Along strike variations in temperature, sediment diagenesis and dewatering in the Costa Rica subduction zone

For the subduction zone off the Nicoya Peninsula, Costa Rica, we have estimated the thermal structure, determined the nature of the sediment entering the subduction zone, and estimated the location of diagenetic fluid sources. A previously documented along-strike offset in the updip limit of seismicity appears to coincide with a transition between subduction of relatively warm crust (seafloor heatflow of ∼100 mW/m²) south of a triple-junction trace and cool crust (seafloor heatflow of ∼20 mW/m²) to the north. The lower heat flow north of the triple junction trace is generally attributed to vigorous fluid circulation within the uppermost (1-2 km) ocean crust. The observed along strike variation in crustal heat flow leads to different temperature histories for subducted sediments. We calculated temperatures in the subducted sediments and the overlying margin wedge using a transient 1-D conductive model that accounts for advection and conduction of heat during progressive burial of subducted sediments beneath the wedge as well as frictional heating along the plate interface. Temperature profiles through the subduction zone were calculated for cases with warm crust (no hydrothermal cooling of the crust) and cool crust (with varying depths of hydrothermal cooling in the crust). The hemipelagic sediment on the incoming plate is approximately 10% opal and 60% smectite by weight, with no significant variations along strike. We then use previously published kinetic expressions for opal and smectite dehydration to estimate the reaction progress and distribution of fluid production within the subducting sediments. Overlying the warm crust (in the south), most of the opal-to-quartz and smectite-to-illite reaction progress (and associated release of fluid) occurs ∼25-45 km landward of the trench. Overlying the hydrothermally cooled crust, most of the sediment dewatering reaction progress occurs ∼35-55 km landward of the trench. At the peak of the reactions, the combined volume of fluid released from opal and smectite is comparable to the volume of fluid expelled from the sediment by compaction at the same locations. Our results indicate that the spatial distribution of fluid released from kinetically controlled sediment dewatering reactions (i.e. opal -> quartz, and smectite -> illite) mimics the pattern of the updip limit of seismicity, which occurs ∼10-20 km landward of the modeled release of most of the fluid from the smectite-to-illite transition. The release of fluid from diagenetic reactions probably causes fluid overpressures and affects the pattern of fluid flow along the plate interface. The location of the updip limit of seismicity may be influenced by the dissipation of fluid overpressures landward of the smecitite-to-illite dehydration front.