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The presence of salt within a sedimentary basin can potentially modify its temperature distribution and history. In particular, the positive thermal anomaly associated with the top of salt domes has attracted considerable attention in the past. The role of the lesser appreciated negative thermal anomaly associated with the base of salt domes in modifying the maturation history of subsalt source rocks is explored. The finite element method was used to model the transient and steady-state conductive temperature perturbations induced by salt layers, domes and pillows. The results indicate that the modification of the thermal regime due to evolving salt domes may significantly affect the maturation level of source rocks in the vicinity of the domes. Modelling the temperature structure of various salt structures has shown that, in general, the refraction of heat flow induces a dipole-shaped temperature anomaly; a positive anomaly located towards the top of the salt structure and a negative anomaly located towards its base. These dipole anomalies can be strongly asymmetrical, the degree of asymmetry depending on the shape of the salt structure and the proximity of the top of the salt structure to the surface of the basin. However, when the salt structure reaches the surface, the dipole-shaped temperature anomaly collapses to a monopole. Below the salt structure, all sediments, independent of their depth and lithology, are colder relative to a section with no salt. Similarly, salt domes that reach the surface very efficiently drain the heat from below and from the side of the dome. These negative thermal anomalies may extend for a considerable depth beneath the base of the salt dome and may reach values of −85°C locally. Because of the large contrast in thermal conductivity between the highly porous sediments and salt at lower temperatures, the efficiency of a salt dome to channel heat increases the closer the salt dome is to the surface. These results indicate that deep sedimentary basins containing salt are more prospective than basins without salt and/or salt diapirism. In addition to the structural traps associated with salt tectonics, salt domes and tongues connected to their source dissipate heat more efficiently and thus keep deeper regions of the basin relatively colder and potentially within the oil window for a longer time. This cooling effect is maximized when the top of the salt dome remains close to the surface of the sedimentary basin for a significant period of time and may be especially important for continental margins such as Brazil and offshore West Africa, where most of the source rocks lie beneath extensive evaporite deposits. In contrast, it was found that for the Gulf of Mexico basin, pre- and Early Tertiary salt diapirism and sheet emplacement may have caused significant delays in the maturation of subsalt source rocks in the deeper regions of the Gulf basin, but the maturation is likely to be relatively insensitive to the Late Miocene-Pliocene stage of salt mobilization because the time interval has been too short (

A one-dimensional model of fluid pressure and porosity evolution is used to investigate the physical processes that control the development and maintenance of overpressure in a compacting sedimentary basin. We show that for shale-dominated sequences the variation of the hydraulic diffusivity in both space and time is such that it produces a minimum between 2 and 4 km depth, consistent with observations from the Gulf Coast basin. This minimum inhibits the upward flow of fluid by acting as a “bottleneck” and thus determines the shallowest position of the depth to the top of overpressure. Above this region of bottleneck, overpressure does not develop because the porosity is sufficiently large to maintain high values of hydraulic diffusivity that are conducive to the rapid dissipation of excess fluid pressure. Within the overpressured shales, compaction propagates downward through the section, releasing fluids from the upper part of the section while continuing to restrain the upward flow of fluids from deeper within the section. As such, overpressures are predicted to be maintained within the deeper regions of a basin for tens to hundreds of millions of years. Further, fluid viscosity plays an important role in defining the depth behavior of hydraulic diffusivity as a function of time. Assuming a temperature-dependent fluid viscosity guarantees that the hydraulic diffusivity minimum will always exist during the development of the basin. On the basis of our results, we find that the depth at which the porosity equals 14±4% correlates with the depth to the local hydraulic diffusivity minimum and thus the depth to the top of overpressure. Moreover, we interpret that the 14±4% represents the threshold porosity for which a shale actually begins to act as a seal. Within the Gulf Coast basin, the gross sediment facies consists of lower massive shales across which deltaic systems have prograded allowing the deposition of an alternating series of sandstones and shales that grade vertically into massive sandstones. The massive sandstones are highly permeable and are connected hydrologically to the surface. We conclude that these sandstones play little role in the development of overpressure because of their high permeability except to the extent that the base of the massive sandstones marks the minimum depth possible for the top of overpressure. In contrast, overpressuring is observed to develop within either the shale-dominated sequence or the region of interspersed/interfingering sands and clays. The clay-encompassed sands play only a passive role in the development and maintenance of overpressure because it is the low-permeability clays that control the movement of fluids into and out of the sands.

