LEG 141

Chile Triple Junction

The Chile Triple Junction represents the only presently active ridge-trench collision where the overriding plate is composed of continental lithosphere. Spreading-ridge subduction events leave structural and stratigraphic signatures in the geological record of the overlying plate. In addition, hot fluids venting from the subducting ridge cause hydrothermal alteration, anomalous diagenesis, and mineralization of the forearc materials. During Leg 141, five sites were drilled to study these processes (Site 859 to Site 861 and Site 863) as well as the origin of the Taitao Ridge and its affinity to an onshore ophiolite complex (Site 862).

Downhole temperature measurements at the base of the trench slope 0 and 30 km from the subducting ridge showed similar thermal gradients (~100 deg C/km) but different degrees of lithification and hydrothermal activity. At 0 km, a lack of Quaternary sediment incorporated into the thrust system implies that subduction accretion has ceased and that this segment of the margin is in transition from accretion to erosion. Sediments at 30 km, above the subducted spreading axis, are undergoing extensional deformation as the latest phase of their deformation history, indicating that subduction erosion is taking place; thus the transition from subduction accretion to subduction erosion occurs within ~ 3 m.y. and over a distance along strike of ~ 30 km. Distinct, narrow salinity and chloride minima in interstitial and in-situ water chemistry are probably the result of dissociation of gas hydrate during drilling. The difference between background and minimum chloride values implies that originally ~ 25% of the pore space was filled with hydrate. The middle and upper slope region of the forearc is undergoing complex depositional patterns and tectonic uplift prior to ridge subduction. At the upper trench slope, the chlorinity of interstitial water remains close to seawater values with two negative anomalies corresponding to high-permeability zones; high concentration gradients surrounding these anomalies require current, or recently active, advection of exotically-derived water. Chlorinity at the middle trench slope increases steadily with depth, a possible result of long-term steady diffusion of high-salinity brines from underlying continental basement, and indirect evidence of the farthest extent of continental crust from the South American margin.

Instead of the expected uniform tholeiitic basalt, drilling on the Taitao Ridge recovered interlayered basaltic and rhyolitic to dacitic lavas which may be the result of seaward migration of arc magmas into the oceanic plate through a gap in the subducting slab opened by the Taitao Fracture Zone