Tertiary sediment from the East pacific rise

More than 50 cores ranging in age from Pliocene to Lower Miocene have been recovered from the East Pacific Rise. Near the crestal regions the sediment cover is thin or lacking, and only Pleistocene sediments were recovered. On the flanks, the sediment thickness increases and pre-Pleistocene sediments are encountered. This pattern of increasing age and increasing sediment thickness away from the axis of the rise is in agreement with that predicted for spreading of the ocean floor.     

Ages of pacific deep-sea basalts, and spreading of the sea floor

Potassium-argon determinations of age from whole-rock samples of tholeiitic basalts, dredged from the crest of the East Pacific Rise and from the flanks of three seamounts at varying distances from the crest, show that the crest is younger than 1 million years and that age does not correlate with distance from the crest. Our data, however, do not necessarily oppose the general concept of spreading of the ocean floor.     

Origin of mountains on Io by thrust faulting and large-scale mass movements

Voyager stereoimages of Euboea Montes, Io, indicate that this mountain formed when a large crustal block was uplifted 10.5 kilometers and tilted by approximately 6 degrees. Uplift triggered a massive slope failure on the northwest flank, forming one of the largest debris aprons in the solar system. This slope failure probably involved relatively unconsolidated layers totaling approximately 2 kilometers in thickness, overlying a rigid crust (or lithosphere) at least 11 kilometers thick. Mountain formation on Io may involve localized deep-rooted thrust faulting and block rotation, due to compression at depth induced during vertical recycling of Io's crust.     

Stimulus control in pigeons based on proprioceptive stimuli from floor inclination

Two groups of pigeons trained to peck a dimly illuminated disc in an otherwise dark experimental chamber with the floor horizontal (0 degrees tilt) or inclined 30 degrees to the left, respectively, show decremental generalization gradients of response rate when the floor inclination is varied from its training position. Discrimination training in which food reinforcement is available under one floor tilt condition but not under another steepens the slope of such gradients. In a secondv experiment, pigeons reinforced for pecking when the floor was tilted 10 degrees or 20 degrees and not reinforced under the alternative condition showed steep gradients with maximal responding displaced from the reinforced stimulus.     

Biological communities at the Florida escarpment resemble hydrothermal vent taxa

Dense biological communities of large epifaunal taxa similar to those found along ridge crest vents at the East Pacific Rise were discovered in the abyssal Gulf of Mexico. These assemblages occur on a passive continental margin at the base of the Florida Escarpment, the interface between the relatively impermeable hemipelagic clays of the distal Mississippi Fan and the jointed Cretaceous limestone of the Florida Platform. The fauna apparently is nourished by sulfide rich hypersaline waters seeping out at near ambient temperatures onto the sea floor.     

Petroleum associated with polymetallic sulfide in sediment from gorda ridge

A sediment sample, impregnated with asphaltic petroleum and polymetallic sulfide, was dredged from the southern end of Gorda Ridge (the Escanaba Trough) off northern California, within the offshore Exclusive Economic Zone of the United States. The molecular distributions of hydrocarbons in this petroleum show that it was probably derived from terrestrial organic matter in turbidite sediment filling the Escanaba Trough. Hydrothermal activity at the Gorda Ridge spreading center provided the heat for petroleum formation and was the source of fluids for sulfide mineralization.     

Geothermal System at 21{degrees}N, East Pacific Rise: Physical Limits on Geothermal Fluid and Role of Adiabatic Expansion

Pressure-volume-temperature relations for water at the depth of the magma chamber at 21 degrees N on the East Pacific Rise suggest that the maximum subsurface temperature of the geothermal fluid is about 420 degrees C. Both the chemistry of the discharging fluid and thermal balance considerations indicate that the effective water/rock ratios in the geothermal system are between 7 and 16. Such low ratios preclude effective metal transport at temperatures below 350 degrees C, but metal solubilization at 400 degrees C and above is effective even at such low ratios. It is proposed that the 420 degrees C fluid ascends essentially adiabatically and in the process expands, cools, and precipitates metal sulfides within the upper few hundred meters of the sea floor and on the sea floor itself.     

Radiolarian evidence consistent with spreading of the pacific floor

In sediments west of the East Pacific Rise between the equator and 20 degrees N, Eocene microfossils are recorded no farther east than 134 degrees W; Oligocene, no farther east than 125 degrees W; while Miocene and Pliocene occurrences extend closer to the crest of the rise. This distribution may result from post-Eocene spreading of the sea floor here averaging about 8 centimeters per year.     

