HR: 1340h
AN: T13C-1380    [Abstracts]
TI: Active Faulting, Surface Deformation and Subduction Earthquakes at Isla Santa Maria, South-Central Chile
AU: * Melnick, D
EM: melnick@gfz-potsdam.de
AF: GFZ Potsdam, Telegrafenberg, Potsdam, 14473 Germany
AU: Bohm, M
AF: GFZ Potsdam, Telegrafenberg, Potsdam, 14473 Germany
AU: Bookhagen, B
AF: Institut of Geosciences, University of Potsdam, Karl Liebknecht Str. 24, Potsdam, 14415 Germany
AU: Echtler, H
AF: GFZ Potsdam, Telegrafenberg, Potsdam, 14473 Germany
AU: Krawczyk, C
AF: GFZ Potsdam, Telegrafenberg, Potsdam, 14473 Germany
AU: Manzanares, A
AF: GFZ Potsdam, Telegrafenberg, Potsdam, 14473 Germany
AU: Moreno, M
AF: TIGO Observatory, University of Concepcion, Camino Einstein, Concepcion, 4036 Chile
AU: Strecker, M
AF: Institut of Geosciences, University of Potsdam, Karl Liebknecht Str. 24, Potsdam, 14415 Germany
AB: The Santa Maria island (37S) is situated above the interplate seismic zone of the Nazca-South America convergent margin. The island is in the boundary zone between distinct seismotectonic sectors of the Chile margin: the Valdivia (46-37S) and Concepcion (38-35S) segments, defined based on the recurrent rupture zones of M$>$8 subduction earthquakes. We used offshore industry reflection lines and local network seismicity to identify active structures and decipher their control on localizing surface deformation during great subduction earthquakes. Their effect on the geomorphology and structural evolution of the island was studied through field work and a high-resolution DEM. 14C ages were used to calculate deformation rates. The island comprises two units: Holocene lowlands formed by a flight of up to 25 emerged strandlines between the present sea level and 18 m elevation, coseismically raised during subduction earthquakes; and an upper surface formed by Tertiary rocks unconformably overlain by 53 to 31-kyr coastal and 31 to 10-kyr-old eolian deposits with paleosol horizons at elevations of 15 to 58 m. The transition from marine to continental depositional environments in the Pleistocene unit indicates complete emergence at 31 kyr BP, when sea-level was 82 m lower than at present, inferring an uplift rate of 3.6$\pm$0.5 m/kyr. The differences between the topography of the base of the near-shore unit, eolian unit, and present-day surface indicate progressive eastward tilting of 0.1%/kyr and synkinematic sedimentation. These surfaces are asymmetric, in the north they are oriented NE and dip SE, while in the south NW/NE. The Holocene strandlines yield a maximum uplift rate of 3 m/kyr, assuming they are preserved since the end of sea-level rise 6 kyr ago. Local network seismicity clusters 7 km NE of the island. Across an E-W transect the earthquakes define a continuous W-dipping zone from 2 km depth to the plate interface at 18 km. Focal mechanisms are compatible with a NNE-striking, steep W-dipping reverse fault. An E-W oriented reflection line across the cluster reveals W-dipping blind reverse faults, fault-propagation folding, and a piggy-back basin documenting eastward propagation of the deformation. This section accounts for $\sim$8% of ongoing shortening since the Pliocene. The structure has a ramp-flat-ramp geometry, probably rooted at the plate interface. Seismic lines SE of the island show N-vergent folding above a WNW-oriented reverse fault. Both the WNW- and NNE-oriented fault/fold systems converge at the center of the island. We therefore conclude that these two convergent systems control the asymmetry of the island. The progressive asymmetric tilting deduced from the base of the Pleistocene sequence, Holocene strandlines, and present-day surface can thus be attributed to lateral propagation of two fold systems that converge at the island. The emergent strandlines and the regional distribution of historic coseismic uplift demonstrate that great subduction earthquakes in this environment trigger crustal-scale reverse faults, which in turn control the spatial evolution of surface deformation. A common assumption, when applying dislocation models to invert surface deformation into slip on the plate interface thrust, is that the crust behaves as an elastic block that deforms without internal failure. Previous inversions yielded slip magnitudes that largely exceed the values expected from plate convergence velocity and earthquake recurrence, which might be explained by the discovery of active faulting below the coast.
DE: 9360 South America
DE: 7230 Seismicity and seismotectonics
DE: 8107 Continental neotectonics
DE: 3025 Marine seismics (0935)
DE: 1824 Geomorphology (1625)
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting