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The Mariana forearc is a non-accretionary forearc with numerous seamounts. Dredge hauls from these seamounts are composed primarily of serpentinite, indicating a non-volcanic origin. In addition, dredged carbonate samples reveal (1) the presence of aragonite in water depths greater than the aragonite compensation depth, (2) unusual porosity and cement types, (3) light carbon and heavy oxygen isotopic compositions, and (4) an unusual trace element geochemistry of aragonite and calcite. These mineralogic, petrologic, and geochemical characteristics formed the basis of a prediction that methane-bearing fluids are venting from these seamounts. Subsequent to this prediction, submersible dives on a Mariana forearc seamount found and sampled chimneys seeping cool water that contains dissolved methane. The serpentine substrate beneath the chimneys appears to form from cold gravitational flows of serpentine material. Fluid inclusion contents of chimney samples also show an earlier history of methane, as well as longer chain hydrocarbons, aromatics and acetate ions. Drilling into the flank and the summit of this seamount by the Ocean Drilling Program during Leg 125 confirmed the presence of methane, other light alkanes, and organic acids in the present-day interstitial pore fluid of the cores. The presence of aromatic compounds and organic acids, if derived from the maturation of organic compounds, indicates a thermogenic origin for these volatiles. Strontium isotopic ratios of the aragonitic chimney samples and of the present-day interstitial waters range from present-day seawater values to significantly lower values. The strontium isotopic ratios of the pore waters within the seamount trend toward a relatively less radiogenic composition with increasing depth below the seafloor. This strontium isotopic signature is probably not the consequence of near-surface processes, but more likely a contribution from an igneous source deeper in the lithosphere. The lack of a large accretionary prism in the Mariana forearc removes the complication of fluid contribution from dewatering of the prism by compression during convergence, as in the Oregon-Washington or Barbados margins. The organic compounds in the fluid inclusions and interstitial pore fluids, as well as the isotopic and the geochemical composition of the carbonates and the dissolved constituents, suggest that fluids are derived from dewatering, compression, and dehydration of subducted sediments and the subducting plate. Furthermore, the composition of the fluid is influenced by thermal maturation of organics in subducted sediments as well as by a contribution from a deep igneous source.

Leg 101 of the Ocean Drilling Program (ODP) was devoted discovering the tectonic and sedimentary evolution of the Atlantic Margin of the lberian Peninsula. A transect of five sites, with a total of 14 drill-holes was undertaken to the South of the Galicia Bank on the seaward edge of the margin. The data obtained revealed a complex history of subsidence and rifting preceding the initiation of sea floor spreading between Newfoundland and Iberia. The main findings include: 1) The Upper Jurassic-Lowermost Cretaceous shallow-water carbonate platform are the first Messozoic deposits at the margin. The «basement seismic reflector» is made-up of these carbonates. 2) The platform drowning, tilting of fault blocks and rapid subsidence preceded the spreading by as much as 25 million years. 3) A ridge of serpentiniced peridotites is located near the boundary between the oceanic and continental crusts. 4) The seismic reflector «S» does not, as widely believed represent a ductile-brinle boundary within the continental crust but is instead a reflector at the base of the synrift sediments.La campaña oceanográfica 103 del Ocean Drilling Program (ODP) ha estado dedicada a dilucidar la evolución tectónica y sedimentación del Margen Atlántico-Ibérico. Se realizaron un total de 14 sondeos., en cinco puntos de posicionamiento, sobre el extremo más profundo del margen; al S del Banco de Galicia. Los resultados obtenidos revelan que previamente al inicio de la expansión oceánica entre Terranova e Iberia ocurrió una historia compleja de distensión cortical, fracturación y subsidencia asociadas. Los resultados fundamentales son los siguientes: 1) Carbonatos de plataforma marina somera, de edad Jurásico superior-Cretáceo basal constituyen los primeros depósitos mesozóicos en ese ámbito del margen y dan lugar a un reflector sísmico considerado interiormente como basamento, 2) El hundimiento de la plataforma, fallamiento y basculamiento de los bloques ocurre desde 25 m.a. antes de iniciarse la acreción oceánica. 3) En el límite entre corteza oceánica-corteza continental se ubica una cresta constituida por peridotitas serpetinizadas. 4) El reflector sísmico «S», generalmente considerado como el límite dúctil-frágil en la corteza continental, corresponde realmente a la base de los depósitos sinrift.