A sedimentary basin is a thermochemical reactor; i.e., a complex, dynamically interrelated system that is deformed and faulted by tectonic forces, modified by sedimentation and erosion, influenced by geopressuring and chemical reaction, and subjected to constantly changing temperature, pressure, and fluid composition and movement. We understand little about the coupling among these processes that comprise basin thermochemical reactors despite many recent advances in basin science and technology. However, we are certain that these processes are coupled so strongly that changes in one can completely alter the operation of all the others in a basin.

Heat Flow values, determined from temperature logs and estimates of thermal conductivity from geophysical logs range from 23 to 37 mW/m2 from 800 to 2500 m depth in the Toa Baja scientific drillhole on the north, central coast of Puerto Rico. Near the target seismic reflector at the base of the well, an active hydrothermal system was encountered in which heat flow of up to 90 mW/m2 was found in a mineralized zone beneath a volcanic sill or flow. The heat flow then dropped to 50 mW/m2 beneath this subhorizontal flow zone. The mining of heat from downdip is proposed to account for this thermal anomaly, as well as the scatter in the heat flow determined from the few other wells drilled into Puerto Rico. The time-temperature history of the well indicates that Eocene volcaniclastics of the lower 2 km were deposited into a geothermal gradient of 60°C/km north of an active arc (heat flow estimated to have been 120–180 mW/m2). Uplift, erosion and cooling occurred between 40 and 30 Ma. Reburial and deposition of Oligocene-Miocene Limestones produced the present-day geothermal gradient of 15 °C/km (heat flow of 30–50 mW/m2). Based upon comparisons with slab cooling models, the crustal thickness beneath Puerto Rico is estimated to be closer to continental than oceanic.

Geophysical logs from the Toa Baja drillhole on the north central coast of Puerto Rico are used to distinguish the variation in physical properties with depth in this non-volcanic island arc massif. A marked unconformity separates shallow Oligocene to Miocene limestones with considerable permeability contrast from a deeper sequence of volcaniclastic rocks with decreasing shaliness. A volcanic sill or flow overlying a strongly mineralized zone is found to cause the impedance contrast responsible for the strong seismic reflection anomaly (bright spot) that was the exploration target.

Geochemical well logging provides a continuous record of the variations in elemental abundances of the major rock-forming oxides of Si, Al, Ca, Fe, Ti, and K, as well as S, Gd, U, Th, and the H and Cl in the formation and pore fluid. Through the additional measurement of the photoelectric capture cross section of the rock, the sum of Mg + Na can also be estimated. Though not as accurate as laboratory analyses of recovered core samples, the log-derived abundances are precise enough to define the degree and extent of alteration, to identify igneous lithostratigraphy, and to calculate integrated chemical exchange between the oceanic crust and seawater. In this paper, the elemental yields from geochemical logging in basalts are calibrated against extensive XRF analyses of cutting samples from the Lamont 2 test well into the diabases of the Palisades Sill, New York. Accuracy and precision of the log-derived analyses are determined in the lower part of the well, and calibration equations are derived, which are then tested against core-derived “standards” from the upper part of the well. The calibrated, log-derived, elemental analyses are within one standard deviation of the core-derived results (except for the Mg + Na curve, which is somewhat noisier). These calibrations are then applied to geochemical logs from the oceanic crustal basalts of Ocean Drilling Program hole 504B, where core recovery was less than 20% of the section. The accuracy and precision of the calibrated, log-derived elemental abundances are tested against core-derived standards from seven dike and sill intervals. Then the corrected elemental analyses are used to derive a mineralogy model for hole 504B that shows the oceanic crust to contain secondary mineralization in the form of celadonites and smectites in the pillow basalts and chlorites in the dikes that are largely confined to fracture and breccia zones. Cyclicity in the Al and other elemental logs was found to vary with the abundances of these alteration products and with eruption and intrusion event boundaries. The geochemical logging data are then used to estimate the integrated chemical exchange resulting from hydrothermal alteration of the oceanic crust that has occurred over the last 5.9 m.y. in hole 504B. The primary change is from Ca loss and Mg gain caused by the reaction of basalt with seawater. A large Si increase found in the transition zone between the pillows and dikes is attributed to precipitation of quartz during mixing of hot, up welling hydrothermal fluids and cold, downwelling seawater at what was once a major permeability discontinuity. The present low-to-high permeability transition in hole 504B is found 500 m shallower. The K budget requires significant addition to the uppermost pillow basalts both from high-temperature depletion in the lower pillows and dikes and from low-temperature exchange with seawater. The geochemical logs further document that the total chemical exchange between the oceanic crust and seawater is as important to the long-term composition of the oceans as is the chemical input carried by rivers. Integrated “water/rock ratios” are then derived for the mass of seawater required to add enriched elements and the mass of hydrothermal fluid required to remove depleted elements in the oceanic crust of hole 504B. Whereas Ca, Mg, and K require relatively low water/rock ratios, high values for Si, Al, and Fe suggest that off-axis, ridge-flank exchange is as important to the total cation exchange budget as are ridge-axis processes.

