Function Of Plate Age Research Articles

Densely Sampled Global Dynamic Topographic Observations and Their Significance

AbstractTopography and bathymetry are principally supported by some combination of crustal and sub‐crustal density variations. However, dynamic topography is generated by vertical deflection of the Earth's surface as a result of mantle convection. Isolating and quantifying observable dynamic topography yields valuable information about mantle processes. Here, we investigate global dynamic topography by calculating residual depth anomalies throughout the oceanic realm and residual topographic anomalies across the continents. We correct for sedimentary and crustal loading by exploiting a variety of seismologic datasets that include seismic reflection profiles, wide‐angle/refraction surveys, and receiver functions. In this way, an extensively revised and augmented global compilation of 10,874 oceanic residual depth measurements and 3,777 continental residual topographic measurements is constructed. In the oceanic realm, the methodology has been revised to improve accuracy and resolution. First, quartz/clay content of the sedimentary column is adjusted to remove minor skewness of residual depth anomalies as a function of plate age. Secondly, variation of bulk density as a function of crustal thickness is taken into account. Our global compilation is used to generate spherical harmonic representations of observable dynamic topography out to degree 40 (i.e., ∼1,000 km). Resultant spectra demonstrate that dynamic topographic power varies linearly with inverse wavenumber. The spectral slope directly reflects the way by which dynamic topography is generated by Stokes' flow. Our global results are consistent with independent and diverse geologic markers of uplift and subsidence together with Neogene‐Quaternary intraplate basalt magmatism.

Lithospheric flexure modelling seaward of the Chile trench: implications for oceanic plate weakening in the Trench Outer Rise region

SUMMARY The Chilean subduction zone presents a unique opportunity to study trench outer rise deformation of the subducting oceanic lithosphere at different thermal ages. The shape of the outer rise for plate ages ranging from 0 to 50 Ma is predicted by using an elastic plate model with variable elastic thickness Te(x) as a function of the distance measured from the trench axis. In addition, the uncertainties of our results are estimated by performing a Monte Carlo-type analysis and we considered explicitly the sediment loading effect on the lithospheric flexure in regions where the trench is heavily sedimented. The results show a systematic reduction in Te of up to 50 per cent (or reduction in the flexural rigidity D of up to ∼90 per cent) from the peak of the outer rise to the trench axis. The reduction in Te and D observed in most of the bathymetric profiles is coincident with (i) high plate curvatures (>5 × 10 −7 m −1 ), (ii) strong bending moments (>10 16 N m), (iii) pervasive fracturing and faulting of the oceanic basement (as imaged by high-resolution bathymetry data) and (iv) reduction in crustal and mantle seismic velocities. The reduction in flexural rigidity towards the trench suggests a weakening of the oceanic lithosphere and is interpreted to be caused partially by fracturing and a likely increase in fluid-pore pressure. In general, our estimates do not show consistent increases in elastic thickness as a function of plate age. This result suggests that either Te is independent of plate age or Te depends strongly on other factors. These factors could include lithospheric weakening due to hydro-fracturing and the loading history of the plate prior to subduction.

An analysis of young ocean depth, gravity and global residual topography

The variation of ocean depth with age in the absence of crustal thickening and dynamic support places valuable constraints on the thermal and rheological properties of the lithosphere and asthenosphere. We have attempted to estimate this variation using a global data set of shiptracks, with particular emphasis on young ocean floor. In this respect, this paper extends a previous study published in this journal by the same authors, which concentrated on the older parts of the ocean basins. We find that, prior to 80 Ma, subsidence patterns are reasonably consistent, with gradients of 325 ± 20 m Ma −1/2 and zero-age depths of 2600 ± 200 m. There is a strong inverse correlation between zero-age depth and the gradient of depth with the square root of age which is unrelated to local variations in dynamic support. Global depth-age trends to 160 Ma are not significantly different to those for the individual ocean basins. Within corridors of similar basement age, gravity-topography correlations are consistently 30 ± 5 mGal km−1. Simple isostatic theory and numerical modelling of mantle plumes suggests that, if the minimum depth of convection is defined by the base of the mechanical boundary layer, the admittance should be a function of plate age. The observation that it is not implies that the active convective upwelling beneath young lithosphere ceases at the same depth as it does beneath old oceanic plates. This result is consistent with geochemical modelling of melts near mid-ocean ridges. We have examined the relationship between residual topography and gravity worldwide, and have found that good spatial correlations are restricted to the Atlantic, North Pacific and youngest Indian ocean basins. By contrast, residual topography and gravity are poorly or negatively correlated in the South and young North Pacific Ocean and in the older Indian Ocean. Away from regions of thick crust and flexure, histograms of residual topography and gravity have symmetric distributions about zero. We then use this residual topography to estimate the volume and buoyancy flux of seven major plume swells. In Hawaii, the clear correlation between melt and swell volumes in discrete age corridors is evidence that the horizontal velocity of the hot plume material far downstream from the plume is similar to the plate spreading velocity and that the plume pulses over time. Finally, comparison with seismic tomographic models suggests that the long-wavelength (>2000 km) residual topographic and gravity anomalies have an origin deeper than 250 km. This result is consistent with observations that the admittance is approximately constant at wavelengths longer than 800 km.

