Helmholtz Centre Potsdam
GFZ German Research Centre for Geosciences
Abstract (EDOC: 9154)
The ISSA 2000 (Integrated Seismological experiment in the Southern Andes) and SPOC 2001 (Subduction Processes Off Chile) onshore and offshore projects surveyed the Chilean margin between 26 and 40° S. This area includes the location of the 1960 earthquake (Mw = 9.5) that ruptured the margin from ~38° S southwards for ~1000 km. Together with gravity and magnetotelluric components, the active-passive seismic experiments between 36 and 40° S provide the first, complete, high-resolution coverage of the entire seismogenic plate interface. The observed offshore mode of sediment subduction corresponds well with the landward extension of the reflection seismic profile at 38° S (westernmost portion of line SPOC-Sputh), which shows material transported downwards in a subduction channel. From the slow uplift of the Coastal Cordillera, we conclude that basal accretion of parts of this material controls the seismic architecture and growth of the south Chilean crust. There is almost no seismicity observed along the entire, approximately 130 km wide, seismogenic coupling zone. Furthermore, the study area is characterized by a 25-35 km thick crust beneath the Longitudinal Valley, with high conductivity zones at 20-40 km depth that correspond to large fault zones. Below the volcanic arc, the crust is generally 35-45 km thick, with a maximum thickness of 55 km at ~36° S. The slab steepens southwards along the margin (13°-21°), and a wedge-shaped body at the plate interface can be either interpreted as hydrated mantle with 20-30% serpentinization or, when divided, as mafic crustal material in the upper part and serpentinized mantle in the lower part. The lower plate could suffer slab rollback while the upper-plate kinematic segmentation exhibits fore-arc extension, possibly combined with corner-flow and active lower-plate retreat. At the top of the active subduction channel, underplating, fore-arc uplift and serpentinization are key processes at the south-central Chilean margin.
(2006): Geophysical Signatures and Active Tectonics at the South-Central Chilean Margin. - In: Oncken, O.; Chong, G.; Franz, G.; Giese, P.; Götze, H.-J.; Ramos, V. A.; Strecker, M. R.; Wigger, P. (Eds.), The Andes - Active Subduction Orogeny, Springer, 171-192.