sediment thickness in Nankai Trough averages 1 km. Moated knolls suggest turbidity currents transporting sediment off the Shichito-Iwo Jima Ridge into the Shikoku Basin. Recent turbidites have been deposited in the Nankai Trough and are folded where they abut the continental slope. This and a plunging basement at the foot of the continental slope suggest seafloor spreading.