Reappraising elastic thickness variation at oceanic trenches

We reassess the variation of elastic thickness as a function of lithospheric plate age using a global database of bathymetric and free‐air gravity profiles which are perpendicular to oceanic trenches. As in many previous studies, our starting point is the well‐known floating elastic plate model. In order to remove the influence of short‐wavelength features not associated with lithospheric bending, adjacent profiles from 10‐Myr bins have been stacked together to construct average profiles with standard deviations. Each average profile was then inverted in a two‐stage procedure. First, singular value decomposition was used to determine two unknown flexural parameters, together with a regional slope and offset, for any given elastic thickness. This procedure was repeated for a range of elastic thicknesses. Second, residual misfit was plotted as a function of elastic thickness, and the global minimum was identified. This two‐stage procedure makes no prior assumptions about magnitude of the load, size of the bending moment, or whether the elastic plate is broken/continuous. We obtained excellent fits between theory and observation for both bathymetric and gravity profiles from lithosphere with an age range of 0–150 Ma. The shape of the residual misfit function indicates the degree of confidence we have in our elastic thickness estimates. The lower limit of elastic thickness is usually well determined but upper limits are often poorly constrained. Inverse modeling was carried out using a range of profile lengths (250–300, 500, and 700 km). In general, our estimates show no consistent increase of elastic thickness as a function of plate age. This surprising result is consistent with recent reassessments of elastic thickness beneath seamounts and implies either that elastic thickness is independent of plate age or that elastic thickness cannot be measured with sufficient accuracy to reveal such a relationship. Modeling of short free‐air gravity profiles (250–300 km) does tentatively suggest that elastic thickness increases linearly from 5 to 10 km between 0 and 20 Ma and from 10 to 15 km between 20 and 150 Ma. This variation roughly matches the depth to the 200°C isotherm which corresponds to an homologous temperature of 0.4 for wet peridotite. Unfortunately, for longer profile lengths, there is no temporal dependence, and elastic thicknesses vary considerably for all plate ages. Bathymetric profile modeling yields similar results although uncertainties are larger.

Fluxes of fluid and heat from the oceanic crustal reservoir

Recent discoveries define a global scale fluid reservoir residing within the uppermost igneous oceanic crust, a region of seafloor that is both warm and may harbor a substantial biosphere. This hydrothermal fluid reservoir formed initially within volcanic rocks newly erupted at mid-ocean ridges, but extends to the vastly larger and older ridge flanks. Upper oceanic crust is porous and permeable due to the presence of lava drainbacks, fissuring, and inter-unit voids, and this porosity and permeability allows active fluid circulation to advect measurable quantities of lithospheric heat from the crust to an average age of 65 Myr. A compilation of crustal porosities shows that this fluid reservoir contains nearly 2% of the total volume of global seawater. Heat flow and sediment thickness data allow calculation of reservoir temperatures, predicting 40°C mean temperatures in Cretaceous crust. Utilizing these temperature estimates, heat flow measurements and models for the thermal structure and evolution of the oceanic lithosphere, we have computed mean hydrothermal fluxes into the deep ocean as a function of plate age. The total hydrothermal volume flux into the oceans approaches 20% of the total riverine input and may contribute to the global seawater mass balance.

Increase of seismic velocities in upper oceanic crust: The “superfast” Spreading East Pacific Rise at 14°14'S

One of the most important observations of oceanic crustal evolution is that seismic compressional wave velocities in the upper igneous crust, layer 2A, increase as the crust ages. The most viable explanation for this phenomenon is decreasing porosity due to filling of open void spaces with hydrothermally generated minerals. But the idea that seismic velocity could be a function of plate age, however, depends primarily on airgun/sonobuoy profiles collected more than 20 years ago [Houtz and Ewing, 1976]. Therefore, in late 1995 new seismic refraction measurements were obtained on the eastern flank of the “superfast” spreading East Pacific Rise south of the Garrett transform. We found that velocities rapidly increase by about 45–50% within only 0.5–1 m.y., double within 5 m.y. or less and remain constant thereafter.