Pelagic sedimentation of aragonite: its geochemical significance

The relative importance of the pelagic flux of aragonite, as compared to calcite, to the deep-sea floor has been evaluated by means of a quantitative x-ray diffraction study of samples collected from sediment traps and from an unusually shallow portion of the open Atlantic Ocean (the Rio Grande Rise). The results suggest that the aragonite flux constitutes at least 12 percent of the total flux of calcium carbonate on a worldwide basis. The presence of high-magnesium calcite in several samples suggests that some of the calcareous material falling to the deep-sea floor may be derived from the long-distance transport of debris from shallow-water beenthic organisms as well as from the settling of planktonic remains. This observation supports the contention that 12 percent represents a minimum value. Aragonite and high-magnesium calcite transported laterally from shallow-water regions, upon dissolution during settling into deeper water, may contribute to the neutralization of excess anthropogenic carbon dioxide added to the oceans.     

Transform faulting and growth of the gulf of california since the late pliocene

Seismic-reflection and magnetic profiles over more than 6000 kilometers suggest that spreading of the sea floor on the East Pacific Rise, at the mouth of the Gulf of California, began to broaden a proto-gulf about 4 million years ago. Movement occurred, on transform faults, offsetting the rise and other centers of crustal growth within the gulf, and translated the end of the peninsula about 200 kilometers to the northwest. Thick pelagic sediments on the east flank of the rise indicate that there was a lapse of spreading by crustal growth between 4 and 10 million years ago.     

Hydrothermal germanium over the southern East pacific rise

Germanium enrichment in the oceanic water column above the southern axis of the East Pacific Rise results from hydrothermal solutions emanating from hot springs along the rise crest. This plume signature provides a new oceanic tracer of reactions between seawater and sea floor basalts during hydrothermal alteration. In contrast to the sharp plumes of (3)He and manganese, the germanium plume is broad and diffuse, suggesting the existence of pervasive venting of low-temperature solutions off the ridge axis.     

Controls on the preservation of biogenic opal in sediments of the eastern tropical pacific

A map of the preservation pattern of siliceous microfossils was constructed from an examination of 125 piston and gravity cores of Quaternary sediments from throughout the eastern tropical Pacific. Preservation is enhanced where high surface water productivity supplies a high input rate of siliceous tests to the sea floor, except in a large area north of 5 degrees N, east of the East Pacific Rise. Here a high input rate of terrigenous silicates may adversely affect preservation of biogenic opal in the sea-floor sediments.     

Pliocene-Pleistocene Boundary in Deep-Sea Sediments: Extinctions and evolutionary changes in microfossils clearly define the abrupt onset of the Pleistocene

For a hundred years students of the Pleistocene have looked for evidence of the climatic change which initiated the Pleistocene epoch. Glacial deposits on the continents have been of little help because of their discontinuity and because of the destructive effect of later glaciations. Hence, Pleistocene geologists have turned to the uplifted Pliocene-Pleistocene marine sediments of the Mediterranean region. In the absence of evidence of a single, clearly defined climatic change in this sedimentary section, they have agreed to define the beginning of the Pleistocene by the first appearance of Anomalina baltica, a species of benthic Foraminifera tolerant of cool water. This definition leaves much to be desired: It rests on a single benthic species whose areal distribution must have been partly determined by local conditions on the sea floor; it is useless outside of the Mediterranean region; and there is no convincing evidence that the first appearance of Anomalina baltica in the Mediterranean coincided with the onset of the first glaciation.     

Seismic structure of the southern East pacific rise

Seismic data from the ultrafast-spreading (150 to 162 millimeters per year) southern East Pacific Rise show that the rise axis is underlain by a thin (less than 200 meters thick) extrusive volcanic layer (seismic layer 2A) that thickens rapidly off axis. Also beneath the rise axis is a narrow (less than 1 kilometer wide) melt sill that is in some places less than 1000 meters below the sea floor. The small dimensions of this molten body indicate that magma chamber size does not depend strongly on spreading rate as predicted by many ridge-crest thermal models. However, the shallow depth of this body is consistent with an inverse correlation between magma chamber depth and spreading rate. These observations indicate that the paradigm of ridge crest magma chambers as small, sill-like, midcrustal bodies is applicable to a wide range of intermediate- and fast-spreading ridges.     

Seismic and bathymetric evidence of a fracture zone on gorda ridge

Swarms of seaquakes have been located hydroacoustically on Gorda Ridge near 41.5 degrees N, 127.6 degrees W, where detailed bathymetric charts indicate a slight dextral offset of the rift valley in the center of the ridge. It is suggested that a fracture zone may be nascent in the area.     

