Slow Spreading Rates Research Articles

Ophiolites and the oceanic crust: New evidence from the Tyrrhenian sea and the Western Alps

The succession recovered in ODP hole 107–651 in the young oceanic Vavilov basin (Tyrrhenian Sea) comprises, beneath a thick Pleistocene to Upper Pliocene sedimentary cover (chiefly volcanoclastics), four basement units: (1) MORB-type basaltic pillows and breccias; (2) a complex succession made of dolerites, albitites, basaltic breccias, metadolerite pebbles (including an intercalated sandy layer with periodotite clasts); (3) MORB-type basaltic pillows and breccias; (4) highly serpentinized peridotite. Between units 3 and 4, granitoid pebbles occur. This sequence is surprisingly similar to successions known in the Western Alps' Tethyan ophiolites. There, the sediments (Callovian-Oxfordian radiolarian cherts) lie stratigraphically upon breccias mostly derived from underlying serpentinite, and sometimes gabbroic basement. At some places, thin basaltic (tholeiitic) pillows and breccias occur between the radiolarian cherts and the breccias. From the comparison between a present day setting (the central Tyrrhenian Sea) and a formerly emplaced basement succession (the Western Alps), we stress the following (a) both the here-discussed ophiolites and oceanic basement are different from classical ophiolite sequences; (b) both occurrences imply unroofing of mantle rocks that therefore were directly outcropping on the seafloor; (c) such a comparison may indicate a very slow spreading rate for the Alpine Tethyan ocean.

Spreading rate dependence of three-dimensional structure in oceanic spreading centres

SEGMENTATION by transform faults and other types of along-axis discontinuity1 is a well described but poorly explained characteristic of oceanic spreading centres. Here we use numerical experiments to explore the dynamics of mantle flow and melting beneath a mid-ocean ridge. Buoyant upwelling, driven by compositional density variations resulting from the extraction of the melt that forms the ocean crust, exhibits a spreading-rate-dependent transition between two-dimensional and three-dimensional upwelling structures. For low spreading rates and mantle viscosities an initial two-dimensional structure transforms into a three-dimensional one; at high spreading rates, an initially two-dimensional structure remains two-dimensional. These results suggest that the origin of spreading-centre segmentation may be different at fast and slow spreading rates.

A nonlinear rheology model for mid‐ocean ridge axis topography

This paper describes a mechanical model of the mid‐ocean ridge axis based on a nonlinear rheology. The oceanic lithosphere is modeled as a strongly temperature‐dependent “power law” material with the Byerlee's friction law describing the brittle behavior of the upper oceanic lithosphere. We examined two simple models. A mantle‐only model predicted an axial rift valley 1–2 km deep and 10–15 km wide at slow half‐rates of 5–25 mm/yr which is in fair agreement with observations. However, the rift valley did not completely vanish at fast spreading rates even though its width and relief became much smaller. Thus, the mantle‐only model failed to explain the observed transition in axial topography with spreading rate. By including a uniform thickness oceanic crust with theological properties different from the mantle, our second model produced the first order variation of axial topography with spreading rate: a pronounced rift valley with 1–2 km relief at slow rates, a smaller relief (<500 m) rift valley at intermediate rates, and no rift valley at fast rates. There are two additional results of the nonlinear rheology model which are distinctively different from the previous uniform viscosity mantle models. First, horizontal velocity at the seafloor changes sharply in a very narrow zone at the ridge axis, i.e., the stresses are concentrated in a very narrow zone for the nonlinear rheology. This conclusion is remarkably independent of the boundary conditions at the seafloor. Second, the computed width of the axial plastic failure zone decreases with spreading rate, a result very different from that of a uniform viscosity model. This is caused by the strong temperature dependence of the power law rheology and the fact that the temperature of the oceanic lithosphere depends greatly on the spreading rate. Beyond some distance from the ridge axis, the brittle plate (roughly the region above the 750°C isotherm) deforms very little; both the horizontal strain rate ė11 and the maximum shear strain rate ėmax are less than 10−17 s−1 which is in good agreement with the 10−18 s−1 average for oceanic brittle plate deformation inferred from seismic moment data. Within the ductile region (area beneath the 750°C isotherm) the strain rates are in the range of 10−14–10−16 s−1, except within a narrow band near the ridge axis where higher strain rates are located. We found that the thermal structure of the oceanic lithosphere is not very sensitive to the details of the mantle flow pattern (or the rheology) but is very sensitive to the cooling by hydrothermal circulation near the ridge axis. Axial structure is also influenced by factors other than the spreading rate. Within the range of slow and intermediate rates, our calculations show that the size of an axial rift valley decreases with an increase in either the thickness of the oceanic crust or the mantle temperature; in other words, a thinner crust or a cooler mantle temperature will produce a more pronounced rift valley. In the vicinity of hotspots, the absence of an axial rift valley even at slow spreading rates, such as the Reykjanes Ridge, can be well explained by the anomalously thicker crust and higher mantle temperatures. In contrast, at the Australian‐Antarctic Discordance a deeper than normal section of an intermediate spreading ridge has very rough topography not present on the adjacent sections of the ridge; this can be explained by our model as due to an anomalously low mantle temperature as has been suggested or perhaps also due to a thinner oceanic crust

