During Leg 141 of the Ocean Drilling Program JOIDES Resolution occupied five sites off the coast of southern Chile between 12 November 1991 and 12 January 1992. Leg 141 investigated the tectonic and geologic processes involved in the subduction of a mid-ocean ridge, the thermal and hydrological effects of ridge subduction on the overriding plate, the geology and tectonics of one possible example of ophiolite formation and emplacement, and the chemistry of sub-seafloor frozen gas hydrates. At Sites 859 and 863, holes drilled into the base of the trench slope 0 and 30 km respectively from the subducting ridge showed similar thermal gradients but different degrees of lithification of the sediment, and different levels of hydrothermal activity. The transition from subduction accretion to subduction erosion during ridge subduction was documented at these two sites. Cores from Sites 860 and 861 in the middle and upper slope region of the Chile Trench forearc yielded records of complex depositional patterns and tectonic uplift of the forearc prior to ridge subduction. These sites also yielded data showing the decrease in temperature gradient landward from the trench, and the associated decrease in the rate of hydrothermal circulation. All four sites in the forearc showed traces of hydrocarbon gases that have migrated long distances up the subduction zone. No gas hydrates were recovered, even in the pressure core sampler, but inorganic geochemical anomalies in interstitial fluids from the sediments indicate that gas hydrates were present before drilling and probably occupied approximately 25% of the pore space. Cores from Site 862 on the Taitao Ridge demonstrated the ridge's oceanic crustal affinity and helped constrain models for its origin, with important implications for ophiolite genesis models elsewhere.

The primary objective of ODP Leg 141 is to investigate the processes associated with the subduction of an active spreading ridge along the margin of Southern Chile, including: 1) rapid uplift and subsidence of the arc and forearc, 2) high levels of regional metamorphism and elevated thermal gradients, 3) a hiatus in arc magmatism, 4) anomalous near-trench and forearc magmatism, and 5) localized subsidence and extensional deformation of the forearc in the region of the collision. Also, alteration, diagenesis, and mineralization of forearc materials can be expected, driven by the hot fluids venting from the subducting spreading ridge. A secondary objective of ODP Leg 141 is to investigate the physical properties and geochemistry of gas hydrates in oceanic sediments. Bottom Simulating Reflectors (BSRs), likely produced by frozen gas hydrate layers in the shallow sub-bottom, are common on seismic reflection profiles from the region.