Comment on “Upper crustal structure as a function of plate age” by Robert Houtz and John Ewing

Comment on “Upper crustal structure as a function of plate age” by Robert Houtz and John Ewing

Geoid anomalies across Pacific fracture zones

SUMMARY Cooling of the oceanic lithosphere and related subsidence of the ocean floor are usually understood in terms of simple conductive thermal models: half-space and plate-cooling models, which predict the same behaviour for heat flow, sea-floor depth and geoid on very young sea floor but large deviations at greater ages. Since geoid data are much more sensitive to the thermal structure of the lithosphere at depth than other kinds of data, satellite-derived geoid anomalies, in particular geoid steps observed across fracture zones, are currently analysed to discriminate between the two thermal models. In this study we present results for the geoid step, Δh, estimated along altimeter profiles of the Seasat satellite across five fracture zones (FZs) of the NE Pacific: Clipperton, Clarion, Molokai, Murray and Mendocino, and across the Udintsev FZ in the S Pacific. For each FZ, the quantity geoid offset, Δh, divided by the age offset, Δt, is plotted as a function of plate age. Except for Mendocino and Clipperton FZs along which the variation of Δh/Δt with age is consistent with the prediction of the thermal plate model with a plate thickness of 90-100 km, the dependence on age of Δh/Δt along the other FZs is different from that predicted by conductive cooling models. Large fluctuations of the Δh/Δt-age relationship are observed along Murray, Clarion and Udintsev FZs, suggesting that the thermal structure of the oceanic lithosphere under a FZ is more complex than is to be expected from conductive cooling alone. According to recent convection models, small-scale convective instabilities developed underneath the FZ in a shallow low-viscosity layer could explain the observed Δh/Δt-age behaviour.

Seismic properties of layer 2A in the Pacific

Many new sonobuoy solutions have been combined with published data and incorporated into a revised plot of igneous layer 2A thickness as a function of plate age at the East Pacific Rise. Each solution was redated on the basis of regional magnetic anomaly maps to improve the accuracy of crustal dating. Results show that layer 2A becomes unobservable by seismic methods on crust older than about 15 m.y. Although igneous materials must make up layer 2A at the East Pacific Rise, low-velocity acoustic basement exists elsewhere in the Pacific. Near seamounts and seamount chains, volcanic aprons with the seismic properties of layer 2A thin distally away from eruptive centers and seem to persist indefinitely without being affected by processes that normally increase the sound velocity at the ridge crests; This is apparently due to a lack of fracturing, which prevents hydrothermal solutions from percolating through the volcanic rubble. At active spreading centers, percolation through fractures provides minerals that could fill voids and increase the sound velocity of the basaltic rubble. Statistical studies show that the velocity in layers 2B and 2C is independent of plate age, but layer 3 velocities peak up significantly on the East Pacific Rise. In the older parts of the Pacific, low-velocity acoustic basement is sedimentary or a mixture of sediment with volcanics. An unusually thick sequence (1 km) of material that is probably mostly sediment occurs in the Nauru and Mariana basins.

Upper crustal structure as a function of plate age

Each Lamont-Doherty sonobuoy located on well-dated crust has been carefully analyzed to determine crustal structure down to oceanic layer 3. Results from the Atlantic and the Pacific are compiled separately in order to study crustal structure as a function of plate age in both oceans, since they have very different spreading rates. Layer 2A (refraction velocity about 3.6 km/s) in the North Atlantic is 1.5 km thick at the ridge crest and thins consistently to about 100 m as the crust ages to about 60 m.y. Layer 2A in the east Pacific is 0.7 km thick at the ridge and thins to about 100 m at about 30 m.y. This difference in thickness is probably attributable to the much faster spreading rate in the Pacific. A poorly refractive acoustic basement layer about 200 m thick with similarities to layer 2A but not necessarily composed of the same materials is measured sporadically in the Pacific M-Series plates and even less consistently in the Atlantic. This layer is not recorded in the Cretaceous or the Jurassic quiet zones. Regressions of refraction velocities in layer 2A as a function of age show that its velocity increases from about 3.3 km/s at the ridge crests to that of layer 2B on crust about 40 m.y. old. There is no corresponding increase of velocity with age in any of the deeper layers. The high resolution of the air gun/sonobuoy records shows that the layer 2B refraction line breaks directly to layer 3 velocities (46 times in the present work) or to a line with a velocity of 6.1 km/s (114 times in the present work), which we call layer 2C. The variance of the velocities in 2C is one fourth that of 2A and 2B, which indicates relative lithological uniformity in 2C. It does not seem likely that layer 2A really thins; what appears to be a thinning of the layer may actually be the result of an increase of its refraction velocity with age. However, the 2A/2B interface is well-defined by large amplitude refractions from 2B on crust that is younger than about 30 m.y., which seems to rule out a transitional zone at the base of layer 2A where it ‘converts’ to 2B. The seismic observations seem to require a diagenetic process or repeated basaltic intrusions; both processes raise serious objections.