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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.

In situ electrical resistivity was measured in DSDP Hole 504B to a depth of 1,013 m into oceanic basement. Apparent resistivities are about 10 Ω m in about 600 m of pillow lavas, sharply increasing to nearly 1,000 Ω m in the underlying dykes. Bulk porosities calculated from resistivities show a distinct layering, apparently corresponding to geophysical layers 2A, 2B and 2C.

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

Major element and trace element analyses of samples dredged from six sites along extensional plate boundaries around the Caroline Sea in the western equatorial Pacific define three separate igneous suites. Three dredge sites were located in the Sorol Trough along the Caroline-Pacific plate boundary and three sites were located in the Ayu Trough along the Philippine-Caroline plate boundary. Within the Sorol Trough, fresh transitional-to-alkalic pillow basalts were dredged from a seamount, weathered oceanic tholeiites were sampled at the base of the northern margin, and fresh, Ti-rich ferrobasalts and ultramafics were recovered from a deep valley in the western section. Pyroxenites, high-Mg basalt and a lherzolite, chemically related to the ferrobasalts in the western section of the trough, include schistose fragments and visibly tectonized ultramafics. High-grade metamorphic features observed in these specimens suggest that a significant component of shear accompanies crustal accretion in the Sorol Trough. Fresh pillow-lava fragments from the axial rift and weathered pillows from the eastern margin of the Ayu Trough have compositions similar to Mid-Ocean Ridge basalts (MORBs) and indicate that the Ayu Trough formed through crustal accretion processes. Additionally, an intriguing assemblage of meta-basalts, meta-gabbros and silicic plutonic rocks were recovered from a shallow scarp along the western margin of the Ayu Trough. The chemical characteristics of the meta-basalts and meta-gabbros suggest that they may be related through crystal fractionation. The chemistry of the silicic plutonics, however, indicates that they may not be derived by fractionation of primary magmas with chemical compositions similar to those of the metamorphosed oceanic crustal rocks. Alternatively, the silicic plutonics may represent an assemblage of intrusive island-arc tholeiitic rocks which formed during the initial stages of magma genesis in the island arc that was rifted apart by seafloor spreading in the nascent Ayu Trough.

Heat flow measurements in the eastern Pacific now total over 800, a sufficient number to permit the analysis of their distribution within a wide range of age zones. Major results are that (1) the mid-ocean ridge crestal regions have lower heat flow than that expected from conductive cooling models of the lithospheric plate, (2) an increase in heat flow is observed some distance away from the crest in a zone which appears to delineate a transition from dominantly convective to conductive heat transfer, and (3) the heat flow in crust older than this transition zone closely approximates that predicted by conductive cooling models for oceanic lithospheric plates. The transition from low to high heat flow occurs when 150-200 m of sediment is deposited over basement, apparently preventing convective transfer of heat from the oceanic crust to seawater. Thus the general shape of the empirical heat flow versus age curve for the East Pacific Rise (EPR) agrees with that from the Galapagos Spreading Center, but the age of the transition zone occurs in 5- to 6-m.y.-old crust on the Galapagos Spreading Center, whereas it occurs in 10- to 15-m.y.-old sea floor on the EPR. Proximity to the equatorial high sedimentation regionmore » causes the deposition of a thick layer of sediment much more quickly on the Galapagos Spreading Center than on the East Pacific Rise. Quantitatively, the degree of heat transfer by convection appears to correlate inversely with the ratio of sediment thickness to topographic relief.« less

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.

A finite-difference numerical program has been constructed to model the thermal structure of a subduction zone, including the combined effects of frictional heating and the heat sink created by dehydration of the subducted oceanic crust. Frictional heating is assumed to begin at the surface and to decrease linearly to zero at the depth at which dehydration begins. Dewatering of the oceanic crust occurs in the 80-125 km depth range and removes approximately 50 cal/gm of oceanic crust dehydrated. In young ( 10 Kb shear stress (at the surface) before isotherms below the dehydration zone are elevated above the level predicted by conduction models. For lower stresses, the slab at $$\leq 150$$ km depth never becomes hotter than 600°...

Wireline logs from the Cajon Pass drillhole provide observations of crustal properties and processes near the San Andreas fault. Mineralogical profiles developed from geochemical logs are combined with a technique for calculating thermal conductivity from the volume fractions of component minerals. Variations in the equilibrium temperature gradient correlate with conductivity contrasts, and thus with lithology, rather than with fluid circulation. K, U, and Th contents calculated from gamma spectroscopy logs yield heat production values which range from 1.3 to 2.4 µW/m³, averaging 1.8 µW/m³ in the section from 500 to 1800 m. Evidence for depletion of U relative to Th raises the possibility of extensive fluid circulation in the past. However, discontinuity of depletion across faults suggests that the fluid regime responsible for U depletion predated San Andreas faulting.

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.

The layered structure of the oceanic crust is characterized by changes in geophysical gradients rather than by abrupt layer boundaries. Correlation of geophysical logs and cores recovered from DSDP Hole 504B provides some insight into the physical properties which control these gradient changes. Borehole televiewer logging in Hole 504B provides a continuous image of wellbore reflectivity into the oceanic crust, revealing detailed structures not apparent otherwise, due to the low percentage of core recovery. Physical characteristics of the crustal layers 2A, 2B and 2C such as the detailed sonic velocity and lithostratigraphic structure are obtained through analysis of the sonic, borehole televiewer and electrical resistivity logs. A prediction of bulk hydrated mineral content, consistent with comparison to the recovered material, suggests a change in the nature of the alteration with depth. Data from the sonic, borehole televiewer, electrical resistivity and other porosity-sensitive logs are used to calculate the variation of porosity in the crustal layers 2A, 2B and 2C. Several of the well logs which are sensitive to the presence of fractures and open porosity in the formation indicate many zones of intense fracturing. Interpretation of these observations suggests that there may be a fundamental pattern of cooling-induced structure in the oceanic crust.

In-situ deep and shallow measurements of electrical resistivity recorded with the Dual Laterolog in the basement of the Cajon Pass scientific drillhole are compared with laboratory measurements of resistivity and porosity on fracture-free core samples. This comparison is the basis of a simple electrical model of the pore space of the rock from which both matrix and fracture porosity are derived. In parallel, the analysis of borehole electrical images obtained with the Formation Microscanner provides a means to identify the fractures that intersect the borehole wall, to evaluate their frequency, and to determine their orientation. Further processing of these images enables us to distinguish mineralized from fluid-filled fractures. The comparison of laterolog-derived fracture porosity, computed over a large volume of rock, with a pair of fracture intensity profiles obtained from the analysis of the two images of the borehole wall is the key of a new technique by which drillhole-related fractures (either due to hydraulic fracturing or to stress-induced borehole breakouts) and natural fractures (either open or mineralized) are discriminated. The fractures in the upper part of Cajon Pass basement are inferred from the analysis of in-situ resistivity measurements as mostly steep and mineralized.

Summary The axis of the Galapagos spreading centre has a high but variable heat flow. A distinct local minimum in the heat-flow field exists in a region 4–6 My old on the northern flank of the spreading centre. This minimum is unlikely to have been produced from refraction of heat flux due to either topographic and sediment thickness irregularities or to the transient effect of sediment loading. We suggest that the oceanic crust has an ‘effective’ permeability to fluid flow and that hydrothermal circulation causes much of the heat loss to arise through mass transport. This mechanism also lowers the conductive heat flow from beneath the sediments causing a regional minimum in the observed heat flux. We consider two different models for the control of the ‘effective’ permeability. The first invokes chemical reactions to open and close the cracks which give rise to the ‘effective’ permeability. In the second, no reactions are invoked and the oceanic crust never completely loses its permeability. The latter model implies that in areas of rough topography with outcropping basement highs the regional heat flow cannot be determined solely by conductive measurements.

In situ lithostratigraphy and the distribution of fracture and void zones in oceanic layer 2A were examined with an ultrasonic borehole televiewer at IPOD sites 501 and 504B on the south flank of the Costa Rica Rift. These records indicate a decrease in the size of basalt pillows with depth and a corresponding increase in fracture and void density. The observed increase in P-wave velocity with depth in this hole, as layer 2A merges into 2B, is best explained by an increase in the degree to which clays and zeolites fill fractures and voids. This also explains the order-of-magnitude drop in permeability observed in hole 504B. An increase in alteration-filling with age could explain the transition from convective to conductive heat flow and the correlative disappearance of layer 2A on the flanks of oceanic spreading centres.

Hole 504B provides a reference section for correlation of in situ petrological and geophysical studies with regional crustal models based on dredged samples and remote surveys; and, at least for the medium-spreading-rate Costa Rica Rift, Hole 504B confirms that the main features of the upper part of an idealized ophiolite sequence occurs in the oceanic crust.

