Hydrogeology of Mound Culebra mud volcano offshore Nicaragua: Preliminary results from heat flow surveying.

Mud volcanism is a common dewatering phenomenon in compressional tectonic scenarios such as subduction zones. Offshore Costa Rica and Nicaragua, several of these features have been discovered, the most prominent of which is Mound Culebra north-west of Nicoya Peninsula. A methane anomaly over the mound, authigenic carbonate formation at its crest, and typical cold seep fauna detected by ocean bottom video surveying indicate that the feature is actively venting. During a most recent survey aboard of the German RV METEOR, a first systematic heat flow survey across this actively venting mud dome has been conducted. This approach is supplemented by data from geophysical surveys and coring. Mound Culebra is composed of overconsolidated scaly clays and clast-bearing muds. Backscatter images of the seafloor suggest that authigenic carbonates cover a significant portion of the top. The heat flow transect across the mound and temperature measurements made with outriggers on gravity corers suggest that heat flow on the mound equals approximately background values of 36 mW/m2. However, values at the foot and steep flanks of the dome are significantly elevated (40-50 mW/m2), suggesting that the overconsolidated clays and carbonates form an impermeable lid. Fluids rising from depth underneath the dome are therefore channelled towards the flanks of the mound. Nevertheless, a single heat flow penetration on the top may indicate that at least locally warm fluid venting can occur on the crest, thereby generating the methane anomaly in the bottom water and nurturing the vent fauna.