Abstract
The effect of the relation between heat flow and age for the ocean crust and the increase in radioactivity on the continents tend to counteract each other creating a decrease in heat flow across the Ocean Continent Transition (OCT) at a young and an increase in heat flow across an older margin. Recent advances in the heat flow field related to our much better understanding of hydrothermal circulation in the ocean crust and two recent papers, that show the expected drop or increase in heat flow at the OCT, encouraged us to take a collective approach and look at all published heat flow profiles across the OCT to investigate if there are any processes common to all or a group of the margins.

We examined 16 actual and 2 constructed profiles across the ocean continent transition (OCT). 9 of these profiles were in marginal basins: 6 in the western Mediterranean and 3 on the north margin of the South China Sea. We could not use the measurements in the Mediterranean because of scatter presumably due to unknown salt movement. Problems with the location of the OCT on the northern South China Sea led to our use of only one profile. In deep water we could use 8 profiles that together with the one in the South China Sea led to a total of 9. We examined the difference between the ocean heat flow and that over the stretched margin ( HF-OCT) for all 9 profiles.

For the two margins > 40 Ma, HT-OCT decreased and for the 6 older than 115 MA it increased or did not change. All the margins where it did not change were in the Northeastern Atlantic near ocean floor spreading at < 15 mm/yr and close to regions of exhumed ultramafic crust. By invoking the low radioactivity of the ultra mafic rocks at such boundaries or the diaparism of the ultramafics into the much thinned continental crust we were able to account for the lack of change. On one profile we found convincing evidence for hydrothermal circulation through both the exhumed oceanic and presumed diapiric intruded continental crust.

At the two younger margins we found the abrupt drop in heat flow (HF-OTC) lies 30 to 50 km landward of the seismic OTC. This can only be explained by significant intrusion of magma or exhumation of hot ultra mafic rock into the thinned continent.