Seismic reflection profiles across the Middle America Trench at 20°N show a high amplitude bottom simulating reflector interpreted as marking a phase transition between methane hydrate and free gas in the pore space of both accreted and trench sediments. We determine the depth of the hydrate-gas phase boundary in order to estimate the geothermal gradient and hence the heat flow beneath the trench and the frontal part of the accretionary wedge which overlies the downgoing plate. After correction for sedimentation, heat flow values in the trench and through the accretionary wedge are only about half of the values predicted by plate cooling models for the 10 Ma subducting lithosphere. There is no systematic correlation between heat flow in the accretionary wedge and distance from the trench. A comparison with heat flow predicted by a simple analytical model suggests that there is little shear heating from within or beneath the wedge, despite the high basal friction suggested by the large taper angle of the wedge. The geothermal gradient varies systematically along the margin and is negatively correlated with the frontal slope of the wedge. Some local peaks may be attributed to channelised fluid expulsion.

This work was initiated during a study visit of TAM to Barcelona funded through the Royal Society/CSIC bilateral agreement. We thank the captains and crews of the R/V´ s Hespe´rides, Altair and Humboldt for their support during the experiment, and the Mexican authorities for facilitating work within their exclusive economic zone. Data acquisition was supported by the CICYT project ANT94-0182-C02-01/02 (Spain), the Secretarı´a de Marina of Mexico, and CICESE project 644107, CONACYT 0894PT, and INSU (France). TAM was partially supported by a Royal Society University Research Fellowship. The Department of Geophysics (ICTJA-CSIC) is a Grup d ´ Excellencia de Qualitat (ref. 2001 SGR 00339). We thank C. de Mets for providing digital magnetic anomaly picks, and M.- A. Gutscher and two anonymous reviewers for constructive comments on an earlier version of this manuscript that contributed significantly to the final version.

9 pages, 6 figures.

Peer reviewed