David K. Rea

Late Pliocene onset of glaciation: ice-rafting and diatom stratigraphy of North Pacific DSDP cores

Abstract The revised diatom biostratigraphy for the North Pacific allows the timing of ice-rafting, as recorded in Deep Sea Drilling Project cores, to be determined. Significant ice-rafting began at the time of the Matuyama-Gauss reversal boundary, about 2.48 Ma. Other bio- and lithostratigraphic data from the Pacific, Atlantic, and Arctic Oceans, and the Black Sea all indicate that northern hemispherical cooling and the accumulation of continental ice began at 2.4–2.5 Ma. We interpret all these data to indicate northern hemisphere ice-cap formation at that time. If so, the closing of the Isthmus of Panama, approximately 3.2 m.y. ago, was not the immediate cause of the Plio-Pleistocene glaciations.

New estimates of rapid sea-floor spreading rates and the identification of young magnetic anomalies on the East Pacific rise, 6° and 11° S

Abstract Detailed surveys of the crest of the East Pacific Rise at 6° and 11°S form the basis for estimates of sea-floor spreading rates of 8.2 and 8.3 cm/yr, respectively. These estimates are significantly higher than previous ones of 6.5 to 7.5 cm/yr. The high quality of the magnetic profile at 6°S allows identification of shorter magnetic events such as the two Olduvai events of Cox. DSDP drill-hole data indicate that the present spreading rates have persisted throughout the Cenozoic.

Model for the Formation of Topographic Features of the East Pacific Rise Crest

The topography of the East Pacific Rise axis is characterized by a central axial block 300 to 400 m high and 15 to 20 km wide. The axial block is flanked by asymmetrical abyssal hills tilted so that their steep slopes face the axis. Extrusion of basalt along the spreading center forms the axial block. According to the proposed model, subsidiary peaks or shoulders of the axial block are formed by faulting along surfaces that dip steeply toward the axis at the surface but decrease in dip with increasing depth. As the shoulders of the axial block are moved laterally outward by the spreading process, they are lowered along the curving fault surfaces to form the observed tilted abyssal hills.

Ascension Fracture Zone, Ascension Island, and the Mid-Atlantic Ridge

Near 7° S. latitude, the Ascension fracture zone offsets the Mid-Atlantic Ridge right-laterally over 230 km. North of the fracture zone, which trends about N. 80° E., the ridge crest is perpendicular to its trend, but to the south, near 8° S., the initially perpendicular trend changes to nearly northerly. Ascension Island lies approximately 50 km south of the fracture on magnetic anomaly 4, with an inferred age of 7 m.y. It is not on any major tectonic trend and there is no evidence that it is part of a volcanic chain. Spreading rates in the region increase from north to south, proportional to the distance from the pole of rotation of the African and South American plates, and may be slightly different on the east and west sides of the ridge. Normal to subnormal heat-flow values prevail except for one high value east of the northern ridge axis. The Ascension fracture valley is wide and filled with thick sediments implying an anomalously high age. Earthquake epicenters are aligned along the ridge crest, but near the fracture zone they define an activity belt south of it and more nearly east-west trending. The data suggest a shift of the fracture zone to an east-west trend about 10 m.y. ago, followed by a reorientation of the southern ridge axis that proceeded from south to north and has not been completed. The hypothesis accounts for most observations except the heat-flow pattern, the absence of epicenters on the southernmost ridge crest, and some small structural features.

Analysis of a fast-spreading rise crest: The East Pacific Rise, 9° to 12° south

The axis of the East Pacific Rise is defined by a topographic block about 15 km wide and 300 to 350 m high which is flanked by abyssal hills 100 to 200 m high and 3 to 5 km wide. These hills often are tilted such that their steep slopes face the axis. An empirical model explaining these features combines axial extrusion to form the central block and rotational faulting to lower the shoulders of the axial block to the regional depth and tilt them outward.The axial block is offset about 10 km left-laterally at 10.0°S and a similar amount right-laterally at 11.5°S. Offsets (or lack of offsets) of young magnetic anomalies indicate that these axial displacements occurred between 1.7 and 0.9 m.y. ago and 0.7 m.y. ago and the present in the north and south. respectively. These small axial offsets are interpreted to be the result of either brief episodes of asymmetric see-floor spreading or discrete jumps in the site of spreading activity. Both axial shifts were to the west; a unidirectional sequence of such shifts occurring at the above rate of one per million years would be difficult to differentiate from true regional asymmetric spreading and might explain that phenomenon on other medium-to fast-spreading rises.Reconnaissance data from the east flank of the East Pacific Rise indicate that spreading activity began on that part of the rise between the 9°S and 13.5°S fracture zones approximately 8.2 m.y. ago when the site of crustal accretion jumped westward from the now dormant Galapagos Rise. Slope change in crust approximately 2 and 6 m.y. old imply faster spreading rates between about 6 and 2 m.y. ago than either before or after that time. Identification and correlation of anomaly 3′ allows an estimate of about 90 mm/y for this higher east flank spreading rate. Since 1.7 m.y. ago spreading rates have averaged about 80 mm/y to the west and 77 mm/y to the east.

