Seismic Investigations of the O'Higgins Seamount Group: Aseismic Ridge Emplacement and Hydration of Hot Spot Modified Lithosphere

The segmentation of the South American continental margin has been linked to the subduction of bathymetric elevations on the oceanic plate. Offshore Valparaiso, Chile, the subduction of the hot spot generated aseismic Juan Fernandez Ridge correlates with the southern segment boundary of the Chilean flat slab. A causative relationship has been attributed to the enhanced buoyancy of anomalous crust and the shallowing of the Wadati-Benioff zone. In 2001, the SPOC project set out to investigate the effects of subducting aseismic ridges on the seismicity and structure of the Chilean margin. A seismic study along the easternmost extent of the Juan Fernandez Ridge reveals a magmatic origin dominated by extrusive processes for the O'Higgins Seamount Group. Hot spot magmatic processes have only moderately added to the crust of the Juan Fernandez Ridge. Thus only limited increased buoyancy may be expected from crustal thickening, insufficient to cause the shallowing of the subduction zone in the flat slab segment. The modest size of the volcanic edifices causes a flexural bending of the upper crust underneath O'Higgins Guyot, but does not affect the trend of the crust-mantle boundary. The off-axis aseismic ridge emplacement caused deficiencies of the lithosphere. These syngenetic structures are reactivated in the outer rise setting and evolve to ridge-parallel fracture zones with vertical offsets exceeding 800 m. Tomographic inversion of seismic mantle phases reveals reduced uppermost mantle velocities indicative of mineral alterations in the hot spot modified lithosphere. Uppermost mantle hydration is solely limited to the flawed lithosphere of the Juan Fernandez Ridge and is linked to the reactivation of hot spot induced fractures. Enhanced buoyancy due to crustal and upper mantle hydration may represent an important additional mechanism for shallow subduction.