Circum-pacific late cenozoic structural rejuvenation: implications for sea floor spreading

The hypothesis of sea floor spreading and lithosphere plates seems to unify the origins of both oceanic ridges and volcanic arc-trench systems; therefore knowledge of well-known land areas should shed light upon sea floor tectonics. Impressive evidence of a major mid-Cenozoic discontinuity in the tectonic history of circum-Pacific land areas suggests a roughly synchronous change in sea floor development, more evidence for which may be anticipated in the future.     

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.     

Geothermal convection through oceanic crust and sediments in the Indian ocean

Closely spaced heat flow surveys at four sites on the flanks of the Central Indian Ridge and the Southeast Indian Ridge delineate a pattern of oscillatory heat flow which can only result from cellular convection of oceanic bottom water through the oceanic crust and overlying sediment. These cells have a wavelength of 5 to 10 kilometers and are presently active in sea floor 18 x 10(6), 25 x 10(6), and 45 x 10(6) years old of the Crozet Basin and in sea floor 55 x 10(6) years old of the Madagascar Basin. The precise measurement of nonlinear temperature profiles makes it possible to calculate the conductive and convective heat transfer components through the sea floor. Even in the oldest sites, geothermal convection is still a major component of heat transfer through both the crust and sedimentary layers. These observations coupled with the results of earlier oceanwide geothermal studies indicate that more than one-third of the entire surface area of the world's ocean floor contains presently active geothermal convection that is cellular in plan form.     

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.     

Spreading of the ocean floor: undeformed sediments in the peru-chile trench

None of the expected stratigraphic and structural effects of a spreading sea floor have been imposed on the sedimentary fill of the Peru-Chile Trench. During at least the last several million years, and perhaps during much of the Cenozoic, the trench has not been affected by an oceanic crust thrusting under the continent.     

Sea floor spreading, topography, and the second layer

Local sea floor topography and also the thickness of the second layer of the oceanic rise-ridge system appear related to the spreading rate in the region. Slow spreading, away from the ridge center at 1 to 2 centimeters per year, is associated with a thick second layer, a central rift, and adjacent rift mountains. Fast spreading, 3 to 4.5 centimeters per year, is associated with a thin second layer and subdued topography lacking any central rift. The volume of lava discharged in this layer per unit time and per unit length along the crest of the whole active system is relatively constant regardless of the spreading rate. Total second layer discharge of the system has been about 5 to 6 cubic kilometers per year during the last several million years.     

Block faulting on the gorda rise

A study made of Gorda Rise near 41 degrees 15'N with a novel instrument shows that the rift-valley walls have a tilted steplike profile, often with perched, planar sediments. Topography indicates that the steps were formed by block faulting. Distribution of the steps and the character of their tops suggest that they originated in the central 2 or 3 kilometers of the valley floor and were subsequently moved outward, uplifted, and tilted along with their underlying blocks as the sea floor spread. Gorda Rise is considered a slowly spreading part of the oceanic-rise system. Studies of the Mid-Atlantic Ridge report features that may be similar uplifted blocks.     

Viscosity of the atlantic ocean bottom

Two profiles across the Mid-Atlantic Ridge were analyzed to determine viscosity values. Viscous creeping of bottom features due to gravitational stress was assumed, as was sea floor spreading at a rate of 2 centimeters per year. Values obtained agreed well with previous results obtained on the Fennoscandian Uplift, despite great differences in the horizontal dimensions of the bottom relief.     

Deep-sea erosion and manganese nodule development in the southeast Indian ocean

Features exhibited by a large number of sea floor photographs together with the dating of 187 sediment cores from the southeast Indian Ocean have revealed extensive manganese nodule development and sediment erosion in deep basinal areas. The most extensive nodule field, with an area of 10(6) square kilometers, occurs in the northwestern sector of the South Australian Basin and is named the Southeast Indian Ocean Manganese Pavement. The crests and flanks of the adjacent mid-ocean ridge are, in contrast, free of nodules and marked by much less dynamic bottom water conditions.     

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.     

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.     

Earth tides, global heat flow, and tectonics

The power of a heat engine ignited by tidal energy can account for geologically reasonable rates of average magma production and sea floor spreading. These rates control similarity of heat flux over continents and oceans because of an inverse relationship between respective depth intervals for mass transfer and consequent distributions of radiogenic heat production.     

Supershrimp: deep bioturbation in the strait of canso, nova scotia

Axius serratus, a crustacean thought to be extremely rare, was discovered in large numbers in polluted regions of the Strait of Canso. The shrimp may live deeper than 3 meters in the sediment; burrows are kept open to at least 2.5 meters. Sediment contained in old filled burrows is anomalous in its distribution of particle size and its content of water, organic carbon, and trace elements. These anomalous qualitites affect the geotechnical properties of sediments on the sea floor.     

