Publication Date:
2019-09-23
Description:
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.
Type:
Article
,
PeerReviewed
Format:
text
DOI:
10.1016/j.epsl.2012.02.006
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