Evidence for deep fluid circulation and gas hydrate dissociation using boron and boron isotopes of pore fluids in forearc sediments from Costa Rica (ODP Leg 170)

Abstract

Abstract Drilling a transect of holes across the Costa Rica forearc during ODP Leg 170 demonstrated the margin wedge to be of continental, non-accretionary origin, which is intersected by permeable thrust faults. Pore waters from four drillholes, two of which penetrated the decollement zone and reached the underthrust lower plate sedimentary sequence of the Cocos Plate, were examined for boron contents and boron isotopic signatures. The combined results show dilution of the uppermost sedimentary cover of the forearc, with boron contents lower than half of the present-day seawater values. Pore fluid “refreshening” suggests that gas hydrate water has been mixed with the sediment interstitial water, without profoundly affecting the δ 11 B values. Fault-related flux of a deeply generated fluid is inferred from high B concentration in the interval beneath the decollement, being released from the underthrust sequence with incipient burial. First-order fluid budget calculations over a cross-section across the Costa Rica forearc indicate that no significant fluid transfer from the lower to the upper plate is inferred from boron fluid profiles, at least within the frontal 40 km studied. Expulsed lower plate pore water, which is estimated to be 0.26–0.44 km 3 per km trench, is conducted efficiently along and just beneath the decollement zone, indicating effective shear-enhanced compaction. In the upper plate forearc wedge, dewatering occurs as diffuse transport as well as channelled flow. A volume of approximately 2 km 3 per km trench is expulsed due to compaction and, to a lesser extent, lateral shortening. Pore water chemistry is influenced by gas hydrate instability, so that it remains unknown whether deep processes like mineral dehydration or hydrocarbon formation may play a considerable role towards the hinterland.