Seismic velocity structure of the slab and continental plate in the region of the 1960 Valdivia (Chile) slip maximum - insights into fluid release and plate coupling

Dzierma, Yvonne, Rabbel, Wolfgang, Thorwart, Martin, Koulakov, Ivan, Wehrmann, Heidi, Hoernle, Kaj and Comte, Diana (2012) Seismic velocity structure of the slab and continental plate in the region of the 1960 Valdivia (Chile) slip maximum - insights into fluid release and plate coupling Earth and Planetary Science Letters, 331-332 . pp. 164-176. DOI 10.1016/j.epsl.2012.02.006.

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The south-central Chilean subduction zone has witnessed some of the largest earthquakes in history, making
this region particularly important for understanding plate coupling. Here we present the results of a local
earthquake tomography study from a temporary local seismic network in the Villarrica region between 39
and 40°S, where the largest coseismic displacement of the 1960 Valdivia earthquake occurred. A lowvelocity
anomaly and high Vp/Vs values occur under the coastal region, indicating mantle serpentinisation
and/or underthrusting of forearc material. Further east, a high-velocity anomaly is observed, interpreted as
“normal” high-velocity mantle. Under the active volcanic arc a low-velocity anomaly together with high
Vp/Vs ratios (1.8 and higher) likely images fluid ascent beneath the volcanoes. Close to the subducting
Valdivia Fracture Zone, the coastal low-velocity anomaly extends further inland, where it interrupts and
shifts the high-velocity anomalies associated with “normal” fast mantle velocities. This may indicate enhanced
fluid presence along this part of the margin, probably caused by a stronger hydration of the incoming
plate along the Valdivia Fracture Zone. This is consistent with geochemical fluid proxies (U/Th, Pb/Ce, Ba/Nb)
in young volcanic rocks displaying peak values along the volcanic front at Llaima and Villarrica Volcanoes,
and with recent GPS measurements, which suggested a local reduction in plate coupling in this region. The
shift in the high-velocity anomaly underlying the central part may be caused by a north to south decrease
in plate age and hydration across the Valdivia Fracture Zone, and may explain why a Central Valley is absent
in this segment of the margin. The low La/Yb ratios in the volcanic rocks from Villarrica and Llaima suggest
that the high slab-derived fluid flux causes elevated degrees of melting beneath these volcanoes, providing
an explanation as to why these are amongst the most active volcanoes in South America.

Document Type: Article
Keywords: Geophysics; Seismology ; local earthquake tomography ; subduction zones ; south-central Chile ; Valdivia Fracture Zone plate coupling
Research affiliation: OceanRep > SFB 574 > C2
Kiel University
OceanRep > SFB 574
OceanRep > SFB 574 > A2
OceanRep > GEOMAR > FB4 Dynamics of the Ocean Floor > FB4-MUHS Magmatic and Hydrothermal Systems
OceanRep > The Future Ocean - Cluster of Excellence
Refereed: Yes
DOI etc.: 10.1016/j.epsl.2012.02.006
ISSN: 0012-821X
Projects: Future Ocean
Contribution Number:
SFB 574216
Date Deposited: 29 Jun 2011 08:22
Last Modified: 06 Mar 2017 11:05

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