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.

Summary The spatial pattern of cooling near a spreading ridge crest was investigated with a suite of 71 precisely-navigated heat-flow stations on the Galapagos spreading centre, East Pacific, near 86"W longitude. Stations are on crust less than 1.0 My old on which bathymetry and sediment distribution are well known. Values vary from near zero to greater than 30 HFU (10-6 cal cm-2 s-1). The average over the entire region is significantly less than that predicted by theoretical conduction models of a cooling lithosphere. We observe a regular variation of the heat-flow pattern with a wavelength of 6 & 1 km approximately normal to the ridge crest. Heat-flow maxima are characteristically located near faults and local topographic highs. The locations of fields of small sediment mounds, apparently hydrothermal vents, are also restricted to these faulted, elevated areas of high heat flow. Near-axis, bottom water temperature anomalies of several hundredths "C were detected. The low average, the low minima in the heat-flow pattern, and the water temperature anomalies suggest that hydrothermal circulation accounts for approximately 80 per cent of the geothermal heat released near the ridge crest. We conclude that the hydrothermal circulation pattern is controlled by one or more of the following physical properties of the system : highly developed cellular convection, discrete zones of high permeability, variation in the strength of heat sources near the base of the crust, or bottom topography. Our results imply that heat-flow studies near active oceanic ridges will be of most value if they are sufficiently detailed and well navigated to define the systematic small-scale variations that appear to be caused by hydrothermal circulation.

An analysis of downhole geophysical logs, and their relation to surface geological data constrain the structure in the first 700 m of the Cajon Pass scientific drillhole. Several angular folds, mapped in a small canyon 150 m to the southeast of the drillsite, are associated with a steep northeast dipping reverse fault at depth. This fault is inferred to cut the scientific drillhole in basement at 700 m, and to intercept a nearby oil-exploration well, Arkoma Federal 1-26, within the sediments at 350 m. The folds mapped at the surface are thought to have developed in Late Miocene to Early Pliocene time, during the Squaw Peak thrusting event. The inferred reverse fault offsets the sediment-basement contact 180 m between the two holes.

The frequency-magnitude distribution (b value) in the upper plane of the deep seismic zone beneath Tohoku, Japan, shows significant variation with depth which we attribute to dewatering of the oceanic crust during the subduction process. High b value from the surface to 40 km in the upper seismic plane and overlying mantle wedge is thought to be caused by the advection of free water from the oceanic crust. The fact that the b value drops to 1.0 is attributed to the passing of the hydrous oceanic crust through a dehydration front. The resulting increase in pore pressure lowers the effective stress and increases the b value in the upper plane. Beneath the volcanic line, b drops back to

Magnetic model studies, rock magnetization, and major element chemical analyses show that unusually large amplitude magnetic anomalies found along the easternmost segment of the Southeast Indian Ridge [zone A] are caused by the high intensities of magnetization of basalts intruded at the ridge axis. In turn, these high magnetizations are caused by a threefold increase in weight percent of titanomagnetites from normal midocean ridge basalts intruded in the west [zones B and C] to ferrobasalts intruded in the high magnetic anomaly zone to the east. The Fe-Ti increase in the opaques reflects a major elemental enrichment in Fe and Ti caused by extensive shallow fractionation of plagioclase, clinopyroxene, and olivine. This shallow fractionation appears related to unusually cold magma crystallization temperatures, perhaps caused by proximity to a postulated downwelling zone in the asthenosphere south of Australia. However, examination of these correlations in greater detail points to several ambiguities: although the entire region is deep, and therefore the lithosphere is integrally colder than normal plates, zone A is shallower than zones B and C, but geothermometrically, its axial magmas are colder. We do not know how cold magmas directly relate to ferrobasalt generation. Also, plumelike geochemical signatures are also seen in some of the analyses. It appears that plume mantle sources are mixing with MORB sources and then undergoing extensive shallow fractional crystallization in zone A.

