Submarine weathering and subduction dewatering across the Central Chilean forearc (~36°S)

The accretionary convergent margin off central Chile is an active tectonic setting, yet little is known about its hydrogeology and the distribution of deep-seated diagenetic processes. To overcome this gap, we present here a joint evaluation of data from ODP Legs 141 and 202 as well as new pore water and solid phase data from a gravity core transect at ~36°S. Sediments in the accretionary prism accommodate two different types of submarine weathering processes: (i) Dissolution of reactive silicate minerals with concomitant transformation of CO2 from methanogenesis into bicarbonate and (ii) alteration of silicate minerals and volcanic ash to smectite, which is accompanied by consumption of water thus leading to elevated pore water salinities. Pore waters on the upper slope are characterized by negative chloride anomalies, which likely results from upward transport of clay dehydration fluids along splay faults. Thermal constraints from heat flow modeling suggest that these fluids originate in the overriding plate whereas dehydration of underthrust sediments seems to take place further seaward below the accretionary prism