A simple magma accretion model of the oceanic lithosphere is proposed and its implications for understanding the thermal field of oceanic lithosphere examined. The new model (designated VBA) assumes existence of lateral variations in magma accretion rates and temperatures at the boundary zone between the lithosphere and the asthenosphere. Heat flow and bathymetry variations calculated on the basis of the VBA model provide vastly improved fits to respective observational datasets. The improved fits have been achieved for the entire age range and without the need to invoke the ad-hoc hypothesis of large-scale hydrothermal circulation in stable ocean crust. The results suggest that estimates of global heat loss need to be downsized by at least 25%.
Detailed understanding of large-scale variations in the thermal field of the oceanic lithosphere provides important constraints on deep tectonic processes.
Nevertheless, thermal models of the lithosphere proposed to date have failed to provide a satisfactory account of some of the important features of large-scale variations in oceanic heat flow. For example, both the Half-Space Cooling [55] and Plate [30] models predict heat flow much higher than the observed values, for young (ages less than 55 Ma) ocean crust. Also, the magnitudes of heat flow anomalies associated with the mid-ocean ridge systems are systematically lower by a factor of 6 at younger ages than those predicted by thermal models proposed in the current literature [41]. In addition, the widths of thermally anomalous zones associated with the spreading centers are narrower (less than 23 Ma) than those calculated (~66 Ma) for a wide range of plausible model parameters. Such discrepancies between model predictions and observational data have given rise to the so-called “oceanic heat flow paradox”, for which no satisfactory solution has been found for over the last forty years. The common practice in the current literature is to consider the paradox as originating from eventual perturbing effects of possible regional scale hydrothermal circulation in the ocean crust not accounted for in conventional heat flow measurements (e.g. [41], [54], [56], [27], [60]).
There are however dissenting views on the subject matter of hydrothermal circulation on regional scales [20]. Direct experimental evidences presented to date have confirmed the existence of only isolated pockets of hydrothermal circulation in the central valley and in the rift flanks of spreading centers (e.g. [12], [13], [17], [13], [29], [34]). No direct experimental evidence has so far been presented that point to the existence large scale circulation systems operating in stable ocean crust. Most of the arguments presented to date in favor of the supposed existence of regional-scale convection systems, in parts of ocean floor at large distances from spreading centers, are based on indirect inferences (e.g. [56], [2], [3], [14]).
In the current work, we present a new model of oceanic lithosphere that can overcome the above-mentioned problems and present a satisfactory solution for the heat flow paradox, without the need to invoke the ad-hoc hypothesis of largescale hydrothermal circulation in stable ocean crust. To place the new model in context, we summarize the main characteristics and inherent limitations of currently accepted thermal models of the lithosphere. Next, the characteristics of thermal fields associated with upwelling of asthenospheric materials are outlined and its compatibility with the new model features examined. Following this, details of the new model fits to observational data on heat flow and bathymetry are presented, along with results of numerical simulations exploring the influence of model parameters. We point out, in addition, that empirical relations such as those proposed for GDH reference models are unnecessary. Implications of the new model results for understanding regional scale variations in global heat flow are discussed and the need to downsize the current estimates of global heat loss emphasized.
Thermal models of the lithosphere, with wide acceptance in the current literature, may be classified as falling into essentially two generic groups:
-Half-Space Cooling (HSC) Models; and -Constant Thickness Plate Models.
In the HSC model the basic assumption is that the temperature of the medium at origin time (t = 0) has a constant value T m for all depths. This constant T m then holds for all time at infinite depth. The lithosphere is considered as boundary layer of the mantle convection cells, arising from near surface conductive cooling. The lithosphere (in other words, the boundary layer) grows in thickness continuously as it moves away from the up-welling limb of the mantle convection system. Analytical expressions for temperature variation of the boundary Layer may be obtained as solution to the one-dimensional heat conduction equation [7].
The boundary layer approach has been successful in accounting for first order features in variation of oceanic heat flow with age (e.g. [38], [39], [44]).
Nevertheless, this model cannot be considered as satisfactory for several reasons.
To begin with the HSC model is strictly valid for heat flux arising from cooling of a stagnant body and not one in which lateral movements occur in response to thermal convection. This is in direct contradiction with one of the essential ingredients of thermal convection, that of lateral movements. In addition, the model predicts infinite heat flow at the ridge axis (the well known problem of singularity in heat flow at time of origin) and heat flow values about five-fold higher than those observed in regions close to the ridge axis. The problem of high model heat flow for yo
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