Correlation of the logs from the Izu-Bonin forearc sedimentary sections at Sites 787, 792, and 793 with the core data from Holes 787A-787B, 792A-792E, and 793A-793B allows the development of a more detailed lithostratigraphic model for those sites, and a more precise correlation of lithologic boundaries to basin-wide seismic reflections. Early Oligocene arc volcanics form the basement strata (Unit 5) sampled at Sites 792 and 793. Downdropped and rotated blocks of Eocene forearc may form a sub-basement beneath these flows in the central forearc basin; mid-Eocene basement was recovered at Sites 782 and 786 on the outer-arc high during Leg 125. Basement at Site 792 was defined using the vertical seismic profile (VSP) and logging data. Deep reflectors observed on the vertical seismic profile may originate in the Eocene sub-basement. Thick sequences of coarse-grained volcaniclastic and hemipelagic sediment fill the 70to 140-km-wide forearc sedimentary basin. Unrecovered (early Oligocene) strata beneath an unconformity, imaged by the multichannel seismic (MCS) line passing over Site 792, fill the deepest grabens of the central forearc and constitute Unit 4. The rapid deposition of volcaniclastics (Unit 3) during a dominant eruptive phase spanning much of the Oligocene, together with erosion of the basement highs bounding the basin, contributed to rapid subsidence and infilling. An inspection of cored materials from Unit 3 and logging data from Sites 792 and 793 reveals microfaults and other structural evidence for extension; on a much larger scale, MCS data show large normal faults near the frontal-arc high and outer-arc high that downfault the sediment section towards the central basin. Much of the largely pelagic or hemipelagic early Miocene section (Unit 2) has been removed by submarine valley formation and erosion, as at Sites 787 and 792. Middle Miocene to Holocene volcaniclastics and hemipelagics (Unit 1) top the forearc sedimentary section.

A continuous section of Pliocene marine sediments was recovered at Ocean Drilling Program Site 978, located in the Alboran Sea between Spain and Morocco. Three Pliocene subunits have been defined at Site 978: the lowermost (Subunit IC, 129.2 In thick) is characterized by alternating beds of lighter, more calcareous, and darker, less calcareous, claystone with bioturbated upper and lower contacts (Type 1 cycles); the middle (Subunit IB, 67.1 m thick) is composed of relatively homogeneous nannofossil claystone; and the uppermost (Subunit IA, 211.6 m thick) contains abrupt-based darker, terrigenous layers interpreted as turbidites that are interstratified with lighter nannofossil claystone (Type 2 cycles). The rhythmically bedded light and dark layers in Subunit IC correlate with those in the Rosello Composite Section of Sicily, a global reference standard for the Pliocene time scale. These sedimentary cycles are products of variations in precession and resulting continental runoff. Missing cycles occur during eustatic highstands. The shift to more homogeneous sedimentation in Subunit IB is represented in similar-aged sequences throughout the Mediterranean which display evidence of submarine mass wasting. Mediterranean-wide slope degradation was likely a response to rapid sea-level change at approximately 3 Ma. This change in sedimentation style was accompanied by an upsection increase in sediment accumulation rates associated with turbidite influx in Subunit IA. Turbidite frequency throughout the Pliocene section can be linked to eustatic changes in sea level, with turbidite maxima corresponding with mid-sequence downlap surfaces and their associated condensed sections.

During the ODP leg 149, three of the five drilling sites have sampled basement rocks from the ocean-continent transition of the Iberia Abyssal Plain margin. The preliminary results indicate that: (1) the transition between oceanic and thinned continental crusts is not sharp; it spreads over a 150 km-wide zone including ultramafic (spinel and plagioclase-bearing serpentinized peridotites) and mafic (flaser gabbros) rocks highly sheared at high, then low, temperature, and (2) the west iberia margin is bounded by serpentinized peridotites over a distance of more than 250 km, confirming that mantle denudation can extensively occur during continental rupture at the end of a rifting event.

Extensively altered brines, with a composition different from that of any previously reported natural water, have been recovered by deep coring of sediments in the Izu-Bonin fore-arc sedimentary basin. Samples from successive depths document the chemical evolution of these unusual pore waters, from sea water to highly enriched CaCl2 brines. The origin of these pore waters seems to be related to the special environment of a fore-arc sedimentary basin, where accumulation rates of volcanogenic sediment are extremely high and the sediments are susceptible to alteration.

Eruption of 1-million-year-old tholeiitic basalt >1800 meters below sea level (>18 megapascals) in a backarc rift behind the Bonin arc produced a scoriaceous breccia similar in some respects to that formed during subaerial eruptions. Explosion of the magma is thought to have produced frothy agglutinate which welded either on the sea floor or in a submarine eruption column. The resulting 135-meter-thick pyroclastic deposit has paleomagnetic inclinations that are random at a scale of <2.5 meters. High magmatic water content, which is about 1.3 percent by weight after vesiculation, contributed to the explosivity.