Introduction to Special Section on Logging and Downhole Measurements in Deep Sea Drilling Project/Ocean Drilling Program Deep Crustal Holes

One of the most important achievements during the first two years of the Ocean Drilling Program (ODP) was the concerted effort to conduct major programs of logging and downhole measurements upon revisiting three of the deepest holes drilled into the oceanic crust by the Deep Sea Drilling Project (DSDP). This effort established holes 395A, 418A, and 504B (Table 1) as our best reference sites to “ground truth” the structure and physical properties of the upper oceanic crust. The results of logging and downhole measurements at these and other ODP holes and their relationship to regional geophysical studies were the subject of a poster symposium at the AGU 1988 Fall Meeting, from which this special section developed.Although it was not the intent to restrict this special section to results from holes 395A, 418A, and 504B, all of the papers here deal with at least one of these holes. This emphasizes their importance as reference sites, which arises not only because of the extensive downhole measurements but simply because they are among the four deepest DSDP/ODP penetrations into genuine upper oceanic crust (i.e., crust formed at a mid‐ocean ridge; Figure 1). Holes 395A and 418A were drilled in North Atlantic crust formed at a slow spreading rate, and 504B was drilled in eastern Pacific crust formed at a moderate spreading rate. Thus the set of reference sites lacks deep holes in crust formed at fast spreading rates, which is a serious shortcoming that will hopefully be addressed by upcoming ODP drilling in the Pacific Ocean. (The set of deep reference holes has already been complemented by the drilling and logging success in 500 m of tectonically exposed gabbros in hole 735B during leg 118 to the southwest Indian Ocean in 1987 [Robinson et al., 1989]; however, interpretation of the logging results from hole 735B had not progressed enough to be included here and will appear first in the work by Robinson et al. [1990].)