Geophysical investigations of the tectonic boundary between East and west antarctica

The Transantarctic Mountains (TAM), which separate the West Antarctic rift system from the stable shield of East Antarctica, are the largest mountains developed adjacent to a rift. The cause of uplift of mountains bordering rifts is poorly understood. One notion based on observations of troughs next to many uplifted blocks is that isostatic rebound produces a coeval uplift and subsidence. The results of an over-snow seismic experiment in Antarctica do not show evidence for a trough next to the TAM but indicate the extension of rifted mantle lithosphere under the TAM. Furthermore, stretching preceded the initiation of uplift, which suggests thermal buoyancy as the cause for uplift.     

Geological exploration in an East coast submarine canyon from a research submersible

Large talus blocks litter the flat floor of Oceanographer Canyon at a depth of 1460 meters; they indicate down-axis mass transport of floor sediment at an unknown time and rate. From 1460 to 1310 meters the sidewall is covered by unconsolidated sediment lying at 35 degrees to 40 degrees from the horizontal. An outcrop of Pleistocene or younger sediment at 1460 meters is probably a remnant of a former fill.     

East pacific rise: hot springs and geophysical experiments

Hydrothermal vents jetting out water at 380 degrees +/- 30 degrees C have been discovered on the axis of the East Pacific Rise. The hottest waters issue from mineralized chimneys and are blackened by sulfide precipitates. These hydrothermal springs are the sites of actively forming massive sulfide mineral deposits. Cooler springs are clear to milky and support exotic benthic communities of giant tube worms, clams, and crabs similar to those found at the Galápagos spreading center. Four prototype geophysical experiments were successfully conducted in and near the vent area: seismic refraction measurements with both source (thumper) and receivers on the sea floor, on-bottom gravity measurements, in situ magnetic gradiometer measurements from the submersible Alvin over a sea-floor magnetic reversal boundary, and an active electrical sounding experiment. These high-resolution determinations of crustal properties along the spreading center were made to gain knowledge of the source of new oceanic crust and marine magnetic anomalies, the nature of the axial magma chamber, and the depth of hydrothermal circulation.     

Crustal processes of the mid-ocean ridge

Independent geological and geophysical investigations of the Mid-Ocean Ridge system have begun to focus on the nature of the magma chamber system underlying its central axis. Thermal models predict the existence of a steady-state chamber beneath a thin crustal lid ranging in thickness from 2 to 13 kilometers. The only aspect of the system that these models fail to account for is the extremely slow spreading rates. Seismological studies reveal the existence of a low-velocity zone beneath segments of the East Pacific Rise, which is thought to correspond to a chamber system having a half-width of approximately 5 to 10 kilometers. These estimates compare favorably with those derived separately through petrological investigations of deep-sea drilling results, various sampling programs, and field and laboratory studies of ophiolites. The chamber is thought to be wing-shaped and to remain continuously open; it is thought to be fed from the center while simultaneously solidifying at the sides as spreading carries the two halves apart. Progressive fractionation occurs by crystal settling coupled with repeated replenishment and magma mixing in an open steady-state system. Near-bottom studies reveal that the zone of extrusion above the chamber is narrow, but its eruptive history is cyclic in nature, in conflict with the predictions of a steady-state model. On-bottom gravity data at 21 degrees N on the East Pacific Rise reveal a negative gravity anomaly that may be related to the uppermost part of the chamber. The anomaly is only 2 kilometers wide and 1 kilometer below the sea floor. This feature may be associated with a short-term upper magma reservoir. The cyclic volcanic activity is directly related to the active phase of hydrothermal circulation responsible for the observed negative thermal anomaly. The volume of water associated with this circulation is equal to the entire ocean volume passing through the accretion zone approximately every 8 million years. This is about 0.5 percent of the world's rivers, but the effective transport rates of elements are comparable to those of rivers in that anomalies for individual elements are frequently between 100 and 1000 times the average river composition. The degree of subsurface dilution determines the final exit temperature and composition of the hydrothermal fluids, ranging from manganese domination at extreme dilution to iron at intermediate levels to sulfide deposition when low dilution occurs. The discovery of massive sulfide deposits on the East Pacific Rise is destined to have a profound impact on our understanding of ore-forming processes. Whether it will have any economic significance remains to be seen.     

Late Cenozoic Underthrusting of the Continental Margin off Northernmost California

The presence of magnetic anomaly 3, age 5 million years, beneath the continental slope off northernmost California, is evidence for underthrusting of the continental margin during the late Cenozoic. Folded and faulted strata near the base of the slope attest to deformation of the eastern edge of the turbidite sedimzents in the Gorda Basin; the deformation observed is exactly that expected from underthrusting. The relative motions of three crustal plates also suggest underthrusting, possibly with a major component of right-lateral slip.