Oroclinal bending of the Juan Fernández Ridge suggested by geohistory analysis of the Bahía Inglesa Formation, north-central Chile

Abstract

The stratigraphy of the Bahía Inglesa Formation in the Caldera Basin west of Copiapó, (north-central Chile) is revised, based on hitherto unpublished stratigraphic sections and 87Sr/86Sr dating. The depositional environment varied from a rocky shoreline to the upper continental slope, with sea-level oscillations and tectonic movements causing numerous local unconformities as well as lateral and vertical facies changes. Geohistory and sedimentological analyses show that the area was close to the concurrent sea level at about 15.3Ma, but at the same time being elevated about 100m above the present sea level. Although the basin then subsided at least 350m until around 6Ma, marine deposition was only recorded after 10.4Ma. This suggests that the sea level initially dropped faster than the rate of subsidence so that subaerial erosion occurred. The period of subaerial exposure before 10.4Ma can be attributed to the presence of a NE-trending branch of the Juan Fernández Ridge below the continental crust at this time, whereas the ensuing subsidence was due to subduction erosion and crustal accommodation in its wake as it migrated south along the South American coastline. The subsequent uplift of at least 250m can be explained by an acceleration in plate expansion and isostatic rebound of the continental crust after being partially submerged in the upper mantle. The uplift–subsidence–uplift pattern mirrors those recorded around the Nazca Ridge in Peru, as well as in similar basins to the south of Caldera. However, a higher southward migration rate of the Juan Fernández Ridge against the edge of the South American Plate and less intense uplift–subsidence–uplift cycles are recorded in the latter basins. This can possibly be attributed to oroclinal bending of the ridge due to friction with the overlying continental plate, which diminished the angle of incidence and the intensity of the stress field, but increased the migration velocity of the ridge relative to the coastline.