HR: 1340h
AN: T13C-1389    [Abstracts]
TI: Quaternary Tectonic Tilting Governed by Rupture Segments Controls Surface Morphology and Drainage Evolution along the South-Central Coast of Chile
AU: Echtler, H P
EM: helle@gfz-potsdam.de
AF: GFZ Potsdam, Telegrafenberg, Potsdam, 14473 Germany
AU: * Bookhagen, B
EM: bodo@geo.uni-potsdam.de
AF: Univ. Potsdam, Golm, Potsdam, 14415 Germany
AU: Melnick, D
EM: melnick@gfz-potsdam.de
AF: GFZ Potsdam, Telegrafenberg, Potsdam, 14473 Germany
AU: Strecker, M
EM: strecker@geo.uni-potsdam.de
AF: Univ. Potsdam, Golm, Potsdam, 14415 Germany
AB: The Chilean coast represents one of the most active convergent margins in the Pacific rim, where major earthquakes (M>8) have repeatedly ruptured the surface, involving vertical offsets of several meters. Deformation along this coast takes place in large-scale, semi-independent seismotectonic segments with partially overlapping transient boundaries. They are possibly related to reactivated inherited crustal anisotropies; internal seismogenic deformation may be accommodated by structures that have developed during accretionary wedge evolution. Seismotectonic segmentation and the identification of large-scale rupture zones, however, are based on limited seismologic und geodetic observations over short timespans. In order to better define the long-term behavior and deformation rates of these segments and to survey the tectonic impact on the landscape on various temporal and spatial scales, we investigated the south-central coast of Chile (37-38S). There, two highly active, competing seismotectonic compartments influence the coastal and fluvial morphology. A rigorous analysis of the geomorphic features is a key for an assessment of the tectonic evolution during the Quaternary and beyond. We studied the N-S oriented Santa Marˇa Island (SMI), 20 km off the coast and only ~70km off the trench, in the transition between the two major Valdivia (46-37S) and Concepci˘n (38-35S) rupture segments. The SMI has been tectonically deformed throughout the Quaternary and comprises two tilt domains with two topographic highs in the north and south that are being tilted eastward. The low-lying and flat eastern part of the island is characterized by a set of emergent Holocene strandlines related to coseismic uplift. We measured detailed surface morphology of these strandlines and E-W traversing ephemeral stream channels with a laser-total station and used these data to calibrate and validate high-resolution, digital imagery. In addition, crucial geomorphic markers were dated by the radiocarbon and optical stimulation methods to better constrain deformation rates. In response to the ongoing deformation, formerly W flowing streams constituting small drainages (< 0.25km2) were inverted and formed closed basins. In contrast, larger streams were reversed or were able to maintain their channels, but formed distinct knickpoints along their longitudinal profiles. In order to reconstruct the Holocene tectonic tilting axis, we connected drainage boundaries of reversed channels and deformation-related knickpoints along more mature rivers. Interestingly, topography clearly indicates that the direction of Pleistocene tectonic tilting was different than that of recent conditions. The Holocene inversion of stream flow associated with continuous uplift may be related to the progressive migration of the tectonic tilting axis in the course of active folding (Melnick et al., this session). The classification of knickpoints and the overall tectonic development also the mainland coast on the Arauco peninsula, during the Quaternary clearly document the surface signature of tectonic segmentation and its spatial evolution through time. The migration of the tilting axes is discussed in relation with active basal accretion and active shortening in the South-Central Chilean forearc.
DE: 5475 Tectonics (8149)
DE: 1744 Tectonophysics
DE: 1625 Geomorphology and weathering (1824, 1886)
DE: 1645 Solid Earth
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting