Convergent Tectonics and Coastal Upwelling : A History of the Peru Continental Margin

ODP LEG 112 SHIPBOARD SCIENTIFIC PARTY

Abstract

During Leg 112 of the Ocean Drilling Program, ten sites were investigated on the Peruvian margin. Three of them, sites 683, 685 and 688 were located on the middle and lower slope of the continental margin. The other sites were in four slope and shelf basins, the Lima Basin (Sites 679 and 682), the Trujillo Basin (Site 684), the Salaverry Basin (Sites 680 and 681), and the West Pisco Basin (Sites 686 and 687). Two major topics of the leg were the tectonic history of the margin and the paleoenvironmental history. Drilling provided sediment samples of Eocene, Miocene, Pliocene, and Quaternary age.
The tectonic evolution was investigated along two E-W transects at 9° and 11° S latitude. The post-Oligocene slope deposits above an Oligocene unconformity extend to within 15 km of the trenck axis. The vertical tectonic history unique to each segment is superimposed on the general subsidence of the Peru margin since Oligocene. Along the northern transect, the Trujillo Basin subsided with relative stability during the past 10 Ma. The Lima Basin area along the southern transect was uplifted, eroded, and subsided rapidly during the same period. Accretion at the front of the margin began about 5 Ma ago, which corresponds to subduction of the Nazca Ridge, an increase in rate of plate convergence, and the general increase in sedimentation.
The paleoenvironmental history was studied in four basins along a N-S profile on the shelf. The record of paleoenvironmental conditions is closely linked to the tectonic history of the basins. The Lima Basin had a rapid subsidence history during Neogene time and preserved a long-time record of upwelling. The Trujillo Basin remained just below Quaternary sea-level fluctuations. The Salaverry Basin well preserved the Pliocene/Quaternary history. In the West Pisco Basin, slow subsidence was continuous over a long period of time while the oceanographic conditions remained unchanged.
The drill sites were located beneath the centers of coastal upwelling. The dominant lithology in the Quaternary sequences in the four basins is organic-rich diatomaceous mud, which contains alternations of bioturbated and laminated units. The observed cyclicity is climatically forced as evidenced by investigation of diatom assemblages. The major cycles coincide with those of the eustatic sea-level change and the variation in the Earth's orbit.
Sediments from beneath coastal upwelling experience extensive early diagenesis. The sequence proceeds from calcite precipitation to dolomitization of calcite to final co-precipitation of organic dolomite formation. This normal sequence is complicated by the incursion of a hypersaline subsurface brine, which was the most surprising find of Leg 112.