We present new seismic images of Cretaceous crust formed at a fast-spreading center in the Pacific. The high crustal reflectivity observed in these data contradicts the conventional wisdom that accretionary structures formed at fast-spreading centers are not seismically detectable. Subhorizontal reflections can be traced at 600-800 ms two-way time below the top of basement for tens of kilometres, suggesting the presence of a widespread seismic boundary, possibly a structural discontinuity related to the maximum depth of hydrothermal circulation at the spreading center or the base of the sheeted dikes. Lower-crustal reflections, dipping dominantly toward the paleo-spreading center, may represent mafic-ultramafic banding similar to that observed in the lower crust of reconstructed ophiolite sections.

Three multichannel seismic reflection records across the Kuril convergent margin provide the first deep data in this area. The records are located across the southern tip of Kamchatka, the central Kuril arc, and the northern extension of Hokkaido Island platform which show three distinct tectonic regimes. The lower slope contains a wedge‐shaped buttress surrounded by low velocity sediments. Underplating sediment uplifts the buttress, as indicated by faults that displace its upper surface. The middle slope is a block of acoustic basement, which has a rough surface with significant arcward dipping faults. The middle slope is separated from the upper slope along a steep arcward dipping reflection, the “middle‐slope boundary.” The upper slope structure off Kamchatka is different from that off Hokkaido. Off Kamchatka a regular stratified sediment section has been uplifted, tilted, and dips seaward. Off Hokkaido, stronger uplift has tilted regional blocks seaward and arcward. Along the southern line north of Hokkaido, well‐studied nonthrust earthquakes with lateral motions occur beneath the middle slope boundary and on the boundary of the subducting plate. Thrust earthquakes occur under the middle and upper slope, whereas tsunami earthquakes occur under the lower slope. High‐amplitude reflections along the lower boundary of the wedge‐shaped buttress and along the active decollement, indicate high fluid concentrations which reduce the friction along the tectonic units so that through the weak coupling, slow rupture may extend up to the seafloor from a tsunami earthquake.

Seismic reflection data along a flow line of crustal generation in the central Pacific that spans ages of zero to 85 Ma and spreading half rates of 30 to 100 km m.y.−1 shows a nearly constant travel time of 2 s through igneous crust to reflection Moho. The highest‐amplitude and most laterally continuous Moho reflections were recorded over 20–30 Ma crust that was emplaced at “superfast” (75–95 km m.y.−1) spreading rates. The superfast spread portion also records the lowest scatter about the 2‐s average travel time to reflection Moho. Seismic images show that lower crustal reflectors dip consistently eastward toward the ridge crest. These dipping reflectors are truncated by the reflection Moho.

Magnetic anomalies of 52–80 Ma age have been mapped in the equatorial Pacific in a region where they were previously unrecognized. These anomalies, now at 7° N are best modeled with crustal rocks of negative inclination, apparently because the crust was formed at a spreading ridge that was south of the magnetic equator in Late Cretaceous‐Early Tertiary time. Magnetic anomalies recorded along a 5900‐km trackline that follows a flowline of crustal generation show one long period, from 12 to 31 Ma, of apparent constant half‐spreading rate over the 0 to 80 Ma represented.

Seismic profiling together with seismic refraction data and dredging enabled us to study the upper crustal structure of the Tonga-Kermadec trench and the Ozbourn Seamount junction area. Horst-graben structures have been revealed both on the insular and oceanic slopes of the trench. Two systems of faults are characteristic: one transverse and the other parallel to a general trend of the Tonga-Kermadec trench. Grabens of the trench insular slope are partly compensated by sediments unlike those of the oceanic slope which are not ponded by sediments. On both slopes the faults are normal. This testifies to the extensional conditions within the upper crustal part of this region. The Tonga-Kermadec uplift (insular slope of the trench) has undergone intensive (with amplitide to 5–7 km) vertical movements (uplifting and then subsidence) in Eocene, Oligocene, Pliocene and Quaternary times.

The South Okhotsk deep-sea basin is a morphologically-simple, fault-bounded basin located in the Kuril back-arc region. The structure of the crust is distinguished by three principal units: layer 1 ( V p ∼- 2–4 km/sec ) is a layer of sediments 3–4 km thick. Layer 2 ( V p ∼- 4.5–5 km/sec ) has a thickness of 0.5–1.5 km. Layer 3 ( V p ∼- 6.5–7 km/sec ) is about 5 km thick, and has a probable basaltic to gabbroic composition. To a depth of 30 km, the upper mantle beneath the basin consists of alternating layers having velocities of 8.0 and 7.0 km/sec (Starshinova, 1980), which may be indicative of localized partial melting. Heat flow values are 2–2.5 times higher than normal. Sedimentation rates of 5–15 m/Ma for a thick, “transparent” sedimentary unit probably consisting of clays and argillites, and immediately overlying basement, suggest that sedimentation in the basin began in pre-Cretaceous time. There is no evidence of a ridge or rift in the basin, and the overlying sediments are undeformed. It is thus difficult to relate the formation of the South Okhotsk basin to subduction of the Pacific plates along its eastern margin.

