Fundamentals of ridge crest topography

doi:10.1016/0012-821X(74)90180-0

Earth and Planetary Science Letters

Volume 21, Issue 4, March 1974, Pages 405-413

https://doi.org/10.1016/0012-821X(74)90180-0 Get rights and content

Abstract

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.

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An ocean trench is a major surface feature of a subduction zone, formed by the subduction of a lithospheric plate into the mantle. Despite the importance of ocean trench depth, the primary factors controlling it have not been systematically studied. In this study, we collected several global datasets and performed stepwise regression analysis to investigate this issue. Bathymetric data illustrate that the maximum depth of global trenches varies from −3282 m to −10,924 m, with an average depth of −6125.8 m. Global trench depth generally follows a normal distribution, but also shows a bimodal pattern. Our analysis results suggest that slab age (A_s), slab dip (θ_s), sediment thickness (T_sed), and crust thickness (T_c) within a trench are the primary factors controlling global trench depth. The influence of A_s, T_sed, and T_c on trench depth is similar and is much greater than that of θ_s. Collectively, these four factors account for 77 % of the variation in trench depth, and other factors including data resolution, reheating events on the seafloor or dynamic topography may account for the remaining 23 % of the trench depth variation. Hierarchical cluster analysis indicates that when a trench enters a terminal stage, its exceptionally thick T_sed and T_c will cause the trench depth to significantly shallow, irrespective of how strongly the A_s and θ_s vary. Prior to this, the trench depth is mainly controlled by A_s and θ_s because T_sed and T_c are generally small. Our study also reveals varying degrees of correlation between trench depth and subduction zone kinematic parameters including subducting plate velocity (V_subn), upper plate velocity (V_upn), thermal parameter (φ), trench migration velocity (V_tn), subduction velocity (V_sn), back-arc deformation rate (V_d), and subduction polarity (P_sub). There seems to be no correlation between subduction duration (T_sub) and trench depth. We consider these linkages between trench depth and subduction kinematic parameters are the result of the correlations between these parameters and A_s, θ_s or T_sed in trenches.
Singularity of lithosphere mass density over the mid-ocean ridges and implication on floor depth and heat flow
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The relation of heat flow and floor depth across the mid-ocean ridges versus lithosphere age can be described by linear functions of square root of age according to plate thermal conductive Half Space Models (HSM). However, one of the long-standing problems of these classical models is the discrepancies between predicted and observed heat flow and floor depth for very young and very old lithosphere. There have been several recent attempts to overcome this problem: one model incorporates temperature- and pressure-dependent parameters and the second model includes an additional low-conductivity crustal layer or magma rich mantle layer (MRM). Alternatively, in the current paper, the ordinary density of lithosphere in the plate conductive models is substituted with a reduction of lithosphere density towards axis that features the irregularity and nonlinearity of plates across the mid-ocean ridges. A new model is formulated incorporating the new form of density for predicting both peak heat flow and floor depth. Simple solutions of power-law forms derived from the model can significantly improve the predicting results of heat flow and floor depth over the mid-ocean ridges. Several datasets in the literature were reutilized for model validation and comparison. These datasets include both earlier datasets used for original model calibration and the more recently compiled high-quality datasets with both sedimentary and crustal loading corrections. The results indicate that both the heat flow and the slope (first order-derivative) of sea floor approach infinity (undifferentiability or singularities) around the mid-ocean ridges. These singularities are partially due to the boundary condition as it has been already known in the literature and partially to the reduction of density of lithosphere as discovered for the first time in the current research.
Seismic evidence for magmatic underplating along the Kodiak-Bowie Seamount Chain, Gulf of Alaska
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Citation Excerpt :
The crust south of the FZ on profile GOA2 is located at Chron 7 (∼24 Ma), while crust north of the FZ is at Chrons 9–10 (∼28 Ma) (Fig. 2b, Walker et al., 2018). Oceanic lithosphere thickness increases linearly with the square root of age up to ∼70 Ma (Davis and Lister, 1974; Stein and Stein, 1992), and thus lithosphere will be thicker for the older crust north of the FZ. Although the predicted lithospheric thickness difference due to age across the FZ will be small (∼8%), it may be sufficient to limit the majority of the magmatic crustal underplating to the younger, southern side of the FZ.
