Heat flow along the Costa Rica Seismogenesis Project drilling transect: Implications for hydrothermal and seismic processes

Heat flow analysis of the Costa Rica convergent margin is carried out for seven sites drilled during Integrated Ocean Drilling Program (IODP) Expeditions 334 and 344 as part of the Costa Rica Seismogenesis Project (CRISP). These expeditions are designed to better understand erosional subduction zones. Heat flow measurements were made to improve estimates of the thermal structure of this erosive margin and are located on the incoming plate, toe, lower, middle, and upper slopes of the margin. Heat flow values corrected for the effects of seafloor bathymetry and sedimentation are on average 15% higher than uncorrected values and range from approximately 158-200 mW/m(2) on the incoming plate to values of approximately 50 mW/m(2) on the middle and upper slopes of the margin. These values are consistent with previous estimates of heat flow showing a landward decrease in heat flow consistent with subduction of the Cocos plate. Preferred thermal models of the shallow subduction zone successfully predicting observed values of heat flow incorporate fluid flow within the upper oceanic aquifer have an uppermost permeability of 10(-9.5) m(2) and a plate boundary effective coefficient of friction of 0.06. These models suggest that temperatures on the subduction thrust reach 100 degrees C at distances between 30 and 35 km landward of the deformation front. The updip limit of seismicity, as defined by aftershocks events of M-L 1-4 recorded following the Mw 6.9 Quepos earthquake, occurs at 25 km landward of the deformation front at temperatures cooler than the 100-150 degrees C typically predicted.