Variations in heat flow across the ocean—continent transition in the Iberia abyssal plain

New heat flow observations have been made in the Iberia abyssal plain off the Galicia margin along the transect of Ocean Drilling Program Leg 149 drill sites, in order to investigate the nature of this unusually wide and deep continent-ocean transition region. Our results indicate the presence of three separate zones. Average values of 47.5 ± 3 mW m −2 in the westernmost zone III agree with predictions of standard oceanic lithospheric models for its estimated age of 126 Ma. In contrast, the heat flow within zone II is 5–15 mW m −2 higher than predicted, assuming that the mantle heat flow remains constant across the basin. This region of high values is coincident with the location of a major intra-crustal “S”-type reflector east of ODP Site 900, and the anomaly is consistent with the presence of 2–3 km of primarily upper continental crust above the reflector, with concentrations of radiogenic components similar to those from granodiorite samples dredged off Galicia Bank. It is not, however, consistent with the low values of heat production measured on gabbroic samples from its western end at ODP Site 900. In zone I, detailed measurements across the tilted fault block south of ODP Site 901 show consistent variations which closely match predictions due to the effects of basement structure and sediment deposition. There is no evidence for variations due to vertical convective transport along the dipping basement fault block. Once corrected for these variations, measurements in zone 1 yield average values that agree quite well with previous measurements across Galicia Bank, indicating no systematic landward increase in heat flow with decreasing amounts of continental extension.