A system of large submarine canyons on the continental slope off the Kamchatka Peninsula has been studied. These are the Kamachatsky, Storozh, Tyushevsky, Olga, Kronotsky, Zhupanovsky and Avachinsky canyons and they are generally controlled by fault zones and cut the Pre-Miocene basement structural elements in the cross-strike direction; this means that they probably are partly the result of subaerial erosion. The canyons appear to have been excavated again in Miocene to Quaternary time by turbidity currents that debouched onto the base of the continental slope and partially filled the Kuril—Kamchatka Deep-Sea Trench. Turbidity currents which move through the canyons are thought to have been generally triggered by earthquakes which dislodged unstable sediments that had accumulated on the upper continental slope and shelf.

The detailed seismic refraction investigation of the oceanic crust south of Shatsky Rise in the Northwestern Pacific revealed a low velocity zone (LVZ) with an average compressional wave velocity of 6.3 km/s within layer 3. This conclusion is based on the shadow zone for refractions on the travel time curves in their first arrivals from the M discontinuity. The LVZ may be composed of oceanic plagiogranites because serpentinization of peridotites would probably lead to an increase in crustal block volume with a concomitent decrease in density and thereby thickening and upwelling at the place of now “overdeepened” ocean would be expected.

The northern and central parts of the Okhotsk Sea is the epi-Mesozoic platform. The hetero-aged acoustic basement is represented by the deformed geosynclinal rocks from Cretaceous to Paleozoic and, probably, Precambrian. The slightly deformed sedimentary cover leveled the uneven surface of the acoustic basement, and this upper Paleogene-Neogene-cover filled the system of the structural basins. The general northwest to southeast and east-to-west trending taphrogenic horsts and grabens of the acoustic basement were formed due to extending and subsiding of the earth's crust during late Paleogene-Neogene time. End_of_Article - Last_Page 969------------

The upper crust structure of the Kuril-Kamchatka deep-sea trench and continental slope is a system of horst-anticlinorial uplifts of the acoustic basement and separating them partially are compensated graben-synclinorial troughs stretching in the northeastern direction according to the trench general trend. From dredging data the acoustic basement rock associations of the horst-anticlinorial uplifts of the trench continental slope are pre-Neogene (Late Cretaceous and older) complexes of the deformed geosynclinal volcanogenic and sedimentary deposits broken by gabbroids, granodiorites, and granitoids. Graben-synclinal troughs are filled with sedimentary deposits mainly of the Neogene-Quaternary ages, the thickness of which in some basins exceeds 1.8 mi (3 km). The oceanic slope of the trench is composed of the sedimentary End_Page 968------------------------------ layer with thickness of about 300 to 990 ft (100 to 300 m), lying on the second layer of the oceanic crust. According to the dredging, the acoustic basement roof on the marginal oceanic swell (Hokkaido Rise) is mainly composed of metamorphosed basalts and it seems to include the sedimentary rock intercalation. From the whole rock K-Ar data, the period of intensive basaltic volcanism on the Hokkaido Rise is from Cretaceous to Paleogene. Crustal faults along the Kuril-Kamchatka and Aleutian trenches can be characterized as normal faults, especially for the oceanic side, which points out their development in crustal tension conditions. The faults transversal to the trenches are mainly established from magnetic data. The anomalous magnetic field is subdivided generally in two regions in which the trend of anomalies varies from the subparallel (in the southern part of trench) to the subtransversal to the trench (in the northern part). A vast region next to the oceanic plate adjacent to the crustal and northeast parts of the Kuril-Kamchatka trench is characterized by the absence of linear magnetic anomalies which can be associated with the structure and movement of the subducted plate. End_of_Article - Last_Page 969------------

The northern and central parts of the Okhotsk Sea form an epiMesozoic platform. The hetero-aged acoustic basement is represented by deformed geosynclinal rocks from Cretaceous to Precambrian in age. The slightly deformed sedimentary cover levelled the uneven surface of the acoustic basement, and this Upper Paleogene—Neogene cover filled up the system of the structural basins. The general NW—SE and W—E extensions of the taphrogenic horsts and grabens of the acoustic basement were formed due to extension and subsidence of the earth's crust during the late Paleogene—Neogene.