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Title Seismic properties of the Nazca oceanic crust in southern Peruvian subduction system
Author YoungHee Kim, Robert W. Clayton Date 2015-09-04
Attachments Kim_and_Clayton_2015_EPSL.pdf [2828335 byte]

The effects of the aseismic ridge subduction on the subduction dynamics have been long debated. Slab-flattening process have been commonly attributed to the excess buoyancy provided by the subduction of an anomalously thick crust of young oceanic lithosphere, as seismically imaged for Alaska (Kim et al., 2014), or of aseismic ridges, as proposed for South America (Gutscher et al., 2000). We use data from the recent seismic experiment in southern Peru to examine horizontal Nazca slab, which extends over a distance of ~800 km along the trench. Increased buoyancy of the oceanic lithosphere alone due to the subduction of Nazca Ridge is insufficient to fully explain such a lengthy segment.


We find that the subduction-related hydration plays a major role in controlling shear wave velocities within the upper part of the oceanic crust and overlying materials. We observe substantial velocity reductions of 20–40% near the top plate interface along- and perpendicular-to the trench from 40–120 km depths. In particular, significant shear wave velocity reductions and subsequently higher P-to-S velocity ratio (exceeding 2.0) at the flat slab region suggest that the seismically probed layer is fluid-rich and mechanically weak. The dominant source of fluid comes from metasediments and subducted crust (Nazca Ridge). Long-term supply of fluid from the southward migrating Nazca Ridge provides additional buoyancy of the subducting oceanic lithosphere and also lowers the viscosity of the overlying mantle wedge to drive and sustain the flat plate segment of 800 km along the trench. Also, by comparing calculated seismic velocities with experimentally derived mineral physics data, we additionally provide mechanical constraints on the possible changes in frictional behavior across the subduction zone plate interface. Observed low seismic velocities in the seismogenic zone suggest a presence of low strength materials that may be explained by overpressured pore fluids (i.e., accreted sediment included in the subduction channel).


Figure. Geophysical datasets probing the subducted Nazca Plate beneath southern Peru. a. Locations of broadband seismic stations, volcanoes, large-magnitude earthquakes, and bathymetric anomalies (Nazca Ridge and Nazca fracture zone). b. Seismic images from a subset of PG stations, showing a shallow-to-horizontal plate interface of the Nazca slab. c. Seismic images from a subset of PE stations, showing a dipping plate interface of the Nazca slab.


This work is published in Earth and Planetary Science Letters (doi:10.1016/j.epsl.2015.07.055).