Magnetic lineations in the ancient crust of mars

The Mars Global Surveyor spacecraft, in a highly elliptical polar orbit, obtained vector magnetic field measurements above the surface of Mars (altitudes >100 kilometers). Crustal magnetization, mainly confined to the most ancient, heavily cratered martian highlands, is frequently organized in east-west-trending linear features, the longest extending over 2000 kilometers. Crustal remanent magnetization exceeds that of terrestrial crust by more than an order of magnitude. Groups of quasi-parallel linear features of alternating magnetic polarity were found. They are reminiscent of similar magnetic features associated with sea floor spreading and crustal genesis on Earth but with a much larger spatial scale. They may be a relic of an era of plate tectonics on Mars.     

Giant larvacean houses: rapid carbon transport to the deep sea floor

An unresolved issue in ocean science is the discrepancy between the food requirements of the animals living on the deep sea floor and their food supply, as measured by sediment traps. A 10-year time-series study of the water column off Monterey Bay, California, revealed that the discarded mucus feeding structures of giant larvaceans carry a substantial portion of the upper ocean's productivity to the deep seabed. These abundant, rapidly sinking, carbon-rich vectors are not detected by conventional sampling methods and thus have not been included in calculations of vertical nutrient flux or in oceanic carbon budgets.     

Recent sedimentation on the new jersey slope and rise

Radiocarbon dating and sedimentological studies of closely spaced cores indicate movement during the Holocene of sediments on the New Jersey continental slope and upper rise between Wilmington and Lindenkohl canyons. The uneven time-stratigraphic thickness of the late Quaternary sediment sections between cores and the nonuniform deposition rate at any given core site and among core sites show that the sediment blanket in canyon and intercanyon areas has been affected by downslope, gravity-driven pocesses during the Holocene to the present. The reduced rate of deposition on the slope and upper rise between the late Pleistocene and the present is largely due to decreased off-shelf transport in response to the eustatic rise in sea level. Very old radiocarbon dates at core tops result from emplacement of older reworked materials from upslope or from truncation of sections by mass wasting processes exposing older material at the sea floor. These processes also account for an irregular sequence of dated sections within cores and stratigraphic irregularities of the surficial cover from core to core. Marked variability in deposition rates on the slope and upper rise is largely a function of topographic configuration, proximity and accessibility to sediment source, and transport processes seaward of the shelf break. Moreover, higher accumulation rates on the upper rise are attributed primarily to slope bypassing. Bypassing, prevalent during the late Pleistocene, has continued periodically to the present.     

Evidence for sediment eruption on deep sea floor, gulf of Mexico

A large crater has been discovered on the sea floor, Gulf of Mexico, in a water depth of 2176 meters. Deep-tow high-resolution imagery shows that the crater is cut into a low hill surrounded by near-surface concentric faults. Approximately 2 million cubic meters of ejected sediment forms a peripheral debris field. The low hill and faults may be related to mud diapirism or intrusion of gas hydrates into near-surface sediments. A recent eruption evacuated sediments from the crater, apparently because of release of overpressured petrogenic gas.     

Molecular fossil record of elevated methane levels in late Pleistocene coastal waters

Accumulating evidence suggests that methane has been released episodically from hydrates trapped in sea floor sediments during many intervals of rapid climate warming. Here we show that sediments from the Santa Barbara Basin deposited during warm intervals in the last glacial period contain molecular fossils that are diagnostic of aerobic and anaerobic methanotrophs. Sediment intervals with high abundances of these compounds indicate episodes of vigorous methanotrophic activity in methane-laden water masses. Signals for anaerobic methanotrophy in 44,100-year-old sediment are evidence for particularly intense methane emissions and suggest that the basin's methane cycle can profoundly affect oxygen budgets in the water column.     

Arctic Ocean Gravity Field Derived From ERS-1 Satellite Altimetry

The derivation of a marine gravity field from satellite altimetry over permanently ice-covered regions of the Arctic Ocean provides much new geophysical information about the structure and development of the Arctic sea floor. The Arctic Ocean, because of its remote location and perpetual ice cover, remains from a tectonic point of view the most poorly understood ocean basin on Earth. A gravity field has been derived with data from the ERS-1 radar altimeter, including permanently ice-covered regions. The gravity field described here clearly delineates sections of the Arctic Basin margin along with the tips of the Lomonosov and Arctic mid-ocean ridges. Several important tectonic features of the Amerasia Basin are clearly expressed in this gravity field. These include the Mendeleev Ridge; the Northwind Ridge; details of the Chukchi Borderland; and a north-south trending, linear feature in the middle of the Canada Basin that apparently represents an extinct spreading center that "died" in the Mesozoic. Some tectonic models of the Canada Basin have proposed such a failed spreading center, but its actual existence and location were heretofore unknown.     

Ridge spreading, subduction, and sea level fluctuations

A numerical model of mantle convection shows that sea level fluctuations are not simply associated with temporal changes in ocean c plate spreading. In the dynamic model, sea level rises rapidly and then falls toward a steady value (but one still higher than the initial) following increased ridge spreading; this time dependence results from profound changes in the deep thermal structure under ocean and continent. The use of past variations in oceanic spreading to infer sea level fluctuations is called into question. With more realistic models and better continental stratigraphy, constraints may be placed on the viscosity structure of the mantle.