LEG 112

Peru Continental Margin

During Leg 112, 10 sites (Site 679 to Site 688) were drilled along western South American in the Lima, Salaverry, Trujillo, and Pisco forearc basins and the accretionary complex of the Peru continental margin. Drilling verified that continental crust extends to within 15 km of the trench axis of the Peru-Chile Trench. During the Eocene to late Miocene, the front of the overriding continent subsided; unusual in an active continental setting, indicating erosion of the continental plate and subduction of the sediment on the oceanic plate rather than accretion to the edge of the continent. In the accretionary wedge, the oldest accreted sediments recovered are diatomaceous mudstones, deposited between 6.8 and 6.1 Ma, and accreted to the continent between 5 and 4 Ma. That period of subsequent continental growth, as opposed to initial continental erosion, occurred just after subduction of the Nazca Ridge.

Superimposed on the crustal subsidence are a rapid uplift and an accelerated Pliocene subsidence of the Lima Basin not evident in the Trujillo Basin. The basin sediments comprise sequences of diatomites and diatomaceous muds derived from the shelf and upper slope at centers of coastal upwelling. The occurrence of upper Miocene sediment in upwelling facies 150 km from shore indicates that the critical depth zone of coastal upwelling migrated landward in response to margin subsidence and sea-level fluctuation. The sediments deposited on the slope and accreted complex are predominantly produced in the shifting coastal zone characterized by upwelling and intense currents and are subsequently reworked, transported by slumping, and then redeposited in deeper water on the continental slope. The centers of sedimentation of the distinct upwelling facies are located on the shelf and upper slope where the oceanographic oxygen-minimum zone intercepts the seafloor at depths of 100-400 m. The Plio-Pleistocene upwelling facies is characterized by cycles of laminated and non-laminated, meter-scale units recording global-scale changes in climate and sea-level fluctuations and local tectonism. The adjacent Lima and Trujillo basins were at approximately the same water depth until the middle Miocene, when the basins began to respond differently to deep crustal tectonism. The position of coastal upwelling regimes has not been stationary but is controlled by the tectonics of each segment of the margin; thus some modern coastal upwelling centers are ephemeral. An abundant supply of organic-rich matter to the sediments that underlie areas of coastal upwelling on the shelf allows sulfate-reducing bacteria and methanogens to dominate diagenesis and creates environments ideal for the formation of sulfides, calcite, dolomite, and phosphorites. Pore- water sampling indicated a saline brine underlying the upper 300 m of sediments which replenishes the supply of dissolved sulfate and enhances bacterial degradation.