Geologic Evolution of the Northern Nazca Plate

The structural features and evolution of the northern Nazca plate can be explained by the existence of an east-trending spreading center in the eastern Pacific from approximately 40 to 25 m.y. ago, and reorganization of spreading centers about 10 m.y. ago. East-west anomalies from the Oligocene spreading center are found between the Carnegie Ridge and Grijalva scarp to the south. The offset along the scarp is the result of a northeastward-increasing age difference between crust to the north and south. Volcanic extrusions during a period of very slow spreading along this center may have built the ancestral Carnegie-Cocos Ridge. This east-trending spreading center was reactivated during late Miocene time along the present Galapagos rift zone.

Hemipelagic sedimentation in a region of crustal doming between the Mendocino and Pioneer fracture zones

Abstract Pacific Study Area W-N lies 300 km west of California, south of the Gorda Ridge, between the Mendocino and Pioneer fracture zones. Hemipelagic mud covers the seafloor over a broad low rise and grades eastward into distal turbidite deposits of the Delgada Fan. 3.5 kHz profiles show three kinds of acoustic character for the seafloor: strongly reflecting turbidite deposits, depositionally draped hemipelagic sediment and an unusual morphology characterized by low-relief, closely spaced hills. These small hills occur in the same area as the low rise, implying a common origin. Calculations of the plate motions for this small corner of the Pacific Plate suggest that it is overriding the upper mantle thermal trace of the Gorda Ridge magma chamber. This heat source apparently causes doming, offsets in the layered hemipelagic sediment and formation of the closely spaced small hills.

Oceanographic studies supporting the assessment of deep-sea disposal of defueled decommissioned nuclear submarines

Based on criteria developed by the international Atomic Energy Agency (IAEA), potential disposal sites for defueled, decommissioned nuclear submarines appear to exist in deep water south of the Mendocino Fracture Zone within 200 nautical miles of the United States Oceanographic measurements in the water column and at the sea floor in a study area (W-N) at 39 5°N, 127 5°W will allow the operational and radiological consequences of deep-sea disposal to be compared with land burial of old submarines. The W-N studies also are yielding new data that will provide insights to the deposition and early diagenesis of distal hemipelagic sediments

Downslope Transportation of Metalliferous Sediments Along East Pacific Rise During Messinian: ABSTRACT

The distribution of metalliferous sediments adjacent to active spreading centers is of both scientific and economic interest. Metal-rich waters emanating from active hydrothermal vents have been traced in intermediate level water masses far beyond the ridge crest, but the greatest concentrations of metal oxides in sediments occur near the vents. There, however, it is possible that the oxides may be redistributed and possibly further concentrated by redeposition. We document one such case of redeposition for Messinian sediments cored at Deep Sea Drilling Project Site 599, which, along with the other DSDP Leg 92 sites, was the first on the East Pacific Rise to be drilled using the hydraulic piston corer. Site 599 (19°27.09^primeS, 119°52.88^primeW; water depth = 3,654 m), drilled in a small basin about 600 km from the present ridge crest, recovered 41 m of mostly Miocene calcareous oozes characterized by alternating light (mostly yellowish brown to dark yellowish brown) and dark (mostly dark reddish brown) zones from 10 to 100 cm thick and/or bands 2-5 cm thick. A sharp contact at sample point 599-3-3, 21 cm, separates fine-grained light-colored in-situ sediments of calcareous nannofossil Zone CN9b below from a coarser grained and darker metalliferous-rich unit above, which contains older nannofossils derived from Zone CN8b. Indicative of downslope transport of metalliferous materials during the Messininan, this example may explain much of the sediment banding seen throughou the section. End_of_Article - Last_Page 274------------