A quantitative study of the axial topography of the Mid‐Atlantic Ridge

Some basic characteristics of ridge axis topography are related to spreading rate and distance from neighboring transform faults. For example, the presence of an axial depression coincides in most cases with slow spreading rates, and the overall depth of the ridge axis increases toward ridge‐transform intersections (RTIs). On the other hand, it is also well known that the relief and width of the axial valley on, say, the Mid‐Atlantic Ridge (MAR) vary along strike in an unpredictable manner. The purpose of the present study is to quantify how much of the observed variation in the first‐order topography at the axis is related to changes in other parameters, such as spreading rate and distance from RTIs. To carry out this test, the zero‐age depth and the relief and width of the axial valley have been estimated on 46 profiles that cross the axis of the MAR between the equator and 50°N (full spreading rates 22–36 km/Ma). Zero‐age depth is here defined to be the depth at age zero of the best fit thermal subsidence trend. Axial valley relief and width have been measured with respect to the ridge flanks by the least squares fit of a Gaussian bell. The measured axial valley relief varies between 600 and 2100 m (average ∼1300 m), while the valley width varies between 16 and 62 km (average ∼35 km). The correlation between zero‐age depth, axial valley relief and width, latitude of axial crossing, spreading rate, distance from nearest RTT, and offset on the nearby transforms has been investigated using linear regression techniques. The main results of the present study are that (1) zero‐age depth significantly correlates with latitude of crossing, distance from nearest RTI, and offset on the nearby transforms; and (2) the variation in axial valley relief and width is essentially uncorrelated with spreading rate, zero‐age depth, distance from nearest RTT, and offset on nearby transforms. The preferred explanation for the observed spatial variation in axial valley geometry is that it reflects a temporal variation. In fact, if the rough abyssal hill topography typical of the MAR flanks is created within the axial valley, the shape of the axial valley cannot be steady state (although the existence of an axial valley may be a steady state phenomenon). This hypothesis is supported by the observation that the variability in axial valley relief is similar to the overall amplitude of abyssal hill topography, measured as the residual on the thermal subsidence trend.

Structure of an atypical ophiolite: The Trinity complex, eastern Klamath Mountains, California

Research Article| June 01, 1989 Structure of an atypical ophiolite: The Trinity complex, eastern Klamath Mountains, California F. BOUDIER; F. BOUDIER 1Laboratoire de Tectonophysique, Université des Sciences et Techniques du Languedoc Montpellier, Place Eugène Bataillon, 34060 Montpellier, France Search for other works by this author on: GSW Google Scholar E. LE SUEUR; E. LE SUEUR 1Laboratoire de Tectonophysique, Université des Sciences et Techniques du Languedoc Montpellier, Place Eugène Bataillon, 34060 Montpellier, France Search for other works by this author on: GSW Google Scholar A. NICOLAS A. NICOLAS 1Laboratoire de Tectonophysique, Université des Sciences et Techniques du Languedoc Montpellier, Place Eugène Bataillon, 34060 Montpellier, France Search for other works by this author on: GSW Google Scholar GSA Bulletin (1989) 101 (6): 820–833. https://doi.org/10.1130/0016-7606(1989)101<0820:SOAAOT>2.3.CO;2 Article history first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation F. BOUDIER, E. LE SUEUR, A. NICOLAS; Structure of an atypical ophiolite: The Trinity complex, eastern Klamath Mountains, California. GSA Bulletin 1989;; 101 (6): 820–833. doi: https://doi.org/10.1130/0016-7606(1989)101<0820:SOAAOT>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract The Trinity complex consists of an ophiolite section displaying some distinctive characteristics. The mantle section is represented mainly by a relatively fertile plagioclase Iherzolite, and the crustal section is rather thin owing to the limited extent of the mafic cumulates, which may explain the occurrence of hydrothermal reactions as far down as the uppermost peridotites. The high-temperature flow plane in the peridotites is vertical, and the flow line horizontal, parallel to the ridge orientation as defined by the sheeted dikes. Similar attitudes have been encountered elsewhere in other plagioclase Iherzolite bodies. This particular type of lithosphere may result from a small degree of partial melting associated with mantle diapirism. The ophiolite may have been formed at a spreading center characterized by a slow spreading rate. The Trinity complex represents a fragment of Ordovician oceanic lithosphere possibly trapped in a marginal-sea environment during the Devonian, just before its accretion by northwestward thrusting onto the Central Metamorphic belt. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Shear wave constraints on crustal structure of the pole abyssal plain

