HR: 1340h
AN: GP43A-0843    [Abstracts]
TI: Late Miocene block rotations in the South Chilean offshore forearc (38.5$\deg$S) linked to backarc shortening: paleomagnetic and bathymetric evidence
AU: * Vietor, T
EM: tvietor@gfz-potsdam.de
AF: GFZ, Telgrafenberg, Potsdam, 14473 Germany
AU: Echtler, H P
EM: helle@gfz-potsdam.de
AF: GFZ, Telgrafenberg, Potsdam, 14473 Germany
AU: Mueller, H
EM: mueller@gfz-potsdam.de
AF: GFZ, Telgrafenberg, Potsdam, 14473 Germany
AU: Oncken, O
EM: oncken@gfz-potsdam.de
AF: GFZ, Telgrafenberg, Potsdam, 14473 Germany
AU: Ladage, S
EM: S.Ladage@bgr.de
AF: BGR, Stilleweg 2, Hannover, 30655 Germany
AB: Oblique subduction inducing dextral shear along the southern Chilean forearc has prevailed for more than 30 Ma with rates between 15 and 5 cm/yr. At around 38øS the onshore-geology is dominated by numerous NW-striking faults parallel to the inherited Paleozoic basement structure. The swath bathymetry of cruise SONNE 161 emphasizes the continuity of these structures to the trench and shows that they segment the forearc slope and shelf into 20 to 80 km wide slices. The trench displays a pronounced saw-tooth morphology in map-view that suggests dextral rotations of these segments. However, both onshore and offshore there is no evidence for active strike-slip deformation along the NW-striking faults and the available moment-tensor solutions indicate a pre-dominance of thrust-type earthquakes within the forearc wedge. This suggests that the dextral block rotations are presently inactive or proceed at very low rates. Recently collected paleomagnetic data from 12 sites on Mocha Island 60 km east of the trench support this notion. Sites in Eocene and Miocene sediments show pre-dominantly dextral rotations of 20 to 30 $\deg$. In contrast, Pliocene rocks are not significantly rotated but only tilted to the NE in agreement with folding and thrusting over NE-dipping reverse faults. Hence, the available data imply that a major fraction of the block rotations has been accumulated between the deposition of the Miocene and the Pliocene sediments. These time constraints fit the deformation chronology of the upper-plate. At this latitude, shortening in the back arc of the Andes has been restricted to the interval between 10 and 5 Ma. Inspection of the upper-plate deformation field shows that the along-strike variations in shortening have resulted in an extension of the forearc that can account for the magnitude of the dextral rotations. In summary, forearc deformation is obviously driven by oblique subduction. However, considerable block rotations in map-view require along-strike-extension. The coincidence of the block-rotations and the Late Miocene upper-plate deformation at this latitude suggests that the required space has been provided by the deformation of the plate-boundary due to differential, southward diminishing shortening in the back-arc.
DE: 9360 South America
DE: 8105 Continental margins and sedimentary basins
DE: 1525 Paleomagnetism applied to tectonics (regional, global)
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2004 AGU Fall Meeting