Geochemical well logs recorded in the Cajon Pass Scientific drillhole provide an estimate of the elemental concentrations of Si, Al, Fe, Ca, K, Th, Ti, S, and the HREE Gd. The continuous geochemical profiles were used to define a lithostratigraphy in the cystalline basement from 501 to 1,829 m. The interval between the faulted top of basement and a thrust fault at 689 m consists of six alternating slivers of granite and granodiorite. Interlayered granitic and granodioritic units were then encountered to another fault at 1290 m, whereas gneisses of granitic to tonalitic composition were logged in the basal portion of the hole. Linear discriminant analysis of major element chemistry, as well as variations in Gd, Th, Ti, and S, were used to demonstrate that each of the major lithostratigraphic units is chemically distinct. Different trace element signatures and variable degrees of metamorphism indicate a complex thermal and hydrological history for the basement rocks encountered in the Cajon Pass well.

Intraplate seismicity, deformed oceanic crust and sediments, and local areas of abnormally high heat flow characterize significant internal deformation of the Indo–Australian plate in the northeastern Indian Ocean. The deformation appears analogous to the buckling of an elastic plate with brittle failure at its surface. Itis not clear whether the Indo–Australian plate can be resolvedinto two component parts or whether a new convergent plate boundary is likely to form in the deformed region. The onset of intraplate deformation in the late Miocene seems to be connected to the Himalayan orogenic stage of the collision between India and Asia.

The well logging activity of the Ocean Drilling Programme, which deploys the most technically advanced suite of downhole measurements available for routine use, is providing new opportunities for advancement in marine geoscience. Particular examples are cited of the application of wireline data to problems associated with global environmental changes, crust/ mantle interactions, crustal fluid circulation in the context of a global geochemical budget, lithospheric stress and deformation, and evolutionary processes in oceanic communities. Further technological developments will expand the scientific role of downhole measurements still further, especially in terms of the integration of geophysical data at different scales of measurement, and the interpretation of these data in accordance with; global scientific themes.

Twenty-one new heat flow measurements in the Mariana Trough delineate a central zone of high heat flow corresponding to an axial topographic high and a flank zone of low heat flow. The high heat flow is an indication that the axial high is a narrow zone of magmatlc intrusion and that sea floor spreading is occurring in the Mariana Trough in a manner similar to its occurrence on midocean ridges. The low heat flow values appear to be the result of the occurrence of hydrothermal circulation in the oceanic crust of the Mariana Marginal Basin.

We present 206 new heat flow measurements in the Indian Ocean. These and approximately 300 previously published heat flow values are individually evaluated for sedimentary environment and instrumental performance. The relationship between average heat flow and age is found to be little affected by selection of the most reliable experiments, although the scatter about the mean is significantly lowered. The variation of mean heat flow with age is found to be very similar to that in the eastern Pacific and Atlantic oceans: There is a crestal low heat flow zone with large variability, a transition zone within which the heat flow increases from values considerably below to values in agreement with predictions from thermal models of the oceanic lithosphere, and a region where heat flow values are in accord with theoretical predictions. However, the transition zone occurs over different crustal ages from ocean to ocean: 40–60 m.y. in the Indian Ocean, 4–6 m.y. in the Galapagos spreading center, 10–15 m.y. on the East Pacific Rise, and 50–70 m.y. on the Mid-Atlantic Ridge. The transition zone generally corresponds to a sea floor age where (1) sedimentary thickness increases to ≥300 m, (2) sea floor roughness is significantly smoothed by sediment blanketing, and (3) the carbonate content of surface sediments decreases to ≤40%. The transition zone occurs where water circulation in the oceanic crust stops affecting the surface heat flow strongly. There are two possible explanations for the transition. First, a change in composition from carbonate to siliceous sediments results in a decrease in bulk permeability. This combined with general thickening of the sedimentary blanket with aging results in the deposition of an impermeable layer which prevents the convective exchange of heat from the oceanic crust to the ocean. Second, hydrothermal flow within the oceanic crust is plugged by filling of circulation cracks in the oceanic crust. The fact that in several basins of the Indian Ocean the heat flow transition corresponds with the carbonatesiliceous boundary is support for the former mechanism. However, the fact that locations of increases in velocity of seismic layer 2A generally correspond to the transition regions in the Atlantic and Pacific oceans provides support for the latter mechanism. Heat flow measurements in the world's oceans allow us to calculate the variations of bulk permeability and basal temperature in the oceanic crust as a function of age and to evaluate the geochemical implications of the variation in these parameters between oceans. The combination of conductive heat flow and elevation versus age observations in old lithosphere demonstrates the deviation from t1/2 cooling in the Indian Ocean and indicates that the Mozambique and western Somali basins are considerably older than preliminary deep-sea drilling results suggest. 1 Supplementary table is available with entire article on microfiche. Order from American Geophysical Union, Suite 1000, 1909 K Street, N.W., Washington, D.C. 20006. Document J77-003; $1.00. Payment must accompany order.

