The Central Chilean Margin: Lower Plate Structure and Subduction Zone Geometry
Abstract
The central Chilean margin was the target of a combined on-/offshore seismic experiment using RV SONNE as platform for the marine data acquisition during cruise SO161. The along-strike segmentation of the margin results in areas of reduced slab dip ('Flat slab' segments). Segment boundaries frequently coincide with the active subduction of bathymetric features on the lower plate. There may exist a correlation between the buoyancy of the subducted seafloor relief and the occurrence of shallow subduction. The fundamental effects of ridge subduction on the margin suggest a linkage of the subducting Juan Fernandez chain on the Nazca plate to the flat slab segment of the central Chilean margin, which poses one aspect investigated in the scope of the SPOC project presented here. The study area covers the eastern part of the aseismic Juan Fernandez Ridge and the continental slope off Valparaiso. Four wide-angle profiles were laid out: Two profiles are W-E oriented and located across the subduction complex at latitudes 32S and 31S, respectively. A small accretionary prism has accumulated against a backstop of increased shear strength. The upper slope is underlain by continental basement. 'Normal' oceanic crust enters the trench except where the lower plate is altered by the O'Higgins seamount group which marks the easternmost termination of the Juan Fernandez Ridge. This hotspot ridge is currently entering the trench and poses the target of a tomographic investigation using two perpendicular, densely spaced wide-angle lines. The inversion uses a top-to-bottom approach using first arrivals as well as later phases and includes a mantle inversion to obtain upper mantle velocities. Extrusive type volcanism formed the O'Higgins volcanoes when the plate moved over the hotspot that is currently forming Alexander Selkirk Island at the western termination of Juan Fernandez Ridge. A localized crustal root has evolved, but a downflexing of the crust cannot be observed.
- Publication:
-
EGS - AGU - EUG Joint Assembly
- Pub Date:
- April 2003
- Bibcode:
- 2003EAEJA.....1361K