A study of the crustal structure in the Pole Abyssal Plain of the Arctic Ocean was carried out using P-waves constrained with converted shear waves. The data, recorded with a single ocean bottom seismometer (O.B.S.) on three channels, were modelled for travel time and amplitude variations with WKBJ synthetic seismograms. The study confirms that converted shear waves can be usefully employed to place limits on P-wave data. Shear wave velocities of sediments ranging from 0.3 km/s on the sea bed to 0.9 km/s at the sediment-basement interface with corresponding P-wave velocities of 1.6 to 2.1 km/s were obtained. The resulting Poisson's ratios for the sediments varies between 0.48 and 0.39, and indicate a poorly consolidated sedimentary layer. Well determined P- and S-wave velocities from the PPP and PSP phases give a Poisson's ratio of 0.31 for the lower crust in agreement with results from other studies. The models developed lead to the conclusion that there is considerable lateral heterogeneity in structure, and that the thickness of the crust (layers 2 and 3) under the Pole Abyssal Plain varies between 3.2 km and 4.1 km. The crust here is therefore much thinner than average oceanic crust, a thinning which may be related to the slow spreading rate at the Arctic Mid-Ocean Ridge.

Depth and age in the Cayman Trough

Caribbean plate boundaries are almost everywhere either strike-slip or convergent, so oceanic crust accreted at the spreading axis within the Cayman Trough provides information critical to determining the relative motions and tectonic interactions among the Caribbean and surrounding plates. The tectonic evolution of the Trough as inferred from magnetic anomaly identifications is not entirely compatible with either its crustal subsidence or the geological evidence for time of opening and rate of offset movement along the North American-Caribbean plate boundary. A close examination of basement topography and depth within the Trough suggests that the total opening of the Cayman Trough is about 1100 km and that this opening occurred in two stages. Crust generated by the present, ongoing stage of very slow spreading has extensive relief and shows a distinct increase of crustal depth with increasing distance from the spreading center. Crust generated by an earlier episode of faster spreading is characterized by reduced topographic relief and no appreciable differential subsidence, and is separated from the younger crust by distinct topographic breaks. The rate of spreading of the current slow spreading episode is15 ± 5mm/yr, based upon a new crustal subsidence curve which accommodates lateral as well as vertical cooling of the accreting slab. The Trough has opened at this rate for about 30 Ma. The rate of opening prior to 30 Ma cannot be determined with any reliability from crustal subsidence. A reexamination of magnetic anomalies in the central part of the Trough shows that these anomalies can be generated at a much slower spreading rate than previously thought. This revised interpretation of Cayman Trough evolution brings magnetic, subsidence and geological evidence into much better agreement.

A possible explanation of gravity anomalies over mid‐ocean ridges

Gravity fields over the crests of slow spreading mid‐ocean ridges are characterized by small amplitude free air gravity anomaly lows (30–70 mGal) with wavelengths of 40–60 km, flanked by smaller gravity highs. In general, the amplitude and wavelength of these gravity lows decrease with increasing spreading rate until, at fast spreading ridges (&gt; 5 cm/yr), free air gravity highs (10–20 mGal) are observed. Previous explanations of these anomalies, which involve uncompensated ridges, thermally expanded ridges and elastic plate compensation models, are generally not consistent with the geological evidence regarding the nature of the crust and upper mantle beneath the ridge crest. In particular, the postulated presence of wide zones of partial melt in the uppermost mantle is inconsistent with petrologic data from mid‐ocean ridge basalts, which indicates that primary melts are in equilibrium with and extracted from residual mantle at depths of 30 km or more. In addition, similar gravity lows observed over extinct spreading ridges suggest that the anomalies are not mainly due to any dynamic aspect of the accretion process taking place at active spreading centers. Geometrical models of mid‐ocean ridges derived from detailed structural and petrologic studies of the Bay of Islands ophiolite complex have been used to generate gravity anomalies over ridges with various spreading rates. Modeling results show that four main sources serve to define the amplitude and wavelength of the gravity anomaly: the ridge topography, the crustal level magma chamber, the configuration of the isotherms near the ridge crest, and a low‐density, partially gabbroic cumulate root extending vertically into the mantle beneath the ridge. Systematic variations in the contributions from these sources with increasing spreading rate control the changes in character of the gravity anomaly from slow to fast spreading centers. Major contributions are found to come from the topography and from the lowdensity root, which broadens with slower spreading rates, resulting in both increasing amplitude and increasing wavelength with decreasing spreading rate. At extinct spreading centers, contributions from the magma chamber, isotherms, and topography gradually diminish with time once the ridge ceases to spread, whereas the low‐density root remains a permanent part of the oceanic lithosphere and continues its contribution to the gravity field. Seismic evidence from slow spreading centers appears to support the existence of the lowdensity gabbroic root zone inferred from the study of the Bay of Islands ophiolite and the gravity analysis presented here.