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.

Twenty-four heat flow measurements are clustered in 5–20 m.y. and 60–80 m.y. old seafloor on the crest and northern flank of the Pacific-Antarctic Ridge. The crestal heat flow stations are characterized by (1) low mean heat flow relative to that predicted by theoretical models, (2) thin sediment cover, and (3) high ratio of standard deviation to mean heat flow, all of which indicate a system dominated by convective heat transfer. The measurements made on older seafloor of the northern flank have (1) mean heat flow equal to the theoretical predictions of conductively-cooling lithospheric plate models, (2) thick sediment cover, and (3) low ratio of standard deviation to mean heat flow. Thus convective loss associated with hydrothermal circulation is not considered to be important in 60–80 m.y. old seafloor on the Pacific-Antarctic Ridge. The pattern of heat flow on this ridge is thus similar to that in the Atlantic, Indian and Pacific Oceans: hydrothermal circulation is dominant on the ridge crest but is suppressed on the flanks, possibly due to a difference in the hydraulic admittance of the sediment between the two regions.

Gabbro, gabbroic anorthosite, basaltic breccia, diabase and basalt samples were dredged from a fracture zone of the Galapagos Rise, a fossil spreading center on the Nazca plate. Serpentinite was dredged from the Ecuador fracture zone of the Galapagos Spreading Center. Although the rocks are variably altered, minerals and chemistry of gabbroic rocks indicate that extensive fractionation occurred throughout the suite. Basaltic breccias have textures indicating prefracture zone brecciation.

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.

Measurements of temperature gradient and excess pore pressure in the surficial sediment of a fault block basin in the Guatemala Basin correlate with sediment thickness. The temperature gradient is smaller and the excess pore pressure gradient is more negative in areas of thinner sediment. This correlation is explained by postulating downward pore water advection within the sediments, with flow velocities on the order of 10−9 to 10−8 m/s in the thinnest sediments and much less flow in the thickest sediments. Sediment physical properties and pore water chemistry also support this interpretation. Since the conductive heat flow of the basin as a whole is less than one third that predicted by sea floor spreading models, the oceanic basement may be the site of a vigorous hydrothermal circulation system. The pore water advection in the sediments may be driven by this larger scale circulation.

One of the major problems in the use of heat flow data in a quantitative manner has been the variability of closely spaced measurements. It is suggested that this variability is directly related to hydrothermal circulation in an ''effectively permeable'' oceanic crust. As a consequence, only where this crust is sealed from the seawater by an extensive and thick layer of impermeable sediment can reliable estimates be made of the flux at depth. Heat flow data from such areas are analyzed. It is shown that the scatter in the data is low and that in most areas the mean heat flow through a province of known age is close to that predicted by plate models which account for the increase in depth with increasing age of the ocean floor. From this analysis it is argued that these heat flow means are a reliable estimate of the flux at depth. In order to be able to use heat flow measurements to investigate the age of the crust, the thickness of the lithospheric plate, upper mantle convection, and local anomalies, it is necessary to substantiate this analysis and to find an adequate explanation of the low mean heat flow observed at themore » center of the sediment bulge in the equatorial Pacific.« less

Estimates of elemental and mineralogical abundances from geochemical logs are compared to preliminary chemical and modal analyses from cores in the Cajon Pass Scientific Drillhole. Accuracies of log-computed weight percent oxide and mineralogy determinations range from 10 to 30%.

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...

Rare earths, Fe and Ti concentrations in basalts dredged along the Galapagos spreading centre reveal that a juvenile mantle plume is welling up beneath the Galapagos Islands and mixing with the depleted asthenosphere to the north-east and north-west beneath the ridge. Unusually extensive shallow-depth, crystal fractionation is also apparent on the ridge between 85 and 87°W. The ‘magnetic telechemistry’ hypothesis is further clarified.

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.

In the absence of convincing magnetic anomaly information, topographic profiles have been used to infer the tectonic history of the East Pacific Rise between 5°S and 20°S. Profiles projected at right angles to the rise crest show a sharp drop in elevation at roughly the same distance on either side of the crest. Profiles to the east of the rise also show a second topographic high at 95°W. Comparison of these profiles with empirical depth versus age curves for the North Pacific suggests that this rise, the Galapagos Rise, is the fossil East Pacific Rise which terminated close to 6 mybp by the spreading center jumping 900 km to the west. The extreme youthfulness of the present East Pacific Rise, the step structure of its flanks, and the similarity in age of the top of this step and the crest of the Galapagos Rise substantiate this interpretation. This jump coincided with a similar readjustment involving the Mathematicians ridge at 5° to 20°N and the opening of the Gulf of California. This wholesale realignment of the active spreading centers in the eastern Pacific from 20°N to 20°S may have arisen because of the break-up of the Farallon Plate into several smaller plates and/or the complex interaction of the East Pacific Rise with the North American continent.

