C. R. B. Lister

Fundamentals of ridge crest topography

A linear relationship between the sea floor depth and the square root of age has been found for ocean lithosphere spreading from mid-ocean ridges. The asymptotic solution of depth as a function of age for the thermally contracting lithosphere predicts a linear dependence of depth ontwith a proportionality involving the initial lithosphere temperature, the thermal diffusivity, and the isostatic expansion coefficient averaged to include any temperature dependent phase changes. Empirical depth observations, when plotted as a function of the square root of age, bear out this prediction well, but there is a variation in the gradient,ht, along the ridge on a fine scale (up to 20% over 200 km). This implies a fundamental variation of the contraction parameter over the same scale, most probably of compositional origin. Details of a more complete cooling model near the ridge crest, including a crust of different thermal parameters than those of the mantle, predict a crestal height about 0.2 km below that of the simplified model. Individual profiles from the southeast Pacific show no such crestal deviation, and it is concluded that by quickly cooling the new crust, hydrothermal circulation may remove any effects of the crust which would be seen in the topography of a lithosphere cooled totally by conduction. The straightness of depth versust for older ocean data (to 80 m.y.) precludes any basal isothermal boundary shallower than 100 km.

Topographic forcing of supercritical convection in a porous medium such as the oceanic crust

Abstract We have performed laboratory experiments using a Hele-Shaw cell to model a saturated, porous layer with various sinusoidal upper boundaries. Our intent was to determine the range of conditions over which boundary topography can control the pattern of thermal convection within a porous layer, and thereby take the first step toward understanding why heat flow seems correlated with hypsography in many areas of the ocean floor. These experiments indicate that above the critical Rayleigh number, topography does not control the convection pattern, except when the topographic wavelength is comparable to the depth of water penetration. Scaled to the depth of the layer, the convective wavenumbers are restricted to values between 2.5 and 4.8—a range which brackets π, the natural wavenumber for convection in a porous slab with planar, isothermal, impermeable boundaries. Topographies within this range control the circulation pattern perfectly, with downwelling under valleys and upwelling aligned with topographic highs. Other topographies do not force the pattern, although in some cases, the convection wavenumber may be a harmonic of the topographic wavenumber. Unforced circulation cells wander and vary in size, because they are not locked to the topography. For these experiments we employed eight different topographies with non-dimensional wavenumbers between 1.43 and 8.17, and we studied the flow at Rayleigh numbers between zero and five times the critical Rayleigh number. The amplitude of each topography tapered linearly (over a factor of three to six) from one end of the apparatus to the other, and the mean topographic amplitude was 0.05 times the depth of the layer. Under these conditions, amplitude has only a minor effect on the structural form and vigor of supercritical convection. Our results may apply to submarine geothermal systems, sealed by a thin layer of impermeable sediment draped over the basement topography. In this case, the convection wavelength—as measured perhaps by the spatial periodicity of conductive heat flow—may be a good measure of the depth to which water penetrates the crust. Where the circulation correlates with the bottom topography, it may be because the topographic wavelength is comparable to the depth to which water penetrates the porous crust.

Reheating of old oceanic lithosphere: Deductions from observations

Deep, wide oceanic basins are the only regions of old seafloor where depth is truly representative of thermal isostasy. When the depths of these basins are corrected for the effect of sediment accumulation, and variation in crustal thickness, the principal non-thermal factors have been eliminated. We collect the most precise and reliable values of heat flow for the same basins, from multi-penetration measurements with in situ thermal conductivity, or deep sea drilling thermal gradients backed up by surface surveys. The 9 data points that result from this selection process have been plotted on a depth versus heat flow graph and compared to published thermal models of lithosphere. When considered without regard to age, all the points fall at greater depths than predicted by the ‘plate’ models with constant temperature lower boundaries, and remarkably close to boundary-layer cooling with parameters determined from the pre-80 Ma depth and heat flow history of the ocean floor. They are differentiated by their heat flows not much by their depths and the order they plot in along the heat flow axis is random with respect to crustal age. Modeling of discrete reheating events shows that near boundary layer conditions are re-established after about 40 Myr, but corresponding to a younger-than-real age. The data therefore favor discrete reheating events rather than a continuously hot basal boundary, as implicitly assumed by the plate model. Lithospheric reheating appears to start only on ocean floor > 100 Ma. The data alone cannot discriminate between a few discrete reheating events due to convective peel-off at the base of the lithosphere or one or more catastrophic events. However, the distribution of points from the Blake-Bahama basin is more consistent with distal reheating associated with the Bermuda hotspot than local convective peel-off.

A direct recording ocean bottom seismometer

We have designed a simple, cheap and reliable ocean-bottom seismometer. Signals from three-component geophones are recorded directly on magnetic tape running continuously at a speed of 1 mm s1. Time reference is derived from a temperature-compensated quartz crystal oscillator and encoded on a fourth channel as an amplitude modulation of a 20 Hz carrier. A bipolar square-root signal-compression scheme doubles the tape dynamic range to 80 db, and the available bandwidth is 2 to 100 Hz. Tape and batteries are capable of 500-hr operation, and the unique magnetic release comes close to being a fail-safe system. A heavy, high-drag concrete anchor shaped like a flower-pot provides easy launching, fast stable descent and good coupling to the ocean floor. We have had numerous successful field emplacements which have yielded good earthquake and shot-refraction data.

An ocean-bottom seismometer suitable for arrays

Abstract We have developed a cheap and simple three-component ocean-bottom seismometer. One vertical and two horizontal seismometers are levelled in a boat floating on thick silicone oil in the lower half of a buoyant spherical pressure case. Signals are compressed by recording the bipolar square root directly on magnetic tape moving at 1 mm s −1 . The nominalbandwidth is 2 to 100 Hz, and a special 24-cm reel of tape will run for 500 h. Fast emplacement is obtained by lodging the buoyant spheres in heavy flower-pot shaped concrete anchors that have stable descent characteristics. Five successful drops have been made with two prototypes in epoxy resin spheres, and clear arrivals from shots and earthquakes have been received.

Thermal leakage from beneath sedimentary basins; an experimental test of the contribution of convective flow structures

In many places heat flow decreases towards the edges of young submarine sedimentary basins, and toward basement outcrops within them. Perhaps this fact contains some information on the thickness and permeability of the convective zone in the ocean crust. A laboratory experiment was conducted by covering 88% of an 8 m2 porous bed with an insulating layer made up of polymethylmethacrylate slabs. Heat flow through these slabs was obtained by pushing a thermistor-tipped snake between them and the bead-and-water porous bed beneath. At all convective fluxes studied from 24 to 2987 W, heat-flow depression only extended one-half of the porous-bed thickness into the insulated zone. The proportion of the total heat flux passing through the insulated area declined from 62% near the onset of convection to 27% at the highest fluxes attained. However, the flux transported convectively to the base of the insulating layer remains close to the value expected from the temperature difference, in spite of the larger flux delivered to the thermally-exposed zone nearby. The ability of the experimental apparatus to deliver the two separate fluxes is due to its highly-conductive, constant-temperature base: enhanced heat transfer due to cold flows from the thermally-exposed area leaves enough physical space for the modest plume flux needed by the resistive cover. Were the bottom boundary resistive, as in nature, the high heat flux from the exposed area would depress the surface-temperature of the base and thus the flux available for the insulated zone. There appears to be no mechanism in the porous flow itself that causes heat flow depression away from the edge of the resistive zone, and thus the marine heat-flow data contain little information about the thickness of the permeable zone in the oceanic crust.