Description: ottom simulating reflectors (BSRs) were detected in multichannel seismic reflection data acquired in the vicinity of Isla Mocha across the southern Chile margin and near 33°S. Geothermal gradients were determined from the depth of the BSR that is interpreted to mark the thermally controlled base of a gas hydrate layer. Ground truth for the assessment and additional thermal constraints were provided by downhole measurements obtained during Ocean Drilling Program (ODP) Leg 202 in Site 1233 at 41°S and Sites 1234 and 1235 near 36°S. Both BSR-derived data and downhole temperatures were used to calculate heat flow anomalies and provide new constraints on the thermal regime of the continental slope and downgoing slab in Chile between 32°S and 41°S. Downhole chemical logs of Th, U, and K from Site 859 of ODP Leg 141 have been used to assess the radiogenic heat production in the margin wedge. Heat production is low (∼0.8 μW/m3). However, knowledge of this reduces the errors of estimating the contribution from frictional heating along the subduction thrust fault. With respect to the Eocene age of the incoming oceanic lithosphere, heat flow appears to decrease landward of the deformation front as expected due to the advective transport of heat into the subduction zone by the downgoing slab. Calculations of conductive fore-arc heat flow show that the modelled seafloor heat flow agrees with the measured heat flow only if there is negligible frictional heating. At 33°S, temperatures in the fault zone reach 100°C approximately 60 km landward of the deformation front and are coincident with the onset of earthquake activity and hence mark the up-dip limit of the seismogenic zone. The up-dip limit shifts seaward going to the south, reflecting the progressive southward decrease of lithospheric age of the subducting plate.