Abstract

The Guatemala Basin has long been a tectonic puzzle because of its anomalously low heat flow. Recent numerical investigations of convective flow within a Newtonian fluid have shown that a downwelling asthenospheric convection limb should produce anomalously low heat flow, anomalously deep depth, and a negative free-air gravity anomaly on the upper boundary. Observational confirmation of the correlation, predicted by the model, between depth anomalies and gravity anomalies in other oceanic regions leads to the suggestion that the low heat flow of the Guatemala Basin might show a correlation with negative gravity. Because of the time constant for conductive heat transfer through the lithospheric plate, one must know where this portion of the Cocos plate was relative to an assumed fixed gravity field 25 to 40 m.y. ago.

The lithosphere of the Guatemala Basin traversed a 10- to 15-mgal negative gravity anomaly 30 to 40 m.y. ago. This correlation, along with that between depth and gravity, is evidence for upper-mantle convection. Furthermore, the portion of the long-wavelength free-air gravity field of the Earth that is not dominated by plate downthrusting at trenches might represent a map of asthenospheric flow.

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