ANOMALOUS OCEAN LITHOSPHERE IMPACTS THERMAL COOLING

One of the fundamental sources of information for the nature and physical properties of the earth's lithosphere is the cooling history of the global oceans. We have revisited this relation applying updated age and depth data. We focus here on the impact of anomalous oceanic lithosphere.

Anomalous lithosphere can be generated by large igneous provinces, thermal plumes, and hot spots or by tectonics at subduction zones (both the trench and the flexural bulge). We defined anomalous crust through the seafloor gravity data. Where the gravity remained constant over large regions the crust was assumed to be isostatically compensated. However rapid changes in gravity, as well as very high or very low values (in excess of ±1.5 mGal) were assumed to be anomalous.

Finally, seafloor shallower than 2000 m or deeper than 8000 m was eliminated from both data sets. Age vs. depth relations were derived using both the Airy-isostatic ocean (AIO) data set and the entire ocean (EO) data set. Using data from the entire global ocean, with the exception of the Arctic Ocean where data quality is poor, the following relations were derived:

AIO: depth (m) = 2859 + 279 √time (ocean 0 – 96 Ma) R=0.84 pts = 320,163

EO: depth (m) = 2709 + 305 √time (ocean 0 – 76 Ma) R=0.78 pts = 1,277,722

AIO: depth (m) = 5664 – 2484 exp(-time/43.8) R=0.82 pts = 393,211

EO: depth (m) = 5775 – 2594 exp(-time/50.7) R=0.75 pts = 1,904,171

The two cooling half space models (√time curves) are most different. The intercepts of the two plate models (exponential curves) are virtually identical, and deeper than the actual plate. These models should not be applied to sea floor younger than 5 Ma. However, the thermal decay constant for the EO data set is larger, suggesting slower subsidence than that of the AIO data set. This implies that older anomalous sea floor tends towards shallower rather than deeper depths. These relations suggest that the ocean lithosphere is between 80 and 110 m thick and that increased anomalous crust with age may be one reason why the cooling half space relation (√time) does not fit after 76 Ma when the entire seafloor database is used.