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.

The procedure of lowering geophysical instruments down boreholes on a wire has, over the past 60 years, been a useful aid in defining the physical nature of the rock formations through which the borehole passes. Current developments, however, herald the advent of the use of such techniques for the measurement of geochemical concentrations in a quantitative way. This opens the door to a whole new series of applications in a wide range of geological disciplines.

The Bauer Deep, in the south-eastern equatorial Pacific, is the only deep ocean basin so far known to be characterised by metalliferous sediments throughout the depositional column. The basin was formed when the eastern Pacific spreading centre jumped from the now fossil Galapagos Rise to the newly formed East Pacific Rise. The distinctly bimodal distribution of heat flow in the Bauer Deep indicates that precipitation from hydrothermal fluids emanating from the oceanic crust may be the mechanism for metallogenesis in the Bauer Deep. The hydrothermal circulation may have been initiated by the emplacement, approximately ~ 6 Myr BP, of a heat source associated with the ridge jump into that vicinity.

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).

The southern Appalachian ultradeep core hole (ADCOH) is designed to test modern ideas about the formation of mountain chains along the edges of continents. The faulting, stratigraphy, lithology and out-crop patterns observed during detailed studies of the surface geology in this region have been interpreted in terms of a series of thrust sheets of diverse ages, which were transported northwestward (Fig. 1) over the Iapetan continental margin of North America (Hatcher 1978). Seismic reflection data and interpretations by Clark and others (1978), Cook and others (1979, 1983) and Harris and Bayer (1979) support this interpretation, and reveal a zone of high reflectivity which dips to the southeast beneath portions of the Blue Ridge and Piedmont physiographic provinces. This zone of reflections is correlative with a similar interval known in the sedimentary rocks of the Appalachian foreland some 150 km northwest of the ADCOH study area, and has been interpreted as a regional decollement, serving as the root zone for major thrust faults such as the Brevard.

The principal goal of the Appalachian Ultradeep Core Hole (ADCOH) Project is to study the processes related to the formation and reactivation of large faults in the internal parts of an intact composite crystalline thrust sheet formed by continent-continent collision along an ancient continental margin. Composite crystalline thrust sheets are some of the largest structures in orogenic belts, comparable in size to the large accreted terranes. They are therefore of central importance in the overall processes related to formation of mountain chains and the evolution of continental crust.

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.

Most geologic studies of orogenic terranes are primarily two-dimensional, with some dependence upon down-plunge and down-dip projections of structures. Potential field and seismic reflection/refraction studies provide some information aobut the third dimension, but these data may be interpreted in a variety of ways without direct calibration using drilling. Drilling is therefore a necessary tool to provide the ultimate test of a scientific hypothesis regarding crustal evolution and structure. A hole may be drilled as an attempt to solve a problem or, more commonly, to prove existence of some potentially economic deposit. If we totally accepted the interpretations based upon structural crosssections, seismic refraction and reflection studies, potential field studies, or studies which combine results from these and other techniques, it would never be necessary to drill except to recover economic minerals or hydrocarbons. However, the results of these types of analysis are generally not sufficiently precise to achieve to unique resolution of crustal structure and reduce the possibilities for alternative interpretations.

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

One of the most complex problems in earth sciences is that of the determination of fluid flow behavior such as flow pathways, flow velocity and/or hydrologic conductivity in a heterogeneous subsurface. Information on fluid flow behavior (both present and predicted) is needed for the determination of the size and shape of groundwater recharge areas, the assessment of contamination impact of spills and plumes and the planning and assessment of remediation efforts. For all these problems it is recognized that a good knowledge of the 3D values of hydrologic conductivity is essential, yet most often the only knowledge available on the hydrologic conductivity is that deduced from a number of sampling wells and pumping tests experiments. These experiments only give a (poor) bulk approximation of the hydrologic conductivity, and consequently the predictions of fluid flow behavior based on these approximations are more often than not unsuccessful.