Abstract
Guided wave dispersion is observed from earthquakes at 180–280 km depth recorded at stations in the fore-arc of Northern Chile, where the 44 Ma Nazca plate subducts beneath South America. Characteristic P-wave dispersion is observed at several stations in the Chilean fore-arc with high frequency energy (>5 Hz) arriving up to 3 s after low frequency (<2 Hz) arrivals. This dispersion has been attributed to low velocity structure within the subducting Nazca plate which acts as a waveguide, retaining and delaying high frequency energy. Full waveform modelling shows that the single LVL proposed by previous studies does not produce the first motion dispersion observed at multiple stations, or the extended P-wave coda observed in arrivals from intermediate depth events within the Nazca plate. These signals can however be accurately accounted for if dipping low velocity fault zones are included within the subducting lithospheric mantle. A grid search over possible LVL and faults zone parameters (width, velocity contrast and separation distance) was carried out to constrain the best fitting model parameters. Our results imply that fault zone structures of 0.5–1.0 km thickness, and 5–10 km spacing, consistent with observations at the outer rise are present within the subducted slab at intermediate depths. We propose that these low velocity fault zone structures represent the hydrated structure within the lithospheric mantle. They may be formed initially by normal faults at the outer rise, which act as a pathway for fluids to penetrate the deeper slab due to the bending and unbending stresses within the subducting plate. Our observations suggest that the lithospheric mantle is 5–15% serpentinised, and therefore may transport approximately 13–42 Tg/Myr of water per meter of arc. The guided wave observations also suggest that a thin LVL (∼1 km thick) interpreted as un-eclogitised subducted oceanic crust persists to depths of at least 220 km. Comparison of the inferred seismic velocities with those predicted for various MORB assemblages suggest that this thin LVL may be accounted for by low velocity lawsonite-bearing assemblages, suggesting that some mineral-bound water within the oceanic crust may be transported well beyond the volcanic arc. While older subducting slabs may carry more water per metre of arc, approximately one third of the oceanic material subducted globally is of a similar age to the Nazca plate. This suggests that subducting oceanic lithosphere of this age has a significant role to play in the global water cycle.
Highlights
Subducting oceanic crust is widely thought to deliver large amounts of water to the mantle, some of which is thought to be released from the slab and rise through the mantle wedge to cause arc volcanism
The results presented here offer a new constraint on the fine scale velocity structure of the subducting Nazca plate beneath Northern Chile, and suggest that the low velocity structures related to both metastable oceanic crust, and hydrated lithospheric mantle persists to depths of at least 220 km
Guided wave observations suggest that low velocity structures related to hydration at the outer rise may be present in the lithospheric mantle at intermediate depths, in addition to a thin low velocity layer (LVL) previously inferred
Summary
Subducting oceanic crust is widely thought to deliver large amounts of water to the mantle, some of which is thought to be released from the slab (causing intermediate depth earthquakes) and rise through the mantle wedge to cause arc volcanism. Observations from lower plane WBZ seismicity in Northern Japan suggest that these events may be directly associated with low velocity channels, caused by serpentinised outer rise normal faults that penetrate the subducting slab mantle (Garth and Rietbrock, 2014a, 2014b). Geodynamic modelling of the formation of outer rise normal faults suggests that the extent to which these faults penetrate into the subducting plate may increase with depth within the subduction zone, as fluids are forced down as the subducting slab unbends (Faccenda et al, 2009, 2012) These models demonstrate the mechanism by which the lithospheric mantle may become hydrated, allowing lower plane WBZ seismicity to occur due to the associated dehydration reactions. The addition of dipping low velocity normal fault structures accounts for the extended P-wave coda, and allows the clear dispersion of P-wave arrivals observed to be accurately simulated
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