Harzburgite and lherzolite subtypes in ophiolitic and oceanic environments

In most ophiolites the ultramafic section is harzburgitic. It is rarely composed of residual lherzolites, except for the local occurrence of impregnation lherzolites within harzburgitic massifs. Several characters (environmental formations, thickness and composition of the crust, geometry of the asthenospheric flow, serpentinization) validate the distinction between an harzburgite and a lherzolite ophiolitic subtype. The harzburgite subtype can be derived from any oceanic spreading center, provided the rate is larger than 1 cm/yr. The lherzolite subtype would correspond to situations (vicinity of transform fault, very slow spreading rates) where the lithospheric front penetrates at 20–30 km into the mantle below the spreading center. These lherzolitic massifs are best explained as being derived from slow-spreading rifts. Finally the particular asthenospheric flow discovered in these massifs is discussed.

Rough-smooth basement boundary in the western North Atlantic basin: Evidence for a seafloor-spreading origin

A distinctive change in the character of acoustic basement has been mapped within the Mesozoic (Keathley) magnetic anomaly sequence in the western North Atlantic basin. This boundary, known as the rough-smooth basement boundary, separates relatively smooth acoustic basement on the older (western) side of the boundary from much rougher basement to the east. South of Bermuda, the boundary is well defined and nearly isochronous, corresponding to anomaly M-12 or M-13 (∼128 m.y. B.P.). North of Bermuda the boundary is diachronous and more transitional, with the change in basement relief occurring in a zone between anomaly M-21 and M-16–age crust (132 to 142 m.y. B.P.). Both the well-defined and transitional changes in basement relief found in the western basin are also identifiable in seismic-reflection records from the eastern (Canary) basin, and an analysis of magnetic-anomaly patterns in the central North Atlantic basin indicates that these changes occurred simultaneously on both sides of the ridge. These results strongly suggest a primary seafloor-spreading origin for the rough-smooth basement boundary, with the greater basement relief associated with slower spreading rates, changing directions of relative plate motion, and ridge jumps of isolated accretionary segments beginning in Late Jurassic–Early Cretaceous time.

Crustal processes at spreading centers

Recent discoveries and research on mid‐ocean ridges (MOR's) in the last four years have greatly increased our knowledge of this fundamental type of plate boundary. The last IUGG review of MOR research was in 1975 [Aumento et al., 1975], and its omission as a topic in 1978 may have reflected a feeling that spreading centers were reasonably well understood. Since that time however, there has been a surfeit of important new observations and exciting research, and there is now a turmoil of innovative ideas about the processes which create and leave their imprint on two‐thirds of the earth's crust.New seismic measurements have been made and interpreted as evidence for the existence of shallow axial magma chambers (AMC) beneath intermediate (total opening rate 5–9 cm/yr) and fast (9–18 cm/yr) spreading centers (Fig. 1). Yet there is controversy as to whether they are steady state, whether they persist beneath slow ridges as well in the steady state, what the dimensions and shape of the chamber are, its relation to oceanic crustal stratigraphy, and its relation to observations in ophiolites. “Black smoker” hydrothermal vents have been discovered in the Pacific, spewing forth 350°C fluids blackened by polymetallic sulfide precipitates. This discovery is embroiled in debate as to how long the vents last, whether they occur at slow spreading rates as well, whether the enormous heat flux of the vents freeze the magma chamber even at fast spreading rates, their influence on geochemical balances in the ocean, and the value of the sulfide deposits commercially. It has been found that our initial view of MOR'S as two dimensional linear structures has served us well as an initial working model, but must be significantly modified to account for variations in tectonic, volcanic and hydrothermal activity along strike as well as for episodicity of these processes. Deep Sea Drilling Project (DSDP) samples have raised fundamental questions about formation of the magnetic source layer, hydrothermal circulation, and fault mechanisms along the MOR. Spreading centers which penetrate existing lithosphere along strike as “propagating rifts” have modified our views of how plate boundaries reorient themselves and how V‐shaped magnetic and topographic trends are formed. We have also found that transform faults may modify spreading center processes for some distance away from their intersections with MOR's, and that “rigid plate” transform faults may fail to develop at fast spreading centers under certain conditions, resulting in en echelon overlapping of spreading centers.This review will focus on these developments and several others. Only example references to these topics will be given in the text, and many important topics will not be discussed directly (e.g. petrology and geochemistry, mantle convection, research on lithosphere older than 1 m.y.). However, I have tried to compile and have included an extensive bibliography for U. S. research on a broader range of MOR topics for mid‐1978 to mid‐1982, which is the main purpose of this report. Pre‐1978 references are occasionally included where appropriate, abstracts are not included.