Summary Geochemical constraints based upon compositions and mechanisms of formation and eruption of island arc volcanic rocks strongly suggest hydrous conditions in the region of primary magma generation. The source of most of the water is probably dehydration of the oceanic crust during underthrusting. Chemical reactions involving dehydration of representative oceanic crustal assemblages absorb enough heat to possibly negate any frictional heat which may be generated along the Benioff-Wadati zone in the upper 75 to 100 km of the underthrust zone. Calculations of thermodynamic parameters of relevant reactions at elevated pressures and temperatures show that N 50cal of heat are absorbed per gramme of oceanic crust dehydrated. The result of this heat absorption is suppression of isotherms not only in the oceanic crustal section of the downgoing slab but also in the overlying mantle wedge. Therefore, low heat fiow measured from the trenches to the volcanic arcs may be caused in part by the heat-sink effect of dehydration in the downgoing slab. The water released by dehydration either remains in the slab promoting its melting and the generation of andesitic primary magmas and/or escapes into the overlying wedge of mantle promoting its melting and the generation of the basaltic or andesitic primary magmas. The major thermal effect of the released water is to cool the wedge overlying the slab. The progressive geochemical changes in arc volcanism both in space and time may be controlled by the thermal conditions at the downgoing slab-peridotite wedge interface.

The temperature structure of a subduction zone, including convection in the surrounding asthenosphere and dehydration and frictional heating in the descending slab, has been calculated by a finite difference procedure. Models of ocean-ocean (thin landward lithosphere) and ocean-continent (thick landward lithosphere) convergence have been constructed. In the ocean-ocean case, melting is predicted only in the wet-peridotite of the asthenosphere overlying the downgoing slab. In the ocean-continent case, however, melting of the lower continental mantle, the basal continental crust, and the subducted oceanic crust as well as the asthenosphere above the slab could occur. A complex origin for continental volcanic arc magmas is permitted by this model. Additionally, a change in focal mechanisms from interplate thrust to intraplate compression and a zone of high b-values (the frequency-magnitude relation of earthquakes) in the upper of two seismic planes delineating the subducting Pacific Plate beneath northeaster...

Sixty-seven new heat flow measurements on the Nazca Plate are reported, and the thermal regimes of three specific areas on the plate are examined. The Nazca Ridge is an aseismic ridge which may have been generated as an “island trail” from the Easter Island “hot spot” and/or may be a fossil transform fault. The Nazca Ridge has lower heat flow than the surrounding sea floor implying that the ridge might have low “effective” thermal conductivity causing heat to preferentially flow or refract to surrounding ocean crust which has higher conductivity, or, the low heat flow values may be caused by hydrothermal circulation on the ridge. The Carnegie Plateau is an elevated region south of the Carnegie Ridge on the northeastern Nazca Plate with high heat flow and shallow topography consistent with an age of less than 20 m.y. B.P. The central Nazca Plate is an area of highly variable heat flow which is possibly related to thin sediment and to rough regional topography.

The temperature structure of a subduction zone during subduction of an oceanic spreading ridge has been calculated by a finite-difference procedure. The plates preceding and following the ridge crest into the trench had convergence rates of 90 and 20 mm/yr, respectively, corresponding to a spreading half-rate for the ridge of 35 mm/yr and to the velocity balance for a simple three-plate model of ridge subduction. Abrupt temperature increases in the range 50–500°C occur at shallow locations(≤35km) in the outer 40 km of the arc-trench gap within a few million years of the time that the ridge crest enters the trench. Geological manifestations of these changes could include anomalous near-trench magmatism (gabbroic or granitic), termination of high-pressure metamorphism, enhancement of low-pressure metamorphism, and generation of hydrocarbons. Along the slab-wedge interface the depth of dehydration of hydrous phases in the subducted oceanic crust rapidly migrates upward as the ridge approaches. Consequently, substantial volumes of water may be released at depths of 50–75 km just before the ridge crest is subducted, and relatively shallow dehydration(

