Ages of the ocean floor
The thermal state of the oceanic lithosphere is largely controlled by its age. Physical models invoking a cooling halfspace predict that oceanic heat flow and sea floor depth are proportional to s1/(sqrt(age) and sqrt(age), respectively (Davis and Lister, 1974); whereas plate cooling models assume a constant thickness after a certain age (Parsons and Sclater, 1977; Stein and Stein, 1992). Although the plate models fit the observed heat flow and bathymetry better than the halfspace model, both models exhibit systematic misfits to the data at young and old lithosphere (Carlson and Johnson, 1994). Hydrothermal circulation is believed to cause the lower than predicted heat flow values observed for young oceanic regions (Anderson and Hobart, 1976). For older oceanic regions, however, the observed heat flow is "anomalously" high and the sea floor depth-age curve flattens out more than predicted by these cooling models. Both effects have been attributed to thermal rejuvenation of the lithosphere by interaction with hotspots (e.g., Detrick and Crough, 1978; Heestand and Crough, 1981), yet seismic studies along the Hawaiian hotspot track show little support for any large scale thermal effects (Woods et al., 1991). Furthermore, heat flow anomalies across the Hawaiian Swell are small (Von Herzen et al., 1989). Alternatively, pressure-induced asthenospheric flow has been proposed for the flattening of the depth-age curve (Phipps Morgan and Smith, 1992), but does not account for elevated heat flow. We propose that power dissipated within the asthenosphere may contribute to heat flow anomalies, as well as to the flattening of the depth-age curve, as power dissipation for oceanic plates is much larger under older lithosphere than under younger lithosphere.
(From Sachnoff & Stoddard, 1995).
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