Establishment of the European Rabbit Flea on Macquarie Island.

Since December 1968, 241 600 rabbit fleas (Spilopsyllus cuniculi (Dale)) have been released on Macquarie Island (between Australia and Antarctica) as a vector of myxomatosis for the biological control of rabbits. The flea is now established but is unevenly distributed due to its low rate of reproduction and slow rate of spread, especially into small isolated pockets of rabbits. The number of rabbits infested was greatest following the rabbit breeding season. Levels of infestation on individual rabbits were low; of 5135 rabbits shot in 3 years, only 21 carried more than 100 rabbit fleas. Limits to the spread and survival of the fleas on the island are discussed.

The Nansen ridge, Arctic Ocean; some geophysical observations of the rift valley at slow spreading rate

We have examined continous echo soundings across the Nansen Rift Valley and analysed the first gravity and bathymetric data available across the northern flank of the Nansen Ridge for comparison with other parts of the mid-ocean ridge system. The observed width of the inner floor (1.7–5.5 km) at a total opening rate of 1.5 cm/yr supports the idea of progressive narrowing of the inner floor with decreasing spreading rate for slow spreading ridges. However, the observed distance between the outer walls (18–22 km) or rift valley width (30–33 km) suggests that these parameters at least to a first approximation tend towards a minimum value with decreasing spreading rate. The relation between gravity and bathymetry along the drift track of ice station FRAM-I tentatively suggests that the sea-floor topography of this part of the Nansen Ridge is supported by a plate with an elastic thickness less than what is observed on the Mid-Atlantic Ridge.

Crustal structure near the Arctic Mid‐Ocean Ridge

Seismic reflection and crustal refraction studies were carried out at the FRAM I ice station near the Arctic Mid‐Ocean Ridge crest. This ridge is spreading at the very slow rate of 5 mm yr−1. Upper mantle Pn arrivals with apparent velocities averaging 7.9 km/s are observed at distance ranges less than 15 km, with corresponding intercept times of typically 1.1 s of which 0.5 s can be attributed to the travel path through a sedimentary layer. Anisotropy of about 6% appears to be present. These early Pn arrivals are observed on seven of the eight refraction lines studied, but crustal velocities are not well constrained. Modeling was done by computing travel time curves for a number of possible velocity structures for comparison with the data and suggests a crustal thickness between 2 and 3 km. This thin crust is associated with low‐amplitude magnetic anomalies. The remaining refraction line was shot within a region of enhanced magnetic anomaly amplitudes the Yermak H zone, where the data interpretation indicates a crustal thickness of about 8 km. This thicker crust and associated strong magnetic anomalies may be due to the ‘Yermak hot spot,’ a region of high magmatic activity. The generally thin crust is probably due to low basaltic productivity at the ridge crest, as a result of increased cooling at the very slow spreading rate.

