HEAT-FLOW IN THE WESTERN ABYSSAL-PLAIN OF THE GULF-OF-MEXICO - IMPLICATIONS FOR THERMAL EVOLUTION OF THE OLD OCEANIC LITHOSPHERE

The seafloor depth of an oceanic basin reflects the average temperatur e of the lithosphere. Thus the western abyssal plain of the Gulf of Me xico, which has tectonically subsided much (> 1 lan) deeper than other basins of comparable ages (late Jurassic), should be underlain by an anomalously cold lithosphere. In order to examine this hypothesis, we made suites of high-accuracy heat flow measurements at 10 sites along a line connecting Deep Sea Drilling Project (DSDP) sites 90 and 91 in the Sigsbee abyssal plain. The new hear flow sites were initially surv eyed by 3.5-kHz echo sounding, 4-channel seismic reflection, seismic r efraction with eight ocean bottom seismometers, and nine piston cores. We occupied a total of 48 heat flow stations along the seismic survey line (3 to 6 at each site), including 28 where we measured in situ th ermal conductivities over the practical depth interval (4 m) of the ne w multioutrigger bow heat flow probe. We determined the heat flow asso ciated with the lithosphere by correcting the values measured at the s eafloor (41 to 45 mW/m(2)) for (1) the thermal effect of the sedimenta tion and (2) the additional heat from the radioactive elements within the sediments. The sedimentation history, required for the first, was reconstructed at each heat flow site based on ages and thicknesses of the major seismic stratigraphical sequences, age data from the DSDP co res, 3.5-kHz subbottom reflectors, and correlation of turbidite units found in the piston cores. Radiogenic heat production was measured for 55 sediment samples from four DSDP holes in the gulf, whose age range d from present to Early Cretaceous (0.83 mu W/m(3) on the average). Th is provided the correction for the second. The effects of these two se condary factors approximately cancel one another. The lithospheric hea t flow under the abyssal plain thus estimated ranges from 40 to 47 mW/ m(2). These heat flow values are among the lowest in the Mesozoic ocea n basins where highly reliable data (45 to 55 mW/m(2)) have been repor ted. Therefore the lithosphere under the gulf seems indeed colder than that under other old ocean basins. However, it is not as cold as expe cted from the large tectonic subsidence. The inconsistency between the depth and heat flow may imply an anomaly in the regional thermal isos tasy.