Oceanic crust formed at mid-ocean ridges may be later modified by off-ridge magmatism forming seamounts, guyots, and islands. We investigate processes associated with seamount formation in the Gulf of Alaska Seamount Province using two coincident seismic reflection/wide-angle profiles. A north-south profile crosses the Kodiak-Bowie Seamount Chain and Aja fracture zone (FZ), and an orthogonal east-west profile is located about 90 km south of the seamount chain over Pacific plate oceanic crust. Structure along the profile away from the seamount chain is consistent with typical oceanic crust. Crust in our study region is thinnest (about 5.6 km) at the Aja FZ. Unlike observations from active transform faults, no low-velocity anomaly is observed at the Aja FZ suggesting that the crustal velocities have recovered to normal values through crack closure and crack healing. Higher lower crustal velocities (∼7.3 and > 7.5 km/s) and thicker crust (∼8.5 and ∼7.0 km) are observed near the Pratt and Durgin Seamounts and at the intersection of the Kodiak-Bowie Seamount Chain linear trend, respectively. These observations are attributed to magmatic underplating associated with seamount province magmatism. Lithospheric thickness variations across the Aja FZ may form a barrier or impediment to magmatic flow. The thickest crust (8.5 km) along our two profiles is located on the younger side of the FZ, and we suggest that the majority of magmatism jumped south of the Aja FZ when thinner lithosphere was encountered by the Bowie hot spot. The crustal structure near the Kodiak-Bowie Seamount Chain is most similar to that of other seamounts and guyots that formed on similarly young lithosphere (8–12 Ma). Our results suggest that lithospheric thickness at the time of hot spot interaction has a large control on magmatic underplating at seamounts and seamount provinces.
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The tectonic plate under the eastern Mediterranean Sea shows a remarkable variability as it comprises Earth's oldest oceanic lithosphere as well as the transition towards continental lithosphere beneath the Levant Basin. Its thickness and other properties offer essential information on the lithospheric evolution but have been difficult to determine seismically due to the high heterogeneity of the region and its complex crustal structure. Here, we combine a large, new surface wave dataset with published wide-angle data in order to determine lithospheric properties in the eastern Mediterranean. Our stochastic inversions of broad-band, phase-velocity dispersion measurements resolve the crust-mantle structural trade-offs and yield robust, 1-D shear-wave velocity models down to 300 km depth beneath the Ionian and Levant Basins. The thickness of the crust beneath the two locations is 16.4 ± 3 km and 22.3 ± 2 km, respectively. The Poisson's ratio (σ) of 0.32 and V_p/V_s of 1.93 in the crystalline crust confirm the presence of serpentinized oceanic crust beneath the Ionian Basin. Beneath the Levant Basin, low crustal V_p/V_s (∼ 1.7) and Poisson's (∼ 0.24) ratios indicate continental crust. Beneath the Ionian Basin, the lithosphere is about 180 km thick. By contrast, thin, 75 km thick lithosphere is found beneath the Levant Basin. S-velocity tomography based on surface wave data also shows thick, spatially variable oceanic mantle lithosphere beneath the eastern Mediterranean. Thickness of the oceanic lithosphere increases eastwards from the Triassic Ionian towards the Permo-Carboniferous lithosphere in the Central Eastern Mediterranean. These results demonstrate that oceanic lithosphere can thicken by cooling substantially beyond the limits suggested by the plate cooling model. Beneath the eastern Herodotus oceanic Basin, lithospheric thickness is decreasing to about 180 km. Thin continental lithosphere and shallow asthenosphere are present beneath the Dead Sea Fault, demonstrating that the localization of the lithospheric deformation and crustal seismicity along the fault correlates spatially with the thinning of the underlying continental lithosphere.
Age of the oceans
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Accurate seafloor age grids are crucial for modern geodynamic and paleoceanographic studies. They illustrate temporal and spatial changes in the direction and rates of seafloor spreading and also provide the keys to modelling seafloor depth through time.
Over the past few decades, the improvement in magnetic anomaly data quality and increase in quantity and the proliferation of sophisticated plate kinematic models have resulted in analogue maps of seafloor age, giving way to computer-based gridded products with quantified uncertainties. Modern age grids are built on the basis of the identification and integration of dated magnetic anomaly isochrons together with seafloor fabric features such as fracture zone traces, transform faults and active and extinct spreading centres.
Age grid uncertainty will be highest for grid cells not coincident with data points or in the vicinity of fracture zone traces. Further reliability tests can be performed by comparing age grid predictions with age determinations obtained from other sources such as drilled rock samples.
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Citation Excerpt :
The second issue has become evident relatively recently, and this is about the younger part of seafloor (though it also concerns the older part indirectly). As noted above, the simple half-space cooling model yields the subsidence rate of ~355 m Ma−1∕2, and the subsidence of young (<80 Ma) seafloor had been thought to exhibit a similar rate (e.g., 350 m Ma−1∕2 by Davis and Lister (1974) and 345 m Ma−1∕2 by Carlson and Johnson (1994)). This apparent similarity between theory and observation turns out to be coincidental.
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Fundamentals of ridge crest topography

Abstract

Tectonophysics

Earth Planet. Sci. Lett.

Earth Planet. Sci. Lett.

The implications of terrestrial heat flow observations on current tectonic and geochemical models of the crust and upper mantle of the earth

Geophys. J.R. Astr. Soc.

Sensitivity of heat flow and gravity to the mechanism of sea-floor spreading

J. Geophys. Res.

Elevation of ridges and evolution of the central eastern Pacific

J. Geophys. Res.

Geophysical tests of petrological models of the spreading lithosphere

J. Geophys. Res.

Heat flow, bathymetry, and the Mid-Atlantic Ridge at 43°N

J. Geophys. Res.

A detailed heat flow, topographic, and magnetic survey across the Galapagos spreading center at 86°W.

J. Geophys. Res.

Thermal model of ocean ridges

Nature

Conduction of heat in solids

Temperatures and compositions of magmas ascending along mid-ocean ridges

J. Geophys. Res.