Identification des anomalies magnétiques sur les dorsales à faible taux d'expansion: Méthode des taux fictifs

The fit between calculated and observed magnetic anomalies from slow-spreading centers is improved when allowing for a transition zone between two inversely magnetized blocks. In this paper it is shown that these models are very easily computed by choosing a fictitious spreading rate which is slower than the real spreading rate and by changing the distance scale appropriately. With these slow fictitious spreading rates, the models are very sensitive to the relative position of the successive inversions and could be used to adjust these positions in the magnetic time scales.

Crustal Processes of the Mid-Ocean Ridge

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

New geophysical evidence for sea-floor spreading in central Baffin Bay

Geophysical data collected during a detailed survey in Baffin Bay have shown that lineated magnetic anomalies trending north-northwest occupy the deep central region. These anomalies exhibit maximum amplitudes of about 300 nT and can be modelled by a 1-km thick magnetic source layer divided into blocks of normal and reversed polarity. The magnetizations required are comparable with those of oceanic basalts. A striking feature of the gravity field is a 20 mGal gravity low, about 20 km wide, which runs through the centre of the bay with approximately the same trend as the magnetic lineations. The gravity low is associated with a change in crustal structure measured from seismic refraction data and sometimes with a deepening of the sediment-basement interface, reminiscent of a median valley. These results suggest that the magnetic anomalies were produced by sea-floor spreading and that the gravity low marks an extinct spreading centre in Baffin Bay. Comparisons of the magnetic anomaly profiles with a model profile computed for magnetic anomalies 13–24 (38 to 60 Ma), show good correlation between the observed and computed anomalies in the time period represented by anomalies 13–21, with slow spreading rates of about 0.3–0.4 cm yr−1 perpendicular to the spreading axis. These results are in reasonable agreement with magnetic anomaly identifications and spreading rates deduced from geophysical data in the Labrador Sea. The direction of plate motion in Baffin Bay is not well defined from the data, but the Labrador Sea data require plate motions at a highly oblique angle to the spreading centre in the bay. Peculiarities of the postulated spreading centre, including the change in crustal structure beneath the gravity low along its strike from south to north, and the decrease in coherence and amplitude of the magnetic anomalies immediately north of the survey area, may be the result of these very low spreading rates, oblique spreading and changes in spreading direction, or the proximity of this area to the junction with a possible major transform fault through the Nares Strait.

Necking of the lithosphere and the mechanics of slowly accreting plate boundaries

Data on ridges with slow spreading rates (1–3 cm/yr), obtained through detailed studies with Deep‐Tow instruments and manned submersibles (French‐American Mid‐Ocean Undersea Study), or where the active structures are not submarine (Afar triangle) yield a precise picture of the axial region. There is evidence for a thinned lithosphere to be present at the axis with a thickness of 4–5 km. The thermal structure and composition of this solid layer can be estimated from seismic data and thermal and petrologic models. Extensional tectonics prevails in a belt some 10 km wide and is expressed at the surface by normal faults and fissures. In most previous mechanical models the existence of the lithosphere at the axis is neglected, and the rise of viscous asthenospheric material in a narrow vertical cleft beneath the axis is considered the main cause for the steady state presence of an axial valley and the development of normal faults. In contrast with these models we suggest here that these features depend on the rheological behavior of the lithosphere at the axis: the lithosphere is continually thinned by tectonic strain but also thickened by cooling at the base and by volcanism at the surface. In the steady state this process can be viewed as a succession of tectonic ‘neckings’ in the central active part of the axial valley (10 km) followed by doming of the lithosphere over the whole width of the axial valley (30 km), in response to isostatic disequilibrium. Creep is likely to control the process at depth and because of the high temperature and large strains will be of the steady state type. The application of experimental flow laws for material constituting layer 3 and the uppermost mantle to this problem, where both temperature profile and strain rate can be estimated, allows an order of magnitude of the ‘strength’ of the lithosphere at a given strain rate to be calculated. With this strength, isostatic recovery in response to vertical shear stresses will occur at a distance of about 8–15 km from the axis. The resulting ‘simple shear’ strain progressively ‘levels off’ the mean topographic slope until it becomes horizontal in the rift mountains. Whereas for rifted ridges or slowly accreting plate boundaries the behavior of the lithosphere controls the mechanics of the axial region, where only small discontinuous transient magma chambers exist, we suggest that in the case of nonrifted ridges the behavior of the asthenosphere is more important, with axial crust in isostatic equilibrium over a large continuous permanent magma chamber.

