HR: 0800h
AN: T41C-1243    [Abstracts]
TI: Along-strike Variations of Subduction Parameters at the Chilean Plate Boundary
AU: * Hoffmann-Rothe, A
EM: ahoro@gfz-potsdam.de
AF: GeoForschungsZentrum Potsdam, Telegrafenberg, Potsdam, D-14473 Germany
AU: Kukowski, N
EM: nina@gfz-potsdam.de
AF: GeoForschungsZentrum Potsdam, Telegrafenberg, Potsdam, D-14473 Germany
AU: Oncken, O
EM: oncken@gfz-potsdam.de
AF: GeoForschungsZentrum Potsdam, Telegrafenberg, Potsdam, D-14473 Germany
AB: Newly compiled data of the geometric, kinematic and mechanic properties and their variations along-strike the oblique Chilean subduction margin between $20\deg$S and $46\deg$S are used to weigh their competing influence on forearc deformation. Special emphasis lies on the formation of margin-parallel strike-slip systems. Among the parameters considered are the convergence rate and obliquity, the ocean floor age, the dip of the down-going and the slope of the overriding plate, the geodetic and seismic coupling depth, the interplate seismicity, the depth of the trench-fill and the mass transfer mode at the subduction front. Commonly discussed control factors for forearc deformation can be attributed to three major elements of a subduction system, namely (1) the plate kinematic boundary conditions, (2) the plate coupling properties that govern the effectiveness of force transmission from the subducting plate to the overriding plate, and (3) the upper plate heterogeneities affecting its rheology (e.g. elasticity, shear strength) or resistance to block motion (buttressing). An example is given for each of these elements: (1) Oblique convergence is a pre-requisite for the activation of margin-parallel strike-slip systems, but apparently not a sufficient condition. For example, strike-slip motion can presently be observed along the Liqui\~{n}e-Ofqui Fault Zone in southern Chile, while neither the Atacama Fault Zone nor the Precordilleran Fault System in northern Chile accommodate significant amounts of margin-parallel slip since the Pliocene. This difference can not be explained by variations of convergence rate or obliquity as the plate kinematic framework is almost constant along the Chilean trench. (2) The plate coupling force is a function of the frictionally coupled area on the plate interface and of the shear friction that needs to be overcome. Along the Chilean margin various factors affect coupling in opposing manner: The slab-dip is shallower in southern Chile compared to northern Chile, resulting in a greater plate contact area. On the other hand, subduction of younger and hotter oceanic plate in the south could limit the frictionally coupled area (counter acted by increased buoyancy forces?). Subduction of wet sediments in the accretive margin of southern Chile compared to the erosive margin in the north may additionally weaken the interface. (3) The trenchward concave-shaped margin in North-Chile likely hampers margin-parallel motion of a forearc sliver, while strike-slip faulting may be supported in southern Chile due to the lateral proximity of the downdip end of coupling on the plate interface and the rheologically weakened zone of the active volcanic arc. Establishing the current state of plate coupling in southern Chile compared to northern Chile thus remains ambiguous. Margin-parallel strike-slip activity in southern Chile, however, may be facilitated by superposition of two conditions: a shallow-dipping slab that transfers stresses at the base of the overriding plate further arcward and an exceptionally close position of the arc to the trench.
DE: 9360 South America
DE: 8150 Plate boundary--general (3040)
DE: 8158 Plate motions--present and recent (3040)
DE: 3040 Plate tectonics (8150, 8155, 8157, 8158)
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting