REHEATING OF OLD OCEANIC LITHOSPHERE - DEDUCTIONS FROM OBSERVATIONS

Deep, wide oceanic basins are the only regions of old seafloor where d epth is truly representative of thermal isostasy. When the depths of t hese basins are corrected for the effect of sediment accumulation, and variation in crustal thickness, the principal non-thermal factors hav e been eliminated. We collect the most precise and reliable values of heat flow for the same basins, from multi-penetration measurements wit h 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 conside red without regard to age, all the points fall al greater depths than predicted by the 'plate' models with constant temperature lower bounda ries, and remarkably close to boundary-layer cooling with parameters d etermined 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 d epths and the order they plot in along the heat flow axis is random wi th 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 rehea ting appears to start only on ocean floor > 100 Ma. The data alone can not discriminate between a few discrete reheating events due to convec tive peel-off at the base of the lithosphere or one or more catastroph ic events. However, the distribution of points from the Blake-Bahama b asin is more consistent with distal reheating associated with the Berm uda hotspot than local convective peel-off.