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