Geological and geophysical studies along the entire length of the Line Islands were undertaken in order to test the hot spot model for the origin of this major linear island chain. Volcanic rocks were recovered in 21 dredge hauls and fossiliferous sedimentary rocks were recovered in 19 dredge hauls. Volcanic rocks from the Line Islands are alkalic basalts and hawaiites. In addition, a tholeiitic basalt and a phonolite have been recovered from the central part of the Line chain. Microprobe analyses of groundmass augite in the alkalic basalts indicate that they contain high TiO2 (1.0–4.0 wt %) and Al2O3 (3.4–9.1 wt %) and are of alkaline to peralkaline affinities. Major element compositions of the Line Islands volcanic rocks are very comparable to Hawaiian volcanic rocks. Trace element and rare earth element analyses also indicate that the rocks are typical of oceanic island alkalic lavas; the Line Islands lavas are very much unlike typical mid-ocean ridge or fracture zone basalts. Dating of these rocks by 40Ar-39Ar, K-Ar, and paleontological methods, combined with Deep Sea Drilling Project data from sites 165, 315, and 316 and previously dated dredged rocks, provide ages of volcanic events at 20 localities along the chain from 18°N to 9°S, a distance of almost 4000 km. All of these dates define mid-Cretaceous to late Eocene edifice or ridge-building volcanic events. Eocene volcanic events took place from 15°N to 9°S, and Late Cretaceous events took place from 18°N to 9°S. In the southern Line Islands both Cretaceous and Eocene events took place on the same edifice or ridge, indicating recurrent volcanism at a single locality. The irregular distribution of atolls in the chain, the fact that Late Cretaceous reefs flourished along a distance of approximately 2500 km in the central and southern Line Islands, and the observation that spatially closely related seamounts exhibit different subsidence histories are interpreted as indicating that large segments of the chain have not followed a t1/2 related subsidence path. Magnetic surveys of 11 seamounts show that four of the seamounts, from the central Line chain, give virtual geomagnetic poles which fall well to the north of virtual geomagnetic poles of Cretaceous seamounts. These four poles agree with other paleomagnetic data of middle-late Eocene-early Oligocene age from the Pacific. One of these four seamounts yielded a 40Ar-39Ar total fusion age of 39 Ma. Because the poles of all four seamounts fall into a tight group we infer that they are probably of middle-late Eocene age to early Oligocene age. The other three seamounts as well as one seamont from the Line Islands analyzed previously all give virtual geomagnetic poles which agree with Late Cretaceous paleomagnetic data from the Pacific. Of these four seamounts, three give Late Cretaceous 40Ar-39Ar ages ranging from 71 to 85 Ma; another, in the southern Line Islands, is interpreted, on paleontological evidence, to be of Late Cretaceous age. The origin of the Line Islands has been ascribed by previous workers to the effects of a single hot spot and to the action of four hot spots. The single hot spot model cannot account for all of the volcanic edifices in the Line Islands, although it does explain a general age progression of 9.6±0.4 cm/yr from north to south along the chain derived from a number of dated edifices. The four hot spot model accounts for more of the dated volcanic edifices but still does not explain all of the available data. The petrologic data argue against a mid-ocean ridge or transform fault origin proposed by earlier workers. The complex volcanic province represented by the Line Islands remains a challenge to existing models for the origin of midplate volcanism in the Pacific. An atoll drilling program which could determine edifice building histories, paleolatitudes of seamount formation, and subsidence rates and patterns in the Line Islands is needed.

