Contreras-Reyes, Eduardo (2008) Evolution of the seismic structure of the incoming/subducting oceanic Nazca Plate off South Central Chile. (PhD/ Doctoral thesis), Christian-Albrechts-Universität, Kiel, Germany, 152 pp.

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Abstract

The release of fluids in subduction zones is believed to be an important process controlling earthquakes both in the downgoing plate and in the seismogenic megathrust fault zone. Depending on the depth of slab dehydration, water may nurture rupture propagation and trigger arc magmatism. Growing number of geophysical evidence suggest that alteration and hydration of the oceanic plate is most vigorous at the trench-outer rise, where extensional bending-related faulting affects the hydrogeology of the oceanic crust and mantle. To better understand the processes of hydration and alteration affecting the oceanic lithosphere prior to and during subduction, I have studied the seismic velocity structure of the oceanic Nazca plate offshore of southern central Chile. A combination of swath bathymetric, wide-angle and reflection seismic data was used to derive 2D velocity-depth models, using joint refraction and reflection travel-time tomography along two main corridors: (i) offshore Isla de Chiloe (~43°S) and (ii) southern Arauco peninsula (~38°S). The study area corresponds to the southern central accretionary Chile margin, where the trench is heavily filled with up to 2.2 km of sediments. The velocity models show P-wave velocities typical for mature fast-spreading crust in the seaward section of the profiles, with uppermost mantle velocities as fast as 8.3-8.4 km/s. Approaching the Chile trench seismic velocities are significantly reduced, however, suggesting that the structure of both the oceanic crust and uppermost mantle have been altered, possibly due to a certain degree of fracturing and hydration. The decrease of the velocities roughly starts at the outer rise; ~100 km from the deformation front, and continues into the trench. Anomalously low heat flow values near outcropping basement highs were founded at the outer rise offshore Isla de Chiloe, suggesting an efficient inflow of cold seawater into the oceanic crust. Hydration and crustal cracks, activated by extensional tectonic stresses, are suggested to govern the reduced velocities in the vicinity of the trench. Considering typical flow distances of 50 km, water might be redistributed over most of the trench-outer rise area. In addition, Poisson's ratios at the lowermost crust and uppermost mantle reach values of ~0.26 and ~0.29, respectively. These features can be explained by an oceanic crust partially weathered, altered and fractured. Relatively high Poisson's ratios in the uppermost mantle may be likely related to partially serpentinized mantle. On the other hand, the comparison of the uppermost mantle P-wave velocities at the crossing point between perpendicular profiles at ~43°S (~90 km seaward from the trench axis) reveals a low degree of Pn anisotropy (<2 %). Offshore southern Arauco peninsula, the Mocha Fracture Zone is obliquely subducting underneath the South American plate and coincides with an area of even slower velocities and thinning of the oceanic crust (10-15 % thinning), suggesting that the incoming fracture zone may enhance the flux of the chemically-bound water into the subduction zone. The low $P_n$ velocities found in the outer rise area span along the subducting plate and reach a maximum depth of 6-8 km in the uppermost mantle, suggesting that as the oceanic plate subducts at the trench, bending and faulting continues to affect the oceanic lithosphere. The restoring to 'normal' mantle velocities of ~8.4 km/s coincide with the 400-430°C isotherm. This depth is interpreted as the depth limit of hydro-alteration within the upper part of the oceanic lithosphere, where extensional stresses dominate.

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