An empirical relationship between ridge elevation and age of the oceanic crust is presented for the Pacific, Atlantic, and Indian oceans. This relationship is accounted for by the thermal contraction of a cooling lithosphere as it moves away from a center of spreading, and thus is compatible with plate theory. Hence, it is possible to use topographic profiles to predict the age of the ocean floor. Detailed examination of profiles in different areas indicates that slow-spreading ridges (half-rate 3 cm/yr). However, all ridges appear to show a uniform subsidence rate near the ridge crest. This uniform rate makes it possible to use topographic profiles in regions of smooth topography to predict the age of oceanic crust, less than 40 m.y. old, to better than ±2 m.y. Topographic profiles will be particularly useful for predicting age where magnetic anomaly patterns are absent or difficult to interpret. Magnetic, topographic, seismic reflection, twenty-nine new, and all other heat-flow observations across the east Pacific rise and Mathematician and Clipperton seamount chains in the central eastern Pacific are examined. The magnetic and heat-flow observations are too inconclusive to enable a tectonic reconstruction of the area. However, topographic profiles at right angles to the rise between 20°N and the equator show that the Mathematician and Clipperton seamount chains are the old crest of the east Pacific rise. These parts of the rise crest terminated approximately 5 m.y. B.P. by the spreading center jumping 4° to the east. Seismic reflection profiles at right angles to the crest support this conclusion. The topography west of these two chains is used as the basis for a proposed evolution of the central eastern Pacific during the past 20 m.y.

All active midocean ridges show a uniform relationship between depth and age of the oceanic crust. Recently, it has been shown by numerical methods that convective flow in a Newtonian fluid will have a positive gravity anomaly and an upward surface deformation associated with an ascending limb. If there is thermal convection in the upper mantle, these calculations predict that there may be a correlation between free air gravity anomalies and differences from the uniform relationship between oceanic depth and age. To investigate such a correlation, we considered the crestal elevation and free air gravity anomaly over the crest of the midocean ridges. It has been suggested that the differences from the depth versus age relationship are related to spreading rate. Thus, we also considered a correlation between crestal elevation and changes in rate along the ridge axis. We found a positive correlation between free air gravity and differences in crestal depth of the midocean ridge system. We found no correlation between spreading rate and gravity and no uniform relationship which holds in all the oceans between spreading rate and observed crestal depths. The long wavelength gravity anomalies which are correlated with the differences in crestal depth cannot be supported by an 80 km thick lithosphere. Thus, they are considered evidence of flow within the aesthenosphere. Further, the correlation between gravity anomaly and differences in crestal depth has the same sign and gradient as predicted by the investigations of convection in a Newtonian fluid.

WITHIN the framework of plate tectonics, dating of the ocean floor has been almost exclusively achieved by studies of magnetic anomalies. But a north–south striking mid-ocean ridge system generating new seafloor in the equatorial region produces magnetic anomalies which are of small amplitude and are frequently distorted and disguised by magnetic “noise”. The correspondence between the absolute depth of oceanic crust and its age1 has provided an alternative method of age dating the oceanic crust anywhere in the world's oceans, provided, of course, that the crust was generated by a seafloor spreading system. Such a correlation tool has been used on the East Pacific Rise (Fig. 1) between 20° N and the equator1 and between 0° and 20° S (ref. 2).

A detailed study of a 60×150 km area at 60°W, 24°N at the eastern end of the Nares Abyssal Plain indicates that hydrothermal circulation is still active in the 80 m.y. B.P. oceanic crust. The 58 heat flow measurements made at five stations in the area have revealed (1) constant heat flow over the abyssal plain (56 mW m−2), (2) a cyclic heat flow over the abyssal hills (mean of 77 mW m−2), and (3) a large anomaly of 710 m W m−2 over one of several small domes which protrude from the abyssal plain. The domes are 0.5–1.0 km in diameter near the top and rise 50 m above the level of the abyssal plain. They are recognized from surface echo sounders by an abrupt disappearance in the abyssal plain subbottom reflectors, but on near-bottom pinger records they appear as steep-walled structures which are covered by ∼10 m of sediment (compared to ∼75 m on the surrounding abyssal hills). From analogy with active ridge crests, these features are probably small volcanoes. The heat flow anomaly over one of the domes is matched well by a finite element convection model with the following characteristics: (1) recharge at one basement outcrop and discharge at another, (2) 300 m of sediment fill between outcrops, and (3) permeabilities of 10−10 cm2 for basalt and 10−13 cm2 for sediment. In other words, we believe that there is very effective convective heat transfer within the crust and out of the relatively permeable, thinly sedimented basement dome, resulting in the local high heat flow. Overall, the results from the Nares survey vividly show the age independent muting effect of sediment on the surface manifestation of crustal convection. In our survey area the mode of heat transfer varies from purely conductive in the more thickly sedimented abyssal plain areas (∼300 m sediment cover) to moderate amplitude convection pattern beneath the abyssal hills (∼75 m sediment cover) to a very large thermal anomaly over the small dome or ‘chimneylike’ structure (∼10 m sediment cover). The domes are possibly active analogues to the presently inactive basement chimney drilled at DSDP site 417A.

The upper layers of the oceanic crust are characterized by large changes in geophysical properties both sub-areally and with submarine depth which belie their volcanic modes of origin. Feeder dikes connecting surface-flow basalts with the magma chamber below, produce new seafloor with similar volcanic structure worldwide within the spreading systems active on all mid-ocean ridges. All ocean crust consists of pillow basalts overlying sheeted dikes which in turn rest upon gabbroes which are the frozen remains of old magma chambers. Even the chemistry of the oceanic crust is essentially the same from ocean to ocean. It is the rare ocean basalt which differs from MORB, or mid-ocean ridge basalt composition. Yet geophysical properties of Mid-Atlantic Ridge flank crust can be very different from East Pacific Rise crust. For example, the seismic velocity in the very uppermost crust is often slower in the Atlantic than in the Pacific (Houtz and Ewing 1976). The most highly altered metamorphic rocks are invariably found in the Atlantic.

This section contains three papers on the chemistry and physics of hydrothermal systems on Iceland and on mid-ocean ridges. These contributions detail the contrasting styles of the two systems in detail. But before examining the specifics of these hydrothermal circulation systems, it is appropriate to step back and examine the differences from a broad perspective. The physical and chemical processes of a subaereal and a deep subaqueous hot spring are similar, except that the boundary conditions are dramatically different. These changing boundary conditions produce very different surface expressions of hydrothermal convection, and it is important to identify cause and effect. Below two examples are given of differences caused by contrasting land versus deep sea hydrothermal activity that result from the same physical and chemical principles operating under atmospheric and greater than 200 atmospheres of pressure, respectively. First, the effects of boiling or the lack thereof in the two systems is quite different. Second, the pattern of fracturing which controls permeability is, itself, controlled by very different pore pressure and thus effective stress regimes on land versus in the deep sea.

Brown clay cores from the Madagascar and Crozet basins show the following evidence of excess pore pressures: large amounts of flow-in, increasing average sedimentation rate with age, and nonlinear temperature gradients. Additionally, many hilltops in these basins have no visible sediment cover. The bare hilltops may result from periodic slumping caused by excess pore pressures. Calculated excess pore pressures which equal or exceed the overburden pressure were inferred from water fluxes predicted by nonlinear temperature gradients and laboratory permeability measurements by using Darcy's law. Since pore pressures which exceed the overburden pressure are unreasonable, we attribute this discrepancy to laboratory measures which underestimate the in situ permeability. The widespread presence of overpressured sediments in areas of irregular topography provides a process for resuspension of clay-sized particles. This mechanism does not require high current velocities for the erosion of clay and therefore can be applied to many areas where no strong currents are evident. Carbonate-rich sediments from the Madagascar Ridge, the Mozambique Ridge, and the Agulhas Plateau had almost no flow-in and occurred in areas where all topography was thickly draped with sediment, Since the age and tectonic location of the ridges and plateaus preclude water circulation in the basement, wemore » attribute these differences between the brown clay and the carbonate-rich material to an absence of significant excess pore pressures in the plateau and ridge sediments.« less

La cresta de la dorsal del Pacifico oriental entre los 17° N a los 40° S se caracteriza por un horst central. Este rasgo se atribuye a! producto estacionario de una deriva constante en torno a la cresta, en que la varianza de la velocidad de inyección es menor que la varianza de la velocidad de deformación horizontal. Se elabora un modelo basado en estos dos parámetros, con el que se logra explicar tanto la topografía del horst central como de un graben central. Este último ocurre cuando en el modelo se postula una velocidad del fondo marino suficientemente baja, de modo que la varianza de la velocidad de inyección vertical es mayor que la varianza de la velocidad de deformación horizontal. Se supone que esto ocurrirá solamente cuando la deriva es intermitente, debido a la solidificación y ruptura alternada de la zona central de intrusión. Por lo tanto, la configuración del horst central sería la "normal". Dicho de otra manera, hay formación de graben central cuando el material asciende por la cresta sobre distancias mayores que las necesarias para acelerar el material de la cresta desde una velocidad horizontal nula hasta la velocidad de dispersión. De lo contrario, se forma un horst central.

Hole 504B is by far the deepest hole yet drilled into the oceanic crust in situ, and it therefore provides the most complete “ground truth” now available to test our models of the structure and evolution of the upper oceanic crust. Cored in the eastern equatorial Pacific Ocean in 5.9-m.y.-old crust that formed at the Costa Rica Rift, hole 504B now extends to a total depth of 1562.3 m below seafloor, penetrating 274.5 m of sediments and 1287.8 m of basalts. The site was located where the rapidly accumulating sediments impede active hydrothermal circulation in the crust. As a result, the conductive heat flow approaches the value of about 200 mW/m² predicted by plate tectonic theory, and the in situ temperature at the total depth of the hole is about 165°C. The igneous section was continuously cored, but recovery was poor, averaging about 20%. The recovered core indicates that this section includes about 575 m of extrusive lavas, underlain by about 200 m of transition into over 500 m of intrusive sheeted dikes; the latter have been sampled in situ only in hole 504B. The igneous section is composed predominantly of magnesium-rich olivine tholeiites with marked depletions in incompatible trace elements. Nearly all of the basalts have been altered to some degree, but the geochemistry of the freshest basalts is remarkably uniform throughout the hole. Successive stages of on-axis and off-axis alteration have produced three depth zones characterized by different assemblages of secondary minerals: (1) the upper 310 m of extrusives, characterized by oxidative “seafloor weathering“; (2) the lower extrusive section, characterized by smectite and pyrite; and (3) the combined transition zone and sheeted dikes, characterized by greenschist-facies minerals. A comprehensive suite of logs and downhole measurements generally indicate that the basalt section can be divided on the basis of lithology, alteration, and porosity into three zones that are analogous to layers 2A, 2B, and 2C described by marine seismologists on the basis of characteristic seismic velocities. Many of the logs and experiments suggest the presence of a 100- to 200-m-thick layer 2A comprising the uppermost, rubbly pillow lavas, which is the only significantly permeable interval in the entire cored section. Layer 2B apparently corresponds to the lower section of extrusive lavas, in which original porosity is partially sealed as a result of alteration. Nearly all of the logs and experiments showed significant changes in in situ physical properties at about 900–1000 m below seafloor, within the transition between extrusives and sheeted dikes, indicating that this lithostratigraphic transition corresponds closely to that between seismic layers 2B and 2C and confirming that layer 2C consists of intrusive sheeted dikes. A vertical seismic profile conducted during leg 111 indicates that the next major transition deeper than the hole now extends—that between the sheeted dikes of seismic layer 2C and the gabbros of seismic layer 3, which has never been sampled in situ—may be within reach of the next drilling expedition to hole 504B. Therefore despite recent drilling problems deep in the hole, current plans now include revisiting hole 504B for further drilling and experiments when the Ocean Drilling Program returns to the eastern Pacific in 1991.

Although sea floor spreading of some sort has created marginal basins, it is not known whether this spreading is similar to that at midocean ridges. Back arc spreading in the Mariana Trough occurs along an axial topographic high, but there is rapid variation in style along strike and less well defined symmetry than there is at midocean ridges. The topography is very rough, despite an apparently fast spreading rate. This lack of correlation between spreading rate and topographic relief also occurs in other marginal basins of the western Pacific. In addition, the apparently different curvatures between the frontal and remnant arcs of some marginal basins, such as the Mariana Trough, might result from nonrigid deformation of the frontal arc. There is no single finite pole of rotation about which the Mariana trough can be closed without seriously violating observed fracture zone directions and the geology at both ends of the arc. South of 14°N the frontal arc is truncated by the east-west fractures so that marginal basin crust is juxtaposed against the Pacific plate near the possible triple junction between the Philippine and the Pacific plates and the Mariana platelet. The present data suggest that the Mariana Trough has opened as if the frontal arc were a horizontal beam pinned at its northern end and deforming laterally so that fracture zones parallel to the spreading direction have themselves undergone minor extension. That is, the Mariana Trough has pulled loose from the Philippine plate at its southern end and has, been bent and fractured so that extension has occurred not only parallel but also perpendicular to the direction of principal rotation.

Cenozoic marine sediments were drilled and cored by the Deep Sea Drilling Project at Site 613 in the Baltimore Canyon Trough, and four‐channel sonic waveforms were recorded from 115 to 585 m below sea floor. Compressional wave velocities and spectra were calculated from the sonic waveforms. In situ porosity was estimated and compared to core measurements. As expected, velocity increased with depth as porosity decreased. However, the energy loss between near and far receivers increased and the peak crosspower frequency decreased with depth, suggesting that attenuation increases over the same interval. The large changes in energy and frequency cannot be explained solely by changes in the elastic propeties of the sediments. We model the observed compressional wave spectra using measured values of velocity and density. Reasonable fits to the observed spectra are obtained using either a constant Q (CO) or nonconstant Q model. For the CO model, O decreases from 100 to 20 over this depth interval. Diagenesis obs...