Deep Sea Drilling Project Sites 604 and 60S on the upper continental rise are the first of a series of cored holes along the “New Jersey transect” which, when completed, will provide the first comprehensive dipwise suite of drill holes across a passive margin from the coastal plain to the abyssal plain. Our drilling results document the age of important seismic sequence boundaries and allow their correlation with wells on the continental shelf and slope as well as with the regional oceanic seismic stratigraphy. Hole 605,156 km (97 mi) southeast of Atlantic City, New Jersey, and drilled 816.7 m down to mid-Maestrichtian limestones, penetrated a near-complete Cretaceous/Tertiary boundary section overlain by a 200-m expanded Paleocene sequence. Unusually high amounts of terrigenous silts and glauconite are present at the boundary and immediately above. Among the several hypotheses discussed, we suggest that the terrigenous silts and glauconite may represent a high-energy event such as a tsunami caused by a Cretaceous/Tertiary impact. Site 604, 5 km (3 mi) seaward of Site 605, was terminated in upper Miocene glauconitic sands and debris flows at 294.5 m by unstable hole conditions. These sediments contain shelf-derived gravels and exotic blocks of Eocene chalk (up to 50 cm across) eroded from bedrock that is today widely exposed on the adjacent slope. Our drilling results show that denudation of the Eocene units was not limited to the Oligocene A u erosional event, but that major loss occurred during late Miocene and later glacial sea-level lowstands.

Rocks dredged from a seamount 100 km northwest of Caroline Island, at the southern end of the Line Islands chain, contain Late Cretaceous fossils associated with volcanic debris. This association is evidence for the existence of a reef-bearing volcanic edifice with a minimum age of Late Cretaceous, 70 to 75 m.y., near Caroline Island. With the discovery of this seamount, the known occurrences of Late Cretaceous, reef-capped, volcanic edifices now extend a distance of 2,500 km, from Deep Sea Drilling Project Site 165 to 100 km northwest of Caroline Island. The apparent synchroneity of Late Cretaceous volcanism over this distance argues against the proposition that a single hotspot of the Hawaiian-Emperor type produced the Line Islands chain. Biochronologic data from the Line Islands indicate that the chain is not the temporal equivalent of the Emperor chain. Volcanic edifices of Cretaceous age are now known to extend from the Line Islands through the Mid-Pacific Mountains to the Marshall Islands and the western margin of the Pacific plate from Japan to the Marianas. A volcanic event occurred in the southern Line Islands during middle Eocene time; Eocene sediments were engulfed and altered by a volcanic eruption. The occurrence of both Cretaceous and Eocene volcanism in the southern Line Islands indicates that the history of the Line Islands is similar to that of the Marshall Islands.

Drilling at Deep Sea Drilling Project Site 603 has provided the first deep (>1000-m) penetration of strata beneath the continental rise off the Atlantic margin of North America. Nearly continuously cored through 1585 m of section down to Berriasian pelagic limestones, the site 435 km (270 mi) east of Cape Hatteras intersected an extensive Lower Cretaceous deep-sea fan complex, which provides new information on the petroleum potential of the continental rise. Hauterivian to early Aptian in age, this 208-m interval of interbedded limestones, sand, and black shale turbidites begs the existence of any post-Valanginian reefs along the Baltimore Canyon Trough. Less extensive terrigenous turbidites were encountered higher in the section up to the Cretaceous/Tertiary boundary, which is marked by a current-laminated sand rich in dark spherules. Pelagic early Paleogene clays are disconformably overlain by Miocene pelagic mud. Turbiditic silts and clays began to accumulate rapidly at this site during the middle Miocene, leading to deposition of muddy contourites that formed the Lower Continental Rise Hills of the Hatteras Outer Ridge as sand turbidites were ponded concurrently on its landward side. The section at Site 603 confirms the concept that eustatic and other large-scale events subdivide Earth history into distinct chapters allowing the correlation of deep-sea seismic sequence boundaries with continental shelf and margin unconformities.