Summary

All available ship and aeroplane tracks across the Indian Ocean were searched for identifiable magnetic anomalies and transform faults, and hence the age and direction of motion at the time of formation of about two-thirds of the floor of the ocean established. The magnetic lineations show that India moved away from Antarctica at about 18 cm/y for 20 My in the Early Tertiary. This rapid motion ceased in the Eocene and was followed by a period in which little or no spreading took place west of the Ninety East Ridge. Australia separated from Antarctica during this period. The present spreading episode began about 36 My ago. This detailed study has permitted instantaneous poles of rotation to be obtained, and has established that Africa is now moving northward at 2cm/y relative to Antarctica in the South West Indian Ocean. The evolution of the triple junction between the South East, South West and Central Indian Ridges is clearly reflected in the topography and magnetic lineations. The depth of parts of the ocean formed since the Late Cretaceous increases with age in the manner expected from the temperature structure of a cooling plate, and supports the evolution determined from the magnetic lineations in a most remarkable way. Heat flow observations are more scattered but also consistent with the same thermal model. The proposed evolution agrees with the distribution of known continental fragments and with the Late Cretaceous palaeomagnetic poles from surrounding continents and one obtained from the shape of the magnetic lineations south of India. It is, however, not yet clear how to reconstruct Gondwanaland from the Late Cretaceous reconstructions.

The Evolution of the Indian Ocean since the Late Cretaceous

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.

Elevation of ridges and evolution of the central Eastern Pacific.

A geophysical survey employing satellite navigation was carried out over the Reykjanes submarine ridge southwest of Iceland. Water depth, sediment thickness, and the gravity and magnetic fields were continuously measured. In addition, bottom cores and measurements of sediment and water temperatures were obtained at stations. Expendable radio sonobuoys were used to make seismic refraction measurements. This paper combines these various geophysical data to obtain information about phenomena in the water layer, about details of crustal structure, and about mechanisms operating at the ridge axis. The satellite navigation results and water temperature data are used to deduce current directions and magnitude over the ridge. These currents play a role in the observed distribution of sediment. Variations in these currents are inferred from sediment temperature measurements. Magnetic profiles parallel to the ridge crest are used to demonstrate the presence of a thin, highly magnetized layer (termed layer 2A, since it constitutes the top of layer 2 of refraction seismology), as well as to directly infer the presence of normally and reversely magnetized rocks in bands on the ridge. Seismic refraction measurements reveal: (1) a 6.5-km/sec layer under the ridge; (2) a flankward increase in seismic velocity in the crust; and (3) evidence for a surface layer of relatively low velocity (about 3 km/sec) corresponding to layer 2A. Geothermal measurements revealed two zones of low heat flow, one within 10 km of the ridge axis and the other about 75 km from the axis. The maximum values of heat flow were observed in a zone from 15 to 50 km. The over-all average of heat flow over the ridge is not significantly different from that observed in the adjacent oceanic basins. Free-air gravity anomalies over the Reykjanes ridge range from +25 to +60 mgal. Compared to the mid-Atlantic ridge, the Bouguer anomalies over the Reykjanes ridge are about 60 mgal less, but the gradients are nearly the same. The narrow axial magnetic anomaly can be traced with minor offsets to the Reykjanes Peninsula. On Iceland, positive magnetic anomalies occur over much wider areas, implying that the active zone is much wider in Iceland than over the Reykjanes ridge.

Reykjanes ridge crest: A detailed geophysical study

Summary

The northernmost expressions of the East Pacific Rise spreading centre, the Juan de Fuca Ridge and Explorer Trough, have unusual heat-flow distributions Crestal values are only moderately high, but flank areas that are models of undisturbed sedimentation have mean heat flows near 7 HFU (290 mW m−2) The values in these areas retain considerable scatter, but the detailed distribution is close to the opposite of that expected from conductive heat-flow refraction The high mean values confirm that sea-floor spreading occurs by the upwelling of hot (magma temperature) material The low crestal measurements and the non-refractive scatter in the flank areas strongly suggest that hydrothermal circulation is the dominant heat transfer process in fresh crestal material Much of the heat loss from the new lithospheric material must occur through open circulation near the ridge crest and is never reflected in measurements of surface conduction

On the Thermal Balance of a Mid‐Ocean Ridge

Water-rock interactions within the seafloor are responsible for significant energy and solute fluxes between basaltic oceanic crust and the overlying ocean. Permeability is the primary hydrologic property controlling the form, intensity, and duration of seafloor fluid circulation, but after several decades of characterizing shallow oceanic basement, we are still learning how permeability is created and distributed and how it changes as the crust ages. Core-scale measurements of basaltic oceanic crust yield permeabilities that are quite low (generally 10−22 to 10−17 m²), while in situ measurements in boreholes suggest an overlapping range of values extending several orders of magnitude higher (10−18 to 10−13 m²). Additional indirect estimates include calculations made from borehole temperature and flow meter logs (10−16 to 10−11 m²), numerical models of coupled heat and fluid flow at the ridge crest and within ridge flanks (10−16 to 10−9 m²), and several other methods. Qualitative indications of permeability within the basaltic oceanic crust come from an improved understanding of crustal stratigraphy and patterns of alteration and tectonic modification seen in ophiolites, seafloor samples and boreholes. Difficulties in reconciling the wide range of estimated permeabilities arise from differences in experimental scale and critical assumptions regarding the nature and distribution of fluid flow. Many observations and experimental and modeling results are consistent with permeability varying with depth into basement and with primary basement lithology. Permeability also seems to be highly heterogeneous and anisotropic throughout much of the basaltic crust, as within crystalline rocks in general. A series of focused experiments is required to resolve permeability in shallow oceanic basement and to directly couple upper crustal hydrogeology to magmatic, tectonic, and geochemical crustal evolution.

Permeability within basaltic oceanic crust

Heat flow from the Mid-Ocean Ridges and sea-floor spreading

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Heat flow from the Mid-Ocean Ridges and sea-floor spreading