Tectonic evolution of the Cocos-Nazca spreading center

Magnetic and bathymetric data from the eastern Pacific have been analyzed and a model for the evolution of the Galapagos region developed. The Farallon plate appears to have broken apart along a pre-existing Pacific-Farallon fracture zone, possibly the Marquesas fracture zone, at about 25 m.y. B.P. to form the Cocos and Nazca plates. This break is marked on the Nazca plate topographically by the Grijalva scarp and magnetically by a rough-smooth boundary coincident with the scarp. The oldest Cocos-Nazca magnetic anomalies parallel this boundary, implying that the early Cocos-Nazca spreading center trended east-northeast. This system soon reorganized into an approximately east-west rise–north-south transform configuration, which has persisted until the present, and the Pacific-Cocos-Nazca triple junction has since migrated north from its original location near lat 5°S. If correct, the combination of these simple geometric constraints produced the “enigmatic” east-trending anomalies south of the Carnegie Ridge. The axes of the Cocos-Nazca spreading center and the Carnegie Ridge are essentially parallel; this can lead to paradoxical conclusions about interpretation of the Cocos and Carnegie Ridges as hotspot tracks. Hey and others (1977) have shown that recent accretion on the Cocos-Nazca spreading center has been asymmetric, resulting at least in part from small discrete jumps of the rise axis. I show here that the geometric objections to both the “hot-spot” and “ancestral-ridge” hypotheses on the origin of the Cocos and Carnegie Ridges can be resolved with an asymmetric-accretion model. However, all forms of the ancestral-ridge hypothesis encounter more severe geometric difficulties, and these results support the hotspot hypothesis. After further elaboration of the hotspot hypothesis by Johnson and Lowrie (1972) and Hey and others (1973), Sclater and Klitgord (1973) examined both the hotspot and ancestral-ridge hypotheses and decided that both should be rejected, concluding that the Cocos and Carnegie Ridges “are not tectonically related” (p. 6973). The difficulty in the Galapagos area arises primarily because the older magnetic anomalies have proven extremely difficult to correlate, surprisingly so considering the high data density and the ease with which the very young anomalies are correlated. An important problem is the reason for this difficulty in correlating older anomalies — either clearly recognizable anomalies were never formed here, or some mechanism has acted in this area to destroy them after they were formed. On the basis of all available evidence, including new data presented here, I conclude that a model based on the hotspot hypothesis, with the modification of asymmetric accretion resulting at least in part from discrete jumps of the rise axis as discussed by Hey and others (1973) and demonstrated by Hey and others (1977), successfully meets the objection of Sclater and Klitgord (1973) and allows us to outline the history of the area from the break-up of the Farallon plate and birth of the Cocos-Nazca spreading center to the present. The “instantaneous” (a term Hey and others, 1977, have examined) configuration of plate boundaries and motions (Fig. 1) has generated a wedge of crust spread from the Cocos-Nazca spreading center, which is characterized by a slow spreading rate, rough topography, and strong magnetic anomalies. This wedge, termed the Galapagos gore by Holden and Dietz (1972) and discussed in detail by Hey and others (1977), is surrounded by crust spread from the Pacific-Cocos and Pacific-Nazca spreading centers which has the smooth morphology common to fast-spreading rises and low-amplitude magnetic anomalies, as both these segments of the East Pacific Rise are oriented nearly parallel to the Earth's magnetic field vector. My model explains the location and orientation of the magnetic and bathymetric rough-smooth boundaries that thus bound the gore and implies that there are two genetically different magnetic and bathymetric boundaries in the area.